ABSTRACT
To anticipate changes in the environmental light and temperature, the organisms developed a “clock” system, which allows to properly time the physiological events. The daily period of the “circadian clock” is close to 24 hours (circa – near; dia – day) and can be precisely entrained to 24-hour period by light perceived through the photoreceptors. One of the intrinsic features of the photoreceptors is their ability to synthesize melatonin (N-acetyl-5-methoxytryptamine). The circulating amino acid L-tryptophan is the precursor for melatonin synthesis; it is converted to serotonin (5HT) by a two-step process, catalyzed by the enzymes tryptophan hydroxylase and 5-hydroxytryptophan decarboxylase. This process involves serotonin’s N-acetylation, catalysed by N-acetyltransferease (NAT), and then its methylation by hydroxyindole-O-methyltransferase (HIOMT) to produce melatonin. Melatonin production is strictly periodic, occurring only at night, and is acutely suppressed by nighttime light exposure. Apparently, photoreceptors use melatonin for their local purposes, i.e., as a paracrine agent. However, the evolution of one of the major photoreceptory organs, the pineal gland (epiphysis cerebri), led it to produce the excessive amounts of melatonin and release it into the blood stream and cerebrospinal fluid. Due to high lipid solubility and, thus, ability to cross the cell membranes, melatonin can be rapidly distributed throughout the entire organism. As a result, a periodic production of melatonin by the pineal gland became a universal endocrine message of nighttime.
