DISTRIBUTION AND COLOCALIZATION OF 5-HYDROXYTRYPTAMINE, THYROTROPIN-RELEASING-HORMONE AND SUBSTANCE-P IN THE CAT MEDULLA

This study demonstrates the co-existence of three neurochemicals in ve ntral medullary neurons of the cat utilizing fluorescence immunohistoc hemistry. Neurons containing 5-hydroxytryptamine. thyrotropin-releasin g hormone and substance P were identified within the rostrocaudal exte nt of the medulla, specifically within the raphe pallidus and raphe ma gnus and in the reticular formation of the ventrolateral medulla in th e nucleus paragigantocellularis lateralis. Within the raphe pallidus t he majority of 5-hydroxytroptamine-containing neurons were co-localize d with thyrotropin-releasing hormone and substance P. However, in the raphe magnus the majority of stained neurons contained 5-hydroxytrypta mine and thyrotropin-releasing hormone but were devoid of substance P. In the ventrolateral medulla two major populations of neurons were id entified rostral to the inferior olivary nuclei, one containing 5-hydr oxytryptamine and thyrotropin-releasing hormone, while a more lateral group contained substance P alone. More caudally, at the level of the inferior olives, the majority of 5-hydroxytryptamine-containing cells also displayed immunoreactivity for thyrotropin-releasing hormone and substance P. A consistent finding in both the ventromedial and ventrol ateral regions of the medulla was a population of 5-hydroxytryptamine- containing cells which did not stain for either thyrotropin-releasing hormone or substance P. The functional role of co-localized neurochemi cals remains unknown but co-existence of neurotransmitter substances i n medullary neurons may allow for specific and multiple actions in the spinal cord.