Effect of naloxone in the median eminence on gonadotrophin secretion in intact male rats.

To evaluate whether the median eminence (ME) is a site of action of opioids on gonadotropin secretion, we injected naloxone into the ME in male rats and measured LH and FSH serum levels. Injection into the ME of either 0.5 or 1 microgram of naloxone significantly decreased serum LH levels 10 and 30 min postinjection. A dose of 0.1 microgram had not effect. Naloxone at the doses used had no significant effect on FSH secretion. From these data it can be concluded that endogenous opioids exert a tonic stimulation on LH secretion under our experimental conditions.