Melatonin is secreted by the pineal gland. The pineal gland was described for the first time by Herophile, in the third century. He speculated it to be the sphincter that regulated flow of thought in the ventricular system of the brain. It was four and a half centuries later that Galen observed that the pineal structure appeared different to that of nervous tissue but very similar to that of the other glands. Vesal considered the gland as the center of a fine vascular system.
Descartes took up studying the gland in the 17th century. He described the pineal gland as the third eye. He described its role in the control of the photoperiod and the perception of environmental light by the body clock.
At the end of the 19th century Ahlborn and Rabl-Ruckhardt, then Graaf, Korschelt, and Spencer went further to describe the anatomy, tissue structure, nerve inputs and outputs and fetal growth and development of the mammalian pineal gland. They noted that the mammalian pineal gland had similarities with epiphysis organ of lower vertebrates. In 1905, Studnicka established that phylogenetically the pineal gland derived from a photoreceptor organ. The function was not yet discovered.
At the beginning of the 20th century the physiological role of the pineal gland was studied. Heubner found that destruction of the pineal by the tumor prevents normal reproductive development. In 1943, Bargman suggested that the endocrine function of the pineal gland was regulated by light, via the central nervous system.
In mammals, melatonin produced in the pineal gland is secreted outside the blood-brain barrier. It acts as an endocrine hormone and regulates a number of bodily functions. Melatonin produced by the retina and the gastrointestinal (GI) tract acts as a paracrine hormone.
The primary function of melatonin is regulation of the circadian rhythm. The information of environmental light or darkness reaches the suprachiasmatic nuclei (SCN) via retinal photosensitive ganglion cells. These are photosensitive photoreceptor cells. These cells represent approximately 2% of the retinal ganglion cells in humans.
Melatonin is secreted in darkness in both day-active (diurnal) and night-active (nocturnal) animals. In mammals, thus, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of neurotransmitters or chemical messengers like norepinephrine from the dense pineal sympathetic afferents. In mammals, melatonin synthesis in the retina is elevated at night and reduced during the day in a fashion similar to events in the pineal gland.
In mammals, melatonin can suppress the libido by inhibiting secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland. This is true for mammals that have a breeding season when daylight hours are long.
Those animals that are long day-breeders have melatonin as a repressor and those which are short-day breeders, the reproduction is stimulated by melatonin. At night melatonin also lowers the levels of the hormone leptin that regulated appetite and satiety.