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

The final common output from the telencephalic vocal control system in songbirds is the projection from nucleus RA, which drives respiratory and syringeal muscles via medullary nuclei. We examined the possible role of GABAergic inhibition in RA of adult male zebra finches by micro-injecting bicuculline, an antagonist of inhibitory GABA(A) receptors, while recording simultaneously with multiple microelectrodes. Following bicuculline injection, the normally high spontaneous activity of RA neurons exhibited a pattern of rhythmic bursting lasting up to 30 min. The bursts were often accompanied by involuntary vocalizations: monosyllabic notes resembling calls. Other experiments used microinjections that were below threshold for involuntary vocalization. When the bird sang to a female during a period after the injection, song structure was degraded: song duration was lengthened, noisiness increased, and novel syllables appeared. The results suggest that GABA normally contributes to regulating excitability in RA. When this regulation is blocked, activity increases sufficiently to engage the respiratory and vocal musculature. Synaptic inputs that affect GABAergic interneurons in RA could thus play a role in initiation and control of vocalization. The abnormal vocalizations produced in the presence of bicuculline suggest that GABAergic inhibition may normally help to shape the pattern of learned vocalizations, as well as to regulate overall RA activity.