Gonadotropin secretion in rats bearing a prolactin-secreting pituitary tumor: effects of naloxone administration.

In this study the effects of the implantation of the transplantable rat prolactin (PRL)-secreting pituitary tumor 7315a on gonadotropin secretion were investigated. Hyperprolactinemia was accompanied by suppressed plasma levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The total PRL content of the host's pituitary gland was decreased, but the glands of tumor-bearing animals contained greatly increased amounts of LH and FSH. Chronic administration of naloxone to tumor-bearing rats for 5 days further diminished the already suppressed total PRL content of the pituitary gland, normalized the total LH content, and did not affect the FSH content. The pituitary glands from tumor-bearing rats given naloxone showed a higher ability to release LH in vitro. Hyperprolactinemia in rats is accompanied by an increased total gonadotropin content of the pituitary gland with lowered circulating gonadotropin levels. Some of the PRL-induced changes on LH synthesis and release are mediated by opioid receptors in the hypothalamus, as naloxone administration reversed some of these effects.