Pitutitary

Effects of Prolactin on the Luteinizing Hormone Response to Gonadotropin-Releasing Hormone in Primary Pituitary Cell Cultures During the Annual Ovine Reproductive Cycle

Abstract
In the sheep pituitary, the localization of PRL receptors in gonadotrophs and the existence of gonadotroph- lactotroph associations have provided morphological evidence for possible direct effects of PRL on gonadotropin secretion. Here, we investigated whether PRL can readily modify the LH response to GnRH throughout the ovine annual reproductive cycle. Cell populations were obtained from sheep pituitaries during the breeding season (BS) and non-breeding season (NBS), plated to monolayer cultures for 7 d and assigned to receive one of the following treatments: i) nil (Control); ii) acute (90 min) bromocriptine (ABr); iii) chronic (7 d) bromocriptine (CBr); iv) ABr and PRL; v) CBr and PRL; vi) PRL alone; or vii) TRH. Cells were treated as above, with the aim of decreasing or increasing the concentrations of PRL in the culture, and simultaneously treated with GnRH for 90 min. LH concentrations in the medium were then determined by RIA. GnRH stimulated LH in a dose-dependent manner during both stages of the annual reproductive cycle. During the NBS, single treatments did not significantly affect the LH response to GnRH. However, when PRL was combined with bromocriptine either acutely or chronically, GnRH failed to stimulate LH release at all doses tested (P<0.01). In contrast, during the BS the LH response to GnRH was not affected by any of the experimental treatments. The results reveal no apparent effects of PRL alone, but an interaction between PRL and dopamine in the regulation of LH secretion within the pituitary gland, and a seasonal modulation of this mechanism.

Key words: Pituitary • Dopamine • Luteinizing hormone • Prolactin • Seasonal reproduction

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