Melatonin has been found to inhibit the response of neonatal rat pituitary tissue to luteinizing hormone releasing hormone (LH-RH) in vitro (Martin & Klein, 1976;Martin, Engel & Klein, 1977). This inhibitory effect of melatonin has been observed previously in the pituitary gland of mature male dogs in vivo (Yamashita, Mieno, Shimizu & Yamashita, 1978), but it is not known whether the inhibition is manifested in immature animals. The response of the pituitary gland of the immature male dog to exogenous LH-RH has therefore been investigated ; the rate of secretion of 17-oxosteroids by the testis in vivo was used as an index of LH release (Yamashita, 1966). Melatonin (Sigma Chemical Co.; 100 µg/kg body weight dissolved in 0-5 ml 1-6% ethanol-isotonic saline solution) was administered into the left carotid artery of immature male dogs (approximately 2-4 months old, 1-9-4-5 kg) under pentobarbitone (25 mg/kg body weight, administered i.v.) anaesthesia in acute experiments of 4 h duration using methods described previously (Yamashita et al. 1978); control dogs received vehicle only. At the same time, the left spermatic vein was cannulated (Yamashita, 1966). Three hours after the administration of melatonin, LH-RH (Protein Research Foundation; 5 or 10 µg/kg body weight in 0-5 ml 0-9% saline) was injected into the left carotid artery for 15 s. In other experiments, human chorionic gonadotrophin (HCG; CG-10, Sigma Chemical Co.; 20 i.u./kg body weight) was administered i.v. instead of LH-RH. At intervals, the entire blood flow (about 8 ml) through the left spermatic vein was collected for 8-12 min periods and the concentrations of 17-oxosteroids in samples of plasma (3 ml) were measured (Gardner, 1953;Yamashita, 1966). With the micro-Zimmermann reaction used here, the lower limit of detectability of 17-oxosteroids in a 3 ml sample of plasma was 30 ng. The rate of secretion of 17-oxosteroids by one testis was expressed as ng kg body weight-1 min-1, calculated from the concentration of 17-oxosteroids (ng/ml) in spermatic venous plasma and the rate of flow of spermatic venous plasma (ml kg body weight-1 min-1). Student's r-test was used for statistical comparisons and a value of < 005 was regarded as significant.In the control (vehicle-treated) animals, intracarotid injection of LH-RH (5 or 10 µg/kg body weight) resulted in a slight but definite increase in the testicular output of 17-oxo¬ steroids (Table 1) and the response was greater after administration of the higher dose of hormone. When either dose of LH-RH was given to animals pretreated with 100 µg melatonin/kg body weight, a decrease in the testicular responsiveness to LH-RH was observed; the increase in the secretion of 17-oxosteroids after treatment with LH-RH was smaller than that recorded in control animals not pretreated with melatonin (Table 1). The testicular responses to HCG (20 i.u./kg body weight) were of almost the same magnitude in untreated and melatonin-treated animals (Table 1).These results show that in immature, as in mature dogs, melatonin inhibits ...
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