Thalamostriatal and cerebellothalamic pathways in a songbird,the Bengalese finch

The thalamostriatal system is a major network in the mammalian brain, originating principally from the intralaminar nuclei of thalamus. Its functions remain unclear, but a subset of these projections provides a pathway through which the cerebellum communicates with the basal ganglia. Both the cerebellum and basal ganglia play crucial roles in motor control. Although songbirds have yielded key insights into the neural basis of vocal learning, it is unknown whether a thalamostriatal system exists in the songbird brain. Thalamic nucleus DLM is an important part of the song system, the network of nuclei required for learning and producing song. DLM receives output from song system basal ganglia nucleus Area X and sits within dorsal thalamus, the proposed avian homolog of the mammalian intralaminar nuclei that also receives projections from the cerebellar nuclei. Using a viral vector that specifically labels presynaptic axon segments, we show in Bengalese finches that dorsal thalamus projects to Area X, the basal ganglia nucleus of the song system, and to surrounding medial striatum. To identify the sources of thalamic input to Area X, we map DLM and cerebellar‐recipient dorsal thalamus (DT_CbN). Surprisingly, we find both DLM and dorsal anterior DT_CbN adjacent to DLM project to Area X. In contrast, the ventral medial subregion of DT_CbN projects to medial striatum outside Area X. Our results suggest the basal ganglia in the song system, like the mammalian basal ganglia, integrate feedback from the thalamic region to which they project as well as thalamic regions that receive cerebellar output.     

Differential androgen receptor expression and DNA methylation state in striatum song nucleus Area X between wild and domesticated songbird strains

In songbirds, a specialized neural system, the song system, is responsible for acquisition and expression of species‐specific vocal patterns. We report evidence for differential gene expression between wild and domesticated strains having different learned vocal phenotypes. A domesticated strain of the wild white‐rumped munia, the Bengalese finch, has a distinct song pattern with a more complicated syntax than the wild strain. We identified differential androgen receptor (AR) expression in basal ganglia nucleus Area X GABAergic neurons between the two strains, and within different domesticated populations. Differences in AR expression were correlated with the mean coefficient of variation of the inter‐syllable duration in the two strains. Differential AR expression in Area X was observed before the initiation of singing, suggesting that inherited and/or early developmental mechanisms may affect expression within and between strains. However, there were no distinct differences in regions upstream of the AR start codon among all the birds in the study. In contrast, an epigenetic modification, DNA methylation state in regions upstream of AR in Area X, was observed to differ between strains and within domesticated populations. These results provide insight into the molecular basis of behavioral evolution through the regulation of hormone‐related genes and demonstrate the potential association between epigenetic modifications and behavioral phenotype regulation.     

The song system of the human brain

Although sophisticated insights have been gained into the neurobiology of singing in songbirds, little comparable knowledge exists for humans, the most complex singers in nature. Human song complexity is evidenced by the capacity to generate both richly structured melodies and coordinated multi-part harmonizations. The present study aimed to elucidate this multi-faceted vocal system by using 15O-water positron emission tomography to scan "listen and respond" performances of amateur musicians either singing repetitions of novel melodies, singing harmonizations with novel melodies, or vocalizing monotonically. Overall, major blood flow increases were seen in the primary and secondary auditory cortices, primary motor cortex, frontal operculum, supplementary motor area, insula, posterior cerebellum, and basal ganglia. Melody repetition and harmonization produced highly similar patterns of activation. However, whereas all three tasks activated secondary auditory cortex (posterior Brodmann Area 22), only melody repetition and harmonization activated the planum polare (BA 38). This result implies that BA 38 is responsible for an even higher level of musical processing than BA 22. Finally, all three of these "listen and respond" tasks activated the frontal operculum (Broca's area), a region involved in cognitive/motor sequence production and imitation, thereby implicating it in musical imitation and vocal learning.     

Singing-related neural activity in a dorsal forebrain-basal ganglia circuit of adult zebra finches.

The anterior forebrain pathway (AFP) of songbirds, a specialized dorsal forebrain-basal ganglia circuit, is crucial for song learning but has a less clear function in adults. We report here that neurons in two nuclei of the AFP, the lateral magnocellular nucleus of the anterior neostriatum (LMAN) and Area X, show marked changes in neurophysiological activity before and during singing in adult zebra finches. The presence of modulation before song output suggests that singing-related AFP activity originates, at least in part, in motor control nuclei. Some neurons in LMAN of awake birds also responded selectively to playback of the bird's own song, but neural activity during singing did not completely depend on auditory feedback in the short term, because neither the level nor the pattern of this activity was strongly affected by deafening. The singing-related activity of neurons in AFP nuclei of songbirds is consistent with a role of the AFP in adult singing or song maintenance, possibly related to the function of this circuit during initial song learning.     

A novel basal ganglia pathway forms a loop linking a vocal learning circuit with its dopaminergic input

Dopamine has been implicated in mediating contextual modulation of motor behaviors and learning in many species. In songbirds, dopamine may act on the basal ganglia nucleus Area X to influence the neural activity that contributes to vocal learning and contextual changes in song variability. Neurons in midbrain dopamine centers, the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA), densely innervate Area X and show singing-related changes in firing rate. In addition, dopamine levels in Area X change during singing. It is unknown, however, how song-related information could reach dopaminergic neurons. Here we report an anatomical pathway that could provide song-related information to the SNc and VTA. By using injections of bidirectionally transported fluorescent tracers in adult male zebra finches, we show that Area X and other song control nuclei do not project directly to the SNc or VTA. Instead, we describe an indirect pathway from Area X to midbrain dopaminergic neurons via a connection in the ventral pallidum (VP). Specifically, Area X projects to the VP via axon collaterals of Area X output neurons that also project to the thalamus. Dual injections revealed that the area of VP receiving input from Area X projects to the SNc and VTA. Furthermore, VP terminals in the SNc and VTA overlap with cells that project back to Area X. A portion of the arcopallium also projects to the SNc and VTA and could carry auditory information. These data demonstrate an anatomical loop through which Area X activity could influence its dopaminergic input.     

Dopamine physiology in the basal ganglia of male zebra finches during social stimulation

Accumulating evidence suggests that dopamine (DA) is involved in altering neural activity and gene expression in a zebra finch cortical–basal ganglia circuit specialized for singing, upon the shift between solitary singing and singing as a part of courtship. Our objective here was to sample changes in the extracellular concentrations of DA in Area X of adult and juvenile birds, to test the hypothesis that DA levels would change similarly during presentation of a socially salient stimulus in both age groups. We used microdialysis to sample the extracellular milieu of Area X in awake, behaving adult and juvenile male zebra finches, and analysed the dialysate using high‐performance liquid chromatography coupled with electrochemical detection. The extracellular levels of DA in Area X increased significantly during both female presentation to adult males and tutor presentation to juvenile males. DA levels were not correlated with the time spent singing. We also reverse‐dialysed Area X with pharmacologic agents that act either on DA systems directly or on norepinephrine, and found that all of these agents significantly increased DA levels (3‐ to 10‐fold) in Area X. These findings suggest that changes in extracellular DA levels can be stimulated similarly by very different social contexts (courtship and interaction with tutor), and influenced potently by dopaminergic and noradrenergic drugs. These results raise the possibility that the arousal level or attentional state of the subject (rather than singing behavior) is the common feature eliciting changes in extracellular DA concentration.     

Singing activity‐driven Arc expression associated with vocal acoustic plasticity in juvenile songbird

Learned vocalization, including birdsong and human speech, is acquired through self‐motivated vocal practice during the sensitive period of vocal learning. The zebra finch (Taeniopygia guttata) develops a song characterized by vocal variability and crystallizes a defined song pattern as adulthood. However, it remains unknown how vocal variability is regulated with diurnal singing during the sensorimotor learning period. Here, we investigated the expression of activity‐dependent neuroplasticity‐related gene Arc during the early plastic song phase to examine its potential association with vocal plasticity. We first confirmed that multiple acoustic features of syllables in the plastic song were dramatically and simultaneously modulated during the first 3 hr of singing in a day and the altered features were maintained until sleep. In a concurrent manner, Arc was intensely induced during morning singing and a subsequent attenuation during afternoon singing in the robust nucleus of the arcopallium (RA) and the interfacial nucleus of the nidopallium (NIf). The singing‐driven Arc expression was not altered by circadian rhythm, but rather reduced during the day as juveniles produced more songs. Song stabilization accelerated by testosterone administration in juveniles was accompanied with attenuation of Arc induction in RA and NIf. In contrast, although early‐deafened birds produced highly unstable song even at adulthood, singing‐driven Arc expression was not different between intact and early‐deafened adults. These results suggest a potential functional link between Arc expression in RA and NIf and vocal plasticity during the sensorimotor phase of song learning. Nonetheless, Arc expression did not reflect the quality of bird's own song or auditory feedback.     

A comparative study of the behavioral deficits following lesions of various parts of the zebra finch song system: implications for vocal learning

Song production in song birds is controlled by an efferent pathway. Appended to this pathway is a "recursive loop" that is necessary for song acquisition but not for the production of learned song. Since zebra finches learn their song by imitating external models, we speculated that the importance of the recursive loop for learning might derive from its processing of auditory feedback during song acquisition. This hypothesis was tested by comparing the effects on song in birds deafened early in life and birds with early lesions in either of two nuclei--Area X and the lateral magnocellular nucleus of the anterior neostriatum (LMAN). These nuclei are part of the recursive loop. The three treatments affected song development differently, as reflected by various parameters of the adult song of these birds. Whereas LMAN lesions resulted in songs with monotonous repetitions of a single note complex, songs of Area X-lesioned birds consisted of rambling series of unusually long and variable notes. Furthermore, whereas song of LMAN lesioned birds stabilized early, song stability as seen in intact birds was never achieved in Area X-lesioned birds. Early deafness also resulted in poorly structured and unstable song. We conclude that Area X and LMAN contribute differently to song acquisition: the song variability that is typical of vocal development persists following early deafness or lesions of Area X but ends abruptly following removal of LMAN. Apparently, LMAN plays a crucial role in fostering the kinds of circuit plasticity necessary for learning.     

Seasonal change in neuron size and spacing but not neuronal recruitment in a basal ganglia nucleus in the avian song control system

Neural plasticity in the song control system of seasonally breeding songbirds accompanies seasonal changes in singing behavior. The volume of Area X, a song control nucleus that forms a portion of the avian basal ganglia, is 75% larger in the spring than it is in the fall. The neuronal basis of the seasonal plasticity in Area X is largely unknown, however. We examined neuronal attributes of Area X in wild adult male song sparrows (Melospiza melodia) captured during the spring and the fall after being implanted for 30 days with osmotic pumps containing [3H]thymidine. We measured the volume of Area X from thionin-stained sections, and neuronal density and number, and average area of the soma from sections labeled with an antibody against Hu, a neuron-specific protein. We sampled two neuron classes: "small" neurons that were most likely striatal-like spiny neurons and "large" neurons, which most likely included pallidal-like projection neurons. We also analyzed seasonal patterns of neuronal recruitment to Area X. The average area of the soma and neuronal spacing for both neuronal classes were greater in breeding birds. There was no difference in total neuron number for both neuronal classes between seasons. The average area of the soma and density and number of newly recruited neurons did not vary across seasons. These results demonstrate that seasonal plasticity in Area X includes changes in neuron size and neuronal density, but not changes in the rate at which new neurons are recruited.     

Neural encoding of auditory temporal context in a songbird basal ganglia nucleus, and its independence of birds' song experience

Some of the most complex auditory neurons known are found in the songbird forebrain, throughout the 'song system', including its basal ganglia nucleus Area X. These cells are selective for the temporal order of the bird's own song (BOS): they typically respond strongly to BOS, but more weakly when the syllable sequence of BOS is played in reverse order (roBOS), indicating that they integrate auditory information over more than single syllables. Here, studying the zebra finch Area X, we found that order selectivity strongly depends on the mean syllable duration of individual songs, decreasing markedly as this duration approaches 150–200 ms. Simply segmenting the same songs differently, creating an increase in average syllable length towards 150 ms, caused a similar decrease in order selectivity. This suggests that song neurons integrate acoustic information over a relatively limited time window, predominantly less than 150 ms. We provided further support for this by showing that a significant fraction of Area X order selectivity was accounted for by the acoustic similarity between each BOS and roBOS, measured using cross-correlation with fixed window sizes, but only when the correlation windows were at least 50 ms and no more than 200 ms long. All the same findings were evident in birds raised without tutor exposure, indicating that tutor learning has little effect on neural mechanisms underlying song temporal selectivity. Our results suggest that song-selective neurons encode much of the temporal context of song using a short, constant time window that is conserved across differences in songs, birds and learning.     

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 GABA_A reversal potential.     

Organization of the songbird basal ganglia, including area X

Area X is a songbird basal ganglia nucleus that is required for vocal learning. Both Area X and its immediate surround, the medial striatum (MSt), contain cells displaying either striatal or pallidal characteristics. We used pathway-tracing techniques to compare directly the targets of Area X and MSt with those of the lateral striatum (LSt) and globus pallidus (GP). We found that the zebra finch LSt projects to the GP, substantia nigra pars reticulata (SNr) and pars compacta (SNc), but not the thalamus. The GP is reciprocally connected with the subthalamic nucleus (STN) and projects to the SNr and motor thalamus analog, the ventral intermediate area (VIA). In contrast to the LSt, Area X and surrounding MSt project to the ventral pallidum (VP) and dorsal thalamus via pallidal-like neurons. A dorsal strip of the MSt contains spiny neurons that project to the VP. The MSt, but not Area X, projects to the ventral tegmental area (VTA) and SNc, but neither MSt nor Area X projects to the SNr. Largely distinct populations of SNc and VTA dopaminergic neurons innervate Area X and surrounding the MSt. Finally, we provide evidence consistent with an indirect pathway from the cerebellum to the basal ganglia, including Area X. Area X projections thus differ from those of the GP and LSt, but are similar to those of the MSt. These data clarify the relationships among different portions of the oscine basal ganglia as well as among the basal ganglia of birds and mammals.     

Song-learning behavior: the interface with neuroethology.

Behavioral studies of song learning in birds continue to raise new problems for neuroethological investigation. Evidence is emerging for a new form of vocal plasticity, called 'action-based learning'. Motor patterns are overproduced during a particular phase of ontogeny, and are then subjected to attrition and selective reinforcement by various kinds of social stimulation as the young bird matures. This form of learning, analogous to operant conditioning, can occur at phases of development when the more traditional form of 'memory-based learning' is no longer possible. There is evidence that different physiological mechanisms are involved in the development and the maintenance of mature singing, with a transition occurring at the time of song crystallization. Neural events associated with the developmental stabilization of motor patterns are worthy of more study.     

Partial lesions in the anterior forebrain pathway affect song production in adult Bengalese finches

The avian song system consists of two main parts: the descending motor pathway and the anterior forebrain (AF) pathway. The former directly controls the motor act of singing, but the role of the AF pathway in real-time song production is not understood; lesioning a nucleus in the AF pathway in adult zebra finches did not cause any notable effects. Here we show that in a related species of songbirds, the Bengalese finch, a partial lesion located in Area X, a nucleus of the avian basal ganglia that is part of the AF pathway, causes a transient but substantial song motor deficit. This is the first demonstration that the AF pathway is involved in real-time song production in adulthood.     

The pallial basal ganglia pathway modulates the behaviorally driven gene expression of the motor pathway

The discrete neural network for songbird vocal communication provides an effective system to study neural mechanisms of learned motor behaviors in vertebrates. This system consists of two pathways--a vocal motor pathway used to produce learned vocalizations and a vocal pallial basal ganglia loop used to learn and modify the vocalizations. However, it is not clear how the loop exerts control over the motor pathway. To study the mechanism, we used expression of the neural activity-induced gene ZENK (or egr-1), which shows singing-regulated expression in a social context-dependent manner: high levels in both pathways when singing undirected and low levels in the lateral part of the loop and in the robust nucleus of the arcopallium (RA) of the motor pathway when singing directed to another animal. Here, we show that there are two parallel interactive parts within the pallial basal ganglia loop, lateral and medial, which modulate singing-driven ZENK expression of the motor pathway nuclei RA and HVC, respectively. Within the loop, the striatal and pallial nuclei appear to have opposing roles; the striatal vocal nucleus lateral AreaX is required for high ZENK expression in its downstream nuclei, particularly during undirected singing, while the pallial vocal lateral magnocellular nucleus of the anterior nidopallium is required for lower expression, particularly during directed singing. These results suggest a dynamic molecular interaction between the basal ganglia pathway and the motor pathway during production of a learned motor behavior.     

Basal ganglia lateralization in different types of reward

Reward processing is a fundamental human activity. The basal ganglia are recognized for their role in reward processes; however, specific roles of the different nuclei (e.g., nucleus accumbens, caudate, putamen and globus pallidus) remain unclear. Using quantitative meta-analyses we assessed whole-brain and basal ganglia specific contributions to money, erotic, and food reward processing. We analyzed data from 190 fMRI studies which reported stereotaxic coordinates of whole-brain, within-group results from healthy adult participants. Results showed concordance in overlapping and distinct cortical and sub-cortical brain regions as a function of reward type. Common to all reward types was concordance in basal ganglia nuclei, with distinct differences in hemispheric dominance and spatial extent in response to the different reward types. Food reward processing favored the right hemisphere; erotic rewards favored the right lateral globus pallidus and left caudate body. Money rewards engaged the basal ganglia bilaterally including its most anterior part, nucleus accumbens. We conclude by proposing a model of common reward processing in the basal ganglia and separate models for money, erotic, and food rewards.

     

Cortical inter‐hemispheric circuits for multimodal vocal learning in songbirds

Vocal learning in songbirds and humans is strongly influenced by social interactions based on sensory inputs from several modalities. Songbird vocal learning is mediated by cortico‐basal ganglia circuits that include the SHELL region of lateral magnocellular nucleus of the anterior nidopallium (LMAN), but little is known concerning neural pathways that could integrate multimodal sensory information with SHELL circuitry. In addition, cortical pathways that mediate the precise coordination between hemispheres required for song production have been little studied. In order to identify candidate mechanisms for multimodal sensory integration and bilateral coordination for vocal learning in zebra finches, we investigated the anatomical organization of two regions that receive input from SHELL: the dorsal caudolateral nidopallium (dNCL_SHELL) and a region within the ventral arcopallium (Av). Anterograde and retrograde tracing experiments revealed a topographically organized inter‐hemispheric circuit: SHELL and dNCL_SHELL, as well as adjacent nidopallial areas, send axonal projections to ipsilateral Av; Av in turn projects to contralateral SHELL, dNCL_SHELL, and regions of nidopallium adjacent to each. Av on each side also projects directly to contralateral Av. dNCL_SHELL and Av each integrate inputs from ipsilateral SHELL with inputs from sensory regions in surrounding nidopallium, suggesting that they function to integrate multimodal sensory information with song‐related responses within LMAN‐SHELL during vocal learning. Av projections share this integrated information from the ipsilateral hemisphere with contralateral sensory and song‐learning regions. Our results suggest that the inter‐hemispheric pathway through Av may function to integrate multimodal sensory feedback with vocal‐learning circuitry and coordinate bilateral vocal behavior.     

Localized immediate early gene expression related to the strength of song learning in socially reared zebra finches

Recent evidence showed that exposure of tape-tutored zebra finch (Taeniopygia guttata castanotis) males to the tutor song involves neuronal activation in brain regions outside the conventional 'song control pathways', particularly the caudal part of the neostriatum (NCM) and of the hyperstriatum ventrale (CMHV). Zebra finch males were reared with a live tutor during the sensitive period for song learning. When, as adults, they were re-exposed to the tutor song, the males showed increased expression of Fos, the protein product of the immediate early gene c-fos, in the NCM and CMHV, compared with expression in two conventional 'song control nuclei', high vocal centre (HVC) and Area X. The strength of the Fos response (which is a reflection of neuronal activation) in the NCM (but not in the other three regions) correlated significantly and positively with the number of song elements that the birds had copied from the tutor song. Thus, socially tutored zebra finch males show localized neural activation in response to tutor song exposure, which correlates with the strength of song learning.     

Evidence for "direct" and "indirect" pathways through the song system basal ganglia

Song learning in oscine birds relies on a circuit known as the "anterior forebrain pathway," which includes a specialized region of the avian basal ganglia. This region, area X, is embedded within a telencephalic structure considered homologous to the striatum, the input structure of the mammalian basal ganglia. Area X has many features in common with the mammalian striatum, yet has distinctive traits, including largely aspiny projection neurons that directly innervate the thalamus and a cell type that physiologically resembles neurons recorded in the mammalian globus pallidus. We have proposed that area X is a mixture of striatum and globus pallidus and has the same functional organization as circuits in the mammalian basal ganglia. Using electrophysiological and anatomical approaches, we found that area X contains a functional analog of the "direct" striatopallidothalamic pathway of mammals: axons of the striatal spiny neurons make close contacts on the somata and dendrites of pallidal cells. A subset of pallidal neurons project directly to the thalamus. Surprisingly, we found evidence that many pallidal cells may not project to the thalamus, but rather participate in a functional analog of the mammalian "indirect" pathway, which may oppose the effects of the direct pathway. Our results deepen our understanding of how information flows through area X and provide more support for the notion that song learning in oscines employs physiological mechanisms similar to basal ganglia-dependent forms of motor learning in mammals.