A melatonin receptor is a G protein-coupled receptor

G protein-coupled receptor

G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...

 (GPCR) which binds melatonin

Melatonin

Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

.

Three types of melatonin receptor have been cloned. The MT₁ (or Mel_1A

Melatonin receptor 1A

Melatonin receptor type 1A is a protein that in humans is encoded by the MTNR1A gene.-Further reading:...

 or MTNR1A) and MT₂ (or Mel_1B

Melatonin receptor 1B

Melatonin receptor 1B, also known as MTNR1B, is a protein that in humans is encoded by the MTNR1B gene.-Function:This gene encodes the MT2 protein, one of two high-affinity forms of a receptor for melatonin, the primary hormone secreted by the pineal gland. This gene product is an integral membrane...

 or MTNR1B) receptor subtypes are present in human

Human

Humans are the only living species in the Homo genus...

s and other mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, while an additional melatonin receptor subtype MT₃ (or Mel_1C

Melatonin receptor 1C

Melatonin receptor 1C, also known as MTNR1C, is a protein that is encoded by the Mtnr1c gene. This receptor has been identified in fish, amphibia, and birds, but not in humans.- References :...

 or MTNR1C) has been identified in amphibia and bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s.

Expression patterns

In mammals, melatonin receptors are found in the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 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

Pars tuberalis

The pars tuberalis is part of the anterior lobe of the pituitary gland, and wraps the pituitary stalk in a highly vascularized sheath.-Histology:...

 of the pituitary gland

Pituitary gland

In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...

 and the suprachiasmatic nuclei of the hypothalamus

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

. The MT₂ subtype is mainly present in the retina

Retina

The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. The MT₃ subtype of many non-mammalian vertebrates is expressed in various brain areas.