5-HT immunoreactive hypothalamic neurons in rat and cat after 5-HTP administration

Using immunohistochemistry for serotonin (5-HT) after administration of 5-hydroxytryptophan (5-HTP), we have demonstrated the existence of a particular group of 5-HT accumulating neurons in the posterior hypothalamus of the rat and cat. This cluster of neurons, which we termed "5-HT immunoreactive (IR) posterior hypothalamic magnocellular neurons," was characterized by: (1) diffuse 5-HT-IR staining; (2) rather large-sized cell bodies; (3) a marked activity of monoamine oxydase (MAO); (4) capacity of uptake and decarboxylation of 5-HTP; and (5) the absence of capacity of uptake and hydroxylation of tryptophan. These 5-HT-IR magnocellular neurons extended from the suprachiasmatic nucleus area to the caudal end of the hypothalamus. In the rat, the majority of these neurons was found in the tuberal and caudal magnocellular nuclei, whereas in the cat, it was found more widespread in the ventrolateral part of the posterior hypothalamus including the tuberomamillary nucleus. The characteristics of the 5-HT-IR magnocellular neurons which project directly both to the cerebral cortex and the pontine tegmentum are discussed in comparison with those of "APUD" cell series or "paraneurons," as well as with those of other 5-HT-IR neurons demonstrated in the hypothalamus after administration of 5-HTP or tryptophan.