Histamine synthesis inhibition reduces light-induced phase shifts of circadian rhythms.

Circadian rhythms are generated by the hypothalamic suprachiasmatic nuclei, a site of dense histaminergic innervation. Histamine can phase shift circadian rhythms in a manner similar to light. In this experiment, we administered alpha-fluoromethylhistidine (FMH), an inhibitor of histamine synthesis, prior to a light pulse in hamsters housed under constant darkness. Photic phase shifts in wheel-running rhythms were significantly attenuated by FMH pretreatment. These results suggest that histamine may modulate photic input to the circadian clock.