Effects of morphine on the serum prolactin levels of morphine-tolerant and nontolerant male rats and of the in vitro release of pituitary prolactin.

Morphine increased the serum prolactin (PRL) levels of male rats in a dose response manner. This effect was abolished by naloxone and apomorphine, but was not affected by diphenhydramine. The increase in the serum PRL levels by haloperidol was abolished by apomorphine, but not by naloxone. Repeated administrations of increasing doses of morphine attenuated the response of serum PRL to morphine. Naloxone did not alter the serum PRL levels of morphine-tolerant rats, while it precipitated full withdrawal signs on these rats. Although neither haloperidol nor morphine increased the release of PRL from the isolated anterior pituitary, haloperidol, but not morphine, reversed the inhibition by dopamine of the in vitro release of pituitary PRL. These results indicate that tolerance develops regarding the effect of morphine with a resulting increase in the serum PRL levels, abstinence precipitated by naloxone has no effect on the serum PRL levels, the mechanism of morphine involved in increase in the serum PRL is different from that of haloperidol as the effect of haloperidol is not antagonized by naloxone and morphine does not antagonize the effect of dopamine which inhibits the release of PRL from the anterior pituitary in vitro.