Injection of alpha-bungarotoxin near the suprachiasmatic nucleus blocks the effects of light on nocturnal pineal enzyme activity.

Environmental lighting can prevent or quickly reverse the nocturnal elevation of serotonin N-acetyltransferase activity in the rat pineal gland. Intraventricular injections of carbachol, acting via a 'nicotonic' cholinergic receptor, mimic the acute effect of light. Injections of alpha-bungarotoxin near the suprachiasmatic nucleus of the hypothalamus prevent the effects of light. These results suggest that acetylcholine is involved in the effects of light on the rat pineal's circadian rhythm.