Electrophysiological effects of cocaine and procaine on dorsal raphe serotonin neurons

The effects of cocaine and procaine on the spontaneous activity of serotonin (5-hydroxytryptamine, 5-HT) neurons in the dorsal raphe nucleus of the chloral hydrate anesthetized male rat were characterized using extracellular single-unit and microiontophoretic recording techniques. Intravenous cocaine elicited a potent, dose-dependent inhibition of 5-HT cell firing with a cumulative dose to 50% inhibition of cell firing of 0.66 ± 0.11 mg/kg ( x ± S.E.M.; N =17) . In contrast, systemic administration of the local anesthetic procaine did not significantly alter cell firing incumulative doses up to 16 mg/kg (N = 10). Microiontophoretic application of cocaine, but not procaine, alsodepressed spontaneous cell firing of 5-HT-containing neurons and potentiated the inhibitory effects of 5-HT in thedorsal raphe nucleus. The depressant effects on 5-HT neurons following systemic or microiontophoretic application of cocaine may result from autoinhibition of 5-HT neurons, presumably as a consequence of cocaine-induced 5-HT euptake inhibition.