Abstract
The combined horseradish peroxidase retrograde transport-peroxidase- antiperoxidase immunohistochemical procedure was utilized in the present study to ascertain the sites of origin of serotonin and neurotensin projections to the rodent nucleus raphe magnus. The major serotonin inputs to the raphe magnus arise from the B-8 and B-9 groups of Dahlstrom and Fuxe (Dahlstrom, A., and K. Fuxe (1964) Acta Physiol. Scand. Suppl. 232 62: 1–55), the nucleus reticularis paragigantocellularis, and the nucleus reticularis gigantocellularis pars alpha. Neurotensinergic projections to the raphe magnus originate predominantly from the periaqueductal gray, the nucleus solitarius, the dorsal and ventral parabrachial nuclei, and the nucleus cuneiformis. The periaqueductal gray and the nucleus paragigantocellularis were found to provide both a neurotensin and a serotonin projection to this raphe nucleus. The present results indicate that several brain stem nuclei, which have been implicated previously in endogenous analgesia mechanisms, provide serotonergic and neurotensinergic input to the nucleus raphe magnus.