Are there non-monoaminergic paradoxical sleep-off neurons in the brainstem?

Using extracellular single unit recording in the medulla of freely moving cats, we have found a population of PS-off ("Type II") neurons that are distinct from the classically described monoaminergic PS-off ("Type I") neurons. The presumed non-monoaminergic Type II PS-off neurons (n=22) showed a relatively high rate of tonic discharge during both quiet waking and slow-wave sleep (10.4+/-4.1 and 9.3+/-3.1 spikes/sec, mean +/- S.D., respectively) and a marked overall decrease in discharge rate during PS (0.3+/-0.4 spikes/sec). In contrast to the presumed monoaminergic PS-off neurons (n=62), Type II PS-off neurons showed short-lasting phasic discharges during PS, often in association with rapid eye movement and PGO wave bursts. These Type II neurons were all characterized by a short action potential which was significantly different from that of the monoaminergic PS-off neurons described so far. Five out of 22 neurons were identified antidromically by stimulation of the ventrolateral reticulospinal tract (vlRST) at the caudal medulla, while 2 of the 22 were identified antidromically by stimulation of the peri-locus coeruleus alpha of the mediodorsal pontine tegmentum. Their mean conduction velocity (7.2+/-1.9 m/sec) was significantly higher than that (0.9+/-0.3 m/sec) of the presumed monoaminergic PS-off neurons which were identified exclusively by stimulation of the vlRST. In addition, when examined during the sleep-waking cycle, the antidromic responses of Type II PS-off neurons were either completely blocked or reduced, with a prolongation of antidromic latency during PS. Most of these neurons were located in medullary structures containing no, or virtually no, monoaminergic neurons, and none responded by inhibition to systemic administration of serotonergic or adrenergic autoreceptor agonists. These findings indicate the existence, in the medulla, of non-monoaminergic PS-off neurons that would play an important role in PS generation.