A telencephalic nucleus essential for song learning contains neurons with physiological characteristics of both striatum and globus pallidus

ERα Stimulation Rapidly Modulates Excitatory Synapse Properties in Female Rat Nucleus Accumbens Core

INTRODUCTION

The nucleus accumbens core (NAcc) is a sexually differentiated brain region that is modulated by steroid hormones such as 17β-estradiol (estradiol), with consequential impacts on relevant motivated behaviors and disorders such as addiction, anxiety, and depression. NAcc estradiol levels naturally fluctuate, including during the estrous cycle in adult female rats, which is analogous to the menstrual cycle in adult humans. Across the estrous cycle, excitatory synapse properties of medium spiny neurons rapidly change, as indicated by analysis of miniature excitatory postsynaptic currents (mEPSCs). mEPSC frequency decreases during estrous cycle phases associated with high estradiol levels. This decrease in mEPSC frequency is mimicked by acute topical exposure to estradiol. The identity of the estrogen receptor (ER) underlying this estradiol action is unknown. Adult rat NAcc expresses three ERs, all extranuclear: membrane ERα, membrane ERβ, and GPER1.

METHODS

In this brief report, we take a first step toward addressing this challenge by testing whether activation of ERs via acute topical agonist application is sufficient for inducing changes in mEPSC properties recorded via whole-cell patch clamp.

RESULTS

An agonist of ERα induced large decreases in mEPSC frequency, while agonists of ERβ and GPER1 did not robustly modulate mEPSC properties.

CONCLUSIONS

These data provide evidence that activation of ERα is sufficient for inducing changes in mEPSC frequency and is a likely candidate underlying the estradiol-induced changes observed during the estrous cycle. Overall, these findings extend our understanding of the neuroendocrinology of the NAcc and implicate ERα as a primary target for future studies.

Analogies of human speech and bird song: From vocal learning behavior to its neural basis

Vocal learning is a complex acquired social behavior that has been found only in very few animals. The process of animal vocal learning requires the participation of sensorimotor function. By accepting external auditory input and cooperating with repeated vocal imitation practice, a stable pattern of vocal information output is eventually formed. In parallel evolutionary branches, humans and songbirds share striking similarities in vocal learning behavior. For example, their vocal learning processes involve auditory feedback, complex syntactic structures, and sensitive periods. At the same time, they have evolved the hierarchical structure of special forebrain regions related to vocal motor control and vocal learning, which are organized and closely associated to the auditory cortex. By comparing the location, function, genome, and transcriptome of vocal learning-related brain regions, it was confirmed that songbird singing and human language-related neural control pathways have certain analogy. These common characteristics make songbirds an ideal animal model for studying the neural mechanisms of vocal learning behavior. The neural process of human language learning may be explained through similar neural mechanisms, and it can provide important insights for the treatment of language disorders.

Durations of preparatory motor activity in the avian basal ganglia for songs and calls in a species of songbirds

Male songbirds are highly motivated to sing undirected song (US) as juveniles during song learning, and as adults. Given that singing US is a self-driven, elaborated behavior, we would expect to see preparatory activity in the striatal area prior to vocalization, and this preparatory activity could have different characteristics compared to activity driven by calls. In general, songs are longer, complex and influenced by learning while calls are shorter, simpler, and less influenced by experience. The present study recorded neural activity in Area X, a nucleus of the basal ganglia, in male Java sparrows (Lonchura oryzivora) in a sound-proof box and analyzed differences in activity change before US and trill-calls. Trill-calls were often emitted in social arousal, but occasionally emitted when alone. We saw a gradual increase in firing rate for about 2.3seconds prior to the onset of US, and a shorter increase of about 1.3seconds in firing rate prior to the onset of trill-calls. The results reveal that initiating US may be influenced by a prolonged and specific activity increase in the extent that is not seen with trill-calls. Results suggest that direct or indirect projections to Area X, which may reflect motivational state, could be the cause of this activity change.

Striatal Injury Induces Overall Brain Alteration at the Pallial, Thalamic, and Cerebellar Levels

Simple Summary

Magnetic resonance imaging showed that striatal injury leads to structural changes within several brain areas. Here, we specify these changes via gene expression of synaptic plasticity markers, neuronal markers, assessing the number of newborn cells as well as cell densities. We found that the injury resulted in long-lasting modifications involving plasticity and neural protection mechanisms in areas directly as well as indirectly connected with the damaged striatum, including the cerebellum.

Abstract

The striatal region Area X plays an important role during song learning, sequencing, and variability in songbirds. A previous study revealed that neurotoxic damage within Area X results in micro and macrostructural changes across the entire brain, including the downstream dorsal thalamus and both the upstream pallial nucleus HVC (proper name) and the deep cerebellar nuclei (DCN). Here, we specify these changes on cellular and gene expression levels. We found decreased cell density in the thalamic and cerebellar areas and HVC, but it was not related to neuronal loss. On the contrary, perineuronal nets (PNNs) in HVC increased for up to 2 months post-lesion, suggesting their protecting role. The synaptic plasticity marker Forkhead box protein P2 (FoxP2) showed a bi-phasic increase at 8 days and 3 months post-lesion, indicating a massive synaptic rebuilding. The later increase in HVC was associated with the increased number of new neurons. These data suggest that the damage in the striatal vocal nucleus induces cellular and gene expression alterations in both the efferent and afferent destinations. These changes may be long-lasting and involve plasticity and neural protection mechanisms in the areas directly connected to the injury site and also to distant areas, such as the cerebellum.

Computational benefits of structural plasticity, illustrated in songbirds

The plasticity of nervous systems allows animals to quickly adapt to a changing environment. In particular, the structural plasticity of brain networks is often critical to the development of the central nervous system and the acquisition of complex behaviors. As an example, structural plasticity is central to the development of song-related brain circuits and may be critical for song acquisition in juvenile songbirds. Here, we review current evidences for structural plasticity and their significance from a computational point of view. We start by reviewing evidence for structural plasticity across species and categorizing them along the spatial axes as well as the along the time course during development. We introduce the vocal learning circuitry in zebra finches, as a useful example of structural plasticity, and use this specific case to explore the possible contributions of structural plasticity to computational models. Finally, we discuss current modeling studies incorporating structural plasticity and unexplored questions which are raised by such models.

Deafening-induced rapid changes to spine synaptic connectivity in the adult avian vocal basal ganglia

The basal ganglia have been implicated in auditory-dependent vocal learning and plasticity in human and songbirds, but the underlying neural phenotype remains to be clarified. Here, using confocal imaging and three-dimensional electron microscopy, we investigated striatal structural plasticity in response to hearing loss in Area X, the avian vocal basal ganglia, in adult male zebra finch (Taeniopygia guttata). We observed a rapid elongation of dendritic spines, by approximately 13%, by day 3 after deafening, and a considerable increase in spine synapse density, by approximately 61%, by day 14 after deafening, compared with the controls with an intact cochlea. These findings reveal structural sensitivity of Area X to auditory deprivation and suggest that this striatal plasticity might contribute to deafening-induced changes to learned vocal behavior.

What Is the Role of Thalamostriatal Circuits in Learning Vocal Sequences?

Basal ganglia (BG) circuits integrate sensory and motor-related information from the cortex, thalamus, and midbrain to guide learning and production of motor sequences. Birdsong, like speech, is comprised of precisely sequenced vocal elements. Learning song sequences during development relies on Area X, a vocalization related region in the medial striatum of the songbird BG. Area X receives inputs from cortical-like pallial song circuits and midbrain dopaminergic circuits and sends projections to the thalamus. It has recently been shown that thalamic circuits also send substantial projections back to Area X. Here, we outline a gated-reinforcement learning model for how Area X may use signals conveyed by thalamostriatal inputs to direct song learning. Integrating conceptual advances from recent mammalian and songbird literature, we hypothesize that thalamostriatal pathways convey signals linked to song syllable onsets and offsets and influence striatal circuit plasticity via regulation of cholinergic interneurons (ChIs). We suggest that syllable sequence associated vocal-motor information from the thalamus drive precisely timed pauses in ChIs activity in Area X. When integrated with concurrent corticostriatal and dopaminergic input, this circuit helps regulate plasticity on medium spiny neurons (MSNs) and the learning of syllable sequences. We discuss new approaches that can be applied to test core ideas of this model and how associated insights may provide a framework for understanding the function of BG circuits in learning motor sequences.

FireMaster® 550 (FM 550) exposure during the perinatal period impacts partner preference behavior and nucleus accumbens core medium spiny neuron electrophysiology in adult male and female prairie voles, Microtus ochrogaster

One of the most widely used flame retardant (FR) mixtures in household products is Firemaster 550 (FM 550). FM 550 leaches from items such as foam-based furniture and infant products, resulting in contamination of the household environment and biota. Previous studies indicate sex-specific behavioral deficits in rodents and zebrafish in response to developmental FM 550 exposure. These deficits include impacts on social and attachment behaviors in a prosocial rodent: the prairie vole (Microtus ochrogaster). The prairie vole is a laboratory-acclimated rodent that exhibits spontaneous attachment behaviors including pair bonding. Here we extend previous work by addressing how developmental exposure to FM 550 impacts pair bonding strength via an extended-time partner preference test, as well as neuron electrophysiological properties in a region implicated in pair bond behavior, the nucleus accumbens (NAcc) core. Dams were exposed to vehicle or 1000 μg of FM 550 via subcutaneous injections throughout gestation, and female and male pups were directly exposed beginning the day after birth until weaning. Pair bond behavior of adult female and male offspring was assessed using a three hour-long partner preference test. Afterwards, acute brain slices of the NAcc core were produced and medium spiny neuron electrophysiological attributes recorded via whole cell patch-clamp. Behavioral impacts were sex-specific. Partner preference behavior was increased in exposed females but decreased in exposed males. Electrophysiological impacts were similar between sexes and specific to attributes related to input resistance. Input resistance was decreased in neurons recorded from both sexes exposed to FM 550 compared to vehicle. This study supports the hypothesis that developmental exposure to FM 550 impacts attachment behaviors and demonstrates a novel FM 550 effect on neural electrophysiology.

New roles for dopamine in motor skill acquisition: lessons from primates, rodents, and songbirds

Motor learning is a core aspect of human life and appears to be ubiquitous throughout the animal kingdom. Dopamine, a neuromodulator with a multifaceted role in synaptic plasticity, may be a key signaling molecule for motor skill learning. Though typically studied in the context of reward-based associative learning, dopamine appears to be necessary for some types of motor learning. Mesencephalic dopamine structures are highly conserved among vertebrates, as are some of their primary targets within the basal ganglia, a subcortical circuit important for motor learning and motor control. With a focus on the benefits of cross-species comparisons, this review examines how "model-free" and "model-based" computational frameworks for understanding dopamine's role in associative learning may be applied to motor learning. The hypotheses that dopamine could drive motor learning either by functioning as a reward prediction error, through passive facilitating of normal basal ganglia activity, or through other mechanisms are examined in light of new studies using humans, rodents, and songbirds. Additionally, new paradigms that could enhance our understanding of dopamine's role in motor learning by bridging the gap between the theoretical literature on motor learning in humans and other species are discussed.

Perinatal activation of ER and ER but not GPER-1 masculinizes female rat caudate-putamen medium spiny neuron electrophysiological properties

Exposure to steroid sex hormones such as 17β-estradiol (estradiol) during early life potentially permanently masculinize neuron electrophysiological phenotype. In rodents, one crucial component of this developmental process occurs in males, with estradiol aromatized in the brain from testes-sourced testosterone. However, it is unknown whether most neuron electrophysiological phenotypes are altered by this early masculinization process, including medium spiny neurons (MSNs) of the rat caudate-putamen. MSNs are the predominant and primary output neurons of the caudate-putamen and exhibit increased intrinsic excitability in females compared to males. Here, we hypothesize that since perinatal estradiol exposure occurs in males, then a comparable exposure in females to estradiol or its receptor agonists would be sufficient to induce masculinization. To test this hypothesis, we injected perinatal female rats with estradiol or its receptor agonists and then later assessed MSN electrophysiology. Female and male rats on postnatal day 0 and 1 were systemically injected with either vehicle, estradiol, the estrogen receptor (ER)α agonist PPT, the ERβ agonist DPN, or the G-protein-coupled receptor 1 (GPER-1) agonist G1. On postnatal days 19 ± 2, MSN electrophysiological properties were assessed using whole cell patch clamp recordings. Estradiol exposure abolished increased intrinsic excitability in female compared to male MSNs. Exposure to either an ERα or ERβ agonist masculinized female MSN evoked action potential firing rate properties, whereas exposure to an ERβ agonist masculinized female MSN inward rectification properties. Exposure to ER agonists minimally impacted male MSN electrophysiological properties. These findings indicate that perinatal estradiol exposure masculinizes MSN electrophysiological phenotype via activation of ERα and ERβ.NEW & NOTEWORTHY This study is the first to demonstrate that estradiol and estrogen receptor α and β stimulation during early development sexually differentiates the electrophysiological properties of caudate-putamen medium spiny neurons, the primary output neuron of the striatal regions. Overall, this evidence provides new insight into the neuroendocrine mechanism by which caudate-putamen neuron electrophysiology is sexually differentiated and demonstrates the powerful action of early hormone exposure upon individual neuron electrophysiology.

The Adjustment of Anterior Forebrain Pathway (AFP) to Birdsong Is Phased during Song Learning and Maintenance

Anterior forebrain pathway (AFP), a basal ganglia-dorsal forebrain circuit, significantly impacts birdsong, specifically in juvenile or deaf birds. Despite many physiological experiments supporting AFP's role in song production, the mechanism underlying it remains poorly understood. Using a computational model of the anterior forebrain pathway and song premotor pathway, we examined the dynamic process and exact role of AFP during song learning and distorted auditory feedback (DAF). Our simulation suggests that AFP can adjust the premotor pathway structure and syllables based on its delayed input to the robust nucleus of the archistriatum (RA). It is also indicated that the adjustment to the synaptic conductance in the song premotor pathway has two phases: normal phases where the adjustment decreases with an increasing number of trials and abnormal phases where the adjustment remains stable or even increases. These two phases alternate and impel a specific effect on birdsong based on AFP's specific structures, which may be associated with auditory feedback. Furthermore, our model captured some characteristics shown in birdsong experiments, such as similarities in pitch, intensity, and duration to real birds and the highly abnormal features of syllables during DAF.

Blocking Opioid Receptors in a Songbird Cortical Region Modulates the Acoustic Features and Levels of Female-Directed Singing

The organization of the anterior forebrain pathway (AFP) of songbirds important for context-dependent singing is similar to that of cortical basal ganglia loops (CBG) in mammals, which underlie motor behaviors including vocalization. Since different components of the AFP express high levels of μ-opioid receptors (μ-ORs) as do CBG loops, songbirds act as model systems to study the role of opioid modulation on vocalization and the motivation to sing. The AFP in songbirds includes the cortical/pallial region LMAN (lateral magnocellular nucleus of the anterior nidopallium) which projects to Area X, a nucleus of the avian basal ganglia. In the present study, microdialysis was used to infuse different doses of the opioid antagonist naloxone in LMAN of adult male zebra finches. Whereas all doses of naloxone led to significant decreases in the number of FD (female-directed) songs, only 100 and 200 ng/ml of naloxone affected their acoustic properties. The decrease in FD song was not accompanied by changes in levels of attention toward females or those of neurotransmitters (dopamine, glutamate, and GABA) in LMAN. An earlier study had shown that similar manipulations in Area X did not lead to alterations in the number of FD songs but had significantly greater effects on their acoustic properties. Taken together, our results suggest that there are reciprocal effects of OR modulation on cortical and basal ganglia components of the AFP in songbirds.

Estradiol decreases medium spiny neuron excitability in female rat nucleus accumbens core

The menstrual cycle in humans and its analogous cycle in rodents, the estrous cycle, modulate brain function and behavior. Both cycles are characterized by the cyclical fluctuation of ovarian hormones including estrogens such as estradiol. Estradiol induces cycle- and sex-dependent differences in the phenotype and incidence of many behaviors, including those related to reward and motivation. The nucleus accumbens core (AcbC), a limbic and premotor system nexus region, directly regulates these behaviors. We previously showed that the estrous cycle modulates intrinsic excitability and excitatory synapse properties of medium spiny neurons (MSNs) in the AcbC. The identity of the underlying hormone mechanism is unknown, with estradiol being a prime candidate. The present study tests the hypothesis that estradiol induces estrous cycle-relevant differences in MSN electrophysiology. To accomplish this goal, a time- and dose-dependent estradiol replacement paradigm designed to simulate the rise of circulating estradiol levels across the estrous cycle was employed in ovariectomized adult female rats as well as a vehicle control group. Estradiol replacement decreased MSN excitability by modulating properties such as resting membrane potential, input resistance in both the linear and rectified ranges, and rheobase compared with vehicle-treated females. These differences in MSN excitability mimic those previously described regarding estrous cycle effects on MSN electrophysiology. Excitatory synapse properties were not modulated in response to this estradiol replacement paradigm. These data are the first to demonstrate that an estrous cycle-relevant estradiol exposure modulates MSN electrophysiology, providing evidence of the fundamental neuroendocrine mechanisms regulating the AcbC.NEW & NOTEWORTHY The present study shows, for the first time, that an estrous cycle-relevant estradiol exposure modulates nucleus accumbens neuron excitability. This evidence provides insight into the neuroendocrine mechanisms by which estradiol cyclically alters neuron properties during the estrous cycle. Overall, these data emphasize the significant influence of hormone action in the brain and especially individual neuron physiology.

Differential and synergistic roles of 17β-estradiol and progesterone in modulating adult female rat nucleus accumbens core medium spiny neuron electrophysiology

Naturally occurring cyclical changes in sex steroid hormones such as 17β-estradiol and progesterone can modulate neuron function and behavior in female mammals. One example is the estrous cycle in rats, which is composed of multiple phases. We previously reported evidence of differences between estrous cycle phases in excitatory synapse and intrinsic electrophysiological properties of rat nucleus accumbens core (AcbC) medium spiny neurons (MSNs). The AcbC is a nexus between the limbic and premotor systems and is integral for controlling motivated and reward-associated behaviors and disorders, which are sensitive to the estrous cycle and hormones. The present study expands our prior findings by testing whether circulating levels of estradiol and progesterone correlate with changes in MSN electrophysiology across estrous cycle phases. As part of this project, the excitatory synapse and intrinsic excitability properties of MSNs in late proestrus of adult female rats were assessed. Circulating levels of estradiol correlate with resting membrane potential, the time constant of the membrane, and rheobase. Circulating levels of progesterone correlate with miniature excitatory postsynaptic current (mEPSC) frequency and amplitude. Circulating levels of estradiol and progesterone together correlate with mEPSC amplitude, resting membrane potential, and input resistance. The late proestrus phase features a prominent and unique decrease in mEPSC frequency. These data indicate that circulating levels of estradiol and progesterone alone or in combination interact with specific MSN electrophysiological properties, indicating differential and synergistic roles of these hormones. Broadly, these findings illustrate the underlying endocrine actions regarding how the estrous cycle modulates MSN electrophysiology.NEW & NOTEWORTHY This research indicates that estradiol and progesterone act both differentially and synergistically to modulate neuron physiology in the nucleus accumbens core. These actions by specific hormones provide key data indicating the endocrine mechanisms underlying how the estrous cycle modulates neuron physiology in this region. Overall, these data reinforce that hormones are an important influence on neural physiology.

The expression of DARPP-32 in adult male zebra finches (Taenopygia guttata)

Although the catecholaminergic circuitry in the zebra finch brain has been well studied, there is little information regarding the postsynaptic targets of dopamine. To answer this question, we looked at overall patterns of immunoreactivity for DARPP-32 (a dopamine and cAMP-regulated phosphoprotein, present mostly in dopaminoceptive neurons) in adult male zebra finches. Our results demonstrated that as in mammals and other avian species, DARPP-32 expression was highest in both medial and lateral striatum. Interestingly, a specific pattern of immunoreactivity was observed in the song control system, with 'core' song control regions, that is, LMANcore (lateral magnocellular nucleus of the anterior nidopallium), RA (nucleus robustus arcopallialis) and HVC being less immunoreactive for DARPP-32 than 'shell' areas such as LMANshell, RAcup, AId (intermediate arcopallium) and HVCshelf. Our results suggest that whereas dopamine may modulate the shell pathways at various levels of the AFP, dopaminergic modulation of the core pathway occurs mainly through Area X, a basal ganglia nucleus. Further, secondary sensory cortices including the perientopallial belt, Fields L1 and L3 had higher DARPP-32-immunoreactivity than primary sensory cortical areas such as the pallial basolateral nucleus, entopallium proper and Field L2, corresponding to somatosensory, visual and auditory systems, respectively. We also found DARPP-32-rich axon terminals surrounding dopaminergic neurons in the ventral tegmental area-substantia nigra complex which in turn project to the striatum, suggesting that there may be a reciprocal modulation between these regions. Overall, DARPP-32 expression appears to be higher in areas involved in integrating sensory information, which further supports the role of this protein as a molecular integrator of different signal processing pathways.

Estradiol rapidly modulates excitatory synapse properties in a sex- and region-specific manner in rat nucleus accumbens core and caudate-putamen

Estradiol acutely facilitates sex differences in striatum-dependent behaviors. However, little is understood regarding the underlying mechanism. In striatal regions in adult rodents, estrogen receptors feature exclusively extranuclear expression, suggesting that estradiol rapidly modulates striatal neurons. We tested the hypothesis that estradiol rapidly modulates excitatory synapse properties onto medium spiny neurons (MSNs) of two striatal regions, the nucleus accumbens core and caudate-putamen in adult female and male rats. We predicted there would be sex-specific differences in pre- and postsynaptic locus and sensitivity. We further analyzed whether MSN intrinsic properties are predictive of estrogen sensitivity. Estradiol exhibited sex-specific acute effects in the nucleus accumbens core: miniature excitatory postsynaptic current (mEPSC) frequency robustly decreased in response to estradiol in female MSNs, and mEPSC amplitude moderately increased in response to estradiol in both male and female MSNs. This increase in mEPSC amplitude is associated with MSNs featuring increased intrinsic excitability. No MSN intrinsic electrical property associated with changes in mEPSC frequency. Estradiol did not acutely modulate mEPSC properties in the caudate-putamen of either sex. This is the first demonstration of acute estradiol action on MSN excitatory synapse function. This demonstration of sex and striatal region-specific acute estradiol neuromodulation revises our understanding of sex hormone action on striatal physiology and resulting behaviors.NEW & NOTEWORTHY This study is the first to demonstrate rapid estradiol neuromodulation of glutamatergic signaling on medium spiny neurons (MSNs), the major output neuron of the striatum. These findings emphasize that sex is a significant biological variable both in MSN sensitivity to estradiol and in pre- and postsynaptic mechanisms of glutamatergic signaling. MSNs in different regions exhibit diverse responses to estradiol. Sex- and region-specific estradiol-induced changes to excitatory signaling on MSNs explain sex differences partially underlying striatum-mediated behaviors and diseases.

Exploring the molecular basis of neuronal excitability in a vocal learner

BACKGROUND

Vocal learning, the ability to learn to produce vocalizations through imitation, relies on specialized brain circuitry known in songbirds as the song system. While the connectivity and various physiological properties of this system have been characterized, the molecular genetic basis of neuronal excitability in song nuclei remains understudied. We have focused our efforts on examining voltage-gated ion channels to gain insight into electrophysiological and functional features of vocal nuclei. A previous investigation of potassium channel genes in zebra finches (Taeniopygia guttata) revealed evolutionary modifications unique to songbirds, as well as transcriptional specializations in the song system [Lovell PV, Carleton JB, Mello CV. BMC Genomics 14:470 2013]. Here, we expand this approach to sodium, calcium, and chloride channels along with their modulatory subunits using comparative genomics and gene expression analysis encompassing microarrays and in situ hybridization.

RESULTS

We found 23 sodium, 38 calcium, and 33 chloride channel genes (HGNC-based classification) in the zebra finch genome, several of which were previously unannotated. We determined 15 genes are missing relative to mammals, including several genes (CLCAs, BEST2) linked to olfactory transduction. The majority of sodium and calcium but few chloride channels showed differential expression in the song system, among them SCN8A and CACNA1E in the direct motor pathway, and CACNG4 and RYR2 in the anterior forebrain pathway. In several cases, we noted a seemingly coordinated pattern across multiple nuclei (SCN1B, SCN3B, SCN4B, CACNB4) or sparse expression (SCN1A, CACNG5, CACNA1B).

CONCLUSION

The gene families examined are highly conserved between avian and mammalian lineages. Several cases of differential expression likely support high-frequency and burst firing in specific song nuclei, whereas cases of sparse patterns of expression may contribute to the unique electrophysiological signatures of distinct cell populations. These observations lay the groundwork for manipulations to determine how ion channels contribute to the neuronal excitability properties of vocal learning systems.

The estrous cycle modulates rat caudate-putamen medium spiny neuron physiology

The neuroendocrine environment in which the brain operates is both dynamic and differs by sex. How differences in neuroendocrine state affect neuron properties has been significantly neglected in neuroscience research. Behavioral data across humans and rodents indicate that natural cyclical changes in steroid sex hormone production affect sensorimotor and cognitive behaviors in both normal and pathological contexts. These behaviors are critically mediated by the caudate-putamen. In the caudate-putamen, medium spiny neurons (MSNs) are the predominant and primary output neurons. MSNs express membrane-associated estrogen receptors and demonstrate estrogen sensitivity. However, how the cyclical hormone changes across the estrous cycle may modulate caudate-putamen MSN electrophysiological properties remains unknown. Here, we performed whole-cell patch-clamp recordings on male, diestrus female, proestrus female, and estrus female caudate-putamen MSNs. Action potential, passive membrane, and miniature excitatory post-synaptic current properties were assessed. Numerous MSN electrical properties robustly differed by cycle state, including resting membrane potential, rheobase, action potential threshold, maximum evoked action potential firing rate, and inward rectification. Strikingly, when considered independent of estrous cycle phase, all but one of these properties do not significantly differ from male MSNs. These data indicate that female caudate-putamen MSNs are sensitive to the estrous cycle, and more broadly, the importance of considering neuroendocrine state in studies of neuron physiology.

Altering Opioid Neuromodulation in the Songbird Basal Ganglia Modulates Vocalizations

Although the interplay between endogenous opioids and dopamine (DA) in the basal ganglia (BG) is known to underlie diverse motor functions, few studies exist on their role in modulating speech and vocalization. Vocal impairment is a common symptom of Parkinson’s disease (PD), wherein DA depletion affects striosomes rich in μ-opioid receptors (μ-ORs). Symptoms of opioid addiction also include deficiencies in verbal functions and speech. To understand the interplay between the opioid system and BG in vocalization, we used adult male songbirds wherein high levels of μ-ORs are expressed in Area X, a BG region which is part of a circuit similar to the mammalian thalamocortical-basal ganglia loop. Changes in DA, glutamate and GABA levels were analyzed during the infusion of different doses of the μ-OR antagonist naloxone (50 and 100 ng/ml) specifically in Area X. Blocking μ-ORs in Area X with 100 ng/ml naloxone led to increased levels of DA in this region without altering the number of songs directed toward females (FD). Interestingly, this manipulation also led to changes in the spectro-temporal properties of FD songs, suggesting that altered opioid modulation in the thalamocortical-basal ganglia circuit can affect vocalization. Our study suggests that songbirds are excellent model systems to explore how the interplay between μ-ORs and DA modulation in the BG affects speech/vocalization.

Electrophysiological properties of medium spiny neurons in the nucleus accumbens core of prepubertal male and female Drd1a-tdTomato line 6 BAC transgenic mice

The nucleus accumbens core (AcbC) is a striatal brain region essential for integrating motivated behavior and reward processing with premotor function. In humans and rodents, research has identified sex differences and sex steroid hormone sensitivity in AcbC-mediated behaviors, in disorders, and in rats in the electrophysiological properties of the AcbC output neuron type, the medium spiny neuron (MSN). It is unknown whether the sex differences detected in MSN electrophysiological properties extend to mice. Furthermore, MSNs come in distinct subtypes with subtle differences in electrophysiological properties, and it is unknown whether MSN subtype-specific electrophysiology varies by sex. To address these questions, we used male and female Drd1a-tdTomato line 6 bacterial artificial chromosome transgenic mice. We made acute brain slices of the AcbC, and performed whole cell patch-clamp recordings across MSN subtypes to comprehensively assess AcbC MSN subtype electrophysiological properties. We found that ( 1 mice MSNs did not exhibit the sex differences detected in rat MSNs, and 2) electrophysiological properties differed between MSN subtypes in both sexes, including rheobase, resting membrane potential, action potential properties, intrinsic excitability, input resistance in both the linear and rectified ranges, and miniature excitatory postsynaptic current properties. These findings significantly extend previous studies of MSN subtypes performed in males or animals of undetermined sex and indicate that the influence of sex upon AcbC MSN properties varies between rodent species. NEW & NOTEWORTHY This research provides the most comprehensive assessment of medium spiny neuron subtype electrophysiological properties to date in a critical brain region, the nucleus accumbens core. It additionally represents the first evaluation of whether mouse medium spiny neuron subtype electrophysiological properties differ by sex.

A subcortical circuit linking the cerebellum to the basal ganglia engaged in vocal learning

Speech is a complex sensorimotor skill, and vocal learning involves both the basal ganglia and the cerebellum. These subcortical structures interact indirectly through their respective loops with thalamo-cortical and brainstem networks, and directly via subcortical pathways, but the role of their interaction during sensorimotor learning remains undetermined. While songbirds and their song-dedicated basal ganglia-thalamo-cortical circuitry offer a unique opportunity to study subcortical circuits involved in vocal learning, the cerebellar contribution to avian song learning remains unknown. We demonstrate that the cerebellum provides a strong input to the song-related basal ganglia nucleus in zebra finches. Cerebellar signals are transmitted to the basal ganglia via a disynaptic connection through the thalamus and then conveyed to their cortical target and to the premotor nucleus controlling song production. Finally, cerebellar lesions impair juvenile song learning, opening new opportunities to investigate how subcortical interactions between the cerebellum and basal ganglia contribute to sensorimotor learning.

Estrous cycle-induced sex differences in medium spiny neuron excitatory synaptic transmission and intrinsic excitability in adult rat nucleus accumbens core

Naturally occurring hormone cycles in adult female humans and rodents create a dynamic neuroendocrine environment. These cycles include the menstrual cycle in humans and its counterpart in rodents, the estrous cycle. These hormone fluctuations induce sex differences in the phenotypes of many behaviors, including those related to motivation, and associated disorders such as depression and addiction. This suggests that the neural substrate instrumental for these behaviors, including the nucleus accumbens core (AcbC), likewise differs between estrous cycle phases. It is unknown whether the electrophysiological properties of AcbC output neurons, medium spiny neurons (MSNs), change between estrous cycle phases. This is a critical knowledge gap given that MSN electrophysiological properties are instrumental for determining AcbC output to efferent targets. Here we test whether the intrinsic electrophysiological properties of adult rat AcbC MSNs differ across female estrous cycle phases and from males. We recorded MSNs with whole cell patch-clamp technique in two experiments, the first using gonad-intact adult males and females in differing phases of the estrous cycle and the second using gonadectomized males and females in which the estrous cycle was eliminated. MSN intrinsic electrophysiological and excitatory synaptic input properties robustly changed between female estrous cycle phases and males. Sex differences in MSN electrophysiology disappeared when the estrous cycle was eliminated. These novel findings indicate that AcbC MSN electrophysiological properties change across the estrous cycle, providing a new framework for understanding how biological sex and hormone cyclicity regulate motivated behaviors and other AcbC functions and disorders. NEW & NOTEWORTHY This research is the first demonstration that medium spiny neuron electrophysiological properties change across adult female hormone cycle phases in any striatal region. This influence of estrous cycle engenders sex differences in electrophysiological properties that are eliminated by gonadectomy. Broadly, these findings indicate that adult female hormone cycles are an important factor for neurophysiology.

Expression of the potassium-chloride co-transporter, KCC2, within the avian song system

Songbirds learn to produce vocalizations early in life by listening to, then copying the songs of conspecific males. The anterior forebrain pathway, homologous to a basal ganglia-forebrain circuit, is essential for song learning. The projection between the striato-pallidal structure, Area X, and the medial portion of the dorsolateral thalamic nucleus (DLM) is strongly hyperpolarizing in adults, due to a very negative chloride reversal potential (Person & Perkel, Neuron 46:129-140, 2005). The chloride reversal potential is determined, in part, by the expression level of a neuron-specific potassium-chloride cotransporter, KCC2, which is developmentally upregulated in mammals. To determine whether a similar upregulation in KCC2 expression occurs at the Area X to DLM synapse during development, we examined the expression level of KCC2 in adult zebra finches across the song system as well as during development in the Area X - DLM synapse. We demonstrate that KCC2 is expressed in a subset of neurons throughout the song system, including HVC (used as a proper name), robust nucleus of the arcopallium (RA), lateral magnocellular nucleus of the anterior nidopallium (LMAN), Area X, and DLM. The majority of pallidal-like projection neurons in Area X showed KCC2 immunoreactivity. In adults, KCC2 expression was robust within DLM, and was upregulated between 14 and 24 days post hatching, before the onset of song learning. Light and electron microscopic analysis indicated that KCC2 immunoreactivity is strongly associated with the plasma membrane. Thus, in the song system as in the mammalian brain, KCC2 expression is well placed to modulate the GABAA reversal potential.

Nucleus accumbens core medium spiny neuron electrophysiological properties and partner preference behavior in the adult male prairie vole, Microtus ochrogaster

Medium spiny neurons (MSNs) in the nucleus accumbens have long been implicated in the neurobiological mechanisms that underlie numerous social and motivated behaviors as studied in rodents such as rats. Recently, the prairie vole has emerged as an important model animal for studying social behaviors, particularly regarding monogamy because of its ability to form pair bonds. However, to our knowledge, no study has assessed intrinsic vole MSN electrophysiological properties or tested how these properties vary with the strength of the pair bond between partnered voles. Here we performed whole cell patch-clamp recordings of MSNs in acute brain slices of the nucleus accumbens core (NAc) of adult male voles exhibiting strong and weak preferences for their respective partnered females. We first document vole MSN electrophysiological properties and provide comparison to rat MSNs. Vole MSNs demonstrated many canonical electrophysiological attributes shared across species but exhibited notable differences in excitability compared with rat MSNs. Second, we assessed male vole partner preference behavior and tested whether MSN electrophysiological properties varied with partner preference strength. Male vole partner preference showed extensive variability. We found that decreases in miniature excitatory postsynaptic current amplitude and the slope of the evoked action potential firing rate to depolarizing current injection weakly associated with increased preference for the partnered female. This suggests that excitatory synaptic strength and neuronal excitability may be decreased in MSNs in males exhibiting stronger preference for a partnered female. Overall, these data provide extensive documentation of MSN electrophysiological characteristics and their relationship to social behavior in the prairie vole. NEW & NOTEWORTHY This research represents the first assessment of prairie vole nucleus accumbens core medium spiny neuron intrinsic electrophysiological properties and probes the relationship between cellular excitability and social behavior.

Dopaminergic modulation of basal ganglia output through coupled excitation-inhibition

Learning and maintenance of skilled movements require exploration of motor space and selection of appropriate actions. Vocal learning and social context-dependent plasticity in songbirds depend on a basal ganglia circuit, which actively generates vocal variability. Dopamine in the basal ganglia reduces trial-to-trial neural variability when the bird engages in courtship song. Here, we present evidence for a unique, tonically active, excitatory interneuron in the songbird basal ganglia that makes strong synaptic connections onto output pallidal neurons, often linked in time with inhibitory events. Dopamine receptor activity modulates the coupling of these excitatory and inhibitory events in vitro, which results in a dynamic change in the synchrony of a modeled population of basal ganglia output neurons receiving excitatory and inhibitory inputs. The excitatory interneuron thus serves as one biophysical mechanism for the introduction or modulation of neural variability in this circuit.

To transduce a zebra finch: interrogating behavioral mechanisms in a model system for speech

The ability to alter neuronal gene expression, either to affect levels of endogenous molecules or to express exogenous ones, is a powerful tool for linking brain and behavior. Scientists continue to finesse genetic manipulation in mice. Yet mice do not exhibit every behavior of interest. For example, Mus musculus do not readily imitate sounds, a trait known as vocal learning and a feature of speech. In contrast, thousands of bird species exhibit this ability. The circuits and underlying molecular mechanisms appear similar between disparate avian orders and are shared with humans. An advantage of studying vocal learning birds is that the neurons dedicated to this trait are nested within the surrounding brain regions, providing anatomical targets for relating brain and behavior. In songbirds, these nuclei are known as the song control system. Molecular function can be interrogated in non-traditional model organisms by exploiting the ability of viruses to insert genetic material into neurons to drive expression of experimenter-defined genes. To date, the use of viruses in the song control system is limited. Here, we review prior successes and test additional viruses for their capacity to transduce basal ganglia song control neurons. These findings provide a roadmap for troubleshooting the use of viruses in animal champions of fascinating behaviors—nowhere better featured than at the 12th International Congress!

Automated synaptic connectivity inference for volume electron microscopy

Teravoxel volume electron microscopy data sets from neural tissue can now be acquired in weeks, but data analysis requires years of manual labor. We developed the SyConn framework, which uses deep convolutional neural networks and random forest classifiers to infer a richly annotated synaptic connectivity matrix from manual neurite skeleton reconstructions by automatically identifying mitochondria, synapses and their types, axons, dendrites, spines, myelin, somata and cell types. We tested our approach on serial block-face electron microscopy data sets from zebrafish, mouse and zebra finch, and computed the synaptic wiring of songbird basal ganglia. We found that, for example, basal-ganglia cell types with high firing rates in vivo had higher densities of mitochondria and vesicles and that synapse sizes and quantities scaled systematically, depending on the innervated postsynaptic cell types.

Singing modulates parvalbumin interneurons throughout songbird forebrain vocal control circuitry

Across species, the performance of vocal signals can be modulated by the social environment. Zebra finches, for example, adjust their song performance when singing to females ('female-directed' or FD song) compared to when singing in isolation ('undirected' or UD song). These changes are salient, as females prefer the FD song over the UD song. Despite the importance of these performance changes, the neural mechanisms underlying this social modulation remain poorly understood. Previous work in finches has established that expression of the immediate early gene EGR1 is increased during singing and modulated by social context within the vocal control circuitry. Here, we examined whether particular neural subpopulations within those vocal control regions exhibit similar modulations of EGR1 expression. We compared EGR1 expression in neurons expressing parvalbumin (PV), a calcium buffer that modulates network plasticity and homeostasis, among males that performed FD song, males that produced UD song, or males that did not sing. We found that, overall, singing but not social context significantly affected EGR1 expression in PV neurons throughout the vocal control nuclei. We observed differences in EGR1 expression between two classes of PV interneurons in the basal ganglia nucleus Area X. Additionally, we found that singing altered the amount of PV expression in neurons in HVC and Area X and that distinct PV interneuron types in Area X exhibited different patterns of modulation by singing. These data indicate that throughout the vocal control circuitry the singing-related regulation of EGR1 expression in PV neurons may be less influenced by social context than in other neuron types and raise the possibility of cell-type specific differences in plasticity and calcium buffering.

Social context differentially modulates activity of two interneuron populations in an avian basal ganglia nucleus

Basal ganglia circuits are critical for the modulation of motor performance across behavioral states. In zebra finches, a cortical-basal ganglia circuit dedicated to singing is necessary for males to adjust their song performance and transition between spontaneous singing, when they are alone ("undirected" song), and a performance state, when they sing to a female ("female-directed" song). However, we know little about the role of different basal ganglia cell types in this behavioral transition or the degree to which behavioral context modulates the activity of different neuron classes. To investigate whether interneurons in the songbird basal ganglia encode information about behavioral state, I recorded from two interneuron types, fast-spiking interneurons (FSI) and external pallidal (GPe) neurons, in the songbird basal ganglia nucleus area X during both female-directed and undirected singing. Both cell types exhibited higher firing rates, more frequent bursting, and greater trial-by-trial variability in firing when male zebra finches produced undirected songs compared with when they produced female-directed songs. However, the magnitude and direction of changes to the firing rate, bursting, and variability of spiking between when birds sat silently and when they sang undirected and female-directed song varied between FSI and GPe neurons. These data indicate that social modulation of activity important for eliciting changes in behavioral state is present in multiple cell types within area X and suggests that social interactions may adjust circuit dynamics during singing at multiple points within the circuit.

Neonatal Masculinization Blocks Increased Excitatory Synaptic Input in Female Rat Nucleus Accumbens Core

Steroid sex hormones and genetic sex regulate the phenotypes of motivated behaviors and relevant disorders. Most studies seeking to elucidate the underlying neuroendocrine mechanisms have focused on how 17β-estradiol modulates the role of dopamine in striatal brain regions, which express membrane-associated estrogen receptors. Dopamine action is an important component of striatal function, but excitatory synaptic neurotransmission has also emerged as a key striatal substrate and target of estradiol action. Here, we focus on excitatory synaptic input onto medium spiny neurons (MSNs) in the striatal region nucleus accumbens core (AcbC). In adult AcbC, miniature excitatory postsynaptic current (mEPSC) frequency is increased in female compared with male MSNs. We tested whether increased mEPSC frequency in female MSNs exists before puberty, whether this increased excitability is due to the absence of estradiol or testosterone during the early developmental critical period, and whether it is accompanied by stable neuron intrinsic membrane properties. We found that mEPSC frequency is increased in female compared with male MSNs before puberty. Increased mEPSC frequency in female MSNs is abolished after neonatal estradiol or testosterone exposure. MSN intrinsic membrane properties did not differ by sex. These data indicate that neonatal masculinization via estradiol and/or testosterone action is sufficient for down-regulating excitatory synaptic input onto MSNs. We conclude that excitatory synaptic input onto AcbC MSNs is organized long before adulthood via steroid sex hormone action, providing new insight into a mechanism by which sex differences in motivated behavior and other AbcC functions may be generated or compromised.

Focal expression of mutant huntingtin in the songbird basal ganglia disrupts cortico-basal ganglia networks and vocal sequences

The basal ganglia (BG) promote complex sequential movements by helping to select elementary motor gestures appropriate to a given behavioral context. Indeed, Huntington's disease (HD), which causes striatal atrophy in the BG, is characterized by hyperkinesia and chorea. How striatal cell loss alters activity in the BG and downstream motor cortical regions to cause these disorganized movements remains unknown. Here, we show that expressing the genetic mutation that causes HD in a song-related region of the songbird BG destabilizes syllable sequences and increases overall vocal activity, but leave the structure of individual syllables intact. These behavioral changes are paralleled by the selective loss of striatal neurons and reduction of inhibitory synapses on pallidal neurons that serve as the BG output. Chronic recordings in singing birds revealed disrupted temporal patterns of activity in pallidal neurons and downstream cortical neurons. Moreover, reversible inactivation of the cortical neurons rescued the disorganized vocal sequences in transfected birds. These findings shed light on a key role of temporal patterns of cortico-BG activity in the regulation of complex motor sequences and show how a genetic mutation alters cortico-BG networks to cause disorganized movements.

Reduced vocal variability in a zebra finch model of dopamine depletion: implications for Parkinson disease

Midbrain dopamine (DA) modulates the activity of basal ganglia circuitry important for motor control in a variety of species. In songbirds, DA underlies motivational behavior including reproductive drive and is implicated as a gatekeeper for neural activity governing vocal variability. In the zebra finch, Taeniopygia guttata, DA levels increase in Area X, a song-dedicated subregion of the basal ganglia, when a male bird sings his courtship song to a female (female-directed; FD). Levels remain stable when he sings a less stereotyped version that is not directed toward a conspecific (undirected; UD). Here, we used a mild dose of the neurotoxin 6-hydroxydopamine (6-OHDA) to reduce presynaptic DA input to Area X and characterized the effects on FD and UD behaviors. Immunoblots were used to quantify levels of tyrosine hydroxylase (TH) as a biomarker for DA afferent loss in vehicle- and 6-OHDA-injected birds. Following 6-OHDA administration, TH signals were lower in Area X but not in an adjacent subregion, ventral striatal-pallidum (VSP). A postsynaptic marker of DA signaling was unchanged in both regions. These observations suggest that effects were specific to presynaptic afferents of vocal basal ganglia. Concurrently, vocal variability was reduced during UD but not FD song. Similar decreases in vocal variability are observed in patients with Parkinson disease (PD), but the link to DA loss is not well-understood. The 6-OHDA songbird model offers a unique opportunity to further examine how DA loss in cortico-basal ganglia pathways affects vocal control.

Origins of basal ganglia output signals in singing juvenile birds

Across species, complex circuits inside the basal ganglia (BG) converge on pallidal output neurons that exhibit movement-locked firing patterns. Yet the origins of these firing patterns remain poorly understood. In songbirds during vocal babbling, BG output neurons homologous to those found in the primate internal pallidal segment are uniformly activated in the tens of milliseconds prior to syllable onsets. To test the origins of this remarkably homogenous BG output signal, we recorded from diverse upstream BG cell types during babbling. Prior to syllable onsets, at the same time that internal pallidal segment-like neurons were activated, putative medium spiny neurons, fast spiking and tonically active interneurons also exhibited transient rate increases. In contrast, pallidal neurons homologous to those found in primate external pallidal segment exhibited transient rate decreases. To test origins of these signals, we performed recordings following lesion of corticostriatal inputs from premotor nucleus HVC. HVC lesions largely abolished these syllable-locked signals. Altogether, these findings indicate a striking homogeneity of syllable timing signals in the songbird BG during babbling and are consistent with a role for the indirect and hyperdirect pathways in transforming cortical inputs into BG outputs during an exploratory behavior.

Convergent transcriptional specializations in the brains of humans and song-learning birds

Song-learning birds and humans share independently evolved similarities in brain pathways for vocal learning that are essential for song and speech and are not found in most other species. Comparisons of brain transcriptomes of song-learning birds and humans relative to vocal nonlearners identified convergent gene expression specializations in specific song and speech brain regions of avian vocal learners and humans. The strongest shared profiles relate bird motor and striatal song-learning nuclei, respectively, with human laryngeal motor cortex and parts of the striatum that control speech production and learning. Most of the associated genes function in motor control and brain connectivity. Thus, convergent behavior and neural connectivity for a complex trait are associated with convergent specialized expression of multiple genes.

Intrinsic excitability varies by sex in prepubertal striatal medium spiny neurons

Sex differences in neuron electrophysiological properties were traditionally associated with brain regions directly involved in reproduction in adult, postpubertal animals. There is growing acknowledgement that sex differences can exist in other developmental periods and brain regions as well. This includes the dorsal striatum (caudate/putamen), which shows robust sex differences in gene expression, neuromodulator action (including dopamine and 17β-estradiol), and relevant sensorimotor behaviors and pathologies such as the responsiveness to drugs of abuse. Here we examine whether these sex differences extend to striatal neuron electrophysiology. We test the hypothesis that passive and active medium spiny neuron (MSN) electrophysiological properties in prepubertal rat dorsal striatum differ by sex. We made whole cell recordings from male and females MSNs from acute brain slices. The slope of the evoked firing rate to current injection curve was increased in MSNs recorded from females compared with males. The initial action potential firing rate was increased in MSNs recorded from females compared with males. Action potential after-hyperpolarization peak was decreased, and threshold was hyperpolarized in MSNs recorded from females compared with males. No sex differences in passive electrophysiological properties or miniature excitatory synaptic currents were detected. These findings indicate that MSN excitability is increased in prepubertal females compared with males, providing a new mechanism that potentially contributes to generating sex differences in striatal-mediated processes. Broadly, these findings demonstrate that sex differences in neuron electrophysiological properties can exist prepuberty in brain regions not directly related to reproduction.

Two tales of how expectation of reward modulates behavior

Expectation of reward modulates many types of behaviors. Here we highlight two lines of research on reward-modulated perceptual decision making in primates and social context-modulated singing in songbirds, respectively. These two seemingly distinct behaviors are both known to involve cortico-basal ganglia-thalamic circuits. The underlying computations may be conceptualized using a simple, common framework. We summarize and compare our current knowledge of the two fields to motivate new experiments for each field, with the goal of finding general principles for how the brain implements reward-modulated behavior.

Emergence of context-dependent variability across a basal ganglia network

Context dependence is a key feature of cortical-basal ganglia circuit activity, and in songbirds the cortical outflow of a basal ganglia circuit specialized for song, LMAN, shows striking increases in trial-by-trial variability and bursting when birds sing alone rather than to females. To reveal where this variability and its social regulation emerge, we recorded stepwise from corticostriatal (HVC) neurons and their target spiny and pallidal neurons in Area X. We find that corticostriatal and spiny neurons both show precise singing-related firing across both social settings. Pallidal neurons, in contrast, exhibit markedly increased trial-by-trial variation when birds sing alone, created by highly variable pauses in firing. This variability persists even when recurrent inputs from LMAN are ablated. These data indicate that variability and its context sensitivity emerge within the basal ganglia network, suggest a network mechanism for this emergence, and highlight variability generation and regulation as basal ganglia functions.

Auditory synapses to song premotor neurons are gated off during vocalization in zebra finches

Songbirds use auditory feedback to learn and maintain their songs, but how feedback interacts with vocal motor circuitry remains unclear. A potential site for this interaction is the song premotor nucleus HVC, which receives auditory input and contains neurons (HVCX cells) that innervate an anterior forebrain pathway (AFP) important to feedback-dependent vocal plasticity. Although the singing-related output of HVCX cells is unaltered by distorted auditory feedback (DAF), deafening gradually weakens synapses on HVCX cells, raising the possibility that they integrate feedback only at subthreshold levels during singing. Using intracellular recordings in singing zebra finches, we found that DAF failed to perturb singing-related synaptic activity of HVCX cells, although many of these cells responded to auditory stimuli in non-singing states. Moreover, in vivo multiphoton imaging revealed that deafening-induced changes to HVCX synapses require intact AFP output. These findings support a model in which the AFP accesses feedback independent of HVC. DOI: http://dx.doi.org/10.7554/eLife.01833.001.

A Bird's Eye View of Sleep-Dependent Memory Consolidation

How new experiences are solidified into long-lasting memories is a central question in the study of brain and behavior. One of the most intriguing discoveries in memory research is that brain activity during sleep helps to transform newly learned information and skills into robust memories. Though the first experimental work linking sleep and memory was conducted 90 years ago by Jenkins and Dallenbach, the case for sleep-dependent memory consolidation has only garnered strong support in the last decade. Recent studies in humans provide extensive behavioral, imaging, and polysomnographic data supporting sleep consolidation of a broad range of memory tasks. Likewise, studies in a few animal model systems have elucidated potential mechanisms contributing to sleep consolidation such as neural reactivation and synaptic homeostasis. Here, we present an overview of sleep-dependent memory consolidation, focusing on how investigations of sleep and learning in birds have complemented the progress made in mammalian systems by emphasizing a strong connection between behavior and physiology. We begin by describing the behavioral approach that has been utilized to demonstrate sleep consolidation in humans. We then address neural reactivation in the rodent hippocampal system as a putative mechanism of sleep consolidation. Next, we discuss the role of sleep in the learning and maintenance of song in zebra finches. We note that while both the rodent and zebra finch systems provide evidence for sleep-dependent memory changes in physiology and behavior, neither duplicates the pattern of changes most commonly observed in humans. Finally, we present a recently developed model of sleep consolidation involving auditory classification learning in European starlings , which has the potential to connect behavioral evidence of sleep consolidation as developed in humans with underlying neural mechanisms observable in animals.

Food rewards modulate the activity of song neurons in Bengalese finches

Vocal learning, a critical component of speech acquisition, is a rare trait in animals. Songbirds are a well-established animal model in vocal learning research; male birds acquire novel vocal patterns and have a well-developed 'song system' in the brain. Although this system is unique to songbirds, anatomical and physiological studies have reported similarities between the song system and the thalamo-cortico-basal ganglia circuit that is conserved among reptiles, birds, and mammals. Here, we focused on the similarity of the neural response between these two systems while animals were engaging in operant tasks. Neurons in the basal ganglia of vertebrates are activated in response to food rewards and reward predictions in behavioral tasks. A striatal nucleus in the avian song system, Area X, is necessary for vocal learning and is considered specialized for singing. We found that the spiking activity of singing-related Area X neurons was modulated by food rewards and reward signals in an operant task. As previous studies showed that Area X is not critical for general cognitive tasks, the role of Area X in general learning might be limited and vestigial. However, our results provide a new viewpoint to investigate the independence of the vocal learning system from neural systems involved in other cognitive tasks.

Differential contributions of basal ganglia and thalamus to song initiation, tempo, and structure

Basal ganglia-thalamocortical circuits are multistage loops critical to motor behavior, but the contributions of individual components to overall circuit function remain unclear. We addressed these issues in a songbird basal ganglia-thalamocortical circuit (the anterior forebrain pathway, AFP) specialized for singing and critical for vocal plasticity. The major known afferent to the AFP is the premotor cortical nucleus, HVC. Surprisingly, previous studies found that lesions of HVC alter song but do not eliminate the ability of the AFP to drive song production. We therefore used this AFP-driven song to investigate the role of basal ganglia and thalamus in vocal structure, tempo, and initiation. We found that lesions of the striatopallidal component (Area X) slowed song and simplified its acoustic structure. Elimination of the thalamic component (DLM) further simplified the acoustic structure of song and regularized its rhythm but also dramatically reduced song production. The acoustic structure changes imply that sequential stages of the AFP each add complexity to song, but the effects of DLM lesions on song initiation suggest that thalamus is a locus of additional inputs important to initiation. Together, our results highlight the cumulative contribution of stages of a basal ganglia-thalamocortical circuit to motor output along with distinct involvement of thalamus in song initiation or "gating."

The basal ganglia is necessary for learning spectral, but not temporal, features of birdsong

Executing a motor skill requires the brain to control which muscles to activate at what times. How these aspects of control-motor implementation and timing-are acquired, and whether the learning processes underlying them differ, is not well understood. To address this, we used a reinforcement learning paradigm to independently manipulate both spectral and temporal features of birdsong, a complex learned motor sequence, while recording and perturbing activity in underlying circuits. Our results uncovered a striking dissociation in how neural circuits underlie learning in the two domains. The basal ganglia was required for modifying spectral, but not temporal, structure. This functional dissociation extended to the descending motor pathway, where recordings from a premotor cortex analog nucleus reflected changes to temporal, but not spectral, structure. Our results reveal a strategy in which the nervous system employs different and largely independent circuits to learn distinct aspects of a motor skill.

Ultrastructural and electrophysiological analysis of Area X in the untutored and deafened Bengalese finch in relation to normally reared birds

Birdsong learning bears many similarities to human speech acquisition. Although the anterior forebrain pathway (AFP) is believed to be involved in birdsong learning, the underlying neural mechanisms are unclear. We produced two types of abnormal song learning: young birds untutored from adult "song tutors", or birds deafened by bilateral cochlear removal before the onset of sensory learning. We then studied how ultrastructure and electrophysiological activity changed in an AFP nucleus, Area X, among these birds at adulthood. Our results showed that, although the size of Area X did not change significantly, the numbers of synapses per unit area and compound synapses and the percent of concave synapses increased significantly in the untutored or deafened birds. The percent of perforated synapses or axo-spinous synapses decreased compared to the normally reared birds, suggesting a decreased efficiency of synaptic transmission in the untutored or deafened birds. We then identified several types of spontaneously firing cells in Area X. Cells with fast and slow firing rates did not show significant electrophysiological differences among the groups, but cells with moderate firing rates, most likely DLM-projecting neurons, fired at significantly lower rates in the untutored and deafened birds. In addition, cells firing irregularly were only found in the deafened birds. Thus, the decreased or irregular electrophysiological activity in the untutored or deafened birds, together with the corresponding ultrastructural findings, could be implicated in the abnormal song production in these two types of birds.

Rhythmic cortical neurons increase their oscillations and sculpt basal ganglia signaling during motor learning

The function and modulation of neural circuits underlying motor skill may involve rhythmic oscillations (Feller, 1999; Marder and Goaillard, 2006; Churchland et al., 2012). In the proposed pattern generator for birdsong, the cortical nucleus HVC, the frequency and power of oscillatory bursting during singing increases with development (Crandall et al., 2007; Day et al., 2009). We examined the maturation of cellular activity patterns that underlie these changes. Single unit ensemble recording combined with antidromic identification (Day et al., 2011) was used to study network development in anesthetized zebra finches. Autocovariance quantified oscillations within single units. A subset of neurons oscillated in the theta/alpha/mu/beta range (8-20 Hz), with greater power in adults compared to juveniles. Across the network, the normalized oscillatory power in the 8-20 Hz range was greater in adults than juveniles. In addition, the correlated activity between rhythmic neuron pairs increased with development. We next examined the functional impact of the oscillators on the output neurons of HVC. We found that the firing of oscillatory neurons negatively correlated with the activity of cortico-basal ganglia neurons (HVC(X)s), which project to Area X (the song basal ganglia). If groups of oscillators work together to tonically inhibit and precisely control the spike timing of adult HVC(X)s with coordinated release from inhibition, then the activity of HVC(X)s in juveniles should be decreased relative to adults due to uncorrelated, tonic inhibition. Consistent with this hypothesis, HVC(X)s had lower activity in juveniles. These data reveal network changes that shape cortical-to-basal ganglia signaling during motor learning.

Genomics analysis of potassium channel genes in songbirds reveals molecular specializations of brain circuits for the maintenance and production of learned vocalizations

Background

A fundamental question in molecular neurobiology is how genes that determine basic neuronal properties shape the functional organization of brain circuits underlying complex learned behaviors. Given the growing availability of complete vertebrate genomes, comparative genomics represents a promising approach to address this question. Here we used genomics and molecular approaches to study how ion channel genes influence the properties of the brain circuitry that regulates birdsong, a learned vocal behavior with important similarities to human speech acquisition. We focused on potassium (K-)Channels, which are major determinants of neuronal cell excitability.

Starting with the human gene set of K-Channels, we used cross-species mRNA/protein alignments, and syntenic analysis to define the full complement of orthologs, paralogs, allelic variants, as well as novel loci not previously predicted in the genome of zebra finch (Taeniopygia guttata). We also compared protein coding domains in chicken and zebra finch orthologs to identify genes under positive selective pressure, and those that contained lineage-specific insertions/deletions in functional domains. Finally, we conducted comprehensive in situ hybridizations to determine the extent of brain expression, and identify K-Channel gene enrichments in nuclei of the avian song system.

Results

We identified 107 K-Channel finch genes, including 6 novel genes common to non-mammalian vertebrate lineages. Twenty human genes are absent in songbirds, birds, or sauropsids, or unique to mammals, suggesting K-Channel properties may be lineage-specific. We also identified specific family members with insertions/deletions and/or high dN/dS ratios compared to chicken, a non-vocal learner. In situ hybridization revealed that while most K-Channel genes are broadly expressed in the brain, a subset is selectively expressed in song nuclei, representing molecular specializations of the vocal circuitry.

Conclusions

Together, these findings shed new light on genes that may regulate biophysical and excitable properties of the song circuitry, identify potential targets for the manipulation of the song system, and reveal genomic specializations that may relate to the emergence of vocal learning and associated brain areas in birds.

Calbindin expression in developing striatum of zebra finches and its relation to the formation of area X

A sexually dimorphic network of brain regions controls learning and production of song in zebra finches. How this specialized song system evolved is unknown. To start addressing this question, we focused on developmental differences between the sexes, using the expression of the calcium-binding protein calbindin (CB) during embryonic to adult stages to map out the early development of Area X, a male-specific striatal structure. We related this pattern to the expression of three transcription factors, Pax6 and Islet1 to delineate the striatal radial domains, and Nkx2.1 as a marker for cells of pallidal origin. An incipient Area X-CB+ domain became discernable at embryonic day 13 in the Islet1-ventral striatal field. This region contained many Nkx2.1-expressing cells with a morphology characteristic of migrating cells. Eight days after hatching (PHD) CB staining clearly delineated Area X. Another CB+ structure formed around PHD5 at the subpallial/pallial boundary. We call it the CB+striatal capsule (CB-StC) and discuss its relation with the previously described striatal capsule in vertebrates. The CB cell population in both Area X and CB-StC, but not in the surrounding striatum, colocalized with the striatal medium spiny neurons (MSNs) marker, D1-receptor associated signaling protein dopamine-and-cAMP-regulated phosphoprotein of 32 kDa, DARPP32. In females, CB-positive cells were also present in the rostral striatum but did not coalesce into an Area X-like structure. We discuss possible reasons for CB expression in MSNs in songbirds and mammals, but not described in chicken striatum.