Pineal Gland Melatonin Secretes Hormones

The pineal gland in humans is a small (100-150mg), highly vascularized, secretory neuroendocrine organ that produces a variety of hormones. Located in the middle of the brain, outside of the blood-brain barrier, and attached to the roof of the third ventricle by a short stalk, it is a structure that has evolved. With increasing age in humans, the pineal gland typically exhibits some degree of calcification, which can be used as a good imaging marker. The sympathetic nervous system provides the majority of the innervation, which originates in the superior cervical ganglia. The pineal gland’s arterial vascularization is supplied by both the anterior and posterior circulations, with the main artery supplying the lateral pineal artery, which originates from the posterior circulation, being supplied by the anterior circulation. Pinealocytes account for 95% of the total number of cells in mammals, with scattered glial cells (astrocytic and phagocytic subtypes) accounting for the remaining 5%. Melatonin is produced and secreted by pinealocytes, which are found in the pineal gland.

Functions of  Pineal Gland

The pineal gland’s primary function is to receive and transmit information about the current light-dark cycle from the environment. Melatonin is produced and secreted by the pineal gland cyclically during the night as a result of this information transmission (dark period). Even though the pineal gland is photosensitive in cold-blooded vertebrates (lower-vertebrate species), this property is not retained in higher vertebrates such as humans. Light is sensed by the inner retina (retinal ganglion cells) in higher vertebrates, which sends neural signals to the visual areas of the brain. However, a small number of retinal ganglion cells contain melanopsin and have intrinsic photoreceptor capability, which allows them to send neural signals to non-image-forming areas of the brain, such as the pineal gland, via complex neuronal connections. The retina sends photic information to the suprachiasmatic nucleus (SCN), which is the major rhythm-generating system or “clock” in mammals, which then sends the information to the hypothalamus for processing. It is believed that when the light signal is positive, the SCN secretes the neurotransmitter gamma-aminobutyric acid, which is responsible for the inhibition of neurons that synapse in the paraventricular nucleus (PVN) of the hypothalamus. As a result, the signal from the SCN to the pineal gland is interrupted, and thus no melatonin is produced. When there is no light (darkness), on the other hand, the SCN secretes glutamate, which is responsible for the transmission of the signal from the PVN to the pineal gland through the pathway of the SCN. This nucleus communicates with the higher thoracic segments of the spinal column, transmitting information to the superior cervical ganglion, which then transmits the final signal to the pineal gland through sympathetic postsynaptic fibres, resulting in the release of norepinephrine from the pineal gland (NE). N-acetyltransferase (AA-NAT) is the enzyme that initiates melatonin production in the pinealocytes by activating transcription of the mRNA encoding the enzyme arylalkylamine N-acetyltransferase (AA-NAT), which is the first molecular step in melatonin synthesis.

Melatonin Synthesis and Metabolism  are ImportantMethylphenidate Synthesis

Melatonin (N-acetyl-5-hydroxytryptamine) is synthesised within the pinealocytes from tryptophan, with the majority of the production occurring during the dark phase of the day, when there is a significant increase in the activity of serotonin-N-acetyltransferase (arylalkylamine N-acetyltransferase, AA-NAT), which is responsible for the transformation of 5-hydroxytrypt (NAS). Finally, the enzyme acetylserotonin O-methyltransferase converts N-acetylserotonin to melatonin, which is responsible for sleep. Proteasomal proteolysis is thought to be responsible for the rapid decline in synthesis observed after exposure to light at night. Both AA-NAT and the availability of serotonin may be responsible for limiting melatonin production. The pineal gland and retina are the primary sites of AA-NAT mRNA expression, with some other brain areas, the pituitary, and the testis expressing it to a lesser extent. The activation of AA-NAT is triggered by the activation of adrenergic receptors 1 and 1b induced by NE. NE is the primary transmitter, which is released through adrenoceptors-1, with potentiation resulting from adrenoceptor-1 stimulation. NE levels are higher at night, and they are approximately 180 degrees out of phase with the serotonin rhythm, which occurs during the day. Melatonin synthesis is stimulated by the availability of both N-acetylcysteine (NE) and serotonin. The administration of -adrenergic antagonists or pathological or traumatic sympathetic denervation of the pineal gland results in the cessation of the rhythmic synthesis of melatonin as well as the control of its production by light and dark.

In addition to the skin, the gastrointestinal tract, the retina, the bone marrow, and the placenta, evidence suggests that melatonin can be synthesised in other organs and tissues of the body, acting in an autocrine or paracrine manner. Nonetheless, except for the pineal gland, these structures have little effect on circulating melatonin concentrations in mammals, as evidenced by the fact that after pinealectomy, melatonin levels are undetectable.

Conclusion

The pineal gland is a small, pea-sized organ in the centre of the brain that is an outward development of the third ventricle. It consists of two main components: hormone-producing cells known as pinealocytes and supporting cells known as glial cells that convey information.