madman
Super Moderator
Again would tread lightly on how high you run your trough/steady-state FT level!
This can easily backfire in the long-run for many especially when it comes to mood, libido and erectile function!
Too high a FT level can be just as bad in many ways as having too low a FT level.
Libido starts in the brain.
Neurotransmitters have a big impact especially dopamine.
There is a fine balance here when it comes to the dopamine system!
This is key here..... dopamine circuits are powerfully regulated by androgens!
*dopamine circuits are powerfully regulated by androgens
*androgens as potent modulators of prefrontal cortical operations and of closely related, functionally critical measures of prefrontal dopamine level or tone
*androgens dynamically control meso prefrontal dopamine systems and impact prefrontal states of hypo- and hyper-dopaminergia
*dopamine-dependent prefrontal operations appear to universally follow inverted U shaped functions
* androgens maintain a lifelong capacity to bidirectionally modulate prefrontal dopamine tone
*By targeting enzymes and signaling molecules associated with androgenic metabolites of testosterone (Fig 1), these studies more directly implicate androgens in modulating prefrontal function. They also show that both supranormal androgen stimulation and androgen deficiency negatively affects prefrontal operations (Fig 2A). This inverted U- shaped function is similar to that described for functional meso prefrontal dopamine settings (Cools R and D'Esposito M, 2011; Cools R et al.,2019; Floresco SB, 2013; Floresco SB and Magyar O, 2006)
*The data also demonstrate an inverted U-shaped function that describes these dopamine effects. According to this function, prefrontal dopamine levels— often referred to as prefrontal dopamine tone- that are either higher or lower than a functionally optimal set point are detrimental to behavior and circuit function (Fig 2B).
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Figure 2. Schematic diagrams showing inverted U-shaped functions for levels of androgens (A:X-axis) and levels of dopamine (B, C: X-axes) relative to performance on cognitive tasks that are dependent on the prefrontal cortex (A, B, C: Y-axes). A. Physiological levels of circulating androgens contribute to optimal cognitive performance whereas androgen deprivation and supranormal androgen signaling both result in cognitive impairment. B. Physiological levels of prefrontal dopamine are required for optimal cognitive performance and both hypo- (Hypo-[DA]) and hyperdopaminergia (Hyper-[DA]) impair cognitive performance. C. The data included in this review indicate that physiological androgen levels hold prefrontal dopamine concentrations within a functionally optimal range; that supranormal androgen levels, e.g.,related to anabolic androgenic steroid (AAS) use/misuse impairs cognition by producing hypodopaminergia; and that androgen deprivation, e.g., following gonadectomy (GDX), impairs prefrontal function by producing hyper-dopaminergia.
ABSTRACT
Epidemiological data show that males are more often and/or more severely affected by symptoms of prefrontal cortical dysfunction in schizophrenia, Parkinson’s disease and other disorders in which dopamine circuits associated with the prefrontal cortex are dysregulated. This review focuses on research showing that these dopamine circuits are powerfully regulated by androgens. It begins with a brief overview of the sex differences that distinguish prefrontal function in health and prefrontal dysfunction or decline in aging and/or neuropsychiatric disease. This review article then spotlights data from human subjects and animal models that specifically identify androgens as potent modulators of prefrontal cortical operations and of closely related, functionally critical measures of prefrontal dopamine level or tone. Candidate mechanisms by which androgens dynamically control meso prefrontal dopamine systems and impact prefrontal states of hypo- and hyper-dopaminergia in aging and disease are then considered. This is followed by discussion of a working model that identifies a key locus for androgen modulation of meso prefrontal dopamine systems as residing within the prefrontal cortex itself. The last sections of this review critically consider the ways in which the organization and regulation of mesoprefrontal dopamine circuits differ in the adult male and female brain, and highlights gaps where more research is needed.
Introduction
The prefrontal cortices in humans and animals orchestrate the brain’s highest order executive functions. These include goal setting, decision making, reward processing, behavioral flexibility and emotional regulation (Burgess PW and Stuss DT, 2017;Dalley JW et al., 2004; Friedman NP and Robbins TW, 2022; Kesner RP and Churchwell JC, 2011; Levy R, 2024; Miller EK, 2000). These functions are essential for the planning and execution of the activities of daily living and the negative consequences of prefrontal dysfunction in aging and in neuropsychiatric disease, e.g.,Alzheimer’s disease, schizophrenia, Parkinson’s disease, are profound (Amanzio M et al.,2020; Jones DT and Graff-Radford J, 2021; McDonald AP et al., 2018; Rabinovici GD et al.,2015 ;Smucny J et al., 2022;Xu P et al., 2019). In Parkinson’s disease, for example, over the course of illness, roughly 80% of patients will experience deficits in decision-making, behavioral flexibility, impulse control and other executive functions (Aarsland D et al., 2003; Aarsland D etal., 2021; Broeders M et al., 2013; Fang C et al., 2020; Gonzalez-Latapi P et al., 2021; Litvan | et al., 2011;Petkus AJ et al., 2020) which are often described by patients and caregivers as exacting an equal or greater toll on quality of life than the motor deficits associated with this condition (Kudlicka A et al., 2014; Schrag A et al., 2000; WinterY et al., 2011). Even more distressing is that treatment options offering clinical relief from prefrontal symptoms in Parkinson’s disease and other disorders are lacking. As a consequence, the cognitive symptoms of neuropsychiatric disease are often enduring and progressively worsening (Arnsten AF et al., 2015; Begemann MJ et al., 2020). This renders urgent the need to better understand the neural substrates that mediate prefrontal cortical operations in health and that underpin prefrontal cortical dysfunction in disease. Key factors that have been identified to date include the neurotransmitter dopamine and gonadal steroid hormones, e.g., estrogens, progestins,androgens. Here, evidence for androgen modulation of mesoprefrontal dopamine systems and the effects of these actions on the adult male brain are reviewed.
This article begins with a brief consideration of the sex differences that distinguish both prefrontal function in health and prefrontal dysfunction or decline in aging and neuropsychiatric disease. The patterns of male vulnerability revealed for the latter have motivated formulation of hypotheses for estrogen protections and intensive study of estrogen effects on prefrontal structure, function and pathophysiology. However, this review spotlights data from human subjects and animal models that identify androgens as potent modulators of prefrontal cortical operations and potential drivers of male susceptibility to prefrontal dysfunction in Alzheimer’s and Parkinson’s disease, schizophrenia and other conditions. Further, while the mechanisms underlying these actions are likely numerous, particular attention is paid to the data demonstrating a bidirectional androgen influence over functionally critical measures of prefrontal dopamine level or tone and how these could account for operationally disruptive states of prefrontal hypo- and hyper-dopaminergia that occur in aging or disease. Candidate mechanisms by which androgens may exert this dynamic control are then considered, including a working model that identifies a key, previously unsuspected locus for androgen modulation of prefrontal cortical dopamine levels that resides within the prefrontal cortex itself. Finally, this review also critically considers the striking and perhaps underappreciated ways in which the organization and regulation of meso prefrontal dopamine circuits differ in the male and female brain.
Part I. Androgen Influence Over Prefrontal Function and Dysfunction in Aging and Disease:
Sex differences
Follow-up investigations into roles for activational and/or organizational effects of gonadal hormones in shaping these and other sex differences have focused heavily on ovarian hormones, female subjects and on hypotheses for estrogen protections. These studies have shown that estrogens provide critical support for prefrontal cortical structure, chemistry and function in healthy subjects (Almey A et al., 2015; Jacobs E and D'Esposito M, 2011; Luine VN, 2014; Shanmugan Sand Epperson CN, 2014;Shughrue PJ and Merchenthaler |, 2000); exert significant protections from prefrontal disturbance induced by stress, aging and neuropsychiatric disease (Androvicova R et al., 2021; Bourque M et al., 2011; Garcia-Segura LM et al., 2001; Gillies GE and McArthur S, 2010;Mu E et al., 2024; Seeman MV, 2012; Sherwin BB, 2012); and appear to be beneficial as adjunct treatments in mitigating age- and disease-related cognitive decline (Ali N et al., 2023; Gava G et al., 2019; Gillies GE and McArthur S, 2010; Mu E,Gurvich C and Kulkarni J, 2024).While some of these effects are relevant for prefrontal function in males (Hogervorst E, 2012; Wibowo E, 2017; Zimmerman ME et al., 2011), estrogen-mediated actions alone do not fully explain the sex differences observed for prefrontal function or the alarming vulnerability of males to prefrontal cortical dysfunction in aging and disease. Rather, a more modest literature identifies specific roles for androgen signaling in modulating prefrontal function and dysfunction in human subjects and animal models. This article focuses on these modulatory actions as they relate to the adult male brain.
Hormone effects:
Human studies
Once associated mainly with reproductive functions or aggression, testosterone is now understood to exert a much wider influence over brain and behavior (Celec P et al., 2015; Nguyen TV, 2018; Ostatnikova D et al., 2020; Yaffe K, 2004; Zitzmann M, 2006). However, establishing testosterone actions as androgenic is complicated by systemic, local and brain region-specific patterns of testosterone catabolism that yield metabolites capable of signaling through androgen and/or estrogen receptor mediate 4d pathways (Fig 1) (Durdiakova J et al., 2011; Handelsman DJ, 2000). For these reasons, studies showing correlations between levels of testosterone and/or its precursors [e.g.,dehydroepiandrosterone (DHEA), DHEA sulfate] and performance on prefrontal cortical dependent tasks in healthy adults (de Menezes KJ et al., 2016; Elpers AL and Steptoe A,2020; Gouchie C and Kimura D, 1991; Lu PH et al., 2006) and in hypogonadal and aging male populations (Cai Z and Li H, 2020; Dong X et al., 2021; Gurvich C, Thomas N and Kulkarni J, 2020; Janowsky JS, 2006; Janowsky JS et al., 1994; Lasaite L et al., 2014; Lv W et al., 2016) leave uncertain whether relevant brain actions are androgenic, estrogenic or both. Similarly, while some promise for testosterone augmentation has been reported for prefrontal deficits in aging, Alzheimer’s disease and schizophrenia (Gregori G et al., 2021; Jung HJ and Shin HS, 2016 ;Ko Y Het al., 2008; Pike CJ et al., 2009; Wahjoepramono EJ et al., 2016), the formulations used often leave unanswered whether benefits are derived from testosterone’s estrogenic and/or androgenic metabolites. However, fewer in number are other studies that have identified cognitive deficits in male subjects: with genetic disruptions of androgen receptor function, e.g., CAG repeats, congenital androgen-insensitivity (Lee DM et al., 2010; Tan S et al., 2021); taking anti-androgen medications, e.g., for treatment of prostate cancers (Cherrier MM and Higano CS, 2020; Jamadar RJ et al., 2012; McGinty HL et al., 2014); undergoing hormone augmentation using non-aromatizable androgens (Cherrier MM et al., 2003); or using or misusing anabolic androgenic steroids (AAS (Bjornebekk A et al., 2019; Frati P et al., 2015; Hauger LE et al., 2020; Kanayama G et al., 2013; Scarth M and Bjornebekk A, 2021)). By targeting enzymes and signaling molecules associated with androgenic metabolites of testosterone (Fig 1), these studies more directly implicate androgens in modulating prefrontal function. They also show that both supranormal androgen stimulation and androgen deficiency negatively affects prefrontal operations (Fig 2A). This inverted U- shaped function is similar to that described for functional meso prefrontal dopamine settings (Cools R and D'Esposito M, 2011; Cools R et al.,2019; Floresco SB, 2013; Floresco SB and Magyar O, 2006). As this review proceeds, data supporting causal relationships between these two functions are considered. Unusually low levels of plasma testosterone, free testosterone and/or DHEA/DHEA-S have been reported in studies of male patients diagnosed with Alzheimer’s disease, schizophrenia and Parkinson’s compared to age-matched healthy subjects (Bianchi VE, 2022; Lv W, Du N, Liu Y ,Fan X, Wang Y,Jia X, Hou X and Wang B, 2016; Nitkowska M et al., 2015; Okun MS et al., 2004; Taherianfard M and Shariaty M, 2004; Yeap BB and Flicker L, 2022) and in some cases ,significant correlations have been noted between these levels and the severity of cognitive or negative symptoms (Akhondzadeh S et al., 2006; Bianchi VE, 2022; Bourque M, Soulet D and DiPaolo T, 2021; Driscoll | and Resnick SM, 2007; Jaeger ECB et al., 2020; Jimenez-Rubio G et al., 2017; Jurado-Coronel JC, Cabezas R, Avila Rodriguez MF, Echeverria V, Garcia-Segura LM and Barreto GE, 2018; Moore .et al., 2013; Pike CJ, Carroll JC, Rosario ER and Barron AM, 2009; Russillo MC, Andreozzi V, Erro R, Picillo M, Amboni M,Cuoco S, Barone P and Pellecchia MT,2022;Yeap BB and Flicker L, 2022). However, there are also studies finding no significant correlations between testosterone and cognitive health, neuropsychiatric diagnoses or cognitive impairment in aging or disease (Cherrier MM et al., 2015; Kaufman JM and Lapauw B, 2020; Ulubaev A et al., 2009). The results from clinical trials examining the effectiveness of testosterone augmentation in mitigating prefrontal dysfunction in aging and disease are also mixed (Lisco G et al., 2020; Okun MS et al., 2006; Okun MS et al., 2002; Yalamanchi S and Dobs A, 2017). Nonetheless, the balance of information to date has been sufficiently promising and the need for more information sufficiently pressing to prompt continued investigation of testosterone effects in human subjects. Importantly, this work both informs and is informed by studies in animal models and preclinical animal models of neuropsychiatric disease where sample sizes, population variance and other factors that often challenge human studies can be well controlled and flexible tools are available to facilitate investigations that focus specifically on androgen effects.
Hormone effects:
Animal studies
As in the human studies (above), these data demonstrate that androgen effects on prefrontal function in rats follow inverted U-shaped functions (Fig 2A). In addition, a growing literature further shows that male vulnerability to prefrontal dysfunction in disease is recapitulated in several leading rodent models of Alzheimer’s disease (Jiao SS et al., 2016; Pike CJ, 2017; Yang JT et al., 2018), Parkinson’s disease (Bourque M, Soulet D and Di Paolo T, 2021; Gillies GE, Pienaar IS, Vohra S and Qamhawi Z, 2014; Pinizotto CC, Patwardhan, A.Aldarondo, D. Kritzer, M.F., 2022; Russillo MC, Andreozzi V, Erro R, Picillo M, Amboni M, Cuoco S, Barone P and Pellecchia MT, 2022) and schizophrenia (Hill RA, 2016; Kokras N and Dalla C, 2014). For some of these models, potent and potentially therapeutic effects of androgens on brain pathologies and/or behaviors have also been described.
Part Il. Androgen Influence Over Prefrontal Dopamine Tone:
Organizational and activational hormone effects
Estrogens, progestins, androgens and other steroid hormones exert a range of permanent organization effects on the pre-, early post-natal and pubertal cerebral cortex (Karaismailoglu S and Erdem A, 2013; Rebuli ME and Patisaul HB, 2016). These hormones also have diverse activational effects that continue to modulate cortical structure and function for the remainder of the lifespan (Meyer CE et al., 2023; Pompili A et al., 2020; Singh M, 2006; Singh M et al., 2006). The effects of testosterone, its non-aromatizable derivatives and/or AAS on the cerebral cortex or cultured cortical neurons range from regulation of cell survival, apoptosis and adult neurogenesis (Spritzer MD and Roy EA, 2020; Zelleroth S et al., 2019), to acute effects on neuronal excitability and synaptic plasticity to sculpting circuit substrate, including dendritic spine morphology and synaptic density (Hajszan Tet al., 2008). However, the focus of this review is on androgen modulation of the meso prefrontal dopamine systems and the functionally critical measure of prefrontal dopamine tone. The cells of origin of the meso prefrontal dopamine system are anatomically intermingled with dopamine neurons projecting to neostriatum, nucleus accumbens and other target regions in the ventral tegmental area (VTA), rostral linear nucleus and medial substantia nigra (Lammel S et al., 2011; Yamaguchi T et al., 2011; Yetnikoff L et al., 2014). However, the dopamine neurons projecting to the prefrontal cortex form a discrete subset of cells that do not send axon collaterals to other cortical, subcortical or allocortical brain regions. Dynamic features of mesoprefrontal dopamine signaling, e.g., uptake/reuptake, autoregulation, also differ markedly from those of the nigrostriatal and mesolimbic systems (Cass WA and Gerhardt GA, 1995; GarrisPA et al., 1993;Lammel S, lon DI, Roeper J and Malenka RC, 2011). Accordingly, readers are referred to separate reviews describing androgen effects on nigrostriatal and mesolimbic dopamine circuits (Becker JB, 1999; Becker JB and Chartoff E, 2019; Gillies GE et al., 2014; Low KL et al., 2020; Seib DR et al., 2023; Tobiansky DJ et al., 2018; Zachry JE et al., 2021) and are cautioned about the generalizability of findings across these systems.
Dopamine effects on prefrontal function
The primacy of the meso prefrontal dopamine system for frontal lobe operations is illustrated in studies using chemical lesioning, pharmacological and other strategies to selectively manipulate dopamine signaling (Arnsten AF, Wang M and Paspalas CD, 2015; Floresco SB, 2013; Floresco SB and Magyar O, 2006; Ott T and Nieder A, 2019; Puig MV et al., 2014); in all cases, evidence points to selective perturbations of prefrontal dopamine pathways as significantly impairing working memory, attention, goal-directed behavior, behavioral flexibility, impulsivity and other classical prefrontal cortical functions. The data also demonstrate an inverted U-shaped function that describes these dopamine effects. According to this function, prefrontal dopamine levels— often referred to as prefrontal dopamine tone- that are either higher or lower than a functionally optimal set point are detrimental to behavior and circuit function (Fig 2B). This rule explains the prefrontal impairments associated with hyper-dopaminergic states, e.g., those induced by mild stress (Arnsten AF, 2000; Arnsten AF, 2015; Arnsten AFT et al., 2023; Luine V, 2002; Luine V et al., 2017; Shansky RM and Lipps J, 2013), as well as the prefrontal dysfunction associated with hypodopaminergia in aging, Alzheimer’s disease and schizophrenia (Ciampa CJ et al., 2022; Guillin Oet al., 2007; Pan X et al., 2019). However, clinical attempts to restore optimal prefrontal dopamine levels are often complicated by competing needs to rectify imbalances in other, e.g.,nigrostriatal, mesolimbic, dopamine systems. For example, the dopamine-blocking actions of antipsychotic medications that effectively quell positive symptoms in schizophrenia and the dopamine potentiating drugs that reduce motor symptoms in patients with Parkinson’s disease have little benefit for the prefrontal impairments associated with these disorders (Martinez ALet al., 2021;Zhang Q et al., 2020). However, as reviewed below, androgens maintain a lifelong capacity to bidirectionally modulate prefrontal dopamine tone which could ultimately be useful in improving treatment for cognitive deficits in these and other conditions.
Androgen effects on prefrontal dopamine level/tone
Studies examining the effects that sex hormones have on frontal cortex dopamine levels in adult male rats reveal consensus findings for androgens as tonically suppressing meso-prefrontal dopamine systems. First, gonadectomy of more than a few week’s duration has been shown: to increase dopamine, but not serotonin and/or norepinephrine concentrations in frontal cortical homogenates (Battaner E et al., 1987; Kokras N et al., 2018); to nearly double the density of prefrontal cortical axons that are immunoreactive for the dopamine marker tyrosine hydroxylase (TH), but not axons labeled with markers for serotonin, norepinephrine or acetylcholine (Adler A et al., 1999; Kritzer MF, 2000; Kritzer MF, 2003; Kritzer MF et al., 1999; Kritzer MF, Brewer A, Montalmant F, Davenport Mand Robinson JK, 2007); and to significantly increase extracellular dopamine concentrations measured in vivo using microdialysis interfaced with high pressure liquid chromatography (Aubele T and Kritzer MF, 2011; Aubele T and Kritzer MF, 2012). Gonadectomy on postnatal day 60 has also been shown to increase the numbers of dopamine neurons present in the VTA at postnatal day 90 relative to sham operated controls, possibly by attenuating developmental pruning these dopamine neurons (Johnson ML et al., 2010). Importantly, the studies of axon density, dopamine cell number and in vivo dopamine levels further showed that the effects of gonadectomy on these endpoints are attenuated in gonadectomized rats supplemented with testosterone or dihydrotestosterone but are not affected by treatment with estradiol. On the other hand, studies in gonadally intact adult male rats have shown that treatment with AAS significantly decreases concentrations of dopamine and serotonin, but not noradrenalin, measured in homogenates of prefrontal cortical tissue (Tucci P et al., 2012). Together these findings show that the same androgen-specific experimental manipulations that negatively affect prefrontal function also induce changes in prefrontal dopamine. Further, they identify androgen depletion vs. augmentation as inducing behaviorally disruptive states of prefrontal hyper- and hypo-dopaminergia, respectively (Fig 2C). Accordingly, age or disease-related changes in these bidirectional effects could well be important in rendering males the significantly more vulnerable sex to prefrontal cortical dysfunction in disorders where pathophysiology includes meso prefrontal dopamine over- or underactivity. Moreover,manipulations of androgen signaling may have therapeutic effects on prefrontal deficits. Both possibilities have motivated deeper investigations into the mechanisms that underpin androgenic control over prefrontal dopamine tone.
Part Ill: Mechanisms, Substrates and Working Models for Androgen Effects on Meso prefrontal Dopamine Systems:
Mechanisms: Androgen effects on prefrontal dopamine uptake/reuptake, turnover and catabolism.
The kinetics and dynamics of dopamine signaling in the prefrontal cortices differ markedly from the characteristics of dopamine signaling in other brain regions. For example, in the prefrontal cortex tonic levels of dopamine release are relatively low, dopamine clearance from the synaptic space is relatively slow and dopamine turnover rates are relatively high compared to corresponding measures in neostriatum and other major target regions of the nigrostriatal and mesolimbic dopamine systems (Cass WA and Gerhardt GA, 1995; Garris PA, Collins LB, Jones SR and Wightman RM, 1993). Androgens have been shown to influence several of the mechanisms that contribute to these unique settings—some in ways that could explain androgens’ long-term biasing of prefrontal dopamine levels. For example, gonadectomy in adult rats and mice has been shown to reduce ligand binding affinity for the prefrontal norepinephrine transporter and reduce prefrontal dopamine transporter expression,respectively, in ways that are attenuated by supplementing animals with testosterone or dihydrotestosterone but not estradiol (Du X et al., 2019; Meyers B and Kritzer MF, 2009). However, dopamine uptake and reuptake machineries in the prefrontal cortex are notably sparse (Miner LH et al., 2003; Sesack SR et al., 1998) and pharmacological blockade of transporter function has only modest effects on prefrontal dopamine levels (Cass WA and Gerhardt GA, 1995; Garris PA and Wightman RM, 1994). Thus, it is unlikely that androgen effects on transporter proteins alone account for the significant effects that this steroid hormone has on readouts of prefrontal dopamine level. However, the extant literature for androgen modulation of prefrontal dopamine turnover includes studies showing prefrontal dopamine turnover is unaffected (Handa RJ et al., 1997) or decreased by gonadectomy (KokrasN, Pastromas N,Papasava D, de Bournonville C, Cornil CA and Dalla C, 2018) and is decreased by treatment with aromatase inhibitors (Kokras N, Pastromas N, Papasava D,de Bournonville C, Cornil CA and Dalla C, 2018) or increased by AAS (Tucci P, Morgese MG, Colaianna M ,Zotti M, Schiavone S, Cuomo V and Trabace L, 2012) in gonadally intact males. Similarly, studies examining the effects of gonadectomy or AAS treatment on the activity of catechol-O-methyltransferase and monoaminoxidase in prefrontal homogenates found either no effects, effects that were estrogen-dependent or effects that were androgenic but targeted enzyme isoforms with preferred substrates of serotonin or norepinephrine rather than dopamine (Birgner C et al., 2008; Luine VN and Rhodes JC, 1983; Meyers B et al., 2010). Thus, there is currently little consensus support for androgen effects on turnover or catabolism as contributing to observed regulation of prefrontal dopamine tone.
Mechanisms: Androgen effects on meso prefrontal dopamine cell firing
Action potential firing rates in meso prefrontal and other populations of midbrain dopamine neurons normally vacillate between sustained periods of slow, irregular 2-5 Hz single spiking that are punctuated by brief periods of higher frequency (10-20 Hz) burst-firing (Grace AA and Bunney BS, 1984; Overton PG and Clark D, 1997; Seamans JK and Yang CR, 2004). The slower, single spiking modes of activity set basal dopamine levels that are thought to stabilize prefrontal circuits and maintain internal representations of behaviorally relevant events, strategies and goals (Durstewitz D and Seamans JK, 2008; Ellwood IT et al., 2017; Seamans JK and Yang CR, 2004). Burst firing, on the other hand, is often event related and serves to transiently increase prefrontal dopamine release and raise extracellular dopamine concentrations; these phasic events that are viewed as important for updating internal representations and changing behaviors and behavioral strategies (Durstewitz D and Seamans JK, 2008; Ellwood IT, Patel T,Wadia V, Lee AT, Liptak AT, Bender KJ and Sohal VS, 2017; Seamans JK and Yang CR, 2004). Single unit extracellular recordings in adult male rats have shown that gonadectomy has no effect on single spiking activity (Locklear MN et al., 2017). However, it significantly and selectively increases the incidence and the frequency of burst firing in dopamine neurons of the ventral tegmental area but not the adjacent substantia nigra in a testosterone-reversible, estrogen-insensitive manner (Locklear MN, Michealos M, Collins WF and Kritzer MF, 2017). This could explain observed effects of gonadectomy and hormone replacement on prefrontal dopamine levels. Moreover, it is tempting to speculate that the updating of prefrontal information normally associated with phasic dopamine burst firing is facilitated in rats where androgens are depleted and impaired in subjects where androgen signaling is augmented. Both predictions align with behavioral data showing that perseverative responding is reduced in set shifting tasks in male rats treated with blockers of androgen synthesis or receptors (Tomm RJ et al., 2022) and is increased in rats treated with exogenous testosterone or AAS (Wallin KG and Wood RI, 2015; Wood RI and Serpa RO, 2020). This underscores the importance of knowing where and how androgens modulate midbrain dopamine cell activity. As discussed below, the question of where relevant action is levied has been addressed in part by mapping the distributions of the intracellular androgen receptors that mediate the genomic actions that are presumed to be involved.
Substrates: Intracellular androgen receptors as proxies for realms of influence
One way that androgens could influence the tempo of cell firing in the VTA is by modifying channel, membrane or other intrinsic properties of the dopamine cells themselves. However, the precise distributions of intracellular androgen receptors (AR) within TH-immunoreactive, presumed dopaminergic ventral midbrain neurons do not match the selectivity or the magnitude of androgen effects on meso prefrontal dopamine systems. Rather, immunoreactivity for AR is present in comparable, and comparably low, proportions of TH-immunoreactive neurons in the VTA, substantia nigra and the retrorubral fields (Kritzer MF, 1997; Simerly RB et al., 1990). Further, triple labeling studies (Fig 3A) that concurrently defined midbrain neurons as dopaminergic, as containing AR and as projecting to the prefrontal cortex, showed that only about a third of midbrain dopamine neurons projecting to prefrontal cortex contain AR (KritzerMF and Creutz LM, 2008). These proportions were not appreciably different from the percentages of AR-containing midbrain dopamine neurons projecting to primary or premotor cortices (Kritzer MF and Creutz LM, 2008)or to subcortical targets including amygdala, nucleus accumbens and neostriatum (Creutz LM and Kritzer MF, 2004). However, a survey of AR immunoreactivity among more than 27 ipsilateral and contralateral cortical and subcortical regions projecting to the VTA (back-labeled by retrograde tracer injections in the VTA) revealed that projections arising from the prefrontal cortex itself were far more and were by far the most AR-enriched (Aubele T and Kritzer MF, 2012). Thus, while the percentages of AR immunoreactive VTA afferents arising from brainstem, midbrain and forebrain nuclei were at most only about 30%, between 50 and 60% of prefrontal neurons retrogradely labeled by VTA injections were also immunoreactive for AR (Fig 3B). This places an abundance of androgen influence within cortical efferents known to make monosynaptic synapses onto meso prefrontal dopamine neurons (Beier KT et al., 2015; Carr DB and Sesack SR, 2000) and to exert top-down control over midbrain dopamine cell firing and dopamine release back in the prefrontal cortex (Gariano RF and Groves PM, 1988; Overton PG et al., 1996;Tong ZY et al., 1996). The section below focuses on a working model for androgen effects that is centered on this critical corticofugal pathway and the data that support it.
Working Model
Studies in male rodents using in vivo microdialysis or electrophysiology have shown that prefrontal cortical pyramidal cells projecting to the VTA exert dynamic control over cell firing rates in meso prefrontal (and other) dopamine neurons (Gariano RF and Groves PM, 1988; Overton PG, Tong ZY and Clark D, 1996; Tong ZY, Overton PG and Clark D, 1996). These actions in turn modulate the amount of transmitter that is released from the axon terminal of midbrain dopamine neurons projecting back to the prefrontal cortex (Garris PA, Collins LB, Jones SR and Wightman RM, 1993; Garris PA and Wightman RM, 1994; Overton PG and Clark D, 1997). Studies carried out in gonadally intact male rats that combined in vivo microdialysis with reverse dialysis drug administration further showed that excitability in this pivotal pathway—and ultimately its influence over prefrontal dopamine levels— is regulated by functionally offsetting intracortical glutamatergic influences (Feenstra MG et al., 1995; Jedema HP and Moghaddam B, 1996;Takahata R and Moghaddam B, 1998). These include direct activation of VTA-projecting prefrontal pyramidal cells by a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors; and N-methyl-D-aspartate (NMDA) receptor-mediated influences that excite prefrontal inhibitory interneurons and di-synaptically inhibit cell firing in VTA-projecting prefrontal pyramidal cells (Fig 4A). Experimentally, when AMPA agonists have been intra cortically applied, increases in the prefrontal cortex’s descending drive, increases in burst firing in meso prefrontal dopamine neurons cells and increases in dopamine levels/release back in the prefrontal cortex have all been observed (Jedema HP and Moghaddam B, 1996; TakahataR and Moghaddam B, 1998). However, when NMDA agonists are intra cortically delivered,prefrontal interneurons are activated; this in turn inhibits firing in VTA-projecting prefrontal pyramidal cells, slows firing in midbrain dopamine cell neurons and reduces dopamine levels/release in the prefrontal cortex (Feenstra MG et al., 2002; Jedema HP and Moghaddam B, 1996; Takahata R and Moghaddam B, 1998). Studies in gonadectomized and hormone-replaced male rats showed that circulating androgens modulate these intra cortical glutamatergic influences and have especially strong effects on NMDA-mediated mechanisms. Specifically,reverse dialysis administration of the NMDA receptor subtype-selective antagonist 2-amino-5-phosphonovaleric acid (APV) normally serves to di-synaptically disinhibit VTA-projecting prefrontal pyramidal cells and increase extracellular prefrontal dopamine concentrations (Fig 4A, C) (Feenstra MG, Botterblom MH and van Uum JF, 2002; Jedema HP and Moghaddam B, 1996;Takahata R and Moghaddam B, 1998). However, in gonadectomized males APV infusion decreases prefrontal dopamine levels in an androgen sensitive, estrogen insensitive manner(Aubele T and Kritzer MF, 2012) (Fig 4 C, D). This suggests that androgen deprivation functionally re-directs NVUDA-receptor mediated activation to VTA-projecting prefrontal pyramidal cells (Fig 4B)—a hypothesis supported by data from microdialysis/reverse dialysis dual drug challenge studies in which either the voltage gated sodium channel blocker tetrodotoxin (TTX) or the selective GABA- B receptor antagonist CGP52432 was intra cortically co-applied with NMDA or APV. While these infusion paradigms blocked the effects of NUDA and APV on prefrontal dopamine levels in gonadally intact animals (Fig 4E, G) (Locklear MN etal., 2016), neither affected NUDA-mediated stimulation or APV’s paradoxical inhibition of prefrontal dopamine levels in gonadectomized males (Fig 4F, H). This confirms the sphere of functional NMDA influence in gonadectomized rats as downstream of prefrontal interneurons and directly impacting VTA-projecting prefrontal pyramids (Fig 4B). Additional support for this model includes data from single unit extracellular recordings studies showing that gonadectomy significantly and selectively increases burst-firing only in infragranular pyramidal cells identified by collision testing as projecting to the VTA; these effects were androgen sensitive and estrogen insensitive (Locklear MN, Michealos M, Collins WF and Kritzer MF, 2017). However, most exciting are the data that align this working model with androgen-sensitive effects of gonadectomy on dopamine-dependent prefrontal behaviors.
Treatment with NMDA antagonists induce prefrontal hyper dopaminergic and to impair delayed spatial alternation performance in gonadally intact male rats in a manner that is attenuated by dopamine receptor antagonists (Verma A and Moghaddam B, 1996). However, In gonadectomized and gonadectomized rats given estradiol where VTA-projecting neurons are excessively bursty (Locklear MN, Michealos M,Collins WF and Kritzer MF, 2017) and prefrontal dopamine levels are excessively high (Aubele T and Kritzer MF, 2011; Aubele T and Kritzer MF, 2012), the actions of intracortical APV infusion serve to return meso prefrontal dopamine settings to more normal values. Assuming that the androgen- and NMDA-sensitive, hyper dopaminergic states induced by gonadectomy are responsible for the negative impacts of this manipulation on prefrontal function, intracortical infusion of APV should rescue behavioral deficits in gonadectomized and gonadectomized rats given estradiol and induce them in control and gonadectomized rats given testosterone. This prediction was tested using processes of spatial cognition measured in Barnes maze testing as readouts (Fig 5A). Previous studies showed that commission of primary and perseverative errors, path lengths and times taken to discover a goal location are significantly increased in gonadectomized compared to control male rats and mitigated by supplementing gonadectomized rats with either testosterone or estradiol, i.e., were estrogen-sensitive (Locklear MN and Kritzer MF, 2014). However,gonadectomy also induced deficits in task solving strategies that were androgen-sensitive (Locklear MN and Kritzer MF, 2014). Specifically, gonadally intact and gonadectomized males treated with testosterone progressively refined what were initially random searches to ultimately navigate directly or nearly directly to the goal location. However, gonadectomized and gonadectomized rats given estradiol showed little spatial organization to their searches and persisted in randomly investigating possible goal locations across all four trials (Locklear MN and Kritzer MF, 2014). These group specific strategies were also seen in Barnes maze testing in animals given bilateral intra-prefrontal infusions of saline (Fig 5B, left). However, when APV was infused, the gonadectomized and gonadectomized rats given estradiol were the ones to successfully develop efficient, spatially directed search strategies while the gonadally intact controls and gonadectomized rats given testosterone did not (Locklear MNaK, M.F., 2012) (Fig 5B, right). These data indicate that androgen regulation of NMDA receptor-mediated effects on meso prefrontal dopamine physiology contributes significantly to androgen’s impacts on prefrontal cortex-dependent cognition in the adult male brain. However, as discussed below, at least some of these actions may be unique to this biological sex.
PART IV. Sex Differences in Meso prefrontal Dopamine Systems:
The theme of this review—the effects of androgens on meso prefrontal dopamine systems in the adult male brain— was stimulated in large part by epidemiological data showing that males are more often and/or more severely affected by deficits in prefrontal cortical function in neuropsychiatric disorders where meso prefrontal dopamine circuits are also dysregulated. However, it seems important to conclude this review by considering some of the ways in which meso prefrontal dopamine systems are similar and some of the ways in which they are fundamentally different in the female brain.
While dopamine-dependent prefrontal operations appear to universally follow inverted U shaped functions, the question of whether functionally optimal resting dopamine levels differ between the sexes is currently unresolved. Thus, while one study that measured dopamine and its major metabolites in prefrontal homogenates in mice reported nearly two-fold higher levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in male compared to female subjects (Kawakami SE et al., 2013), two similar studies in rats identified higher concentrations of dopamine but not dopamine metabolites in females compared to males (Duchesne A et al., 2009; Kokras N, Pastromas N, Papasava D, de Bournonville C, Cornil CA and Dalla C, 2018) and two others found no sex differences in any of dopamine measures (Laatikainen LM et al., 2013; Raftogianni A et al., 2014). Further, of two studies that quantified dopamine levels in rat prefrontal cortex using in vivo microdialysis, one found no sex differences (Locklear MN, Cohen AB, Jone A and Kritzer MF, 2016) and the other reported higher extracellular dopamine levels in females compared to males (Staiti AM et al., 2011). It should be noted, however, that the female subjects in the second study were ovariectomized and injected every other day subcutaneously with estradiol. Because the male subjects were unoperated and uninjected, it is unknown whether group differences in mild stress, which is well known to elevate prefrontal dopamine levels in sex- and sex- hormone dependent ways (Luine V, Gomez J, Beck K and Bowman R, 2017; Shansky RM and Lipps J, 2013), may have influenced the results. However, recent studies have also shown that sex differences in meso accumbal dopamine systems are rat strain-specific (Rivera-Garcia MT et al., 2020). Thus, the use of Sprague Dawley, Long Evans, Wistar and Lister hooded rats across the studies noted above must also be considered as a potential source of variance in the data.
While sex differences in resting prefrontal dopamine levels are uncertain, robust sex differences have been identified for other aspects of the meso prefrontal dopamine systems. For example, while this system is classically known to be comprised of afferents that are roughly one-third dopaminergic and two-thirds non-dopaminergic (Swanson LW, 1982), tract tracing studies in male and female rats suggest that these ratios only apply to the male brain (Kritzer MF and Creutz LM, 2008). Specifically, experiments that combined cortical injections of retrograde tracers with TH-immunocytochemistry showed that fewer than 30% of ventral midbrain neurons contributing to meso prefrontal pathways were dopaminergic in adult male rats while more than 50% of retrogradely labeled cells in females were TH-immunopositive (Kritzer MF and Creutz LM, 2008) (Fig 6). Even more striking are sex differences found for the ways in which intracortical NUDA-receptor mediated mechanisms regulate prefrontal dopamine tone. Specifically, in vivo microdialysis/reverse dialysis drug challenges carried out in the prefrontal cortices showed that the effects of NMDA-receptor antagonists in female rats were diametrically different from those present in males. Thus, whereas NMDA antagonists release a GABA-mediated disynaptic brake that ultimately elevates prefrontal dopamine levels in males (Fig 7A C), in females NMDA antagonists acted similarly to AMPA receptor antagonists to decrease prefrontal dopamine levels (Locklear MN, Cohen AB, Jone A and Kritzer MF, 2016) (Fig 7B, D). This resembles the dysregulated state induced in males by gonadectomy. However,studies in which the selective GABA-B receptor antagonist CGP52432 was co-infused with APV identified a significantly more powerful GABA-mediated inhibitory influence at work in the female prefrontal cortex (Fig, 7E, F) that prevents synergistic NVDA/AMPA activation of VTA projecting prefrontal pyramidal cells from driving their meso prefrontal dopamine systems to hyper-dopaminergic states. It is possible that these sex differences bias the frontal lobes in females toward hyper-dopaminergia, which could be a basis for understanding and ultimately overcoming the greater vulnerability of prefrontal function in females to stress (Luine V, Gomez J, Beck K and Bowman R, 2017; Luine VN, 2007; Shansky RM and Lipps J, 2013). While further study is needed to substantiate this and other hypotheses, the critical involvement of NUDA receptor-mediated signaling and dopamine transmission in cognitive function and dysfunction in aging and disease combines with the prominent use of NMDA-selective drugs in mitigating these symptoms in schizophrenia, Parkinson’s disease, Alzheimer’s disease (Balu DT,2016;Painuli S et al., 2023;VanleB et al., 2018) to underscore the importance of fully clarifying sex differences and sex hormone effects on the amino acid transmitter and dopamine systems that powerfully influence frontal lobe function.
Summary and Conclusions
The sum of findings presented here suggests that in the adult male prefrontal cortex, circulating androgens exert tonic, suppressive influences over meso prefrontal dopamine systems that hold prefrontal dopamine levels within functionally optimal ranges. When this influence is diminished, cognitive deficits are induced as a result of multiple processes that selectively drive prefrontal dopamine levels to dysfunctional ‘too much’, i.e., hyper-dopaminergic states. On the other hand, when androgen stimulation is increased, the resultant cognitive deficits observed coincide with prefrontal dopamine levels that fall below a functionally optimal range. These bidirectional effects are largely explained by a working model for androgen-mediated modulation of the NMDA-sensitivity in VTA-projecting prefrontal pyramidal cells. Supporting data include evidence showing that gonadectomy increases burst-firing in these pyramidal cell populations and in the midbrain dopamine neurons they project, and also induces paradoxical elevations in local dopamine levels in response to intra-prefrontal infusion of NMDA antagonists—all in testosterone-sensitive, estrogen-insensitive ways. These impacts are likely to contribute to the susceptibility of human males and male animal subjects to prefrontal dysfunction in Parkinson’s disease, schizophrenia and other conditions. Moreover, there seems to be hope that the bidirectional effects of androgen deprivation and augmentation might ultimately be tailored to re-balance meso prefrontal dopamine systems that are differentially dysregulated across these conditions. However, while this potential adds impetus to further study and better understand androgen effects on dopamine-dependent cognitive processes in males, it is important to emphasize that sex differences also distinguish the ways in which the meso prefrontal dopamine systems are organized and modulated. Thus, the data argue equally strongly for the need for parallel, independent investigations in female subjects.
This can easily backfire in the long-run for many especially when it comes to mood, libido and erectile function!
Too high a FT level can be just as bad in many ways as having too low a FT level.
Libido starts in the brain.
Neurotransmitters have a big impact especially dopamine.
There is a fine balance here when it comes to the dopamine system!
This is key here..... dopamine circuits are powerfully regulated by androgens!
*dopamine circuits are powerfully regulated by androgens
*androgens as potent modulators of prefrontal cortical operations and of closely related, functionally critical measures of prefrontal dopamine level or tone
*androgens dynamically control meso prefrontal dopamine systems and impact prefrontal states of hypo- and hyper-dopaminergia
*dopamine-dependent prefrontal operations appear to universally follow inverted U shaped functions
* androgens maintain a lifelong capacity to bidirectionally modulate prefrontal dopamine tone
*By targeting enzymes and signaling molecules associated with androgenic metabolites of testosterone (Fig 1), these studies more directly implicate androgens in modulating prefrontal function. They also show that both supranormal androgen stimulation and androgen deficiency negatively affects prefrontal operations (Fig 2A). This inverted U- shaped function is similar to that described for functional meso prefrontal dopamine settings (Cools R and D'Esposito M, 2011; Cools R et al.,2019; Floresco SB, 2013; Floresco SB and Magyar O, 2006)
*The data also demonstrate an inverted U-shaped function that describes these dopamine effects. According to this function, prefrontal dopamine levels— often referred to as prefrontal dopamine tone- that are either higher or lower than a functionally optimal set point are detrimental to behavior and circuit function (Fig 2B).
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Figure 2. Schematic diagrams showing inverted U-shaped functions for levels of androgens (A:X-axis) and levels of dopamine (B, C: X-axes) relative to performance on cognitive tasks that are dependent on the prefrontal cortex (A, B, C: Y-axes). A. Physiological levels of circulating androgens contribute to optimal cognitive performance whereas androgen deprivation and supranormal androgen signaling both result in cognitive impairment. B. Physiological levels of prefrontal dopamine are required for optimal cognitive performance and both hypo- (Hypo-[DA]) and hyperdopaminergia (Hyper-[DA]) impair cognitive performance. C. The data included in this review indicate that physiological androgen levels hold prefrontal dopamine concentrations within a functionally optimal range; that supranormal androgen levels, e.g.,related to anabolic androgenic steroid (AAS) use/misuse impairs cognition by producing hypodopaminergia; and that androgen deprivation, e.g., following gonadectomy (GDX), impairs prefrontal function by producing hyper-dopaminergia.
ABSTRACT
Epidemiological data show that males are more often and/or more severely affected by symptoms of prefrontal cortical dysfunction in schizophrenia, Parkinson’s disease and other disorders in which dopamine circuits associated with the prefrontal cortex are dysregulated. This review focuses on research showing that these dopamine circuits are powerfully regulated by androgens. It begins with a brief overview of the sex differences that distinguish prefrontal function in health and prefrontal dysfunction or decline in aging and/or neuropsychiatric disease. This review article then spotlights data from human subjects and animal models that specifically identify androgens as potent modulators of prefrontal cortical operations and of closely related, functionally critical measures of prefrontal dopamine level or tone. Candidate mechanisms by which androgens dynamically control meso prefrontal dopamine systems and impact prefrontal states of hypo- and hyper-dopaminergia in aging and disease are then considered. This is followed by discussion of a working model that identifies a key locus for androgen modulation of meso prefrontal dopamine systems as residing within the prefrontal cortex itself. The last sections of this review critically consider the ways in which the organization and regulation of mesoprefrontal dopamine circuits differ in the adult male and female brain, and highlights gaps where more research is needed.
Introduction
The prefrontal cortices in humans and animals orchestrate the brain’s highest order executive functions. These include goal setting, decision making, reward processing, behavioral flexibility and emotional regulation (Burgess PW and Stuss DT, 2017;Dalley JW et al., 2004; Friedman NP and Robbins TW, 2022; Kesner RP and Churchwell JC, 2011; Levy R, 2024; Miller EK, 2000). These functions are essential for the planning and execution of the activities of daily living and the negative consequences of prefrontal dysfunction in aging and in neuropsychiatric disease, e.g.,Alzheimer’s disease, schizophrenia, Parkinson’s disease, are profound (Amanzio M et al.,2020; Jones DT and Graff-Radford J, 2021; McDonald AP et al., 2018; Rabinovici GD et al.,2015 ;Smucny J et al., 2022;Xu P et al., 2019). In Parkinson’s disease, for example, over the course of illness, roughly 80% of patients will experience deficits in decision-making, behavioral flexibility, impulse control and other executive functions (Aarsland D et al., 2003; Aarsland D etal., 2021; Broeders M et al., 2013; Fang C et al., 2020; Gonzalez-Latapi P et al., 2021; Litvan | et al., 2011;Petkus AJ et al., 2020) which are often described by patients and caregivers as exacting an equal or greater toll on quality of life than the motor deficits associated with this condition (Kudlicka A et al., 2014; Schrag A et al., 2000; WinterY et al., 2011). Even more distressing is that treatment options offering clinical relief from prefrontal symptoms in Parkinson’s disease and other disorders are lacking. As a consequence, the cognitive symptoms of neuropsychiatric disease are often enduring and progressively worsening (Arnsten AF et al., 2015; Begemann MJ et al., 2020). This renders urgent the need to better understand the neural substrates that mediate prefrontal cortical operations in health and that underpin prefrontal cortical dysfunction in disease. Key factors that have been identified to date include the neurotransmitter dopamine and gonadal steroid hormones, e.g., estrogens, progestins,androgens. Here, evidence for androgen modulation of mesoprefrontal dopamine systems and the effects of these actions on the adult male brain are reviewed.
This article begins with a brief consideration of the sex differences that distinguish both prefrontal function in health and prefrontal dysfunction or decline in aging and neuropsychiatric disease. The patterns of male vulnerability revealed for the latter have motivated formulation of hypotheses for estrogen protections and intensive study of estrogen effects on prefrontal structure, function and pathophysiology. However, this review spotlights data from human subjects and animal models that identify androgens as potent modulators of prefrontal cortical operations and potential drivers of male susceptibility to prefrontal dysfunction in Alzheimer’s and Parkinson’s disease, schizophrenia and other conditions. Further, while the mechanisms underlying these actions are likely numerous, particular attention is paid to the data demonstrating a bidirectional androgen influence over functionally critical measures of prefrontal dopamine level or tone and how these could account for operationally disruptive states of prefrontal hypo- and hyper-dopaminergia that occur in aging or disease. Candidate mechanisms by which androgens may exert this dynamic control are then considered, including a working model that identifies a key, previously unsuspected locus for androgen modulation of prefrontal cortical dopamine levels that resides within the prefrontal cortex itself. Finally, this review also critically considers the striking and perhaps underappreciated ways in which the organization and regulation of meso prefrontal dopamine circuits differ in the male and female brain.
Part I. Androgen Influence Over Prefrontal Function and Dysfunction in Aging and Disease:
Sex differences
Follow-up investigations into roles for activational and/or organizational effects of gonadal hormones in shaping these and other sex differences have focused heavily on ovarian hormones, female subjects and on hypotheses for estrogen protections. These studies have shown that estrogens provide critical support for prefrontal cortical structure, chemistry and function in healthy subjects (Almey A et al., 2015; Jacobs E and D'Esposito M, 2011; Luine VN, 2014; Shanmugan Sand Epperson CN, 2014;Shughrue PJ and Merchenthaler |, 2000); exert significant protections from prefrontal disturbance induced by stress, aging and neuropsychiatric disease (Androvicova R et al., 2021; Bourque M et al., 2011; Garcia-Segura LM et al., 2001; Gillies GE and McArthur S, 2010;Mu E et al., 2024; Seeman MV, 2012; Sherwin BB, 2012); and appear to be beneficial as adjunct treatments in mitigating age- and disease-related cognitive decline (Ali N et al., 2023; Gava G et al., 2019; Gillies GE and McArthur S, 2010; Mu E,Gurvich C and Kulkarni J, 2024).While some of these effects are relevant for prefrontal function in males (Hogervorst E, 2012; Wibowo E, 2017; Zimmerman ME et al., 2011), estrogen-mediated actions alone do not fully explain the sex differences observed for prefrontal function or the alarming vulnerability of males to prefrontal cortical dysfunction in aging and disease. Rather, a more modest literature identifies specific roles for androgen signaling in modulating prefrontal function and dysfunction in human subjects and animal models. This article focuses on these modulatory actions as they relate to the adult male brain.
Hormone effects:
Human studies
Once associated mainly with reproductive functions or aggression, testosterone is now understood to exert a much wider influence over brain and behavior (Celec P et al., 2015; Nguyen TV, 2018; Ostatnikova D et al., 2020; Yaffe K, 2004; Zitzmann M, 2006). However, establishing testosterone actions as androgenic is complicated by systemic, local and brain region-specific patterns of testosterone catabolism that yield metabolites capable of signaling through androgen and/or estrogen receptor mediate 4d pathways (Fig 1) (Durdiakova J et al., 2011; Handelsman DJ, 2000). For these reasons, studies showing correlations between levels of testosterone and/or its precursors [e.g.,dehydroepiandrosterone (DHEA), DHEA sulfate] and performance on prefrontal cortical dependent tasks in healthy adults (de Menezes KJ et al., 2016; Elpers AL and Steptoe A,2020; Gouchie C and Kimura D, 1991; Lu PH et al., 2006) and in hypogonadal and aging male populations (Cai Z and Li H, 2020; Dong X et al., 2021; Gurvich C, Thomas N and Kulkarni J, 2020; Janowsky JS, 2006; Janowsky JS et al., 1994; Lasaite L et al., 2014; Lv W et al., 2016) leave uncertain whether relevant brain actions are androgenic, estrogenic or both. Similarly, while some promise for testosterone augmentation has been reported for prefrontal deficits in aging, Alzheimer’s disease and schizophrenia (Gregori G et al., 2021; Jung HJ and Shin HS, 2016 ;Ko Y Het al., 2008; Pike CJ et al., 2009; Wahjoepramono EJ et al., 2016), the formulations used often leave unanswered whether benefits are derived from testosterone’s estrogenic and/or androgenic metabolites. However, fewer in number are other studies that have identified cognitive deficits in male subjects: with genetic disruptions of androgen receptor function, e.g., CAG repeats, congenital androgen-insensitivity (Lee DM et al., 2010; Tan S et al., 2021); taking anti-androgen medications, e.g., for treatment of prostate cancers (Cherrier MM and Higano CS, 2020; Jamadar RJ et al., 2012; McGinty HL et al., 2014); undergoing hormone augmentation using non-aromatizable androgens (Cherrier MM et al., 2003); or using or misusing anabolic androgenic steroids (AAS (Bjornebekk A et al., 2019; Frati P et al., 2015; Hauger LE et al., 2020; Kanayama G et al., 2013; Scarth M and Bjornebekk A, 2021)). By targeting enzymes and signaling molecules associated with androgenic metabolites of testosterone (Fig 1), these studies more directly implicate androgens in modulating prefrontal function. They also show that both supranormal androgen stimulation and androgen deficiency negatively affects prefrontal operations (Fig 2A). This inverted U- shaped function is similar to that described for functional meso prefrontal dopamine settings (Cools R and D'Esposito M, 2011; Cools R et al.,2019; Floresco SB, 2013; Floresco SB and Magyar O, 2006). As this review proceeds, data supporting causal relationships between these two functions are considered. Unusually low levels of plasma testosterone, free testosterone and/or DHEA/DHEA-S have been reported in studies of male patients diagnosed with Alzheimer’s disease, schizophrenia and Parkinson’s compared to age-matched healthy subjects (Bianchi VE, 2022; Lv W, Du N, Liu Y ,Fan X, Wang Y,Jia X, Hou X and Wang B, 2016; Nitkowska M et al., 2015; Okun MS et al., 2004; Taherianfard M and Shariaty M, 2004; Yeap BB and Flicker L, 2022) and in some cases ,significant correlations have been noted between these levels and the severity of cognitive or negative symptoms (Akhondzadeh S et al., 2006; Bianchi VE, 2022; Bourque M, Soulet D and DiPaolo T, 2021; Driscoll | and Resnick SM, 2007; Jaeger ECB et al., 2020; Jimenez-Rubio G et al., 2017; Jurado-Coronel JC, Cabezas R, Avila Rodriguez MF, Echeverria V, Garcia-Segura LM and Barreto GE, 2018; Moore .et al., 2013; Pike CJ, Carroll JC, Rosario ER and Barron AM, 2009; Russillo MC, Andreozzi V, Erro R, Picillo M, Amboni M,Cuoco S, Barone P and Pellecchia MT,2022;Yeap BB and Flicker L, 2022). However, there are also studies finding no significant correlations between testosterone and cognitive health, neuropsychiatric diagnoses or cognitive impairment in aging or disease (Cherrier MM et al., 2015; Kaufman JM and Lapauw B, 2020; Ulubaev A et al., 2009). The results from clinical trials examining the effectiveness of testosterone augmentation in mitigating prefrontal dysfunction in aging and disease are also mixed (Lisco G et al., 2020; Okun MS et al., 2006; Okun MS et al., 2002; Yalamanchi S and Dobs A, 2017). Nonetheless, the balance of information to date has been sufficiently promising and the need for more information sufficiently pressing to prompt continued investigation of testosterone effects in human subjects. Importantly, this work both informs and is informed by studies in animal models and preclinical animal models of neuropsychiatric disease where sample sizes, population variance and other factors that often challenge human studies can be well controlled and flexible tools are available to facilitate investigations that focus specifically on androgen effects.
Hormone effects:
Animal studies
As in the human studies (above), these data demonstrate that androgen effects on prefrontal function in rats follow inverted U-shaped functions (Fig 2A). In addition, a growing literature further shows that male vulnerability to prefrontal dysfunction in disease is recapitulated in several leading rodent models of Alzheimer’s disease (Jiao SS et al., 2016; Pike CJ, 2017; Yang JT et al., 2018), Parkinson’s disease (Bourque M, Soulet D and Di Paolo T, 2021; Gillies GE, Pienaar IS, Vohra S and Qamhawi Z, 2014; Pinizotto CC, Patwardhan, A.Aldarondo, D. Kritzer, M.F., 2022; Russillo MC, Andreozzi V, Erro R, Picillo M, Amboni M, Cuoco S, Barone P and Pellecchia MT, 2022) and schizophrenia (Hill RA, 2016; Kokras N and Dalla C, 2014). For some of these models, potent and potentially therapeutic effects of androgens on brain pathologies and/or behaviors have also been described.
Part Il. Androgen Influence Over Prefrontal Dopamine Tone:
Organizational and activational hormone effects
Estrogens, progestins, androgens and other steroid hormones exert a range of permanent organization effects on the pre-, early post-natal and pubertal cerebral cortex (Karaismailoglu S and Erdem A, 2013; Rebuli ME and Patisaul HB, 2016). These hormones also have diverse activational effects that continue to modulate cortical structure and function for the remainder of the lifespan (Meyer CE et al., 2023; Pompili A et al., 2020; Singh M, 2006; Singh M et al., 2006). The effects of testosterone, its non-aromatizable derivatives and/or AAS on the cerebral cortex or cultured cortical neurons range from regulation of cell survival, apoptosis and adult neurogenesis (Spritzer MD and Roy EA, 2020; Zelleroth S et al., 2019), to acute effects on neuronal excitability and synaptic plasticity to sculpting circuit substrate, including dendritic spine morphology and synaptic density (Hajszan Tet al., 2008). However, the focus of this review is on androgen modulation of the meso prefrontal dopamine systems and the functionally critical measure of prefrontal dopamine tone. The cells of origin of the meso prefrontal dopamine system are anatomically intermingled with dopamine neurons projecting to neostriatum, nucleus accumbens and other target regions in the ventral tegmental area (VTA), rostral linear nucleus and medial substantia nigra (Lammel S et al., 2011; Yamaguchi T et al., 2011; Yetnikoff L et al., 2014). However, the dopamine neurons projecting to the prefrontal cortex form a discrete subset of cells that do not send axon collaterals to other cortical, subcortical or allocortical brain regions. Dynamic features of mesoprefrontal dopamine signaling, e.g., uptake/reuptake, autoregulation, also differ markedly from those of the nigrostriatal and mesolimbic systems (Cass WA and Gerhardt GA, 1995; GarrisPA et al., 1993;Lammel S, lon DI, Roeper J and Malenka RC, 2011). Accordingly, readers are referred to separate reviews describing androgen effects on nigrostriatal and mesolimbic dopamine circuits (Becker JB, 1999; Becker JB and Chartoff E, 2019; Gillies GE et al., 2014; Low KL et al., 2020; Seib DR et al., 2023; Tobiansky DJ et al., 2018; Zachry JE et al., 2021) and are cautioned about the generalizability of findings across these systems.
Dopamine effects on prefrontal function
The primacy of the meso prefrontal dopamine system for frontal lobe operations is illustrated in studies using chemical lesioning, pharmacological and other strategies to selectively manipulate dopamine signaling (Arnsten AF, Wang M and Paspalas CD, 2015; Floresco SB, 2013; Floresco SB and Magyar O, 2006; Ott T and Nieder A, 2019; Puig MV et al., 2014); in all cases, evidence points to selective perturbations of prefrontal dopamine pathways as significantly impairing working memory, attention, goal-directed behavior, behavioral flexibility, impulsivity and other classical prefrontal cortical functions. The data also demonstrate an inverted U-shaped function that describes these dopamine effects. According to this function, prefrontal dopamine levels— often referred to as prefrontal dopamine tone- that are either higher or lower than a functionally optimal set point are detrimental to behavior and circuit function (Fig 2B). This rule explains the prefrontal impairments associated with hyper-dopaminergic states, e.g., those induced by mild stress (Arnsten AF, 2000; Arnsten AF, 2015; Arnsten AFT et al., 2023; Luine V, 2002; Luine V et al., 2017; Shansky RM and Lipps J, 2013), as well as the prefrontal dysfunction associated with hypodopaminergia in aging, Alzheimer’s disease and schizophrenia (Ciampa CJ et al., 2022; Guillin Oet al., 2007; Pan X et al., 2019). However, clinical attempts to restore optimal prefrontal dopamine levels are often complicated by competing needs to rectify imbalances in other, e.g.,nigrostriatal, mesolimbic, dopamine systems. For example, the dopamine-blocking actions of antipsychotic medications that effectively quell positive symptoms in schizophrenia and the dopamine potentiating drugs that reduce motor symptoms in patients with Parkinson’s disease have little benefit for the prefrontal impairments associated with these disorders (Martinez ALet al., 2021;Zhang Q et al., 2020). However, as reviewed below, androgens maintain a lifelong capacity to bidirectionally modulate prefrontal dopamine tone which could ultimately be useful in improving treatment for cognitive deficits in these and other conditions.
Androgen effects on prefrontal dopamine level/tone
Studies examining the effects that sex hormones have on frontal cortex dopamine levels in adult male rats reveal consensus findings for androgens as tonically suppressing meso-prefrontal dopamine systems. First, gonadectomy of more than a few week’s duration has been shown: to increase dopamine, but not serotonin and/or norepinephrine concentrations in frontal cortical homogenates (Battaner E et al., 1987; Kokras N et al., 2018); to nearly double the density of prefrontal cortical axons that are immunoreactive for the dopamine marker tyrosine hydroxylase (TH), but not axons labeled with markers for serotonin, norepinephrine or acetylcholine (Adler A et al., 1999; Kritzer MF, 2000; Kritzer MF, 2003; Kritzer MF et al., 1999; Kritzer MF, Brewer A, Montalmant F, Davenport Mand Robinson JK, 2007); and to significantly increase extracellular dopamine concentrations measured in vivo using microdialysis interfaced with high pressure liquid chromatography (Aubele T and Kritzer MF, 2011; Aubele T and Kritzer MF, 2012). Gonadectomy on postnatal day 60 has also been shown to increase the numbers of dopamine neurons present in the VTA at postnatal day 90 relative to sham operated controls, possibly by attenuating developmental pruning these dopamine neurons (Johnson ML et al., 2010). Importantly, the studies of axon density, dopamine cell number and in vivo dopamine levels further showed that the effects of gonadectomy on these endpoints are attenuated in gonadectomized rats supplemented with testosterone or dihydrotestosterone but are not affected by treatment with estradiol. On the other hand, studies in gonadally intact adult male rats have shown that treatment with AAS significantly decreases concentrations of dopamine and serotonin, but not noradrenalin, measured in homogenates of prefrontal cortical tissue (Tucci P et al., 2012). Together these findings show that the same androgen-specific experimental manipulations that negatively affect prefrontal function also induce changes in prefrontal dopamine. Further, they identify androgen depletion vs. augmentation as inducing behaviorally disruptive states of prefrontal hyper- and hypo-dopaminergia, respectively (Fig 2C). Accordingly, age or disease-related changes in these bidirectional effects could well be important in rendering males the significantly more vulnerable sex to prefrontal cortical dysfunction in disorders where pathophysiology includes meso prefrontal dopamine over- or underactivity. Moreover,manipulations of androgen signaling may have therapeutic effects on prefrontal deficits. Both possibilities have motivated deeper investigations into the mechanisms that underpin androgenic control over prefrontal dopamine tone.
Part Ill: Mechanisms, Substrates and Working Models for Androgen Effects on Meso prefrontal Dopamine Systems:
Mechanisms: Androgen effects on prefrontal dopamine uptake/reuptake, turnover and catabolism.
The kinetics and dynamics of dopamine signaling in the prefrontal cortices differ markedly from the characteristics of dopamine signaling in other brain regions. For example, in the prefrontal cortex tonic levels of dopamine release are relatively low, dopamine clearance from the synaptic space is relatively slow and dopamine turnover rates are relatively high compared to corresponding measures in neostriatum and other major target regions of the nigrostriatal and mesolimbic dopamine systems (Cass WA and Gerhardt GA, 1995; Garris PA, Collins LB, Jones SR and Wightman RM, 1993). Androgens have been shown to influence several of the mechanisms that contribute to these unique settings—some in ways that could explain androgens’ long-term biasing of prefrontal dopamine levels. For example, gonadectomy in adult rats and mice has been shown to reduce ligand binding affinity for the prefrontal norepinephrine transporter and reduce prefrontal dopamine transporter expression,respectively, in ways that are attenuated by supplementing animals with testosterone or dihydrotestosterone but not estradiol (Du X et al., 2019; Meyers B and Kritzer MF, 2009). However, dopamine uptake and reuptake machineries in the prefrontal cortex are notably sparse (Miner LH et al., 2003; Sesack SR et al., 1998) and pharmacological blockade of transporter function has only modest effects on prefrontal dopamine levels (Cass WA and Gerhardt GA, 1995; Garris PA and Wightman RM, 1994). Thus, it is unlikely that androgen effects on transporter proteins alone account for the significant effects that this steroid hormone has on readouts of prefrontal dopamine level. However, the extant literature for androgen modulation of prefrontal dopamine turnover includes studies showing prefrontal dopamine turnover is unaffected (Handa RJ et al., 1997) or decreased by gonadectomy (KokrasN, Pastromas N,Papasava D, de Bournonville C, Cornil CA and Dalla C, 2018) and is decreased by treatment with aromatase inhibitors (Kokras N, Pastromas N, Papasava D,de Bournonville C, Cornil CA and Dalla C, 2018) or increased by AAS (Tucci P, Morgese MG, Colaianna M ,Zotti M, Schiavone S, Cuomo V and Trabace L, 2012) in gonadally intact males. Similarly, studies examining the effects of gonadectomy or AAS treatment on the activity of catechol-O-methyltransferase and monoaminoxidase in prefrontal homogenates found either no effects, effects that were estrogen-dependent or effects that were androgenic but targeted enzyme isoforms with preferred substrates of serotonin or norepinephrine rather than dopamine (Birgner C et al., 2008; Luine VN and Rhodes JC, 1983; Meyers B et al., 2010). Thus, there is currently little consensus support for androgen effects on turnover or catabolism as contributing to observed regulation of prefrontal dopamine tone.
Mechanisms: Androgen effects on meso prefrontal dopamine cell firing
Action potential firing rates in meso prefrontal and other populations of midbrain dopamine neurons normally vacillate between sustained periods of slow, irregular 2-5 Hz single spiking that are punctuated by brief periods of higher frequency (10-20 Hz) burst-firing (Grace AA and Bunney BS, 1984; Overton PG and Clark D, 1997; Seamans JK and Yang CR, 2004). The slower, single spiking modes of activity set basal dopamine levels that are thought to stabilize prefrontal circuits and maintain internal representations of behaviorally relevant events, strategies and goals (Durstewitz D and Seamans JK, 2008; Ellwood IT et al., 2017; Seamans JK and Yang CR, 2004). Burst firing, on the other hand, is often event related and serves to transiently increase prefrontal dopamine release and raise extracellular dopamine concentrations; these phasic events that are viewed as important for updating internal representations and changing behaviors and behavioral strategies (Durstewitz D and Seamans JK, 2008; Ellwood IT, Patel T,Wadia V, Lee AT, Liptak AT, Bender KJ and Sohal VS, 2017; Seamans JK and Yang CR, 2004). Single unit extracellular recordings in adult male rats have shown that gonadectomy has no effect on single spiking activity (Locklear MN et al., 2017). However, it significantly and selectively increases the incidence and the frequency of burst firing in dopamine neurons of the ventral tegmental area but not the adjacent substantia nigra in a testosterone-reversible, estrogen-insensitive manner (Locklear MN, Michealos M, Collins WF and Kritzer MF, 2017). This could explain observed effects of gonadectomy and hormone replacement on prefrontal dopamine levels. Moreover, it is tempting to speculate that the updating of prefrontal information normally associated with phasic dopamine burst firing is facilitated in rats where androgens are depleted and impaired in subjects where androgen signaling is augmented. Both predictions align with behavioral data showing that perseverative responding is reduced in set shifting tasks in male rats treated with blockers of androgen synthesis or receptors (Tomm RJ et al., 2022) and is increased in rats treated with exogenous testosterone or AAS (Wallin KG and Wood RI, 2015; Wood RI and Serpa RO, 2020). This underscores the importance of knowing where and how androgens modulate midbrain dopamine cell activity. As discussed below, the question of where relevant action is levied has been addressed in part by mapping the distributions of the intracellular androgen receptors that mediate the genomic actions that are presumed to be involved.
Substrates: Intracellular androgen receptors as proxies for realms of influence
One way that androgens could influence the tempo of cell firing in the VTA is by modifying channel, membrane or other intrinsic properties of the dopamine cells themselves. However, the precise distributions of intracellular androgen receptors (AR) within TH-immunoreactive, presumed dopaminergic ventral midbrain neurons do not match the selectivity or the magnitude of androgen effects on meso prefrontal dopamine systems. Rather, immunoreactivity for AR is present in comparable, and comparably low, proportions of TH-immunoreactive neurons in the VTA, substantia nigra and the retrorubral fields (Kritzer MF, 1997; Simerly RB et al., 1990). Further, triple labeling studies (Fig 3A) that concurrently defined midbrain neurons as dopaminergic, as containing AR and as projecting to the prefrontal cortex, showed that only about a third of midbrain dopamine neurons projecting to prefrontal cortex contain AR (KritzerMF and Creutz LM, 2008). These proportions were not appreciably different from the percentages of AR-containing midbrain dopamine neurons projecting to primary or premotor cortices (Kritzer MF and Creutz LM, 2008)or to subcortical targets including amygdala, nucleus accumbens and neostriatum (Creutz LM and Kritzer MF, 2004). However, a survey of AR immunoreactivity among more than 27 ipsilateral and contralateral cortical and subcortical regions projecting to the VTA (back-labeled by retrograde tracer injections in the VTA) revealed that projections arising from the prefrontal cortex itself were far more and were by far the most AR-enriched (Aubele T and Kritzer MF, 2012). Thus, while the percentages of AR immunoreactive VTA afferents arising from brainstem, midbrain and forebrain nuclei were at most only about 30%, between 50 and 60% of prefrontal neurons retrogradely labeled by VTA injections were also immunoreactive for AR (Fig 3B). This places an abundance of androgen influence within cortical efferents known to make monosynaptic synapses onto meso prefrontal dopamine neurons (Beier KT et al., 2015; Carr DB and Sesack SR, 2000) and to exert top-down control over midbrain dopamine cell firing and dopamine release back in the prefrontal cortex (Gariano RF and Groves PM, 1988; Overton PG et al., 1996;Tong ZY et al., 1996). The section below focuses on a working model for androgen effects that is centered on this critical corticofugal pathway and the data that support it.
Working Model
Studies in male rodents using in vivo microdialysis or electrophysiology have shown that prefrontal cortical pyramidal cells projecting to the VTA exert dynamic control over cell firing rates in meso prefrontal (and other) dopamine neurons (Gariano RF and Groves PM, 1988; Overton PG, Tong ZY and Clark D, 1996; Tong ZY, Overton PG and Clark D, 1996). These actions in turn modulate the amount of transmitter that is released from the axon terminal of midbrain dopamine neurons projecting back to the prefrontal cortex (Garris PA, Collins LB, Jones SR and Wightman RM, 1993; Garris PA and Wightman RM, 1994; Overton PG and Clark D, 1997). Studies carried out in gonadally intact male rats that combined in vivo microdialysis with reverse dialysis drug administration further showed that excitability in this pivotal pathway—and ultimately its influence over prefrontal dopamine levels— is regulated by functionally offsetting intracortical glutamatergic influences (Feenstra MG et al., 1995; Jedema HP and Moghaddam B, 1996;Takahata R and Moghaddam B, 1998). These include direct activation of VTA-projecting prefrontal pyramidal cells by a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors; and N-methyl-D-aspartate (NMDA) receptor-mediated influences that excite prefrontal inhibitory interneurons and di-synaptically inhibit cell firing in VTA-projecting prefrontal pyramidal cells (Fig 4A). Experimentally, when AMPA agonists have been intra cortically applied, increases in the prefrontal cortex’s descending drive, increases in burst firing in meso prefrontal dopamine neurons cells and increases in dopamine levels/release back in the prefrontal cortex have all been observed (Jedema HP and Moghaddam B, 1996; TakahataR and Moghaddam B, 1998). However, when NMDA agonists are intra cortically delivered,prefrontal interneurons are activated; this in turn inhibits firing in VTA-projecting prefrontal pyramidal cells, slows firing in midbrain dopamine cell neurons and reduces dopamine levels/release in the prefrontal cortex (Feenstra MG et al., 2002; Jedema HP and Moghaddam B, 1996; Takahata R and Moghaddam B, 1998). Studies in gonadectomized and hormone-replaced male rats showed that circulating androgens modulate these intra cortical glutamatergic influences and have especially strong effects on NMDA-mediated mechanisms. Specifically,reverse dialysis administration of the NMDA receptor subtype-selective antagonist 2-amino-5-phosphonovaleric acid (APV) normally serves to di-synaptically disinhibit VTA-projecting prefrontal pyramidal cells and increase extracellular prefrontal dopamine concentrations (Fig 4A, C) (Feenstra MG, Botterblom MH and van Uum JF, 2002; Jedema HP and Moghaddam B, 1996;Takahata R and Moghaddam B, 1998). However, in gonadectomized males APV infusion decreases prefrontal dopamine levels in an androgen sensitive, estrogen insensitive manner(Aubele T and Kritzer MF, 2012) (Fig 4 C, D). This suggests that androgen deprivation functionally re-directs NVUDA-receptor mediated activation to VTA-projecting prefrontal pyramidal cells (Fig 4B)—a hypothesis supported by data from microdialysis/reverse dialysis dual drug challenge studies in which either the voltage gated sodium channel blocker tetrodotoxin (TTX) or the selective GABA- B receptor antagonist CGP52432 was intra cortically co-applied with NMDA or APV. While these infusion paradigms blocked the effects of NUDA and APV on prefrontal dopamine levels in gonadally intact animals (Fig 4E, G) (Locklear MN etal., 2016), neither affected NUDA-mediated stimulation or APV’s paradoxical inhibition of prefrontal dopamine levels in gonadectomized males (Fig 4F, H). This confirms the sphere of functional NMDA influence in gonadectomized rats as downstream of prefrontal interneurons and directly impacting VTA-projecting prefrontal pyramids (Fig 4B). Additional support for this model includes data from single unit extracellular recordings studies showing that gonadectomy significantly and selectively increases burst-firing only in infragranular pyramidal cells identified by collision testing as projecting to the VTA; these effects were androgen sensitive and estrogen insensitive (Locklear MN, Michealos M, Collins WF and Kritzer MF, 2017). However, most exciting are the data that align this working model with androgen-sensitive effects of gonadectomy on dopamine-dependent prefrontal behaviors.
Treatment with NMDA antagonists induce prefrontal hyper dopaminergic and to impair delayed spatial alternation performance in gonadally intact male rats in a manner that is attenuated by dopamine receptor antagonists (Verma A and Moghaddam B, 1996). However, In gonadectomized and gonadectomized rats given estradiol where VTA-projecting neurons are excessively bursty (Locklear MN, Michealos M,Collins WF and Kritzer MF, 2017) and prefrontal dopamine levels are excessively high (Aubele T and Kritzer MF, 2011; Aubele T and Kritzer MF, 2012), the actions of intracortical APV infusion serve to return meso prefrontal dopamine settings to more normal values. Assuming that the androgen- and NMDA-sensitive, hyper dopaminergic states induced by gonadectomy are responsible for the negative impacts of this manipulation on prefrontal function, intracortical infusion of APV should rescue behavioral deficits in gonadectomized and gonadectomized rats given estradiol and induce them in control and gonadectomized rats given testosterone. This prediction was tested using processes of spatial cognition measured in Barnes maze testing as readouts (Fig 5A). Previous studies showed that commission of primary and perseverative errors, path lengths and times taken to discover a goal location are significantly increased in gonadectomized compared to control male rats and mitigated by supplementing gonadectomized rats with either testosterone or estradiol, i.e., were estrogen-sensitive (Locklear MN and Kritzer MF, 2014). However,gonadectomy also induced deficits in task solving strategies that were androgen-sensitive (Locklear MN and Kritzer MF, 2014). Specifically, gonadally intact and gonadectomized males treated with testosterone progressively refined what were initially random searches to ultimately navigate directly or nearly directly to the goal location. However, gonadectomized and gonadectomized rats given estradiol showed little spatial organization to their searches and persisted in randomly investigating possible goal locations across all four trials (Locklear MN and Kritzer MF, 2014). These group specific strategies were also seen in Barnes maze testing in animals given bilateral intra-prefrontal infusions of saline (Fig 5B, left). However, when APV was infused, the gonadectomized and gonadectomized rats given estradiol were the ones to successfully develop efficient, spatially directed search strategies while the gonadally intact controls and gonadectomized rats given testosterone did not (Locklear MNaK, M.F., 2012) (Fig 5B, right). These data indicate that androgen regulation of NMDA receptor-mediated effects on meso prefrontal dopamine physiology contributes significantly to androgen’s impacts on prefrontal cortex-dependent cognition in the adult male brain. However, as discussed below, at least some of these actions may be unique to this biological sex.
PART IV. Sex Differences in Meso prefrontal Dopamine Systems:
The theme of this review—the effects of androgens on meso prefrontal dopamine systems in the adult male brain— was stimulated in large part by epidemiological data showing that males are more often and/or more severely affected by deficits in prefrontal cortical function in neuropsychiatric disorders where meso prefrontal dopamine circuits are also dysregulated. However, it seems important to conclude this review by considering some of the ways in which meso prefrontal dopamine systems are similar and some of the ways in which they are fundamentally different in the female brain.
While dopamine-dependent prefrontal operations appear to universally follow inverted U shaped functions, the question of whether functionally optimal resting dopamine levels differ between the sexes is currently unresolved. Thus, while one study that measured dopamine and its major metabolites in prefrontal homogenates in mice reported nearly two-fold higher levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in male compared to female subjects (Kawakami SE et al., 2013), two similar studies in rats identified higher concentrations of dopamine but not dopamine metabolites in females compared to males (Duchesne A et al., 2009; Kokras N, Pastromas N, Papasava D, de Bournonville C, Cornil CA and Dalla C, 2018) and two others found no sex differences in any of dopamine measures (Laatikainen LM et al., 2013; Raftogianni A et al., 2014). Further, of two studies that quantified dopamine levels in rat prefrontal cortex using in vivo microdialysis, one found no sex differences (Locklear MN, Cohen AB, Jone A and Kritzer MF, 2016) and the other reported higher extracellular dopamine levels in females compared to males (Staiti AM et al., 2011). It should be noted, however, that the female subjects in the second study were ovariectomized and injected every other day subcutaneously with estradiol. Because the male subjects were unoperated and uninjected, it is unknown whether group differences in mild stress, which is well known to elevate prefrontal dopamine levels in sex- and sex- hormone dependent ways (Luine V, Gomez J, Beck K and Bowman R, 2017; Shansky RM and Lipps J, 2013), may have influenced the results. However, recent studies have also shown that sex differences in meso accumbal dopamine systems are rat strain-specific (Rivera-Garcia MT et al., 2020). Thus, the use of Sprague Dawley, Long Evans, Wistar and Lister hooded rats across the studies noted above must also be considered as a potential source of variance in the data.
While sex differences in resting prefrontal dopamine levels are uncertain, robust sex differences have been identified for other aspects of the meso prefrontal dopamine systems. For example, while this system is classically known to be comprised of afferents that are roughly one-third dopaminergic and two-thirds non-dopaminergic (Swanson LW, 1982), tract tracing studies in male and female rats suggest that these ratios only apply to the male brain (Kritzer MF and Creutz LM, 2008). Specifically, experiments that combined cortical injections of retrograde tracers with TH-immunocytochemistry showed that fewer than 30% of ventral midbrain neurons contributing to meso prefrontal pathways were dopaminergic in adult male rats while more than 50% of retrogradely labeled cells in females were TH-immunopositive (Kritzer MF and Creutz LM, 2008) (Fig 6). Even more striking are sex differences found for the ways in which intracortical NUDA-receptor mediated mechanisms regulate prefrontal dopamine tone. Specifically, in vivo microdialysis/reverse dialysis drug challenges carried out in the prefrontal cortices showed that the effects of NMDA-receptor antagonists in female rats were diametrically different from those present in males. Thus, whereas NMDA antagonists release a GABA-mediated disynaptic brake that ultimately elevates prefrontal dopamine levels in males (Fig 7A C), in females NMDA antagonists acted similarly to AMPA receptor antagonists to decrease prefrontal dopamine levels (Locklear MN, Cohen AB, Jone A and Kritzer MF, 2016) (Fig 7B, D). This resembles the dysregulated state induced in males by gonadectomy. However,studies in which the selective GABA-B receptor antagonist CGP52432 was co-infused with APV identified a significantly more powerful GABA-mediated inhibitory influence at work in the female prefrontal cortex (Fig, 7E, F) that prevents synergistic NVDA/AMPA activation of VTA projecting prefrontal pyramidal cells from driving their meso prefrontal dopamine systems to hyper-dopaminergic states. It is possible that these sex differences bias the frontal lobes in females toward hyper-dopaminergia, which could be a basis for understanding and ultimately overcoming the greater vulnerability of prefrontal function in females to stress (Luine V, Gomez J, Beck K and Bowman R, 2017; Luine VN, 2007; Shansky RM and Lipps J, 2013). While further study is needed to substantiate this and other hypotheses, the critical involvement of NUDA receptor-mediated signaling and dopamine transmission in cognitive function and dysfunction in aging and disease combines with the prominent use of NMDA-selective drugs in mitigating these symptoms in schizophrenia, Parkinson’s disease, Alzheimer’s disease (Balu DT,2016;Painuli S et al., 2023;VanleB et al., 2018) to underscore the importance of fully clarifying sex differences and sex hormone effects on the amino acid transmitter and dopamine systems that powerfully influence frontal lobe function.
Summary and Conclusions
The sum of findings presented here suggests that in the adult male prefrontal cortex, circulating androgens exert tonic, suppressive influences over meso prefrontal dopamine systems that hold prefrontal dopamine levels within functionally optimal ranges. When this influence is diminished, cognitive deficits are induced as a result of multiple processes that selectively drive prefrontal dopamine levels to dysfunctional ‘too much’, i.e., hyper-dopaminergic states. On the other hand, when androgen stimulation is increased, the resultant cognitive deficits observed coincide with prefrontal dopamine levels that fall below a functionally optimal range. These bidirectional effects are largely explained by a working model for androgen-mediated modulation of the NMDA-sensitivity in VTA-projecting prefrontal pyramidal cells. Supporting data include evidence showing that gonadectomy increases burst-firing in these pyramidal cell populations and in the midbrain dopamine neurons they project, and also induces paradoxical elevations in local dopamine levels in response to intra-prefrontal infusion of NMDA antagonists—all in testosterone-sensitive, estrogen-insensitive ways. These impacts are likely to contribute to the susceptibility of human males and male animal subjects to prefrontal dysfunction in Parkinson’s disease, schizophrenia and other conditions. Moreover, there seems to be hope that the bidirectional effects of androgen deprivation and augmentation might ultimately be tailored to re-balance meso prefrontal dopamine systems that are differentially dysregulated across these conditions. However, while this potential adds impetus to further study and better understand androgen effects on dopamine-dependent cognitive processes in males, it is important to emphasize that sex differences also distinguish the ways in which the meso prefrontal dopamine systems are organized and modulated. Thus, the data argue equally strongly for the need for parallel, independent investigations in female subjects.
