Exploring the molecular basis of neuronal excitability in a vocal learner

Abstract

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. Electronic supplementary material The online version of this article (10.1186/s12864-019-5871-2) contains supplementary material, which is available to authorized users.

Molecular specializations of deep cortical layer analogs in songbirds

“…This finding argues that over half of the shared RA/LMC markers may not be uniquely associated with the vocal motor pathway or vocal control, but could perhaps subserve a broader array of somatic motor control functions. Among identified enriched pathways for these RA and AId markers is the rapid depolarization pathway containing sodium and calcium channel genes known to be differentially regulated in the song system 81 , and the axon guidance pathway containing GAP43 , which is a shared marker of RA and LMC 21 , 82 . Particularly noticeable was the very low expression in RA and AId of SNCA , previously shown to be transcriptionally regulated in LMAN during song learning but constitutively downregulated in HVC and RA 83 .…”

“…Another pathway likely of functional relevance to RA is potassium channels, which are major determinants of neuronal excitability. Potassium channels are broadly expressed throughout the brain but song nuclei exhibit unique expression profiles of both potassium channels 96 and other ion channel families 81 . These observations suggest that vocal nuclei are sites of differential regulation of intrinsic excitability, consistent with growing evidence that modulation of intrinsic excitable features may play important roles in regulating properties of the vocal learning circuitry 97 , 98 .…”

Resurgent Na+ currents promote ultrafast spiking in projection neurons that drive fine motor control

“…We therefore hypothesized that Navβ4 might be a key determinant of excitability properties in RA. High expression of Navβ4 mRNA is restricted to RA in the arcopallium of adult male zebra finches 49 , contrasting sharply with Navβ3, which is all but absent (Fig. 2a-b).…”

Widespread false gene gains caused by duplication errors in genome assemblies

“…5d). The calcium voltage-gated channel subunit alpha1 H (CACNA1H), a gene with specialized expression in vocal learning circuits of the zebra finch 37,38 , had a FEG in the second exon (Fig. 4f).…”