**3. Melatonin receptors and their functions**

It has been reported that melatonin synthesized by the pineal gland has 3 groups of receptors. These are MT1 receptors with high affinity. They are MT2 and MT3 receptors with low affinity [23–25]. MT1 stimulation from these receptors suppresses adenylate cyclase activity in the target tissue. It has been determined that these receptors are involved in retinal function, cerebral artery contraction, circadian rhythm, renal function, and reproduction. As a result of the stimulation of MT2 and MT3 receptors, phosphoinositol hydrolysis takes place in the target tissue [24, 25]. These receptors are located in the body of the brain, cardiovascular system, coronary and cerebral arteries, retina, ventricular wall, aorta, liver, and gall bladder, enterocytes, cecum, skin, colon, and appendix vermiformis, parotid gland, immune system cells, exocrine pancreas, kidney, platelets, ovary/granulosa cells, myometrium, placenta, and fetal kidney.
