Peripheral and CNS Effects of the Pineal Gland: Target Enzymes Common to Tissues and Species

The pineal gland is an important transducer mediating environmental influences on endocrine organs. It has direct effects on the hypothalamus and pancreas and indirect effects on the pituitary, adrenal, thyroid, and testes. Thus, specific endocrine rhythms are affected by alterations in trophic hormone secretion by the hypothalamus-pituitary complex, direct effects on biochemical transformations within target organs, or by alterations in metabolism and excretion of hormones by the liver. Target enzymes common to the above organs for melatonin and arginine vasotocin (AVT) are 5α-reductase, monoamine oxidase (MAO), and smooth muscle enzymes. Melatonin selectively inhibited 17rβ ol-dehydrogenase activity and 5α-reductase activity while 17rβ ol-dehydrogenase was stimulated by serotonin (5-HT). Other steroid biotransformations were inhibited by both 5-HT and melatonin. Evidence from the pancreas and insulin secretion, liver and glucuronosyl transferase activity, and hypothalamic and pituitary studies indicate that melatonin mediated some of its effects on these organs through MAO activity and 5-HT levels. There were some species and tissue differences with respect to the effects of melatonin and AVT on MAO activity and steroid biotransformations. Melatonin stimulated steroid biotransformations in the duck, while MAO activity and 5α-reductase activity in the hamster responded differently to melatonin than did similar preparations from the rat.