Inter- and intracellular relationship of substance P-containing neurons with serotonin and GABA in the dorsal raphe nucleus: combination of autoradiographic and immunocytochemical techniques.

Double-labeling experiments were performed at the electron microscopic level in the dorsal raphe nucleus of rat, in order to study the inter- and intracellular relationship of substance P with gamma-aminobutyric acid (GABA) and serotonin. Autoradiography for either [3H]serotonin or [3H]GABA was coupled, on the same tissue section, with peroxidase-antiperoxidase immunocytochemistry for substance P in colchicine-treated animals. Intercellular relationships were represented by synaptic contacts made by [3H]serotonin-labeled terminals on substance P-containing somata and dendrites, and by substance P-containing terminals on [3H]GABA-labeled cells. Intracellular relationships were suggested by the occurrence of the peptide within [3H]serotonin-containing and [3H]GABA-containing cell bodies and fibers. Doubly labeled varicosities of the two kinds were also observed in the supraependymal plexus adjacent to the dorsal raphe nucleus. The results demonstrated that, in addition to reciprocal synaptic interactions made by substance P with serotonin and GABA, the dorsal raphe nucleus is the site of intracellular relationships between the peptide and either the amine or the amino acid.