Diurnal and circadian changes of serotonin in the suprachiasmatic nuclei: regulation by light and an endogenous pacemaker.

Daily variations of serotonin (5-HT) in the suprachiasmatic nuclei (SCN) were measured in rats kept under various lighting conditions to elucidate the serotonergic contribution to the mechanism underlying SCN function on circadian rhythmicity. Animals kept in 12-h light-12-h dark (LD) cycles showed a peak 5-HT level during the light period and a trough during the dark period. In constant darkness (DD), rhythmic 5-HT variation was out of phase to changes observed in LD. Rats that have been kept in DD and then exposed to constant light (LL) showed transitory increases in 5-HT just after lights on. Taken together, these results show that 5-HT variation in the SCN is generated by an endogenous pacemaker and is also influenced by photic cues.