Androgen metabolites impact CSF amines and axonal serotonin via MAO-A and -B in male macaques

Effects of gender-affirming hormone therapy on gray matter density, microstructure and monoamine oxidase A levels in transgender subjects

MAO-A catalyzes the oxidative degradation of monoamines and is thus implicated in sex-specific neuroplastic processes that influence gray matter (GM) density (GMD) and microstructure (GMM). Given the exact monitoring of plasma hormone levels and sex steroid intake, transgender individuals undergoing gender-affirming hormone therapy (GHT) represent a valuable cohort to potentially investigate sex steroid-induced changes of GM and concomitant MAO-A density. Here, we investigated the effects of GHT over a median time period of 4.5 months on GMD and GMM as well as MAO-A distribution volume. To this end, 20 cisgender women, 11 cisgender men, 20 transgender women and 10 transgender men underwent two MRI scans in a longitudinal design. PET scans using [11C]harmine were performed before each MRI session in a subset of 35 individuals. GM changes determined by diffusion weighted imaging (DWI) metrics for GMM and voxel based morphometry (VBM) for GMD were estimated using repeated measures ANOVA. Regions showing significant changes of both GMM and GMD were used for the subsequent analysis of MAO-A density. These involved the fusiform gyrus, rolandic operculum, inferior occipital cortex, middle and anterior cingulum, bilateral insula, cerebellum and the lingual gyrus (post-hoc tests: pFWE+Bonferroni < 0.025). In terms of MAO-A distribution volume, no significant effects were found. Additionally, the sexual desire inventory (SDI) was applied to assess GHT-induced changes in sexual desire, showing an increase of SDI scores among transgender men. Changes in the GMD of the bilateral insula showed a moderate correlation to SDI scores (rho = - 0.62, pBonferroni = 0.047). The present results are indicative of a reliable influence of gender-affirming hormone therapy on 1) GMD and GMM following an interregional pattern and 2) sexual desire specifically among transgender men.

High-dose testosterone treatment reduces monoamine oxidase A levels in the human brain: A preliminary report

The sex hormones testosterone and estradiol influence brain structure and function and are implicated in the pathogenesis, prevalence and disease course of major depression. Recent research employing gender-affirming hormone treatment (GHT) of gender dysphoric individuals and utilizing positron emission tomography (PET) indicates increased serotonin transporter binding upon high-dosages of testosterone treatment. Here, we investigated the effects of GHT on levels of monoamine oxidase A (MAO-A), another key target of antidepressant treatment. Participants underwent PET with the radioligand [¹¹C]harmine to assess cerebral MAO-A distribution volumes (V_T) before and four months after initiation of GHT. By the time this study was terminated for technical reasons, 18 transgender individuals undergoing GHT (11 transmen, TM and 7 transwomen, TW) and 17 cis-gender subjects had been assessed. Preliminary analysis of available data revealed statistically significant MAO-A V_T reductions in TM under testosterone treatment in six of twelve a priori defined regions of interest (middle frontal cortex (-10%), anterior cingulate cortex (-9%), medial cingulate cortex (-10.5%), insula (-8%), amygdala (-9%) and hippocampus (-8.5%, all p<0.05)). MAO-A V_T did not change in TW receiving estrogen treatment. Despite the limited sample size, pronounced MAO-A V_T reduction could be observed, pointing towards a potential effect of testosterone. Considering MAO-A's central role in regulation of serotonergic neurotransmission, changes to MAO-A V_T should be further investigated as a possible mechanism by which testosterone mediates risk for, symptomatology of, and treatment response in affective disorders.

The role of monoamine oxidase enzymes in the pathophysiology of neurological disorders

Monoamine oxidase enzymes are responsible for the degredation of serotonin, dopamine, and norepinephrine in the central neurvous system. Although it has been nearly 100 years since they were first described, we are still learning about their role in the healthy brain and how they are altered in various disease states. The present review provides a survey of our current understanding of monoamine oxidases, with a focus on their contributions to neuropsychiatric, neurodevelopmental, and neurodegenerative disease. Important species differences in monoamine oxidase function and development in the brain are highlighted. Sex-specific monoamine oxidase regulatory mechanisms and their implications for various neurological disorders are also discussed. While our understanding of these critical enzymes has expanded over the last century, gaps exist in our understanding of sex and species differences and the roles monoamine oxidases may play in conditions often comorbid with neurological disorders.

Sex and the serotonergic underpinnings of depression and migraine

Most psychiatric disorders demonstrate sex differences in their prevalence and symptomatology, and in their response to treatment. These differences are particularly pronounced in mood disorders. Differences in sex hormone levels are among the most overt distinctions between males and females and are thus an intuitive underpinning for these clinical observations. In fact, treatment with estrogen and testosterone was shown to exert antidepressant effects, which underscores this link. Changes to monoaminergic signaling in general, and serotonergic transmission in particular, are understood as central components of depressive pathophysiology. Thus, modulation of the serotonin system may serve as a mechanism via which sex hormones exert their clinical effects in mental health disorders. Over the past 20 years, various experimental approaches have been applied to identify modes of influence of sex and sex hormones on the serotonin system. This chapter provides an overview of different molecular components of the serotonin system, followed by a review of studies performed in animals and in humans with the purpose of elucidating sex hormone effects. Particular emphasis will be placed on studies performed with positron emission tomography, a method that allows for human in vivo molecular imaging and, therefore, assessment of effects in a clinically representative context. The studies addressed in this chapter provide a wealth of information on the interaction between sex, sex hormones, and serotonin in the brain. In general, they offer evidence for the concept that the influence of sex hormones on various components of the serotonin system may serve as an underpinning for the clinical effects these hormones demonstrate.

Exogenous testosterone and the monoamine-oxidase A polymorphism influence anger, aggression and neural responses to provocation in males

Testosterone and the monoamine oxidase-A (MAOA) polymorphism are potential neuromodulators for aggression. By acting on similar brain circuits, they might interactively influence human behavior. The current study investigates the causal role of testosterone on aggression-related brain activity and the potential interaction with the MAOA polymorphism. In a double-blind process, 93 healthy males received a testosterone or placebo gel. In an fMRI session, participants performed a Taylor aggression paradigm in which they received provoking feedback and could afterwards decide how aggressively they would react. Testosterone and cortisol levels as well as subjective anger were assessed prior and after the task. Circulating testosterone levels were higher in carriers of the long compared to the short MAOA allele. An interaction of the MAOA polymorphism and testosterone administration was identified in the cuneus, where short allele carriers in the placebo group showed diminished activity in the decision period. Task-related anger was significantly higher in this group. Overall, a mesocorticolimbic network was implicated in processing of high versus low provoking feedback, and core hubs of the default mode network were implicated in the subsequent decision after high versus low provocation. Testosterone administration increased activation in this network. The data provides evidence for an interaction of the MAOA polymorphism and exogenous testosterone on anger and suggests that interactive effects on the brain signal could underlie differential emotional reactivity. The increased default mode activation in the testosterone group suggests an enhanced engagement of social cognition related regions possibly supporting responsivity towards social provocation. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity'.

Alzheimer Disease and Selected Risk Factors Disrupt a Co-regulation of Monoamine Oxidase-A/B in the Hippocampus, but Not in the Cortex

Monoamine oxidase-A (MAO-A) and MAO-B have both been implicated in the pathology of Alzheimer disease (AD). We examined 60 autopsied control and AD donor brain samples to determine how well MAO function aligned with two major risk factors for AD, namely sex and APOE ε4 status. MAO-A activity was increased in AD cortical, but not hippocampal, samples. In contrast, MAO-B activity was increased in both regions (with a strong input from female donors) whether sample means were compared based on: (a) diagnosis alone; (b) diagnosis-by-APOE ε4 status (i.e., carriers vs. non-carriers of the ε4 allele); or (c) APOE ε4 status alone (i.e., ignoring ‘diagnosis’ as a variable). Sample means strictly based on the donor’s sex did not reveal any difference in either MAO-A or MAO-B activity. Unexpectedly, we found that cortical MAO-A and MAO-B activities were highly correlated in both males and females (if focussing strictly on the donor’s sex), while in the hippocampus, any correlation was lost in female samples. Stratifying for sex-by-APOE ε4 status revealed a strong correlation between cortical MAO-A and MAO-B activities in both non-carriers and carriers of the allele, but any correlation in hippocampal samples was lost in carriers of the allele. A diagnosis of AD disrupted the correlation between MAO-A and MAO-B activities in the hippocampus, but not the cortex. We observed a novel region-dependent co-regulation of MAO-A and MAO-B mRNAs (but not proteins), while a lack of correlation between MAO activities and the respective proteins corroborated previous reports. Overexpression of human APOE4 increased MAO activity (but not mRNA/protein) in C6 and in HT-22 cell cultures. We identified a novel co-regulation of MAO-A and MAO-B activities that is spared from any influence of risk factors for AD or AD itself in the cortex, but vulnerable to these same factors in the hippocampus. Sex- and region-dependent abilities to buffer influences on brain MAO activities could have significant bearing on ambiguous outcomes when monoaminergic systems are targeted in clinical populations.

The association between endogenous testosterone level and behavioral flexibility in young men - Evidence from stimulus-outcome reversal learning

The capacity to flexibly adapt responding to unexpected changes in the environment is crucial for survival. Several neurotransmitters have been implicated in stimulus-outcome reversal learning. Yet, it remains an open question whether inter-individual differences in the neuroactive hormone testosterone may also be related to this type of behavioral flexibility. In this study we assessed the association between endogenous testosterone level and reversal learning in young healthy men. We used an observer reversal learning task, in which subjects viewed computer-based decisions between two stimuli, of which one was currently rewarded while the other one was punished. Contingencies reversed unpredictably every 5-9 trials. Subjects had to indicate the current outcome association before the actual outcome was revealed. In the trial following an unexpected reversal either the same stimulus from the reversal (experienced reversal), or its alternative, for which the reversal had not yet been shown (inferred reversal), could be chosen by the computer, and subjects had to adapt responding accordingly. We found that testosterone predicted better post-reversal performance. This correlation was strongest in the more difficult inferred reversal condition, particularly in impulsive individuals. Collectively, these data support the view that endogenous testosterone may enhance behavioral flexibility in men, particularly when working memory demand is high and subjects have to update several stimulus-outcome contingencies at the same time. It remains to be further elucidated whether this testosterone effect was achieved through an interaction with dopaminergic transmission or through direct interplay with androgen receptors in the brain regions implicated in reversal learning.

Reproductive steroid receptors and actions in the locus coeruleus of male macaques: Part of an aggression circuit?

This study was initiated to determine whether the noradrenergic (NE) neurons of the locus coeruleus (LC) could mediate the stimulatory action of androgens on serotonin-related gene expression in male macaques. These experiments follow our observations that serotonin neurons lack androgen receptors (ARs), and yet respond to androgens. Male Japanese macaques (Macaca fuscata) were castrated for 5-7months and then treated for 3months with [1] placebo, [2] T (testosterone), [3] DHT (dihydrotestosterone; non-aromatizable androgen) plus ATD (steroidal aromatase inhibitor), or [4] FLUT (Flutamide; androgen antagonist) plus ATD (n=5/group). The noradrenergic (NE) innervation of the raphe was determined with immunolabeling of axons with an antibody to dopamine-β-hydroxylase (DBH). Immunolabeling of tyrosine hydroxylase (TH) dendrites and corticotropin releasing hormone (CRH) axons innervating the LC was also determined. Due to the longer treatment period employed, the expression of the cognate nuclear receptors was sought. Androgen receptor (AR), estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) immunostaining was accomplished. Quantitative image analysis was applied and immunopositive neurons or axons with boutons were measured. Double-label of NE neurons for each receptor plus TH determined whether the receptors were localized in NE neurons. Androgens with or without aromatase activity significantly stimulated DBH axon density in the raphe (ANOVA, p=0.006), and LC dendritic TH (ANOVA, p<0.0001), similar to serotonin-related mRNA expression in the raphe. There were significantly more AR-positive neurons in T- and DHT+ATD-treated groups compared to placebo or FLUT+ATD-treated groups (ANOVA, p=0.0014). There was no difference in the number of positive-neurons stained for ERα or ERβ. The CRH axon density in the LC was significantly reduced with aromatase inhibition, suggesting that CRH depends on estrogen, not androgens (ANOVA, p=0.0023). Double-immunohistochemistry revealed that NE neurons did not contain AR. Rather, AR-positive nuclei were found in neighboring cells that are likely neurons. However, >80% of LC NE neurons contained ERα or ERβ. In conclusion, the LC NE neurons may transduce the stimulatory effect of androgens on serotonin-related gene expression. Since LC NE neurons lack AR, the androgenic stimulation of dendritic TH and axonal DBH may be indirectly mediated by other neurons. Estrogen, either from metabolism of T or from de novo synthesis, appears necessary for robust CRH innervation of the LC, which differs from female macaques.

Metabolite profiles of repeatedly sampled urine from male fathead minnows (Pimephales promelas) contain unique lipid signatures following exposure to anti-androgens

The purpose of this study was twofold. First, we sought to identify candidate markers of exposure to anti-androgens by analyzing endogenous metabolite profiles in the urine of male fathead minnows (mFHM, Pimephales promelas). Based on earlier work, we hypothesized that unidentified lipids in the urine of mFHM were selectively responsive to exposure to androgen receptor antagonists, which is otherwise difficult to confirm using established fish toxicity assays. A second goal was to evaluate the feasibility of non-lethally and repeatedly sampling urine from individual mFHMs over the time course of response to a chemical exposure. Accordingly, we exposed mFHM to the model anti-androgens vinclozolin or flutamide. Urine was collected from each fish at 48hour intervals over the course of a 14day exposure. Parallel experiments were conducted with mFHM exposed to bisphenol A or control water. The frequent handling/sampling regime did not cause apparent adverse effects on the fish. Endogenous metabolite profiling was conducted with gas chromatography-mass spectrometry (GC-MS), which exhibited lower variation for the urinary metabolome than was found in earlier work with nuclear magnetic resonance (NMR) spectroscopy. Specifically, for inter- and intra-individual variations, the median spectrum-wide relative standard deviation (RSD) was 32.6% and 33.3%, respectively, for GC-MS analysis of urine from unexposed mFHM. These results compared favorably with similar measurements of urine from other model species, including the Sprague Dawley rat. In addition, GC-MS allowed us to identify several lipids (e.g., certain saturated fatty acids) in mFHM urine as candidate markers of exposure to androgen receptor antagonists.