Forebrain steroid levels fluctuate rapidly during social interactions

First-Principles Structure Search Study of 17-β-Estradiol Adsorption on Graphene

17-Beta-estradiol (E2), a steroid hormone synthesized from cholesterol, has various impacts on health and the environment. Currently, the gold standard for its measurement in the body is a conventional blood test (mass spectrometry), but carbon-based electrochemical sensors have been proposed as an alternative due to their advantages, such as rapid analysis time and sensitivity. To improve the atomic-level understanding of the interactions at the substrate surface, we performed density functional theory (DFT) simulations to study the nature of the adsorption of E2 on pristine graphene. Bayesian Optimization Structure Search (BOSS) was employed to reduce human bias in the determination of the most favorable adsorption configurations. Two stable adsorption minimum configurations were found. Analysis of their electronic properties indicates that E2 physisorbs on graphene. Embarking upon complex carbonaceous materials, the importance of finding all possible minimum candidates with automated structure search tools is highlighted. Computational investigations facilitate tailoring substrate materials with outstanding performance and applications in neuroscientific research, fertility monitoring, and clinical trials. Combining them with experimental research carries significant potential to advance sensor design beyond the current state-of-the-art.

G protein-coupled estrogen receptor (GPER) in the dorsal hippocampus regulates memory consolidation in gonadectomized male mice, likely via different signaling mechanisms than in female mice

Studies in ovariectomized (OVX) female rodents suggest that G protein-coupled estrogen receptor (GPER) is a key regulator of memory, yet little is known about its importance to memory in males or the cellular mechanisms underlying its mnemonic effects in either sex. In OVX mice, bilateral infusion of the GPER agonist G-1 into the dorsal hippocampus (DH) enhances object recognition and spatial memory consolidation in a manner dependent on rapid activation of c-Jun N-terminal kinase (JNK) signaling, cofilin phosphorylation, and actin polymerization in the DH. However, the effects of GPER on memory consolidation and DH cell signaling in males are unknown. Thus, the present study first assessed effects of DH infusion of G-1 or the GPER antagonist G-15 on object recognition and spatial memory consolidation in gonadectomized (GDX) male mice. As in OVX mice, immediate post-training bilateral DH infusion of G-1 enhanced, whereas G-15 impaired, memory consolidation in the object recognition and object placement tasks. However, G-1 did not increase levels of phosphorylated JNK (p46, p54) or cofilin in the DH 5, 15, or 30 min after infusion, nor did it affect phosphorylation of ERK (p42, p44), PI3K, or Akt. Levels of phospho-cAMP-responsive element binding protein (CREB) were elevated in the DH 30 min following G-1 infusion, indicating that GPER in males activates a yet unknown signaling mechanism that triggers CREB-mediated gene transcription. Our findings show for the first time that GPER in the DH regulates memory consolidation in males and suggests sex differences in underlying signaling mechanisms.

Estrogen and testosterone secretion from the mouse brain

Estrogen and testosterone are typically thought of as gonadal or adrenal derived steroids that cross the blood brain barrier to signal via both rapid nongenomic and slower genomic signalling pathways. Estrogen and testosterone signalling has been shown to drive interlinked behaviours such as social behaviours and cognition by binding to their cognate receptors in hypothalamic and forebrain nuclei. So far, acute brain slices have been used to study short-term actions of 17β-estradiol, typically using electrophysiological measures. For example, these techniques have been used to investigate, nongenomic signalling by estrogen such as the estrogen modulation of long-term potentiation (LTP) in the hippocampus. Using a modified method that preserves the slice architecture, we show, for the first time, that acute coronal slices from the prefrontal cortex and from the hypothalamus maintained in aCSF over longer periods i.e. 24 h can be steroidogenic, increasing their secretion of testosterone and estrogen. We also show that the hypothalamic nuclei produce more estrogen and testosterone than the prefrontal cortex. Therefore, this extended acute slice system can be used to study the regulation of steroid production and secretion by discrete nuclei in the brain.

Acute Aromatase Inhibition Impairs Neural and Behavioral Auditory Scene Analysis in Zebra Finches

Auditory perception can be significantly disrupted by noise. To discriminate sounds from noise, auditory scene analysis (ASA) extracts the functionally relevant sounds from acoustic input. The zebra finch communicates in noisy environments. Neurons in their secondary auditory pallial cortex (caudomedial nidopallium, NCM) can encode song from background chorus, or scenes, and this capacity may aid behavioral ASA. Furthermore, song processing is modulated by the rapid synthesis of neuroestrogens when hearing conspecific song. To examine whether neuroestrogens support neural and behavioral ASA in both sexes, we retrodialyzed fadrozole (aromatase inhibitor, FAD) and recorded in vivo awake extracellular NCM responses to songs and scenes. We found that FAD affected neural encoding of songs by decreasing responsiveness and timing reliability in inhibitory (narrow-spiking), but not in excitatory (broad-spiking) neurons. Congruently, FAD decreased neural encoding of songs in scenes for both cell types, particularly in females. Behaviorally, we trained birds using operant conditioning and tested their ability to detect songs in scenes after administering FAD orally or injected bilaterally into NCM. Oral FAD increased response bias and decreased correct rejections in females, but not in males. FAD in NCM did not affect performance. Thus, FAD in the NCM impaired neuronal ASA but that did not lead to behavioral disruption suggesting the existence of resilience or compensatory responses. Moreover, impaired performance after systemic FAD suggests involvement of other aromatase-rich networks outside the auditory pathway in ASA. This work highlights how transient estrogen synthesis disruption can modulate higher-order processing in an animal model of vocal communication.

Brain and circulating steroids in an electric fish: Relevance for non-breeding aggression

Steroids play a crucial role in modulating brain and behavior. While traditionally it is thought that the brain is a target of sex steroids produced in endocrine glands (e.g. gonads), the brain itself produces steroids, known as neurosteroids. Neurosteroids can be produced in regions involved in the regulation of social behaviors and may act locally to regulate social behaviors, such as reproduction and aggression. Our model species, the weakly electric fish Gymnotus omarorum, displays non-breeding aggression in both sexes. This is a valuable natural behavior to understand neuroendocrine mechanisms that differ from those underlying breeding aggression. In the non-breeding season, circulating sex steroid levels are low, which facilitates the study of neurosteroids. Here, for the first time in a teleost fish, we used liquid chromatography-tandem mass spectrometry (LC-MS/MS) to quantify a panel of 8 steroids in both plasma and brain to characterize steroid profiles in wild non-breeding adult males and females. We show that: 1) systemic steroid levels in the non-breeding season are similar in both sexes, although only males have detectable circulating 11-ketotestosterone, 2) brain steroid levels are sexually dimorphic, as females display higher levels of androstenedione, testosterone and estrone, and only males had detectable 11-ketotestosterone, 3) systemic androgens such as androstenedione and testosterone in the non-breeding season are potential precursors for neuroestrogen synthesis, and 4) estrogens, which play a key role in non-breeding aggression, are detectable in the brain (but not the plasma) in both sexes. These data are consistent with previous studies of G. omarorum that show non-breeding aggression is dependent on estrogen signaling, as has also been shown in bird and mammal models. Overall, our results provide a foundation for understanding the role of neurosteroids, the interplay between central and peripheral steroids and potential sex differences in the regulation of social behaviors.

Sex differences in aggressive intensities and brain steroids during status resolution in a sex changing fish, Lythrypnus dalli

For vertebrates living in social hierarchies, the neuroendocrine system regulates temporal aspects of aggressive interactions during status establishment. In teleost fishes, the sex steroids 17β-estradiol (E₂) and 11-ketotestosterone (KT), and the glucocorticoid, cortisol (CORT) are associated with aggression in distinct phases of their life history. Bluebanded gobies, Lythrypnus dalli, exhibit bidirectional sexual plasticity by responding to changes in their social structure by escalating aggression associated with neural changes that precede gonadal reorganization to the opposite sex. Here, we used a novel experimental design to investigate systemic (waterborne) and neural steroids associated with the earliest behavioral changes associated with feminization and masculinization during protandrous and protogynous sex change respectively. In stable social groups of wild-caught L. dalli comprising of one male and two females, we disrupted hierarchy by adding or removing a male, providing a social context for intrasexual aggression. Within only 30 min, males exhibited high rates of physical aggression inside the nest to maintain their territory, while females exhibited high rates of chases outside the nest to reestablish social status. During this period of instability, while waterborne steroids were not affected, brain E₂ was higher in all fish and CORT was lower in male brains. Brain KT was higher in males who emerged as dominant compared to dominant females. Overall, a combination of differences in brain E₂, CORT, and KT were important in the regulation of hierarchy re-establishment and maintenance. Rapid responses during conspecific aggressive encounters are likely mediated by neural steroid synthesis that precede changes in systemic steroids.

Correlated evolution of social organization and lifespan in mammals

Discerning the relationship between sociality and longevity would permit a deeper understanding of how animal life history evolved. Here, we perform a phylogenetic comparative analysis of ~1000 mammalian species on three states of social organization (solitary, pair-living, and group-living) and longevity. We show that group-living species generally live longer than solitary species, and that the transition rate from a short-lived state to a long-lived state is higher in group-living than non-group-living species, altogether supporting the correlated evolution of social organization and longevity. The comparative brain transcriptomes of 94 mammalian species identify 31 genes, hormones and immunity-related pathways broadly involved in the association between social organization and longevity. Further selection features reveal twenty overlapping pathways under selection for both social organization and longevity. These results underscore a molecular basis for the influence of the social organization on longevity.

Sex-Linked Biology and Gender-Related Research Is Essential to Advancing Hearing Health

There is robust evidence that sex (biological) and gender (behavioral/social) differences influence hearing loss risk and outcomes. These differences are noted for animals and humans-in the occurrence of hearing loss, hearing loss progression, and response to interventions. Nevertheless, many studies have not reported or disaggregated data by sex or gender. This article describes the influence of sex-linked biology (specifically sex-linked hormones) and gender on hearing and hearing interventions, including the role of sex-linked biology and gender in modifying the association between risk factors and hearing loss, and the effects of hearing loss on quality of life and functioning. Most prevalence studies indicate that hearing loss begins earlier and is more common and severe among men than women. Intrinsic sex-linked biological differences in the auditory system may account, in part, for the predominance of hearing loss in males. Sex- and gender-related differences in the effects of noise exposure or cardiovascular disease on the auditory system may help explain some of these differences in the prevalence of hearing loss. Further still, differences in hearing aid use and uptake, and the effects of hearing loss on health may also vary by sex and gender. Recognizing that sex-linked biology and gender are key determinants of hearing health, the present review concludes by emphasizing the importance of a well-developed research platform that proactively measures and assesses sex- and gender-related differences in hearing, including in understudied populations. Such research focus is necessary to advance the field of hearing science and benefit all members of society.

A brain for all seasons: An in vivo MRI perspective on songbirds

Seasonality in songbirds includes not only reproduction but also seasonal changes in singing behavior and its neural substrate, the song control system (SCS). Prior research mainly focused on the role of sex steroids on this seasonal SCS neuroplasticity in males. In this review, we summarize the advances made in the field of seasonal neuroplasticity by applying in vivo magnetic resonance imaging (MRI) in male and female starlings, analyzing the entire brain, monitoring birds longitudinally and determining the neuronal correlates of seasonal variations in plasma hormone levels and song behavior. The first MRI studies in songbirds used manganese enhanced MRI to visualize the SCS in a living bird and validated previously described brain volume changes related to different seasons and testosterone. MRI studies with testosterone implantation established how the consequential boost in singing was correlated to structural changes in the SCS, indicating activity-induced neuroplasticity as song proficiency increased. Next, diffusion tensor MRI explored seasonal neuroplasticity in the entire brain, focusing on networks beyond the SCS, revealing that other sensory systems and even the cerebellum, which is important for the integration of sensory perception and song behavior, experience neuroplasticity starting in the photosensitive period. Functional MRI showed that olfactory, and auditory processing was modulated by the seasons. The convergence of seasonal variations in so many sensory and sensorimotor systems resembles multisensory neuroplasticity during the critical period early in life. This sheds new light on seasonal songbirds as a model for unlocking the brain by recreating seasonally the permissive circumstances for heightened neuroplasticity.

Evolutionary endocrinology and the problem of Darwin's tangled bank

Like Darwin's tangled bank of biodiversity, the endocrine mechanisms that give rise to phenotypic diversity also exhibit nearly endless forms. This tangled bank of mechanistic diversity can prove problematic as we seek general principles on the role of endocrine mechanisms in phenotypic evolution. A key unresolved question is therefore: to what degree are specific endocrine mechanisms re-used to bring about replicated phenotypic evolution? Related areas of inquiry are booming in molecular ecology, but behavioral traits are underrepresented in this literature. Here, I leverage the rich comparative tradition in evolutionary endocrinology to evaluate whether and how certain mechanisms may be repeated hotspots of behavioral evolutionary change. At one extreme, mechanisms may be parallel, such that evolution repeatedly uses the same gene or pathway to arrive at multiple independent (or, convergent) origins of a particular behavioral trait. At the other extreme, the building blocks of behavior may be unique, such that outwardly similar phenotypes are generated via lineage-specific mechanisms. This review synthesizes existing case studies, phylogenetic analyses, and experimental evolutionary research on mechanistic parallelism in animal behavior. These examples show that the endocrine building blocks of behavior have some elements of parallelism across replicated evolutionary events. However, support for parallelism is variable among studies, at least some of which relates to the level of complexity at which we consider sameness (i.e. pathway vs. gene level). Moving forward, we need continued experimentation and better testing of neutral models to understand whether, how - and critically, why - mechanism A is used in one lineage and mechanism B is used in another. We also need continued growth of large-scale comparative analyses, especially those that can evaluate which endocrine parameters are more or less likely to undergo parallel evolution alongside specific behavioral traits. These efforts will ultimately deepen understanding of how and why hormone-mediated behaviors are constructed the way that they are.

Estrogens rapidly shape synaptic and intrinsic properties to regulate the temporal precision of songbird auditory neurons

Sensory neurons parse millisecond-variant sound streams like birdsong and speech with exquisite precision. The auditory pallial cortex of vocal learners like humans and songbirds contains an unconventional neuromodulatory system: neuronal expression of the estrogen synthesis enzyme aromatase. Local forebrain neuroestrogens fluctuate when songbirds hear a song, and subsequently modulate bursting, gain, and temporal coding properties of auditory neurons. However, the way neuroestrogens shape intrinsic and synaptic properties of sensory neurons remains unknown. Here, using a combination of whole-cell patch clamp electrophysiology and calcium imaging, we investigate estrogenic neuromodulation of auditory neurons in a region resembling mammalian auditory association cortex. We found that estradiol rapidly enhances the temporal precision of neuronal firing via a membrane-bound G-protein coupled receptor and that estradiol rapidly suppresses inhibitory synaptic currents while sparing excitation. Notably, the rapid suppression of intrinsic excitability by estradiol was predicted by membrane input resistance and was observed in both males and females. These findings were corroborated by analysis of in vivo electrophysiology recordings, in which local estrogen synthesis blockade caused acute disruption of the temporal correlation of song-evoked firing patterns. Therefore, on a modulatory timescale, neuroestrogens alter intrinsic cellular properties and inhibitory neurotransmitter release to regulate the temporal precision of higher-order sensory neurons.

Sex-specific differences in zebrafish brains

In this systematic review, we highlight the differences between the male and female zebrafish brains to understand their differentiation and their use in studying sex-specific neurological diseases. Male and female brains display subtle differences at the cellular level which may be important in driving sex-specific signaling. Sex differences in the brain have been observed in humans as well as in non-human species. However, the molecular mechanisms of brain sex differentiation remain unclear. The classical model of brain sex differentiation suggests that the steroid hormones derived from the gonads are the primary determinants in establishing male and female neural networks. Recent studies indicate that the developing brain shows sex-specific differences in gene expression prior to gonadal hormone action. Hence, genetic differences may also be responsible for differentiating the brain into male and female types. Understanding the signaling mechanisms involved in brain sex differentiation could help further elucidate the sex-specific incidences of certain neurological diseases. The zebrafish model could be appropriate for enhancing our understanding of brain sex differentiation and the signaling involved in neurological diseases. Zebrafish brains show sex-specific differences at the hormonal level, and recent advances in RNA sequencing have highlighted critical sex-specific differences at the transcript level. The differences are also evident at the cellular and metabolite levels, which could be important in organizing sex-specific neuronal signaling. Furthermore, in addition to having one ortholog for 70% of the human gene, zebrafish also shares brain structural similarities with other higher eukaryotes, including mammals. Hence, deciphering brain sex differentiation in zebrafish will help further enhance the diagnostic and pharmacological intervention of neurological diseases.

Uncovering the seasonal brain: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a biochemical approach for studying seasonal social behaviors

Many animals show pronounced changes in physiology and behavior across the annual cycle, and these adaptations enable individuals to prioritize investing in the neuroendocrine mechanisms underlying reproduction and/or survival based on the time of year. While prior research has offered valuable insight into how seasonal variation in neuroendocrine processes regulates social behavior, the majority of these studies have investigated how a single hormone influences a single behavioral phenotype. Given that hormones are synthesized and metabolized via complex biochemical pathways and often act in concert to control social behavior, these approaches provide a limited view of how hormones regulate seasonal changes in behavior. In this review, we discuss how seasonal influences on hormones, the brain, and social behavior can be studied using liquid chromatography-tandem mass spectrometry (LC-MS/MS), an analytical chemistry technique that enables researchers to simultaneously quantify the concentrations of multiple hormones and the activities of their synthetic enzymes. First, we examine studies that have investigated seasonal plasticity in brain-behavior interactions, specifically by focusing on how two groups of hormones, sex steroids and nonapeptides, regulate sexual and aggressive behavior. Then, we explain the operations of LC-MS/MS, highlight studies that have used LC-MS/MS to study the neuroendocrine mechanisms underlying social behavior, both within and outside of a seasonal context, and discuss potential applications for LC-MS/MS in the field of behavioral neuroendocrinology. We propose that this cutting-edge technology will provide a more comprehensive understanding of how the multitude of hormones that comprise complex neuroendocrine networks affect seasonal variation in the brain and behavior.

Motivational increase of androgens and behavior by infant distress calls in highly responsive common marmoset fathers, Callithrix jacchus

Common marmoset fathers are highly involved in care of their infants. However, variability exists in their response to infant behavior even in paternally experienced fathers. Using infant distress cries as a motivation test, we investigated: 1. the differences in paternally experienced fathers' motivation to search for the infant vocalization stimuli; 2. the relationship between a father's motivation to search for the source of the infant cries and testosterone levels; and 3. if there is a rapid steroidogenesis pathway leading to increased testosterone and estradiol in the peripheral circulation. Only 44% of the paternally experienced fathers showed a high frequency of searching for the source of the infant distress cries. Through the use of multisteroid analysis, we found high responsive fathers had significantly higher levels of progesterone and testosterone in response to infant distress cries compared to a control stimulus with progesterone and androstenedione correlating with testosterone, while no differences were seen in low responders. The frequency to search for the infant stimuli was positively correlated with higher testosterone compared to control vocal levels. These results suggest that searching for the source of infant cries represents a motivation behavior for fathers that is activated by testosterone and reflects rapid circulating testosterone.

Sex Differences in Social Cognition

In this review we explore the sex differences underlying various types of social cognition. Particular focus will be placed on the behaviors of social recognition, social learning, and aggression. Known similarities and differences between sexes in the expressions of these behaviors and the known brain regions where these behaviors are mediated are discussed. The role that the sex hormones (estrogens and androgens) have as well as possible interactions with other neurochemicals, such as oxytocin, vasopressin, and serotonin is reviewed as well. Finally, implications about these findings on the mediation of social cognition are mediated and the sex differences related to humans are considered.

Specialized androgen synthesis in skeletal muscles that actuate elaborate social displays

Androgens mediate the expression of many reproductive behaviors, including the elaborate displays used to navigate courtship and territorial interactions. In some vertebrates, males can produce androgen-dependent sexual behavior even when levels of testosterone (T) is low in the bloodstream. One idea is that select tissues make their own androgens from scratch to support behavioral performance. We first study this phenomenon in the skeletal muscles that actuate elaborate sociosexual displays in downy woodpeckers and two songbirds. We show that the woodpecker display muscle maintains elevated T when the testes are regressed in the non-breeding season. Both the display muscles of woodpeckers, as well as the display muscles in the avian vocal organ (syrinx or SYR) of songbirds, express all transporters and enzymes necessary to convert cholesterol into bioactive androgens locally. In a final analysis, we broaden our study by looking for these same transporters and enzymes in mammalian muscles that operate at different speeds. Using RNA-seq data, we find that the capacity for de novo synthesis is only present in "superfast" extraocular muscle. Together, our results suggest that skeletal muscle specialized to generate extraordinary twitch-times and/or extremely rapid contractile speeds may depend on androgenic hormones produced locally within the muscle itself. Our study therefore uncovers an important new dimension of androgenic regulation of behavior.

Seasonal variation in gonadal hormones, spatial cognition, and hippocampal attributes: More questions than answers

A large body of research has been dedicated to understanding the factors that modulate spatial cognition and attributes of the hippocampus, a highly plastic brain region that underlies spatial processing abilities. Variation in gonadal hormones impacts spatial memory and hippocampal attributes in vertebrates, although the direction of the effect has not been entirely consistent. To add complexity, individuals in the field must optimize fitness by coordinating activities with the appropriate environmental cues, and many of these behaviors are correlated tightly with seasonal variation in gonadal hormone release. As such, it remains unclear if the relationship among systemic gonadal hormones, spatial cognition, and the hippocampus also exhibits seasonal variation. This review presents an overview of the relationship among gonadal hormones, the hippocampus, and spatial cognition, and how the seasonal release of gonadal hormones correlates with seasonal variation in spatial cognition and hippocampal attributes. Additionally, this review presents other neuroendocrine mechanisms that may be involved in modulating the relationship among seasonality, gonadal hormone release, and the hippocampus and spatial cognition, including seasonal rhythms of steroid hormone binding globulins, neurosteroids, sex steroid hormone receptor expression, and hormone interactions. Here, endocrinology, ecology, and behavioral neuroscience are brought together to present an overview of the research demonstrating the mechanistic effects of systemic gonadal hormones on spatial cognition and the hippocampus, while, at a functional level, superimposing seasonal effects to examine ecologically-relevant circannual changes in gonadal hormones and spatial behaviors.

Mapping the vocal circuitry of Alston's singing mouse with pseudorabies virus

Vocalizations are often elaborate, rhythmically structured behaviors. Vocal motor patterns require close coordination of neural circuits governing the muscles of the larynx, jaw, and respiratory system. In the elaborate vocalization of Alston's singing mouse (Scotinomys teguina) each note of its rapid, frequency-modulated trill is accompanied by equally rapid modulation of breath and gape. To elucidate the neural circuitry underlying this behavior, we introduced the polysynaptic retrograde neuronal tracer pseudorabies virus (PRV) into the cricothyroid and digastricus muscles, which control frequency modulation and jaw opening, respectively. Each virus singly labels ipsilateral motoneurons (nucleus ambiguus for cricothyroid, and motor trigeminal nucleus for digastricus). We find that the two isogenic viruses heavily and bilaterally colabel neurons in the gigantocellular reticular formation, a putative central pattern generator. The viruses also show strong colabeling in compartments of the midbrain including the ventrolateral periaqueductal gray and the parabrachial nucleus, two structures strongly implicated in vocalizations. In the forebrain, regions important to social cognition and energy balance both exhibit extensive colabeling. This includes the paraventricular and arcuate nuclei of the hypothalamus, the lateral hypothalamus, preoptic area, extended amygdala, central amygdala, and the bed nucleus of the stria terminalis. Finally, we find doubly labeled neurons in M1 motor cortex previously described as laryngeal, as well as in the prelimbic cortex, which indicate these cortical regions play a role in vocal production. The progress of both viruses is broadly consistent with vertebrate-general patterns of vocal circuitry, as well as with circuit models derived from primate literature.

A neural circuit perspective on brain aromatase

This review explores the role of aromatase in the brain as illuminated by a set of conserved network-level connections identified in several vertebrate taxa. Aromatase-expressing neurons are neurochemically heterogeneous but the brain regions in which they are found are highly-conserved across the vertebrate lineage. During development, aromatase neurons have a prominent role in sexual differentiation of the brain and resultant sex differences in behavior and human brain diseases. Drawing on literature primarily from birds and rodents, we delineate brain regions that express aromatase and that are strongly interconnected, and suggest that, in many species, aromatase expression essentially defines the Social Behavior Network. Moreover, in several cases the inputs to and outputs from this core Social Behavior Network also express aromatase. Recent advances in molecular and genetic tools for neuroscience now enable in-depth and taxonomically diverse studies of the function of aromatase at the neural circuit level.

Non-sensory Influences on Auditory Learning and Plasticity

Distinguishing between regular and irregular heartbeats, conversing with speakers of different accents, and tuning a guitar-all rely on some form of auditory learning. What drives these experience-dependent changes? A growing body of evidence suggests an important role for non-sensory influences, including reward, task engagement, and social or linguistic context. This review is a collection of contributions that highlight how these non-sensory factors shape auditory plasticity and learning at the molecular, physiological, and behavioral level. We begin by presenting evidence that reward signals from the dopaminergic midbrain act on cortico-subcortical networks to shape sound-evoked responses of auditory cortical neurons, facilitate auditory category learning, and modulate the long-term storage of new words and their meanings. We then discuss the role of task engagement in auditory perceptual learning and suggest that plasticity in top-down cortical networks mediates learning-related improvements in auditory cortical and perceptual sensitivity. Finally, we present data that illustrates how social experience impacts sound-evoked activity in the auditory midbrain and forebrain and how the linguistic environment rapidly shapes speech perception. These findings, which are derived from both human and animal models, suggest that non-sensory influences are important regulators of auditory learning and plasticity and are often implemented by shared neural substrates. Application of these principles could improve clinical training strategies and inform the development of treatments that enhance auditory learning in individuals with communication disorders.

Neuroendocrine underpinning of social recognition in males and females

Social recognition is an essential skill for the expression of appropriate behaviors towards conspecifics in most social species. Several studies point to oxytocin (OT) and arginine vasopressin (AVP) as key mediators of social recognition in males and females. However, sex differences in social cognitive behaviors highlight an important interplay between OT, AVP and the sex steroids. Estrogens facilitate social recognition by regulating OT action in the hypothalamus and that of OT receptor in the medial amygdala. The role of OT in these brain regions appears to be essential for social recognition in both males and females. Conversely, social recognition in male rats and mice is more dependent on AVP release in the lateral septum than in females. The AVP system comprises a series of highly sexually dimorphic brain nuclei, including the bed nucleus of the stria terminalis, the amygdala and the lateral septum. Various studies suggest that testosterone and its metabolites, including estradiol, influence social recognition in males by modulating the activity of the AVP at V1a receptor. Intriguingly, both estrogens and androgens can affect social recognition very rapidly, through non-genomic mechanisms. In addition, the androgen metabolites, namely 3α-diol and 3β-diol, may also have an impact on social behaviors either by interacting with the estrogen receptors or through other mechanisms. Overall, the regulation of OT and AVP by sex steroids fine tunes social recognition and the behaviors that depend upon it (e.g., social bond, hierarchical organization, aggression) in a sex-dependent manner. Elucidating the sex-dependent interaction between sex steroids and neuroendocrine systems is essential for understanding sex differences in the normal and abnormal expression of social behaviors.

The form, function, and evolutionary significance of neural aromatization

Songbirds have emerged as exceptional research subjects for helping us appreciate and understand estrogen synthesis and function in brain. In the context of recognizing the vertebrate-wide importance of brain aromatase expression, in this review we highlight where we believe studies of songbirds have provided clarification and conceptual insight. We follow by focusing on more recent studies of aromatase and neuroestrogen function in the hippocampus and the pallial auditory processing region NCM of songbirds. With perspectives drawn from this body of work, we speculate that the evolution of enhanced neural estrogen signaling, including in the mediation of social behaviors, may have given songbirds the resilience to radiate into one of the most successful vertebrate groups on the planet.

Neuron-Derived Estrogen-A Key Neuromodulator in Synaptic Function and Memory

In addition to being a steroid hormone, 17β-estradiol (E₂) is also a neurosteroid produced in neurons in various regions of the brain of many species, including humans. Neuron-derived E₂ (NDE₂) is synthesized from androgen precursors via the action of the biosynthetic enzyme aromatase, which is located at synapses and in presynaptic terminals in neurons in both the male and female brain. In this review, we discuss evidence supporting a key role for NDE₂ as a neuromodulator that regulates synaptic plasticity and memory. Evidence supporting an important neuromodulatory role of NDE₂ in the brain has come from studies using aromatase inhibitors, aromatase overexpression in neurons, global aromatase knockout mice, and the recent development of conditional forebrain neuron-specific knockout mice. Collectively, these studies demonstrate a key role of NDE₂ in the regulation of synapse and spine density, efficacy of excitatory synaptic transmission and long-term potentiation, and regulation of hippocampal-dependent recognition memory, spatial reference memory, and contextual fear memory. NDE₂ is suggested to achieve these effects through estrogen receptor-mediated regulation of rapid kinase signaling and CREB-BDNF signaling pathways, which regulate actin remodeling, as well as transcription, translation, and transport of synaptic proteins critical for synaptic plasticity and function.

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

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

Brain-derived estrogen and neural function

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

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

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

Brain-Generated 17β-Estradiol Modulates Long-Term Synaptic Plasticity in the Primary Auditory Cortex of Adult Male Rats

Neuron-derived 17β-estradiol (E2) alters synaptic transmission and plasticity in brain regions with endocrine and non-endocrine functions. Investigations into a modulatory role of E2 in synaptic activity and plasticity have mainly focused on the rodent hippocampal formation. In songbirds, E2 is synthesized by auditory forebrain neurons and promotes auditory signal processing and memory for salient acoustic stimuli; however, the modulatory effects of E2 on memory-related synaptic plasticity mechanisms have not been directly examined in the auditory forebrain. We investigated the effects of bidirectional E2 manipulations on synaptic transmission and long-term potentiation (LTP) in the rat primary auditory cortex (A1). Immunohistochemistry revealed widespread neuronal expression of the E2 biosynthetic enzyme aromatase in multiple regions of the rat sensory and association neocortex, including A1. In A1, E2 application reduced the threshold for in vivo LTP induction at layer IV synapses, whereas pharmacological suppression of E2 production by aromatase inhibition abolished LTP induction at layer II/III synapses. In acute A1 slices, glutamate and γ-aminobutyric acid (GABA) receptor-mediated currents were sensitive to E2 manipulations in a layer-specific manner. These findings demonstrate that locally synthesized E2 modulates synaptic transmission and plasticity in A1 and suggest potential mechanisms by which E2 contributes to auditory signal processing and memory.

Aromatase and nonaromatase neurons in the zebra finch secondary auditory forebrain are indistinct in their song-driven gene induction and intrinsic electrophysiological properties

Estrogens support major brain functions including cognition, reproduction, neuroprotection and sensory processing. Neuroestrogens are synthesized within some brain areas by the enzyme aromatase and can rapidly modulate local circuit functions, yet the cellular physiology and sensory-response profiles of aromatase neurons are essentially unknown. In songbirds, social and acoustic stimuli drive neuroestrogen elevations in the auditory forebrain caudomedial nidopallium (NCM). In both males and females, neuroestrogens rapidly enhance NCM auditory processing and auditory learning. Estrogen-producing neurons in NCM may therefore exhibit distinguishing profiles for sensory-activation and intrinsic electrophysiology. Here, we explored these questions using both immunocyctochemistry and electrophysiological recordings. Immunoreactivity for aromatase and the immediate early gene EGR1, a marker of activity and plasticity, were quantified in NCM of song-exposed animals versus silence-exposed controls. Using whole-cell patch clamp recordings from NCM slices, we also documented the intrinsic excitability profiles of aromatase-positive and aromatase-negative neurons. We observed that a subset of aromatase neurons were significantly activated during song playback, in both males and females, and in both hemispheres. A comparable population of non-aromatase-expressing neurons were also similarly driven by song stimulation. Membrane properties (i.e., resting membrane potential, rheobase, input resistance and multiple action potential parameters) were similarly indistinguishable between NCM aromatase and non-aromatase neurons. Together, these findings demonstrate that aromatase and non-aromatase neurons in NCM are indistinct in terms of their intrinsic electrophysiology and responses to song. Nevertheless, such similarities in response properties may belie more subtle differences in underlying conductances and/or computational roles that may be crucial to their function.

Arbiters of endogenous opioid analgesia: role of CNS estrogenic and glutamatergic systems

Nociception and opioid antinociception in females are pliable processes, varying qualitatively and quantitatively over the reproductive cycle. Spinal estrogenic signaling via membrane estrogen receptors (mERs), in combination with multiple other signaling molecules [spinal dynorphin, kappa-opioid receptors (KOR), glutamate and metabotropic glutamate receptor 1 (mGluR₁)], appears to function as a master coordinator, parsing functionality between pronociception and antinociception. This provides a window into pharmacologically accessing intrinsic opioid analgesic/anti-allodynic systems. In diestrus, membrane estrogen receptor alpha (mERα) signals via mGluR₁ to suppress spinal endomorphin 2 (EM2) analgesia. Strikingly, in the absence of exogenous opioids, interfering with this suppression in a chronic pain model elicits opioid anti-allodynia, revealing contributions of endogenous opioid(s). In proestrus, robust spinal EM2 analgesia is manifest but this requires spinal dynorphin/KOR and glutamate-activated mGluR₁. Furthermore, spinal mGluR₁ blockade in a proestrus chronic pain animal (eliminating spinal EM2 analgesia) exacerbates mechanical allodynia, revealing tempering by endogenous opioid(s). A complex containing mu-opioid receptor, KOR, aromatase, mGluRs, and mERα are foundational to eliciting endogenous opioid anti-allodynia. Aromatase-mERα oligomers are also plentiful, in a central nervous system region-specific fashion. These can be independently regulated and allow estrogens to act intracellularly within the same signaling complex in which they are synthesized, explaining asynchronous relationships between circulating estrogens and central nervous system estrogen functionalities. Observations with EM2 highlight the translational relevance of extensively characterizing exogenous responsiveness to endogenous opioids and the neuronal circuits that mediate them along with the multiplicity of estrogenic systems that concomitantly function in phase and out-of-phase with the reproductive cycle.

Dopamine D1 Receptor Activation Drives Plasticity in the Songbird Auditory Pallium

Vocal learning species must form and extensively hone associations between sounds and social contingencies. In songbirds, dopamine signaling guides song motor production, variability, and motivation, but it is unclear how dopamine regulates fundamental auditory associations for learning new sounds. We hypothesized that dopamine regulates learning in the auditory pallium, in part by interacting with local neuroestradiol signaling. Here, we show that zebra finch auditory neurons frequently coexpress D1 receptor (D1R) protein, neuroestradiol-synthase, GABA, and parvalbumin (PV). Auditory classical conditioning increased neuroplasticity gene induction in D1R-positive neurons. In vitro, D1R pharmacological activation reduced the amplitude of GABAergic and glutamatergic currents and increased the latter's frequency. In vivo, D1R activation reduced the firing of putative interneurons, increased the firing of putative excitatory neurons, and made both neuronal types unable to adapt to novel stimuli. Together, these findings support the hypothesis that dopamine acting via D1Rs modulates auditory association in the songbird sensory pallium.SIGNIFICANCE STATEMENT Our key finding is that auditory forebrain D1 receptors (D1Rs) modulate auditory plasticity, in support of the hypothesis that dopamine modulates the formation of associations between sounds and outcomes. Recent work in songbirds has identified roles for dopamine in driving reinforcement learning and motor variability in song production. This leaves open whether dopamine shapes the initial events that are critical for learning vocalizations, e.g., auditory learning. Our study begins to address this question in the songbird caudomedial nidopallium (NCM), an analog of the mammalian secondary auditory cortex. Our findings indicate that dopamine receptors are important modulators of excitatory/inhibitory balance and sound association learning mechanisms in the NCM, a system that could be a fundamental feature of vertebrate ascending auditory pathways.

To hum or not to hum: Neural transcriptome signature of male courtship vocalization in a teleost fish

For many animal species, vocal communication is a critical social behavior and often a necessary component of reproductive success. Additionally, vocalizations are often demanding motor acts. Wanting to know whether a specific molecular toolkit might be required for vocalization, we used RNA-sequencing to investigate neural gene expression underlying the performance of an extreme vocal behavior, the courtship hum of the plainfin midshipman fish (Porichthys notatus). Single hums can last up to 2 h and may be repeated throughout an evening of courtship activity. We asked whether vocal behavioral states are associated with specific gene expression signatures in key brain regions that regulate vocalization by comparing transcript expression levels in humming versus non-humming males. We find that the circadian-related genes period3 and Clock are significantly upregulated in the vocal motor nucleus and preoptic area-anterior hypothalamus, respectively, in humming compared with non-humming males, indicating that internal circadian clocks may differ between these divergent behavioral states. In addition, we identify suites of differentially expressed genes related to synaptic transmission, ion channels and transport, neuropeptide and hormone signaling, and metabolism and antioxidant activity that together may support the neural and energetic demands of humming behavior. Comparisons of transcript expression across regions stress regional differences in brain gene expression, while also showing coordinated gene regulation in the vocal motor circuit in preparation for courtship behavior. These results underscore the role of differential gene expression in shifts between behavioral states, in this case neuroendocrine, motor and circadian control of courtship vocalization.

Profiling of systemic and brain steroids in male songbirds: Seasonal changes in neurosteroids

Steroids are secreted by the gonads and adrenal glands into the blood to modulate neurophysiology and behaviour. In addition, the brain can metabolise circulating steroids and synthesise steroids de novo. Songbirds show high levels of neurosteroid synthesis. In the present study, we developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for the measurement of 10 steroids in whole blood, plasma and microdissected brain tissue (1-2 mg) of song sparrows. Our assay is highly accurate, precise, specific and sensitive. Moreover, the liquid-liquid extraction is fast, simple and effective. We quantified steroids in the blood and brain of wild male song sparrows in both breeding and non-breeding seasons. As expected, systemic androgen levels were higher in the breeding season than in the non-breeding season. Brain androgens were detectable only in the breeding season; androstenedione and 5α-dihydrotestosterone levels were up to 20-fold higher in specific brain regions than in blood. Oestrogens were not detectable in blood in both seasons. Oestrone and 17β-oestradiol were detectable in brain in the breeding season only (up to 1.4 ng g^-1 combined). Progesterone levels in several regions were higher in the non-breeding season than the breeding season, despite the lack of seasonal changes in systemic progesterone. Corticosterone levels in the blood were higher in the breeding season than in the non-breeding season but showed few seasonal differences in the brain. In general, the steroid levels presented here are lower than those in previous reports using immunoassays, because of the higher specificity of mass spectrometry. We conclude that (i) brain steroid levels can differ greatly from circulating steroid levels and (ii) brain steroid levels show region-specific seasonal patterns that are not a simple reflection of circulating steroid levels. This approach using ultrasensitive LC-MS/MS is broadly applicable to other species and allows steroid profiling in microdissected brain regions.

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.

Sex differences and similarities in the neural circuit regulating song and other reproductive behaviors in songbirds

In the 1970s, Nottebohm and Arnold reported marked male-biased sex differences in the volume of three song control nuclei in songbirds. Subsequently a series of studies on several songbird species suggested that there is a positive correlation between the degree to which there is a sex difference in the volume of these song control nuclei and in song behavior. This correlation has been questioned in recent years. Furthermore, it has become clear that the song circuit is fully integrated into a more comprehensive neural circuit that regulates multiple courtship and reproductive behaviors including song. Sex differences in songbirds should be evaluated in the context of the full complement of behaviors produced by both sexes in relation to reproduction and based on the entire circuit in order to understand the functional significance of variation between males and females in brain and behavior. Variation in brain and behavior exhibited among living songbird species provides an excellent opportunity to understand the functional significance of sex differences related to social behaviors.

Aggression: Perspectives from social and systems neuroscience

Exhibiting behavioral plasticity in order to mount appropriate responses to dynamic and novel social environments is crucial to the survival of all animals. Thus, how animals regulate flexibility in the timing, duration, and intensity of specific behaviors is of great interest to biologists. In this review, we discuss how animals rapidly respond to social challenges, with a particular focus on aggression. We utilize a conceptual framework to understand the neural mechanisms of aggression that is grounded in Wingfield and colleagues' Challenge Hypothesis, which has profoundly influenced how scientists think about aggression and the mechanisms that allow animals to exhibit flexible responses to social instability. Because aggressive behavior is rooted in social interactions, we propose that mechanisms modulating prosocial behavior may be intricately tied to mechanisms of aggression. Therefore, in order to better understand how aggressive behavior is mediated, we draw on perspectives from social neuroscience and discuss how social context, species-typical behavioral phenotype, and neural systems commonly studied in relation to prosocial behavior (i.e., neuropeptides) contribute to organizing rapid responses to social challenges. Because complex behaviors are not the result of one mechanism or a single neural system, we consider how multiple neural systems important for prosocial and aggressive behavior (i.e., neuropeptides and neurosteroids) interact in the brain to produce behavior in a rapid, context-appropriate manner. Applying a systems neuroscience perspective and seeking to understand how multiple systems functionally integrate to rapidly modulate behavior holds great promise for expanding our knowledge of the mechanisms underlying social behavioral plasticity.

How research on female vertebrates contributes to an expanded challenge hypothesis

The bi-directional links between hormones and behavior have been a rich area of research for decades. Theory on the evolution of testosterone (T) was greatly advanced by the challenge hypothesis, which presented a framework for understanding interspecific, seasonal, and social variation in T levels in males, and how they are shaped by the competing demands of parental care and male-male competition. Female competition is also widespread in nature, although it is less clear whether or how the challenge hypothesis applies to females. Here, we evaluate this issue in four parts: (1) We summarize and update prior analyses of seasonal plasticity and interspecific variation in T in females. (2) We evaluate experimental links between T and female aggression on shorter timescales, asking how T manipulations affect aggression and conversely, how social manipulations affect T levels in female mammals, birds, lizards, and fishes. (3) We examine alternative mechanisms that may link aggression to the social environment independently of T levels in circulation. (4) We present a case study, including new data analyses, in an aggressive female bird (the tree swallow, Tachycineta bicolor) to explore how variation in tissue-level processing of T may bridge the gap between circulating T and variation in behavior that is visible to natural selection. We close by connecting these multivariate levels of sex steroid signaling systems alongside different temporal scales (social, seasonal, and evolutionary) to generate broadly applicable insights into how animals respond to their social environment, regardless of whether they are male or female.

Reprint of "Concepts derived from the Challenge Hypothesis"

The Challenge Hypothesis was developed to explain why and how regulatory mechanisms underlying patterns of testosterone secretion vary so much across species and populations as well as among and within individuals. The hypothesis has been tested many times over the past 30years in all vertebrate groups as well as some invertebrates. Some experimental tests supported the hypothesis but many did not. However, the emerging concepts and methods extend and widen the Challenge Hypothesis to potentially all endocrine systems, and not only control of secretion, but also transport mechanisms and how target cells are able to adjust their responsiveness to circulating levels of hormones independently of other tissues. The latter concept may be particularly important in explaining how tissues respond differently to the same hormone concentration. Responsiveness of the hypothalamo-pituitary-gonad (HPG) axis to environmental and social cues regulating reproductive functions may all be driven by gonadotropin-releasing hormone (GnRH) or gonadotropin-inhibiting hormone (GnIH), but the question remains as to how different contexts and social interactions result in stimulation of GnRH or GnIH release. These concepts, although suspected for many decades, continue to be explored as integral components of environmental endocrinology and underlie fundamental mechanisms by which animals, including ourselves, cope with a changing environment. Emerging mass spectrometry techniques will have a tremendous impact enabling measurement of multiple steroids in specific brain regions. Such data will provide greater spatial resolution for studying how social challenges impact multiple steroids within the brain. Potentially the Challenge Hypothesis will continue to stimulate new ways to explore hormone-behavior interactions and generate future hypotheses.

Estradiol induces synaptic rearrangements

17β-Estradiol (E2) is a potent steroid hormone of both gonadal and neuronal origin that exerts profound effects on neuroplasticity in several brain regions. Dendritic spine and synapse formation and rearrangements are modulated and mediated by estrogens. In this chapter, we highlighted the essential background concerning the effects of E2 on synaptic rearrangements accompanied by synaptic plasticity in E2-sensitive brain regions that mediate learning and memory, i.e., cortex and hippocampus. We also address details of the molecular mechanisms underlying E2 regulation of spine dynamics. The proposed models of action of E2 overlaps with that for well-established synaptic modulators, such as adenosine. Thus, the possible synergistic effects of those two molecules in respect to synaptic rearrangement and plasticity were presented.

An updated field guide for snark hunting: Comparative contributions to behavioral neuroendocrinology in the era of model organisms

Studying neuroendocrine behavioral regulatory mechanisms in a variety of species across vertebrate groups is critical for determining how they work in natural contexts, how they evolved, and ultimately what can be generalized from them, potentially even to humans. All of the above are difficult, at best, if work within our field is exclusively done in traditional laboratory organisms. The importance of comparative approaches for understanding the relationships between hormones and behavior has been recognized and advocated for since our field's inception through a series of papers centered upon a poetic metaphor of Snarks and Boojums, all of which have articulated the benefits that come from studying a diverse range of species and the risks associated with a narrow focus on "model organisms." This mini-review follows in the footsteps of those powerful arguments, highlighting some of the comparative work since the latest interactions of the metaphor that has shaped how we think about three major conceptual frameworks within our field, two of them formalized - the Organization/Activation Model of sexual differentiation and the Social Brain Network - and one, context-dependency, that is generally associated with virtually all modern understandings of how hormones affect behavior. Comparative approaches are broadly defined as those in which the study of mechanism is placed within natural and/or evolutionary contexts, whether they directly compare different species or not. Studies are discussed in relation to how they have either extended or challenged generalities associated with the frameworks, how they have shaped subsequent work in model organisms to further elucidate neuroendocrine behavioral regulatory mechanisms, and how they have stimulated work to determine if and when similar mechanisms influence behavior in our own species.

Forebrain Transcriptional Response to Transient Changes in Circulating Androgens in a Cichlid Fish

It has been hypothesized that androgens respond to the social interactions as a way to adjust the behavior of individuals to the challenges of the social environment in an adaptive manner. Therefore, it is expected that transient changes in circulating androgen levels within physiological scope should impact the state of the brain network that regulates social behavior, which should translate into adaptive behavioral changes. Here, we examined the effect that a transient peak in androgen circulating levels, which mimics socially driven changes in androgen levels, has on the forebrain state, which harbors most nuclei of the social decision-making network. For this purpose, we successfully induced transient changes in circulating androgen levels in an African cichlid fish (Mozambique tilapia, Oreochromis mossambicus) commonly used as a model in behavioral neuroendocrinology by injecting 11-ketotestosterone or testosterone, and compared the forebrain transcriptome of these individuals to control fish injected with vehicle. Forebrain samples were collected 30 min and 60 min after injection and analyzed using RNAseq. Our results showed that a transient peak in 11-ketotestosterone drives more accentuated changes in forebrain transcriptome than testosterone, and that transcriptomic impact was greater at the 30 min than at the 60 min post-androgen administration. Several genes involved in the regulation of translation, steroid metabolism, ion channel membrane receptors, and genes involved in epigenetic mechanisms were differentially expressed after 11-ketotestosterone or testosterone injection. In summary, this study identified specific candidate genes that may regulate socially driven changes in behavioral flexibility mediated by androgens.

Testosterone synthesis in the female songbird brain

Decades of work have established the brain as a source of steroid hormones, termed 'neurosteroids'. The neurosteroid neuroestradiol is produced in discrete brain areas and influences cognition, sensory processing, reproduction, neurotransmission, and disease. A prevailing research focus on neuroestradiol has essentially ignored whether its immediate synthesis precursor - the androgen testosterone - is also dynamically regulated within the brain. Testosterone itself can rapidly influence neurophysiology and behavior, and there is indirect evidence that the female brain may synthesize significant quantities of testosterone to regulate cognition, reproduction, and behavior. In songbirds, acoustic communication is regulated by neuroestrogens. Neuroestrogens are rapidly synthetized in the caudomedial nidopallium (NCM) of the auditory cortex of zebra finches in response to song and can influence auditory processing and song discrimination. Here, we examined the in vivo dynamics of NCM levels of the neuroestrogen synthesis precursor, testosterone. Unlike estradiol, testosterone did not appear to fluctuate in the female NCM during song exposure. However, a substantial song-induced elevation of testosterone was revealed in the left hemisphere NCM of females when local aromatization (i.e., conversion to estrogens) was locally blocked. This elevation was eliminated when local androgen synthesis was concomitantly blocked. Further, no parallel elevation was observed in the circulation in response to song playback, consistent with a local, neural origin of testosterone synthesis. To our knowledge, this study provides the first direct demonstration that testosterone fluctuates rapidly in the brain in response to socially-relevant environmental stimuli. Our findings suggest therefore that locally-derived 'neuroandrogens' can dynamically influence brain function and behavior. SIGNIFICANCE STATEMENT: This study demonstrates that androgen synthesis occurs rapidly in vivo in the brain in response to social cues, in a lateralized manner. Specifically, testosterone synthesis occurs within the left secondary auditory cortex when female zebra finches hear male song. Therefore, testosterone could act as a neuromodulator to rapidly shape sensory processing. Androgens have been linked to functions such as the control of female libido, and many steroidal drugs used for contraception, anti-cancer treatments, and sexual dysfunction likely influence the brain synthesis and action of testosterone. The current findings therefore establish a clear role for androgen synthesis in the female brain with implications for understanding neural circuit function and behavior in animals, including humans.

Auditory learning in an operant task with social reinforcement is dependent on neuroestrogen synthesis in the male songbird auditory cortex

Animals continually assess their environment for cues associated with threats, competitors, allies, mates or prey, and experience is crucial for those associations. The auditory cortex is important for these computations to enable valence assignment and associative learning. The caudomedial nidopallium (NCM) is part of the songbird auditory association cortex and it is implicated in juvenile song learning, song memorization, and song perception. Like human auditory cortex, NCM is a site of action of estradiol (E2) and is enriched with the enzyme aromatase (E2-synthase). However, it is unclear how E2 modulates auditory learning and perception in the vertebrate auditory cortex. In this study we employ a novel, auditory-dependent operant task governed by social reinforcement to test the hypothesis that neuro-E2 synthesis supports auditory learning in adult male zebra finches. We show that local suppression of aromatase activity in NCM disrupts auditory association learning. By contrast, post-learning performance is unaffected by either NCM aromatase blockade or NCM pharmacological inactivation, suggesting that NCM E2 production and even NCM itself are not required for post-learning auditory discrimination or memory retrieval. Therefore, neuroestrogen synthesis in auditory cortex supports the association between sounds and behaviorally relevant consequences.

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

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

Biotic and abiotic sounds affect calling activity but not plasma testosterone levels in male frogs (Batrachyla taeniata) in the field and in captivity

In animals, the expression of diverse reproductive behaviors is hormonally regulated. In particular, vocalizing during courtship has been related to circulating androgen levels, and reciprocally, conspecific vocalizations are known to modulate androgen secretion in vertebrates. The effect of natural sounds of abiotic origin on hormonal status has virtually not received attention. Therefore, we evaluated the vocal responses of male Batrachyla taeniata frogs to conspecific chorus and rainfall sounds in natural and controlled laboratory settings, measuring the testosterone levels of exposed individuals. In field and laboratory conditions, testosterone levels of frogs exposed to 31.5 min of chorus and rain sounds and non-exposed individuals were similar. In the field, frogs increased their call rate in response to playbacks of chorus and rain sound, but the evoked calling activity was unrelated to plasma testosterone. In contrast to the field, frogs showed limited responsiveness to 31.5-min acoustic exposures in the laboratory. Similarly to the field, for vocally active males tested in the laboratory there was no association between call rate and testosterone levels. Additionally, in this group, testosterone levels were higher in vocally active males relative to non-calling individuals. Overall, these results indicate that in B. taeniata testosterone levels are not altered following a short-term exposure to conspecific biotic and to abiotic sounds. Our results are suggestive of a threshold influence of testosterone on the vocal activity of the species studied. Further explorations of the influence of abiotic sounds on endocrine activation are required to understand how animals respond to variable acoustic environmental conditions.

Acute neuroestrogen blockade attenuates song-induced immediate early gene expression in auditory regions of male and female zebra finches

Neuron-derived estrogens are synthesized by aromatase and act through membrane receptors to modulate neuronal physiology. In many systems, long-lasting hormone treatments can alter sensory-evoked neuronal activation. However, the significance of acute neuroestrogen production is less understood. Both sexes of zebra finches can synthesize estrogens rapidly in the auditory cortex, yet it is unclear how this modulates neuronal cell signaling. We examined whether acute estrogen synthesis blockade attenuates auditory-induced expression of early growth response 1 (Egr-1) in the auditory cortex of both sexes. cAMP response element-binding protein phosphorylation (pCREB) induction by song stimuli and acute estrogen synthesis was also examined. We administered the aromatase inhibitor fadrozole prior to song exposure and measured Egr-1 across several auditory regions. Fadrozole attenuated Egr-1 in the auditory cortex greater in males than females. Females had greater expression and clustering of aromatase cells than males in high vocal center (HVC) shelf. Auditory-induced Egr-1 expression exhibited a large sex difference following fadrozole treatment. We did not observe changes in pCREB expression with song presentation or aromatase blockade. These findings are consistent with the hypothesis that acute neuroestrogen synthesis can drive downstream transcriptional responses in several cortical auditory regions, and that this mechanism is more prominent in males.

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

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

Anterior cingulate cortex and dorsal hippocampal glutamate receptors mediate generalized fear in female rats

Exhibiting fear to non-threatening cues or contexts-generalized fear-is a shared characteristic of several anxiety disorders, which afflict women more than men. Female rats generalize contextual fear at a faster rate than males and this is due, in part, to actions of estradiol in the dorsal CA1 hippocampus (dCA1). To understand the mechanisms underlying estradiol's effects on generalization, we infused estradiol into the anterior cingulate cortex (ACC) or ventral CA1 hippocampus (vCA1) of ovariectomized (OVX) female rats. Estradiol acts within the ACC, but not the vCA1, to promote generalized fear. We next examined if AMPA or NMDA receptor antagonists (NBQX, APV) infused into the dCA1 or the ACC of female rats could block generalized fear induced by systemic injections of estradiol. Immediate pre-testing infusions of NBQX or APV into either region eliminated estradiol-induced generalization. Specific blockade of GluN2B receptors with infusions of Ro 25-6981 into the dCA1 or ACC also eliminated generalized fear. Our results suggest that in addition to the dCA1, the ACC is an important locus for the effects of estradiol on fear generalization. Moreover, within these regions, AMPA and NMDA-GluN2B receptors are necessary for estradiol-induced generalization of fear responses, suggesting a critical involvement of glutamatergic transmission. Furthermore, we identified a novel role for GluN2B in mediating the effects of estradiol on generalized fear in female rats. These data potentially implicate GluN2B receptors in more general forms of memory retrieval inaccuracies, and form the foundation for exploration of glutamate receptor pharmacology for treatments of anxiety disorders involving generalization.

Concepts derived from the Challenge Hypothesis

The Challenge Hypothesis was developed to explain why and how regulatory mechanisms underlying patterns of testosterone secretion vary so much across species and populations as well as among and within individuals. The hypothesis has been tested many times over the past 30years in all vertebrate groups as well as some invertebrates. Some experimental tests supported the hypothesis but many did not. However, the emerging concepts and methods extend and widen the Challenge Hypothesis to potentially all endocrine systems, and not only control of secretion, but also transport mechanisms and how target cells are able to adjust their responsiveness to circulating levels of hormones independently of other tissues. The latter concept may be particularly important in explaining how tissues respond differently to the same hormone concentration. Responsiveness of the hypothalamo-pituitary-gonad (HPG) axis to environmental and social cues regulating reproductive functions may all be driven by gonadotropin-releasing hormone (GnRH) or gonadotropin-inhibiting hormone (GnIH), but the question remains as to how different contexts and social interactions result in stimulation of GnRH or GnIH release. These concepts, although suspected for many decades, continue to be explored as integral components of environmental endocrinology and underlie fundamental mechanisms by which animals, including ourselves, cope with a changing environment. Emerging mass spectrometry techniques will have a tremendous impact enabling measurement of multiple steroids in specific brain regions. Such data will provide greater spatial resolution for studying how social challenges impact multiple steroids within the brain. Potentially the Challenge Hypothesis will continue to stimulate new ways to explore hormone-behavior interactions and generate future hypotheses.