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

The classic estrogen 17ß-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 ß (ERß), 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.

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.

Noradrenergic control of gene expression and long-term neuronal adaptation evoked by learned vocalizations in songbirds.

Norepinephrine (NE) is thought to play important roles in the consolidation and retrieval of long-term memories, but its role in the processing and memorization of complex acoustic signals used for vocal communication has yet to be determined. We have used a combination of gene expression analysis, electrophysiological recordings and pharmacological manipulations in zebra finches to examine the role of noradrenergic transmission in the brain's response to birdsong, a learned vocal behavior that shares important features with human speech. We show that noradrenergic transmission is required for both the expression of activity-dependent genes and the long-term maintenance of stimulus-specific electrophysiological adaptation that are induced in central auditory neurons by stimulation with birdsong. Specifically, we show that the caudomedial nidopallium (NCM), an area directly involved in the auditory processing and memorization of birdsong, receives strong noradrenergic innervation. Song-responsive neurons in this area express α-adrenergic receptors and are in close proximity to noradrenergic terminals. We further show that local α-adrenergic antagonism interferes with song-induced gene expression, without affecting spontaneous or evoked electrophysiological activity, thus dissociating the molecular and electrophysiological responses to song. Moreover, α-adrenergic antagonism disrupts the maintenance but not the acquisition of the adapted physiological state. We suggest that the noradrenergic system regulates long-term changes in song-responsive neurons by modulating the gene expression response that is associated with the electrophysiological activation triggered by song. We also suggest that this mechanism may be an important contributor to long-term auditory memories of learned vocalizations.

The mouse primary visual cortex is a site of production and sensitivity to estrogens.

The classic female estrogen, 17ß-estradiol (E2), has been repeatedly shown to affect the perceptual processing of visual cues. Although gonadal E2 has often been thought to influence these processes, the possibility that central visual processing may be modulated by brain-generated hormone has not been explored. Here we show that estrogen-associated circuits are highly prevalent in the mouse primary visual cortex (V1). Specifically, we cloned aromatase, a marker for estrogen-producing neurons, and the classic estrogen receptors (ERs) ERα and ERß, as markers for estrogen-responsive neurons, and conducted a detailed expression analysis via in-situ hybridization. We found that both monocular and binocular V1 are highly enriched in aromatase- and ER-positive neurons, indicating that V1 is a site of production and sensitivity to estrogens. Using double-fluorescence in-situ hybridization, we reveal the neurochemical identity of estrogen-producing and -sensitive cells in V1, and demonstrate that they constitute a heterogeneous neuronal population. We further show that visual experience engages a large population of aromatase-positive neurons and, to a lesser extent, ER-expressing neurons, suggesting that E2 levels may be locally regulated by visual input in V1. Interestingly, acute episodes of visual experience do not affect the density or distribution of estrogen-associated circuits. Finally, we show that adult mice dark-reared from birth also exhibit normal distribution of aromatase and ERs throughout V1, suggesting that the implementation and maintenance of estrogen-associated circuits is independent of visual experience. Our findings demonstrate that the adult V1 is a site of production and sensitivity to estrogens, and suggest that locally-produced E2 may shape visual cortical processing.

Neurochemical organization and experience-dependent activation of estrogen-associated circuits in the songbird auditory forebrain.

The classic steroid hormone estradiol is rapidly produced by central auditory neurons in the songbird brain and instantaneously modulates auditory coding to enhance the neural and behavioral discrimination of acoustic signals. Although recent advances highlight novel roles for estradiol in the regulation of central auditory processing, current knowledge on the functional and neurochemical organization of estrogen-associated circuits, as well as the impact of sensory experience in these auditory forebrain networks, remains very limited. Here we show that both estrogen-producing and -sensitive neurons are highly expressed in the caudomedial nidopallium (NCM), the zebra finch analog of the mammalian auditory association cortex, but not other auditory forebrain areas. We further demonstrate that auditory experience primarily engages estrogen-producing, and to a lesser extent, estrogen-responsive neurons in NCM, that these neuronal populations moderately overlap and that acute episodes of sensory experience do not quantitatively affect these circuits. Finally, we show that whereas estrogen-producing cells are neurochemically heterogeneous, estrogen-sensitive neurons are primarily glutamatergic. These findings reveal the neurochemical and functional organization of estrogen-associated circuits in the auditory forebrain, demonstrate their activation and stability in response to sensory experience in behaving animals, and highlight estrogenic circuits as fundamental components of central networks supporting sensory processing.

Expression and rapid experience-dependent regulation of type-A GABAergic receptors in the songbird auditory forebrain.

GABAergic transmission influences sensory processing and experience-dependent plasticity in the adult brain. Little is known about the functional organization of inhibitory circuits in the auditory forebrain of songbirds, a robust model extensively used in the study of central auditory processing of behaviorally relevant communication signals. In particular, no information is currently available on the expression and organization of GABAA receptor-expressing neurons. Here, we studied the distribution and regulation of GABAA receptors in the songbird auditory forebrain, with a specific focus on α5, a subunit implicated in tonic inhibition and sensory learning. We obtained a zebra finch cDNA that encodes the α5-subunit (GABRA5) and carried out a detailed analysis of its expression via in situ hybridization. GABRA5 was highly expressed in the caudomedial nidopallium (NCM), caudomedial mesopallium, and field L2. Using double fluorescence in situ hybridization, we demonstrate that a large fraction of GABRA5-expressing neurons is engaged by auditory experience, as revealed by the song-induced expression of the activity-dependent gene zenk. Remarkably, we also found that α5 expression is rapidly regulated by sensory stimulation: 30 min of conspecific song playbacks significantly increase the number of GABRA5-expressing neurons in NCM, but not in other auditory areas. This effect is selective for α5, but not γ2 transcripts. Our results suggest that α5-containing GABAA receptors likely play a key role in central auditory processing and may contribute to the experience-dependent plasticity underlying auditory learning.

Sensory Experience in Development Balances Excitation and Inhibition to Stabilize Frequency Tuning in Central Auditory Neurons.

The balance between excitation and inhibition is critical in shaping receptive field tuning properties in sensory neurons and, ultimately, in determining how sensory cues are extracted, transformed and interpreted by brain circuits. New findings suggest that developmentally-regulated, experience-dependent changes in intracortical inhibitory networks are key to defining receptive field tuning properties of auditory cortical neurons.

Brain-generated estradiol drives long-term optimization of auditory coding to enhance the discrimination of communication signals.

Auditory processing and hearing-related pathologies are heavily influenced by steroid hormones in a variety of vertebrate species, including humans. The hormone estradiol has been recently shown to directly modulate the gain of central auditory neurons, in real time, by controlling the strength of inhibitory transmission via a nongenomic mechanism. The functional relevance of this modulation, however, remains unknown. Here we show that estradiol generated in the songbird homolog of the mammalian auditory association cortex, rapidly enhances the effectiveness of the neural coding of complex, learned acoustic signals in awake zebra finches. Specifically, estradiol increases mutual information rates, coding efficiency, and the neural discrimination of songs. These effects are mediated by estradiol's modulation of both the rate and temporal coding of auditory signals. Interference with the local action or production of estradiol in the auditory forebrain of freely behaving animals disrupts behavioral responses to songs, but not to other behaviorally relevant communication signals. Our findings directly show that estradiol is a key regulator of auditory function in the adult vertebrate brain.

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.

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.

Double fluorescence in situ hybridization in fresh brain sections.

Here we describe a modified version of a double fluorescence in situ hybridization (dFISH) method optimized for detecting two mRNAs of interest in fresh frozen brain sections. Our group has successfully used this approach to study gene co-regulation. More specifically, we have used this dFISH method to explore the anatomical organization, neurochemical properties, and the impact of sensory experience in central sensory circuits, at single cell resolution. This protocol has been validated in brain tissue from mice, rats and songbirds but is expected to be easily adaptable to other vertebrate species, as well as to an array of non-neural tissues. In this film we provide a detailed demonstration of the main steps of this procedure.

Genome of a songbird unveiled.

An international collaborative effort has recently uncovered the genome of the zebra finch, a songbird model that has provided unique insights into an array of biological phenomena.

Bilateral multielectrode neurophysiological recordings coupled to local pharmacology in awake songbirds.

Here we describe a protocol for bilateral multielectrode neurophysiological recordings during intracerebral pharmacological manipulations in awake songbirds. This protocol encompasses fitting adult animals with head-posts and recording chambers, and acclimating them to periods of restraint. The adaptation period is followed by bilateral penetrations of multiple electrodes to obtain acute, sensory-driven neurophysiological responses before versus during the application of pharmacological agents of interest. These local manipulations are achieved by simultaneous and restricted drug infusions carried out independently for each hemisphere. We have used this protocol to elucidate how neurotransmitter and neuroendocrine systems shape the auditory and perceptual processing of natural, learned communication signals. However, this protocol can be used to explore the neurochemical basis of sensory processing in other small vertebrates. Representative results and troubleshooting of key steps of this protocol are presented. Following the animal's recovery from head-post and recording chamber implantation surgery, the length of the procedure is 2 d.

Duplex in situ hybridization in the study of gene co-regulation in the vertebrate brain.

We describe here a high-sensitivity in situ hybridization protocol, optimized for fresh-frozen brain sections, that enables the detection of two transcripts, at single cell resolution. Riboprobes directed against two mRNAs of interest are synthesized with nucleotides tagged with different haptens (digoxigenin- or biotin-UTP), via in vitro transcription, hybridized simultaneously to brain sections, and independently detected through immunocytochemistry. Sequential detection of each probe involves peroxidase-mediated precipitation of tyramide-linked fluorophores of separate emission wavelengths. In addition, we demonstrate how classic non-fluorescent chromogens, such as 3,3'-diaminobenzidine, can be successfully combined with fluorescence-based detection, to yield reliable detection of two transcript populations. We provide examples of representative results obtained with this protocol and describe necessary controls. Additionally, we discuss common problems associated with this methodology, and detail troubleshooting recommendations. Although this method has been optimized for brain sections, it may be useful to detect two mRNA species in a variety of tissues.

Estradiol shapes auditory processing in the adult brain by regulating inhibitory transmission and plasticity-associated gene expression.

Estradiol impacts a wide variety of brain processes, including sex differentiation, mood, and learning. Here we show that estradiol regulates auditory processing of acoustic signals in the vertebrate brain, more specifically in the caudomedial nidopallium (NCM), the songbird analog of the mammalian auditory association cortex. Multielectrode recordings coupled with local pharmacological manipulations in awake animals reveal that both exogenous and locally generated estradiol increase auditory-evoked activity in NCM. This enhancement in neuronal responses is mediated by suppression of local inhibitory transmission. Surprisingly, we also found that estradiol is both necessary and sufficient for the induction of multiple mitogen-activated protein kinase (MAPK)-dependent genes thought to be required for synaptic plasticity and memorization of birdsong. Specifically, we show that local blockade of estrogen receptors or aromatase activity in awake birds decrease song-induced MAPK-dependent gene expression. Infusions of estradiol in acoustically isolated birds induce transcriptional activation of these genes to levels comparable with song-stimulated animals. Our results reveal acute and rapid nongenomic functions for estradiol in central auditory physiology and suggest that such roles may be ubiquitously expressed across sensory systems.

Anatomical and Functional Organization of Inhibitory Circuits in the Songbird Auditory Forebrain.

Recent studies on the anatomical and functional organization of GABAergic networks in central auditory circuits of the zebra finch have highlighted the strong impact of inhibitory mechanisms on both the central encoding and processing of acoustic information in a vocal learning species. Most of this work has focused on the caudomedial nidopallium (NCM), a forebrain area postulated to be the songbird analogue of the mammalian auditory association cortex. NCM houses neurons with selective responses to conspecific songs and is a site thought to house auditory memories required for vocal learning and, likely, individual identification. Here we review our recent work on the anatomical distribution of GABAergic cells in NCM, their engagement in response to song and the roles for inhibitory transmission in the physiology of NCM at rest and during the processing of natural communication signals. GABAergic cells are highly abundant in the songbird auditory forebrain and account for nearly half of the overall neuronal population in NCM with a large fraction of these neurons activated by song in freely-behaving animals. GABAergic synapses provide considerable local, tonic inhibition to NCM neurons at rest and, during sound processing, may contain the spread of excitation away from un-activated or quiescent parts of the network. Finally, we review our work showing that GABA(A)-mediated inhibition directly regulates the temporal organization of song-driven responses in awake songbirds, and appears to enhance the reliability of auditory encoding in NCM.