Immunohistochemical localization of neuropeptides in the vocal control regions of two songbird species.

Immunohistochemistry was used to map the distribution of four neuropeptides in song control regions of two songbird species, the European starling (Sturnus vulgaris) and the song sparrow (Melospiza melodia). We searched for positively stained cell bodies or apparent terminals containing vasoactive intestinal peptide (VIP), methionine-enkephalin (MET), cholecystokinin (CCK), and substance P (SUB P). Intraventricular colchicine pretreatment was administered to enhance the visualization of peptide-containing cell bodies. Four areas implicated in the central control of song were examined. Three of these areas are sexually dimorphic telencephalic nuclei characteristic of songbirds: the caudal nucleus of the ventral hyperstriatum (HVc), the robust nucleus of the archistriatum (RA), and the magnocellular nucleus of the anterior neostriatum (MAN). The fourth region is the mesencephalic nucleus intercollicullaris (ICo), common to all birds, which contains the dorsomedial nucleus (DM) that appears to be specifically involved in the motor control of song. The pattern of neuropeptide localization was similar between the two species. However, the neuropeptides were heterogeneously dispersed among the four areas. VIP and MET were the most widely distributed, whereas CCK and SUB P were seen only in DM. MAN and HVc revealed remarkably similar patterns of staining for both MET and VIP. Fine varicosities immunolabeled for both these peptides appear to encircle nonreactive somata. In both these nuclei positively stained somata were observed for MET but not for VIP. In RA there was a dense accumulation of MET-positive multipolar cell bodies. VIP-containing neurons were seen in the surrounding archistriatum and caudal neostriatum but not in RA itself. Cell bodies and fibers for all four peptides were observed in DM; in no case were they limited to this subregion, but rather seemed to encompass the surrounding intercollicular area as well. The widespread distribution of VIP and MET strongly suggests a role for these peptides in the acquisition or production of passerine song.