Effects of histamine on circadian rhythms and hibernation

Histamine appears to play a role in regulation of sleep and arousal as well as in synchronizing endogenous circadian rhythms with exogenous photic cue s. Direct application of histamine to the suprachiasmatic nucleus (SCN), th e site of the mammalian circadian pacemaker, phase shifts the circadian rhy thm in neural activity. Intraventricular injections of histamine also phase shift circadian rhythms as do micro-injections directed towards the SCN. T he magnitude and direction of the phase shifting effects of histamine depen d on circadian phase in a manner similar to light. Depletion of brain hista mine levels by inhibition of histamine synthesis reduces phase shifts to li ght. Histamine appears to influence phase shifts to light via a direct modu lation of NMDA receptors in the SCN. Increased histamine levels and turnove r observed in hibernating animals render it possible that histamine is a ke y regulator of hibernation. Thus histamine participates in an important lin k between sleep, circadian rhythms, and hibernation.