Populations of substance P, Met-enkephalin and serotonin immunoreactive neurons in the interpeduncular nucleus of cat: cytoarchitectonics.

The indirect antibody-peroxidase immunostaining method was used to study the distribution of substance P- (SPLI), Met-enkephalin- (MELI) and serotonin-like (5-HTLI) immunoreactivity within the interpeduncular nuclear complex (IPN) of the cat. Populations of immunoreactive cell bodies were found to have a distinct pattern of localization within the IPN. This distribution pattern is described as seen at 4 representative levels rostrocaudally through the IPN. SPLI cell bodies were mainly at rostral levels I, II and III especially in the median region and as a cluster of cells in the intermediate regions of the IPN. MELI cell bodies were seen at all 4 levels, confined mainly to the median and intermediate regions at levels I and II and also in the lateral regions at levels III and IV. There were few 5-HTLI cell bodies at the rostral levels I and II where they were confined to the dorsal zone of the median region, whereas at levels III and IV there were numerous cell bodies located throughout the intermediate region of the IPN. The immunoreactive cell bodies in the IPN may belong to both intrinsic and projection neurons. The distinctive neurochemical cytoarchitectonics of the IPN suggest that this region of the brain which lies along the course of major limbic pathways, may be a center of complex integrative mechanisms of neuronal activity.