Response Modulation by Single Units in the Auditory Neostriatum of Songbirds: Characterization of Responses and Their Relationships to Nuclear Gene Regulation in Adult and Juvenile Zebra Finches
"Presentation of birdsong activates immediate early gene (IEG) expression in the caudomedial neostriatum (NCM) of songbirds that are at least 30 days of age (d30) (Mello, et al., 1992; Jin & Clayton, 1997). However, when songbirds are raised in isolation, playback of birdsong fails to induce these genes. To assess the function of this genomic response, I first analyzed the quantitative and temporal relationships between electrophysiological activity and gene induction in the NCM of adult ($>$d120) zebra finches. Single units in NCM showed large increases in firing during birdsong presentations, whereas pure tones usually inhibited firing. Most cells showed little selectivity for individual songs based on mean firing rates. As novel songs were repeated, cell responses habituated. However, even after many repetitions of a song, cells showed excitatory responses during playback of that stimulus. This is unlike the complete loss of responsiveness previously observed for genomic activity. Habituated responses to a song recovered when other intervening songs were presented. However, song specific habituation persisted when one song was repeated for 200 trials ($\sim$34 minutes). These results suggested that there is a correlation between IEG induction and electrophysiological habituation. Specifically, that the initial phase of the habituation signals the cell to induce IEGs. This model was tested by examining electrophysiological responses of NCM units in isolate d30, and normal d20 and d30 juvenile zebra finches. As in adults, NCM units in all juveniles responded robustly to song playback. Cells fired less strongly to other classes of stimuli (e.g., tones when compared to songs), but tended to fire equally well to different individual songs. In all juveniles, firing rates habituated during repetitions of novel song, even in juveniles that had not yet developed the ZENK induction response. Thus, the initial habituation of electrophysiological responses is not by itself a sufficient trigger for IEG induction, but it may still be necessary. These results indicate dissociation between gross physiological activity and ""immediate early"" gene expression in the brain: genomic activity occurs only during a subset of electrophysiological responses. The possible significance of this as an integrative mechanism in brain function and plasticity is discussed."
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign