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 humans, The MT1 subtype's expression in the pars tuberalis of the pituitary gland and suprachiamatic nuclei of the hypothalamus is indicative of melatonin's circadian and reproductive functional involvement.
In humans, the MT2 subtype's expression in the retina is suggestive of melatonin's effect on the mammalian retina occurring through this receptor. Research suggests that melatonin acts to inhibit the Ca2+-dependent release of dopamine. Melatonin's action in the retina is believed to affect several light-dependent functions, including phagocytosis and photopigment disc shedding.
^Nickelsen T, Samel A, Vejvoda M, Wenzel J, Smith B, Gerzer R (September 2002). "Chronobiotic effects of the melatonin agonist LY 156735 following a simulated 9h time shift: results of a placebo-controlled trial". Chronobiol. Int.19 (5): 915–36. PMID12405554.