Electron microscopic immunocytochemical localization of substance P in the area postrema of rat

Electron microscopic immunocytochemistry was employed to examine the ultrastructure of processes containing substance P-like immunoreactivity (SP-LI) in the area postrema of rat. SP-LI was present within axons and axon terminals which formed synapses with dendrites of intrinsic neurons in the rat area postrema. Specialized contacts between labeled processes and perivascular or ependymal glia cells were not observed. These data suggested that substance P may act as a neurotransmitter involved primarily with neuronal (axodendritic) interactions within the area postrema.