Possible roles for GABAergic inhibition in the vocal control system of the zebra finch

Cell type specializations of the vocal-motor cortex in songbirds

Identifying molecular specializations in cortical circuitry supporting complex behaviors, like learned vocalizations, requires understanding of the neuroanatomical context from which these circuits arise. In songbirds, the robust arcopallial nucleus (RA) provides descending cortical projections for fine vocal-motor control. Using single-nuclei transcriptomics and spatial gene expression mapping in zebra finches, we have defined cell types and molecular specializations that distinguish RA from adjacent regions involved in non-vocal motor and sensory processing. We describe an RA-specific projection neuron, differential inhibitory subtypes, and glia specializations and have probed predicted GABAergic interneuron subtypes electrophysiologically within RA. Several cell-specific markers arise developmentally in a sex-dependent manner. Our interactive apps integrate cellular data with developmental and spatial distribution data from the gene expression brain atlas ZEBrA. Users can explore molecular specializations of vocal-motor neurons and support cells that likely reflect adaptations key to the physiology and evolution of vocal control circuits and refined motor skills.

Manipulations of inhibition in cortical circuitry differentially affect spectral and temporal features of Bengalese finch song

The interplay between inhibition and excitation can regulate behavioral expression and control, including the expression of communicative behaviors like birdsong. Computational models postulate varying degrees to which inhibition within vocal motor circuitry influences birdsong, but few studies have tested these models by manipulating inhibition. Here we enhanced and attenuated inhibition in the cortical nucleus HVC (used as proper name) of Bengalese finches (Lonchura striata var. domestica). Enhancement of inhibition (with muscimol) in HVC dose-dependently reduced the amount of song produced. Infusions of higher concentrations of muscimol caused some birds to produce spectrally degraded songs, whereas infusions of lower doses of muscimol led to the production of relatively normal (nondegraded) songs. However, the spectral and temporal structures of these nondegraded songs were significantly different from songs produced under control conditions. In particular, muscimol infusions decreased the frequency and amplitude of syllables, increased various measures of acoustic entropy, and increased the variability of syllable structure. Muscimol also increased sequence durations and the variability of syllable timing and syllable sequencing. Attenuation of inhibition (with bicuculline) in HVC led to changes to song distinct from and often opposite to enhancing inhibition. For example, in contrast to muscimol, bicuculline infusions increased syllable amplitude, frequency, and duration and decreased the variability of acoustic features. However, like muscimol, bicuculline increased the variability of syllable sequencing. These data highlight the importance of inhibition to the production of stereotyped vocalizations and demonstrate that changes to neural dynamics within cortical circuitry can differentially affect spectral and temporal features of song.NEW & NOTEWORTHY We reveal that manipulations of inhibition in the cortical nucleus HVC affect the structure, timing, and sequencing of syllables in Bengalese finch song. Enhancing and blocking inhibition led to opposite changes to the acoustic structure and timing of vocalizations, but both caused similar changes to vocal sequencing. These data provide support for computational models of song control but also motivate refinement of existing models to account for differential effects on syllable structure, timing, and sequencing.

Differential developmental changes in cortical representations of auditory-vocal stimuli in songbirds

Procedural skill learning requires iterative comparisons between feedback of self-generated motor output and a goal sensorimotor pattern. In juvenile songbirds, neural representations of both self-generated behaviors (each bird's own immature song) and the goal motor pattern (each bird's adult tutor song) are essential for vocal learning, yet little is known about how these behaviorally relevant stimuli are encoded. We made extracellular recordings during song playback in anesthetized juvenile and adult zebra finches ( Taeniopygia guttata) in adjacent cortical regions RA (robust nucleus of the arcopallium), AId (dorsal intermediate arcopallium), and RA cup, each of which is well situated to integrate auditory-vocal information: RA is a motor cortical region that drives vocal output, AId is an adjoining cortical region whose projections converge with basal ganglia loops for song learning in the dorsal thalamus, and RA cup surrounds RA and receives inputs from primary and secondary auditory cortex. We found strong developmental differences in neural selectivity within RA, but not in AId or RA cup. Juvenile RA neurons were broadly responsive to multiple songs but preferred juvenile over adult vocal sounds; in addition, spiking responses lacked consistent temporal patterning. By adulthood, RA neurons responded most strongly to each bird's own song with precisely timed spiking activity. In contrast, we observed a complete lack of song responsivity in both juvenile and adult AId, even though this region receives song-responsive inputs. A surprisingly large proportion of sites in RA cup of both juveniles and adults did not respond to song playback, and responsive sites showed little evidence of song selectivity. NEW & NOTEWORTHY Motor skill learning entails changes in selectivity for behaviorally relevant stimuli across cortical regions, yet the neural representation of these stimuli remains understudied. We investigated how information important for vocal learning in zebra finches is represented in regions analogous to infragranular layers of motor and auditory cortices during vs. after the developmentally regulated learning period. The results provide insight into how neurons in higher level stages of cortical processing represent stimuli important for motor skill learning.

Supervised learning in spiking neural networks with FORCE training

Populations of neurons display an extraordinary diversity in the behaviors they affect and display. Machine learning techniques have recently emerged that allow us to create networks of model neurons that display behaviors of similar complexity. Here we demonstrate the direct applicability of one such technique, the FORCE method, to spiking neural networks. We train these networks to mimic dynamical systems, classify inputs, and store discrete sequences that correspond to the notes of a song. Finally, we use FORCE training to create two biologically motivated model circuits. One is inspired by the zebra finch and successfully reproduces songbird singing. The second network is motivated by the hippocampus and is trained to store and replay a movie scene. FORCE trained networks reproduce behaviors comparable in complexity to their inspired circuits and yield information not easily obtainable with other techniques, such as behavioral responses to pharmacological manipulations and spike timing statistics.

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.

Expression of the GABA(A) receptor gamma4-subunit gene in discrete nuclei within the zebra finch song system

The acquisition, production and maintenance of song by oscine birds is a form of audition-dependent learning that, in many ways, resembles the process by which humans learn to speak. In songbirds, the generation of structured song is determined by the activity of two interconnected neuronal pathways (the anterior forebrain pathway and the vocal motor pathway), each of which contains a number of discrete nuclei that together form the song system. It is becoming increasingly evident that inhibitory GABAergic mechanisms are indispensable in counterbalancing the excitatory actions of glutamate and, thus, likely shape the neuronal firing patterns of neurons within this network. Furthermore, there is compelling evidence for the involvement of GABA(A) receptors, although the molecular composition of these has, to date, remained elusive. Here we describe the isolation of a complementary DNA for the zebra finch GABA(A) receptor gamma4 subunit, and map the expression pattern of the corresponding gene within the zebra finch (Taeniopygia guttata) brain. Our findings show, remarkably, that the gamma4-subunit transcript is highly enriched in the major nuclei of the song system, including the lateral magnocellular nucleus of the anterior nidopallium (LMAN), the medial magnocellular nucleus of the anterior nidopallium (MMAN), Area X, the robust nucleus of the arcopallium (RA) and the HVC (used as the proper name), as well as Field L, which innervates the area surrounding HVC. In summary, we have demonstrated the presence of the mRNA for the gamma4 subunit of the GABA(A) receptor, the major inhibitory receptor in brain, in most of the nuclei of the two neural circuits that mediate song production in the zebra finch. This not only marks the beginning of the characterization of the GABA(A) receptor subtype(s) that mediates the actions of GABA in the song system but it also provides a robust molecular marker with which to distinguish song system-specific brain structures.

GABA immunoreactivity in auditory and song control brain areas of zebra finches

Inhibitory transmission is critical to sensory and motor processing and is believed to play a role in experience-dependent plasticity. The main inhibitory neurotransmitter in vertebrates, GABA, has been implicated in both sensory and motor aspects of vocalizations in songbirds. To understand the role of GABAergic mechanisms in vocal communication, GABAergic elements must be characterized fully. Hence, we investigated GABA immunohistochemistry in the zebra finch brain, emphasizing auditory areas and song control nuclei. Several nuclei of the ascending auditory pathway showed a moderate to high density of GABAergic neurons including the cochlear nuclei, nucleus laminaris, superior olivary nucleus, mesencephalic nucleus lateralis pars dorsalis, and nucleus ovoidalis. Telencephalic auditory areas, including field L subfields L1, L2a and L3, as well as the caudomedial nidopallium (NCM) and mesopallium (CMM), contained GABAergic cells at particularly high densities. Considerable GABA labeling was also seen in the shelf area of caudodorsal nidopallium, and the cup area in the arcopallium, as well as in area X, the lateral magnocellular nucleus of the anterior nidopallium, the robust nucleus of the arcopallium and nidopallial nucleus HVC. GABAergic cells were typically small, most likely local inhibitory interneurons, although large GABA-positive cells that were sparsely distributed were also identified. GABA-positive neurites and puncta were identified in most nuclei of the ascending auditory pathway and in song control nuclei. Our data are in accordance with a prominent role of GABAergic mechanisms in regulating the neural circuits involved in song perceptual processing, motor production, and vocal learning in songbirds.

Widespread capacity for steroid synthesis in the avian brain and song system

Steroids exert powerful effects on the brains and behavior of many species, but measures and manipulations of endocrine physiology in songbirds often reveal unexplained connections between steroids and the brain. The zebra finch song system, a sensorimotor neural circuit sensitive to steroids throughout life, organizes and functions largely in apparent independence from gonadally derived steroids. We tested the hypothesis that the zebra finch brain has the capacity for de novo steroidogenesis and that neurally synthesized steroids, neurosteroids, may impact the song system. Using multiple techniques, we demonstrate that the steroidogenic acute regulatory protein (StAR), cytochrome P450 side-chain cleavage (CYP11A1), and 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase, the first three factors in the steroidogenic pathway, are expressed in both developing and adult zebra finch brain. Detailed expression mapping at posthatch d 20 (P20) and adult reveals widespread area-specific expression and coexpression patterns for steroidogenic acute regulatory protein, CYP11A1, and 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase, which suggest neurosteroids may modulate multiple brain functions, including sensory and motor systems. Notably, whereas expression of other steroidogenic genes such as aromatase has been essentially absent from the song system, each of the major song nuclei express at least a subset of steroidogenic genes described here, establishing the song system as a potential steroidogenic circuit.

Physiological insights into the social-context-dependent changes in the rhythm of the song motor program

Precisely timed behaviors are central to the survival of almost all organisms. Song is an example of a learned behavior under exquisite temporal control. Song tempo in zebra finches (Taeniopygia guttata) is systematically modified depending on social context. When male zebra finches sing to females (directed), it is produced with a faster motor pattern compared with when they sing in isolation (undirected). We measured heart rate and air sac pressure during directed and undirected singing to quantify motivation levels and respiratory timing. Heart rate was significantly higher when male birds sang to females and was negatively correlated with song duration. The change in song tempo between directed and undirected song was accounted for by varying the duration of vocal expiratory events, whereas the duration of silent inspirations was unchanged. Song duration increased with repeated singing during directed song bouts, which was caused by a uniform increase in the duration of both expirations and inspirations. These results illustrate the importance of motivational state in regulating song tempo and demonstrate that multiple timing oscillators are necessary to control the rhythm of song. At least two different neural oscillators are required to control context-dependent changes in song tempo. One oscillator controlling expiratory duration varies as function of social context and another controlling inspiratory duration is fixed. In contrast, the song tempo change affecting expiratory and inspiratory duration within a directed bout of song could be achieved by slowing the output of a single oscillator.

Noradrenergic modulation of activity in a vocal control nucleus in vitro

Norepinephrine (NE) can profoundly modulate sensory processing, but its effect on motor function is less well understood. Birdsong is a learned behavior involving sensory and motor processes that are influenced by NE. A potential site of NE action is the robust nucleus of the arcopallium (RA): RA receives noradrenergic inputs and has adrenergic receptors, and it is a sensorimotor area instrumental to song production. We hypothesized that NE modulates RA neurons, and as a first test, we examined the effect of NE on RA activity in vitro. We recorded spontaneous activity extracellularly from isolated RA neurons in brain slices made from adult male zebra finches. These neurons exhibited regular tonic activity with firing rates averaging 5.5 Hz. Bath application of NE rapidly and reversibly decreased firing for the majority of neurons, to the extent that spontaneous activity was often abolished. This was likely a direct effect on the cell recorded, because it occurred with blockade of fast excitatory and inhibitory synaptic transmission or of all synaptic transmission. The NE-induced suppression involved alpha2-adrenergic receptors: yohimbine, an antagonist, completely reversed the suppression, and clonidine, an agonist, partially mimicked it. Perforated patch recordings revealed that NE induced a conductance increase in RA neurons; however, this did not prevent cells from firing when stimulated by afferents in HVC. For some neurons, NE application resulted in an increase in signal-to-noise ratio for spikes evoked by HVC stimulation. Thus NE could strongly modulate the spontaneous activity of RA cells, potentially enhancing signals relayed through RA.

Bilateral control and interhemispheric coordination in the avian song motor system

Birdsong is a complex learned motor behavior controlled by an interconnected network of vocal control nuclei that are present in both cerebral hemispheres. Unilateral lesions of song nuclei in the left or the right hemisphere result in different effects on song structure, suggesting that normal song output results from the activation of two parallel but functionally different motor pathways. Because each syringeal half is innervated primarily by ipsilateral motor structures and activity in both halves is tightly coordinated during singing, motor commands originating from both hemispheres must be tightly coordinated to produce the appropriate vocal output. This coordination occurs despite the absence of direct interhemispheric connections between song control nuclei. In this article, we discuss how motor commands in nucleus HVC, a key forebrain song control region, are coordinated by precisely timed inputs that act to synchronize premotor activity in both hemispheres. Synchronizing inputs are tightly linked to syllable and note onset, which suggests that bilaterally organized circuits in the midbrain or brainstem act in specifying higher-order song features, such as duration, order, and possibly even structure of individual song syllables. The challenge ahead lies in identifying the networks that generate the synchronizing timing inputs and to determine how these inputs specify the motor commands in HVC. Resolving these issues will help us gain a better understanding of how pattern-generating networks in the midbrain/brainstem interface with forebrain circuits to produce complex learned behaviors.

Mapping neural architectures onto acoustic features of birdsong

The motor pathway responsible for the complex vocalizations of songbirds has been extensively characterized, both in terms of intrinsic and synaptic physiology in vitro and in terms of the spatiotemporal patterns of neural activity in vivo. However, the relationship between the neural architecture of the song motor pathway and the acoustic features of birdsong is not well understood. Using a computational model of the song motor pathway and the songbird vocal organ, we investigate the relationship between song production and the neural connectivity of nucleus HVc (used as a proper name) and the robust nucleus of the archistriatum (RA). Drawing on recent experimental observations, our neural model contains a population of sequentially bursting HVc neurons driving the activity of a population of RA neurons. An important focus of our investigations is the contribution of intrinsic circuitry within RA to the acoustic output of the model. We find that the inclusion of inhibitory interneurons in the model can substantially influence the features of song syllables, and we illustrate the potential for subharmonic behavior in RA in response to forcing by HVc neurons. Our results demonstrate the association of specific acoustic features with specific neural connectivities and support the view that intrinsic circuitry within RA may play a critical role in generating the features of birdsong.

Pattern of interhemispheric synchronization in HVc during singing correlates with key transitions in the song pattern

Many complex voluntary behaviors require that motor commands be tightly coordinated between cerebral hemispheres. The neural mechanisms underlying such coordination, however, remain poorly understood. Song production in birds is a highly stereotyped learned motor behavior that requires finely tuned coordination between hemispheres. In the present study, neural activity was recorded simultaneously from the song control nucleus HVc in each hemisphere of singing adult male zebra finches (Taeniopygia guttata). In all cases, the pattern of recorded multiunit activity in each hemisphere was highly correlated during short segments of the song motor pattern. These correlated segments often consisted of multiple short bursts of activity. Because of the absence of interhemispheric connections between song control nuclei, these observations suggest that HVc activity is "synchronized" by common inputs to both hemispheres. Using sliding-window cross-covariance analyses, periods of high interhemispheric synchronization were found to be time-locked to the acoustic onset of syllables and notes. In some cases, precisely synchronized bursts in both hemispheres were also observed during periods associated with the intersyllable silent interval. In all cases, activity was correlated between hemispheres independently of the recording site, suggesting that all regions of HVc may be globally synchronized during these short segments of the song. Given the anatomical organization of the song system, inputs originating from either thalamus or midbrain are proposed to act as timing signals that initiate and synchronize intrinsic motor networks within each HVc thus allowing for the precise coordination of motor commands across hemispheres.