Immunocytochemical analysis of noradrenaline, substance P and enkephalin axonal contacts on serotonin neurons in the caudal raphe nuclei of the cat.

The present study investigates the anatomical basis for interactions between serotonin immunoreactive neurons in nuclei raphe magnus and pallidus, and either noradrenaline, substance P (SP) or enkephalin immunoreactive axonal varicosities. Using a double-label immunocytochemical method, we found that each of these neurochemicals could be localized to axons which contacted serotonin immunoreactive neurons. The frequency and location of these inputs differed in nuclei raphe magnus and pallidus. SP immunoreactive varicosities formed the greatest number of contacts. These findings suggest that serotonin-containing neurons in the caudal raphe nuclei receive input from multiple putative neurotransmitters.