Rapid effects of aromatase inhibition on male reproductive behaviors in Japanese quail

Role of neuroestrogens in the regulation of social behaviors - From social recognition to mating

In this mini-review, we summarize the brain distribution of aromatase, the enzyme catalyzing the synthesis of estrogens from androgens, and the mechanisms responsible for regulating estrogen production within the brain. Understanding this local synthesis of estrogens by neurons is pivotal as it profoundly influences various facets of social behavior. Neuroestrogen action spans from the initial processing of socially pertinent sensory cues to integrating this information with an individual's internal state, ultimately resulting in the manifestation of either pro-affiliative or - aggressive behaviors. We focus here in particular on aggressive and sexual behavior as the result of correct individual recognition of intruders and potential mates. The data summarized in this review clearly point out the crucial role of locally synthesized estrogens in facilitating rapid adaptation to the social environment in rodents and birds of both sexes. These observations not only shed light on the evolutionary significance but also indicate the potential implications of these findings in the realm of human health, suggesting a compelling avenue for further investigation.

Rapid nongenomic estrogen signaling controls alcohol drinking behavior

The sex steroid hormone estrogen is a key modulator of numerous physiological processes and adaptive behaviors, but it may also be co-opted to drive maladaptive behaviors. While many behavioral roles for estrogen signaling have been shown to occur through canonical genomic signaling mechanisms via nuclear receptors, estrogen can also act in a neurotransmitter-like fashion at membrane-associated estrogen receptors to rapidly regulate neuronal function. Early alcohol drinking confers greater risk for alcohol use disorder in women than men, and binge alcohol drinking is correlated with high circulating estrogen but a causal role for estrogen in alcohol drinking has not been established. Here, we demonstrate that gonadally intact female mice consume more alcohol and display an anxiolytic phenotype when they have elevated levels of ovarian-derived estrogen across the estrous cycle. We found that rapid, nongenomic estrogen signaling at membrane-associated estrogen receptor alpha in the bed nucleus of the stria terminalis (BNST) is necessary and sufficient for the pro-alcohol drinking effects of ovarian estrogen signaling, regardless of the transcriptional program of a high ovarian estrogen state. We further show that a population of corticotropin-releasing factor (CRF) BNST neurons (BNSTCRF) is a critical mediator of these effects, as high estrogen rapidly enhances synaptic excitation of BNSTCRF neurons and promotes their role in driving binge alcohol drinking. These findings show a causal role for endogenous, ovarian-derived estrogen in hormonal modulation of risky alcohol consumption and provide the first demonstration of a purely rapid, nongenomic signaling mechanism of ovarian estrogen in the brain controlling behavior in gonadally intact females.

Steroid hormone-mediated regulation of sexual and aggressive behaviour by non-genomic signalling

Sex and aggression are well studied examples of social behaviours that are common to most animals and are mediated by an evolutionary conserved group of interconnected nuclei in the brain called the social behaviour network. Though glucocorticoids and in particular estrogen regulate these social behaviours, their effects in the brain are generally thought to be mediated by genomic signalling, a slow transcriptional regulation mediated by nuclear hormone receptors. In the last decade or so, there has been renewed interest in understanding the physiological significance of rapid, non-genomic signalling mediated by steroids. Though the identity of the membrane hormone receptors that mediate this signalling is not clearly understood and appears to be different in different cell types, such signalling contributes to physiologically relevant behaviours such as sex and aggression. In this short review, we summarise the evidence for this phenomenon in the rodent, by focusing on estrogen and to some extent, glucocorticoid signalling. The use of these signals, in relation to genomic signalling is manifold and ranges from potentiation of transcription to the possible transduction of environmental signals.

Role of membrane estrogen receptor alpha (ERα) in the rapid regulation of male sexual behavior

The activation of male sexual behavior depends on brain estrogen synthesis. Estrogens act through nuclear and membrane receptors producing effects within hours/days or seconds/minutes, respectively. In mice, estrogen receptor alpha (ERα) is the main estrogen receptor (ER) controlling the activation of male sexual behavior. Although neuroestrogens rapidly modulate mouse sexual behavior, it is not known whether these effects involve membrane ERα (mERα). This study combines two complementary approaches to address this question. C451A-ERα mice carry an ERα that cannot signal at the membrane, while estetrol (E4) is a natural estrogen acting as an agonist on nuclear ERα but as an antagonist on membrane ERα. In wild-type males, E4 decreased the number of mounts and intromissions after 10 min. In C451A-ERα males, E4 also altered sexual performance but after 30 min. E4 did not affect time spent near the female in both wild-type and C451A-ERα mice. However, regardless of genotype, the aromatase inhibitor 1,4,6-Androstatriene-3,17-dione (ATD) decreased both sexual performance and the time spent near the female after 10 and 30 min, confirming the key role of aromatization in the rapid control of sexual behavior and motivation. In conclusion, the shift in timing at which the effect of E4 is observed in mice lacking mERα suggests a role for mERα in the regulation of rapid effects of neuroestrogens on sexual performance, thus providing the first demonstration that E4 acts as an antagonist of a mER in the brain. The persisting effect of ATD on behavior in C451A-ERα mice also suggests the implication of another ER.

Brain-Derived Estrogen and Neurological Disorders

Astrocytes and neurons in the male and female brains produce the neurosteroid brain-derived 17β-estradiol (BDE₂) from androgen precursors. In this review, we discuss evidence that suggest BDE₂ has a role in a number of neurological conditions, such as focal and global cerebral ischemia, traumatic brain injury, excitotoxicity, epilepsy, Alzheimer's disease, and Parkinson's disease. Much of what we have learned about BDE₂ in neurological disorders has come from use of aromatase inhibitors and global aromatase knockout mice. Recently, our group developed astrocyte- and neuron-specific aromatase knockout mice, which have helped to clarify the precise functions of astrocyte-derived 17β-estradiol (ADE₂) and neuron-derived 17β-estradiol (NDE₂) in the brain. The available evidence to date suggests a primarily beneficial role of BDE₂ in facilitating neuroprotection, synaptic and cognitive preservation, regulation of reactive astrocyte and microglia activation, and anti-inflammatory effects. Most of these beneficial effects appear to be due to ADE₂, which is induced in most neurological disorders, but there is also recent evidence that NDE₂ exerts similar beneficial effects. Furthermore, in certain situations, BDE₂ may also have deleterious effects, as recent evidence suggests its overproduction in epilepsy contributes to seizure induction. In this review, we examine the current state of this quickly developing topic, as well as possible future studies that may be required to provide continuing growth in the field.

Beyond sex and aggression: testosterone rapidly matches behavioural responses to social context and tries to predict the future

Although androgens are widely studied in the context of aggression, androgenic influences on prosocial behaviours have been less explored. We examined testosterone's (T) influence on prosocial and aggressive responses in a positively valenced social context (interacting with a pairbond partner) and a negatively valenced context (interacting with an intruder) in socially monogamous Mongolian gerbils. T increased and decreased prosocial responses in the same individuals towards a pairbond partner and an intruder, respectively, both within 30 min, but did not affect aggression. T also had persistent effects on prosocial behaviour; males in which T initially increased prosocial responses towards a partner continued to exhibit elevated prosocial responses towards an intruder male days later until a second T injection rapidly eliminated those responses. Thus, T surges can rapidly match behaviour to current social context, as well as prime animals for positive social interactions in the future. Neuroanatomically, T rapidly increased hypothalamic oxytocin, but not vasopressin, cellular responses during interactions with a partner. Together, our results indicate that T can facilitate and inhibit prosocial behaviours depending on social context, that it can influence prosocial responses across rapid and prolonged time scales, and that it affects oxytocin signalling mechanisms that could mediate its context-dependent behavioural influences.

From Reductionism Toward Integration: Understanding How Social Behavior Emerges From Integrated Circuits

Complex social behaviors are emergent properties of the brain's interconnected and overlapping neural networks. Questions aimed at understanding how brain circuits produce specific and appropriate behaviors have changed over the past half century, shifting from studies of gross anatomical and behavioral associations, to manipulating and monitoring precisely targeted cell types. This technical progression has enabled increasingly deep insights into the regulation of perception and behavior with remarkable precision. The capacity of reductionist approaches to identify the function of isolated circuits is undeniable but many behaviors require rapid integration of diverse inputs. This review examines progress toward understanding integrative social circuits and focuses on specific nodes of the social behavior network including the medial amygdala, ventromedial hypothalamus (VMH) and medial preoptic area of the hypothalamus (MPOA) as examples of broad integration between multiple interwoven brain circuits. Our understanding of mechanisms for producing social behavior has deepened in conjunction with advances in technologies for visualizing and manipulating specific neurons and, here, we consider emerging strategies to address brain circuit function in the context of integrative anatomy.

Estrogenic signaling and sociosexual behavior in wild sex-changing bluehead wrasses, Thalassoma bifasciatum

Estrogenic signaling is an important focus in studies of gonadal and brain sexual differentiation in fishes and vertebrates generally. This study examined variation in estrogenic signaling (1) across three sexual phenotypes (female, female-mimic initial phase [IP] male, and terminal phase [TP] male), (2) during socially-controlled female-to-male sex change, and (3) during tidally-driven spawning cycles in the protogynous bluehead wrasse (Thalassoma bifasciatum). We analyzed relative abundances of messenger RNAs (mRNAs) for the brain form of aromatase (cyp19a1b) and the three nuclear estrogen receptors (ER) (ERα, ERβa, and ERβb) by qPCR. Consistent with previous reports, forebrain/midbrain cyp19a1b was highest in females, significantly lower in TP males, and lowest in IP males. By contrast, ERα and ERβb mRNA abundances were highest in TP males and increased during sex change. ERβa mRNA did not vary significantly. Across the tidally-driven spawning cycle, cyp19a1b abundances were higher in females than TP males. Interestingly, cyp19a1b levels were higher in TP males close (~1 h) to the daily spawning period when sexual and aggressive behaviors rise than males far from spawning (~10-12 h). Together with earlier findings, our results suggest alterations in neural estrogen signaling are key regulators of socially-controlled sex change and sexual phenotype differences. Additionally, these patterns suggest TP male-typical sociosexual behaviors may depend on intermediate rather than low estrogenic signaling. We discuss these results and the possibility that an inverted-U shaped relationship between neural estrogen and male-typical behaviors is more common than presently appreciated.

Membrane-initiated actions of sex steroids and reproductive behavior: A historical account

It was assumed for a long time that sex steroids are activating reproductive behaviors by the same mechanisms that produce their morphological and physiological effects in the periphery. However during the last few decades an increasing number of examples were identified where behavioral effects of steroids were just too fast to be mediated via changes in DNA transcription. This progressively forced behavioral neuroendocrinologists to recognize that part of the effects of steroids on behavior are mediated by membrane-initiated events. In this review we present a selection of these early data that changed the conceptual landscape and we provide a summary the different types of membrane-associated receptors (estrogens, androgens and progestagens receptors) that are playing the most important role in the control of reproductive behaviors. Then we finally describe in more detail three separate behavioral systems in which membrane-initiated events have clearly been established to contribute to behavior control.

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.

Sexually Dimorphic Neurosteroid Synthesis Regulates Neuronal Activity in the Murine Brain

Sex steroid hormones act on hypothalamic kisspeptin neurons to regulate reproductive neural circuits in the brain. Kisspeptin neurons start to express estrogen receptors in utero, suggesting steroid hormone action on these cells early during development. Whether neurosteroids are locally produced in the embryonic brain and impinge onto kisspeptin/reproductive neural circuitry is not known. To address this question, we analyzed aromatase expression, a key enzyme in estrogen synthesis, in male and female mouse embryos. We identified an aromatase neuronal network comprising ∼6000 neurons in the hypothalamus and amygdala. By birth, this network has become sexually dimorphic in a cluster of aromatase neurons in the arcuate nucleus adjacent to kisspeptin neurons. We demonstrate that male arcuate aromatase neurons convert testosterone to estrogen to regulate kisspeptin neuron activity. We provide spatiotemporal information on aromatase neuronal network development and highlight a novel mechanism whereby aromatase neurons regulate the activity of distinct neuronal populations expressing estrogen receptors.SIGNIFICANCE STATEMENT Sex steroid hormones, such as estradiol, are important regulators of neural circuits controlling reproductive physiology in the brain. Embryonic kisspeptin neurons in the hypothalamus express steroid hormone receptors, suggesting hormone action on these cells in utero Whether neurosteroids are locally produced in the brain and impinge onto reproductive neural circuitry is insufficiently understood. To address this question, we analyzed aromatase expression, a key enzyme in estradiol synthesis, in mouse embryos and identified a network comprising ∼6000 neurons in the brain. By birth, this network has become sexually dimorphic in a cluster of aromatase neurons in the arcuate nucleus adjacent to kisspeptin neurons. We demonstrate that male aromatase neurons convert testosterone to estradiol to regulate kisspeptin neuron activity.

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.

De Novo Synthesized Estradiol: A Role in Modulating the Cerebellar Function

The estrogen estradiol is a potent neuroactive steroid that may regulate brain structure and function. Although the effects of estradiol have been historically associated with gonadal secretion, the discovery that this steroid may be synthesized within the brain has expanded this traditional concept. Indeed, it is accepted that de novo synthesized estradiol in the nervous system (nE2) may modulate several aspects of neuronal physiology, including synaptic transmission and plasticity, thereby influencing a variety of behaviors. These modulations may be on a time scale of minutes via non-classical and often membrane-initiated mechanisms or hours and days by classical actions on gene transcription. Besides the high level, recent investigations in the cerebellum indicate that even a low aromatase expression can be related to the fast nE2 effect on brain functioning. These pieces of evidence point to the importance of an on-demand and localized nE2 synthesis to rapidly contribute to regulating the synaptic transmission. This review is geared at exploring a new scenario for the impact of estradiol on brain processes as it emerges from the nE2 action on cerebellar neurotransmission and cerebellum-dependent learning.

Neuroestrogen synthesis modifies neural representations of learned song without altering vocal imitation in developing songbirds

Birdsong learning, like human speech, depends on the early memorization of auditory models, yet how initial auditory experiences are formed and consolidated is unclear. In songbirds, a putative cortical locus is the caudomedial nidopallium (NCM), and one mechanism to facilitate auditory consolidation is 17β-estradiol (E2), which is associated with human speech-language development, and is abundant in both NCM and human temporal cortex. Circulating and NCM E2 levels are dynamic during learning, suggesting E2’s involvement in encoding recent auditory experiences. Therefore, we tested this hypothesis in juvenile male songbirds using a comprehensive assessment of neuroanatomy, behavior, and neurophysiology. First, we found that brain aromatase expression, and thus the capacity to synthesize neuroestrogens, remains high in the auditory cortex throughout development. Further, while systemic estrogen synthesis blockade suppressed juvenile song production, neither systemic nor unilateral E2 synthesis inhibition in NCM disrupted eventual song imitation. Surprisingly, early life neuroestrogen synthesis blockade in NCM enhanced the neural representations of both the birds’ own song and the tutor song in NCM and a downstream sensorimotor region, HVC, respectively. Taken together, these findings indicate that E2 plays a multifaceted role during development, and that, contrary to prediction, tutor song memorization is unimpaired by unilateral estrogen synthesis blockade in the auditory cortex.

Role for the membrane estrogen receptor alpha in the sexual differentiation of the brain

Estrogens exert pleiotropic effects on multiple physiological and behavioral responses. Male and female sexual behavior in rodents constitutes some of the best-characterized responses activated by estrogens in adulthood and largely depend on ERα. Evidence exists that nucleus- and membrane-initiated estrogen signaling cooperate to orchestrate the activation of these behaviors both in short- and long-term. However, questions remain regarding the mechanism(s) and receptor(s) involved in the early brain programming during development to organize the circuits underlying sexually differentiated responses. Taking advantage of a mouse model harboring a mutation of the ERα palmitoylation site, which prevents membrane ERα signaling (mERα; ERα-C451A), this study investigated the role of mERα on the expression of male and female sexual behavior and neuronal populations that differ between sexes. The results revealed no genotype effect on the expression of female sexual behavior, while male sexual behavior was significantly reduced, but not abolished, in males homozygous for the mutation. Similarly, the number of kisspeptin- (Kp-ir) and calbindin-immunoreactive (Cb-ir) neurons in the anteroventral periventricular nucleus (AVPv) and the sexually dimorphic nucleus of the preoptic area (SDN-POA), respectively, were not different between genotypes in females. In contrast, homozygous males showed increased numbers of Kp-ir and decreased numbers of Cb-ir neurons compared to wild-types, thus leading to an intermediate phenotype between females and wild-type males. Importantly, females neonatally treated with estrogens exhibited the same neurochemical phenotype as their corresponding genotype among males. Together, these data provide evidence that mERα is involved in the perinatal programming of the male brain.

Testosterone causes pleiotropic effects on cleanerfish behaviour

Mathematical modelling regarding evolutionary theory typically assumes that optimal strategies are not constrained through mechanistic processes. In contrast, recent studies on brain anatomy and neurobiology suggest that flexibility in social behaviour is rather constrained by the physiological state of the social decision-making network. Changing its state may yield selective advantages in some social contexts but neutral or even detrimental effects in others. Here we provide field evidence for such physiological trade-offs. We subjected wild female cleaner wrasse to injections of testosterone or of saline solution (control) and then observed both intraspecific interactions and interspecific cleaning behaviour with other reef fish, referred to as clients. Testosterone-treated females intensified intraspecific social interactions, showing more aggression towards smaller females and tendencies of increased aggressive and affiliative contacts with dominant males. Such testosterone-mediated changes fit the hypothesis that an increase in testosterone mediates female's focus on status in this protogynous hermaphrodite species, where females eventually change sex to become males. Moreover, we also identified other effects on interspecific social interactions: testosterone-treated females interacted less with client reef fishes and hence obtained less food. Most importantly, they selectively reduced service quality for species that were less likely to punish after being cheated. Overall, our findings suggest that testosterone causes pleiotropic effects on intra and interspecific social behaviour by broadly influencing female cleaners' decision-making.

New concepts in the study of the sexual differentiation and activation of reproductive behavior, a personal view

Since the beginning of this century, research methods in neuroendocrinology enjoyed extensive refinements and innovation. These advances allowed collection of huge amounts of new data and the development of new ideas but have not led to this point, with a few exceptions, to the development of new conceptual advances. Conceptual advances that took place largely resulted from the ingenious insights of several investigators. I summarize here some of these new ideas as they relate to the sexual differentiation and activation by sex steroids of reproductive behaviors and I discuss how our research contributed to the general picture. This selective review clearly demonstrates the importance of conceptual changes that have taken place in this field since beginning of the 21st century. The recent technological advances suggest that our understanding of hormones, brain and behavior relationships will continue to improve in a very fundamental manner over the coming years.

Rapid effects of testosterone on social decision-making in a monogamous California mice (Peromyscus californicus)

Social animals must cope with challenges and opportunities by adjusting how they react to a salient stimulus. Here we use California mice (Peromyscus californicus) and investigate the mechanisms underlying social decision-making by studying (i) rapid effects of testosterone (T) pulses on a male's decisions to approach a novel male (challenge) versus a receptive female (opportunity), and (ii) whether social experience shapes how such effects are manifested. In Experiment 1, we found that sexually naïve males administered saline injections preferentially approached unfamiliar females over unfamiliar males, in contrast, 10 min after receiving a single T-injection, males expressed a preference for approaching unfamiliar males. Such an effect of T only occurred in sexually naïve males, but not pair-bonded males, suggesting that the rapid effects of T on approach behavior may rely on the pair-bonding experiences. Experiment 2 investigated social decision-making across three repeated exposures to the challenge/opportunity situations. Only the initial decision, approach to the challenge, predicted future aggressive behaviors, and such an effect relied on the rapid actions of T. We also found that experience with the controlled challenge situation (the male intruder was restrained behind a wire mesh) dampened the approach to the male side (potential threat) when later exposed to the same conditions. This suggests that a resident's motivation to defend against a threatening individual may decrease as the threat posed by the "neighbors" is reduced. Overall rapid effects of post-encounter T pulses may play important roles in influencing behavioral decisions during social interactions.

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.

Rapid sex steroid effects on reproductive responses in male goldfish: Sensory and motor mechanisms

Contribution to Special Issue on Fast effects of steroids. Although we have learned a great deal about the molecular mechanisms through which sex steroids rapidly affect cellular physiology, we still know little about the links between those mechanisms and behavioral output, nor about their functional consequences in natural contexts. In this review, we first briefly discuss the contexts associated with rapid effects of sex steroids on reproductive behaviors and their likely functional outcomes, as well the sensory, motor, and motivational mechanisms associated with those effects. We then discuss our recent studies on the rapid effects of testosterone in goldfish. Those studies indicate that testosterone, through its aromatization and the subsequent activation of estrogen receptors, rapidly stimulates physiological processes related to the release of milt/sperm through likely influences on motor pathways, as well as behavioral responses to female visual stimuli that may reflect, in part, influences on early stages of sensory processing. Such motor and sensory mechanism are likely important for sperm competition and mate detection / tracking, respectively, in competitive mating contexts. We also present preliminary data on rapid effects of testosterone on responses to pheromones that may not involve estrogen receptors, suggesting a dissociation in the receptor mechanisms that mediate behavioral responses in different sensory modalities. Lastly, we briefly discuss the implications of our work on unresolved questions about rapid sex steroid neuromodulation in fish.

Neuroestradiol in regulation of GnRH release

Contribution to Special Issue on Fast effects of steroids. The concept that the positive feedback effect of ovarian estradiol (E₂) results in GnRH and gonadotropin surges is a well-established principle. However, a series of studies investigating the rapid action of E₂ in female rhesus monkeys has led to a new concept that neuroestradiol, synthesized and released in the hypothalamus, also contributes to regulation of the preovulatory GnRH surge. This unexpected finding started from our surprising observation that E₂ induces rapid stimulatory action in GnRH neurons in vitro. Subsequently, we confirmed that a similar rapid stimulatory action of E₂ occurs in vivo. Unlike subcutaneous injection of E₂ benzoate (EB), a brief (10-20 min), direct infusion of EB into the median eminence in ovariectomized (OVX) female monkeys rapidly stimulates release of GnRH and E₂ in a pulsatile manner, and the EB-induced GnRH and E₂ release is blocked by simultaneous infusion of the aromatase inhibitor, letrozole. This suggests that stimulated release of E₂ is of hypothalamic origin. To further determine the role of neuroestradiol we examined the effects of letrozole on EB-induced GnRH and LH surges in OVX females. Results indicate that letrozole treatment greatly attenuated the EB-induced GnRH and LH surges. Collectively, neuroestradiol released from the hypothalamus appears to be necessary for the positive feedback effect of E₂ on the GnRH/LH surge.

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.

Steroid Transport, Local Synthesis, and Signaling within the Brain: Roles in Neurogenesis, Neuroprotection, and Sexual Behaviors

Sex steroid hormones are synthesized from cholesterol and exert pleiotropic effects notably in the central nervous system. Pioneering studies from Baulieu and colleagues have suggested that steroids are also locally-synthesized in the brain. Such steroids, called neurosteroids, can rapidly modulate neuronal excitability and functions, brain plasticity, and behavior. Accumulating data obtained on a wide variety of species demonstrate that neurosteroidogenesis is an evolutionary conserved feature across fish, birds, and mammals. In this review, we will first document neurosteroidogenesis and steroid signaling for estrogens, progestagens, and androgens in the brain of teleost fish, birds, and mammals. We will next consider the effects of sex steroids in homeostatic and regenerative neurogenesis, in neuroprotection, and in sexual behaviors. In a last part, we will discuss the transport of steroids and lipoproteins from the periphery within the brain (and vice-versa) and document their effects on the blood-brain barrier (BBB) permeability and on neuroprotection. We will emphasize the potential interaction between lipoproteins and sex steroids, addressing the beneficial effects of steroids and lipoproteins, particularly HDL-cholesterol, against the breakdown of the BBB reported to occur during brain ischemic stroke. We will consequently highlight the potential anti-inflammatory, anti-oxidant, and neuroprotective properties of sex steroid and lipoproteins, these latest improving cholesterol and steroid ester transport within the brain after insults.

Dual action of neuro-estrogens in the regulation of male sexual behavior

Estrogens derived from brain testosterone aromatization (neuro-estrogens) are critical for the activation of male sexual behavior. Their effects on this behavior are typically associated with long-term changes in circulating levels of testosterone and the transcriptional activity of their liganded nuclear receptors. According to this view, neuro-estrogens would prime the neural circuits controlling the long-term expression of behavior, which would then be acutely regulated by neurotransmitter systems conveying information from the social environment. In parallel, neuro-estrogens are also able to produce much faster effects than previously anticipated. Our recent investigations in Japanese quail revealed an interesting dichotomy in the regulation of male sexual behavior by membrane- and nuclear-initiated estrogen signaling providing respectively an acute modulation of sexual motivation and a long-term control of the capacity to display the copulatory sequence. In parallel, a similar dichotomy applies to the regulation of brain aromatase whose expression depends on the transcriptional activity of testosterone metabolites while its enzymatic activity is rapidly regulated in a region- and context-dependent manner. Recent evidences suggest that rapid changes in sexual motivation result from rapid changes in local estrogen production. Together, these data support the idea that the acute regulation of some aspects of male sexual behavior depends not only on classical neurotransmitter systems, but also on rapid and spatially restricted changes in local estrogen availability. The existing literature suggests that this acute regulation by neuro-estrogens of the motivational aspects of behavior could be generalized to other systems such as singing behavior in songbirds.

Pattern of aromatase mRNA expression in the brain of a weakly electric fish, Apteronotus leptorhynchus

Aromatase is a steroidogenic enzyme involved in the conversion of testosterone into estradiol. Teleosts are unique among vertebrates in possessing two distinct aromatase genes that show different expression patterns within the body. Since the brain is the essential organ underlying the control of behavior, an understanding of the expression pattern of aromatase in the brain can help to identify neural circuits and behaviors that are most likely to be affected by aromatase activity. In addition, identifying species differences in aromatase expression in the brain can further our understanding of divergence in behaviors regulated by local estradiol production and estrogen signaling. Apteronotus leptorhynchus is a species of weakly electric fish in which little is known about sex steroid expression within the brain and its role in electric signaling behavior. The goal of this study was to identify the mRNA expression pattern of aromatase in the brain of A. leptorhynchus. Aromatase mRNA was detected in several parts of the forebrain and in the pituitary gland; however, no aromatase expression was detected in the midbrain or hindbrain. These findings in A. leptorhynchus support a role for aromatase activity in reproduction, but no direct role in electric signaling behavior in non-breeding fish. The findings of this study help to broaden the basis for making phylogenetic comparisons of aromatase expression across teleost lineages as well as different signaling systems, and provide information on behaviors and neural circuits that are potentially affected by local estradiol production in A. leptorhynchus.

Acute effects of sex steroids on visual processing in male goldfish

Elevations of sex steroids induced by social cues can rapidly modulate social behavior, but we know little about where they act within the nervous system to produce such effects. In male goldfish, testosterone (T) rapidly increases approach responses to the visual cues of females through its conversion to estradiol. Because aromatase is expressed in the retina, we tested if T can acutely influence retina responses to visual stimuli, and investigated the receptor mechanisms that may mediate such effects. Specifically, we measured FOS protein immunoreactivity to determine if T affects cellular responses to visual stimuli that include females, and used electrophysiology to investigate whether T can generally affect light sensitivity. We found that T acutely increased FOS responses to the simultaneous onset of light and the presence of female visual stimuli, both of which would normally be associated with early morning spawning, and increased electrophysiological responses to low intensity light pulses. Both effects were blocked by an estrogen receptor beta (ERβ) antagonist, indicating that T is likely being converted to estradiol (E2) and acting through an ERβ mediated mechanism to acutely modulate visual processing. Changes in sensory processing could subsequently influence approach behavior to increase reproductive success in competitive mating environments.

Actions of Steroids: New Neurotransmitters

Over the past two decades, the classical understanding of steroid action has been updated to include rapid, membrane-initiated, neurotransmitter-like functions. While steroids were known to function on very short time spans to induce physiological and behavioral changes, the mechanisms by which these changes occur are now becoming more clear. In avian systems, rapid estradiol effects can be mediated via local alterations in aromatase activity, which precisely regulates the temporal and spatial availability of estrogens. Acute regulation of brain-derived estrogens has been shown to rapidly affect sensorimotor function and sexual motivation in birds. In rodents, estrogens and progesterone are critical for reproduction, including preovulatory events and female sexual receptivity. Membrane progesterone receptor as well as classical progesterone receptor trafficked to the membrane mediate reproductive-related hypothalamic physiology, via second messenger systems with dopamine-induced cell signals. In addition to these relatively rapid actions, estrogen membrane-initiated signaling elicits changes in morphology. In the arcuate nucleus of the hypothalamus, these changes are needed for lordosis behavior. Recent evidence also demonstrates that membrane glucocorticoid receptor is present in numerous cell types and species, including mammals. Further, membrane glucocorticoid receptor influences glucocorticoid receptor translocation to the nucleus effecting transcriptional activity. The studies presented here underscore the evidence that steroids behave like neurotransmitters to regulate CNS functions. In the future, we hope to fully characterize steroid receptor-specific functions in the brain.

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.

Glutamate released in the preoptic area during sexual behavior controls local estrogen synthesis in male quail

Estrogens are known to act rapidly, probably via membrane estrogen receptors, to induce fast effects on physiological and behavioral processes. Engaging in some of these behaviors, such as sexual behavior, results in an acute modulation of the production of estrogens in the brain by regulating the efficiency of the estrogen synthase enzyme, aromatase. We recently demonstrated that aromatase activity (AA) in the male quail brain is rapidly inhibited in discrete brain regions including the medial preoptic nucleus (POM) following exposure to a female. Evidence from in vitro studies point to glutamate release as one of the mechanisms controlling these rapid regulations of the aromatase enzyme. Here, we show that (a) the acute injection of the glutamatergic agonist kainate into the POM of anesthetized male quail inhibits AA and (b) glutamate is released in the POM during copulation. These results provide the first set of in vivo data demonstrating a role for glutamate release in the rapid control of AA in the context of sexual behavior.

Topography and Lateralized Effect of Acute Aromatase Inhibition on Auditory Processing in a Seasonal Songbird

It is increasingly recognized that brain-derived estrogens (neuroestrogens) can regulate brain physiology and behavior much faster than what was previously known from the transcriptional action of estrogens on nuclear receptors. One of the best examples of such neuromodulation by neuroestrogens concerns the acute regulation of sensory coding by the auditory cortex as demonstrated by electrophysiological studies of selected neurons in zebra finches. Yet, the spatial extent of such modulation by neuroestrogens is not known. Using functional magnetic resonance imaging, we demonstrate here that acute estrogen depletion alters within minutes auditory processing in male European starlings. These effects are confined to very specific but large areas of the auditory cortex. They are also specifically lateralized to the left hemisphere. Interestingly, the modulation of auditory responses by estrogens was much larger (both in amplitude and in topography) in March than in December or May/June. This effect was presumably independent from changes in circulating testosterone concentrations since levels of the steroid were controlled by subcutaneous implants, thus suggesting actions related to other aspects of the seasonal cycle or photoperiodic manipulations. Finally, we also show that estrogen production specifically modulates selectivity for behaviorally relevant vocalizations in a specific part of the caudomedial nidopallium. These findings confirm and extend previous conclusions that had been obtained by electrophysiological techniques. This approach provides a new very powerful tool to investigate auditory responsiveness in songbirds and its fast modulation by sex steroids.SIGNIFICANCE STATEMENT Neuroestrogens can acutely modulate sensory processing in a manner similar to neuromodulators. We report that acute estrogen depletion rapidly disrupts auditory processing in large areas of the male starling brain. Effects were larger in March than in December or May/June, lateralized to the left hemisphere and specific to behaviorally relevant stimuli. These findings confirm and extend previous data that identified an acute regulation of auditory neurons in zebra finches by (1) delineating the extent of the brain region affected, (2) confirming its lateralization, and (3) demonstrating that a large part of the auditory brain regions are acutely affected by estrogens. These findings provide a very powerful tool to investigate auditory responsiveness in songbirds and its fast modulation by sex steroids.

Aromatase inhibition rapidly affects in a reversible manner distinct features of birdsong

Recent evidence has implicated steroid hormones, specifically estrogens, in the rapid modulation of cognitive processes. Songbirds have been a useful model system in the study of complex cognitive processes including birdsong, a naturally learned vocal behavior regulated by a discrete steroid-sensitive telencephalic circuitry. Singing behavior is known to be regulated by long-term actions of estrogens but rapid steroid modulation of this behavior has never been examined. We investigated if acute actions of estrogens regulate birdsong in canaries (Serinus canaria). In the morning, male canaries sing within minutes after light onset. Birds were injected with fadrozole, a potent aromatase inhibitor, or vehicle within 2–5 minutes after lights on to implement a within-subjects experimental design. This single injection of fadrozole reduced the motivation to sing as well as song acoustic stereotypy, a measure of consistency over song renditions, on the same day. By the next day, however, all song measures that were affected had returned to baseline. This study indicates that estrogens also act in a rapid fashion to regulate two distinct features of song, a learned vocal behavior.

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.

Estrogen Receptor β Activation Rapidly Modulates Male Sexual Motivation through the Transactivation of Metabotropic Glutamate Receptor 1a

In addition to the transcriptional activity of their liganded nuclear receptors, estrogens, such as estradiol (E2), modulate cell functions, and consequently physiology and behavior, within minutes through membrane-initiated events. The membrane-associated receptors (mERs) underlying the acute effects of estrogens on behavior have mostly been documented in females where active estrogens are thought to be of ovarian origin. We determined here, by acute intracerebroventricular injections of specific agonists and antagonists, the type(s) of mERs that modulate rapid effects of brain-derived estrogens on sexual motivation in male Japanese quail. Brain aromatase blockade acutely inhibited sexual motivation. Diarylpropionitrile (DPN), an estrogen receptor β (ERβ)-specific agonist, and to a lesser extent 17α-estradiol, possibly acting through ER-X, prevented this effect. In contrast, drugs targeting ERα (PPT and MPP), GPR30 (G1 and G15), and the Gq-mER (STX) did not affect sexual motivation. The mGluR1a antagonist LY367385 significantly inhibited sexual motivation but mGluR2/3 and mGluR5 antagonists were ineffective. LY367385 also blocked the behavioral restoration induced by E2 or DPN, providing functional evidence that ERβ interacts with metabotropic glutamate receptor 1a (mGluR1a) signaling to acutely regulate male sexual motivation. Together these results show that ERβ plays a key role in sexual behavior regulation and the recently uncovered cooperation between mERs and mGluRs is functional in males where it mediates the acute effects of estrogens produced centrally in response to social stimuli. The presence of an ER-mGluR interaction in birds suggests that this mechanism emerged relatively early in vertebrate history and is well conserved. Significance statement: The membrane-associated receptors underlying the acute effects of estrogens on behavior have mostly been documented in females, where active estrogens are thought to be of ovarian origin. Using acute intracerebroventricular injections of specific agonists and antagonists following blockade of brain aromatase, we show here that brain-derived estrogens acutely facilitate male sexual motivation through the activation of estrogen receptor β interacting with the metabotropic glutamate receptor 1a. This behavioral effect occurring within minutes provides a mechanistic explanation of how an estrogen receptor not intrinsically coupled to intracellular effectors can signal from the membrane to govern behavior in a very rapid fashion. It suggests that different subtypes of estrogen receptors could regulate the motivation versus performance aspects of behavior.

Hypothalamic inhibition of socio-sexual behaviour by increasing neuroestrogen synthesis

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that inhibits gonadotropin secretion and socio-sexual behaviours. Oestrogen (neuroestrogen) synthesized in the brain from androgen by aromatase regulates male socio-sexual behaviours. Here we show that GnIH directly activates aromatase and increases neuroestrogen synthesis in the preoptic area (POA) and inhibits socio-sexual behaviours of male quail. Aromatase activity and neuroestrogen concentration in the POA are low in the morning when the birds are active, but neuroestrogen synthesis gradually increases until the evening when the birds become inactive. Centrally administered GnIH in the morning increases neuroestrogen synthesis in the POA and decreases socio-sexual behaviours. Centrally administered 17β-oestradiol at higher doses also inhibits socio-sexual behaviours in the morning. These results suggest that GnIH inhibits male socio-sexual behaviours by increasing neuroestrogen synthesis beyond its optimum concentration for the expression of socio-sexual behaviours. This is the first demonstration of any hypothalamic neuropeptide that directly regulates neuroestrogen synthesis.

Sex differences and rapid estrogen signaling: A look at songbird audition

The actions of estrogens have been associated with brain differentiation and sexual dimorphism in a wide range of vertebrates. Here we consider the actions of brain-derived 'neuroestrogens' in the forebrain and the accompanying differences and similarities observed between males and females in a variety of species. We summarize recent evidence showing that baseline and fluctuating levels of neuroestrogens within the auditory forebrain of male and female zebra finches are largely similar, and that neuroestrogens enhance auditory representations in both sexes. With a comparative perspective we review evidence that non-genomic mechanisms of neuroestrogen actions are sexually differentiated, and we propose a working model for nonclassical estrogen signaling via the MAPK intracellular signaling cascade in the songbird auditory forebrain that is informed by the way sex differences may be compensated. This view may lead to a more comprehensive understanding of how sex influences estradiol-dependent modulation of sensorimotor representations.

The dual action of estrogen hypothesis

Estradiol (E2) can act in the brain in a relatively fast manner (i.e., seconds to minutes) usually through signaling initiated at the cell membrane. Brain-derived E2 has thus been considered as another type of neurotransmitter. Recent work found that behaviors indicative of male sexual motivation are activated by estrogenic metabolites of testosterone (T) in a fast manner, while sexual performance (copulatory behavior per se) is regulated by brain E2 in a slower manner via nucleus-initiated actions. This functional division between these two types of action appears to generalize to other behavioral systems regulated by E2. We propose the dual action of estrogen hypothesis to explain this functional distinction between these two different modes of action.

Local modulation of steroid action: rapid control of enzymatic activity

Estrogens can induce rapid, short-lived physiological and behavioral responses, in addition to their slow, but long-term, effects at the transcriptional level. To be functionally relevant, these effects should be associated with rapid modulations of estrogens concentrations. 17β-estradiol is synthesized by the enzyme aromatase, using testosterone as a substrate, but can also be degraded into catechol-estrogens via hydroxylation by the same enzyme, leading to an increase or decrease in estrogens concentration, respectively. The first evidence that aromatase activity (AA) can be rapidly modulated came from experiments performed in Japanese quail hypothalamus homogenates. This rapid modulation is triggered by calcium-dependent phosphorylations and was confirmed in other tissues and species. The mechanisms controlling the phosphorylation status, the targeted amino acid residues and the reversibility seem to vary depending of the tissues and is discussed in this review. We currently do not know whether the phosphorylation of the same amino acid affects both aromatase and/or hydroxylase activities or whether these residues are different. These processes provide a new general mechanism by which local estrogen concentration can be rapidly altered in the brain and other tissues.

Effects of testosterone and estradiol on anxiety and depressive-like behavior via a non-genomic pathway

Besides their known slow genomic effects, testosterone and estradiol have rapid effects in the brain. However, their impact on mood-related behavior is not clear. The aim of this study was to investigate the non-genomic pathway of testosterone and estradiol in the amygdala in relation to anxiety and depressive-like behavior. Sham-operated and gonadectomized male rats (GDX) supplemented with testosterone propionate, estradiol, or olive oil were used. Five minutes after administration, anxiety and depression-like behavior were tested. Estradiol increased anxiolytic behavior in the open-field test compared to the GDX group, but administration of testosterone had no significant effect. Besides, c-Fos expression in the medial nucleus of the amygdala significantly increased after testosterone treatment compared to the GDX group, while no significant difference was observed in the central and the basolateral nuclei of the amygdala in the testosterone-treated group compared to the GDX group. In conclusion, estradiol had an anxiolytic effect via a rapid pathway, but no rapid effect of testosterone on anxiety was found. Further studies elucidating whether the rapid effect is mediated by a non-genomic pathway are needed.

Modelling inhibition of avian aromatase by azole pesticides

The potential effects of pesticides and their metabolites on the endocrine system are of major concern to wildlife and human health. In this context, the azole pesticides have earned special attention due to their cytochrome P450 aromatase inhibition potential. Cytochrome P450 aromatase (CYP19) catalyses the conversion of androstenedione and testosterone into oestrone and oestradiol, respectively. Thus, aromatase modulates the oestrogenic balance essential not only for females, but also for male physiology, including gonadal function. Its inhibition affects reproductive organs, fertility and sexual behaviour in humans and wildlife species. Several studies have shown that azole pesticides are able to inhibit human and fish aromatases but the information on birds is lacking. Consequently, it appeared to be of interest to estimate the aromatase inhibition of azoles in three different avian species, namely Gallus gallus, Coturnix coturnix japonica and Taeniopygia guttata. In the absence of the crystal structure of the aromatase enzyme in these bird species, homology models for the individual avian species were constructed using the crystal structure of human aromatase (hAr) (pdb: 3EQM) that showed high sequence similarity for G. gallus (82.0%), T. guttata (81.9%) and C. japonica (81.2%). A homology model with Oncorhynchus mykiss (81.9%) was also designed for comparison purpose. The homology-modelled aromatase for each avian and fish species and crystal structure of human aromatase were selected for docking 46 structurally diverse azoles and related compounds. We showed that the docking behaviour of the chemicals on the different aromatases was broadly the same. We also demonstrated that there was an acceptable level of correlation between the binding score values and the available aromatase inhibition data. This means that the homology models derived on bird and fish species can be used to approximate the potential inhibitory effects of azoles on their aromatase.

Neuroestrogen signaling in the songbird auditory cortex propagates into a sensorimotor network via an 'interface' nucleus

Neuromodulators rapidly alter activity of neural circuits and can therefore shape higher order functions, such as sensorimotor integration. Increasing evidence suggests that brain-derived estrogens, such as 17-β-estradiol, can act rapidly to modulate sensory processing. However, less is known about how rapid estrogen signaling can impact downstream circuits. Past studies have demonstrated that estradiol levels increase within the songbird auditory cortex (the caudomedial nidopallium, NCM) during social interactions. Local estradiol signaling enhances the auditory-evoked firing rate of neurons in NCM to a variety of stimuli, while also enhancing the selectivity of auditory-evoked responses of neurons in a downstream sensorimotor brain region, HVC (proper name). Since these two brain regions are not directly connected, we employed dual extracellular recordings in HVC and the upstream nucleus interfacialis of the nidopallium (NIf) during manipulations of estradiol within NCM to better understand the pathway by which estradiol signaling propagates to downstream circuits. NIf has direct input into HVC, passing auditory information into the vocal motor output pathway, and is a possible source of the neural selectivity within HVC. Here, during acute estradiol administration in NCM, NIf neurons showed increases in baseline firing rates and auditory-evoked firing rates to all stimuli. Furthermore, when estradiol synthesis was blocked in NCM, we observed simultaneous decreases in the selectivity of NIf and HVC neurons. These effects were not due to direct estradiol actions because NIf has little to no capability for local estrogen synthesis or estrogen receptors, and these effects were specific to NIf because other neurons immediately surrounding NIf did not show these changes. Our results demonstrate that transsynaptic, rapid fluctuations in neuroestrogens are transmitted into NIf and subsequently HVC, both regions important for sensorimotor integration. Overall, these findings support the hypothesis that acute neurosteroid actions can propagate within and between neural circuits to modulate their functional connectivity.

Rapid effects of estrogens on behavior: environmental modulation and molecular mechanisms

Estradiol can modulate neural activity and behavior via both genomic and nongenomic mechanisms. Environmental cues have a major impact on the relative importance of these signaling pathways with significant consequences for behavior. First we consider how photoperiod modulates nongenomic estrogen signaling on behavior. Intriguingly, short days permit rapid effects of estrogens on aggression in both rodents and song sparrows. This highlights the importance of considering photoperiod as a variable in laboratory research. Next we review evidence for rapid effects of estradiol on ecologically-relevant behaviors including aggression, copulation, communication, and learning. We also address the impact of endocrine disruptors on estrogen signaling, such as those found in corncob bedding used in rodent research. Finally, we examine the biochemical mechanisms that may mediate rapid estrogen action on behavior in males and females. A common theme across these topics is that the effects of estrogens on social behaviors vary across different environmental conditions.

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.

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.