Distribution of immunoreactivity for enkephalin, substance P and vasoactive intestinal peptide in fibres surrounding splanchnic sympathetic preganglionic neurons in rats

Abstract

The distribution of substance P, enkephalin and vasoactive intestinal peptide in fibres and cells was examined in the autonomic nuclei of the lower thoracic and lumbar segments of the rat spinal cord. Attention was focussed on the location of the peptides in sympathetic preganglionic neurons contributing to the greater and lesser splanchnic nerves and in fibres surrounding these neurons. To identify splanchnic preganglionic neurons, Fluoro-Gold was applied to the left splanchnic nerve in anaesthetized rats and some of these animals received intrathecal administration of colchicine at thoracic segments 6, 9 and 12, 24–48 h before perfusion with fixative. Immunoreactivity for substance P, enkephalin and vasoactive intestinal peptide in fibres and cells of the sixth thoracic to second lumbar spinal cord was detected with fluorescent immunocytochemical techniques. Most retrogradely labelled cells (90%) were located in the intermediolateral nucleus and the rest were situated in the nucleus intercalatus and the central autonomic nucleus of the gray matter. Terminals of fibres containing immunoreactivity to all three peptides were found in all autonomic regions. Fibres immunoreactive for substance P and enkephalin were seen projecting in the white matter to the region of the intermediolateral nucleus and extending from this nucleus to the central autonomic nucleus. Terminals containing each of the three peptides were also found surrounding the retrogradely 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 immunoreactive for substance P and enkephalin were observed (approximately one per section) and 70% were outside laminae VII and X. Although cells immunoreactive for substance P, enkephalin and vasoactive intestinal peptide were located in all autonomic regions of the spinal cord, cells doubly labelled with retrograde dye and with the antisera to either of the peptides could not be identified. The data suggest that (i) substance P, enkephalin and vasoactive intestinal peptide are contained in fibres of neurons regulating preganglionic sympathetic control of the abdominal viscera and its vasculature; and (ii) these peptides may not be major transmitters within splanchnic preganglionic neurons.