Neurochemical identification of afferents onto spinomedullary neurons in the rat spinal cord central gray matter.

The synaptic relationship between spinal cord central gray projection neurons and immunocytochemically identified afferents in the rat were examined at the light microscopic level using the combined techniques of retrogradely transported True blue and serotonin (5-HT), enkephalin (ENK), and substance P (SP) immunocytochemistry. At L4-L6, numerous retrogradely labeled neurons could be identified around the central canal after large bulbar injections of True blue. Of these projection neurons, 75% were apposted by 5-HT varicosities, 57% by ENK varicosities and 58% by SP varicosities. Hemisection of the spinal cord produced a marked reduction in the amount of 5-HT immunoreactivity and the number of putative 5-HT contacts observed on neurons of the spinal cord central gray. A small decrease in SP immunoreactivity and putative contacts was seen after dorsal rhizotomy. Neither rhizotomy nor hemisection produced discernable changes in ENK immunofluorescence. Based on the distributions of 5-HT, ENK and SP in the spinal cord, we suggest that a more precise delineation of lamina X in the rat can be made according to immunocytochemical rather than strictly morphological criteria.