[Possible mechanisms of the interaction between gonadotropins and prolactin in an experimental hyperprolactinemia model].

Abstract

The effects of grafting one extra pituitary gland beneath the kidney capsule of prepubertal male and female rats on plasma levels and pituitary content of gonadotropins and prolactin (PRL) in the adult age were studied. Further treatments with dopamine agonistic or antagonistic drugs have been performed in grafted and sham operated controls. PRL and gonadotropin values were measured by specific RIA methods using materials provided by NIAMDD. Plasma prolactin levels showed increased values 48 hr after the grafting, and maintained this elevation throughout the whole studied period. These high PRL levels in grafted rats resulted in a significant decrease of plasma LH values over the whole studied period compared with sham operated controls. Surprisingly an elevation of plasma FSH levels was detected in grafted animals, being the increase only significant in the first 3 months after the grafting. No differences could be detected from control values beyond this period. Female rats showed a constant diestrous anovulatory syndrome. Both male and female grafted rats showed diminished plasma LH and FSH responses to the i.p. LHRH administration. After EB administration control female rats showed a pulsatile response of plasma prolactin, FSH and LH with higher levels found in the afternoon. This pulsatility was abolished for PRL, diminished for LH and exaggerated for FSH in grafted animals. Male grafted rats presented a delay in the EB response pattern compared to control rats. Lisuride treatment (DA agonist) eight months after the grafting resulted in a significant reduction to plasma PRL levels. Acutely Lisuride administration restored LH response to LHRH together with a significant increase in the number of estrus in female rats. Chronic administration of the drug resulted in decreased plasma LH values not only in controls but also in grafted animals, together with an impaired response to LHRH and a significant increase in the number of diestrus in control and experimental rats. On the other hand, both acute or chronic Metoclopramide administration (DA antagonist drug) significantly increased plasma prolactin levels in control and grafted rats. This increase was accompanied by an elevation in the number of estrus in grafted female rats together with a restored LH response to LHRH higher to those observed in control female rats. These data suggest that prolactin influence on gonadotropin secretion and fertility could be mediated by a modification on dopamine secretion.