Reversal by melatonin of the effect of pinealectomy on tumor growth.

Summary Pinealectomy is known to cause accelerated growth of transplanted melanoma in hamsters. The precise mechanisms involved in this action have not yet been identified. Since the pineal is the only organ that produces melatonin, this investigation was undertaken to determine the effect of this indole in the same tumor model. Administration of 0.1 mg of exogenous melatonin i.p. daily for 3 weeks did not influence the growth rate of tumors at 3- and 6-week intervals in intact animals. Administration of the same dosage of melatonin to pinealectomized animals, however, abolished the accelerating effect of pinealectomy on the growth of melanoma implants. Therefore it is concluded that the effect of pinealectomy on tumor growth is due to a deficiency in endogenous melatonin.