Effects of adrenergic drugs on raphe unit activity in freely moving cats

Previous work has suggested that central noradrenergic neurons may have an important influence on the activity of serotonergic neurons located in the dorsal raphe nucleus. Pharmacological studies have indicated that an α-adrenergic input is necessary to maintain the activity of these raphe cells. This issue was examined in freely moving cats by studying the effects of adrenergic drugs on raphe unit activity. Systemic administration of the selective α₁-antagonists WB4101 (0.5 and 1.0 mg/kg) or prazosin (10 mg/kg), or the non-competitive antagonist phenoxybenzamine (10 mg/kg), produced strong behavioral effects but had little effect on raphe unit activity. A low dose of the α-agonist clonidine (0.05 mg/kg), which decreases adrenergic transmission, produced similar results. These same drugs also had negligible effects on the evoked responses of raphe units to auditory or visual stimulation. Administration of d-amphetamine, a catecholamine releaser, produced behavioral stereotypy but had no significant effect on the discharge of raphe units. These data suggest that the adrenergic influence on serotonergic raphe neurons is very small in the freely moving cat. This is in contrast to the reported complete suppression of unit activity produced by α-adrenergic blockade in the chloral hydrate anesthetized rat.