Melatonin's inhibition of pituitary, adrenal, testicular and accessory gland growth in male golden hamsters: pineal dependence and organ differences with shielding and intracranial surgery.

Testes, accessory glands, pituitaries and adrenal glands from 101 male Golden Hamsters (55-65 days old) were weighed after 4 weeks of daily injections of vehicle or 25 microgram or 2500 microgram of melatonin, and 32-33 days after surgery. The surgical groups within each injection group were: (1) nonoperated (NO), (2) sham-pinealectomized (S), (3) sham-pinealectomized with black plastic shielding of the pineal region (S + Pl), (4) pinealectomized (PX), and (5) pinealectomized with black plastic shielding of the pineal region (PX + Pl). All injections were made between L11 and L11.75 in a fixed LD14:10 daily photoperiod. Absolute and relative organ weights were significantly depressed by 25 but not 2500 microgram melatonin. This effect of low dose melatonin was blocked by pinealectomy (PX, PX + Pl) in all four organ groups, but was blocked as well by the sham-operation (S, S + Pl) only in the case of the adrenal glands. Effects and organ weights in S animals were not modified in the S + Pl animals. But in vehicle-injected groups the S + Pl animals had significantly lower accessory organ weights in comparison with those of NO and S groups. These results aid in the further definition of the mechanisms of melatonin's physiological actions as a chemical mediator within neuroendocrine timing controls, and show that the mechanisms for melatonin's actions can differ in relation to eventual endpoint target tissue or organ studied.