Brain estrogen signaling effects acute modulation of acoustic communication behaviors: A working hypothesis

Differential expression of steroid-related genes across electrosensory brain regions in two sexually dimorphic species of electric knifefish

The production of communication signals can be modulated by hormones acting on the brain regions that regulate these signals. However, less is known about how signal perception is regulated by hormones. The electrocommunication signals of weakly electric fishes are sexually dimorphic, sensitive to hormones, and vary across species. The neural circuits that regulate the production and perception of these signals are also well-characterized, and electric fishes are thus an excellent model to examine the neuroendocrine regulation of sensorimotor mechanisms of communication. We investigated (1) whether steroid-related genes are expressed in sensory brain regions that process communication signals; and (2) whether this expression differs across sexes and species that have different patterns of sexual dimorphism in their signals. Apteronotus leptorhynchus and Apteronotus albifrons produce continuous electric organ discharges (EODs) that are used for communication. Two brain regions, the electrosensory lateral line lobe (ELL) and the dorsal torus semicircularis (TSd), process inputs from electroreceptors to allow fish to detect and discriminate electrocommunication signals. We used qPCR to quantify the expression of genes for two androgen receptors (ar1, ar2), two estrogen receptors (esr1, esr2b), and aromatase (cyp19a1b). Four out of five steroid-related genes were expressed in both sensory brain regions, and their expression often varied between sexes and species. These results suggest that expression of steroid-related genes in the brain may differentially influence how EOD signals are encoded across species and sexes, and that gonadal steroids may coordinately regulate central circuits that control both the production and perception of EODs.

The estrous cycle and 17β-estradiol modulate the electrophysiological properties of rat nucleus accumbens core medium spiny neurons

The nucleus accumbens core is a key nexus within the mammalian brain for integrating the premotor and limbic systems and regulating important cognitive functions such as motivated behaviors. Nucleus accumbens core functions show sex differences and are sensitive to the presence of hormones such as 17β-estradiol (estradiol) in normal and pathological contexts. The primary neuron type of the nucleus accumbens core, the medium spiny neuron (MSN), exhibits sex differences in both intrinsic excitability and glutamatergic excitatory synapse electrophysiological properties. Here, we provide a review of recent literature showing how estradiol modulates rat nucleus accumbens core MSN electrophysiology within the context of the estrous cycle. We review the changes in MSN electrophysiological properties across the estrous cycle and how these changes can be mimicked in response to exogenous estradiol exposure. We discuss in detail recent findings regarding how acute estradiol exposure rapidly modulates excitatory synapse properties in nucleus accumbens core but not caudate-putamen MSNs, which mirror the natural changes seen across estrous cycle phases. These recent insights demonstrate the strong impact of sex-specific estradiol action upon nucleus accumbens core neuron electrophysiology.

Endocrine Modulation of Sending and Receiving Signals in Context-Dependent Social Communication

Animal communication requires senders to transmit signals through the environment to conspecific receivers, which then leads to context-dependent behavioral decisions. Sending and receiving sensory information in social contexts, however, can be dramatically influenced by an individual’s internal state, particularly in species that cycle in and out of breeding or other physiological condition like nutritional state or social status. Modulatory substances like steroids, peptides, and biogenic amines can influence both the substrates used for sending social signals (e.g., motivation centers, sensorimotor pathways, and muscles) as well as the peripheral sensory organs and central neural circuitry involved in the reception of this information and subsequent execution of behavioral responses. This issue highlights research from neuroethologists on the topic of modulation of sending and receiving social signals and demonstrates that it can occur in both males and females, in different senses at both peripheral sensory organs and the brain, at different levels of biological organization, on different temporal scales, in various social contexts, and across many diverse vertebrate taxa. Modifying a signal produced by a sender or how that signal is perceived in a receiver provides flexibility in communication and has broad implications for influencing social decisions like mate choice, which ultimately affects reproductive fitness and species persistence. This phenomenon of modulators and internal physiological state impacting communication abilities is likely more widespread than currently realized and we hope this issue inspires others working on diverse systems to examine this topic from different perspectives. An integrative and comparative approach will advance discovery in this field and is needed to better understand how endocrine modulation contributes to sexual selection and the evolution of animal communication in general.

Reproductive- and Social-State Plasticity of Multiple Sensory Systems in a Cichlid Fish

Intra- and inter-sexual communications are vital to the survival and reproductive success of animals. In species that cycle in and out of breeding or other physiological condition, sensory function can be modulated to optimize communication at crucial times. Little is known, however, about how widespread this sensory plasticity is across taxa, whether it occurs in multiple senses or both sexes within a species, and what potential modulatory substances and substrates are involved. Thus, studying modulation of sensory communication in a single species can provide valuable insights for understanding how sensory abilities can be altered to optimize detection of salient signals in different sensory channels and social contexts. The African cichlid fish Astatotilapia burtoni uses multimodal communication in social contexts such as courtship, territoriality, and parental care and shows plasticity in sensory abilities. In this review, we synthesize what is known about how visual, acoustic, and chemosensory communication is used in A. burtoni in inter- and intra-specific social contexts, how sensory funtion is modulated by an individual's reproductive, metabolic, and social state, and discuss evidence for plasticity in potential modulators that may contribute to changes in sensory abilities and behaviors. Sensory plasticity in females is primarily associated with the natural reproductive cycle and functions to improve detection of courtship signals (visual, auditory, chemosensory, and likely mechanosensory) from high-quality males for reproduction. Plasticity in male sensory abilities seems to function in altering their ability to detect the status of other males in the service of territory ownership and future reproductive opportunities. Changes in different classes of potential modulators or their receptors (steroids, neuropeptides, and biogenic amines) occur at both peripheral sensory organs (eye, inner ear, and olfactory epithelium) and central visual, olfactory, and auditory processing regions, suggesting complex mechanisms contributing to plasticity of sensory function. This type of sensory plasticity revealed in males and females of A. burtoni is likely more widespread among diverse animals than currently realized, and future studies should take an integrative and comparative approach to better understand the proximate and ultimate mechanisms modulating communication abilities across taxa.

Life-Stage Dependent Plasticity in the Auditory System of a Songbird Is Signal and Emitter-Specific

Social animals flexibly use a variety of vocalizations to communicate in complex and dynamic environments. However, it remains unknown whether the auditory perception of different vocalizations changes according to the ecological context. By using miniature wireless devices to synchronously record vocal interactions and local neural activity in freely-behaving zebra finches in combination with playback experiments, we investigate whether the auditory processing of vocalizations changes across life-history stages. We show that during breeding, females (but not males) increase their estrogen levels and reply faster to their mates when interacting vocally. These changes are associated with an increase in the amplitude of the female’s neural auditory responses. Furthermore, the changes in auditory response are not general, but specific to a subset of functionally distinct vocalizations and dependent on the emitter’s identity. These results provide novel insights into auditory plasticity of communication systems, showing that the perception of specific signals can shift according to ecologically-determined physiological states.

The difference a day makes: Breeding remodels hearing, hormones and behavior in female Cope's gray treefrogs (Hyla chrysoscelis)

In seasonal breeders, there are behavioral, endocrine, and neural adaptations that promote the sexual receptivity of females and tune their sensory systems to detect and discriminate among advertising males and to successfully copulate. What happens immediately after this key life history event is unclear, but this transitional moment offers a window into the mechanisms that remodel sexual phenotypes. In this study of wild female Cope's gray treefrogs (Hyla chrysoscelis), we tested the hypothesis that oviposition results in a suite of coordinated changes in the sexual phenotype. Specifically, we predicted that sexual receptivity and discrimination behaviors would decline along with circulating concentrations of steroid hormones (corticosterone, estradiol, testosterone) and auditory sensitivity to the acoustic frequencies emphasized in male advertisement calls. We conducted these trait measurements before and after oviposition (ca. 24-h period). There was a 100% decrease in behavioral responsiveness after oviposition, and the concentrations of all three steroids plummeted during this brief window of time, especially testosterone. Moreover, higher concentrations of corticosterone-an important component of the endocrine stress response-were associated with longer response latencies, suggesting that adrenal hormones should be considered in future studies on the hormonal basis of mate choice. Counter to our prediction, auditory sensitivity increased following oviposition, and the amplitude of the auditory brainstem response was influenced by concentrations of estradiol. In pre-oviposition females auditory sensitivity diminished with increasing estradiol concentrations, while sensitivity increased with increasing estradiol concentrations in post-oviposition females, suggesting non-linear estrogenic modulation of peripheral auditory neural recruitment. Overall, our results indicate that there is considerable remodeling of behavioral output following oviposition that co-occurs with changes in both endocrine and sensory physiology.

The expression of select genes necessary for membrane-associated estrogen receptor signaling differ by sex in adult rat hippocampus

17β-estradiol can rapidly modulate neuron function via membrane estrogen receptors (ERs) in a sex-specific manner. For example, female rat hippocampal neurons express palmitoylated versions of ERα and ERβ that associate with the plasma membrane. These membrane-associated ERs are organized by caveolin proteins into functional signaling microdomains with metabotropic glutamate receptors (mGluRs). ER/mGluR signaling mediates several sex-specific estradiol actions on hippocampal neuron function. An important unanswered question regards the mechanism by which sex-specific membrane-associated ER signaling is generated, especially since it has been previously demonstrated that mGluR action is not sex-specific. One possibility is that the genes necessary for the ER membrane complex are differentially expressed between males and females, including genes that encode ERα and β, caveolin 1 and 3, and/or the palmitoylacyltransferases DHHC-7 and -21. Thus we used qPCR to test the hypothesis that these genes show sex differences in expression in neonatal and adult rat hippocampus. As an additional control we tested the expression of the 20 other DHHC palmitoylacyltransferases with no known connections to ER. In neonatal hippocampus, no sex differences were detected in gene expression. In adult hippocampus, the genes that encode caveolin 1 and DHHC-7 showed decreased expression in females compared to males. Thus, select genes differ by sex at specific developmental stages, arguing for a more nuanced model than simple widespread perinatal emergence of sex differences in all genes enabling sex-specific estradiol action. These findings enable the generation of new hypotheses regarding the mechanisms by which sex differences in membrane-associated ER signaling are programmed.

Rapid changes in auditory processing in songbirds following acute aromatase inhibition as assessed by fMRI

Contribution to Special Issue on Fast effects of steroids. This review introduces functional MRI (fMRI) as an outstanding tool to assess rapid effects of sex steroids on auditory processing in seasonal songbirds. We emphasize specific advantages of this method as compared to other more conventional and invasive methods used for this purpose and summarize an exemplary auditory fMRI study performed on male starlings exposed to different types of starling song before and immediately after the inhibition of aromatase activity by an i.p. injection of Vorozole™. We describe how most challenges that relate to the necessity to anesthetize subjects and minimize image- and sound-artifacts can be overcome in order to obtain a voxel-based 3D-representation of changes in auditory brain activity to various sound stimuli before and immediately after a pharmacologically-induced depletion of endogenous estrogens. Analysis of the fMRI data by assumption-free statistical methods identified fast specific changes in activity in the auditory brain regions that were stimulus-specific, varying over different seasons, and in several instances lateralized to the left side of the brain. This set of results illustrates the unique features of fMRI that provides opportunities to localize and quantify the brain responses to rapid changes in hormonal status. fMRI offers a new image-guided research strategy in which the spatio-temporal profile of fast neuromodulations can be identified and linked to specific behavioral inputs or outputs. This approach can also be combined with more localized invasive methods to investigate the mechanisms underlying the observed neural changes.

Neuroestrogens rapidly shape auditory circuits to support communication learning and perception: Evidence from songbirds

Contribution to Special Issue on Fast effects of steroids. Steroid hormones, such as estrogens, were once thought to be exclusively synthesized in the ovaries and enact transcriptional changes over the course of hours to days. However, estrogens are also locally synthesized within neural circuits, wherein they rapidly (within minutes) modulate a range of behaviors, including spatial cognition and communication. Here, we review the role of brain-derived estrogens (neuroestrogens) as modulators within sensory circuits in songbirds. We first present songbirds as an attractive model to explore how neuroestrogens in auditory cortex modulate vocal communication processing and learning. Further, we examine how estrogens may enhance vocal learning and auditory memory consolidation in sensory cortex via mechanisms similar to those found in the hippocampus of rodents and birds. Finally, we propose future directions for investigation, including: 1) the extent of developmental and hemispheric shifts in aromatase and membrane estrogen receptor expression in auditory circuits; 2) how neuroestrogens may impact inhibitory interneurons to regulate audition and critical period plasticity; and, 3) dendritic spine plasticity as a candidate mechanism mediating estrogen-dependent effects on vocal learning. Together, this perspective of estrogens as neuromodulators in the vertebrate brain has opened new avenues in understanding sensory plasticity, including how hormones can act on communication circuits to influence behaviors in other vocal learning species, such as in language acquisition and speech processing in humans.

Reciprocal interactions between prostaglandin E2- and estradiol-dependent signaling pathways in the injured zebra finch brain

Background

Astrocytic aromatization and consequent increases in estradiol are neuroprotective in the injured brain. In zebra finches, cyclooxygenase-activity is necessary for injury-induced aromatase expression, and increased central estradiol lowers neuroinflammation. The mechanisms underlying these influences are unknown. Here, we document injury-induced, cyclooxygenase-dependent increases in glial aromatase expression and replicate previous work in our lab showing increases in central prostaglandin E2 and estradiol following brain damage. Further, we describe injury-dependent changes in E-prostanoid and estrogen receptor expression and reveal the necessity of E-prostanoid and estrogen receptors in the injury-dependent, reciprocal interactions of neuroinflammatory and neurosteroidogenic pathways.

Methods

Adult male and female birds were shams or received bilateral injections of the appropriate drug or vehicle into contralateral telencephalic lobes.

Results

Injuries sustained in the presence of indomethacin (a cyclooxygenase inhibitor) had fewer aromatase-expressing reactive astrocytes relative to injuries injected with vehicle suggesting that cyclooxygenase activity is necessary for the induction of glial aromatase around the site of damage. Injured hemispheres had higher prostaglandin E2 and estradiol content relative to shams. Importantly, injured hemispheres injected with E-prostanoid- or estrogen receptor-antagonists showed elevated prostaglandin E2 and estradiol, respectively, but lower prostaglandin E2 or estradiol-dependent downstream activity (protein kinase A or phosphoinositide-3-kinase mRNA) suggesting that receptor antagonism did not affect injury-induced prostaglandin E2 or estradiol, but inhibited the effects of these ligands. Antagonism of E-prostanoid receptors 3 or 4 prevented injury-induced increases in neural estradiol in males and females, respectively, albeit this apparent sex-difference needs to be tested more stringently. Further, estrogen receptor-α, but not estrogen receptor-β antagonism, exaggerated neural prostaglandin E2 levels relative to the contralateral lobe in both sexes.

Conclusion

These data suggest injury-induced, sex-specific prostaglandin E2-dependent estradiol synthesis, and estrogen receptor-α dependent decreases in neuroinflammation in the vertebrate brain.

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.

Differential sensitivity to estrogen-induced opsin expression in two poeciliid freshwater fish species

The sensory system shapes an individual's perception of the world, including social interactions with conspecifics, habitat selection, predator detection, and foraging behavior. Sensory signaling can be modulated by steroid hormones, making these processes particularly vulnerable to environmental perturbations. Here we examine the influence of exogenous estrogen manipulation on the visual physiology of female western mosquitofish (Gambusia affinis) and sailfin mollies (Poecilia latipinna), two poeciliid species that inhabit freshwater environments across the southern United States. We conducted two experiments to address this aim. First, we exposed females from both species to a one-week dose response experiment with three treatments of waterborne β-estradiol. Next, we conducted a one-week estrogen manipulation experiment with a waterborne estrogen (β-Estradiol), a selective estrogen receptor modulator (tamoxifen), or combination estrogen and tamoxifen treatment. We used quantitative PCR (qPCR) to examine the expression of cone opsins (SWS1, SWS2b, SWS2a, Rh2, LWS), rhodopsin (Rh1), and steroid receptor genes (ARα, ARβ, ERα, ERβ2, GPER) in the eyes of individual females from each species. Results from the dose response experiment revealed estradiol-sensitivity in opsin (SWS2a, Rh2, Rh1) and androgen receptor (ARα, ARβ) gene expression in mosquitofish females, but not sailfins. Meanwhile, our estrogen receptor modulation experiments revealed estrogen sensitivity in LWS opsin expression in both species, along with sensitivity in SWS1, SWS2b, and Rh2 opsins in mosquitofish. Comparisons of control females across experiments reveal species-level differences in opsin expression, with mosquitofish retinas dominated by short-wavelength sensitive opsins (SWS2b) and sailfins retinas dominated by medium- and long-wavelength sensitive opsins (Rh2 and LWS). Our research suggests that variation in exogenous levels of sex hormones within freshwater environments can modify the visual physiology of fishes in a species-specific manner.

Effects of estradiol on neural responses to social signals in female túngara frogs

Estradiol plays an important role in mediating changes in female sexual behavior across reproductive cycles. In the túngara frog [Physalaemus (=Engystomops) pustulosus], the relationship between gonadal activity and female sexual behavior, as expressed by phonotaxis, is mediated primarily by estradiol. Estradiol receptors are expressed in auditory and motivational brain areas and the hormone could serve as an important modulator of neural responses to conspecific calls. To better understand how estradiol modifies neural responses to conspecific social signals, we manipulated estradiol levels and measured expression of the immediate early gene egr-1 in the auditory midbrain, thalamus and limbic forebrain in response to conspecific or heterospecific calls. We found that estradiol and conspecific calls increased egr-1 expression in the auditory midbrain and limbic forebrain, but in the thalamus, only conspecific calls were effective. In the preoptic area, estradiol enhanced the effect of the conspecific call on egr-1 expression, suggesting that the preoptic area could act as a hormonal gatekeeper to phonotaxis. Overall, the results suggest that estradiol has broad influences on the neural circuit involved in female reproduction, particularly those implicated in phonotaxis.

Neurohormones, Brain, and Behavior: A Comparative Approach to Understanding Rapid Neuroendocrine Action

The definition of a hormone has been in part delineated by its journey to distant receptor targets. Following activation of a receptor, a subsequent reaction facilitates the regulation of physiology and, ultimately, behavior. However, a growing number of studies report that hormones can influence these events at a previously underappreciated high speed. With the potential to act as neurotransmitters, the definition of a hormone and its mechanisms of action are evolving. In this symposium, we united scientists who use contemporary molecular, electrophysiological, and biochemical approaches to study aspects of rapid hormone action in a broad array of systems across different levels of biological organization. What emerged was an overwhelming consensus that the use of integrative and comparative approaches fuels discovery and increases our understanding of de novo hormone synthesis, local actions of neurohormones, and subsequent effects on neuroplasticity and behavior.

Male fidelity expressed through rapid testosterone suppression of ultrasonic vocalizations to novel females in the monogamous California mouse

The steroid hormone testosterone (T) is a well-known mediator of male sexual behavior in vertebrates. However, less is known about T's rapid effects on sexual behavior, particularly those involving ultrasonic vocalizations (USVs), a mode of communication that can influence mate acquisition in rodents. Using the monogamous California mouse, Peromyscus californicus, we tested whether T rapidly alters male USV production by giving T or saline injections to non-paired (sexually naïve) males and paired (paternally experienced and pair-bonded) males immediately prior to a brief exposure to an unrelated, novel female. Among non-paired males, no differences in the total number of USVs were observed; however, T increased the proportion of simple sweeps produced. Among paired males, T decreased the number of USVs produced, and this change was driven by a reduction in simple sweeps. These results suggest a differential rapid effect of T pulses between non-paired and paired males upon exposure to a novel female. Additionally, we observed a positive correlation in the production of USVs made between males and novel females, and this relationship was altered by T. Given the importance of USVs in sexual communication, our study supports an essential concept of monogamy in that mate fidelity is reinforced by decreased responsiveness to prospective mates outside of the pair bond. The central mechanism in pair bonded males that decreases their responsiveness to novel females appears to be one that T can trigger. This is among the first studies to demonstrate that T can inhibit sexually related behaviors and do so rapidly.

Dynamic variation in forebrain estradiol levels during song learning

Estrogens shape brain circuits during development, and the capacity to synthesize estrogens locally has consequences for both sexual differentiation and the acute modulation of circuits during early learning. A recently optimized method to detect and quantify fluctuations in brain estrogens in vivo provides a direct means to explore how brain estrogen production contributes to both differentiation and neuromodulation during development. Here, we use this method to test the hypothesis that neuroestrogens are sexually differentiated as well as dynamically responsive to song tutoring (via passive video/audio playback) during the period of song learning in juvenile zebra finches. Our results show that baseline neuroestradiol levels in the caudal forebrain do not differ between males and females during an early critical masculinization window. Instead, we observe a prominent difference between males and females in baseline neuroestradiol that emerges during the subadult stage as animals approach sexual maturity. Second, we observe that fluctuating neuroestradiol levels during periods of passive song tutoring exhibit a markedly different profile in juveniles as compared to adults. Specifically, neuroestrogens in the caudal forebrain are elevated following (rather than during) tutor song exposure in both juvenile males and females, suggesting an important role for the early consolidation of tutor song memories. These results further reveal a circadian influence on the fluctuations in local neuroestrogens during sensory/cognitive tasks. Taken together, these findings uncover several unexpected features of brain estrogen synthesis in juvenile animals that may have implications for secondary masculinization as well as the consolidation of recent sensory experiences.

Frank Beach Award Winner: Steroids as neuromodulators of brain circuits and behavior

Neurons communicate primarily via action potentials that transmit information on the timescale of milliseconds. Neurons also integrate information via alterations in gene transcription and protein translation that are sustained for hours to days after initiation. Positioned between these two signaling timescales are the minute-by-minute actions of neuromodulators. Over the course of minutes, the classical neuromodulators (such as serotonin, dopamine, octopamine, and norepinephrine) can alter and/or stabilize neural circuit patterning as well as behavioral states. Neuromodulators allow many flexible outputs from neural circuits and can encode information content into the firing state of neural networks. The idea that steroid molecules can operate as genuine behavioral neuromodulators - synthesized by and acting within brain circuits on a minute-by-minute timescale - has gained traction in recent years. Evidence for brain steroid synthesis at synaptic terminals has converged with evidence for the rapid actions of brain-derived steroids on neural circuits and behavior. The general principle emerging from this work is that the production of steroid hormones within brain circuits can alter their functional connectivity and shift sensory representations by enhancing their information coding. Steroids produced in the brain can therefore change the information content of neuronal networks to rapidly modulate sensory experience and sensorimotor functions.

Relationships between rapid changes in local aromatase activity and estradiol concentrations in male and female quail brain

Estradiol-17β (E2) synthesized in the brain plays a critical role in the activation of sexual behavior in many vertebrate species. Because E2 concentrations depend on aromatization of testosterone, changes in aromatase enzymatic activity (AA) are often utilized as a proxy to describe E2 concentrations. Utilizing two types of stimuli (sexual interactions and acute restraint stress) that have been demonstrated to reliably alter AA within minutes in opposite directions (sexual interactions=decrease, stress=increase), we tested in Japanese quail whether rapid changes in AA are paralleled by changes in E2 concentrations in discrete brain areas. In males, E2 in the pooled medial preoptic nucleus/medial portion of the bed nucleus of the stria terminalis (POM/BST) positively correlated with AA following sexual interactions. However, following acute stress, E2 decreased significantly (approximately 2-fold) in the male POM/BST despite a significant increase in AA. In females, AA positively correlated with E2 in both the POM/BST and mediobasal hypothalamus supporting a role for local, as opposed to ovarian, production regulating brain E2 concentrations. In addition, correlations of individual E2 in POM/BST and measurements of female sexual behavior suggested a role for local E2 synthesis in female receptivity. These data demonstrate that local E2 in the male brain changes in response to stimuli on a time course suggestive of potential non-genomic effects on brain and behavior. Overall, this study highlights the complex mechanisms regulating local E2 concentrations including rapid stimulus-driven changes in production and stress-induced changes in catabolism.

Palmitoylation of estrogen receptors is essential for neuronal membrane signaling

In addition to activating nuclear estrogen receptor signaling, 17β-estradiol can also regulate neuronal function via surface membrane receptors. In various brain regions, these actions are mediated by the direct association of estrogen receptors (ERs) activating metabotropic glutamate receptors (mGluRs). These ER/mGluR signaling partners are organized into discrete functional microdomains via caveolin proteins. A central question that remains concerns the underlying mechanism by which these subpopulations of ERs are targeted to the surface membrane. One candidate mechanism is S-palmitoylation, a posttranscriptional modification that affects the subcellular distribution and function of the modified protein, including promoting localization to membranes. Here we test for the role of palmitoylation and the necessity of specific palmitoylacyltransferase proteins in neuronal membrane ER action. In hippocampal neurons, pharmacological inhibition of palmitoylation eliminated 17β-estradiol-mediated phosphorylation of cAMP response element-binding protein, a process dependent on surface membrane ERs. In addition, mutation of the palmitoylation site on estrogen receptor (ER) α blocks ERα-mediated cAMP response element-binding protein phosphorylation. Similar results were obtained after mutation of the palmitoylation site on ERβ. Importantly, mutation of either ERα or ERβ did not affect the ability of the reciprocal ER to signal at the membrane. In contrast, membrane ERα and ERβ signaling were both dependent on the expression of the palmitoylacyltransferase proteins DHHC-7 and DHHC-21. Neither mGluR activity nor caveolin or ER expression was affected by knockdown of DHHC-7 and DHHC-21. These data collectively suggest discrete mechanisms that regulate specific isoform or global membrane ER signaling in neurons separate from mGluR activity or nuclear ER function.

Recent evidence for rapid synthesis and action of oestrogens during auditory processing in a songbird

It is now clear that oestrogens are not only circulating reproductive hormones, but that they also have neurotransmitter-like properties in a wide range of brain circuits. The view of oestrogens as intrinsic neuromodulators that shape behaviour has been bolstered by a series of recent developments from multiple vertebrate model systems. Here, we review several recent findings from studies of songbirds showing how the identified neural circuits that govern auditory processing and sensorimotor integration are modulated by the local and acute production of oestrogens. First, studies using in vivo microdialysis demonstrate that oestrogens fluctuate in the auditory cortex (30-min time bin resolution) when songbirds are hearing song and interacting with conspecifics. Second, oestrogens rapidly boost the auditory-evoked activity of neurones in the same auditory cortical region, enhancing auditory processing. Third, local pharmacological blockade of oestrogen signalling in this region impairs auditory neuronal responsiveness, as well as behavioural song preferences. Fourth, the rapid actions of oestrogens that occur within the auditory cortex can propagate downstream (trans-synaptically) to sensorimotor circuits to enhance the neural representation of song. Lastly, we present new evidence showing that the receptor for the rapid actions of oestradiol is likely in neuronal membranes, and that traditional nuclear oestrogen receptor agonists do not mimic these rapid actions. Broadly speaking, many of these findings are observed in both males and females, emphasising the fundamental importance of oestrogens in neural circuit function. Together, these and other emergent studies provide support for rapid, brain-derived oestrogen signalling in regulating sensorimotor integration, learning and perception.

Localization and divergent profiles of estrogen receptors and aromatase in the vocal and auditory networks of a fish with alternative mating tactics

Estrogens play a salient role in the development and maintenance of both male and female nervous systems and behaviors. The plainfin midshipman (Porichthys notatus), a teleost fish, has two male reproductive morphs that follow alternative mating tactics and diverge in multiple somatic, hormonal, and neural traits, including the central control of morph-specific vocal behaviors. After we identified duplicate estrogen receptors (ERβ1 and ERβ2) in midshipman, we developed antibodies to localize protein expression in the central vocal-acoustic networks and saccule, the auditory division of the inner ear. As in other teleost species, ERβ1 and ERβ2 were robustly expressed in the telencephalon and hypothalamus in vocal-acoustic and other brain regions shown previously to exhibit strong expression of ERα and aromatase (estrogen synthetase, CYP19) in midshipman. Like aromatase, ERβ1 label colocalized with glial fibrillary acidic protein (GFAP) in telencephalic radial glial cells. Quantitative polymerase chain reaction revealed similar patterns of transcript abundance across reproductive morphs for ERβ1, ERβ2, ERα, and aromatase in the forebrain and saccule. In contrast, transcript abundance for ERs and aromatase varied significantly between morphs in and around the sexually polymorphic vocal motor nucleus (VMN). Together, the results suggest that VMN is the major estrogen target within the estrogen-sensitive hindbrain vocal network that directly determines the duration, frequency, and amplitude of morph-specific vocalizations. Comparable regional differences in steroid receptor abundances likely regulate morph-specific behaviors in males and females of other species exhibiting alternative reproductive tactics.

Estradiol modulates neurotransmitter concentrations in the developing zebra finch song system

The neural song system in zebra finches is highly sexually dimorphic; only males sing and the brain regions controlling song are far larger in males than females. Estradiol (E2) administered during development can partially masculinize both structure and function. However, additional mechanisms, including those through which E2 may act, remain unclear. Male and female zebra finches were treated with E2 or control vehicle from post-hatching days 3 through 25, at which time norepinephrine (NE), dopamine (DA) and serotonin (5-HT) were measured in individual nuclei of the song system. Main effects of sex were not detected. However, E2 increased NE in the robust nucleus of the arcopallium (RA). In HVC (proper name), the hormone decreased 5-HT across the two sexes and increased DA in females only. These effects suggest that, while baseline levels of these neurotransmitters may not contribute to sexually dimorphic development of the song system, they could play specific roles in functions common to both sexes and/or in modification of the song system by exogenous E2.

The incentive salience of courtship vocalizations: hormone-mediated 'wanting' in the auditory system

Conspecific vocalizations differ from many other sounds in that they have natural incentive salience. Our thinking about auditory responses to vocalizations may therefore benefit from models originally developed to understand reward. According to those models, the brain attributes incentive salience to rewarding stimuli via the activity of monoaminergic neuromodulators. These neuromodulators, in turn, mediate the effects of experience and internal state. Songbirds lend themselves well to this discussion because the natural incentive salience of song is clearly modulated by both factors. Their auditory responses have been well-studied, particularly the song-induced expression of plasticity-associated genes such as ZENK. Here I review evidence that ZENK responses to song are regulated by monoamine neuromodulators, and I interpret this evidence in the context of incentive salience. First, hearing conspecific song engages monoaminergic activity in the auditory system and elsewhere. Second, in females this activity may be regulated by the same hormones that regulate behavioral preferences for song. Finally, much of the evidence thought to implicate neuromodulators in song discrimination and memory suggests that they may affect incentive salience. Expanding the study of incentive salience beyond the mesolimbic reward system may reveal some new ways of thinking about its underlying neural basis. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".