NEUROKININ-1 RECEPTOR-IMMUNOREACTIVE SYMPATHETIC PREGANGLIONIC NEURONS - TARGET SPECIFICITY AND ULTRASTRUCTURE

Substance P is involved in cardiovascular control at the spinal cord l evel, where it acts through neurokinin-1 receptors. In this study we u sed immunocytochemistry and retrograde tracing to investigate the pres ence of the neurokinin-1 receptor and its ultrastructural localization in rat sympathetic preganglionic neurons that project to the superior cervical ganglion or the adrenal medulla. Immunofluorescence for the neurokinin-1 receptor outlined the somatic and dendritic surfaces of n eurons in autonomic subnuclei of spinal cord segments T1-T12, whereas immunofluorescence for the tracer, cholera toxin B subunit, filled ret rogradely labelled cells. There was a significant difference in the pr oportion of neurokinin-1 receptor-immunoreactive sympathetic pregangli onic neurons supplying the superior cervical ganglion and the adrenal medulla. Thirty-eight percent of the neurons that projected to the sup erior cervical ganglion were immunoreactive for the neurokinin-1 recep tor compared to 70% of neurons innervating the adrenal medulla. Of neu rons projecting to the superior cervical ganglion, significantly diffe rent proportions showed neurokinin-1 receptor immunoreactivity in spin al cord segment T1 (15%) versus segments T2-T6 (45%). At the ultrastru ctural level, neurokinin-1 receptor staining occurred predominantly on the inner leaflets of the plasma membranes of retrogradely labelled s ympathetic preganglionic neurons. Deposits of intracellular label were often observed in dendrites and in the rough endoplasmic reticulum an d Golgi apparatus of cell bodies. Neurokinin-1 receptor immunoreactivi ty was present at many, but not all, synapses as well as at non-synapt ic sites, and occurred at synapses with substance P-positive as well a s substance P-negative nerve fibres. Only 37% of the substance P synap ses occurred on neurokinin-1-immunoreactive neurons in the intermediol ateral cell column. These results show that presence of the neurokinin -1 receptor in sympathetic preganglionic neurons is related to their t arget. The ultrastructural localization of the receptor suggests that sympathetic preganglionic neurons may be affected (i) by substance P r eleased at neurokinin-1 receptor-immunoreactive synapses, (ii) by othe r tachykinins (e.g., neurokinin A), which co-localize in substance P f ibres in the intermediolateral cell column, acting through other neuro kinin receptors, and (iii) by substance P that diffuses to neurokinin- 1 receptors from distant sites. (C) 1997 IBRO. Published by Elsevier S cience Ltd.