Possible involvement of central cholinergic-serotonergic interaction in natural sleep.

Administration of L-5-hydroxytryptophan (L-5-HTP) (200 mg/kg, p.o.) to adult male hamsters (110-120 g) increased non-rapid-eye-movement (NREM) as well as rapid-eye-movement (REM) sleep, with an increase of total sleep time. Methysergide (10 mg/kg, i.p.) facilitated REM sleep and inhibited NREM sleep. Both REM and NREM sleep disappeared with atropine (5 mg/kg, i.p.) both in absence and in presence of L-5-HTP (200 mg/kg, p.o.). Physostigmine (0.1 mg/kg, i.p.) increased REM sleep and decreased NREM sleep without altering the total sleep time. Co-administration of physostigmine (0.1 mg/kg, i.p.) and methysergide (10 mg/kg, i.p.) increased REM but blocked completely NREM sleep. These results suggest that the serotonergic system involved in REM sleep is regulated by the interaction of cholinergic receptor activity and the involvement of the cholinergic system in NREM sleep is regulated by the interaction of serotonergic receptor activity.