Physiological identification of cortico-striatal projection neurons for song control in Bengalese finches

Song-related brain auditory activity in Bengalese finches as examined by immediate early gene expressions: Comparison of arousal states and the correlational analyses between brain regions

Songbirds use auditory feedback to memorize a tutor song in juveniles and to maintain it in adults. In Bengalese finches, electrophysiological studies showed the auditory responses in the premotor area HVC remained active regardless of asleep/awake status, in contrast to auditory gating phenomenon identified in zebra finches. We investigated the correlations in auditory activity between the brain regions and differences in the activity during wakefulness and sleeping in Bengalese finches. We used the immediate early gene egr-1 as a marker of neural activity that can detect regions responding to auditory stimuli in the whole brain. Results showed that auditory response, as measured by egr-1 expression to the bird's own song while sleeping and awake, was similar in HVC and NCM. Higher activity during awake than sleep was found only in the lower auditory area MLd. Analyses showed egr-1 expressions between brain regions induced by the bird's own song playback in awake/sleep conditions, suggesting that auditory information correlated with the inter part, not the outer part, of MLd with the higher song-related regions. Furthermore, the sleep condition suppressed the spontaneous activity, but not the song-induced activity in Area X. Altogether, this study presents a new attempt to explore the auditory-motor network using a molecular tool to map neurons of the nearly whole brain.

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.

Inter- and intra-specific differences in muscarinic acetylcholine receptor expression in the neural pathways for vocal learning in songbirds

Acetylcholine receptors (AChRs) abound in the central nervous system of vertebrates. Muscarinic AChRs (mAChRs), a functional subclass of AChRs, mediate neuronal responses via intracellular signal transduction. They also play roles in sensorimotor coordination and motor skill learning by enhancing cortical plasticity. Learned birdsong is a complex motor skill acquired through sensorimotor coordination during a critical period. However, the functions of AChRs in the neural circuits for vocal learning and production remain largely unexplored. Here, we report the unique expression of mAChRs subunits (chrm2-5) in the song nuclei of zebra finches. The expression of excitatory subunits (chrm3 and chrm5) was downregulated in the song nuclei compared with the surrounding brain regions. In contrast, the expression of inhibitory mAChRs (chrm2 and chrm4) was upregulated in the premotor song nucleus HVC relative to the surrounding nidopallium. Chrm4 showed developmentally different expression in HVC during the critical period. Compared with chrm4, individual differences in chrm2 expression emerged in HVC early in the critical period. These individual differences in chrm2 expression persisted despite testosterone administration or auditory deprivation, which altered the timing of song stabilization. Instead, the variability in chrm2 expression in HVC correlated with parental genetics. In addition, chrm2 expression in HVC exhibited species differences and individual variability among songbird species. These results suggest that mAChRs play an underappreciated role in the development of species and individual differences in song patterns by modulating the excitability of HVC neurons, providing a potential insight into the gating of auditory responses in HVC neurons.