Abstract

The present study investigated the involvement of presynaptic serotonergic mechanisms in the sleep suppressant action of fenfluramine, an indirect serotonin (5-HT) receptor agonist. Rats implanted with cerebrocortical and dorsal neck muscle electrodes were pretreated with either fluoxetine, an inhibitor of 5-HT uptake, or p-chlorophenylalanine (PCPA), an inhibitor of 5-HT synthesis. Animals were continuously monitored by the EEG for 12 hr after an i.p. injection of 5 mg/kg of dl-fenfluramine hydrochloride. Pretreatment with fluoxetine failed to antagonize the fenfluramine-induced suppression of slow-wave sleep and rapid-eye-movement sleep. However, chemical analyses showed that fluoxetine pretreatment completely prevented the depletion of brain 5-HT produced by fenfluramine, suggesting that fenfluramine did not gain entry into serotonergic neurons. Depletion of brain 5-HT by PCPA also failed to antagonize the sleep suppression caused by fenfluramine as well as that observed after administration of quipazine, a direct 5-HT receptor stimulant. Because administration of fenfluramine to PCPA-pretreated rats produced no additional depletion of 5-HT, it appears that 5-HT was no longer available for release by fenfluramine in these animals. Furthermore, neither pretreatment with fluoxetine nor PCPA antagonized the head-shaking induced by fenfluramine, a behavior associated with activation of central 5-HT receptors. These data indicate that the suppression of sleep produced by fenfluramine is not mediated through release of 5-HT as brain 5-HT concentrations were not related to the behavioral effects of the drug. The sleep suppressant action of fenfluramine may therefore result from a direct action of the drug on postsynaptic 5-HT receptors.