Distribution of immunoreactivity for the NK1 receptor on different subpopulations of sympathetic preganglionic neurons in the rat

The distribuktion of immunoreactivity to the recptor for substance P, the neurokinin 1 (NK1) receptor, was ezamined in preganglionic sympathetic neurons of the rat by usingimmunohistochemistry and retroggrade neuronal tracing abour one‐third of all sympathetic preganglionic neurons were NK1 receptor immunoreactiv, and most of the NK1 receptor immunoractive neurons were also nitric oxide synthase immunoreactive the proportions ofsympathetic preganglionic neurons projecting to the superior and inferior mesenteric ganglia, addrenal glnd, and lumbar sympathetic hain which werre nK1 rdeptor‐immunoreactive were determined. Most (89%) of the preganglionic neurons projecting to the addrenal glands were NK1 rceptor immunoreactive few (17%) of the preganglionic neurons projecting to the L5 sympathetic hain ganglion were immunoreactive for the receptor, while preganglionic neurons projecting tothe prevertebral ganglia were NK1 receptor immunoractive at intermediate frequencies (61–64%). Thus, substance P acting on NK1 receptors in likely to be importnt inthe pregnglionic pathways to the addrenal medulla and viscera via the preveertebral ganglia, but the NK1 receptor withthe enzyme nitric oxide synthase was also examined. The co‐localisation of NK1 receptor‐immunoreactive neurons were also nitric oxide synthase immunoreactiv. Thus NK1 receptors occur onpreganglionic neurons over many spinal segments and in a range of prganglionic pathways, as well as in a range of combinations with nitric oxide shnthase. The heterogeneity ofjpreganglionic neurons showing NK1 receeptor immunoreactivity may reglect the involvement of NK1‐mediated trnsmkssion in a variety of functional pathways, most notably thepreganglionic projections to the adrenal medulla and to the viscera. © 1996 Wiley‐Liss, Inc.