AXONAL CONNECTIONS OF THE HIGH VOCAL CENTER AND SURROUNDING CORTICAL REGIONS IN JUVENILE AND ADULT MALE ZEBRA FINCHES

Neuronal connections of the High Vocal Center (HVC), a cortical nucleu s of songbirds necessary for learned vocal behavior, and the region ad jacent to HVC called paraHVC (pHVC), mere studied in adult and juvenil e male zebra finches. Extremely small injections of fluorescent dextra n amines or biocytin were made within subregions of HVC and pHVC to de fine the precise nature and development of these pathways. In adults, all HVC injections produced an even, nontopographic distribution of re trograde label throughout the medial magnocellular nucleus of the ante rior neostriatum (mMAN), the interfacial nucleus (NIf), and the uvaefo rm nucleus of the thalamus (Uva) and an even distribution of anterogra de label within area X of the striatum and the robust nucleus of the a rchistriatum (RA). These same patterns of projections were present in juvenile birds 20-23 days of age, including the projection from HVC to RA, which has previously been reported to develop only after 25-30 da ys of age. Results also establish a novel efferent projection fi om PN C to pHVC in both juvenile and adult birds. Injections into pHVC indic ate that this region receives afferent input from song control areas H VC, mMAN, medial regions of the parvicellular shell of lateral MAN, NI f, and Uva and projects to Area X, caudomedial regions of striatum, an d regions of the caudomedial neostriatum (NCM). Thus, neuronal connect ions of pHVC are highly integrated with circuitry important for vocal behavior and are distinct from those of HVC. Such differences establis h HVC and pHVC as separate brain areas and suggest that each may serve a different function in vocal behavior. Control injections in both ju veniles and adults produced specific patterns of projections from area s outside of HVC to areas outside of RA, illustrating an overall spati al organization of projections from HVC and neighboring cortical areas . Further, although neuronal connections of HVC are not topographic, p rojections of HVC, pHVC, and surrounding areas demonstrate a broad spa tial organization of efferents to striatum and regions surrounding RA, thus defining a level of organization beyond that of individual song control nuclei. (C) 1998 Wiley-Liss, Inc.