An electrophysiological study of the forebrain projection of nucleus commissuralis: preliminary identification of presumed A2 catecholaminergic neurons.

Using a double-labeling technique (HRP combined with catecholamine fluorescence), up to 80% of all CA-containing neurons visualized in the nucleus commissuralis were found to project to or through the median forebrain bundle area (MFB). In addition at least 90% of all nucleus commissuralis neurons projecting through the MFB were found to be catecholaminergic. In a series of chloral hydrate-anesthetized rats, nucleus commissuralis neurons projecting through the MFB were identified with single-unit recordings by antidromic (AD) activation. These cells had a conduction velocity of about 0.5 m/s and a firing rate of 0-14 spikes/s. The pattern of discharge of these neurons was not correlated with the heart rate; they were unaffected by single-unit stimulation applied to the sciatic nerve but were powerfully excited by vagus nerve stimulation. For comparative purposes, NE-containing neurons were also recorded in the locus coeruleus (A6) in the course of the same experiments; in contrast with MFB-activated commissuralis neurons, A6 neurons were excited by both visceral (vagus nerve) and somatic (sciatic nerve) stimulation. The spontaneous firing rate of MFB-activated commissuralis neurons was inhibited by the intravenous administration of a low dose of the centrally acting antihypertensive agent clonidine (ED50: 28 micrograms/kg).