New horizons of pineal research

For years it was assumed that indoles, especially melatonin, were responsible for the ability of the pineal gland to inhibit pituitary gonadotrophins and thus depress sexual physiology. Recent studies have shown, however, that melatonin treatment in two species of hamsters and in the rat is equivalent to pinealectomy in terms of its effect on reproduction. For example, both pinealectomy and the subcutaneous implantation of melatonin prevents darkness from depressing reproductive functions in experimental animals. Furthermore, both treatments also block the changes in pituitary hormones which result from exposure to short daily photoperiods. Such findings suggest that the active pineal principles may be something other than indoles and, indeed, a considerable amount of evidence indicates that polypeptides may account for the pineal's ability to inhibit reproduction. A theory is presented for the cellular release of pineal polypeptides. In this scheme the pineal polypeptide hormones are exocytotically released from cells in conjunction with carrier proteins. The hormone is then exchanged for calcium resulting in the liberation of the hormone into the pineal capillaries and in the eventual deposition of calcium within the pineal gland. This theory provides a working hypothesis for the release of pineal hormonal products and explains the presence of calcified deposits within the pineal gland.