Expression of the GABAA receptor γ4-subunit gene in discrete nuclei within the zebra finch song system

Abstract

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.

The constitutive differential transcriptome of a brain circuit for vocal learning

“…For GABAergic receptors, the non-differential expression of the more abundant and fundamental alpha subunits of type-A receptors contrasted markedly with the differential regulation of multiple beta, delta, and epsilon subunits, suggesting these less prevalent subunits are more prominent targets of differential regulation in the song system. In particular, the up-regulation of delta subunits in all major telencephalic song nuclei, consistent with a previous report [ 70 ], suggests a distinctive feature of inhibitory neurotransmission, but its functional significance is unknown. The data on glycine receptor subunits is also intriguing, as the patterns of alpha and beta subunits seem largely complementary rather than overlapping.…”

Distribution of vesicular glutamate transporter 2 in auditory and song control brain regions in the adult zebra finch (Taeniopygia guttata)

“…It has been shown that the LMAN evokes excitatory inputs in the RA via NMDA-type receptors (Mooney, 1992;Spiro et al, 1999;Sizemore and Perkel, 2008). GABA A receptor mRNA-expressing neurons and GABA-immunoreactive neurons have also been observed in the LMAN (Grisham and Arnold, 1994;Pinaud and Mello, 2007;Thode et al, 2008), although questions about whether glutamatergic and GABAergic neurons in this region interact remain unanswered. The evidence shows that the LMAN projects to area X (Vates and Nottehohm, 1995), and the RA and area X are thought to be the target nuclei of glutamatergic projection neurons.…”

“…Furthermore, immunohistochemical studies found that GABA is localized in somata and axon terminals in song nuclei, such as the HVC, RA, LMAN, and area X (Grisham and Arnold, 1994;Luo and Perkel, 1999;Pinaud and Mello, 2007). GABA receptors have been identified in these nuclei as well (Thode et al, 2008). Other electrophysiological studies have identified ionotropic glutamate receptors in the HVC, LMAN, RA, and caudomedial nidopallium (Mooney and Konishi, 1991;Basham et al, 1999;Pinaud et al, 2008).…”

Characterization of Synaptically Connected Nuclei in a Potential Sensorimotor Feedback Pathway in the Zebra Finch Song System

“…GABA A receptors have been localized in MMAN [36], so to test this idea we inactivated MMAN with GABA while recording auditory evoked activity in HVC (n = 4 in 3 birds). The effect of GABA in MMAN on auditory responses in HVC was calculated using the activity (response) during auditory stimuli (Figure 6), as the response strength was more variable, presumably due to a large variability in baseline activity.…”