The induction by testosterone of aromatase activity in the preoptic area and activation of copulatory behavior

Bidirectional relationships between testosterone and aggression: a critical analysis of four predictions

Experimentally elevated testosterone (T) often leads to enhanced aggression, with examples across many different species, including both males and females. Indeed, the relationship between T and aggression is among the most well-studied and fruitful areas of research at the intersection of behavioral ecology and endocrinology. This relationship is also hypothesized to be bidirectional (i.e., T influences aggression, and aggression influences T), leading to four key predictions: (1) Individuals with higher T levels are more aggressive than individuals with lower T. (2) Seasonal changes in aggression mirror seasonal changes in T secretion. (3) Aggressive territorial interactions stimulate increased T secretion. (4) Temporary elevations in T temporarily increase aggressiveness. These predictions cover a range of timescales, from a single snapshot in time, to rapid fluctuations, and to changes over seasonal timescales. Adding further complexity, most predictions can also be addressed by comparing among individuals or with repeated sampling within-individuals. In our review, we explore how the spectrum of results across predictions shapes our understanding of the relationship between T and aggression. In all cases, we can find examples of results that do not support the initial predictions. In particular, we find that predictions 1-3 have been tested frequently, especially using an among-individual approach. We find qualitative support for all three predictions, though there are also many studies that do not support predictions 1 and 3 in particular. Prediction 4, on the other hand, is something that we identify as a core underlying assumption of past work on the topic, but one that has rarely been directly tested. We propose that when relationships between T and aggression are individual-specific or condition-dependent, then positive correlations between the two variables may be obscured or reversed. In essence, even though T can influence aggression, many assumed or predicted relationships between the two variables may not manifest. Moving forward, we urge greater attention to understanding how and why it is that these bidirectional relationships between T and aggression may vary among timescales and among individuals. In doing so, we will move towards a deeper understanding on the role of hormones in behavioral adaptation.

Brain-derived estrogen and neural function

Although classically known as an endocrine signal produced by the ovary, 17β-estradiol (E₂) is also a neurosteroid produced in neurons and astrocytes in the brain of many different species. In this review, we provide a comprehensive overview of the localization, regulation, sex differences, and physiological/pathological roles of brain-derived E₂ (BDE₂). Much of what we know regarding the functional roles of BDE₂ has come from studies using specific inhibitors of the E₂ synthesis enzyme, aromatase, as well as the recent development of conditional forebrain neuron-specific and astrocyte-specific aromatase knockout mouse models. The evidence from these studies support a critical role for neuron-derived E₂ (NDE₂) in the regulation of synaptic plasticity, memory, socio-sexual behavior, sexual differentiation, reproduction, injury-induced reactive gliosis, and neuroprotection. Furthermore, we review evidence that astrocyte-derived E₂ (ADE₂) is induced following brain injury/ischemia, and plays a key role in reactive gliosis, neuroprotection, and cognitive preservation. Finally, we conclude by discussing the key controversies and challenges in this area, as well as potential future directions for the field.

Rapid changes in brain estrogen concentration during male sexual behavior are site and stimulus specific

Classically, estrogens regulate male sexual behavior through effects initiated in the nucleus. However, neuroestrogens, i.e., estrogens locally produced in the brain, can act within minutes via membrane-initiated events. In male quail, rapid changes in brain aromatase activity occur after exposure to sexual stimuli. We report here that local extracellular estrogen concentrations measured by in vivo microdialysis increase during sexual interactions in a brain site- and stimulus-specific manner. Indeed, estrogen concentrations rose within 10 min of the initiation of sexual interaction with a female in the medial preoptic nucleus only, while visual access to a female led to an increase in estrogen concentrations only in the bed nucleus of the stria terminalis. These are the fastest fluctuations in local estrogen concentrations ever observed in the vertebrate brain. Their site and stimulus specificity strongly confirm the neuromodulatory function of neuroestrogens on behavior.

Effect of cyclo‑oxygenase inhibition on embryonic microglia and the sexual differentiation of the brain and behavior of Japanese quail (Coturnix japonica)

Enduring sex differences in the brain are established during a developmental process known as brain sexual differentiation and are mainly driven by estrogens during a critical period. In rodents, the masculinization of the preoptic area by estrogens derived from the central aromatization of testosterone depends in part on the interaction between microglia and prostaglandin E₂ (PGE₂), a pro-inflammatory hormone of the prostanoid subclass. In contrast, in birds, estrogens produced by females induce a demasculinization, but whether an interaction with the neuro-immune system is involved in this process is unknown. This study addressed this question by testing the effects of blockade of cyclo‑oxygenases (COX), the rate-limiting enzymes for prostanoid synthesis, on embryonic microglia and the sexual differentiation of brain and behavior using the Japanese quail as an animal model. The results show that COX inhibition does not affect the behavior of females, but impairs male sexual behavior and suppresses the sex difference in microglial profiles at embryonic day 12 (E12) in the medial preoptic nucleus by increasing the number of microglia in males only. However, neither prostanoid concentrations nor PGE₂ receptors differed between sexes in the hypothalamus and preoptic area (HPOA) during development. Overall, these results uncovered a potential role of prostanoids in the demasculinization of Japanese quail. Moreover, the parallel effect of COX inhibition on behavior and microglia suggests an interaction between prostanoids and microglia in brain demasculinization, thus fueling the hypothesis of a conserved role of the neuroimmune system in the organization of the brain by estrogens.

Effect of chronic intracerebroventricular administration of an aromatase inhibitor on the expression of socio-sexual behaviors in male Japanese quail

Aromatase converts androgens into estrogens in the brain of vertebrates including humans. This enzyme is also expressed in other tissues where its action may result in negative effects on human health (e.g., promotion of tumor growth). To prevent these effects, aromatase inhibitors were developed and are currently used to block human estrogen-dependent tumors. In vertebrates including quail, aromatase is expressed in a highly conserved set of interconnected brain nuclei known as the social behavior network. This network is directly implicated in the expression of a large range of social behaviors. The primary goal of this study was to characterize in Japanese quail the potential impact of brain aromatase on sexual behavior, aggressiveness and social motivation (i.e., tendency to approach and stay close to conspecifics). An additional goal was to test the feasibility and effectiveness of long-term delivery of an aromatase inhibitor directly into the third ventricle via Alzet™ osmotic minipumps using male sexual behavior as the aromatase dependent measure. We demonstrate that this mode of administration results in the strongest inhibition of both copulatory behavior and sexual motivation ever observed in this species, while other social behaviors were variably affected. Sexual motivation and the tendency to approach a group of conspecifics including females clearly seem to depend on brain aromatase, but the effects of central estrogen production on aggressive behavior and on the motivation to approach males remain less clear.

Key role of estrogen receptor β in the organization of brain and behavior of the Japanese quail

Estrogens play a key role in the sexual differentiation of the brain and behavior. While early estrogen actions exert masculinizing effects on the brain of male rodents, a diametrically opposite effect is observed in birds where estrogens demasculinize the brain of females. Yet, the two vertebrate classes express similar sex differences in the brain and behavior. Although ERα is thought to play a major role in these processes in rodents, the role of ERβ is still controversial. In birds, the identity of the estrogen receptor(s) underlying the demasculinization of the female brain remains unclear. The aim of the present study was thus to determine in Japanese quail the effects of specific agonists of ERα (propylpyrazole triol, PPT) and ERβ (diarylpropionitrile, DPN) administered at the beginning of the sensitive period (embryonic day 7, E7) on the sexual differentiation of male sexual behavior and on the density of vasotocin-immunoreactive (VT-ir) fibers, a known marker of the organizational action of estrogens on the quail brain. We demonstrate that estradiol benzoate and the ERβ agonist (DPN) demasculinize male sexual behavior and decrease the density of VT-ir fibers in the medial preoptic nucleus and the bed nucleus of the stria terminalis, while PPT has no effect on these measures. These results clearly indicate that ERβ, but not ERα, is involved in the estrogen-induced sexual differentiation of brain and sexual behavior in quail.

Reduced cooperative behavior as a cost of high testosterone in a lekking passerine bird

Many studies have identified the reproductive benefits of cooperative behaviors, yet few have identified the mechanisms that underlie these behaviors. Mechanistic studies can inform our understanding of why some individuals are more or less cooperative, as well as identify the physiological constraints imposed upon the evolution of reproductive traits. Male wire-tailed manakins (Pipra filicauda) exhibit cooperative courtship behaviors and more cooperative territory holders have been shown to exhibit higher reproductive success. To begin to understand the proximate basis of cooperative display behaviors, we conducted both an observational study and an experimental study. Because coordinated courtship displays underlie this form of cooperation, our study also examined both the hormonal and social drivers of individual variation in courtship behavior more broadly (e.g., courtship display rates). Our observational study revealed that males with higher testosterone levels performed fewer cooperative display bouts. In addition, our experimental study demonstrated that the proportion of a male’s courtship displays that were cooperative decreased after being administered a testosterone-filled hormone implant. We found no relationship between an individual’s courtship display effort (i.e., display rate and time spent performing courtship displays) and circulating testosterone in either study. However, more cooperative males spent a greater proportion of time performing courtship displays than did less cooperative males, suggesting that testosterone may indirectly mediate courtship display behaviors by influencing a territory holder’s cooperative behavior. Overall, both our observational and experimental results suggest that reduced cooperative behavior is a cost of maintaining high levels of testosterone for territory-holding males.

Why Do Birds Flock? A Role for Opioids in the Reinforcement of Gregarious Social Interactions

The formation of social groups provides safety and opportunities for individuals to develop and practice important social skills. However, joining a social group does not result in any form of obvious, immediate reinforcement (e.g., it does not result in immediate copulation or a food reward), and individuals often remain in social groups despite agonistic responses from conspecifics. Much is known about neural and endocrine mechanisms underlying the motivation to perform mate- or offspring-directed behaviors. In contrast, relatively little is known about mechanisms underlying affiliative behaviors outside of these primary reproductive contexts. Studies on flocking behavior in songbirds are beginning to fill this knowledge gap. Here we review behavioral evidence that supports the hypothesis that non-sexual affiliative, flocking behaviors are both (1) rewarded by positive social interactions with conspecifics, and (2) reinforced because affiliative contact reduces a negative affective state caused by social isolation. We provide evidence from studies in European starlings, Sturnus vulgaris, that mu opioid receptors in the medial preoptic nucleus (mPOA) play a central role in both reward and the reduction of a negative affective state induced by social interactions in flocks, and discuss potential roles for nonapeptide/opioid interactions and steroid hormones. Finally, we develop the case that non-sexual affiliative social behaviors may be modified by two complementary output pathways from mPOA, with a projection from mPOA to the periaqueductal gray integrating information during social interactions that reduces negative affect and a projection from mPOA to the ventral tegmental area integrating information leading to social approach and reward.

Triphenyltin exposure affects mating behaviors and attractiveness to females during mating in male guppies (Poecilia reticulata)

The impacts of triphenyltin (TPT) on ecological health have been of great concern due to their widespread use and ubiquity in aquatic ecosystems. However, little is known about the effects of TPT on the reproductive behaviors of fishes. Therefore, the present study was conducted to investigate the effects of TPT at environmentally relevant concentrations (0, 1 and 10 ng Sn/L) on the mating behaviors and the attractiveness to females during mating in male guppies (Poecilia reticulata). The results showed that TPT exposure disturbed the mating behaviors; the TPT-exposed male fish performed more sneaking attempts, but no changes in sigmoid courtship were displayed. The increases in sneaking attempts might be related to increases in testosterone levels induced by TPT exposure. In the context of a competing male, the TPT-exposed males were less attractive to females during mating. The decreases in attractiveness might be related to decreases in carotenoid-based coloration, shown as decreases in caudal fin redness values and skin carotenoid contents. In addition, TPT-induced total antioxidant capacities, the activities of superoxide dismutase and catalase, and the contents of malondialdehyde in liver and intestinal tissues indicated increases in oxidative stress. Both oxidative stress and coloration are linked to carotenoids. Thus, we speculated that the TPT-exposed males might use carotenoids to cope with increases in oxidative stress at the expense of carotenoid-based coloration. The disruption of mating behaviors and the decrease in attractiveness to females in male fish could result in reproductive failure. The present study underscores the importance of using behavioral tests as a sensitive tool in assessing the impact of pollutants present in aquatic environments.

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.

Site-specific effects of aromatase inhibition on the activation of male sexual behavior in male Japanese quail (Coturnix japonica)

Aromatization within the medial preoptic nucleus (POM) is essential for the expression of male copulatory behavior in Japanese quail. However, several nuclei within the social behavior network (SBN) also express aromatase. Whether aromatase in these loci participates in the behavioral activation is not known. Castrated male Japanese quail were implanted with 2 subcutaneous Silastic capsules filled with crystalline testosterone and with bilateral stereotaxic implants filled with the aromatase inhibitor Vorozole targeting the POM, the bed nucleus of the stria terminalis (BST) or the ventromedial nucleus of the hypothalamus (VMN). Control animals were implanted with testosterone and empty bilateral stereotaxic implants. Starting 2 days after the surgery, subjects were tested for the expression of consummatory sexual behavior (CSB) every other day for a total of 10 tests. They were also tested once for appetitive sexual behavior (ASB) as measured by the rhythmic cloacal sphincter movements displayed in response to the visual presentation of a female. CSB was drastically reduced when the Vorozole implants were localized in the POM, but not in the BST nor in the VMN. Birds with implants in the BST took longer to show CSB in the first 6 tests than controls, suggesting a role of the BST in the acquisition of the full copulatory ability. ASB was not significantly affected by aromatase blockade in any region. These data confirm the key role played by the POM in the control of male sexual behavior and suggest a minor role for aromatization in the BST or VMN.

Differential control of appetitive and consummatory sexual behavior by neuroestrogens in male quail

Contribution to Special Issue on Fast effects of steroids. Estrogens exert pleiotropic effects on multiple physiological and behavioral traits including sexual behavior. These effects are classically mediated via binding to nuclear receptors and subsequent regulation of target gene transcription. Estrogens also affect neuronal activity and cell-signaling pathways via faster, membrane-initiated events. Although the distinction between appetitive and consummatory aspects of sexual behavior has been criticized, this distinction remains valuable in that it facilitates the causal analysis of certain behavioral systems. Effects of neuroestrogens produced by neuronal aromatization of testosterone on copulatory performance (consummatory aspect) and on sexual motivation (appetitive aspect) are described in male quail. The central administration of estradiol rapidly increases expression of sexual motivation, as assessed by two measures of sexual motivation produced in response to the visual presentation of a female but not sexual performance in male Japanese quail. This effect is mimicked by membrane-impermeable analogs of estradiol, indicating that it is initiated at the cell membrane. Conversely, blocking the action of estrogens or their synthesis by a single intracerebroventricular injection of estrogen receptor antagonists or aromatase inhibitors, respectively, decreases sexual motivation within minutes without affecting performance. The same steroid has thus evolved complementary mechanisms to regulate different behavioral components (motivation vs. performance) in distinct temporal domains (long- vs. short-term) so that diverse reproductive activities can be properly coordinated. Changes in preoptic aromatase activity and estradiol as well as glutamate concentrations are observed during or immediately after copulation. The interaction between these neuroendocrine/neurochemical changes and their functional significance is discussed.

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.

Neuroendocrine Underpinnings of Increased Risk for Posttraumatic Stress Disorder in Women

Women are particularly vulnerable to the effects of psychological trauma and the development of trauma-, stressor-, and anxiety-related mental illnesses such as posttraumatic stress disorder (PTSD). In the current chapter, we examine the female hormonal systems that interact with psychobiological stress response systems to elicit maladaptive behavior and mental disease states in traumatized female populations. In addition, we provide a contemporary translational example of a stress vulnerability genomic profile (coding for pituitary adenylate cyclase-activating polypeptide) that may underlie the specific susceptibilities observed in women. Translational scientific investigations such as those described herein may lead to the identification of risk and resilience factors for PTSD as well as enhanced clinical interventions for treating excessive fear and anxiety.

Sex, estradiol, and spatial memory in a food-caching corvid

Estrogens significantly impact spatial memory function in mammalian species. Songbirds express the estrogen synthetic enzyme aromatase at relatively high levels in the hippocampus and there is evidence from zebra finches that estrogens facilitate performance on spatial learning and/or memory tasks. It is unknown, however, whether estrogens influence hippocampal function in songbirds that naturally exhibit memory-intensive behaviors, such as cache recovery observed in many corvid species. To address this question, we examined the impact of estradiol on spatial memory in non-breeding Western scrub-jays, a species that routinely participates in food caching and retrieval in nature and in captivity. We also asked if there were sex differences in performance or responses to estradiol. Utilizing a combination of an aromatase inhibitor, fadrozole, with estradiol implants, we found that while overall cache recovery rates were unaffected by estradiol, several other indices of spatial memory, including searching efficiency and efficiency to retrieve the first item, were impaired in the presence of estradiol. In addition, males and females differed in some performance measures, although these differences appeared to be a consequence of the nature of the task as neither sex consistently out-performed the other. Overall, our data suggest that a sustained estradiol elevation in a food-caching bird impairs some, but not all, aspects of spatial memory on an innate behavioral task, at times in a sex-specific manner.

Endocrine mechanisms, behavioral phenotypes and plasticity: known relationships and open questions

Behavior of wild vertebrate individuals can vary in response to environmental or social factors. Such within-individual behavioral variation is often mediated by hormonal mechanisms. Hormones also serve as a basis for among-individual variations in behavior including animal personalities and the degree of responsiveness to environmental and social stimuli. How do relationships between hormones and behavioral traits evolve to produce such behavioral diversity within and among individuals? Answering questions about evolutionary processes generating among-individual variation requires characterizing how specific hormones are related to variation in specific behavioral traits, whether observed hormonal variation is related to individual fitness and, whether hormonal traits are consistent (repeatable) aspects of an individual's phenotype. With respect to within-individual variation, we need to improve our insight into the nature of the quantitative relationships between hormones and the traits they regulate, which in turn will determine how they may mediate behavioral plasticity of individuals. To address these questions, we review the actions of two steroid hormones, corticosterone and testosterone, in mediating changes in vertebrate behavior, focusing primarily on birds. In the first part, we concentrate on among-individual variation and present examples for how variation in corticosterone concentrations can relate to behaviors such as exploration of novel environments and parental care. We then review studies on correlations between corticosterone variation and fitness, and on the repeatability over time of corticosterone concentrations. At the end of this section, we suggest that further progress in our understanding of evolutionary patterns in the hormonal regulation of behavior may require, as one major tool, reaction norm approaches to characterize hormonal phenotypes as well as their responses to environments.

In the second part, we discuss types of quantitative relationships between hormones and behavioral traits within individuals, using testosterone as an example. We review conceptual models for testosterone-behavior relationships and discuss the relevance of these models for within-individual plasticity in behavior. Next, we discuss approaches for testing the nature of quantitative relationships between testosterone and behavior, concluding that again reaction norm approaches might be a fruitful way forward.

We propose that an integration of new tools, especially of reaction norm approaches into the field of behavioral endocrinology will allow us to make significant progress in our understanding of the mechanisms, the functional implications and the evolution of hormone–behavior relationships that mediate variation both within and among individuals. This knowledge will be crucial in light of already ongoing habitat alterations due to global change, as it will allow us to evaluate the mechanisms as well as the capacity of wild populations to adjust hormonally-mediated behaviors to altered environmental conditions.

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.

Prenatal exposure to low doses of atrazine affects mating behaviors in male guppies

Performing appropriate mating behaviors is crucial to male reproductive success, especially in species where mating is predominantly via female mate choice. Mating behaviors are hormonally regulated and may be sexually selected traits: courtship displays are selected via mate choice, while forced copulations and aggressive behaviors are selected for via intrasexual competition. Endocrine disrupting compounds interfere with proper hormonal functioning in exposed animals. Exposures during developmentally crucial life stages can have irreversible effects lasting through adulthood. I tested the effects of prenatal exposure to environmentally relevant doses of a commonly used herbicide, atrazine (1 and 13.5μg/L) on mating behaviors in male guppies. Guppies were used as a model organism to test the effects of atrazine exposure on wildlife reproductive health. Adult female guppies were mated and exposed to the treatments throughout the gestation period, and offspring born to them were raised without further treatment. At adulthood, the males were tested for the effects of prenatal exposure on their mating behaviors such as courtship displays, gonopodium swings, forced copulatory attempts, and competitive and aggressive behaviors towards rivals who were not exposed to atrazine. I also tested female preference for treated males compared to control males. Atrazine-exposed males were less likely to perform the mating behaviors, and performed them less frequently, than control males. Atrazine exposure also made males less aggressive towards rivals. Females preferred untreated males over atrazine-treated males. In all cases, a non-monotonic pattern was seen, highlighting the significance of low-dose exposures.

Dynamic changes in brain aromatase activity following sexual interactions in males: where, when and why?

It is increasingly recognized that estrogens produce rapid and transient effects at many neural sites ultimately impacting physiological and behavioral endpoints. The ability of estrogens to acutely regulate cellular processes implies that their concentration should also be rapidly fine-tuned. Accordingly, rapid changes in the catalytic activity of aromatase, the limiting enzyme for estrogen synthesis, have been identified that could serve as a regulatory mechanism of local estrogen concentrations. However, the precise anatomical localization, time-course, triggering stimuli and functional significance of these enzymatic changes in vivo are not well understood. To address these issues as to where, when and why aromatase activity (AA) rapidly changes after sexual interactions, AA was assayed in six populations of aromatase-expressing cells microdissected from the brain of male quail that experienced varying durations of visual exposure to or copulation with a female. Sexual interactions resulted in a rapid AA inhibition. This inhibition occurred in specific brain regions (including the medial preoptic nucleus), in a context-dependent fashion and time-scale suggestive of post-translational modifications of the enzyme. Interestingly, the enzymatic fluctuations occurring in the preoptic area followed rather than preceded copulation and were tied specifically to the female's presence. This pattern of enzymatic changes suggests that rapid estrogen effects are important during the motivational phase of the behavior to trigger physiological events essential to activate mate search and copulation.

Neuroestrogens rapidly regulate sexual motivation but not performance

Estrogens exert pleiotropic effects on reproductive traits, which include differentiation and activation of reproductive behaviors and the control of the secretion of gonadotropins. Estrogens also profoundly affect non-reproductive traits, such as cognition and neuroprotection. These effects are usually attributed to nuclear receptor binding and subsequent regulation of target gene transcription. Estrogens also affect neuronal activity and cell-signaling pathways via faster, membrane-initiated events. How these two types of actions that operate in distinct timescales interact in the control of complex behavioral responses is poorly understood. Here, we show that the central administration of estradiol rapidly increases the expression of sexual motivation, as assessed by several measures of sexual motivation produced in response to the visual presentation of a female but not sexual performance in male Japanese quail. This effect is mimicked by membrane-impermeable analogs of estradiol, indicating that it is initiated at the cell membrane. Conversely, blocking the action of estrogens or their synthesis by a single intracerebroventricular injection of estrogen receptor antagonists or aromatase inhibitors, respectively, decreases sexual motivation within minutes without affecting performance. The same steroid has thus evolved complementary mechanisms to regulate different behavioral components (motivation vs performance) in distinct temporal domains (long- vs short-term) so that diverse reproductive activities can be properly coordinated to improve reproductive fitness. Given the pleiotropic effects exerted by estrogens, other responses controlled by these steroids might also depend on a slow genomic regulation of neuronal plasticity underlying behavioral activation and an acute control of motivation to engage in behavior.

Aromatase expression in the brain of the ruffed grouse (Bonasa umbellus) and comparisons with other galliform birds (Aves, Galliformes)

The enzyme aromatase is important for regulating sexual and aggressive behaviors during the reproductive season, including many aspects of courtship. In birds, aromatase is expressed at high levels in a number of different brain regions. Although this expression does vary among species, the extent to which the distribution of aromatase positive cells reflects species differences in courtship and other behaviors is not well established. Here, we examine the distribution of aromatase immunoreactive (ARO) neurons in the brain of a species with a unique courtship display, the ruffed grouse (Bonasa umbellus). Unlike most other galliforms, male ruffed grouse do not vocalize as part of their courtship and instead use their wings to create a non-vocal auditory signal to attract females. Because aromatase is involved in courtship behaviors in several bird species, including other galliforms, we hypothesized that aromatase distribution in the ruffed grouse would differ from that of other galliforms. We used an antibody raised against quail aromatase to examine aromatase immunoreactivity in the ruffed grouse, the closely related spruce grouse (Falcipennis canadensis) and the Japanese quail (Coturnix japonica). In all three species, ARO neurons were identified in the medial preoptic nucleus, the bed nucleus of the stria terminalis and the nucleus ventromedialis hypothalami. Both grouse species had ARO neurons in two regions of the telencephalon, the hyperpallium, and entopallium, and the ruffed grouse also in field L. ARO neurons were only found in one region in the telencephalon of the Japanese quail, the septum. In general, breeding male ruffed grouse had significantly more ARO neurons and those neurons were larger than that of both the non-breeding male and female ruffed grouse. Aromatase expression in the telencephalon of the ruffed grouse suggests that steroid hormones might modulate responses to visual and acoustic stimuli, but how this relates to species differences in courtship displays and co-expression with estrogenic receptors is yet to be determined.

Brain aromatase and circulating corticosterone are rapidly regulated by combined acute stress and sexual interaction in a sex-specific manner

Neural production of 17β-oestradiol via aromatisation of testosterone may play a critical role in rapid, nongenomic regulation of physiological and behavioural processes. In brain nuclei implicated in the control of sexual behaviour, sexual or stressfull stimuli induce, respectively, a rapid inhibition or increase in preoptic aromatase activity (AA). In the present study, we tested quail that were either nonstressed or acutely stressed (15 min of restraint) immediately before sexual interaction (5 min) with stressed or nonstressed partners. We measured nuclei-specific AA changes, corresponding behavioural output, fertilisation rates and corticosterone (CORT) concentrations. In males, sexual interaction rapidly reversed stress-induced increases of AA in the medial preoptic nucleus (POM). This time scale (< 5 min) highlights the dynamic potential of the aromatase system to integrate input from stimuli that drive AA in opposing directions. Moreover, acute stress had minimal effects on male behaviour, suggesting that the input from the sexual stimuli on POM AA may actively preserve sexual behaviour despite stress exposure. We also found distinct sex differences in contextual physiological responses: males did not show any effect of partner status, whereas females responded to both their stress exposure and the male partner's stress exposure at the level of circulating CORT and AA. In addition, fertilisation rates and female CORT correlated with the male partner's exhibition of sexually aggressive behaviour, suggesting that female perception of the male can affect their physiology as much as direct stress. Overall, male reproduction appears relatively simple: sexual stimuli, irrespective of stress, drives major neural changes including rapid reversal of stress-induced changes of AA. By contrast, female reproduction appears more nuanced and context specific, with subjects responding physiologically and behaviourally to stress, the male partner's stress exposure, and female-directed male behaviour.

Steroid receptor coactivator 2 modulates steroid-dependent male sexual behavior and neuroplasticity in Japanese quail (Coturnix japonica)

Steroid receptor coactivators are necessary for efficient transcriptional regulation by ligand-bound nuclear receptors, including estrogen and androgen receptors. Steroid receptor coactivator-2 (SRC-2) modulates estrogen- and progesterone-dependent sexual behavior in female rats but its implication in the control of male sexual behavior has not been studied to our knowledge. We cloned and sequenced the complete quail SRC-2 transcript and showed by semi-quantitative PCR that SRC-2 expression is nearly ubiquitous, with high levels of expression in the kidney, cerebellum and diencephalon. Real-time quantitative PCR did not reveal any differences between intact males and females the medial preoptic nucleus (POM), optic lobes and cerebellum. We next investigated the physiological and behavioral role of this coactivator using in vivo antisense oligonucleotide techniques. Daily injections in the third ventricle at the level of the POM of locked nucleic acid antisense targeting SRC-2 significantly reduced the expression of testosterone-dependent male-typical copulatory behavior but no inhibition of one aspect of the appetitive sexual behavior was observed. The volume of POM, defined by aromatase-immunoreactive cells, was markedly decreased in animals treated with antisense as compared with controls. These results demonstrate that SRC-2 plays a prominent role in the control of steroid-dependent male sexual behavior and its associated neuroplasticity in Japanese quail.

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.

Specific activation of estrogen receptor alpha and beta enhances male sexual behavior and neuroplasticity in male Japanese quail

Two subtypes of estrogen receptors (ER), ERα and ERβ, have been identified in humans and numerous vertebrates, including the Japanese quail. We investigated in this species the specific role(s) of each receptor in the activation of male sexual behavior and the underlying estrogen-dependent neural plasticity. Castrated male Japanese quail received empty (CX) or testosterone-filled (T) implants or were daily injected with the ER general agonist diethylstilbestrol (DES), the ERα-specific agonist PPT, the ERβ-specific agonist DPN or the vehicle, propylene glycol. Three days after receiving the first treatment, subjects were alternatively tested for appetitive (rhythmic cloacal sphincter movements, RCSM) and consummatory aspects (copulatory behavior) of male sexual behavior. 24 hours after the last behavioral testing, brains were collected and analyzed for aromatase expression and vasotocinergic innervation in the medial preoptic nucleus. The expression of RCSM was activated by T and to a lesser extent by DES and PPT but not by the ERβagonist DPN. In parallel, T fully restored the complete sequence of copulation, DES was partially active and the specific activation of ERα or ERβ only resulted in a very low frequency of mount attempts in few subjects. T increased the volume of the medial preoptic nucleus as measured by the dense cluster of aromatase-immunoreactive cells and the density of the vasotocinergic innervation within this nucleus. DES had only a weak action on vasotocinergic fibers and the two specific ER agonists did not affect these neural responses. Simultaneous activation of both receptors or treatments with higher doses may be required to fully activate sexual behavior and the associated neurochemical events.

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.

Effects of egg testosterone on female mate choice and male sexual behavior in the pheasant

Evidence is accumulating that sex steroids in the eggs, besides affecting progeny phenotype and behavior in the short term, also have enduring effects until adulthood, when they may translate into differences in reproductive strategies and success. Maternal steroids transfer may therefore affect both agonistic behavior and mate choice decisions, either through the promotion of body size and condition or through a priming effect on the neuroendocrine system. However, owing to the prevalence of a short-term perspective, relevance of maternal transfer of sex steroids to sexual selection processes has been seldom studied. Here we investigate the effects of an experimental increase in egg testosterone on male dominance and copulation success in the ring-necked pheasant, Phasianus colchicus, a polygynous galliform with multiple male ornamental traits, in captivity. We found that females from testosterone (T) injected eggs copulated less than control females. Males from T-injected eggs obtained more copulations than control males, specifically with control females. The effect of male 'ordinary' and secondary sexual traits on either dominance or copulation frequency did not depend on early exposure to T, nor did T treatment affect male dominance. Present results demonstrate that variation in the early hormonal environment set up by mothers affects sexual behavior of the offspring, which might translate into fitness differences.

17beta-Estradiol levels in male zebra finch brain: combining Palkovits punch and an ultrasensitive radioimmunoassay

Local aromatization of testosterone into 17beta-estradiol (E(2)) is often required for the physiological and behavioral actions of testosterone. In most vertebrates, aromatase is expressed in a few discrete brain regions. While many studies have measured brain aromatase mRNA or activity, very few studies have measured brain E(2) levels, particularly in discrete brain regions, because of technical challenges. Here, we used the Palkovits punch technique to isolate 13 discrete brain nuclei from adult male zebra finches. Steroids were extracted via solid phase extraction. E(2) was then measured with an ultrasensitive, specific and precise radioimmunoassay. Our protocol leads to high recovery of E(2) (84%) and effectively removes interfering brain lipids. E(2) levels were high in aromatase-rich regions such as caudal medial nidopallium and hippocampus. E(2) levels were intermediate in the medial preoptic area, ventromedial nucleus of the hypothalamus, lateral and medial magnocellular nuclei of anterior nidopallium, nucleus taeniae of the amygdala, and Area X. E(2) levels were largely non-detectable in the cerebellum, HVC, lateral nidopallium and optic lobes. Importantly, E(2) levels were significantly lower in plasma than in the caudal medial nidopallium. This protocol allows one to measure E(2) in discrete brain regions and potentially relate local E(2) concentrations to aromatase activity and behavior.

Diversity of mechanisms involved in aromatase regulation and estrogen action in the brain

BACKGROUND

The mechanisms through which estrogens modulate neuronal physiology, brain morphology, and behavior in recent years have proven to be far more complex than previously thought. For example, a second nuclear estrogen receptor has been identified, a new family of coregulatory proteins regulating steroid-dependent gene transcriptions was discovered and, finally, it has become clear that estrogens have surprisingly rapid effects based on their actions on cell membranes, which in turn result in the modulation of intracellular signaling cascades.

SCOPE OF REVIEW

This paper presents a selective review of new findings in this area related to work in our laboratories, focusing on the role of estrogens in the activation of male sexual behavior. Two separate topics are considered. We first discuss functions of the steroid receptor coactivator-1 (SRC-1) that has emerged as a key limiting factor for behavioral effects of estradiol. Knocking-down its expression by antisense oligonucleotides drastically inhibits male-typical sexual behaviors. Secondly, we describe rapid regulations of brain estradiol production by calcium-dependent phosphorylations of the aromatase enzyme, themselves under the control of neurotransmitter activity.

MAJOR CONCLUSIONS

These rapid changes in estrogen bioavailability have clear behavioral consequences. Increases or decreases in estradiol concentrations respectively obtained by an acute injection of estradiol itself or of an aromatase inhibitor lead within 15-30 min to parallel changes in sexual behavior frequencies.

GENERAL SIGNIFICANCE

These new controls of estrogen action offer a vast array of possibilities for discrete local controls of estrogen action. They also represent a formidable challenge for neuroendocrinologists trying to obtain an integrated view of brain function in relation to behavior.

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.

Importance of steroid receptor coactivators in the modulation of steroid action on brain and behavior

Steroid receptors such as estrogen and androgen receptors are nuclear receptors involved in the transcriptional regulation of a large number of target genes. Steroid-dependent protein expression in the brain controls a large array of biological processes including spatial cognition, copulatory behavior and neuroprotection. The discovery of a competition, or squelching, between two different nuclear receptors introduced the notion that common cofactors may be involved in the modulation of transcriptional activity of nuclear receptors. These cofactors or coregulatory proteins are functionally divided into coactivators and corepressors and are involved in chromatin remodeling and stabilization of the general transcription machinery. Although a large amount of information has been collected about the in vitro function of these coregulatory proteins, relatively little is known regarding their physiological role in vivo, particularly in the brain. Our laboratory and others have demonstrated the importance of SRC-1 in the differentiation and activation of steroid-dependent sexual behaviors and the related neural genes. For example, we report that the inhibition of SRC-1 expression blocks the activating effects of exogenous testosterone on male sexual behaviors and increases the volume of the median preoptic area. Other coactivators are likely to be involved in the modulation in vivo of steroid receptor activity and it seems that the presence of a precise subset of coactivators could help define the phenotype of the cell by modulating a specific downstream pathway after steroid receptor activation. The very large number of coactivators and their association into preformed complexes potentially allows the determination of hundreds of different phenotypes. The study of the expression of the coactivator and their function in vivo is required to fully understand steroid action and specificity in the brain.

Behavioral effects of brain-derived estrogens in birds

In birds as in other vertebrates, estrogens produced in the brain by aromatization of testosterone have widespread effects on behavior. Research conducted with male Japanese quail demonstrates that effects of brain estrogens on all aspects of sexual behavior, including appetitive and consummatory components as well as learned aspects, can be divided into two main classes based on their time course. First, estrogens via binding to estrogen receptors regulate the transcription of a variety of genes involved primarily in neurotransmission. These neurochemical effects ultimately result in the activation of male copulatory behavior after a latency of a few days. Correlatively, testosterone and its aromatized metabolites increase the transcription of the aromatase mRNA, resulting in an increased concentration and activity of the enzyme that actually precedes behavioral activation. Second, recent studies with quail demonstrate that brain aromatase activity can also be modulated within minutes by phosphorylation processes regulated by changes in intracellular calcium concentration, such as those associated with glutamatergic neurotransmission. The rapid upregulations or downregulations of brain estrogen concentration (presumably resulting from these changes in aromatase activity) affect, by nongenomic mechanisms with relatively short latencies (frequency increases or decreases respectively within 10-15 min), the expression of male sexual behavior in quail and also in rodents. Brain estrogens thus affect behavior on different time scales by genomic and nongenomic mechanisms similar to those of a hormone or a neurotransmitter.

Experience modulates both aromatase activity and the sensitivity of agonistic behaviour to testosterone in black-headed gulls

In young black-headed gulls (Larus ridibundus), exposure to testosterone increases the sensitivity of agonistic behaviour to a subsequent exposure to this hormone. The aim of this paper is twofold: to analyze whether social experience, gained during testosterone exposure, mediates this increase in hormonal sensitivity (priming), and whether this in turn is mediated by an increase in central aromatase activity. To this end, we performed three experiments. In the first juvenile gulls were exposed to two consecutive treatments with testosterone (T1 and T2), with more than a week interval in between. During T1, half of the birds were housed in social isolation (Iso) and the other half in groups (Soc). All birds were re-housed in a new social situation during the second treatment. The increase in social behaviour during T2 was significantly more rapid in Soc than Iso birds. In experiment 2 we show that 17beta-estradiol treatment facilitates the behaviour measured in experiment 1. In experiment 3 we used a set-up comparable with that of experiment 1, but birds were sacrificed early in the T2 period. Aromatase activity in the preoptic area and the hypothalamus was measured using the tritiated water releasing method. In some parts of the preoptic area and hypothalamus aromatase activity was higher in Soc birds relative to Iso birds. The results indicate that social experience can modulate the increase of social behaviour to testosterone via modulation of aromatase activity and independently of actual hormone levels.

Rapid action on neuroplasticity precedes behavioral activation by testosterone

Testosterone has been shown to increase the volume of steroid-sensitive brain nuclei in adulthood in several vertebrate species. In male Japanese quail the volume of the male-biased sexually dimorphic medial preoptic nucleus (POM), a key brain area for the control of male sexual behavior, is markedly increased by testosterone. Previous studies assessed this effect after a period of 8-14 days but the exact time course of these effects is unknown. We asked here whether testosterone-dependent POM plasticity could be observed at shorter latencies. Brains from castrated male quail were collected after 1, 2, 7 and 14 days of T treatment (CX+T) and compared to brains of untreated castrates (CX) collected after 1 or 14 days. POM volumes defined either by Nissl staining or by aromatase immunohistochemistry increased in a time-dependent fashion in CX+T subjects and almost doubled after 14 days of treatment with testosterone while no change was observed in CX birds. A significant increase in the average POM volume was detected after only one day of testosterone treatment. The optical density of Nissl and aromatase staining was also increased after one or two days of testosterone treatment. Activation of male copulatory behavior followed these morphological changes with a latency of approximately one day. This rapid neurochemical and neuroanatomical plasticity observed in the quail POM thus seems to limit the activation of male sexual behavior and offers an excellent model to analyze features of steroid-regulated brain structure and function that determine behavior expression.

Testosterone control of male courtship in birds

A sequence of behaviours which we call courtship initiates reproduction in a large number of species. In vertebrates, as a component of male sexual behaviour courtship is strongly influenced by testicular androgen. Here I will review some salient issues about the regulation of courtship by testosterone in birds. The first section will briefly summarize the first 100 years of research on this topic. The specific role of testosterone or its oestrogenic metabolites in the control of different components of courtship will be the subject of the second section. Then, I will discuss how behavioural patterns can be recruited into courtship and modified in their structure by testosterone action. In the following section, the role of sexual selection and female choice in shaping the link between testosterone and courtship will be addressed. The problematic nature of the quantitative relationships between testosterone and behaviour will be topic of the fifth section. Finally, I will discuss how courtship traits that are activated by testosterone can be apparently independent of hormone blood concentrations. These issues will be examined in an evolutionary perspective, in an attempt to understand how natural and sexual selection have shaped the links between the hormone and the behaviour.

Individual variation and the endocrine regulation of behaviour and physiology in birds: a cellular/molecular perspective

Investigations of the cellular and molecular mechanisms of physiology and behaviour have generally avoided attempts to explain individual differences. The goal has rather been to discover general processes. However, understanding the causes of individual variation in many phenomena of interest to avian eco-physiologists will require a consideration of such mechanisms. For example, in birds, changes in plasma concentrations of steroid hormones are important in the activation of social behaviours related to reproduction and aggression. Attempts to explain individual variation in these behaviours as a function of variation in plasma hormone concentrations have generally failed. Cellular variables related to the effectiveness of steroid hormone have been useful in some cases. Steroid hormone target sensitivity can be affected by variables such as metabolizing enzyme activity, hormone receptor expression as well as receptor cofactor expression. At present, no general theory has emerged that might provide a clear guidance when trying to explain individual variability in birds or in any other group of vertebrates. One strategy is to learn from studies of large units of intraspecific variation such as population or sex differences to provide ideas about variables that might be important in explaining individual variation. This approach along with the use of newly developed molecular genetic tools represents a promising avenue for avian eco-physiologists to pursue.

Effects of ovarian theca cells on granulosa cell differentiation during gonadotropin-independent follicular growth in cattle

We investigated the effects of theca cells or FSH on granulosa cell differentiation and steroid production during bovine early follicular growth, using a co-culture system in which granulosa and theca cells were cultured on opposite sides of a collagen membrane. Follicular cells were isolated from early antral follicles (2-4 mm) that were assumed to be in gonadotropin-independent phase and just before recruitment into a follicular wave. Granulosa cells were cultured under serum-free conditions with and without theca cells or recombinant human FSH to test their effects on granulosa cell differentiation. Messenger RNA levels for P450 aromatase (aromatase), P450 cholesterol side chain cleavage (P450scc), 3beta-hydroxysteroid dehydrogenase (3beta-HSD), LH receptor (LHr), and steroidogenic acute regulatory protein (StAR) in granulosa cells were measured by real-time quantitative RT-PCR analysis. FSH enhanced aromatase mRNA expression in granulosa cells, but did not alter estradiol production. FSH also enhanced mRNA expression for P450scc, LHr, and StAR in granulosa cells, resulting in an increase in progesterone production. In contrast, theca cells enhanced aromatase mRNA expression in granulosa cells resulting in an increase in estradiol production. Theca cells did not alter progesterone production and mRNA expression in granulosa cells for P450scc, 3beta-HSD, LHr, and StAR. The results of the present study indicate that theca cells are involved in both rate-limiting steps in estrogen production, i.e., androgen substrate production and aromatase regulation, and that theca cell-derived factors regulate estradiol and progesterone production in a way that reflects steroidogenesis during the follicular phase of the estrous cycle.

Functional significance of the rapid regulation of brain estrogen action: where do the estrogens come from?

Estrogens exert a wide variety of actions on reproductive and non-reproductive functions. These effects are mediated by slow and long lasting genomic as well as rapid and transient non-genomic mechanisms. Besides the host of studies demonstrating the role of genomic actions at the physiological and behavioral level, mounting evidence highlights the functional significance of non-genomic effects. However, the source of the rapid changes in estrogen availability that are necessary to sustain their fast actions is rarely questioned. For example, the rise of plasma estrogens at pro-estrus that represents one of the fastest documented changes in plasma estrogen concentration appears too slow to explain these actions. Alternatively, estrogen can be synthesized in the brain by the enzyme aromatase providing a source of locally high concentrations of the steroid. Furthermore, recent studies demonstrate that brain aromatase can be rapidly modulated by afferent inputs, including glutamatergic afferents. A role for rapid changes in estrogen production in the central nervous system is supported by experiments showing that acute aromatase inhibition affects nociception as well as male sexual behavior and that preoptic aromatase activity is rapidly (within min) modulated following mating. Such mechanisms thus fulfill the gap existing between the fast actions of estrogen and their mode of production and open new avenues for the understanding of estrogenic effects on the brain.