Light and propranolol suppress the nocturnal elevation of serotonin in the cerebrospinal fluid of rhesus monkeys.

Markedly elevated nighttime concentrations of serotonin in rhesus monkey cerebrospinal fluid were reduced to daytime levels by exposing the monkeys to continuous light or to the beta-adrenergic antagonist propranolol. Nighttime elevations of melatonin in cerebrospinal fluid were also suppressed by propranolol and light. Serotonin released in large quantities at night appears to be regulated like melatonin, and may act as a cerebroventricular hormone to influence brain and pituitary function at night.