Effect of benzodiazepines on central serotonergic neuron systems

Intracerebral (i.c.) injection of serotonin (5-HT) into mice induced head twitches in a dose-dependent manner at 10 min after injection. The head twitches induced by 5-HT (i.c.) were potentiated by the pretreatment of isocarboxazid (3 mg/kg i.p.), and inhibited by cyproheptadine (0.3 mg/kg i.p.), a 5-HT antagonist. Benzodiazepines such as fludiazepam and diazepam potentiated the head twitches induced by 5-HT (i.c.) in a dose-dependent manner. Mescaline (50 mg/kg i.p.) also induced head twitches in mice at 15 and 30 min after injection. Benzodiazepines potentiated the head twitches induced by mescaline in a dose-dependent manner. Cyproheptadine blocked the potentiating effect of benzodiazepines on the head twitches induced by both 5-HT (i.c.) and mescaline. By repeated administration of fludiazepam or diazepam for 5 days, the potentiating effect of both drugs on the head twitches induced by mescaline was unchanged and their anticonvulsant effects were not modified. In contrast, the potency of both drugs on muscle relaxation was significantly decreased by repeated administration. Benzodiazepines failed to change the uptake of 5-HT into the synaptosomal fractions from the rat brain. These results suggest that the pharmacological action of benzodiazepines is derived at least in part from their activating effect on 5-HT receptors.