Forebrain steroid levels fluctuate rapidly during social interactions

Canonical Wnt signaling is necessary for object recognition memory consolidation

Wnt signaling has emerged as a potent regulator of hippocampal synaptic function, although no evidence yet supports a critical role for Wnt signaling in hippocampal memory. Here, we sought to determine whether canonical β-catenin-dependent Wnt signaling is necessary for hippocampal memory consolidation. Immediately after training in a hippocampal-dependent object recognition task, mice received a dorsal hippocampal (DH) infusion of vehicle or the canonical Wnt antagonist Dickkopf-1 (Dkk-1; 50, 100, or 200 ng/hemisphere). Twenty-four hours later, mice receiving vehicle remembered the familiar object explored during training. However, mice receiving Dkk-1 exhibited no memory for the training object, indicating that object recognition memory consolidation is dependent on canonical Wnt signaling. To determine how Dkk-1 affects canonical Wnt signaling, mice were infused with vehicle or 50 ng/hemisphere Dkk-1 and protein levels of Wnt-related proteins (Dkk-1, GSK3β, β-catenin, TCF1, LEF1, Cyclin D1, c-myc, Wnt7a, Wnt1, and PSD95) were measured in the dorsal hippocampus 5 min or 4 h later. Dkk-1 produced a rapid increase in Dkk-1 protein levels and a decrease in phosphorylated GSK3β levels, followed by a decrease in β-catenin, TCF1, LEF1, Cyclin D1, c-myc, Wnt7a, and PSD95 protein levels 4 h later. These data suggest that alterations in Wnt/GSK3β/β-catenin signaling may underlie the memory impairments induced by Dkk-1. In a subsequent experiment, object training alone rapidly increased DH GSK3β phosphorylation and levels of β-catenin and Cyclin D1. These data suggest that canonical Wnt signaling is regulated by object learning and is necessary for hippocampal memory consolidation.

Estrogenic modulation of auditory processing: a vertebrate comparison

Sex-steroid hormones are well-known regulators of vocal motor behavior in several organisms. A large body of evidence now indicates that these same hormones modulate processing at multiple levels of the ascending auditory pathway. The goal of this review is to provide a comparative analysis of the role of estrogens in vertebrate auditory function. Four major conclusions can be drawn from the literature: First, estrogens may influence the development of the mammalian auditory system. Second, estrogenic signaling protects the mammalian auditory system from noise- and age-related damage. Third, estrogens optimize auditory processing during periods of reproductive readiness in multiple vertebrate lineages. Finally, brain-derived estrogens can act locally to enhance auditory response properties in at least one avian species. This comparative examination may lead to a better appreciation of the role of estrogens in the processing of natural vocalizations and mayprovide useful insights toward alleviating auditory dysfunctions emanating from hormonal imbalances.

Storing maternal memories: hypothesizing an interaction of experience and estrogen on sensory cortical plasticity to learn infant cues

Much of the literature on maternal behavior has focused on the role of infant experience and hormones in a canonical subcortical circuit for maternal motivation and maternal memory. Although early studies demonstrated that the cerebral cortex also plays a significant role in maternal behaviors, little has been done to explore what that role may be. Recent work though has provided evidence that the cortex, particularly sensory cortices, contains correlates of sensory memories of infant cues, consistent with classical studies of experience-dependent sensory cortical plasticity in non-maternal paradigms. By reviewing the literature from both the maternal behavior and sensory cortical plasticity fields, focusing on the auditory modality, we hypothesize that maternal hormones (predominantly estrogen) may act to prime auditory cortical neurons for a longer-lasting neural trace of infant vocal cues, thereby facilitating recognition and discrimination. This couldthen more efficiently activate the subcortical circuit to elicit and sustain maternal behavior.

Insights into rapid modulation of neuroplasticity by brain estrogens

Converging evidence from cellular, electrophysiological, anatomic, and behavioral studies suggests that the remodeling of synapse structure and function is a critical component of cognition. This modulation of neuroplasticity can be achieved through the actions of numerous extracellular signals. Moreover, it is thought that it is the integration of different extracellular signals regulation of neuroplasticity that greatly influences cognitive function. One group of signals that exerts powerful effects on multiple neurologic processes is estrogens. Classically, estrogens have been described to exert their effects over a period of hours to days. However, there is now increasing evidence that estrogens can rapidly influence multiple behaviors, including those that require forebrain neural circuitry. Moreover, these effects are found in both sexes. Critically, it is now emerging that the modulation of cognition by rapid estrogenic signaling is achieved by activation of specific signaling cascades and regulation of synapse structure and function, cumulating in the rewiring of neural circuits. The importance of understanding the rapid effects of estrogens on forebrain function and circuitry is further emphasized as investigations continue to consider the potential of estrogenic-based therapies for neuropathologies. This review focuses on how estrogens can rapidly influence cognition and the emerging mechanisms that underlie these effects. We discuss the potential sources and the biosynthesis of estrogens within the brain and the consequences of rapid estrogenic-signaling on the remodeling of neural circuits. Furthermore, we argue that estrogens act via distinct signaling pathways to modulate synapse structure and function in a manner that may vary with cell type, developmental stage, and sex. Finally, we present a model in which the coordination of rapid estrogenic-signaling and activity-dependent stimuli can result in long-lasting changes in neural circuits, contributing to cognition, with potential relevance for the development of novel estrogenic-based therapies for neurodevelopmental or neurodegenerative disorders.

Estradiol differentially affects auditory recognition and learning according to photoperiodic state in the adult male songbird, European starling (Sturnus vulgaris)

Changes in hormones can affect many types of learning in vertebrates. Adults experience fluctuations in a multitude of hormones over a temporal scale, from local, rapid action to more long-term, seasonal changes. Endocrine changes during development can affect behavioral outcomes in adulthood, but how learning is affected in adults by hormone fluctuations experienced during adulthood is less understood. Previous reports have implicated the sex steroid hormone estradiol (E2) in both male and female vertebrate cognitive functioning. Here, we examined the effects of E2 on auditory recognition and learning in male European starlings (Sturnus vulgaris). European starlings are photoperiodic, seasonally breeding songbirds that undergo different periods of reproductive activity according to annual changes in day length. We simulated these reproductive periods, specifically 1. photosensitivity, 2. photostimulation, and 3. photorefractoriness in captive birds by altering day length. During each period, we manipulated circulating E2 and examined multiple measures of learning. To manipulate circulating E2, we used subcutaneous implants containing either 17-β E2 and/or fadrozole (FAD), a highly specific aromatase inhibitor that suppresses E2 production in the body and the brain, and measured the latency for birds to learn and respond to short, male conspecific song segments (motifs). We report that photostimulated birds given E2 had higher response rates and responded with better accuracy than those given saline controls or FAD. Conversely, photosensitive, animals treated with E2 responded with less accuracy than those given FAD. These results demonstrate how circulating E2 and photoperiod can interact to shape auditory recognition and learning in adults, driving it in opposite directions in different states.

Genome-brain-behavior interdependencies as a framework to understand hormone effects on learned behavior

Hormones have profound effects on the maturation and function of the zebra finch song system. Hormones often signal through receptors that directly or indirectly regulate transcription. In this way, hormones and the genome are functionally connected. Genome-brain-behavior interdependencies are often studied on evolutionary timescales but we can now apply and test these relationships on short timescales, relevant to an individual. Here, we begin to place patterns of hormone-related gene expression into the timeframe of an individual's lifespan to consider how hormones contribute to organization of neural systems necessary for learned behavior, and how they might signal during experience in ways that affect future behavior. This framework illustrates both how much investigations into genome and hormone function are intertwined, and how much we still need to learn.

Rapid modulation of aromatase activity in the vertebrate brain

Numerous steroid hormones, including 17β-estradiol (E2), activate rapid and transient cellular, physiological, and behavioral changes in addition to their well-described genomic effects. Aromatase is the key-limiting enzyme in the production of estrogens, and the rapid modulation of this enzymatic activity could produce rapid changes in local E2 concentrations. The mechanisms that might mediate such rapid enzymatic changes are not fully understood but are currently under intense scrutiny. Recent studies in our laboratory indicate that brain aromatase activity is rapidly inhibited by an increase in intracellular calcium concentration resulting from potassium-induced depolarization or from the activation of glutamatergic receptors. Phosphorylating conditions also reduce aromatase activity within minutes, and this inhibition is blocked by the addition of multiple protein kinase inhibitors. This rapid modulation of aromatase activity by phosphorylating conditions is a general mechanism observed in different cell types and tissues derived from a variety of species, including human aromatase expressed in various cell lines. Phosphorylation processes affect aromatase itself and do not involve changes in aromatase protein concentration. The control of aromatase activity by multiple kinases suggests that several amino acids must be concomitantly phosphorylated to modify enzymatic activity but site-directed mutagenesis of several amino acids alone or in combination has not to date revealed the identity of the targeted residue(s). Altogether, the phosphorylation processes affecting aromatase activity provide a new general mechanism by which the concentration of estrogens can be rapidly altered in the brain.

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.

New biosynthesis and biological actions of avian neurosteroids

De novo neurosteroidogenesis from cholesterol occurs in the brain of various avian species. However, the biosynthetic pathways leading to the formation of neurosteroids are still not completely elucidated. We have recently found that the avian brain produces 7α-hydroxypregnenolone, a novel bioactive neurosteroid that stimulates locomotor activity. Until recently, it was believed that neurosteroids are produced in neurons and glial cells in the central and peripheral nervous systems. However, our recent studies on birds have demonstrated that the pineal gland, an endocrine organ located close to the brain, is an important site of production of neurosteroids de novo from cholesterol. 7α-Hydroxypregnenolone is a major pineal neurosteroid that stimulates locomotor activity of juvenile birds, connecting light-induced gene expression with locomotion. The other major pineal neurosteroid allopregnanolone is involved in Purkinje cell survival during development. This paper highlights new aspects of neurosteroid synthesis and actions in birds.

Cellular mechanisms of developmental and sex differences in the rapid hormonal modulation of a social communication signal

Some gymnotiform electric fish species rapidly modify their electric signal waveforms by altering the action potential (AP) waveforms of their electrocytes, the excitable cells that produce the electric organ discharge (EOD). The fish Brachyhypopomus gauderio alters EOD amplitude and pulse duration as a social signal in accordance with the prevailing social conditions, under the dual regulation of melanocortins and androgens. We show here that B. gauderio uses two distinct cellular mechanisms to change signal amplitude, and its use of these two mechanisms varies with age and sex of the signaler. EOD amplitude and waveform are regulated by the coordinated timing and shaping of two APs generated from two opposing excitable membranes in each electrocyte. The two membranes fire in sequence within 100 μs of each other with the second AP being broader than the first. We have shown previously that mature males increase EOD amplitude and duration when melanocortin peptide hormones act directly on electrocytes to selectively broaden the second AP and increase the delay between the two APs by approximately 25 μs. Here we show that females selectively broaden only the second AP as males do, but increase amplitude of both APs with no change in delay between them, a previously unreported second mechanism of EOD amplitude change in B. gauderio. Juvenile fish broaden both APs and increase the delay between the APs. Cellular mechanisms of EOD plasticity are therefore shaped during development, presumably by sex steroids, becoming sexually dimorphic at maturity.

Neuroestrogens rapidly regulate sexual motivation but not performance

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

Effects of water restriction on reproductive physiology and affiliative behavior in an opportunistically-breeding and monogamous songbird, the zebra finch

Wild zebra finches form long-term monogamous pair-bonds that are actively maintained year-round, even when not in breeding condition. These desert finches are opportunistic breeders, and breeding is highly influenced by unpredictable rainfall. Their high levels of affiliation and complex breeding patterns make zebra finches an excellent model in which to study the endocrine regulation of affiliation. Here, we compared zebra finch pairs that were provided with water ad libitum (control) or water restricted. We examined (1) reproductive physiology, (2) pair-maintenance behaviors in several contexts, and (3) circulating and brain steroid levels. In females, water restriction profoundly reduced largest ovarian follicle size, ovary size, oviduct size, and egg laying. In males, water restriction had no effect on testes size but decreased systemic testosterone levels. However, in the hypothalamus, local testosterone and estradiol levels were unaffected by water restriction in both sexes. Systemic and local levels of the androgen precursor dehydroepiandrosterone (DHEA) were also unaffected by water restriction. Lastly, in three different behavioral paradigms, we examined a variety of pair-maintenance behaviors, and none were reduced by water restriction. Taken together, these correlational data are consistent with the hypothesis that local production of sex steroids in the brain promotes the expression of pair-maintenance behaviors in non-breeding zebra finches.

Traumatized and inflamed--but resilient: glial aromatization and the avian brain

Steroids like estrogens have potent effects on the vertebrate brain, and are provided to neural targets from peripheral and central sources. Estradiol synthesized within the vertebrate CNS modulates neural structure and function, including the pathways involved in neuroprotection, and perhaps, neural repair. Specifically, aromatase; the enzyme responsible for the conversion of testosterone to estradiol, is upregulated in the avian and mammalian brain following disruption of the neuropil by multiple forms of perturbation including mechanical injury, ischemia and excitotoxicity. This injury induced aromatase expression is somewhat unique in that it occurs in astroglia rather than neurons, and is stimulated in response to factors associated with brain damage. In this review, we focus on the induction, expression and consequences of glial aromatization in the songbird brain. We begin with a review of the anatomical consequences of glial estrogen provision followed by a discussion of the cellular mechanisms whereby glial aromatization may affect injury-induced neuroplasticity. We then present the current status of our understanding regarding the inductive role of inflammatory processes in the transcription and translation of astrocytic aromatase. We consider the functional aspects of glial aromatization before concluding with unanswered questions and suggestions for future studies. Birds have long informed us about fundamental questions in endocrinology, immunology, and neuroplasticity; and their unique anatomical and physiological characteristics continue to provide an excellent system in which to learn about brain trauma, inflammation, and neuroprotection.

Social opportunity causes rapid transcriptional changes in the social behaviour network of the brain in an African cichlid fish

Animals constantly integrate external stimuli with their own internal physiological state to make appropriate behavioural decisions. Little is known, however, about where in the brain the salience of these signals is evaluated, or which neural and transcriptional mechanisms link this integration to adaptive behaviours. We used an African cichlid fish Astatotilapia burtoni to test the hypothesis that a new social opportunity activates the conserved 'social behaviour network' (SBN), a collection of brain nuclei known to regulate social behaviours across vertebrates. We measured mRNA levels of immediate early genes (IEGs) in microdissected brain regions as a proxy for neuronal activation, and discovered that IEGs were higher in all SBN nuclei in males that were given an opportunity to rise in social rank compared to control stable subordinate and dominant individuals. Furthermore, because the presence of sex-steroid receptors is one defining criteria of SBN nuclei, we also tested whether social opportunity or status influenced androgen and oestrogen receptor mRNA levels within these same regions. There were several rapid region-specific changes in receptor mRNA levels induced by social opportunity, most notably in oestrogen receptor subtypes in areas that regulate social aggression and reproduction, suggesting that oestrogenic signalling pathways play an important role in regulating male status. Several receptor mRNA changes occurred in regions with putative homologies to the mammalian septum and extended amygdala, two regions shared by SBN and reward circuits, suggesting an important role in the integration of social salience, stressors, hormonal state and adaptive behaviours. We also demonstrated increases in plasma sex- and stress-steroids at 30 min after a rise in social rank. This rapid endocrine and transcriptional response suggests that the SBN is involved in the integration of social inputs with internal hormonal state to facilitate the transition to dominant status, which ultimately leads to improved fitness for the previously reproductively-suppressed individual.

Mechanistic basis and functional roles of long-term plasticity in auditory neurons induced by a brain-generated estrogen

The classic estrogen 17β-estradiol (E2) was recently identified as a novel modulator of hearing function. It is produced rapidly, in an experience-dependent fashion, by auditory cortical neurons of both males and females. This brain-generated E2 enhances the efficiency of auditory coding and improves the neural and behavioral discrimination of auditory cues. Remarkably, the effects of E2 are long-lasting and persist for hours after local rises in hormone levels have subsided. The mechanisms and functional consequences of this E2-induced plasticity of auditory responses are unknown. Here, we addressed these issues in the zebra finch model by combining intracerebral pharmacology, biochemical assays, in vivo neurophysiology in awake animals, and computational and information theoretical approaches. We show that auditory experience activates the MAPK pathway in an E2-dependent manner. This effect is mediated by estrogen receptor β (ERβ), which directly associates with MEKK1 to sequentially modulate MEK and ERK activation, where the latter is required for the engagement of downstream molecular targets. We further show that E2-mediated activation of the MAPK cascade is required for the long-lasting enhancement of auditory-evoked responses in the awake brain. Moreover, a functional consequence of this E2/MAPK activation is to sustain enhanced information handling and neural discrimination by auditory neurons for several hours following hormonal challenge. Our results demonstrate that brain-generated E2 engages, via a nongenomic interaction between an estrogen receptor and a kinase, a persistent form of experience-dependent plasticity that enhances the neural coding and discrimination of behaviorally relevant sensory signals in the adult vertebrate brain.

Context-specific effects of estradiol on spatial learning and memory in the zebra finch

Estradiol is known to impact cognitive function including spatial learning and memory, with studies focused largely on rodent models. Estrogens can be produced peripherally or centrally as neuroestrogens, and the specific role for neuroestrogens in memory processes remains unresolved. Many songbirds possess remarkable spatial memory capabilities and also express the estrogen synthetic enzyme aromatase abundantly in the hippocampus, suggesting that locally-produced estrogens may promote the acquisition or retrieval of spatial memories in these birds. We examined the effect of estradiol on spatial memory in three contexts in the zebra finch: retrieval after discrimination training, retrieval after familiarization but without discrimination training, and memory acquisition, using a combination of estradiol implants and oral dosing with the aromatase inhibitor fadrozole (FAD). Retrieval of spatial memory in both contexts was impaired when estradiol production was blocked. However, spatial memory acquisition was enhanced when estradiol production was inhibited whereas estradiol replacement impaired acquisition. These results provide evidence for a context-specific role of estradiol in songbird spatial memory, results that find accord with some mammalian studies but have not yet been observed in birds.

Differential responses of brain, gonad and muscle steroid levels to changes in social status and sex in a sequential and bidirectional hermaphroditic fish

Sex steroids can both modulate and be modulated by behavior, and their actions are mediated by complex interactions among multiple hormone sources and targets. While gonadal steroids delivered via circulation can affect behavior, changes in local brain steroid synthesis also can modulate behavior. The relative steroid load across different tissues and the association of these levels with rates of behavior have not been well studied. The bluebanded goby (Lythrypnus dalli) is a sex changing fish in which social status determines sexual phenotype. We examined changes in steroid levels in brain, gonad and body muscle at either 24 hours or 6 days after social induction of protogynous sex change, and from individuals in stable social groups not undergoing sex change. For each tissue, we measured levels of estradiol (E(2)), testosterone (T) and 11-ketotestosterone (KT). Females had more T than males in the gonads, and more E(2) in all tissues but there was no sex difference in KT. For both sexes, E(2) was higher in the gonad than in other tissues while androgens were higher in the brain. During sex change, brain T levels dropped while brain KT increased, and brain E(2) levels did not change. We found a positive relationship between androgens and aggression in the most dominant females but only when the male was removed from the social group. The results demonstrate that steroid levels are responsive to changes in the social environment, and that their concentrations vary in different tissues. Also, we suggest that rapid changes in brain androgen levels might be important in inducing behavioral and/or morphological changes associated with protogynous sex change.

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

Although estrogens are widely considered circulating "sex steroid hormones" typically associated with female reproduction, recent evidence suggests that estrogens can act as local modulators of brain circuits in both males and females. The functional implications of this newly characterized estrogen signaling system have begun to emerge. This essay summarizes evidence in support of the hypothesis that the rapid production of estrogens in brain circuits can drive acute changes in both the production and perception of acoustic communication behaviors. These studies have revealed two fundamental neurobiological concepts: (1) estrogens can be locally produced in brain circuits, independent of levels in nearby circuits and in the circulation and (2) estrogens can have very rapid effects within these brain circuits to modulate social vocalizations, acoustic processing, and sensorimotor integration. This vertebrate-wide span of research, including vocalizing fishes, amphibians, and birds, emphasizes the importance of comparative model systems in understanding principles of neurobiology.

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

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

Neuroestrogen, rapid action of estradiol, and GnRH neurons

Estradiol plays a pivotal role in the control of GnRH neuronal function, hence female reproduction. A series of recent studies in our laboratory indicate that rapid excitatory actions of estradiol directly modify GnRH neuronal activity in primate GnRH neurons through GPR30 and STX-sensitive receptors. Similar rapid direct actions of estradiol through estrogen receptor beta are also described in mouse GnRH neurons. In this review, we propose two novel hypotheses as a possible physiological role of estradiol in primates. First, while ovarian estradiol initiates the preovulatory GnRH surge through interneurons expressing estrogen receptor alpha, rapid direct membrane-initiated action of estradiol may play a role in sustaining GnRH surge release for many hours. Second, locally produced neuroestrogens may contribute to pulsatile GnRH release. Either way, estradiol synthesized in interneurons in the hypothalamus may play a significant role in the control of the GnRH surge and/or pulsatility of GnRH release.

Rapid control of male typical behaviors by brain-derived estrogens

Beside their genomic mode of action, estrogens also activate a variety of cellular signaling pathways through non-genomic mechanisms. Until recently, little was known regarding the functional significance of such actions in males and the mechanisms that control local estrogen concentration with a spatial and time resolution compatible with these non-genomic actions had rarely been examined. Here, we review evidence that estrogens rapidly modulate a variety of behaviors in male vertebrates. Then, we present in vitro work supporting the existence of a control mechanism of local brain estrogen synthesis by aromatase along with in vivo evidence that rapid changes in aromatase activity also occur in a region-specific manner in response to changes in the social or environmental context. Finally, we suggest that the brain estrogen provision may also play a significant role in females. Together these data bolster the hypothesis that brain-derived estrogens should be considered as neuromodulators.

Increased glutamate levels in the medial prefrontal cortex in patients with postpartum depression

The medial prefrontal cortex (MPFC) is a key brain area in depressive symptomatology; specifically, glutamate (Glu) has been reported to play a significant role in major depression (MD) in this area. MPFC Glu levels are sensitive to ovarian hormone fluctuations and pregnancy and the postpartum period are associated with the most substantial physiological alterations of female hormones. It is therefore logical to measure MPFC Glu levels in women with postpartum depression (PPD). Using in vivo magnetic resonance spectroscopy (MRS) at a field strength of 3 T, we acquired single-voxel spectra from the MPFC of 12 women with PPD and 12 healthy controls (HCs) matched for postpartum scan timing. Water-referenced MPFC Glu levels were measured using a MRS technique that allowed us to be specific for Glu with very little glutamine contamination. The concentrations of other water-quantified brain metabolites such as glycerophosphorylcholine plus phosphorylcholine, N-acetylaspartate (NAA), and creatine plus phosphocreatine were measured in the same MR spectra. MPFC Glu levels were higher in women with PPD (7.21±1.20) compared to matched HCs (6.04±1.21). There were no differences between groups for other brain metabolites measured. These findings suggest an association between Glu dysregulation in the MPFC and PPD. Whether the pathophysiology of PPD differs from the pathophysiology of MD remains to be determined. Further investigations are needed to determine the chronological associations between the occurrence of symptoms of PPD and the onset of changes in MPFC Glu levels.

Changing neuroestrogens within the auditory forebrain rapidly transform stimulus selectivity in a downstream sensorimotor nucleus

The activity of sensory circuits is shaped by neuromodulators, which can have downstream consequences for both sensorimotor integration and behavioral output. Recent evidence indicates that brain-derived estrogens ("neuroestrogens") can act as local circuit modulators in the songbird auditory forebrain. Specifically, neuroestrogens fluctuate in the auditory caudomedial nidopallium (NCM) during social interactions and in response to song stimuli. Within minutes of elevation, neuroestrogens also enhance auditory response properties of NCM neurons, and acute blockade of estrogen production in NCM disrupts behavioral song preferences. Here, we test the hypothesis that fluctuating neuroestrogens within NCM influence stimulus selectivity in a downstream sensorimotor nucleus (HVC, used as a proper name) that receives indirect auditory input from NCM. Dual extracellular recordings coupled with retrodialysis delivery show that song selectivity in HVC is rapidly enhanced by increasing neuroestrogens in NCM in adult males. Conversely, inhibiting neuroestrogen production in NCM causes a rapid decline in song selectivity in HVC, demonstrating the endogenous nature of this modulatory network. In contrast, HVC selectivity is unaffected by neuroestrogen delivery to either nearby caudomedial mesopallium or into HVC itself, indicating that neuroestrogen actions are restricted to NCM. In juvenile males, identical neuroestrogen treatment in NCM also does not alter HVC selectivity, consistent with a developmental maturation of the auditory network. Lastly, the rapid actions of estrogens leading to enhanced HVC selectivity appear to be mediated by membrane-bound receptors in NCM. These findings indicate that steroid-dependent modulation of sensory processing is not locally restricted and can be transmitted transynaptically to influence downstream sensorimotor and premotor targets.

Estradiol treatment and hormonal fluctuations during the estrous cycle modulate the expression of estrogen receptors in the auditory system and the prepulse inhibition of acoustic startle response

Estrogens' effects on hearing are documented across species, but the responsible molecular mechanisms remain unknown. The presence of estrogen receptors (ER) throughout the auditory system offers a potential pathway of direct estrogenic effects on auditory function, but little is known about how each ER's expression is regulated by the overall hormonal status of the body. In the present study, we determined the effects of ovariectomy and chronic 17β-estradiol treatment on mRNA and protein expression of ERα and ERβ in peripheral (cochlea) and central (inferior colliculus) auditory structures of mice, as well as on auditory-related behavior using the acoustic startle response (ASR), prepulse inhibition (PPI), and habituation of the startle response. 17β-Estradiol treatment down-regulated ERα but not ERβ and increased PPI and latency of the ASR. Neither the magnitude nor the habituation of ASR was affected. Furthermore, ER's mRNA and protein expression in the inner ear were analyzed throughout the estrous cycle (proestrus, estrus, metestrus, and diestrus), revealing a negative correlation of circulating estrogens with ERα expression, whereas ERβ was stable. Our findings show that ER not only are present in both the peripheral and central auditory system but also that circulating estrogen levels down-regulate ERα expression in the auditory system and affect PPI and the latency of ASR, suggesting a key role of ERα as a hormone-induced modulator of the auditory system and behavior.

Two-step wiring plasticity--a mechanism for estrogen-induced rewiring of cortical circuits

Estrogens have been shown to exert powerful effects on cognitive behaviors mediated by several areas of the brain including the cortex. Remodeling of spiny synapses is a key step in the rewiring of neuronal circuitry thought to underlie the processing and storage of information in the forebrain. Whereas estrogen has been shown to regulate synapse structure and function, we are only just starting to understand the molecular and cellular underpinnings of how estrogens can modulate neuronal circuits. Here I will review recent molecular and cellular work that offers a potential mechanism of how estrogen may modulate synapse structure and function of cortical neurons. This mechanism allows cortical neurons to respond to activity-dependent stimuli with greater efficacy in a cellular model termed "Two-Step Wiring Plasticity". This novel form of spine plasticity thus provides insight into how estrogens may modulate the rewiring of neuronal circuits, underlying its ability to influencing cortically based behaviors. This article is part of a Special Issue entitled 'Neurosteroids'.

Control of central auditory processing by a brain-generated oestrogen

Recent discoveries show that behaviourally relevant sensory experience drives the production of oestradiol - the classic sex steroid oestrogen - in auditory neurons in the adult brain of both males and females. This brain-generated oestrogen markedly enhances the efficiency of the neural coding of acoustic cues and shapes auditory-based behaviours on a timescale that is relevant for sensory processing and congruent with the action of rapid neuromodulators. These findings are re-shaping our current understanding of the mechanistic framework that supports sensory processing and the functional roles of hormones in the brain, and have implications for multiple health issues.

Building a better hormone therapy? How understanding the rapid effects of sex steroid hormones could lead to new therapeutics for age-related memory decline

A wealth of data collected in recent decades has demonstrated that ovarian sex-steroid hormones, particularly 17β-estradiol (E2), are important trophic factors that regulate the function of cognitive regions of the brain such as the hippocampus. The loss of hormone cycling at menopause is associated with cognitive decline and dementia in women, and the onset of memory decline in animal models. However, hormone therapy is not currently recommended to prevent or treat cognitive decline, in part because of its detrimental side effects. In this article, it is proposed that investigations of the rapid effects of E2 on hippocampal function be used to further the design of new drugs that mimic the beneficial effects of E2 on memory without the side effects of current therapies. A conceptual model is presented for elucidating the molecular and biochemical mechanisms through which sex-steroid hormones modulate memory, and a specific hypothesis is proposed to account for the rapid memory-enhancing effects of E2. Empirical support for this hypothesis is discussed as a means of stimulating the consideration of new directions for the development of hormone-based therapies to preserve memory function in menopausal women.

Acute and specific modulation of presynaptic aromatization in the vertebrate brain

Estrogens affect a diversity of peripheral and central physiological endpoints. Traditionally, estrogens were thought to be peripherally derived transcription regulators (i.e. slow acting). More recently, we have learned that estrogens are also synthesized in neuronal cell bodies and synaptic terminals and have potent membrane effects, which modulate brain function. However, the mechanisms that control local steroid concentrations in a temporal and spatial resolution compatible with their acute actions are poorly understood. Here, using differential centrifugation followed by enzymatic assay, we provide evidence that estrogen synthesis within synaptosomes can be modulated more dramatically by phosphorylating conditions, relative to microsomes. This is the first demonstration of a rapid mechanism that may alter steroid concentrations within the synapse and may represent a potential mechanism for the acute control of neurophysiology and behavior.

Role of estrogen receptor alpha and beta expression and signaling on cognitive function during aging

This review presents evidence for the idea that the expression of estrogen receptor alpha and beta (ERα and ERβ) interacts with the level of estradiol (E2) to influence the etiology of age-related cognitive decline and responsiveness to E2 treatments. There is a nonmonotonic dose response curve for E2 influences on behavior and transcription. Evidence is mounting to indicate that the dose response curve is shifted according to the relative expression of ERα and ERβ. Recent work characterizing age-related changes in the expression of ERα and ERβ in the hippocampus, as well as studies using mutant mice, and viral mediated delivery of estrogen receptors indicate that an age-related shift in ERα/ERβ expression, combined with declining gonadal E2 can impact transcription, cell signaling, neuroprotection, and neuronal growth. Finally, the role of ERα/ERβ on rapid E2 signaling and synaptogenesis as it relates to hippocampal aging is discussed.

Rising StARs: behavioral, hormonal, and molecular responses to social challenge and opportunity

Across taxa, individuals must respond to a dynamic social environment of challenges and opportunities on multiple biological levels, including behavior, hormone profiles, and gene expression. We investigated the response to a complex social environment including both territorial challenges and reproductive opportunities in the African cichlid fish Astatotilapia burtoni (Burton's mouthbrooder), a species well-known for its phenotypic plasticity. Male A. burtoni are either socially dominant or subordinate and can transition between the two phenotypes. We used this transition to simultaneously study changes in aggression, reproductive behavior, testosterone and estradiol levels, gonadal histology, and testes expression of three genes involved in testosterone synthesis. We have found that males immediately become aggressive and increase testosterone levels when they become dominant in this paradigm of challenge and opportunity. Reproductive behavior and estradiol increase slightly later but are also up-regulated within 24h. Increases in steroid hormone levels are accompanied by an increase in expression of steroidogenic acute regulatory protein (StAR), the rate-limiting enzyme during testosterone synthesis, as well as an increase in testis maturation as measured by histological organization. Reproductive behavior was found to correlate with female gravidity, suggesting that males were able to perceive reproductive opportunity. Our study demonstrates the rapid plasticity at multiple levels of biological organization that animals can display in response to changes in their complex social environment.

Sex matters during adolescence: testosterone-related cortical thickness maturation differs between boys and girls

Age-related changes in cortical thickness have been observed during adolescence, including thinning in frontal and parietal cortices, and thickening in the lateral temporal lobes. Studies have shown sex differences in hormone-related brain maturation when boys and girls are age-matched, however, because girls mature 1–2 years earlier than boys, these sex differences could be confounded by pubertal maturation. To address puberty effects directly, this study assessed sex differences in testosterone-related cortical maturation by studying 85 boys and girls in a narrow age range and matched on sexual maturity. We expected that testosterone-by-sex interactions on cortical thickness would be observed in brain regions known from the animal literature to be high in androgen receptors. We found sex differences in associations between circulating testosterone and thickness in left inferior parietal lobule, middle temporal gyrus, calcarine sulcus, and right lingual gyrus, all regions known to be high in androgen receptors. Visual areas increased with testosterone in boys, but decreased in girls. All other regions were more impacted by testosterone levels in girls than boys. The regional pattern of sex-by-testosterone interactions may have implications for understanding sex differences in behavior and adolescent-onset neuropsychiatric disorders.

Rapid and widespread effects of 17β-estradiol on intracellular signaling in the male songbird brain: a seasonal comparison

Across vertebrate species, 17β-estradiol (E(2)) acts on the brain via both genomic and nongenomic mechanisms to influence neuronal physiology and behavior. Nongenomic E(2) signaling is typically initiated by membrane-associated estrogen receptors that modulate intracellular signaling cascades, including rapid phosphorylation of ERK. Phosphorylated ERK (pERK) can, in turn, rapidly phosphorylate tyrosine hydroxylase (TH) and cAMP response element-binding protein (CREB). Recent data suggest that the rapid effects of E(2) on mouse aggressive behavior are more prominent during short photoperiods (winter) and that acute aromatase inhibition reduces songbird aggression in winter only. To date, seasonal plasticity in the rapid effects of E(2) on intracellular signaling has not been investigated. Here, we compared the effects of acute (15 min) E(2) treatment on pERK, pTH, and pCREB immunoreactivity in male song sparrows (Melospiza melodia) pretreated with the aromatase inhibitor fadrozole during the breeding and nonbreeding seasons. We examined immunoreactivity in 14 brain regions including portions of the song control system, social behavior network, and the hippocampus (Hp). In both seasons, E(2) significantly decreased pERK in nucleus taeniae of the amygdala, pTH in ventromedial hypothalamus, and pCREB in mesencephalic central gray, robust nucleus of the arcopallium, and caudomedial nidopallium. However, several effects were critically dependent upon season. E(2) decreased pERK in caudomedial nidopallium in the breeding season only and decreased pCREB in the medial preoptic nucleus in the nonbreeding season only. Remarkably, E(2) decreased pERK in Hp in the breeding season but increased pERK in Hp in the nonbreeding season. Together, these data demonstrate that E(2) has rapid effects on intracellular signaling in multiple regions of the male brain and also demonstrate that rapid effects of E(2) can be profoundly different across the seasons.

To modulate and be modulated: estrogenic influences on auditory processing of communication signals within a socio-neuro-endocrine framework

Gonadal hormones modulate behavioral responses to sexual stimuli, and communication signals can also modulate circulating hormone levels. In several species, these combined effects appear to underlie a two-way interaction between circulating gonadal hormones and behavioral responses to socially salient stimuli. Recent work in songbirds has shown that manipulating local estradiol levels in the auditory forebrain produces physiological changes that affect discrimination of conspecific vocalizations and can affect behavior. These studies provide new evidence that estrogens can directly alter auditory processing and indirectly alter the behavioral response to a stimulus. These studies show that: 1) Local estradiol action within an auditory area is necessary for socially relevant sounds to induce normal physiological responses in the brains of both sexes; 2) These physiological effects occur much more quickly than predicted by the classical time-frame for genomic effects; 3) Estradiol action within the auditory forebrain enables behavioral discrimination among socially relevant sounds in males; and 4) Estradiol is produced locally in the male brain during exposure to particular social interactions. The accumulating evidence suggests a socio-neuro-endocrinology framework in which estradiol is essential to auditory processing, is increased by a socially relevant stimulus, acts rapidly to shape perception of subsequent stimuli experienced during social interactions, and modulates behavioral responses to these stimuli. Brain estrogens are likely to function similarly in both songbird sexes because aromatase and estrogen receptors are present in both male and female forebrain. Estrogenic modulation of perception in songbirds and perhaps other animals could fine-tune male advertising signals and female ability to discriminate them, facilitating mate selection by modulating behaviors.

Rapid direct action of estradiol in GnRH neurons: findings and implications

Estradiol plays a pivotal role in the control of gonadotropin-releasing hormone (GnRH) neuronal function and female reproduction. While positive and negative feedback actions of estradiol that enhance and suppress release of GnRH and LH are primarily mediated through estrogen receptor alpha located in interneurons, a series of recent studies in our laboratory indicate that rapid excitatory actions of estradiol also directly modify GnRH neuronal activity. We observed this phenomenon in cultured primate GnRH neurons, but similar rapid direct actions of estradiol are also described in cultured GnRH neurons and green fluorescent protein-labeled GnRH neurons of mice. Importantly, rapid direct action of estradiol in GnRH neurons is mediated through membrane or membrane associated receptors, such as GPR30, STX-sensitive receptors, and ERβ. In this review, possible implications of this rapid estradiol action in GnRH neurons are discussed.

Neurosteroidogenesis: insights from studies of songbirds

The long-held dogma that the brain is a target of steroids produced by peripheral organs has delayed the widespread acceptance of the functional importance of neurosteroidogenesis. Comparative studies have been vital for establishing the key actions of gonadal and adrenal hormones on brain and behaviour. No doubt, studies across diverse phyla will continue to be crucial for revealing the true significance of neurosteroidogenesis to proper function of the vertebrate brain. Here, we review work carried out in our laboratory, as well as in others, highlighting advances to our understanding of brain steroid synthesis and action using songbirds as animal models. These studies show that steroidogenic transporters and enzymes are present in the songbird brain and that their expression and/or activities are subject to developmental, seasonal or short-term regulation. Our work in a songbird points to synaptic synthesis of neuroactive steroids and fast, perisynaptic membrane actions. Combined with evidence for rapid steroidal control of behaviour, these studies firmly establish a neuromodulatory role for avian neurosteroids. We hope this work will join with that of other species to embolden the acceptance of neurosteroidal signalling as a core property of vertebrate neurobiology.

Regulation of neurosteroid biosynthesis by neurotransmitters and neuropeptides

The enzymatic pathways leading to the synthesis of bioactive steroids in the brain are now almost completely elucidated in various groups of vertebrates and, during the last decade, the neuronal mechanisms involved in the regulation of neurosteroid production have received increasing attention. This report reviews the current knowledge concerning the effects of neurotransmitters, peptide hormones, and neuropeptides on the biosynthesis of neurosteroids. Anatomical studies have been carried out to visualize the neurotransmitter- or neuropeptide-containing fibers contacting steroid-synthesizing neurons as well as the neurotransmitter, peptide hormones, or neuropeptide receptors expressed in these neurons. Biochemical experiments have been conducted to investigate the effects of neurotransmitters, peptide hormones, or neuropeptides on neurosteroid biosynthesis, and to characterize the type of receptors involved. Thus, it has been found that glutamate, acting through kainate and/or AMPA receptors, rapidly inactivates P450arom, and that melatonin produced by the pineal gland and eye inhibits the biosynthesis of 7α-hydroxypregnenolone (7α-OH-Δ(5)P), while prolactin produced by the adenohypophysis enhances the formation of 7α-OH-Δ(5)P. It has also been demonstrated that the biosynthesis of neurosteroids is inhibited by GABA, acting through GABA(A) receptors, and neuropeptide Y, acting through Y1 receptors. In contrast, it has been shown that the octadecaneuropetide ODN, acting through central-type benzodiazepine receptors, the triakontatetraneuropeptide TTN, acting though peripheral-type benzodiazepine receptors, and vasotocin, acting through V1a-like receptors, stimulate the production of neurosteroids. Since neurosteroids are implicated in the control of various neurophysiological and behavioral processes, these data suggest that some of the neurophysiological effects exerted by neurotransmitters and neuropeptides may be mediated via the regulation of neurosteroid production.

Acute effects of steroid hormones and neuropeptides on human social-emotional behavior: a review of single administration studies

Steroids and peptides mediate a diverse array of animal social behaviors. Human research is restricted by technical-ethical limitations, and models of the neuroendocrine regulation of social-emotional behavior are therefore mainly limited to non-human species, often under the assumption that human social-emotional behavior is emancipated from hormonal control. Development of acute hormone administration procedures in human research, together with the advent of novel non-invasive neuroimaging techniques, have opened up opportunities to systematically study the neuroendocrinology of human social-emotional behavior. Here, we review all placebo-controlled single hormone administration studies addressing human social-emotional behavior, involving the steroids testosterone and estradiol, and the peptides oxytocin and vasopressin. These studies demonstrate substantial hormonal control over human social-emotional behavior and give insights into the underlying neural mechanisms. Finally, we propose a theoretical model that synthesizes detailed knowledge of the neuroendocrinology of social-emotional behavior in animals with the recently gained data from humans described in our review.

Sex-specific, rapid neuroestrogen fluctuations and neurophysiological actions in the songbird auditory forebrain

Recent evidence shows that brain-derived steroids such as estrogens ("neuroestrogens") are controlled in a manner very similar to traditional neurotransmitters. The advent of in vivo microdialysis for steroids in songbirds has provided new information about the spatial and temporal dynamics of neuroestrogen changes in a region of the auditory cortex, the caudomedial nidopallium (NCM). Here, experiments using in vivo microdialysis demonstrate that neuroestradiol (E(2)) fluctuations occur within the auditory NCM during presentation of naturalistic auditory and visual stimuli in males but only to the presentation of auditory stimuli in females. These changes are acute (within 30 min) and appear to be specific to the NCM, because similar treatments elicit no changes in E(2) in a nearby mesopallial region or in circulating plasma. Further experiments coupling in vivo steroid microdialysis with extracellular recordings in NCM show that neuroestrogens rapidly boost auditory responses to song stimuli in females, similar to recent observations in males. We also find that the rapid actions of estradiol on auditory responses are fully mimicked by the cell membrane-impermeable estrogen biotinylestradiol, consistent with acute estrogen actions at the neuronal membrane. Thus we conclude that local and acute E(2) flux is regulated by convergent multimodal sensory input, and that this regulation appears to be sex-specific. Second, rapid changes in local E(2) levels in NCM have consequences for the modulation of auditory processing in females and males. Finally, the rapid actions of neuroestrogens on NCM auditory processing appear to be mediated by a nonclassical, membrane-bound estrogen receptor.

Noninvasive analytical estimation of endogenous GnRH drive: analysis using graded competitive GnRH-receptor antagonism and a calibrating pulse of exogenous GnRH

Homeostatic control of endocrine systems proceeds via feedforward (agonistic, stimulatory) and feedback (antagonistic, inhibitory) interactions mediated via implicit dose-response functions. However, neither the feedback/feedforward pathways nor the dose-response interfaces are directly observed in vivo. Thus, the goal was to formulate and estimate an ensemble construct of time-varying feedback/feedforward interactions among GnRH, LH, and testosterone (T) in the male gonadal axis. The new analytical model revises and extends an earlier construct by: 1) allowing systemic T concentrations to inhibit hypothalamic GnRH output; 2) estimating GnRH outflow after injection of a calibrating pulse of biosynthetic GnRH; 3) framing the pituitary response to GnRH as a secretory burst, rather than continuous LH release; and 4) regressing feedback and feedforward ensemble parameters on age, rather than evaluating age dichotomously. Application of this methodology in 21 men aged 23-72 yr unveiled age-related 1) diminution of GnRH efficacy normalized for the decline in free T with age (P = 0.016), 2) potentiation of maximal T feedback onto (inhibition of) GnRH secretion (P = 0.006), and 3) accentuation of hypothalamic GnRH's sensitivity to T repression (P = 0.003). Outcomes were specific, because injected GnRH agonist and antagonist concentrations were invariant of age. We conclude that combining experimental and analytical strategies may provide a noninvasive means to investigate and decipher feedback determinants of unobserved endocrine signal(s).

Human and quail aromatase activity is rapidly and reversibly inhibited by phosphorylating conditions

Besides their slow genomic actions, estrogens also induce rapid physiological responses. To be functionally relevant, these effects must be associated with rapid changes in local concentrations of estrogens. Rapid changes in aromatase activity (AA) controlled by calcium-dependent phosphorylations of the enzyme can alter in a rapid manner local estrogen concentrations, but so far this mechanism was identified only in the avian (quail) brain. We show here that AA is also rapidly down-regulated by phosphorylating conditions in quail ovary homogenates and in various cell lines transfected with human aromatase (HEK 293, Neuro2A, and C6). Enzymatic activity was also rapidly inhibited after depolarization of aromatase-expressing HEK 293 cells with 100 mM KCl, and activity was fully restored when cells returned to control conditions. Western blot analysis demonstrated that the reduction of enzymatic activity is not due to protein degradation. We next investigated by site-directed mutagenesis the potential implication in the control of AA of specific aromatase residues identified by bioinformatic analysis. Mutation of the amino acids S118, S247, S267, T462, T493, or S497 to alanine, alone or in combination, did not block the rapid inhibition of enzymatic activity induced by phosphorylating conditions, but basal AA was markedly decreased in the S118A mutant. Altogether, these results demonstrate that the rapid inhibition of AA is a widespread and fully reversible process and that phosphorylation of specific residues modulate AA. These processes provide a new general mechanism by which local estrogen concentration can be rapidly altered in the brain and other tissues.

Organization of Estrogen-Associated Circuits in the Mouse Primary Auditory Cortex

Sex steroid hormones influence the perceptual processing of sensory signals in vertebrates. In particular, decades of research have shown that circulating levels of estrogen correlate with hearing function. The mechanisms and sites of action supporting this sensory-neuroendocrine modulation, however, remain unknown. Here we combined a molecular cloning strategy, fluorescence in-situ hybridization and unbiased quantification methods to show that estrogen-producing and -sensitive neurons heavily populate the adult mouse primary auditory cortex (AI). We also show that auditory experience in freely-behaving animals engages estrogen-producing and -sensitive neurons in AI. These estrogen-associated networks are greatly stable, and do not quantitatively change as a result of acute episodes of sensory experience. We further demonstrate the neurochemical identity of estrogen-producing and estrogen-sensitive neurons in AI and show that these cell populations are phenotypically distinct. Our findings provide the first direct demonstration that estrogen-associated circuits are highly prevalent and engaged by sensory experience in the mouse auditory cortex, and suggest that previous correlations between estrogen levels and hearing function may be related to brain-generated hormone production. Finally, our findings suggest that estrogenic modulation may be a central component of the operational framework of central auditory networks.

Undirected (solitary) birdsong in female and male blue-capped cordon-bleus (Uraeginthus cyanocephalus) and its endocrine correlates

Background

Birdsong is a popular model system in research areas such as vocal communication, neuroethology or neuroendocrinology of behaviour. As most research has been conducted on species with male-only song production, the hormone-dependency of male song is well established. However, female singing and its mechanisms are poorly understood.

Methodology/Principal Findings

We characterised the song and its endocrine correlates of blue-capped cordon-bleus (Uraeginthus cyanocephalus), a species in which both sexes sing. Like other estrildids, they produce directed song during courtship and undirected (or solitary) song in isolation, i.e. when the mate is not visible or absent. We compare solitary song of blue-capped cordon-bleus to published descriptions of the song of its relative, the zebra finch (Taeniopygia guttata). Solitary song of cordon-bleus shared some overall song features with that of zebra finches but differed in spectro-temporal song features, sequential stereotypy and sequential organisation. The song of cordon-bleus was dimorphic with respect to the larger size of syllable repertoires, the higher song duration and the lower variability of pitch goodness (measuring the pureness of harmonic sounds) in males. However, in both sexes the overall plasma testosterone concentrations were low (ca. 300 pg/ml) and did not correlate with the sexually dimorphic song motor pattern. Despite such low concentrations, the increase in the rate of solitary song coincided with an increase in the level of testosterone. Furthermore, the latency to start singing after the separation from the mate was related to hormone levels.

Conclusions/Significance

Our findings suggest that the occurrence of solitary song but not its motor pattern might be under the control of testosterone in female and male cordon-bleus.

Presynaptic control of rapid estrogen fluctuations in the songbird auditory forebrain

Within the CNS of vertebrates, estrogens can directly modulate neural circuits that govern a wide range of behaviors, including feeding, spatial navigation, reproduction, and auditory processing. The rapid actions of estrogens in brain (seconds to minutes) have become well established, but it is unclear how estrogens are synthesized and released within restricted temporal and spatial domains in neural circuits. Anatomical localization of the estrogen synthesis enzyme (aromatase) within presynaptic terminals suggests that neuroestrogens can be synthesized directly at the neuronal synapse. A consequent prediction follows that synaptic estrogen production is controlled via classical electrochemical events in neurons. Here, we present evidence that acute fluctuations in local neuroestrogen levels in the forebrain of the zebra finch depend on calcium influx within presynaptic terminals. In vivo experiments using microdialysis linked to a sensitive estrogen ELISA showed that local forebrain neuroestrogens were both suppressed by potassium-evoked excitation and upregulated during 30 min periods of extracellular calcium depletion in a region enriched with presynaptic aromatase. Furthermore, potassium-evoked changes in local neuroestrogens were blocked by targeted delivery of the voltage-gated calcium channel blocker ω-conotoxin GVIA. Together, these experiments indicate that neuroestrogens are controlled by specific, depolarization-sensitive, calcium-dependent events within forebrain presynaptic terminals.

Early Language Learning and Literacy: Neuroscience Implications for Education

The last decade has produced an explosion in neuroscience research examining young children's early processing of language that has implications for education. Noninvasive, safe functional brain measurements have now been proven feasible for use with children starting at birth. In the arena of language, the neural signatures of learning can be documented at a remarkably early point in development, and these early measures predict performance in children's language and pre-reading abilities in the second, third, and fifth year of life, a finding with theoretical and educational import. There is evidence that children's early mastery of language requires learning in a social context, and this finding also has important implications for education. Evidence relating socio-economic status (SES) to brain function for language suggests that SES should be considered a proxy for the opportunity to learn and that the complexity of language input is a significant factor in developing brain areas related to language. The data indicate that the opportunity to learn from complex stimuli and events are vital early in life, and that success in school begins in infancy.

Aggressive interactions differentially modulate local and systemic levels of corticosterone and DHEA in a wild songbird

During the nonbreeding season, when gonadal androgen synthesis is basal, recent evidence suggests that neurosteroids regulate the aggression of male song sparrows. In particular, dehydroepiandrosterone (DHEA) is rapidly converted in the brain to androgens in response to aggressive interactions. In other species, aggressive encounters increase systemic glucocorticoid levels. However, the relationship between aggression and local steroid levels is not well understood. Here, during the breeding and nonbreeding seasons, we tested the effects of a simulated territorial intrusion (STI) on DHEA and corticosterone levels in the brachial and jugular plasma. Jugular plasma is enriched with neurosteroids and provides an indirect index of brain steroid levels. Further, during the nonbreeding season, we directly measured steroid levels in the brain and peripheral tissues. Both breeding and nonbreeding males displayed robust aggressive responses to STI. During the breeding season, STI increased brachial and jugular corticosterone levels and jugular DHEA levels. During the nonbreeding season, STI did not affect plasma corticosterone levels, but increased jugular DHEA levels. During the nonbreeding season, STI did not affect brain levels of corticosterone or DHEA. However, STI did increase corticosterone and DHEA concentrations in the liver and corticosterone concentrations in the pectoral muscle. These data suggest that 1) aggressive social interactions affect neurosteroid levels in both seasons and 2) local steroid synthesis in peripheral tissues may mobilize energy reserves to fuel aggression in the nonbreeding season. Local steroid synthesis in brain, liver or muscle may serve to avoid the costs of systemic increases in corticosterone and testosterone.

Combined liquid and solid-phase extraction improves quantification of brain estrogen content

Accuracy in quantifying brain-derived steroid hormones ("neurosteroids") has become increasingly important for understanding the modulation of neuronal activity, development, and physiology. Relative to other neuroactive compounds and classical neurotransmitters, steroids pose particular challenges with regard to isolation and analysis, owing to their lipid solubility. Consequently, anatomical studies of the distribution of neurosteroids have relied primarily on the expression of neurosteroid synthesis enzymes. To evaluate the distribution of synthesis enzymes vis-à-vis the actual steroids themselves, traditional steroid quantification assays, including radioimmunoassays, have successfully employed liquid extraction methods (e.g., ether, dichloromethane, or methanol) to isolate steroids from microdissected brain tissue. Due to their sensitivity, safety, and reliability, the use of commercial enzyme-immunoassays (EIA) for laboratory quantification of steroids in plasma and brain has become increasingly widespread. However, EIAs rely on enzymatic reactions in vitro, making them sensitive to interfering substances in brain tissue and thus producing unreliable results. Here, we evaluate the effectiveness of a protocol for combined, two-stage liquid/solid-phase extraction (SPE) as compared to conventional liquid extraction alone for the isolation of estradiol (E(2)) from brain tissue. We employ the songbird model system, in which brain steroid production is pronounced and linked to neural mechanisms of learning and plasticity. This study outlines a combined liquid-SPE protocol that improves the performance of a commercial EIA for the quantification of brain E(2) content. We demonstrate the effectiveness of our optimized method for evaluating the region specificity of brain E(2) content, compare these results to established anatomy of the estrogen synthesis enzyme and estrogen receptor, and discuss the nature of potential EIA interfering substances.

Estradiol-dependent modulation of auditory processing and selectivity in songbirds

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

Synaptocrine signaling: steroid synthesis and action at the synapse

Sex steroids have long been recognized for their dramatic impact on brain and behavior, including rapid modulation of membrane excitability. It is a widely held perception that these molecules are largely derived from peripheral sources and lack the spatial and temporal specificity ascribed to classical neuromodulatory systems. Neuromodulatory systems, in contrast, are defined by their regulated neuronal presynaptic secretion and by their functional modulation of perisynaptic events. Here we provide evidence for regulated presynaptic estrogen synthesis and functional postsynaptic actions. These results meet all the criteria for a neuromodulatory system and shift our perception of estrogens from that of peripheral signals exclusively to include that of a signaling system intrinsic to the brain itself. We apply the term synaptocrine to describe this form of neuromodulation.

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

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