Effects of environmental lighting on the function of the pineal gland.

The pineal has undergone marked changes as vertebrates have evolved from amphibians to mammals. The amphibian pineal is a photoreceptive organ which sends nervous information to the brain via pineal nerves: the mammalian and avian pineal have no direct connections to the CNS and their metabolism is controlled by an indirect pathway involving their peripheral sympathetic nerves. In addition, the pinealocytes of some birds also function as photoendocrine transducers, i.e., they convert a photic input reaching them directly through the skull into a hormone output. Through the secretion of melatonin and of some as yet undefined low molecular weight peptides, the pineal participates in the control of other neuroendocrine rhythms such as seasonal cycles in gonadal function.