Analysis of Immediate-Early Gene Expression in the Songbird Brain Following Song Presentation

The song system of oscine songbirds consists of the auditory and motor pathways used in the acquisition and production of learned song and provides powerful material for the study of neuronal and behavioral plasticity, both developmentally and during adult life. The purpose of this thesis is to begin to study molecular mechanisms underlying plasticity in the songbird brain. To test whether genomic mechanisms are involved, the expression of an immediate-early gene (lEG) was studied. The isolation of the canary ZENK homologue is described, an lEG that encodes a zinc-finger protein and which is highly sensitive to growth factors and depolarizing stimulation, including stimuli leading to LTP induction in the mammalian hippocampus. Two methods were used to study gene regulation: administration of metrazole, a strong depolarizing agent, and playbacks of birdsong, a natural stimulus of well-defined behavioral relevance. Both stimuli result in significant increases in ZENK mRNA levels in the brain. Metrazole experiments demonstrate that ZENK is differentially regulated and may be selectively repressed in some neuronal populations, including cells of the song control pathway. Mapping of depolarized areas after song playbacks with ZENK probes defines a series of brain areas activated by song; some of these had not been described as being related to song production or perception and neuroanatomical tract-tracing techniques were used to define their connections. Some of these areas are intimately associated with auditory structures; a subset of these is closely apposed to nuclei of the motor pathway for song control and could represent sites for sensory motor integration, an essential aspect of song learning. ZENK induction in the caudo-medial neostriatum (NCM, the area with the most marked response) shows a preference for conspecific song and can be habituated by repetitive presentations of the same song, but does not enter an absolute refractory period after habituation, since another song will elicit a full genomic response; these results suggest that cells showing ZENK response are related to auditory processing and song discrimination