Comparative Biochemistry of the Pineal Gland

Abstract

The pineal gland is biochemically very active. In mammals, it has the unique capacity to synthesize the hormone melatonin (N-acetyl-5-methoxy-tryptamine). Although the synthesis of melatonin is confined mainly to the pineal gland of all vertebrates, the eyes and brains of amphibians and fish also can form melatonin. Melatonin is synthesized in the pineal as follows: tryptophan → 5-hydroxy-tryptophan → serotonin → N-acetylserotonin → melatonin. The final step is catalyzed by the enzyme, hydroxyindole-O-methyl transferase (HIOMT), which is highly localized in the pineal of all vertebrate species examined. The activity of HIOMT is changed when animals are kept in constant darkness or light. In rats, highest HIOMT activity is present in constant darkness, while the reverse occurs in avian species. In mammals, information about lighting reaches the pineal via the retina → inferior accessory optic tract → preganglionic sympathetic fibers → superior cervical ganglia → postganglionic fibers → pineal parenchymal cells. Lighting messages reach the hen's pineal via a nonretinal pathway. Studies with tissue culture indicate that noradrenaline liberated from sympathetic nerves stimulates synthesis of melatonin. There are circadian rhythms in pineal serotonin content which are endogenous and abolished by removal of superior cervical ganglia or by decentralization. There is also a 24-hour rhythm in pineal noradrenaline. This rhythm is exogenous and is abolished by blinding or cutting the inferior accessory optic tract.