Decrease in testicular testosterone synthesis by melatonin-free pineal extract in the rat.

Administration of 2 ml of melatonin-free pineal extract (30 mg bovine pineal powder per ml) per day per animal for 12 days caused, under basal conditions, a decrease of 31% in the testicular testosterone and 33% in the esterified cholesterol, a fraction used in testosterone biosynthesis, but did not change free cholesterol. Under stimulation with 50 UI HCG for 3 days, testicular testosterone was found to be very high, esterified cholesterol low and free cholesterol unchanged. Simultaneous administration of HCG and pineal extract showed that the latter inhibits partly the testicular response to HCG. The decrease in testosterone after pineal extract administration under exogenous HCG stimulation correlates with an increase in the esterified cholesterol fraction. The effect of the melatonin-free pineal extract on testicular testosterone synthesis in the rat is discussed.