DISTRIBUTION OF IMMUNOREACTIVITY FOR ENKEPHALIN, SUBSTANCE-P AND VASOACTIVE-INTESTINAL-PEPTIDE IN FIBERS SURROUNDING SPLANCHNIC SYMPATHETICPREGANGLIONIC NEURONS IN RATS

The distribution of substance P, enkephalin and vasoactive intestinal peptide in fibres and cells was examined in the autonomic nuclei of th e lower thoracic and lumbar segments of the rat spinal cord. Attention was focussed on the location of the peptides in sympathetic pregangli onic neurons contributing to the greater and lesser splanchnic nerves and in fibres surrounding these neurons. To identify splanchnic pregan glionic neurons, Fluoro-Gold was applied to the left splanchnic nerve in anaesthetized rats and some of these animals received intrathecal a dministration of colchicine at thoracic segments 6, 9 and 12, 24-48 h before perfusion with fixative. Immunoreactivity for substance P, enke phalin and vasoactive intestinal peptide in fibres and cells of the si xth thoracic to second lumbar spinal cord was detected with fluorescen t immunocytochemical techniques. Most retrogradely labelled cells (90% ) were located in the intermediolateral nucleus and the rest were situ ated in the nucleus intercalatus and the central autonomic nucleus of the gray matter. Terminals of fibres containing immunoreactivity to al l three peptides were found in all autonomic regions. Fibres immunorea ctive for substance P and enkephalin were seen projecting in the white matter to the region of the intermediolateral nucleus and extending f rom this nucleus to the central autonomic nucleus. Terminals containin g each of the three peptides were also found surrounding the retrograd ely labelled cells in the intermediolateral nucleus. Approximately two cells immunoreactive for vasoactive intestinal peptide were found per section and 80% were located in the autonomic regions. Fewer cells im munoreactive for substance P and enkephalin were observed (approximate ly one per section) and 70% were outside laminae VII and X. Although c ells immunoreactive for substance P, enkephalin and vasoactive intesti nal peptide were located in all autonomic regions of the spinal cord, cells doubly labelled with retrograde dye and with the antisera to eit her of the peptides could not be identified. The data suggest that (i) substance P, enkephalin and vasoactive intestinal peptide are contain ed in fibres of neurons regulating preganglionic sympathetic control o f the abdominal viscera and its vasculature; and (ii) these peptides m ay not be major transmitters within splanchnic preganglionic neurons.