Orcadian Rhythm of Serotonin Binding in Rat Brain—I. Effect of the Light-Dark Cycle

High affinity serotonin binding to rat brain membranes showed a circadian rhythm with minimal binding at 1000 and a maximal binding at 0000. Brain serotonin levels were almost inverse to the rhythm of serotonin binding. Under reverse light-dark conditions, lights on from 1900 to 0700, a significant phase shift in serotonin binding and concentration of about 8-10 hr was found. The adaptation of the rats to the inverse light-dark cycle was ascertained by plasma ACTH and corticosterone assay.