Spatial memory performance in androgen insensitive male rats

Endocrine-Disrupting Chemicals and Hippocampal Development: The Role of Estrogen and Androgen Signaling

Hormones are important regulators of key processes during fetal brain development. Thus, the developing brain is vulnerable to the action of chemicals that can interfere with endocrine signals. Epidemiological studies have pointed toward sexually dimorphic associations between neurodevelopmental outcomes, such as cognitive abilities, in children and prenatal exposure to endocrine-disrupting chemicals (EDCs). This points toward disruption of sex steroid signaling in the development of neural structures underlying cognitive functions, such as the hippocampus, an essential mediator of learning and memory processes. Indeed, during development, the hippocampus is subjected to the organizational effects of estrogens and androgens, which influence hippocampal cell proliferation, differentiation, dendritic growth, and synaptogenesis in the hippocampal fields of Cornu Ammonis and the dentate gyrus. These early organizational effects correlate with a sexual dimorphism in spatial cognition and are subject to exogenous chemical perturbations. This review summarizes the current knowledge about the organizational effects of estrogens and androgens on the developing hippocampus and the evidence for hippocampal-dependent learning and memory perturbations induced by developmental exposure to EDCs. We conclude that, while it is clear that sex hormone signaling plays a significant role during hippocampal development, a complete picture at the molecular and cellular levels would be needed to establish causative links between the endocrine modes of action exerted by EDCs and the adverse outcomes these chemicals can induce at the organism level.

Androgen Affects the Inhibitory Avoidance Memory by Primarily Acting on Androgen Receptor in the Brain in Adolescent Male Rats

Adolescence is the critical postnatal stage for the action of androgen in multiple brain regions. Androgens can regulate the learning/memory functions in the brain. It is known that the inhibitory avoidance test can evaluate emotional memory and is believed to be dependent largely on the amygdala and hippocampus. However, the effects of androgen on inhibitory avoidance memory have never been reported in adolescent male rats. In the present study, the effects of androgen on inhibitory avoidance memory and on androgen receptor (AR)-immunoreactivity in the amygdala and hippocampus were studied using behavioral analysis, Western blotting and immunohistochemistry in sham-operated, orchiectomized, orchiectomized + testosterone or orchiectomized + dihydrotestosterone-administered male adolescent rats. Orchiectomized rats showed significantly reduced time spent in the illuminated box after 30 min (test 1) or 24 h (test 2) of electrical foot-shock (training) and reduced AR-immunoreactivity in amygdala/hippocampal cornu Ammonis (CA1) in comparison to those in sham-operated rats. Treatment of orchiectomized rats with either non-aromatizable dihydrotestosterone or aromatizable testosterone were successfully reinstated these effects. Application of flutamide (AR-antagonist) in intact adolescent rats exhibited identical changes to those in orchiectomized rats. These suggest that androgens enhance the inhibitory avoidance memory plausibly by binding with AR in the amygdala and hippocampus.

Frank Beach Award Winner: Lessons from poison frogs on ecological drivers of behavioral diversification

Variation in natural behavior is tightly linked to the ecological resources with which they co-evolved. This review discusses poison frog behavior and neuroendocrinology to illustrate how ecological factors drive diversification of behavior and its underlying neural mechanisms. Poison frogs show tremendous diversity in reproductive strategies that are tightly linked to water resources in their environment. Different species utilize particular pool sizes to rear their offspring, which has selected for sex differences in parental behavior among poison frog species. Tadpole behavior reflects the behavioral diversity of adults, where tadpoles can display social group living or violent aggression and begging behavior, which are all associated with pool size and occupancy. Using this behavioral diversity among poison frog species, we have identified core brain regions, like the hippocampus and preoptic area, as being involved in regulating different aspects of amphibian parental behavior. In contrast to core brain regions, the neuromodulators governing these behaviors seem to be more labile across species. This work exemplifies how comparative studies are a prime experimental system to study how evolution tunes neural circuits that give rise to the diversity of behaviors we observe in the natural world. Finally, this review ends on a more important form of diversity - that of our scientific community - and how community outreach, decolonization of field based science, and inclusion of groups historically excluded from conducting research are needed for the scientific enterprise to transform into something truly beneficial for all members of our society.

Androgens and the developing hippocampus

The hippocampus is central to spatial learning and stress responsiveness, both of which differ in form and function in males versus females, yet precisely how the hippocampus contributes to these sex differences is largely unknown. In reproductively mature individuals, sex differences in the steroid hormone milieu undergirds many sex differences in hippocampal-related endpoints. However, there is also evidence for developmental programming of adult hippocampal function, with a central role for androgens as well as their aromatized byproduct, estrogens. These include sex differences in cell genesis, synapse formation, dendritic arborization, and excitatory/inhibitory balance. Enduring effects of steroid hormone modulation occur during two developmental epochs, the first being the classic perinatal critical period of sexual differentiation of the brain and the other being adolescence and the associated hormonal changes of puberty. The cellular mechanisms by which steroid hormones enduringly modify hippocampal form and function are poorly understood, but we here review what is known and highlight where attention should be focused.

Androgen Affects the Dynamics of Intrinsic Plasticity of Pyramidal Neurons in the CA1 Hippocampal Subfield in Adolescent Male Rats

Androgen receptor (AR) is abundantly expressed in the preoptico-hypothalamic area, bed nucleus of stria terminalis, and medial amygdala of the brain where androgen plays an important role in regulating male sociosexual, emotional and aggressive behaviors. In addition to these brain regions, AR is also highly expressed in the hippocampus, suggesting that the hippocampus is another major target of androgenic modulation. It is known that androgen can modulate synaptic plasticity in the CA1 hippocampal subfield. However, to date, the effects of androgen on the intrinsic plasticity of hippocampal neurons have not been clearly elucidated. In this study, the effects of androgen on the expression of AR in the hippocampus and on the dynamics of intrinsic plasticity of CA1 pyramidal neurons were examined using immunohistochemistry, Western blotting and whole-cell current-clamp recording in unoperated, sham-operated, orchiectomized (OCX), OCX + testosterone (T) or OCX + dihydrotestosterone (DHT)-primed adolescent male rats. Orchiectomy significantly decreased AR-immunoreactivity, resting membrane potential, action potential numbers, afterhyperpolarization amplitude and membrane resistance, whereas it significantly increased action potential threshold and membrane capacitance. These effects were successfully reversed by treatment with either aromatizable androgen T or non-aromatizable androgen DHT. Furthermore, administration of the AR-antagonist flutamide in intact rats showed similar changes to those in OCX rats, suggesting that androgens affect the excitability of CA1 pyramidal neurons possibly by acting on the AR. Our current study potentially clarifies the role of androgen in enhancing the basal excitability of the CA1 pyramidal neurons, which may influence selective neuronal excitation/activation to modulate certain hippocampal functions.

On the Role of Testosterone in Anxiety-Like Behavior Across Life in Experimental Rodents

Testosterone affects brain functions and might explain some of the observed behavioral sex differences. Animal models may help in elucidating the possible involvement of sex hormones in these sex differences. The effects of testosterone have been intensively investigated, especially in anxiety models. Numerous experiments have brought inconsistent results with either anxiolytic or anxiogenic effects. Besides methodological variations, contradictory findings might be explained by the divergent metabolism of testosterone and its recognition by neurons during prenatal and postnatal development. Gonadectomy and subsequent supplementation have been used to study the role of sex hormones. However, the variable duration of hypogonadism might affect the outcomes and the effect of long-term androgen deficiency is understudied. Testosterone can be metabolized to dihydrotestosterone strengthening the androgen signaling, but also to estradiol converting the androgen to estrogen activity. Moreover, some metabolites of testosterone can modulate γ-aminobutyric acid and serotonergic neurotransmission. Here we review the currently available experimental data in experimental rodents on the effects of testosterone on anxiety during development. Based on the experimental results, females are generally less anxious than males from puberty to middle-age. The anxiety-like behavior of females and males is likely influenced by early organizational effects, but might be modified by activational effects of testosterone and its metabolites. The effects of sex hormones leading to anxiogenesis or anxiolysis depend on factors affecting hormonal status including age. The biological and several technical issues make the study of effects of testosterone on anxiety very complex and should be taken into account when interpreting experimental results.

Cognitive Impacts of Estrogen Treatment in Androgen-Deprived Males: What Needs to be Resolved

Background:

Many prostate cancer (PCa) patients are on androgen deprivation therapy (ADT) as part of their cancer treatments but ADT may lead to cognitive impairments. ADT depletes men of both androgen and estrogen. Whether estradiol supplementation can improve cognitive impairments in patients on ADT is understudied.

Objective:

To summarize data on the effects of estradiol treatment on cognitive function of androgen-deprived genetic male populations (PCa patients and male-to-female transsexuals) and castrated male animals.

Method:

Publications were identified by a literature search on PubMed and Google Scholar.

Results:

While some studies showed that estradiol improves cognitive function (most notably, spatial ability) for castrated rats, what remains uninvestigated are: 1) whether estradiol can improve cognition after long-term androgen deprivation, 2) how estradiol affects memory retention, and 3) how early vs. delayed estradiol treatment after castration influences cognition. For androgen-deprived genetic males, estradiol treatment may improve some cognitive functions (e.g., verbal and visual memory), but the findings are not consistent due to large variability in the study design between studies.

Conclusion:

Future studies are required to determine the best estradiol treatment protocol to maximize cognitive benefits for androgen-deprived genetic males. Tests that assess comparable cognitive domains in human and rodents are needed. What particularly under-investigated is how the effects of estradiol on cognitive ability intersect with other parameters; sleep, depression and physical fatigue. Such studies have clinical implications to improve the quality of life for both PCa patients on ADT as well as for male-to-female transsexuals.

Role of the androgen receptor in the central nervous system

The involvement of gonadal androgens in functions of the central nervous system was suggested for the first time about half a century ago. Since then, the number of functions attributed to androgens has steadily increased, ranging from regulation of the hypothalamic-pituitary-gonadal axis and reproductive behaviors to modulation of cognition, anxiety and other non-reproductive functions. This review focuses on the implication of the neural androgen receptor in these androgen-sensitive functions and behaviors.

Effects of testosterone dose on spatial memory among castrated adult male rats

Previous research on the activational effects of testosterone on spatial memory has produced mixed results, possibly because such effects are dose-dependent. We tested a wide range of testosterone doses using two spatial memory tasks: a working-reference memory version of the radial-arm maze (RAM) and an object location memory task (OLMT). Adult male Sprague-Dawley rats were castrated or sham-castrated and given daily injections of drug vehicle (Oil Sham and Oil GDX) or one of four doses of testosterone propionate (0.125, 0.250, 0.500, and 1.000 mg T) beginning seven days before the first day of behavioral tests and continuing throughout testing. For the RAM, four arms of the maze were consistently baited on each day of testing. Testosterone had a significant effect on working memory on the RAM, with the Oil Sham, 0.125 mg T, and 0.500 mg T groups performing better than the Oil GDX group. In contrast, there was no significant effect of testosterone on spatial reference memory on the RAM. For the OLMT, we tested long-term memory using a 2 h inter-trial interval between first exposure to two identical objects and re-exposure after one object had been moved. Only the 0.125 and 0.500 mg T groups showed a significant increase in exploration of the moved object during the testing trials, indicating better memory than all other groups. Testosterone replacement restored spatial memory among castrated male rats on both behavioral tasks, but there was a complex dose-response relationship; therefore, the therapeutic value of testosterone is likely sensitive to dose.

Androgenic mechanisms of sexual differentiation of the nervous system and behavior

Testicular androgens are the major endocrine factor promoting masculine phenotypes in vertebrates, but androgen signaling is complex and operates via multiple signaling pathways and sites of action. Recently, selective androgen receptor mutants have been engineered to study androgenic mechanisms of sexual differentiation of the nervous system and behavior. The focus of these studies has been to evaluate androgenic mechanisms within the nervous system by manipulating androgen receptor conditionally in neural tissues. Here we review both the effects of neural loss of AR function as well as the effects of neural overexpression of AR in relation to global AR mutants. Although some studies have conformed to our expectations, others have proved challenging to assumptions underlying the dominant hypotheses. Notably, these studies have called into question both the primacy of direct, neural mechanisms and also the linearity of the relationship between androgenic dose and sexual differentiation of brain and behavior.

The Effects of Bisphenol A Exposure at Different Developmental Time Points in an Androgen-Sensitive Neuromuscular System in Male Rats

The industrial plasticizer bisphenol A (BPA) is a ubiquitous endocrine disruptor to which the general human population is routinely exposed. Although BPA is well known as an estrogenic mimic, there have been some suggestions that this compound may also alter activity at the androgen receptor. To determine whether BPA does have antiandrogenic properties, we evaluated BPA effects in the spinal nucleus of the bulbocavernosus and dorsolateral nucleus, sexually dimorphic groups of motor neurons in the lumbar spinal cord that are critically dependent on androgens for survival and maintenance, as well as the monomorphic retrodorsolateral nucleus. In experiment 1, we administered varying concentrations of BPA to juvenile rats pre- and postnatally and examined both the number and size of motor neurons in adulthood. In experiment 2, different doses of BPA were given to adult rats for 28 days, after which the soma size of motor neurons were measured. Although no effect of BPA on neural survival or soma size was noted after perinatal BPA exposure, BPA exposure did result in a decrease in soma size in all motor neuron pools after chronic exposure in adulthood. These findings are discussed with regard to putative antiandrogenic effects of BPA; we argue that BPA is not antiandrogenic but is acting through nonandrogen receptor-dependent mechanisms.

Sex hormones and adult hippocampal neurogenesis: Regulation, implications, and potential mechanisms

Neurogenesis within the adult hippocampus is modulated by endogenous and exogenous factors. Here, we review the role of sex hormones in the regulation of adult hippocampal neurogenesis in males and females. The review is framed around the potential functional implications of sex hormone regulation of adult hippocampal neurogenesis, with a focus on cognitive function and mood regulation, which may be related to sex differences in incidence and severity of dementia and depression. We present findings from preclinical studies of endogenous fluctuations in sex hormones relating to reproductive function and ageing, and from studies of exogenous hormone manipulations. In addition, we discuss the modulating roles of sex, age, and reproductive history on the relationship between sex hormones and neurogenesis. Because sex hormones have diverse targets in the central nervous system, we overview potential mechanisms through which sex hormones may influence hippocampal neurogenesis. Lastly, we advocate for a more systematic consideration of sex and sex hormones in studying the functional implications of adult hippocampal neurogenesis.

The contribution of the androgen receptor (AR) in human spatial learning and memory: A study in women with complete androgen insensitivity syndrome (CAIS)

Few studies have examined the impact of androgen insensitivity on human spatial learning and memory. In the present study, we tested 11 women with complete androgen insensitivity syndrome (CAIS), a rare genetic disorder characterized by complete absence of AR activity, and compared their performance against 20 comparison males and 19 comparison females on a virtual analog of the Morris Water Maze task. The results replicated a main sex effect showing that men relative to women were faster in finding the hidden platform and had reduced heading error. Furthermore, findings indicated that mean performance of women with CAIS was between control women and control men, though the differences were not statistically significant. Effect size estimates (and corresponding confidence intervals) of spatial learning trials showed little difference between women with CAIS and control women but CAIS women differed from men, but not women, on two variables, latency to find the platform and first-move latency. No differences between groups were present during visible platform trials or the probe trial, a measure of spatial memory. Moreover, groups also did not differ on estimates of IQ and variability of performance. The findings are discussed in relation to androgen insensitivity in human spatial learning and memory.

On the effects of testosterone on brain behavioral functions

Testosterone influences the brain via organizational and activational effects. Numerous relevant studies on rodents and a few on humans focusing on specific behavioral and cognitive parameters have been published. The results are, unfortunately, controversial and puzzling. Dosing, timing, even the application route seem to considerably affect the outcomes. In addition, the methods used for the assessment of psychometric parameters are a bit less than ideal regarding their validity and reproducibility. Metabolism of testosterone contributes to the complexity of its actions. Reduction to dihydrotestosterone by 5-alpha reductase increases the androgen activity; conversion to estradiol by aromatase converts the androgen to estrogen activity. Recently, the non-genomic effects of testosterone on behavior bypassing the nuclear receptors have attracted the interest of researchers. This review tries to summarize the current understanding of the complexity of the effects of testosterone on brain with special focus on their role in the known sex differences.

Evidence for involvement of steroid receptors and coactivators in neuroepithelial and meningothelial tumors

Steroid receptors such as androgen receptor (AR) and estrogen receptors (ER) ER-α and ER-β, and their receptor coactivators (steroid receptor coactivator, SRC) are widely localized in the brain. Although previous studies have investigated the expression of steroid receptors in brain tumors like astrocytoma, the studies on the expression of steroid receptors and SRCs in other brain tumors are lacking. Here, we investigated the expression of AR, ERs, and SRCs in neuroepithelial (medulloblastoma, ependymoma, oligodendroglioma) and meningothelial meningioma using tissue microarray immunohistochemistry. Compared to normal brain tissue, we found that the expression of SRC-1, SRC-3, and ER-α significantly decreased in meningothelial tumor and neuroepithelial tumor, suggesting that the SRC-1/SRC-3 levels may be regulated by ER-α. Moreover, the levels of AR strongly correlated to the levels of ER-β. Furthermore, correlation was also detected between SRC-3 and AR in neuroepithelial tumor, and between ER-α and ER-β in meningothelial tumor. In addition, the decreased ratio of SRC-1/SRC-3 was associated with an increase of ER-β in neuroepithelial tumor. These results indicate that expressions of different steroid receptors and activators may be tumor type dependent. While AR, ER-α, and ER-β may be involved in the pathogenesis of meningothelial tumor, SRCs/ER-β axis and SRC-3/AR axis may play a role in the pathogenesis of neuroepithelial tumor.

Sex dimorphism in seizure-controlling networks

Males and females show a different predisposition to certain types of seizures in clinical studies. Animal studies have provided growing evidence for sexual dimorphism of certain brain regions, including those that control seizures. Seizures are modulated by networks involving subcortical structures, including thalamus, reticular formation nuclei, and structures belonging to the basal ganglia. In animal models, the substantia nigra pars reticulata (SNR) is the best studied of these areas, given its relevant role in the expression and control of seizures throughout development in the rat. Studies with bilateral infusions of the GABA(A) receptor agonist muscimol have identified distinct roles of the anterior or posterior rat SNR in flurothyl seizure control, that follow sex-specific maturational patterns during development. These studies indicate that (a) the regional functional compartmentalization of the SNR appears only after the third week of life, (b) only the male SNR exhibits muscimol-sensitive proconvulsant effects which, in older animals, is confined to the posterior SNR, and (c) the expression of the muscimol-sensitive anticonvulsant effects become apparent earlier in females than in males. The first three postnatal days are crucial in determining the expression of the muscimol-sensitive proconvulsant effects of the immature male SNR, depending on the gonadal hormone setting. Activation of the androgen receptors during this early period seems to be important for the formation of this proconvulsant SNR region. We describe molecular/anatomical candidates underlying these age- and sex-related differences, as derived from in vitro and in vivo experiments, as well as by [(14)C]2-deoxyglucose autoradiography. These involve sex-specific patterns in the developmental changes in the structure or physiology or GABA(A) receptors or of other subcortical structures (e.g., locus coeruleus, hippocampus) that may affect the function of seizure-controlling networks.

New knockout model confirms a role for androgen receptors in regulating anxiety-like behaviors and HPA response in mice

Men are less likely than women to suffer from anxiety disorders. Because gonadal hormones play a crucial role in many behavioral sex differences, they may underlie sex differences in human anxiety. In rodents, testosterone (T) exerts anxiolytic effects via the androgen receptor (AR): we found that male mice with a naturally-occurring mutation rendering the AR dysfunctional, referred to as spontaneous testicular feminization mutation (sTfm), showed more anxiety-like behaviors than wildtype (WT) males. Here, we used Cre-lox recombination technology to create another dysfunctional allele for AR. These induced Tfm (iTfm) animals also displayed more anxiety-like behaviors than WTs. We further found that AR-modulation of these behaviors interacts with circadian phase. When tested in the resting phase, iTfms appeared more anxious than WTs in the open field, novel object and elevated plus maze tests, but not the light/dark box. However, when tested during the active phase (lights off), iTfms showed more anxiety-related behavior than WTs in all four tests. Finally, we confirmed a role of T acting via AR in regulating HPA axis activity, as WT males with T showed a lower baseline and overall corticosterone response, and a faster return to baseline following mild stress than did WT males without T or iTfms. These findings demonstrate that this recombined AR allele is a valuable model for studying androgenic modulation of anxiety, that the anxiolytic effects of AR in mice are more prominent in the active phase, and that HPA axis modulation by T is AR dependent.

Androgen insensitive male rats display increased anxiety-like behavior on the elevated plus maze

Male rats carrying the testicular feminization mutation (Tfm-affected males) are insensitive to androgens, resulting in a female-typical peripheral phenotype despite possession of inguinal testes that are androgen secretory. Androgen-dependent neural and behavioral processes may likewise show atypical sexual differentiation. Interestingly, these mutant rats display elevated serum corticosterone, suggesting a chronic anxiety phenotype and dysregulated hypothalamic-pituitary-adrenal axis. In order to understand if elevated anxiety-like behavior is a possible mediating variable affecting the display of certain androgen-dependent behaviors, we compared the performance of Tfm-affected males to wild type males and females in the elevated plus maze (EPM). Two well-established indicators of anxiety-like behavior in the EPM were analyzed: total percentage of time spent on the open arms, and the percentage of open arm entries. We also analyzed the total number of open arm entries. Interestingly, Tfm-affected males spent less percentage of time on the open arms than both males and females, suggesting increased anxiety-like behavior. Percentage of open arm entries and the total number of arm entries was comparable between the groups, indicating that the observed decrease in the percentage of time spent on the open arms was not due to a global reduction in exploratory behavior. These data, in contrast to earlier reports, thus implicate androgen receptor-mediated functions in the expression of anxiety behaviors in male rats. Given that anxiety is widely reported as a precipitating factor in depression, studying the role of the androgen receptor in anxiety may give insights into the pathogenesis of major depressive disorder.

Amelioratory effects of testosterone treatment on cognitive performance deficits induced by soluble Aβ1-42 oligomers injected into the hippocampus

This study was undertaken to investigate the protective effects and potential mechanism of testosterone (T) on cognitive performance in adult male rats given bilateral intrahippocampal injections of beta amyloid 1-42 oligomers (Aβ1-42) combined with gonadectomy (Aβ+GDX). A series of experiments were designed to verify the optimal administration time and dose of T and to explore its potential protective mechanisms on spatial ability in Aβ+GDX rats in the Morris water maze test. Aβ1-42 was injected only once two weeks before testing, while T and the androgen receptor (AR) antagonist flutamide (F) were administered daily beginning 2 days before and throughout the 6 days of testing. The Aβ1-42 injection and GDX individually impaired cognitive performance, and the combination of these treatments was additive, leading to even greater impairment. The serum T level peaked at 48 h after administration. T doses ranging from 0.25 to 1.00 mg corresponding to serum T levels of 4.5-21.35 ng/ml improved the spatial ability. Animals administered 0.75 mg of T corresponding to the serum T level of 15.2 ng/ml had the most significantly improved behavioral performances. However, higher T doses of 1.50 and 2.00 mg resulting in serum T levels of 34.8 and 45 ng/ml, respectively, impaired the behavioral performances. F had no effect on the serum T level and spatial ability, but it blocked the activational effect of T. These findings indicate that the effect of T on behavioral performances is partly mediated through ARs.

Testosterone influences spatial strategy preferences among adult male rats

Males outperform females on some spatial tasks, and this may be partially due to the effects of sex steroids on spatial strategy preferences. Previous work with rodents indicates that low estradiol levels bias females toward a striatum-dependent response strategy, whereas high estradiol levels bias them toward a hippocampus-dependent place strategy. We tested whether testosterone influenced the strategy preferences in male rats. All subjects were castrated and assigned to one of three daily injection doses of testosterone (0.125, 0.250, or 0.500 mg/rat) or a control group that received daily injections of the drug vehicle. Three different maze protocols were used to determine rats' strategy preferences. A low dose of testosterone (0.125 mg) biased males toward a motor-response strategy on a T-maze task. In a water maze task in which the platform itself could be used intermittently as a visual cue, a low testosterone dose (0.125 mg) caused a significant increase in the use of a cued-response strategy relative to control males. Results from this second experiment also indicated that males receiving a high dose of testosterone (0.500 mg) were biased toward a place strategy. A third experiment indicated that testosterone dose did not have a strong influence on the ability of rats to use a nearby visual cue (floating ball) in the water maze. For this experiment, all groups seemed to use a combination of place and cued-response strategies. Overall, the results indicate that the effects of testosterone on spatial strategy preference are dose dependent and task dependent.

Testosterone modulates spatial recognition memory in male rats

A growing body of research indicates that testosterone influences spatial cognition in male rats; however, the overwhelming majority of studies have been conducted on tasks motivated by either food deprivation or water escape. The hippocampus-dependent version of the Y-maze task, which characterizes spatial recognition memory, capitalizes on the propensity of rats to gravitate toward novel spatial environments and is not contingent upon either appetite or the stress associated with water escape, two factors also affected by testosterone. Accordingly, the aim of the current study was to examine the effects of orchidectomy and subsequent testosterone treatment on spatial recognition memory. Orchidectomy did not impact spatial recognition memory when the delay between the information and retention trials of the Y-maze task was 24h. Alternatively, on the second Y-maze task, which featured a 48-h delay between trials, orchidectomy reduced, and treatments that produced higher levels of testosterone restored, preference for the arm associated with the novel spatial environment. Importantly, there were no differences in activity levels as a function of orchidectomy or testosterone treatment on either of the two tasks. Consistent with previous findings, orchidectomy attenuated, and testosterone treatment restored, both body weight gain and the relative weight of the androgen-sensitive ischiocavernosus muscle, which confirmed the efficacy of orchidectomy and testosterone treatments on physiological outcomes. Therefore, testosterone influenced spatial cognition on a task that minimized the influence of non-mnemonic factors and took advantage of the innate preference of rodents to seek out novel spatial environments.

Perinatal BPA exposure demasculinizes males in measures of affect but has no effect on water maze learning in adulthood

Bisphenol A (BPA) is an endocrine disrupting agent that can alter the normal gonadal steroid-sensitive sexual differentiation of the brain and behavior. While reproductive behavior and physiology are known to be altered by perinatal exposure to this compound, less is known about BPA's effects on sex differences in learning and measures of affect. In order to evaluate the effects of perinatal BPA treatment on learning and affect in adulthood, we exposed rats to one of five doses of BPA through gestation and lactation then examined adult behavior in the Morris Water Maze (MWM), the Elevated Plus Maze (EPM) and the Forced Swim Test (FST). No effect of BPA was observed in the MWM, but on both the EPM and FST, low doses (5 μg/kg) of BPA eliminated sex differences found between controls; furthermore, a non-monotonic dose-response observed in previous studies was confirmed for these tasks. Overall, our study adds to the growing data suggesting that BPA interferes with the normal development of affective behaviors in a non-linear, dose-dependent manner.

Effects of testosterone on spatial learning and memory in adult male rats

A male advantage over females for spatial tasks has been well documented in both humans and rodents, but it remains unclear how the activational effects of testosterone influence spatial ability in males. In a series of experiments, we tested how injections of testosterone influenced the spatial working and reference memory of castrated male rats. In the eight-arm radial maze, testosterone injections (0.500 mg/rat) reduced the number of working memory errors during the early blocks of testing but had no effect on the number of reference memory errors relative to the castrated control group. In a reference memory version of the Morris water maze, injections of a wide range of testosterone doses (0.0625-1.000 mg/rat) reduced path lengths to the hidden platform, indicative of improved spatial learning. This improved learning was independent of testosterone dose, with all treatment groups showing better performance than the castrated control males. Furthermore, this effect was only observed when rats were given testosterone injections starting 7 days prior to water maze testing and not when injections were given only on the testing days. We also observed that certain doses of testosterone (0.250 and 1.000 mg/rat) increased perseverative behavior in a reversal-learning task. Finally, testosterone did not have a clear effect on spatial working memory in the Morris water maze, although intermediate doses seemed to optimize performance. Overall, the results indicate that testosterone can have positive activational effects on spatial learning and memory, but the duration of testosterone replacement and the nature of the spatial task modify these effects.

The organizational role of testicular hormones and the androgen receptor in anxiety-related behaviors and sensorimotor gating in rats

Perinatal exposure to testosterone (T), which can act upon both the androgen receptor (AR) and, via aromatization of T into estrogens, upon estrogen receptors, organizes many adult behaviors in rodents. We compared behaviors in wild-type (WT) male rats and AR-deficient rats with the testicular feminization mutation (Tfm), which on the day of birth were either gonadectomized (Neo-Gdx) or sham operated. In adulthood, all rats were either gonadectomized or sham operated and implanted with T capsules to equilibrate circulating androgens. In each of four tests of behavior related to anxiety (open field, novel object exposure, light/dark box, and elevated plus maze), Neo-Gdx rats showed decreased indices of anxiety and increased activity compared with rats sham operated on the day of birth, with no differences between WT or Tfm males within treatment groups. These results indicate that testicular hormones act in development to increase adult indices of anxiety and decrease activity in males and that functional ARs are not required for this effect. Acoustic startle response was also reduced by Neo-Gdx, suggesting that postnatal testicular secretions potentiate this behavior as well. Adult corticosterone levels and sensorimotor gating, as measured by prepulse inhibition of the acoustic startle response, were increased by neonatal castration in both WT and Tfm rats. These findings indicate a role of T before adulthood in the organization of anxiety-related behaviors, activity, the hypothalamic-pituitary-adrenal axis, and sensorimotor gating in rats, all of which appears to be AR independent.

Partner preference and mount latency are masculinized in androgen insensitive rats

The sexual motivation of male rats may be inferred from a preference to stay in proximity to estrous female partners, and also from a short latency to show mounting behavior. Here, partner preference was assessed in rats carrying the testicular feminization mutation (TFM), and compared to wild type (WT) males in one version of this paradigm, and WT females and males in another version. Additionally, mount latency was quantified in the TFMs and compared to WT males in order to assess arousal levels, as this has not been previously reported. When presented with a choice between proximity to an estrous or non-estrous female, WT males and TFMs demonstrated similar preferences for the estrous female. Estrous females, conversely, preferred to spend time with the WT male. In agreement with previous reports we observed several sexual performance deficits in the TFMs, but mount latencies were in the male range. Given that the TFMs reliably choose to spend time with the estrous female in the partner preference tests, and that they display normal arousal levels (reflected in masculinized mount latencies), the data suggest the motivation to engage in sexual behavior is masculine in the TFM rat and that possession of functional androgen receptors is not crucial in these behaviors.

Social status and sex independently influence androgen receptor expression in the eusocial naked mole-rat brain

Naked mole-rats (Heterocephalus glaber) are eusocial rodents that live in large subterranean colonies including a single breeding female and 1-3 breeding males; all other members of the colony, known as subordinates, are reproductively suppressed. We recently found that naked mole-rats lack many of the sex differences in the brain and spinal cord commonly found in other rodents. Instead, neural morphology is influenced by breeding status, such that breeders, regardless of sex, have more neurons than subordinates in the ventromedial nucleus of the hypothalamus (VMH), and larger overall volumes of the bed nucleus of the stria terminalis (BST), paraventricular nucleus (PVN) and medial amygdala (MeA). To begin to understand how breeding status influences brain morphology, we examined the distribution of androgen receptor (AR) immunoreactivity in gonadally intact breeders and subordinates of both sexes. All animals had AR+ nuclei in many of the same regions positive for AR in other mammals, including the VMH, BST, PVN, MeA, and the ventral portion of the premammillary nucleus (PMv). We also observed diffuse labeling throughout the preoptic area, demonstrating that distribution of the AR protein in presumptive reproductive brain nuclei is well-conserved, even in a species that exhibits remarkably little sexual dimorphism. In contrast to other rodents, however, naked mole-rats lacked AR+ nuclei in the suprachiasmatic nucleus and hippocampus. Males had more AR+ nuclei in the MeA, VMH, and PMv than did females. Surprisingly, breeders had significantly fewer AR+ nuclei than subordinates in all brain regions examined (VMH, BST, PVN, MeA, and PMv). Thus, social status is strongly correlated with AR immunoreactivity in this eusocial species.

Early androgen exposure modulates spatial cognition in congenital adrenal hyperplasia (CAH)

Major questions remain about the exact role of hormones in cognition. Furthermore, the extent to which early perturbation in steroid function affects human brain development continues to be a wide open area of research. Congenital adrenal hyperplasia (CAH), a genetic disorder of steroid dysfunction characterized in part by in utero over-production of testosterone, was used as a natural model for addressing this question. Here, CAH (n=54, mean age=17.53, 31 female) patients were compared to healthy age- and sex-matched individuals (n=55, mean age=19.02, 22 female) on a virtual equivalent of the Morris Water Maze task [Morris, R., 1984. Developments of a water-maze procedure for studying spatial learning in the rat. J. Neurosci. Methods 11, 47-60], an established measure of sex differences in spatial cognition in rodents. Findings revealed that females with CAH with the most severe form of the disease and expected highest level of in utero exposure to androgens were found to perform similarly to both healthy males and CAH males, whereas strong sex differences were apparent in milder forms of the disorder and in controls. Moreover, advanced bone age, an indicator of long-term childhood exposure to testosterone was correlated with improved performance. The results indicate that individuals exposed to both excess androgens prenatally and prolonged exposure during childhood may manifest long-lasting changes in cognitive function. Such finding suggests a pivotal role of hormonal function on brain development in humans, mirroring results from the animal literature.

The androgen receptor is selectively involved in organization of sexually dimorphic social behaviors in mice

It is well established that sexually dimorphic neural regions are organized by steroid hormones during development. In many species, neonatal males are exposed to more testosterone than their female littermates, and ultimately it is the estradiol, produced by aromatization of testosterone, that affects sexual differentiation. However, the androgen receptor also plays an important role in the masculinization of brain and behavior. Here we tested the hypothesis that sexually dimorphic social and odor preference behaviors can be differentiated by a nonaromatizable androgen during development by treating female mice on the day of birth (PN0) with dihydrotestosterone (DHT). Control mice received a single vehicle injection on PN0. Adults were gonadectomized, treated with estradiol, and tested for social behaviors. In contrast with control females, females treated on PN0 with DHT, like male controls, exhibited a preference for female-soiled vs. male-soiled bedding, a preference to investigate a female vs. a male and reduced c-Fos-immunoreactivity (ir) in several neural areas after exposure to male-soiled bedding. However, females treated with DHT on PN0 had normal female-typical sexual behavior. The number of calbindin-ir cells in the preoptic area is sexually dimorphic (males more than females), but females given DHT on PN0 had intermediate numbers of calbindin-ir neurons, not significantly different from control males or females. Our data demonstrate that organization of social and olfactory preferences in mice can be affected by perinatal DHT and lends support to the role of androgen receptor in organization of sexual differentiation of brain and behaviors.

Classical androgen receptors in non-classical sites in the brain

Androgen receptors are expressed in many different neuronal populations in the central nervous system where they often act as transcription factors in the cell nucleus. However, recent studies have detected androgen receptor immunoreactivity in neuronal and glial processes of the adult rat neocortex, hippocampal formation, and amygdala as well as in the telencephalon of eastern fence and green anole lizards. This review discusses previously published findings on extranuclear androgen receptors, as well as new experimental results that begin to establish a possible functional role for androgen receptors in axons within cortical regions. Electron microscopic studies have revealed that androgen receptor immunoreactive processes in the rat brain correspond to axons, dendrites and glial processes. New results show that lesions of the dorsal CA1 region by local administration of ibotenic acid reduce the density of androgen receptor immunoreactive axons in the cerebral cortex and the amygdala, suggesting that these axons may originate in the hippocampus. Androgen receptor immunoreactivity in axons is also decreased by the intracerebroventricular administration of colchicine, suggesting that androgen receptor protein is transported from the perikaryon to the axons by fast axonal transport. Androgen receptors in axons located in the cerebral cortex and amygdala and originating in the hippocampus may play an important role in the rapid behavioral effects of androgens.

The role of androgen receptors in the masculinization of brain and behavior: what we've learned from the testicular feminization mutation

Many studies demonstrate that exposure to testicular steroids such as testosterone early in life masculinizes the developing brain, leading to permanent changes in behavior. Traditionally, masculinization of the rodent brain is believed to depend on estrogen receptors (ERs) and not androgen receptors (ARs). According to the aromatization hypothesis, circulating testosterone from the testes is converted locally in the brain by aromatase to estrogens, which then activate ERs to masculinize the brain. However, an emerging body of evidence indicates that the aromatization hypothesis cannot fully account for sex differences in brain morphology and behavior, and that androgens acting on ARs also play a role. The testicular feminization mutation (Tfm) in rodents, which produces a nonfunctional AR protein, provides an excellent model to probe the role of ARs in the development of brain and behavior. Tfm rodent models indicate that ARs are normally involved in the masculinization of many sexually dimorphic brain regions and a variety of behaviors, including sexual behaviors, stress response and cognitive processing. We review the role of ARs in the development of the brain and behavior, with an emphasis on what has been learned from Tfm rodents as well as from related mutations in humans causing complete androgen insensitivity.

Castration differentially affects spatial working and reference memory in male rats

A male advantage for spatial learning and memory tasks is well documented among humans and rodents. A possible physiological cause for this male advantage is activational effects of androgens among males. The spatial memory of eight castrated and eight sham-castrated adult male rats was compared using a working-reference memory version of the eight-arm radial arm maze followed by a reference memory version of the Morris water maze. After maze testing, blood was collected from each rat, and testosterone levels were determined using radioimmunoassay. In the radial arm maze, castrates committed significantly more working memory errors and significantly fewer reference memory errors than did shams. In the water maze, no statistically significant differences were found for acquisition or retention. There was a trend for shams with higher testosterone levels to have better retention in the water maze, but this seemed to be due to higher levels of perseverance rather than better reference memory. Castration may have affected performance in the radial arm maze and not in the water maze because the radial arm maze was a more difficult task or because the water maze was aversively motivated while the radial arm maze was appetitively motivated. Our results indicate that androgens improve working memory and may impair reference memory, but the effects of androgens on reference memory seem to be task dependent.

Effects of androgens and estradiol on spine synapse formation in the prefrontal cortex of normal and testicular feminization mutant male rats

Recent studies suggest that, in female monkeys and rats, estrogens elicit dendritic spine synapse formation in the prefrontal cortex, an area that, similar to the hippocampus, plays a critical role in cognition. However, whether gonadal hormones induce synaptic remodeling in the male prefrontal cortex remains unknown. Here we report that gonadectomy reduced, whereas administration of 5alpha-dihydrotestosterone or estradiol-benzoate to castrated male rats increased, the number of medial prefrontal cortical (mPFC) spine synapses, with estradiol-benzoate being less effective than 5alpha-dihydrotestosterone. To investigate whether the androgen receptor contributes to the mediation of these changes, we compared the response of testicular feminization mutant (Tfm) male rats to that of wild-type animals. The number of mPFC spine synapses in gonadally intact Tfm rats and 5alpha-dihydrotestosterone-treated castrated Tfm males was considerably reduced compared to intact wild-type animals, whereas the synaptogenic effect of estradiol-benzoate was surprisingly enhanced in Tfm rats. These data are consistent with the hypothesis that remodeling of spine synapses in the prefrontal cortex may contribute to the cognitive effect of gonadal steroids. Our findings in Tfm animals indicate that androgen receptors may mediate a large part of the synaptogenic action of androgens in the mPFC of adult males. However, because this effect of 5alpha-dihydrotestosterone is not completely lost in Tfm rats, additional mechanisms may also be involved.

Effects of the testicular feminization mutation (tfm) of the androgen receptor gene on BSTMPM volume and morphology in rats

The posteromedial bed nucleus of the stria terminalis (BSTMPM) is an important component of the extended amygdala that is sexually dimorphic in rats. We examined the effect of the testicular feminization mutation (tfm), which renders the androgen receptor (AR) dysfunctional, on BSTMPM volume and average somal area. As expected, we found a significant sex difference in the volume of the BSTMPM, with females having a smaller BSTMPM than wild type males. Size of the BSTMPM in tfm males was also significantly smaller than that of wildtype males, although this difference was significant only on the left side. We found no sex difference in BSTMPM somal areas. These findings support the role of androgen receptors in the sexual differentiation of this nucleus.

Gonadal steroids and visuo-spatial abilities in adult males: implications for generalized age-related cognitive decline

The relationship between the gonadal steroids, testosterone and estrogen, and individual and group differences in performance on some cognitive tasks remains unclear but sex differences favoring males on some tests of visuo-spatial ability are large and robust. This aim of this review is to assess evidence for both organizational and activational effects of gonadal steroids as the principle cause of sex difference in visuo-spatial ability. Additionally, the implications of this relationship are discussed in the context of decreasing levels of gonadal steroids in aging males and psychological theories of generalized age-related cognitive decline. Based upon human and non-human research gonadal steroids have organizational effects on visuo-spatial ability in adulthood. Activational effects of gonadal steroids on visuo-spatial ability appear most dominant in older men and are necessary for maintaining optimal visuo-spatial ability; randomized clinical trials show that testosterone supplementation improves performance. Additionally, decreasing gonadal steroid levels in aging males may contribute to generalized age-related cognitive decline. Future supplementation studies in men should attempt to control for constituent abilities related to visuo-spatial task performance, and investigate interactions between dosage levels and baseline gonadal status. Further future animal research is required to investigate changes in gonadal steroid levels and their relationship to neurotransmitter systems, neural plasticity, and behavioral correlates.

Sex steroids and the dentate gyrus

In the late 1980s, the finding that the dentate gyrus contains more granule cells in the male than in the female of certain mouse strains provided the first indication that the dentate gyrus is a significant target for the effects of sex steroids during development. Gonadal hormones also play a crucial role in shaping the function and morphology of the adult brain. Besides reproduction-related processes, sex steroids participate in higher brain operations such as cognition and mood, in which the hippocampus is a critical mediator. Being part of the hippocampal formation, the dentate gyrus is naturally involved in these mechanisms and as such, this structure is also a critical target for the activational effects of sex steroids. These activational effects are the results of three major types of steroid-mediated actions. Sex steroids modulate the function of dentate neurons under normal conditions. In addition, recent research suggests that hormone-induced cellular plasticity may play a larger role than previously thought, particularly in the dentate gyrus. Specifically, the regulation of dentate gyrus neurogenesis and synaptic remodeling by sex steroids received increasing attention lately. Finally, the dentate gyrus is influenced by gonadal hormones in the context of cellular injury, and the work in this area demonstrates that gonadal hormones have neuroprotective potential. The expression of estrogen, progestin, and androgen receptors in the dentate gyrus suggests that sex steroids, which could be of gonadal origin and/or synthesized locally in the dentate gyrus, may act directly on dentate cells. In addition, gonadal hormones could also influence the dentate gyrus indirectly, by subcortical hormone-sensitive structures such as the cholinergic septohippocampal system. Importantly, these three sex steroid-related themes, functional effects in the normal dentate gyrus, mechanisms involving neurogenesis and synaptic remodeling, as well as neuroprotection, have substantial implications for understanding normal cognitive function, with clinical importance for epilepsy, Alzheimer's disease and mental disorders.

Testosterone and photoperiod interact to affect spatial learning and memory in adult male white-footed mice (Peromyscus leucopus)

Gonadal hormones affect spatial learning and memory in mammals and circulating gonadal hormone concentrations fluctuate by season. Most nontropical rodents are spring/summer breeders and males display higher testosterone concentrations during the breeding season compared with the nonbreeding season (fall/winter). Seasonal patterns of testosterone concentration (as well as many other seasonal modifications of physiology, morphology, and behaviour) are induced by manipulation of photoperiod (day length; i.e. short or long days) in the laboratory. Coincident with reducing testosterone concentration, short days also impair spatial learning and memory performance in male white-footed mice (Peromyscus leucopus) compared with long days. We hypothesized that short-day-induced reduction of testosterone concentrations inhibits spatial learning and memory performance compared with long days. Adult male white-footed mice were maintained in long (16 h light/day) or short (8 h light/day) days for 14 weeks following sham-castration, castration plus saline implant, or castration plus testosterone implant treatment. Spatial learning and memory was assessed using a water maze, and photoperiod-evoked changes in gene expression of sex steroid receptors within the hippocampus were also examined. Castrated, short-day mice with testosterone replacement displayed enhanced water maze performance compared with other short-day mice, but no differences among testosterone treatments were observed in long-day mice. Photoperiod did not affect hippocampal androgen, oestrogen alpha, or oestrogen beta receptor gene expression. These results suggest that photoperiod modulates the effects of testosterone on spatial learning performance by mechanisms indirect of the hippocampus.

The role of substantia nigra pars reticulata in modulating clonic seizures is determined by testosterone levels during the immediate postnatal period

GABAergic activation of substantia nigra pars reticulata (SNR) at postnatal day (PN) 15 has sex-specific features on seizure control in vivo and electrophysiological responses in vitro. In males, the GABA(A)-receptor agonist muscimol has proconvulsant effects and induces depolarizing responses. In females, muscimol has no effect on seizures and evokes hyperpolarizing responses. We determined the time period during which sex hormones must be present to produce the sex-specific muscimol effects on seizures and their influence on SNR GABA(A) receptor-mediated postsynaptic currents. Exposure to testosterone or its metabolites (estrogen or dihydrotestosterone) during PN0-2 in females or males castrated at PN0 was sufficient to produce proconvulsant muscimol effects but did not affect the in vitro GABA responses, which remained hyperpolarizing. The data suggest that the PN0-2 period is critical for the development of the seizure-controlling SNR system; the hormonal effect on seizure control is independent from their effect on GABA conductance.

Play fighting in androgen-insensitivetfm rats: Evidence that androgen receptors are necessary for the development of adult playful attack and defense

The frequency of playful attack and the style of playful defense, are modifiable by gonadal steroids and change after puberty in male and female rats. The present study examined the play behavior exhibited by testicular feminized mutation (tfm)-affected males, who are insensitive to androgens but can bind estrogens aromatized from androgens, to determine the relative contributions of androgens and estrogens to the age-related changes in play behavior. tfm males did not exhibit a decrease in playful attack with age and were more likely to maintain the use of complete rotations, a juvenile form of playful defense, into adulthood. tfm males did however, show age related changes in the use of partial rotations and upright postures, two other forms of playful defense, that were similar to normal males. These data suggest that the development of play fighting and defense in males is dependent on both androgen- and estrogen-receptor-mediated effects.