Restoration of pituitary prolactin synthesis and release by the administration of morphine to rats bearing a transplanted prolactin-secreting tumor.

Rats bearing transplanted prolactin-secreting tumors are known to have very high prolactin levels in the serum, reduced hypothalamic opiate concentrations, increasing hypothalamic dopamine synthesis and increased hypophyseal portal blood dopamine levels. Further, it has been shown that incubation in 3H-leucine of anterior pituitary glands from these tumor-bearing rats demonstrates a marked inhibition of 3H-prolactin synthesis and release. Using rats with the prolactin-secreting tumor MtTW15, we have found that 4 days of continuous administration of either morphine sulfate 1.44 mg daily or haloperidol 0.11 mg daily using and osmotic infusion pump caused a significant increase in the synthesis and release of 3H-prolactin from incubated pituitary gland compared to untreated tumor-bearing animals. Morphine may reduce hypothalamic dopamine release, whereas haloperidol is a direct dopamine receptor antagonist in achieving these effects.