An endocrine gland located in the brain which secretes melatonin, is strongly regulated by light stimuli, and is an important component of the circadian timing system. The pineal gland is virtually ubiquitous throughout the vertebrate animal kingdom. In nonmammalian vertebrates, it functions as a photoreceptive third eye and an endocrine organ. In mammals, it serves as an endocrine organ that is regulated by light entering the body via the eyes. Despite extensive species variation in anatomy and physiology, the pineal gland generally serves as an essential component of the circadian system which allows animals to internally measure time and coordinate physiological time-keeping with the external environment. See also Biological clocks; Brain.
The pineal gland is an unpaired organ attached by a stalk to the roof of the diencephalon. In frogs and lizards, one component of the pineal complex (the frontal organ or parietal eye) projects upward through the skull to lie under the skin; in all other vertebrates the pineal is located beneath the roof of the skull. Across evolution, cells within the pineal gland have progressed from classic photoreceptor cells in the earliest vertebrates, to rudimentary photoreceptors in birds, to classic endocrine cells in mammals. See also Photoreception; Sense organ.
In mammals, nerve fibers extend from a variety of sources in the brain to the pineal gland. The best studied of these neural inputs is through the retinohypothalamic tract, which extends from the eyes to the pineal gland in mammals. Originating in the retina, the majority of the retinohypothalamic fibers project to or around the bilateral suprachiasmatic nuclei in the hypothalamus. These nuclei serve as endogenous oscillators with period lengths close to 24 h. Thus, the suprachiasmatic nuclei function as pacemakers for the circadian system, which regulates daily physiological and behavioral rhythms. From the suprachiasmatic nuclei there are short projections to the paired paraventricular hypothalamic nuclei, and then long descending axons project from these nuclei to synapse on preganglionic sympathetic neurons in the upper thoracic spinal cord. These sympathetic neurons then extend out of the central nervous system to the superior cervical ganglia in the neck region. From there, postganglionic sympathetic axons reenter the cranium and ultimately innervate the pineal gland.
In mammals, information about environmental light and darkness is relayed from the eye to entrain circadian neural activity of the suprachiasmatic nuclei. In turn, the suprachiasmatic nuclei synchronize circadian rhythms in the pineal gland through its sympathetic innervation. One of the best-studied rhythms in the pineal gland is the biosynthesis of the hormone melatonin. Pinealocytes also have the necessary enzymes for converting tryptophan into a larger family of indole compounds, and numerous polypeptides have been localized in the pineal gland. The biological functions of these other pineal indole and peptide constituents are currently unknown.
In all vertebrate species studied, high levels of melatonin are produced and secreted during the night, while low levels are released during the day. The melatonin circadian rhythm is produced by the endogenous pacemaking activity of the suprachiasmatic nuclei, while the entrainment of this rhythm is coordinated by signals of light and darkness relayed from the eyes. Day length or photoperiod can influence the duration that melatonin production is elevated during the night. This represents a seasonal effect of light on the pineal gland. Specifically, in the summer when days are longer and nights are shorter, the duration of increased nocturnal melatonin secretion is shorter than during the winter when nights are longer. This effect of photoperiod length influencing the duration of nighttime melatonin rise has been documented in many species, including humans.
There is extensive species diversity in the capacity of melatonin to regulate physiology. Numerous species, ranging from insects to mammals, have yearly cycles of activity, morphology, reproduction, or development which are responsive to seasonal changes in day length (photoperiodism). Among many species that breed seasonally, melatonin has been shown to be a potent regulator of the reproductive axis in both males and females. The effects of melatonin on the regulation of circadian physiology has been elucidated in many vertebrate species, including humans. In addition, melatonin has been studied in different species for its influence on retinal physiology, sleep, body temperature regulation, immune function, and cardiovascular regulation.
