Time course of effects of morphine on hypothalamic content of LHRH and serum testosterone and LH levels of morphine-tolerant and nontolerant male rats.

Time course of the effects of morphine on the hypothalamic content of LHRH in both nontolerant and morphine-tolerant male rats was investigated in relation to the temporal changes of serum testosterone and LH levels. The hypothalamic LHRH content of the nontolerant rats was increased 8 hr after the administration of morphine (100 mg/kg) when serum testosterone levels were depressed. The LHRH content of the tolerant rats was decreased during withdrawal of morphine for 48 hr when the lowered serum testosterone and LH levels had returned to within the control levels. Although the hypothalamic LHRH content does not necessarily reflect the release of LHRH, these results are in favour of the hypothesis that the release of hypothalamic LHRH is inhibited by the administration of morphine and is restored by the withdrawal of the narcotic.