Sex differences and steroid control of testosterone-metabolizing enzyme activity in the quail brain

In ovo corticosterone exposure does not influence yolk steroid hormone relative abundance or skeletal muscle development in the embryonic chicken

In ovo corticosterone (CORT) exposure reportedly reduces growth and alters body composition traits in meat-type chickens. However, the mechanisms governing alterations in growth and body composition remain unclear but could involve myogenic stem cell commitment, and/or the presence of yolk steroid hormones. This study investigated whether in ovo CORT exposure influenced yolk steroid hormone content, as well as embryonic myogenic development in meat-type chickens. Fertile eggs were randomly divided at embryonic day (ED) 11 and administered either a control (CON; 100 µL of 10 mM PBS) or CORT solution (100 µL of 10 mM PBS containing 1 µg CORT) into the chorioallantoic membrane. Yolk samples were collected at ED 0 and ED 5. At ED 15 and hatch, embryos were humanely killed, and yolk and breast muscle (BM) samples were collected. The relative abundance of 15 steroid hormones, along with total lipid content was measured in yolk samples collected at ED 0, ED 5, ED 15, and ED 21. Muscle fiber number, cross-sectional area, and fascicle area occupied by muscle fibers were measured in BM samples collected at hatch. Relative expression of MyoD, MyoG, Pax7, PPARγ, and CEBP/β, and the sex steroid receptors were measured in BM samples collected at hatch. The administration of CORT had a limited effect on yolk steroid hormones. In ovo CORT significantly reduced fascicle area occupied by muscle fibers and CEBP/β expression was increased in CORT exposed birds at hatch. In addition, the quantity of yolk lipid was significantly reduced in CORT-treated birds. In conclusion, in ovo exposure to CORT does not appear to influence early muscle development through yolk steroid hormones in embryonic meat-type chickens however, the results provide a comprehensive analysis of the composition of yolk steroid hormones in ovo at different developmental time points. The findings may suggest increased mesenchymal stem cell commitment to the adipogenic lineage during differentiation and requires further investigation.

Characterizing the timing of yolk testosterone metabolism and the effects of etiocholanolone on development in avian eggs

Maternal transfer of steroids to eggs can elicit permanent effects on offspring phenotype. Although testosterone was thought to be a key mediator of maternal effects in birds, we now know that vertebrate embryos actively regulate their exposure to maternal testosterone through steroid metabolism, suggesting testosterone metabolites, not testosterone, may elicit the observed phenotypic effects. To address the role steroid metabolism plays in mediating yolk testosterone effects, we used European starling (Sturnus vulgaris) eggs to characterize the timing of testosterone metabolism and determine whether etiocholanolone, a prominent metabolite of testosterone in avian embryos, is capable of affecting early embryonic development. Tritiated testosterone was injected into freshly laid eggs to characterize steroid movement and metabolism during early development. Varying levels of etiocholanolone were also injected into eggs, with incubation for either 3 or 5 days, to test whether etiocholanolone influences the early growth of embryonic tissues. The conversion of testosterone to etiocholanolone was initiated within 12 h of injection, but the increase in etiocholanolone was transient, indicating that etiocholanolone is also subject to metabolism, and that exposure to maternal etiocholanolone is limited to a short period during early development. Exogenous etiocholanolone manipulation had no significant effect on the growth rate of the embryos or extra-embryonic membranes early in development. Thus, the conversion of testosterone to etiocholanolone may be an inactivation pathway that buffers the embryo from maternal steroids, with any effects of yolk testosterone resulting from testosterone that escapes metabolism; alternatively, etiocholanolone may influence processes other than growth or take additional time to manifest.

Blood Testosterone Concentration and Testosterone-induced Aggressive Behavior in Male Layer Chicks: Comparison between Isolated- and Grouped-Raising

Testosterone (T) is known to induce aggressive behavior, mainly in male animals. Subcutaneous implantation of T-filled silastic tubes, rather than intramuscular injection of T, is generally recommended for long-term treatment using exogenous T. However, the effect of T implantation on chicken aggressive behavior has not been investigated. In addition, the concentration of T required to induce aggressive behavior or whether rearing conditions such as isolated- or grouped-raising affect T-induced aggressive behavior in chickens is not known. The present study aimed to examine the relationship between the lengths of T-filled tubes, blood T concentration, and aggressive behavior in group- and isolation-raised male layer chicks. The testes were bilaterally removed and silactic tubes of various lengths filled with crystalline T were subcutaneously implanted at 14 days of age. A social interaction test was performed to quantitatively assess chick aggressive behavior at 32 days of age. Comb weight and size were used to assess the activation of endogenous androgen receptors. Total aggression frequencies (TAF) and aggression establishment rate (AER) were used to evaluate aggressiveness. Significant positive correlations (P<0.001) were observed between the comb parameters and plasma T concentration. In the isolation-raised chicks, the TAF and AER were high irrespective of the lengths of the implanted T tubes or the corresponding plasma T concentrations. However, in the group-raised chicks, the AER tended to differ between the T-implanted aggressors (P=0.0902), and the AER significantly increased with implantation of 1.0-cm-long T-filled tubes (P<0.05), which corresponded to approximately 47 pg/mL plasma T concentration. These results suggest that both grouped raising and approximately 47 pg/mL plasma T concentration are required for the induction of T-dependent aggressive behavior, and that isolation-induced aggressive behavior is T-independent in male layer chicks.

Effects of a novel partner and sexual satiety on the expression of male sexual behavior and brain aromatase activity in quail

This study was designed to determine whether changes in sexual motivation acutely regulate brain estrogen synthesis by aromatase. Five experiments (Exp.1-5) were first conducted to determine the effect of recent mating and of the presentation of a new female (Coolidge effect) on sexual motivation. Exp.1-2 showed that 10 min or overnight access to copulation decreases measures of male sexual motivation when male subjects were visually exposed to the female they had copulated with and this effect is not counteracted by the view of a new female. Exp.3 showed that sexual motivation is revived by the view of a new female in previously unmated males only allowed to see another female for 10 min. After mating for 10 min (Exp.4) or overnight (Exp.5) with a female, males showed a decrease in copulatory behavior that was not reversed by access to a new female. Exp.6 and 7 confirmed that overnight copulation (Exp.6) and view of a novel female (Exp.7) respectively decreases and increases sexual behavior and motivation. Yet, these manipulations did not affect brain aromatase activity except in the tuberal hypothalamus. Together these data confirm that copulation or prolonged view of a female decrease sexual motivation but a reactivation of sexual motivation by a new female can only be obtained if males had only seen another female but not copulated with her, which is different in some degree from the Coolidge effect described in rodents. Moreover changes in brain aromatase do not simply reflect changes in motivation and more complex mechanisms must be considered.

Steroid profiles in quail brain and serum: Sex and regional differences and effects of castration with steroid replacement

Both systemic and local production contribute to the concentration of steroids measured in the brain. This idea was originally based on rodent studies and was later extended to other species, including humans and birds. In quail, a widely used model in behavioural neuroendocrinology, it was demonstrated that all enzymes needed to produce sex steroids from cholesterol are expressed and active in the brain, although the actual concentrations of steroids produced were never investigated. We carried out a steroid profiling in multiple brain regions and serum of sexually mature male and female quail by gas chromatography coupled with mass spectrometry. The concentrations of some steroids (eg, corticosterone, progesterone and testosterone) were in equilibrium between the brain and periphery, whereas other steroids (eg, pregnenolone (PREG), 5α/β-dihydroprogesterone and oestrogens) were more concentrated in the brain. In the brain regions investigated, PREG sulphate, progesterone and oestrogen concentrations were higher in the hypothalamus-preoptic area. Progesterone and its metabolites were more concentrated in the female than the male brain, whereas testosterone, its metabolites and dehydroepiandrosterone were more concentrated in males, suggesting that sex steroids present in quail brain mainly depend on their specific steroidogenic pathways in the ovaries and testes. However, the results of castration experiments suggested that sex steroids could also be produced in the brain independently of the peripheral source. Treatment with testosterone or oestradiol restored the concentrations of most androgens or oestrogens, respectively, although penetration of oestradiol in the brain appeared to be more limited. These studies illustrate the complex interaction between local brain synthesis and the supply from the periphery for the steroids present in the brain that are either directly active or represent the substrate of centrally located enzymes.

Cognitive Effects of Aromatase and Possible Role in Memory Disorders

Diverse cognitive functions in many vertebrate species are influenced by local conversion of androgens to 17β-estradiol (E2) by aromatase. This enzyme is highly expressed in various brain regions across species, with some inter-species variation in terms of regional brain expression. Since women with breast cancer and men and women with other disorders are often treated with aromatase inhibitors (AI), these populations might be especially vulnerable to cognitive deficits due to low neuroE2 synthesis, i.e., synthesis of E2 directly within the brain. Animal models have been useful in deciphering aromatase effects on cognitive functions. Consequences of AI administration at various life cycle stages have been assessed on auditory, song processing, and spatial memory in birds and various aspects of cognition in rodent models. Additionally, cognitive deficits have been described in aromatase knockout (ArKO) mice that systemically lack this gene throughout their lifespan. This review will consider evidence to date that AI treatment in male and female rodent models, birds, and humans results in cognitive impairments. How brain aromatase regulates cognitive function throughout the lifespan, and gaps in current knowledge will be considered, along with future directions to better define how aromatase might guide learning and memory from early development through the geriatric period. Better understanding the importance of E2 synthesis on neurobehavioral responses at various ages will likely aid in the discovery of therapeutic strategies to prevent potential cognitive deficits, including Alzheimer's Disease, in individuals treated with AI or those possessing CYP19 gene polymorphisms, as well as cognitive effects of normal aging that may be related to changes in brain aromatase activity.

Rapid changes in brain aromatase activity in the female quail brain following expression of sexual behaviour

In male quail, oestrogens produced in the brain (neuro-oestrogens) exert a dual action on male sexual behaviour: they increase sexual motivation within minutes via mechanisms activated at the membrane but facilitate sexual performance by slower, presumably nuclear-initiated, mechanisms. Recent work indicates that neuro-oestrogens are also implicated in the control of female sexual motivation despite the presence of high circulating concentrations of oestrogens of ovarian origin. Interestingly, aromatase activity (AA) in the male brain is regulated in time domains corresponding to the slow "genomic" and faster "nongenomic" modes of action of oestrogens. Furthermore, rapid changes in brain AA are observed in males after sexual interactions with a female. In the present study, we investigated whether similar rapid changes in brain AA are observed in females allowed to interact sexually with males. A significant decrease in AA was observed in the medial preoptic nucleus after interactions that lasted 2, 5 or 10 minutes, although this decrease was no longer significant after 15 minutes of interaction. In the bed nucleus of the stria terminalis, a progressive decline of average AA was observed between 2 and 15 minutes, although it never reached statistical significance. AA in this nucleus was, however, negatively correlated with the sexual receptivity of the female. These data indicate that sexual interactions affect brain AA in females as in males in an anatomically specific manner and suggest that rapid changes in brain oestrogens production could also modulate female sexual behaviour.

On the role of brain aromatase in females: why are estrogens produced locally when they are available systemically?

The ovaries are often thought of as the main and only source of estrogens involved in the regulation of female behavior. However, aromatase, the key enzyme for estrogen synthesis, although it is more abundant in males, is expressed and active in the brain of females where it is regulated by similar mechanisms as in males. Early work had shown that estrogens produced in the ventromedial hypothalamus are involved in the regulation of female sexual behavior in musk shrews. However, the question of the role of central aromatase in general had not received much attention until recently. Here, I will review the emerging concept that central aromatization plays a role in the regulation of physiological and behavioral endpoints in females. The data support the notion that in females, brain aromatase is not simply a non-functional evolutionary vestige, and provide support for the importance of locally produced estrogens for brain function in females. These observations should also have an impact for clinical research.

Steroid metabolism in the brain: From bird watching to molecular biology, a personal journey

Since Arnold Adolph Berthold established in 1849 the critical role of the testes in the activation of male sexual behavior, intensive research has identified many sophisticated neurochemical and molecular mechanisms mediating this action. Studies in Japanese quail demonstrated the critical role of testosterone action and of testosterone aromatization in the sexually dimorphic medial preoptic nucleus in the activation of male copulatory behavior. The development of an immunohistochemical visualization of brain aromatase in quail then allowed further refinement in the localization of the sites of neuroestrogens production. Testosterone aromatization is required for the activation of both appetitive and consummatory aspects of male sexual behavior. Brain aromatase activity is modulated by steroid-induced changes in the transcription of the corresponding gene but also more rapidly by phosphorylation processes. Sexual interactions with a female also rapidly regulate brain aromatase activity in an anatomically specific manner presumably via the release and action of endogenous glutamate. These rapid changes in estrogen production modulate sexual behavior and in particular its motivational component with latencies ranging between 15 and 30min. Brain estrogens seem to act in a manner akin to a neurotransmitter or at least a neuromodulator. More recently, assays of brain estradiol concentrations in micropunched samples or in dialysis samples obtained from behaviorally active males suggested that aromatase activity measured ex vivo might not be an accurate proxy to the rapid changes in local neuroestrogens production and concentrations. Studies of brain testosterone metabolism are thus not over and will keep scientists busy for a little longer. Elsevier SBN Keynote Address, Montreal.

Non-ovarian aromatization is required to activate female sexual motivation in testosterone-treated ovariectomized quail

Although aromatase is expressed in both male and female brains, its functional significance in females remains poorly understood. In female quail, sexual receptivity is activated by estrogens. However it is not known whether sexual motivation is similarly estrogen-dependent and whether estrogens locally produced in the brain contribute to these behavioral responses. Four main experiments were designed to address these questions. In Experiment 1 chronic treatment of females with the anti-estrogen tamoxifen decreased their receptivity, confirming that this response is under the control of estrogens. In Experiment 2 chronic treatment with tamoxifen significantly decreased sexual motivation as treated females no longer approached a sexual partner. In Experiment 3 (a) ovariectomy (OVX) induced a significant decrease of time spent near the male and a significantly decreased receptivity compared to gonadally intact females, (b) treatment with testosterone (OVX+T) partially restored these responses and (c) this effect of T was prevented when estradiol synthesis was inhibited by the potent aromatase inhibitor Vorozole (OVX+T+VOR). Serum estradiol concentration was significantly higher in OVX+T than in OVX or OVX+T+VOR females. Together these data demonstrate that treatment of OVX females with T increases sexual motivation and that these effects are mediated at least in part by non-gonadal aromatization of the androgen. Finally, assays of aromatase activity on brain and peripheral tissues (Experiment 4) strongly suggest that brain aromatization contributes to behavioral effects observed here following T treatment but alternative sources of estrogens (e.g. liver) should also be considered.

Age-dependent and age-independent effects of testosterone in male quail

Various studies in rodents recently concluded that puberty should be considered as a second period of organization of brain and behavior and that action of sex steroids at that time is long lasting and possibly permanent. We tested this notion in male Japanese quail that had been castrated before 3weeks post-hatch by analyzing whether a similar treatment with exogenous testosterone initiated at 3, 5 or 7weeks post-hatch has a differential influence on the development of testosterone-dependent morphological, behavioral and neural characteristics that are known to be sexually differentiated. The growth of the androgen-dependent cloacal gland was significantly faster when testosterone treatment was initiated later in life indicating that the target tissue is not ready to fully respond to androgens at 3weeks post-hatch. The three groups of birds nevertheless developed a gland of the same size typical of intact sexually mature birds. When adults, all birds expressed copulatory behavior with the same frequencies and latencies and they displayed the same level of aromatase activity and of vasotocinergic innervation in the preoptic area as gonadally intact males despite the fact that they had been treated with testosterone for different durations starting at different ages. Surprisingly, the frequency of cloacal sphincter contractions, a measure of appetitive sexual behavior, was significantly higher when testosterone treatment had been initiated later. Together these data provide no clear evidence for an organizational action of testosterone during sexual maturation of male quail but additional experiments should investigate whether estrogens have such an action in females.

Rapid control of reproductive behaviour by locally synthesised oestrogens: focus on aromatase

Oestrogens activate nucleus- and membrane-initiated signalling. Nucleus-initiated events control a wide array of physiological and behavioural responses. These effects generally take place within relatively long periods of time (several hours to days). By contrast, membrane-initiated signalling affects a multitude of cellular functions in a much shorter timeframe (seconds to minutes). However, much less is known about their functional significance. Furthermore, the origin of the oestrogens able to trigger these acute effects is rarely examined. Finally, these two distinct types of oestrogenic actions have often been studied independently such that we do not exactly know how they cooperate to control the same response. The present review presents a synthesis of recent work carried out in our laboratory that aimed to address these issues in the context of the study of male sexual behaviour in Japanese quail, which is a considered as a suitable species for tackling these issues. The first section presents data indicating that 17β-oestradiol, or its membrane impermeable analogues, acutely enhances measures of male sexual motivation but does not affect copulatory behaviour. These effects depend on the activation of membrane-initiated events and local oestrogen production. The second part of this review discusses the regulation of brain oestrogen synthesis through post-translational modifications of the enzyme aromatase. Initially discovered in vitro, these rapid and reversible enzymatic modulations occur in vivo following variations in the social and environment context and therefore provide a mechanism of acute regulation of local oestrogen provision with a spatial and time resolution compatible with the rapid effects observed on male sexual behaviour. Finally, we discuss how these distinct modes of oestrogenic action (membrane- versus nucleus-initiated) acting in different time frames (short- versus long-term) interact to control different components (motivation versus performance) of the same behavioural response and improve reproductive fitness.

Aromatase and 5α-reductase type 2 mRNA in the green anole forebrain: an investigation of the effects of sex, season and testosterone manipulation

Aromatase and 5α-reductase (5αR) catalyze the synthesis of testosterone (T) metabolites: estradiol and 5α-dihydrotestosterone, respectively. These enzymes are important in controlling sexual behaviors in male and female vertebrates. To investigate factors contributing to their regulation in reptiles, male and female green anole lizards were gonadectomized during the breeding and non-breeding seasons and treated with a T-filled or blank capsule. In situ hybridization was used to examine main effects of and interactions among sex, season, and T on expression of aromatase and one isozyme of 5αR (5αR2) in three brain regions that control reproductive behaviors: the preoptic area, ventromedial nucleus of the amygdala and ventromedial hypothalamus (VMH). Patterns of mRNA generally paralleled previous evaluations of intact animals. Although no main effects of T were detected, interactions were present in the VMH. Specifically, the density of 5αR2 expressing cells was greater in T-treated than control females in this region, regardless of season. Among breeding males, blank-treated males had a denser population of 5αR2 positive cells than T-treated males. Overall, T appears to have less of a role in the regulation of these enzymes than in other vertebrate groups, which is consistent with the primary role of T (rather than its metabolites) in regulation of reproductive behaviors in lizards. However, further investigation of protein and enzyme activity levels are needed before specific conclusions can be drawn.

Estradiol-dependent modulation of auditory processing and selectivity in songbirds

The steroid hormone estradiol plays an important role in reproductive development and behavior and modulates a wide array of physiological and cognitive processes. Recently, reports from several research groups have converged to show that estradiol also powerfully modulates sensory processing, specifically, the physiology of central auditory circuits in songbirds. These investigators have discovered that (1) behaviorally-relevant auditory experience rapidly increases estradiol levels in the auditory forebrain; (2) estradiol instantaneously enhances the responsiveness and coding efficiency of auditory neurons; (3) these changes are mediated by a non-genomic effect of brain-generated estradiol on the strength of inhibitory neurotransmission; and (4) estradiol regulates biochemical cascades that induce the expression of genes involved in synaptic plasticity. Together, these findings have established estradiol as a central regulator of auditory function and intensified the need to consider brain-based mechanisms, in addition to peripheral organ dysfunction, in hearing pathologies associated with estrogen deficiency.

Rapid effects of aggressive interactions on aromatase activity and oestradiol in discrete brain regions of wild male white-crowned sparrows

Testosterone is critical for the activation of aggressive behaviours. In many vertebrate species, circulating testosterone levels rapidly increase after aggressive encounters during the early or mid-breeding season. During the late breeding season, circulating testosterone concentrations did not change in wild male white-crowned sparrows after an aggressive encounter and, in these animals, changes in local neural metabolism of testosterone might be more important than changes in systemic testosterone levels. Local neural aromatisation of testosterone into 17β-oestradiol (E(2)) often mediates the actions of testosterone, and we hypothesised that, in the late breeding season, brain aromatase is rapidly modulated after aggressive interactions, leading to changes in local concentrations of E(2). In the present study, wild male white-crowned sparrows in the late breeding season were exposed to simulated territorial intrusion (STI) (song playback and live decoy) or control (CON) for 30 min. STI significantly increased aggressive behaviours. Using the Palkovits punch technique, 13 brain regions were collected. There was high aromatase activity in several nuclei, although enzymatic activity in the CON and STI groups did not differ in any region. E(2) concentrations were much higher in the brain than the plasma. STI did not affect circulating levels of E(2) but rapidly reduced E(2) concentrations in the hippocampus, ventromedial nucleus of the hypothalamus and bed nucleus of the stria terminalis. Unexpectedly, there were no correlations between aromatase activity and E(2) concentrations in the brain, nor were aromatase activity or brain E(2) correlated with aggressive behaviour or plasma hormone levels. This is one of the first studies to measure E(2) in microdissected brain regions, and the first study to do so in free-ranging animals. These data demonstrate that social interactions have rapid effects on local E(2) concentrations in specific brain regions.

Organizing effects of sex steroids on brain aromatase activity in quail

Preoptic/hypothalamic aromatase activity (AA) is sexually differentiated in birds and mammals but the mechanisms controlling this sex difference remain unclear. We determined here (1) brain sites where AA is sexually differentiated and (2) whether this sex difference results from organizing effects of estrogens during ontogeny or activating effects of testosterone in adulthood. In the first experiment we measured AA in brain regions micropunched in adult male and female Japanese quail utilizing the novel strategy of basing the microdissections on the distribution of aromatase-immunoreactive cells. The largest sex difference was found in the medial bed nucleus of the stria terminalis (mBST) followed by the medial preoptic nucleus (POM) and the tuberal hypothalamic region. A second experiment tested the effect of embryonic treatments known to sex-reverse male copulatory behavior (i.e., estradiol benzoate [EB] or the aromatase inhibitor, Vorozole) on brain AA in gonadectomized adult males and females chronically treated as adults with testosterone. Embryonic EB demasculinized male copulatory behavior, while vorozole blocked demasculinization of behavior in females as previously demonstrated in birds. Interestingly, these treatments did not affect a measure of appetitive sexual behavior. In parallel, embryonic vorozole increased, while EB decreased AA in pooled POM and mBST, but the same effect was observed in both sexes. Together, these data indicate that the early action of estrogens demasculinizes AA. However, this organizational action of estrogens on AA does not explain the behavioral sex difference in copulatory behavior since AA is similar in testosterone-treated males and females that were or were not exposed to embryonic treatments with estrogens.

Aromatase mRNA in the brain of adult green anole lizards: effects of sex and season

Neural testosterone metabolism, particularly the synthesis of oestradiol (E(2)) via the aromatase enzyme, is important for sexual behaviours in many vertebrates. In green anole lizards, E(2) metabolised from testosterone facilitates female receptivity and increases sexual motivation in males. Testosterone treatment increases aromatase activity in the whole brain homogenates of gonadectomised male, but not female, anoles, which is an effect limited to the breeding season (BS). To investigate the potential for local effects of this enzyme in reproductive behaviour, we used in situ hybridisation for aromatase mRNA to examine expression during the BS and nonbreeding season (NBS) in areas of the brain that control male sexual behaviours [preoptic area (POA) and amygdala (AMY)], as well as one regulating female reproductive behaviours ventromedial hypothalamus (VMH). Males had a greater total number of aromatase-expressing cells in the POA than females, and the density of aromatase-expressing cells (number per unit volume) was greater in the VMH and AMY of females. This density was also higher during the BS than NBS in the POA. Expression of aromatase in the AMY appeared to be lateralised because trends were detected for the left side to have more total cells and more cells per unit volume than the right. These results suggest that, similar to other vertebrates, regional aromatisation of testosterone may be important for the control of sex-specific reproductive behaviours.

Effects of sex steroids on aromatase mRNA expression in the male and female quail brain

Castrated male quail display intense male-typical copulatory behavior in response to exogenous testosterone but ovariectomized females do not. The behavior of males is largely mediated by the central aromatization of testosterone into estradiol. The lack of behavioral response in females could result from a lower rate of aromatization. This is probably not the case because although the enzymatic sex difference is clearly present in gonadally intact sexually mature birds, it is not reliably found in gonadectomized birds treated with testosterone, in which the behavioral sex difference is always observed. We previously discovered that the higher aromatase activity in sexually mature males as compared to females is not associated with major differences in aromatase mRNA density. A reverse sex difference (females>males) was even detected in the bed nucleus of the stria terminalis. We analyzed here by in situ hybridization histochemistry the density of aromatase mRNA in gonadectomized male and female quail that were or were not exposed to a steroid profile typical of their sex. Testosterone and ovarian steroids (presumably estradiol) increased aromatase mRNA concentration in males and females respectively but mRNA density was similar in both sexes. A reverse sex difference in aromatase mRNA density (females>males) was detected in the bed nucleus of subjects exposed to sex steroids. Together these data suggest that although the induction of aromatase activity by testosterone corresponds to an increased transcription of the enzyme, the sex difference in enzymatic activity results largely from post-transcriptional controls that remain to be identified.

Sex differences in brain aromatase activity: genomic and non-genomic controls

Aromatization of testosterone into estradiol in the preoptic area plays a critical role in the activation of male copulation in quail and in many other vertebrate species. Aromatase expression in quail and in other birds is higher than in rodents and other mammals, which has facilitated the study of the controls and functions of this enzyme. Over relatively long time periods (days to months), brain aromatase activity (AA), and transcription are markedly (four- to sixfold) increased by genomic actions of sex steroids. Initial work indicated that the preoptic AA is higher in males than in females and it was hypothesized that this differential production of estrogen could be a critical factor responsible for the lack of behavioral activation in females. Subsequent studies revealed, however, that this enzymatic sex difference might contribute but is not sufficient to explain the sex difference in behavior. Studies of AA, immunoreactivity, and mRNA concentrations revealed that sex differences observed when measuring enzymatic activity are not necessarily observed when one measures mRNA concentrations. Discrepancies potentially reflect post-translational controls of the enzymatic activity. AA in quail brain homogenates is rapidly inhibited by phosphorylation processes. Similar rapid inhibitions occur in hypothalamic explants maintained in vitro and exposed to agents affecting intracellular calcium concentrations or to glutamate agonists. Rapid changes in AA have also been observed in vivo following sexual interactions or exposure to short-term restraint stress and these rapid changes in estrogen production modulate expression of male sexual behaviors. These data suggest that brain estrogens display most if not all characteristics of neuromodulators if not neurotransmitters. Many questions remain however concerning the mechanisms controlling these rapid changes in estrogen production and their behavioral significance.

Testosterone recruits new aromatase-imunoreactive cells in neonatal quail brain

It was shown earlier that, in Japanese quail the mechanism controlling the induction by testosterone of aromatase activity develops between embryonic days 10 and 14. The cellular processes underlying this activation have, however, not been investigated in detail. Here, we demonstrate that the increase in aromatase activity observed in neonates treated with testosterone propionate between postnatal days 1 and 3 results from the recruitment of additional populations of aromatase-immunoreactive cells that were not expressing the enzyme at detectable levels before. This recruitment concerns all brain nuclei normally expressing the enzyme even if it is more prominent in the ventromedial hypothalamus than in other nuclei.

Testosterone selectively affects aromatase and 5alpha-reductase activities in the green anole lizard brain

Testosterone (T) and its metabolites are important in the regulation of reproductive behavior in males of a variety of vertebrate species. Aromatase converts T to estradiol and 5alpha-reductase converts T to 5alpha-dihydrotestosterone (DHT). Male green anole reproduction depends on androgens, yet 5alpha-reductase in the brain is not sexually dimorphic and does not vary with season. In contrast, aromatase activity in the male brain is increased during the breeding compared to non-breeding season, and males have higher levels than females during the breeding season. Aromatase is important for female, but not male, sexual behaviors. The present experiment was conducted to determine whether 5alpha-reductase and aromatase are regulated by T. Enzyme activity was quantified in whole brain homogenates in both the breeding and non-breeding seasons in males and females that had been treated with either a T or blank implant. In males only, T increased 5alpha-reductase activity regardless of season and up-regulated aromatase during the breeding season specifically. Thus, regulation of both enzymes occurs in males, whereas females do not show parallel sensitivity to T. When considered with previous results, the data suggest that aromatase might influence a male function associated with the breeding season other than sexual behavior. 5alpha-Reductase can be mediated by T availability, but this regulation may not serve a sex- or season-specific purpose.

Japanese quail as a model system for studying the neuroendocrine control of reproductive and social behaviors

Japanese quail (Coturnix japonica; referred to simply as quail in this article) readily exhibit sexual behavior and related social behaviors in captive conditions and have therefore proven valuable for studies of how early social experience can shape adult mate preference and sexual behavior. Quail have also been used in sexual conditioning studies illustrating that natural stimuli predict successful reproduction via Pavlovian processes. In addition, they have proven to be a good model to study how variation in photoperiod regulates reproduction and how variation in gonadal steroid hormones controls sexual behavior. For example, studies have shown that testosterone activates male-typical behaviors after being metabolized into estrogenic and androgenic metabolites. A critical site of action for these metabolites is the medial preoptic nucleus (POM), which is larger in males than in females. The enzyme aromatase converts testosterone to estradiol and is enriched in the POM in a male-biased fashion. Quail studies were the first to show that this enzyme is regulated both relatively slowly via genomic actions of steroids and more quickly via phosphorylation. With this base of knowledge and the recent cloning of the entire genome of the closely related chicken, quail will be valuable for future studies connecting gene expression to sexual and social behaviors.

Sex differences in the expression of sex steroid receptor mRNA in the quail brain

In Japanese quail, males will readily exhibit the full sequence of male-typical sexual behaviors but females never show this response, even after ovariectomy and treatment with male-typical concentrations of exogenous testosterone. Testosterone aromatisation plays a key-limiting role in the activation of this behavior but the higher aromatase activity in the brain of males compared to females is not sufficient to explain the behavioural sex difference. The cellular and molecular bases of this prominent sex difference in the functional consequences of testosterone have not been identified so far. We hypothesised that the differential expression of sex steroid receptors in specific brain areas could mediate this behavioural sex difference. Therefore, using radioactive in situ hybridisation histochemistry, we quantified the expression of the mRNA coding for the androgen receptor (AR) and the oestrogen receptors (ER) of the alpha and beta subtypes. All three receptors were expressed in an anatomically discrete manner in various nuclei of the hypothalamus and limbic system and, at usually lower densities, in a few other brain areas. In both sexes, the intensity of the hybridisation signal for all steroid receptors was highest in the medial preoptic nucleus (POM), a major site of testosterone action that is related to the activation of male sexual behaviour. Although no sex difference in the optical density of the AR hybridisation signal could be found in POM, the area covered by AR mRNA was significantly larger in males than in females, indicating a higher overall degree of AR expression in this region in males. By contrast, females tended to have significantly higher levels of AR expression than males in the lateral septum. ERalpha was more densely expressed in females than males throughout the medial preoptic and hypothalamic areas (including the POM and the medio-basal hypothalamus), an area implicated in the control of female receptivity) and in the mesencephalic nucleus intercollicularis. ERbeta was more densely expressed in the medio-basal hypothalamus of females but a difference in the reverse direction (males > females) was observed in the nucleus taeniae of the amygdala. These data suggest that a differential expression of steroid receptors in specific brain areas could mediate at least certain aspects of the sex differences in behavioural responses to testosterone, although they do not appear to be sufficient to explain the complete lack of activation by testosterone of male-typical copulatory behaviour in females.

Embryonic and posthatching treatments with sex steroids demasculinize the motivational aspects of crowing behavior in male Japanese quail

Demasculinizing action of embryonic estrogen on crowing behavior in male Japanese quails was examined. Eggs were treated with either 20 microg of estradiol benzoate (EB) or vehicle on the 10th day of incubation. Chicks hatched from both groups of eggs were injected daily with either testosterone propionate (TP; 10 microg/g b.w.), 5alpha-dihydrotestosterone (DHT, a non-aromatizable androgen; 10 microg/g b.w.), or vehicle from 11 to 50 days after hatching, and during this period their calling behaviors were observed. Irrespective of embryonic treatments, all birds received posthatching treatment with either TP or DHT, but not with vehicle, emitted crows in place of distress calls in a stress (non-sexual) context of being isolated in a recording chamber. The posthatching TP, but not posthatching DHT, induced crowing in a sexual context (crowing in their home-cages) from much earlier age than posthatching vehicle in the birds received control embryonic treatment with vehicle. The same TP treatment, however, completely eliminated the crowing in a sexual context in the birds received EB during their embryonic life. In the birds treated with either posthatching DHT or posthatching vehicle, the crowing in a sexual context was only slightly decreased by embryonic EB treatment. These data suggest that posthatching estrogen, derived from testosterone aromatization, enhances the demasculinizing action of embryonic estrogen, and thus strongly reduces the sexual motivation for crowing behavior. This demasculinizing action, however, would not influence vocal control system which generates acoustic pattern of crowing in the presence of androgens allowing the birds to crow in a non-sexual context.

Neuroanatomical specificity of sex differences in expression of aromatase mRNA in the quail brain

In birds and mammals, aromatase activity in the preoptic-hypothalamic region (HPOA) is usually higher in males than in females. It is, however, not known whether the enzymatic sex difference reflects the differential activation of aromatase transcription or some other control mechanism. Although sex differences in aromatase activity are clearly documented in the HPOA of Japanese quail (Coturnix japonica), only minimal or even no differences at all were observed in the number of aromatase-immunoreactive (ARO-ir) cells in the medial preoptic nucleus (POM) and in the medial part of the bed nucleus striae terminalis (BSTM). We investigated by in situ hybridization the distribution and possible sex differences in aromatase mRNA expression in the brain of sexually active adult quail. The distribution of aromatase mRNA matched very closely the results of previous immunocytochemical studies with the densest signal being observed in the POM, BSTM and in the mediobasal hypothalamus (MBH). Additional weaker signals were detected in the rostral forebrain, arcopallium and mesencephalic regions. No sex difference in the optical density of the hybridization signal could be found in the POM and MBH but the area covered by mRNA was larger in males than in females, indicating a higher overall expression in males. In contrast, in the BSTM, similar areas were covered by the aromatase expression in both sexes but the density of the signal was higher in females than in males. The physiological control of aromatase is thus neuroanatomically specific and with regard to sex differences, these controls are at least partially different if one compares the level of transcription, translation and activity of the enzyme. These results also indirectly suggest that the sex difference in aromatase enzyme activity that is present in the quail HPOA largely results from differentiated controls of enzymatic activity rather than differences in enzyme concentration.

Effects of androgens and estrogens on crowings and distress callings in male Japanese quail, Coturnix japonica

In male Japanese quail, crowing behavior is considered to be strictly androgen-dependent. It was previously shown that in chicks, treatment with either testosterone or 5alpha-dihydrotestosterone (5alpha-DHT; a non-aromatizable androgen) induced crowing with motivation for distress calling in acutely isolated conditions. Many studies, however, have shown that the potencies of testosterone and 5alpha-DHT in activating crowing in castrated males are different. To clarify the effects of androgenic and estrogenic actions on the production of crows and distress calls, we injected quail daily from 11 to 42 days after hatching (Day 11 to 42) with testosterone propionate (TP), 5alpha-DHT, estradiol benzoate (EB) or vehicle and examined their calling behaviors both in a recording chamber (acutely isolated conditions) and in their home-cages (well-acclimated conditions). Both TP- and 5alpha-DHT-treated birds began to crow by Day 13 when isolated in the recording chamber. The TP-treated birds, however, crowed less frequently than 5alpha-DHT-treated ones. This, combined with the observations that distress calling was strongly inhibited in EB-treated birds, suggests that estrogen converted from testosterone may inhibit the motivation for distress calling. On the other hand, after chronic treatment of TP, but not of 5alpha-DHT, birds began to crow intensely in their home-cages earlier than vehicle treated controls, suggesting that estrogen is needed to initiate crowing behavior in sexually active males. Taken together, it is suggested that estrogenic actions affect the motivation underlying vocal behaviors, while the androgenic action is indispensable in generating crowing.

Estrogen receptors and aromatase activity in the hypothalamus of the female frog, Rana esculenta. Fluctuations throughout the reproductive cycle

It is well known that certain actions of androgen are mediated through in situ aromatization to estrogen in neural target tissues. This study was undertaken to investigate androgen utilization in the hypothalamus of the female frog, Rana esculenta, through a quantification of estrogen receptors and aromatase activity during the reproductive cycle. 3H-estradiol-binding molecules were present in both the cytosol and the nuclear extract of the hypothalamus. These molecules bound specifically 3H-estradiol with high affinity (Kd 10(-10) M) and low capacity (cytosol: 1.2+/-0.4 fmol/mg protein; nuclear extract: 7.9+/-0.6 fmol/mg protein). Aromatase activity was detected in the microsomal fraction of the hypothalamus using a sensitive in vitro radiometric assay. Both aromatase activity and nuclear estrogen receptor binding fluctuated in synchrony throughout the reproductive cycle. Western blot analysis of aromatase protein revealed one immunoreactive band with a molecular weight of approximately 56 kDa. In contrast to aromatase enzyme activity, the relative levels of aromatase protein changed little during the reproductive cycle suggesting that post-translational mechanisms may be involved in regulating estrogen synthesis in the frog brain. A possible role for estrogens in the modulation of the reproductive behavior in this species is suggested.

Ontogeny of aromatase and tyrosine hydroxylase activity and of aromatase-immunoreactive cells in the preoptic area of male and female Japanese quail

The aromatization of testosterone into oestrogens plays a key role in the control of many behavioural and physiological aspects of reproduction. In the quail preoptic area (POA), aromatase activity and the number of aromatase-immunoreactive (ARO-ir) cells are sexually differentiated (males > females). This sex difference is implicated in the control of the sexually dimorphic behavioural response of quail to testosterone. We analysed the ontogenetic development of this sex difference by measuring aromatase activity and counting ARO-ir cells in the POA of males and females from day 1 post hatch to sexual maturity. We investigated in parallel another enzyme: tyrosine hydroxylase, the rate limiting step in catecholamine synthesis. Between hatching and 4 weeks of age, aromatase activity levels were low and equal in males and females. Aromatase activity then markedly increased in both sexes when subjects initiated their sexual maturation but this increase was more pronounced in males so that a marked difference in aromatase activity was present in 6 and 8 week-old subjects. Tyrosine hydroxylase activity progressively increased with age starting immediately after hatching and there was no abrupt modification in the slope of this increase when birds became sexually mature. No sex difference was detected in the activity of this enzyme. The number of ARO-ir cells in the POA progressively increased with age starting at hatching. No sex difference in ARO-ir cell numbers could be detected before subjects reached full sexual maturity. The analysis of the three-dimensional organization of ARO-ir cells in the POA revealed that, with increasing ages, ARO-ir cells acquire a progressively more lateral position: they are largely periventricular in young birds but they are found at higher density in the lateral part of the medial preoptic nucleus in adults. These data indicate that aromatase activity differentiates sexually when birds reach sexual maturity presumably under the activating effects of the increased testosterone levels in males. The number of ARO-ir cells, however, begins to increase in a non sexually differentiated manner before the rise in plasma testosterone in parallel with the increased tyrosine hydroxylase activity. Whether this temporal coincidence results from a general ontogenetic pattern or from more direct causal links remains to be established.

Distribution and effects of testosterone on aromatase mRNA in the quail forebrain: a non-radioactive in situ hybridization study

A number of studies have been devoted to the analysis of the anatomical distribution, control by steroids and functional significance of aromatase (the enzyme metabolizing testosterone into 17beta-estradiol) in the quail brain. In particular, the sexually dimorphic nucleus preopticus medialis has been the main focus of investigation because testosterone aromatization in this structure mediates the activation of male sexual behavior and aromatase activity is itself testosterone-dependent in this nucleus. No information on the anatomical distribution of aromatase gene expression is, however, available so far in this avian species. In the present study we applied a non-radioactive in situ hybridization technique to describe the distribution of aromatase mRNA containing neurons in the quail prosencephalon. We also analyzed, at a neuronal level of resolution, the induction by testosterone of this mRNA in the medial preoptic nucleus. Dense clusters of aromatase gene expressing neurons were observed within the medial preoptic nucleus, the nucleus of the stria terminalis, the ventro-medial hypothalamus and the tuberal region. Scattered neurons expressing lower levels of aromatase mRNA were also found in the dorsal thalamic area and central gray. The specificity of the staining was confirmed by demonstrating the absence of signal in sections that had been hybridized with a sense probe. Moreover, the distribution of the aromatase mRNA containing cells completely overlapped with the distribution of the aromatase-immunoreactive cells. Aromatase-mRNA expression was controlled by testosterone (or its metabolites) in the entire medial preoptic nucleus. Castration resulted in a decrease in the number of aromatase mRNA-containing cells and this effect was totally reversed by testosterone treatment. These data further support the idea that testosterone regulates the rate of its own aromatization by modulating the expression of aromatase rather than by acting at a post transcriptional level.

Distribution of aromatase-immunoreactive cells in the forebrain of zebra finches (Taeniopygia guttata): implications for the neural action of steroids and nuclear definition in the avian hypothalamus

Cells immunoreactive for the enzyme aromatase were localized in the forebrain of male zebra finches with the use of an immunocytochemistry procedure. Two polyclonal antibodies, one directed against human placental aromatase and the other directed against quail recombinant aromatase, revealed a heterogeneous distribution of the enzyme in the telencephalon, diencephalon, and mesencephalon. Staining was enhanced in some birds by the administration of the nonsteroidal aromatase inhibitor, R76713 racemic Vorozole) prior to the perfusion of the birds as previously described in Japanese quail. Large numbers of cells immunoreactive for aromatase were found in nuclei in the preoptic region and in the tuberal hypothalamus. A nucleus was identified in the preoptic region based on the high density of aromatase immunoreactive cells within its boundaries that appears to be homologous to the preoptic medial nucleus (POM) described previously in Japanese quail. In several birds alternate sections were stained for immunoreactive vasotocin, a marker of the paraventricular nucleus (PVN). This information facilitated the clear separation of the POM in zebra finches from nuclei that are adjacent to the POM in the preoptic area-hypothalamus, such as the PVN and the ventromedial nucleus of the hypothalamus. Positively staining cells were also detected widely throughout the telencephalon. Cells were discerned in the medial parts of the ventral hyperstriatum and neostriatum near the lateral ventricle and in dorsal and medial parts of the hippocampus. They were most abundant in the caudal neostriatum where they clustered in the dorsomedial neostriatum, and as a band of cells coursing along the dorsal edge of the lamina archistriatalis dorsalis. They were also present in high numbers in the ventrolateral aspect of the neostriatum and in the nucleus taeniae. None of the telencephalic vocal control nuclei had appreciable numbers of cells immunoreactive for aromatase within their boundaries, with the possible exception of a group of cells that may correspond to the medial part of the magnocellular nucleus of the neostriatum. The distribution of immunoreactive aromatase cells in the zebra finch brain is in excellent agreement with the distribution of cells expressing the mRNA for aromatase recently described in the finch telencephalon. This widespread telencephalic distribution of cells immunoreactive for aromatase has not been described in non-songbird species such as the Japanese quail, the ring dove, and the domestic fowl.

Localization of testosterone-sensitive and sexually dimorphic aromatase-immunoreactive cells in the quail preoptic area

The distribution of aromatase-immunoreactive cells was studied in the medial preoptic nucleus of male and female quail that were sexually mature and gonadally intact, or gonadectomized, or gonadectomized and treated with testosterone. The study first confirmed the existence of a significant difference in the number of aromatase-immunoreactive cells between males and females (males > females) and the marked effect of castration and testosterone treatment which, respectively, decrease and restore the number of these cells. An analysis of the distribution in space of this neurochemically defined cell population was also carried out. This study revealed that castration does not uniformly decrease the density of aromatase-immunoreactive cells, but local increases are observed in an area directly adjacent to the third ventricle. A number of new sex differences in the organization of the medial preoptic nucleus and its population of aromatase cells have, in addition, been identified. The density of aromatase-immunoreactive cells is not higher in males than in females throughout the nucleus, but a higher density of immunoreactive cells is present in the ventromedial part of the nucleus in females as compared to males. In addition, the cross-sectional area of the nucleus as defined by the population of aromatase-immunoreactive cells is larger in males than in females in its rostral part and its shape is more elongated in the dorso-ventral direction in females than in males. Some of these differences (e.g. higher density of ARC-ir cells in the ventromedial part of the female POM, shape of the nucleus) appear to be organizational in nature, because they are still present in birds exposed to the same endocrine conditions during adult life (e.g. gonadectomized and treated with a same dose of testosterone). This conclusion should now be tested by experiments manipulating the endocrine environment of quail embryos. The anatomical heterogeneity of the medial preoptic nucleus revealed by this study also suggests a functional heterogeneity and the specific roles of the medial and lateral parts of the nucleus should also be investigated.

Effects of testosterone and its metabolites on aromatase-immunoreactive cells in the quail brain: relationship with the activation of male reproductive behavior

The enzyme aromatase converts testosterone (T) into 17 beta-estradiol and plays a pivotal role in the control of reproduction. In particular, the aromatase activity (AA) located in the preoptic area (POA) of male Japanese quail is a limiting step in the activation by T of copulatory behavior. Aromatase-immunoreactive (ARO-ir) cells of the POA are specifically localized within the cytoarchitectonic boundaries of the medial preoptic nucleus(POM), a sexually dimorphic and steroid-sensitive structure that is a necessary and sufficient site of steroid action in the activation of behavior. Stereotaxic implantation of aromatase inhibitors in but not around the POM strongly decreases the behavioral effects of a systemic treatment with T of castrated males. AA is decreased by castration and increased by aromatizable androgens and by estrogens. These changes have been independently documented at three levels of analysis: the enzymatic activity measured by radioenzymatic assays in vitro, the enzyme concentration evaluated semi-quantitatively by immunocytochemistry and the concentration of its messenger RNA quantified by reverse transcription-polymerase chain reaction (RT-PCR). These studies demonstrate that T acting mostly through its estrogenic metabolites regulates brain aromatase by acting essentially at the transcriptional level. Estrogens produced by central aromatization of T therefore have two independent roles: they activate male copulatory behavior and they regulate the synthesis of aromatase. Double label immunocytochemical studies demonstrate that estrogen receptors(ER) are found in all brain areas containing ARO-ir cells but the extent to which these markers are colocalized varies from one brain region to the other. More than 70% of ARO-ir cells contain detectable ER in the tuberal hypothalamus but less than 20% of the cells display this colocalization in the POA. This absence of ER in ARO-ir cells is also observed in the POA of the rat brain. This suggests that locally formed estrogens cannot control the behavior and the aromatase synthesis in an autocrine fashion in the cells where they were formed. Multi-neuronal networks need therefore to be considered. The behavioral activation could result from the action of estrogens in ER-positive cells located in the vicinity of the ARO-ir cells where they were produced (paracrine action). Alternatively, actions that do not involve the nuclear ER could be important. Immunocytochemical studies at the electron microscope level and biochemical assays of AA in purified synaptosomes indicate the presence of aromatase in presynaptic boutons. Estrogens formed at this level could directly affect the pre-and post-synaptic membrane or could directly modulate neurotransmission namely through their metabolization into catecholestrogens (CE) which are known to be powerful inhibitors of the catechol- omicron - methyl transferase (COMT). The inhibition of COMT should increase the catecholaminergic transmission. It is significant to note, in this respect, that high levels of 2-hydroxylase activity, the enzyme that catalyzes the transformation of estrogens in CE, are found in all brain areas that contain aromatase. On the other hand, the synthesis of aromatase should also be controlled by estrogens in an indirect, transynaptic manner very reminiscent of the way in which steroids indirectly control the production of LHRH. Fibers that are immunoreactive for tyrosine hydroxylase (synthesis of dopamine), dopamine beta-hydroxylase (synthesis of norepinephrine) or vasotocine have been identified in the close vicinity of ARO-ir cells in the POM and retrograde tracing has identified the origin of the dopaminergic and noradrenergic innervation of these areas. A few preliminary physiological experiments suggest that these catecholaminergic inputs regulate AA and presumably synthesis.

Testosterone effects on the neuronal ultrastructure in the medial preoptic nucleus of male Japanese quail

Dorsolateral neurons of the medial preoptic nucleus (POM) of male Japanese quail are sensitive to the plasma levels of testosterone: their volume and optical density in Nissl-stained sections increase in castrated birds treated with testosterone. The present study was performed on castrated male quail treated or not with Silastic implants filled with testosterone to describe the ultrastructural variations induced by testosterone in these neurons. Gonadally intact male birds were included as controls. The ultrastructure of neurons, taken from the dorsolateral portion of the POM, was dramatically affected by the endocrine manipulations. Quantitative evaluations demonstrated a significant decrease in castrated birds of the rough endoplasmic reticulum (RER), of free polyribosomes, of Golgi complexes, and of dense bodies; these changes paralleled the decrease in cell size. The cell size and the percentage of volume occupied by the intracellular organelles in castrated birds treated with testosterone were comparable to values observed in controls. These ultrastructural changes are similar to those observed in neuronal targets for other gonadal hormones, supporting the idea that testosterone stimulates the development of cytoplasmic structures involved in protein synthesis and secretion. In addition, exposure to testosterone affects the synaptic inputs to POM. These ultrastructural changes are presumably related to the physiological effects (e.g., activation of male sexual behavior) exerted by testosterone on this preoptic region.

Effects of steroidal and non steroidal aromatase inhibitors on sexual behavior and aromatase-immunoreactive cells and fibers in the quail brain

Castrated quail were treated with Silastic implants filled with testosterone (T) in association with injections of the aromatase inhibitors, R76713 (racemic vorozole; 1 mg/kg twice a day) or 4-hydroxyandrostenedione (OHA; 5 mg/bird twice a day). Control birds received no treatment (CX group) or were implanted with T capsules only (CX + T group). Both R76713 and OHA strongly inhibited the T-activated male copulatory behavior. This inhibition had the same magnitude in both groups. The growth of the cloacal gland, a strictly androgen-dependent process was not affected by these compounds. The treatments significantly affected the number of aromatase-immunoreactive (ARO-ir) cells in each of the six brain areas that were studied: the anterior and posterior parts of the sexually dimorphic medial preoptic nucleus (POM), the septal region, the bed nucleus of the stria terminalis (BNST) and the anterior and posterior parts of the tuber. This number was significantly increased in all areas by T. In agreement with our previous study, R76713 significantly inhibited this effect of T in the tuberal hypothalamus but not in the anterior POM nor in the BNST. By contrast the effect of T on the number of ARO-ir cells was completely blocked by OHA in all brain nuclei. The two inhibitors had statistically different effects in all brain regions. Like in a previous study, R76713 increased the intensity of the staining of all ARO-ir cells. This effect took several days to develop suggesting a progressive build-up of the enzyme concentration. This was also suggested by the fact that a rebound in aromatase activity was observed 16 to 24 h after a single injection of R76713. The increased immunoreactivity was not observed in OHA-treated birds. The denser immunoreactivity in R76713-treated birds and the better tissue preservation due to the aldehyde fixative that had been used provided here a clearer picture of the cellular and subcellular localization of ARO-ir material. This allowed to identify new groups of immunoreactive cells, namely in the nucleus accumbens, in the area of the paleostriatum ventrale, in the nucleus taeniae, in the medial and caudal hypothalamus and in the medial part of the mesencephalon and of the pons. Most of the immunoreactive material was located in perikarya but some of these cells were also surrounded by dense networks of ARO-ir fibers often associated with immunopositive punctate structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphometric studies demonstrate that aromatase-immunoreactive cells are the main target of androgens and estrogens in the quail medial preoptic nucleus

The volume and cytoarchitectonic organization of the sexually dimorphic medial preoptic nucleus (POM) of the quail are sensitive to plasma levels of testosterone (T). We previously showed that, in castrated quail, T or its estrogenic metabolite, estradiol (E2), increases the size of the large neurons located in the lateral part of POM. Embryonic treatments with estrogens are also known to affect permanently the size of these large neurons. Since the lateral POM also contains a dense population of aromatase-immunoreactive (ARO-ir) cells, and these are known to be a target for steroids, we hypothesized that the effects of steroids identified in previous experiments were primarily directed to these ARO-ir cells. This idea was tested in two experiments in which the size of these cells was measured in male quail under various endocrine conditions. In experiment 1, a detailed analysis of ARO-ir and of non-immunoreactive cells in the POM of adult, sexually mature males revealed that the immunoreactive perikarya are larger than the non-immunoreactive cells and that they constitute the vast majority of the large cells (area > 50 microns 2) in the POM. In experiment 2, it was shown that T and E2 actually increase the size of ARO-ir cells in the POM while the androgenic metabolite of T, dihydrotestosterone has no effect at this level. Taken together, these data suggest that the sex differences and the steroid-induced changes in cell size previously described in the study of POM sections stained for Nissl material largely concern aromatase-containing cells. Since aromatization of T plays a limiting role in the activation of male copulatory behavior, these changes may represent the morphological signature of the mechanisms causally involved in the control of this behavior.

Aromatase-immunoreactive cells in the quail brain: effects of testosterone and sex dimorphism

We previously demonstrated that testosterone (T) increases aromatase activity (AA) and that AA is sexually dimorphic (males > females) in the quail preoptic area (POA). The precise anatomical localization of these effects is, however, impossible to obtain by biochemical assays even when samples are dissected by the Palkovits punch technique. We were recently able to set up an immunocytochemical (ICC) procedure that permits visualization of aromatase-immunoreactive (ARO-ir) cells in the quail brain. This showed that the ARO-ir cells of the quail POA actually outline the sexually dimorphic medial preoptic nucleus (POM). This ICC technique was used here to analyze the sex dimorphism of the quail preoptic aromatase and the localization of T effects on ARO-ir cells. In Experiment 1, the number of ARO-ir cells was counted in one section every 100 microns throughout the rostral to caudal extent of the POM of castrated birds that had been treated with increasing doses of T (5, 10, or 20 mm long Silastic implants). These T-treatments produced a dose-related increase in the sexual behavior of the birds and they increased the number of ARO-ir cells in POM, in the septal regions, and in the bed nucleus of the stria terminalis (BNST). The effect had a particularly large amplitude in the part of the POM located under the anterior commissure (AC). In Experiment 2, the same procedure was used to reanalyze the sex difference of the preoptic aromatase system. This showed that the POM of adult males contains more stained cells than the POM of females but only in a restricted region located just under and rostral to the AC. No significant sex difference was observed in the septum or in the BNST. In Experiment 3, the number of ARO-ir cells was determined in the POM of males and females that had been gonadectomized and treated with a same dose of T (40 mm implants). No sex difference in the number of ARO-ir cells could be detected in these conditions. This suggests that the sex difference in AA that had been previously observed in T-treated birds results either from a difference in aromatase concentration or activity in a similar number of positive cells or from a difference in the number of ARO-ir cells that is very discrete from the anatomical point of view.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution and regulation of estrogen-2-hydroxylase in the quail brain

The anatomical distribution and endocrine regulation of the estrogen-2-hydroxylase activity were investigated in the brain of adult male and female Japanese quail. Significant levels of enzymatic activity were detected in all brain regions that were studied, but the highest levels were observed in preoptic and hypothalamic brain nuclei that are known to contain high levels of aromatase activity. These data are consistent with previous results suggesting that the placental aromatase is also responsible for the estrogen-2-hydroxylase activity. However, there is a marked sex difference and a control by T of aromatase activity in the quail brain, and no such difference in 2-hydroxylase activity could generally be detected except in the VMN. Further studies will be needed to know whether the previously published conclusions concerning the human placenta also apply to the brain. The present data are consistent with the idea that estrogens formed locally in the brain by testosterone aromatization could affect reproduction by interfering with the catecholaminergic transmission after being metabolized into catechol-estrogens.

Aromatase-immunoreactive neurons in the adult female chicken brain detected using a specific antibody

Estrogen formation in the brain catalysed by the cytochrome P450arom is required for the control of estrogen-dependent neural mechanisms regulating reproductive behaviour. A polyclonal antibody was raised against a 15-amino acid fragment of the chicken ovarian P450arom protein, to localise aromatase-immunoreactive (AR-IR) cells in the adult female chicken brain. Specificity of antibody reaction was established by Western blot and by inhibition of aromatase activity in homogenates of chicken ovarian follicles determined by a radiometric assay. The AR-IR material in the brain was localised in the perikarya and some of their adjacent cytoplasmatic processes. Intense immunoreactivity was observed in the preoptic region as well as in other hypothalamic nuclei. AR-IR cells were also found in extrahypothalamic areas; in particular, in the area entorhinalis and hippocampus. These results confirm histologically that aromatization of testosterone in the adult female chicken brain occurs in preoptic nuclei closely associated with the regulation of reproductive behaviour. The mapping of AR-IR cells in the female chicken brain now allows study of its regulation under different physiological and environmental conditions, and its relation to classic target areas expressing estrogen receptors.

Synergism between androgens and estrogens in the induction of aromatase and its messenger RNA in the brain

It is established that testosterone (T) increases aromatase activity (AA) in the quail brain and that this induction of AA represents a limiting factor in the activation of male copulatory behavior. This action of T presumably results from an induction of aromatase synthesis since the number of aromatase-immunoreactive (ARO-ir) cells increases and, in parallel, there is an increase in aromatase mRNA as measured by reverse transcriptase-polymerase chain reaction (RT-PCR) technology. The specific role of androgenic and estrogenic metabolites of T in this induction is not yet clear but product-formation assays suggest that both types of compounds synergize to increase AA. The exact role of androgens and estrogens in the induction of aromatase was examined by studying both the aromatase protein by immunocytochemistry and the aromatase mRNA by RT-PCR in castrated quail that had been treated with T or its androgenic metabolite, 5 alpha-dihydrotestosterone (DHT) or its estrogenic metabolite, estradiol-17 beta (E2) or both DHT and E2 simultaneously. A specific quantitative PCR technique using a modified aromatase as internal standard was developed for this purpose. T increased the number of ARO-ir cells in all brain areas and increased the concentration of ARO mRNA in the preoptic area-anterior hypothalamus (POA-aHYP) and in the posterior hypothalamus (pHYP). E2-treated birds had more ARO-ir cells than castrates in the posterior part of the medial preoptic nucleus (POM), in the bed nucleus stria terminalis (BNST) and tuber. Their aromatase mRNA concentration was significantly increased in the POA-aHYP but this effect did not reach significance in the pHYP. DHT by itself had no effect on either the number of ARO-ir cells (all brain regions considered) or the concentration of aromatase mRNA. DHT, however, synergized with E2, both in inducing ARO-ir neurons and in increasing aromatase mRNA concentration. This synergism was shown to be statistically significant in several brain areas. These data demonstrate that both androgens and estrogens regulate aromatase at the pretranslational level. Because the percentage increase in the number of ARO-iR cells was in general very similar to the increase in aromatase mRNA concentration, these data also suggest that these steroids regulate aromatase mostly by changing its mRNA synthesis or catabolism.

Brain aromatase and the control of male sexual behavior

The activational effects of testosterone (T) on male copulatory behavior are mediated by its aromatization into estradiol. In quail, we have shown by stereotaxic implantation of steroids and metabolism inhibitors and by electrolytic lesions that the action of T and its aromatization take place in the sexually dimorphic medial preoptic nucleus (POM). The distribution and regulation of brain aromatase was studied in this species by product-formation assays measuring aromatase activity (AA) in microdissected brain regions and by immunocytochemistry (ICC). Aromatase-immunoreactive (ARO-ir) neurons were found in four brain regions: the POM, the septal region, the bed nucleus of the stria terminals (BNST) and the tuberal hypothalamus. ARO-ir cells actually outline the POM boundaries. ARO-ir material is found not only in the perikarya of neurons but also in the full extension of their cellular processes including the axons and the presynaptic boutons. This is confirmed at the light level by the demonstration of immunoreactive fibers and punctate structures in brain regions that are sometimes fairly distant from the closest ARO-ir cells. A lot of ARO-ir cells in the POM and BNST do not contain immunoreactive estrogen receptors (ER-ir) as demonstrated by double label ICC. These morphological data suggest an unorthodox role for the enzyme or the locally formed estrogens. In parallel with copulatory behavior, the preoptic AA decreases after castration and is restored by T to levels seen in sexually mature males. This probably reflects a change in enzyme concentration rather than a modulation of the activity in a constant number of molecules since the maximum enzymatic velocity (Vmax) only is affected while the affinity (Km) remains unchanged. In addition, T increases the number of ARO-ir neurons in POM and other brain areas suggesting that the concentration of the antigen is actually increased. This probably involves the direct activation of aromatase transcription as demonstrated by RT-PCR studies showing that aromatase mRNA is increased following T treatment of castrates. These activating effects of T seem to result from a synergistic action of androgenic and estrogenic metabolites of the steroid. The anatomical substrate for these regulations remains unclear at present especially in POM where ARO-ir cells do not in general contain ER-ir while androgen receptors appear to be rare based on both [3H] dihydrotestosterone autoradiography and ICC. Transynaptic mechanisms of control may be considered. A modulation of brain aromatase by catecholamines is also suggested by a few pharmacological studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical localization of androgen receptors in the male songbird and quail brain

The distribution of androgen receptors was studied in the brain of the Japanese quail (Coturnix japonica), the zebra finch (Taeniopygia guttata), and the canary (Serinus canaria) by immunocytochemistry with a polyclonal antibody (AR32) raised in rabbit against a synthetic peptide corresponding to a sequence located at the N-terminus of the androgen receptor molecule. In quail, androgen receptor-immunoreactive cells were observed in the nucleus intercollicularis and in various nuclei of the preoptic-hypothalamic complex, namely, the nucleus preopticus medialis, the ventral part of the nucleus anterior medialis hypothalami, the nucleus paraventricularis magnocellularis, the nucleus ventromedialis hypothalami, and the tuberal hypothalamus. In the two songbird species, labeled cells were also observed in various nuclei in the preoptic-hypothalamic region, in the nucleus taeniae, and in the nucleus intercollicularis. Additional androgen receptor-immunoreactive cells were present in the androgen-sensitive telencephalic nuclei that are part of the song control system. These immunoreactive cells filled and outlined the boundaries of the hyperstriatum ventrale, pars caudalis, nucleus magnocellularis neostriatalis anterioris (both in the lateral and medial subdivisions), and nucleus robustus archistriatalis. The immunoreactive material was primarily present in cell nuclei but a low level of immunoreactivity was also clearly detected in cytoplasm in some brain areas. These studies demonstrate, for the first time, that androgen receptors can be detected by immunocytochemistry in the avian brain and the results are in general agreement with the binding data obtained by autoradiography with tritiated dihydrotestosterone. Immunocytochemical methods offer several advantages over autoradiography and their use for the study of the androgen receptor will greatly facilitate the analysis of steroid-sensitive systems in the avian brain.

Neuroanatomical specificity in the autoregulation of aromatase-immunoreactive neurons by androgens and estrogens: an immunocytochemical study

Testosterone (T) increases brain aromatase activity (AA) in quail and other avian and mammalian species. It was shown both in quail and in rat that this enzymatic induction results from a synergistic action of androgens and estrogens. These studies provide little information on possible anatomical or cellular specificity of the effect. Using a polyclonal antiserum against human placental aromatase, we have previously identified aromatase-immunoreactive (ARO-ir) neurons in the quail brain and demonstrated that T increases the number of ARO-ir cells in the quail preoptic area (POA) supporting previous evidence that T increases AA in the brain. However, which T metabolites are involved, the actual mechanism of regulation and the possibility of anatomical specificity for these effects are not yet clear. In the present study, we disassociated the effects of androgens and estrogens in aromatase induction by comparing ARO-ir neurons of quail treated with T alone or T in the presence of a potent aromatase inhibitor (R76713), which has been shown to depress AA levels and to suppress T-activated copulatory behavior. T increased the number of ARO-ir cells in POA, bed nucleus striae terminalis (BNST) and tuberal hypothalamus (Tu). The T effect was inhibited by concurrent treatment with aromatase inhibitor in Tu, but not in POA and BNST. This differential effect of the aromatase inhibitor fits in very well with our previous studies of the co-localization of aromatase and estrogen receptors. The T effect was blocked by R76713 in areas where ARO-ir and estrogen receptor-ir are generally co-localized (Tu) and was not affected in areas with mainly ARO-ir positive, estrogen receptor-ir negative cells (POA, BNST). This suggests anatomical differences in the expression or clearance of aromatase which may be differentially sensitive to androgens and estrogens and dependent upon the presence of sex steroid receptors.

Neuroanatomical specificity in the co-localization of aromatase and estrogen receptors

The relative distributions of aromatase and of estrogen receptors were studied in the brain of the Japanese quail by a double-label immunocytochemical technique. Aromatase immunoreactive cells (ARO-ir) were found in the medial preoptic nucleus, in the septal region, and in a large cell cluster extending from the dorso-lateral aspect of the ventromedial nucleus of the hypothalamus to the tuber at the level of the nucleus inferioris hypothalami. Immunoreactive estrogen receptors (ER) were also found in each of these brain areas but their distribution was much broader and included larger parts of the preoptic, septal, and tuberal regions. In the ventromedial and tuberal hypothalamus, the majority of the ARO-ir cells (over 75%) also contained immunoreactive ER. By contrast, very few of the ARO-ir cells were double-labeled in the preoptic area and in the septum. More than 80% of the aromatase-containing cells contained no ER in these regions. This suggests that the estrogens, which are formed centrally by aromatization of testosterone, might not exert their biological effects through binding with the classical nuclear ER. The fact that significant amounts of aromatase activity are found in synaptosomes purified by differential centrifugation and that aromatase immunoreactivity is observed at the electron microscope level in synaptic boutons suggests that aromatase might produce estrogens that act at the synaptic level as neurohormones or neuromodulators.

Distribution of aromatase-immunoreactive cells in the mouse forebrain

The distribution of aromatase-immunoreactive cells was studied by immunocytochemistry in the mouse forebrain using a purified polyclonal antibody raised against human placental aromatase. Labeled perikarya were found in the dorso-lateral parts of the medial and tuberal hypothalamus. Positive cells filled an area extending between the subincertal nucleus in the dorsal part, the ventromedial hypothalamic nucleus in the ventral part, and the internal capsule and the magnocellular nucleus of the lateral hypothalamus in the lateral part. The same distribution was seen in the two strains of mice that were studied (Jackson and Swiss), and the number of immunoreactive perikarya did not seem to be affected by castration or testosterone treatment. No immunoreactivity could be detected in the medial regions of the preoptic area and hypothalamus; these were expected to contain the enzyme based on assays of aromatase activity performed in rats and on indirect autoradiographic evidence in mice. Our data raise questions concerning the distribution of aromatase in the brain and the mode of action of the centrally produced estrogens.

Testosterone metabolism in the avian hypothalamus

Many central actions of testosterone (T) require the transformation of T into several metabolites including 5 alpha-dihydrotestosterone (5 alpha-DHT) and estradiol (E2). In birds as in mammals, 5 alpha-DHT and E2, alone or in combination, mimic most behavioral effects of T. The avian brain is, in addition, able to transform T into 5 beta-DHT, a metabolite which seems to be devoid of any behavioral or physiological effects, at least in the context of reproduction. By in vitro product-formation assays, we have analyzed the distribution, sex differences and regulation by steroids of the 3 main T metabolizing enzymes (aromatase, 5 alpha- and 5 beta-reductases) in the brain of the Japanese quail (Coturnix c. japonica) and the zebra finch (Taeniopygia guttata castanotis). In the hypothalamus of quail and finches, aromatase activity is higher in males than in females. It is also decreased by castration and increased by T. The activity of the 5 alpha-reductase is not sexually differentiated nor controlled by T. The 5 beta-reductase activity is often higher in females than in males but this difference disappears in gonadectomized birds and no clear effect of T can be observed at this level. The zebra finch brain also contains a number of steroid-sensitive telencephalic nuclei [e.g. hyperstriatum ventrale, pars caudale (HVc) and robustus archistriatalis (RA)] which play a key role in the control of vocalizations. These nuclei also contain T-metabolizing enzymes but the regulation of their activity is substantially different from what has been observed in the hypothalamus. Aromatase activity is for example higher in females than in males in HVc and RA and the enzyme in these nuclei is not affected by castration nor T treatment. In these nuclei, the 5 alpha-reductase activity is higher in males than in females and the reverse is true for the 5 beta-reductase. These sex differences in activity are not sensitive to gonadectomy and T treatment and might therefore be organized by neonatal steroids. We have been recently able to localize aromatase-immunoreactive (AR-ir) neurons by ICC in the brain of the quail and zebra finch. Positive cells are found in the preoptic area, ventromedial and tuberal hypothalamus. AR-ir material is found in the perikarya of cells and fills the entire cellular processes including axons. At the electron microscope level, immunoreactive material can clearly be observed in the synaptic boutons. This observation raises questions concerning the mode of action of estrogens produced by central aromatization of T.

Androgen and estrogen action in the preoptic area and activation of copulatory behavior in quail

The sites of androgen and estrogen action on sexual behavior were studied in the preoptic area of castrated male Japanese quail by stereotaxic implantation of hormones, antihormones and metabolism inhibitors. The first experiment demonstrated that bilateral implantation of the aromatase inhibitor, androstatrienedione (ATD), in the sexually dimorphic nucleus (POM) of the preoptic area can completely suppress the behavioral activation produced by a systemic treatment with testosterone. The effects of ATD were only observed if the implants were located in the POM. In the second experiment, implants in the POM of the synthetic estrogen, diethylstilbestrol, restored copulatory behavior in castrated males while implants of the synthetic nonaromatizable androgen, methyltrienolone, were almost ineffective. During the third experiment, the activating effects of a systemic treatment with testosterone were blocked by stereotaxic implants in the POM of the antiestrogen, tamoxifen, or the antiandrogen, flutamide. The effects of tamoxifen were more pronounced than those of flutamide. In addition, tamoxifen was active in all parts of the POM while a behavioral inhibition was observed only for flutamide implants which were located in the caudal part of the nucleus. Taken together, these results demonstrate that the sexually dimorphic POM is the area where the behaviorally active estrogenic metabolites of T have to be produced. The estradiol derived from T aromatization presumably acts within the POM to activate copulation as demonstrated by the effectiveness of DES implanted in this region. Androgens also have a direct action on sexual behavior as suggested by the partial inhibition observed in flutamide-treated birds. It is, however, suggested that androgens and estrogens do not act in the same brain area to activate behavior.