Depletion of brain serotonin by 5,7-DHT: effects on the 8-OH-DPAT-induced changes of sleep and waking in the rat

The effect of 5-HT_1A receptor agonist 8-OH-DPAT on sleep and wakefulness was studied in rats with selective serotonin depletion after ICV administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). Injection of 8-OH-DPAT to vehicle-treated animals induced biphasic effects, such that low doses (0.010 mg/kg) which act perferentially on the somatodendritic autoreceptor decreased wakefulness (W) and increased slow wave sleep (SWS), while higher doses (0.375 mg/kg) which stimulate postsynaptic receptors caused opposite effects. REM sleep was suppressed irrespective of the dosage given. Injection of the 0.010 mg/kg dose in the 5,7-DHT-treated rats did not result in significant changes in sleep or W. On the other hand, the 0.375 mg/kg dose produced changes in sleep variables which were similar to those described in the vehicle-treated animals. Our findings tend to indicate that increased SWS after low doses of 8-OH-DPAT depends upon the activation of inhibitory somatodendritic 5-HT_1A receptors, while increased W after higher doses of the compound is related to stimulation of postsynaptic receptors.