Three types of melatonin receptor have been cloned. The MT1 (or Mel1A or MTNR1A) and MT2 (or Mel1B or MTNR1B) receptor subtypes are present in humans and other mammals, while an additional melatonin receptor subtype MT3 (or Mel1C or MTNR1C) has been identified in amphibia and birds.
In mammals, melatonin receptors are found in the brain and some peripheral organs. However, there is considerable variation in the density and location of the expression of melatonin receptors between species. The MT1 subtype is present in the pars tuberalis of the pituitary gland and the suprachiasmatic nuclei of the hypothalamus. The MT2 subtype is mainly present in the retina. The MT3 subtype of many non-mammalian vertebrates is expressed in various brain areas.
^Nickelsen T, Samel A, Vejvoda M, Wenzel J, Smith B, Gerzer R. Chronobiotic effects of the melatonin agonist LY 156735 following a simulated 9h time shift: results of a placebo-controlled trial. Chronobiology International. 2002 Sep;19(5):915-36. PMID 12405554