Occurrence of vasoactive intestinal polypeptide (VIP)-like immunoreactivity in certain cholinergic neurons of the cat: Evidence from combined immunohistochemistry and acetylcholinesterase staining

Postganglionic sympathetic and parasympathetic neurons of the cat were studied with the indirect immunofluorescenee technique using antiserum to vasoactive intestinal polypeptide (VIP) and with acetylcholinesterase staining. In the stellate and in L7 and S1 sympathetic ganglia some of the principal ganglion cells (10–15% of the total) contained both VIP-like immunorcactivity and intense acetylcholinesterase activity, suggesting the presence of a VIP-like peptide in a population of sympathetic cholinergic neurons. This was corroborated by (1) overlapping accumulations of VIP immunoreactivity and acetylcholinesterase staining around a ligation of the sciatic nerve, (2) very similar distribution patterns of VIP immunoreactive and acetylcholinesterase-containing fibres around sweat glands of the food pad and around some blood vessels in several tissues, and (3) the disappearance of both types of staining of fibres around sweat glands and skeletal muscle vessels after lumbo-sacral sympathectomy. In other sympathetic ganglia, as well as in the parasympathetic pelvic ganglion plexus and in some ganglia of the bladder wall, VIP immunoreactivity was also found in neurons rich in acetylcholinesterase. On the other hand, it should be emphasized that the occurrence of a VIP-like peptide in presumably cholinergic neurons does not seem to be a general phenomenon. No marked overlap was observed between VIP immunoreactive and acetylcholinesterase-rich cell bodies in most ganglia in the urinary bladder wall and nerve terminals of the smooth muscle layer. Furthermore, no obvious correlation in the number of VIP immunoreactive and acetylcholinesterase-containing fibres were seen in certain other nerves such as the spinal ventral roots, the splanchnic nerve and the perivascular superior mesenteric nerves. Thus, three groups of peripheral autonomic neurons have been defined in the present study: neurons rich in acetylcholinesterase that also contain a VIP-like peptide; VIP immunoreactive neurons; and neurons rich in acetylcholinesterase. The functional significance of a VIP-like peptide in probable cholinergic neurons is at present unclear. Preliminary data indicate that, whereas acetylcholine directly stimulates sweat secretion, VIP may cooperate in the production of sweat by increasing local blood flow. Thus, VIP, released together with acetylcholine, may be responsible for the atropine-resistant vasodilatation in sweat glands and other exocrine glands.