Histamine H2-receptors in the brain and sleep produced by clonidine

Sleep was induced in chicks aged 4-7 days by intravenous injection of clonidine hydrochloride 0.04 mumol/kg. Sleep was not prevented or altered by a preceding intramuscular injection of blockers of histamine H2-receptors which were used in doses (as mumol/kg) of up to 800 (metiamide) and 2400 (cimetidine). Clonidine, therefore, does not cause sleep by stimulating H2-receptors in the brain. The highest dose of cimetidine used had a hypnotic action of its own.