Is histamine the final neurotransmitter in the entrainment of circadian rhythms in mammals?

Adaptation of circadian rhythms to the environmental light-dark cycle is ne cessary for the survival of organisms. This synchronization or entrainment is only caused by light (photic input) during the dark period, according to the internal biological clock of an organism. During the light period, int ernal factors (non-photic input), rather than light, are able to entrain ci rcadian rhythms. In this article, the data that implicate the neurotransmit ter system for histamine in circadian entrainment are reviewed, Furthermore , we speculate that histamine receptors are the final gate at which both ph otic and non-photic entrainment mechanisms converge before sending a resett ing signal to the intra cellular biological clock.