Clonidine inhibits central noradrenergic neurons in unanesthetized cats

Within the feline locus coeruleus complex (LCx) there exists a subpopulation of neurons with a distinct behavioral neurophysiological profile; these neurons fire most during waking, less so during slow wave sleep, and fall virtually silent during paradoxical sleep (PS). These "PS-off' cells have been thought to be noradrenergic, largely because their heterogeneous distribution throughout the LCx parallels the distribution of noradrenergic neurons within the feline dorsolateral pontine tegmentum. The alpha 2-adrenergic agonist clonidine has been shown to markedly inhibit the central noradrenergic neurons of the anesthetized rat, regardless of the route of administration. We therefore tested the PS-off cells of the feline LCx with intravenous clonidine, to determine if they too were inhibited by this agent. We found 13/13 LCx PS-off cells to be inhibited by intravenous clonidine (2-4 micrograms/kg) as compared with saline controls; non-PS-off cells were not inhibited. These data add strength to the argument that the PS-off cells of the feline LCx are noradrenergic.