Melatonin is a neurohormone that plays a key role in the synchronisation of circadian and seasonal functions with cyclic environmental variations. In mammals, two melatonin receptors, MT1 and MT2, have been cloned. Activation of MT2 melatonin receptors phase shift circadian rhythms of neuronal firing in the suprachiasmatic nucleus, inhibit dopamine release in retina, induce vasodilation and inhibition of leukocyte rolling in arterial beds, and enhance immune responses.