Abstract

There is a marked circadian rhythm in the serotonin content of the rat pineal gland. The rhythm can be inverted 180° by reversal of the lighting regimen. Phase inversion is complete in 6 days. The activities of 5-hydroxytryptophan decarboxylase and monoamine oxidase in the pineal gland do not exhibit a circadian variation. The absolute increase in pineal gland serotonin produced by tryptophan or 5-hydroxytryptophan injection is much greater than in other tissues but does not vary at different times of day and night. Treatment with β-phenylisopropyhlmydrazine, a monoamine oxidase inhibitor, prevents the nocturnal decline of pineal serotonin but does not alter daytime levels. Treatment with actinomycin D partially inhibits time daytime increase in pineal serotonin.