Melatonin binds to two specific G-protein coupled receptors (GPCR), MT1 (MTNR1A/MEL1A) and MT2 (MTNR1B/MEL1B). MT1 receptors signal via inhibitory G proteins (Gai and Gao) leading to adenylate cyclase inhibition and possibly inositol phosphate stimulation in recombinant systems. In certain native tissues (e.g. sheep pars tuberalis, rat cerebral and caudal arteries) melatonin responses are presumably mediated through activation of MT1 receptors. The hypothalamic suprachiasmatic nucleus appears to be involved in circadian rhythm while the hypophysial pars tuberalis may be responsible for the reproductive effects of melatonin.