ENKEPHALIN-RELEASING, THYROTROPIN-RELEASING HORMONE-IMMUNOREACTIVE AND SUBSTANCE-P-IMMUNOREACTIVE AXONAL INNERVATION OF THE VENTROLATERAL DENDRITIC BUNDLE IN THE CAT SACRAL SPINAL-CORD - AN ULTRASTRUCTURAL-STUDY

The distribution and synaptic arrangement of thyrotropin-releasing hor mone-, substance P- and enkephalin-immunoreactive axonal boutons have been studied in the ventrolateral nucleus (Onufs nucleus) of the upper sacral spinal cord segments in the cat. For this purpose, the peroxid ase-antiperoxidase immunohistochemical technique was used. Immunoreact ive axonal boutons were traced in complete series of sections in order to reveal synaptic contacts with the bundled dendrites of the ventrol ateral nucleus. As judged from the cross-sectional diameter of the pos tsynaptic dendrites, the distribution of immunoreactive boutons was no nrandom. Enkephalin-immunoreactive axonal boutons, presumed to be most ly of segmental origin, displayed a rather restricted distribution to mainly (> 80%) medium-to-large dendrites. Thyrotropin-releasing hormon e-immunoreactive boutons, that derive from supraspinal levels, were al so found to impinge on medium-to-large dendrites (> 80%), indicating a proximal location within the dendritic trees. The skewness toward lar ge postsynaptic dendrites was even more marked for thyrotropin-releasi ng hormone- than for enkephalin-immunoreactive boutons. Substance P-im munoreactive boutons, that are of either supraspinal or spinal origin, showed a more even distribution throughout the dendritic trees, inclu ding both thin distal branches and thick proximal dendrites. In view o f the well-known fact that virtually all thyrotropin-releasing hormone -immunoreactive boutons in the ventral horn co-contain substance P (an d serotonin) it was assumed that substance P-immunoreactive boutons in synaptic contact with the finest-calibre dendrites as well as most of those with a very proximal juxtasomatic location on the dendritic tre es were of segmental origin, while those impinging on medium-to-large dendrites could be of either spinal or supraspinal origin. Fine-calibr e dendrites (< 1 mu m) represent about 25% of the dendritic branches i n the ventrolateral nucleus, but receive, with the exception of substa nce P (8%), very little (< 3%) peptidergic or GABAergic (Ramirez-Leon and Ulfhake, 1993) input, although the degree of dendritic membrane co vering by bouton profiles in the ventrolateral nucleus does not seem t o vary much with the calibre of the postsynaptic dendrite (Ramirez-Leo n and Ulfhake, 1993). Both substance P- and enkephalin-immunoreactive axonal boutons established synaptic contact with more than one dendrit e. Furthermore, one and the same bouton could be found in contact with two dendrites that were coupled to each other by a dendro-dendritic c ontact of desmosomal or puncta adherentia type. This synaptic arrangem ent was, however, not seen among thyrotropin-releasing hormone-immunor eactive boutons, indicating that these axonal boutons act on a single postsynaptic element, while inputs intrinsic to the spinal cord can sh ow a divergence also at the-terminal level.