Posterior pituitary involvement in the control of luteinizing hormone and prolactin secretion during the estrous cycle.

The effects of removal of the posterior lobe of the pituitary on plasma concentrations of PRL, LH, and FSH were examined under the following conditions: on each morning of the 4-day estrous cycle, 2 weeks after ovariectomy, and 2 h after the administration of a LHRH inhibitory analog on estrus. Blood was collected from a femoral artery immediately before and during the 3 h after posterior pituitary lobectomy or sham lobectomy. Significant elevations of both PRL and LH were seen after removal of the posterior pituitary on estrus and diestrus day 1, while no change was seen in the plasma concentration of either hormone after posterior pituitary removal on diestrus day 2. On proestrus, posterior lobectomy resulted in an elevation of LH only. The posterior lobectomy-induced PRL elevations were rapid and of short duration, while those of LH were delayed and prolonged. No alteration in plasma FSH concentrations was seen after posterior pituitary lobectomy performed on any day of the estrous cycle. In contrast to the hormonal elevations observed after lobectomy during the cycle, no change in plasma PRL, LH, or FSH was seen after posterior lobectomy in ovariectomized rats. The administration of a LHRH inhibitory analog 2 h before posterior lobectomy completely abolished the lobectomy-induced rise of LH normally seen on estrus. The data suggest that the posterior pituitary participates in the regulation of PRL and LH, but not FSH, release during the estrous cycle. Given that the posterior lobectomy-induced elevations of plasma PRL and LH were evident only on certain days of the cycle and were absent in ovariectomized rats, it is likely that posterior pituitary inhibition of PRL and LH release is expressed only in the presence of specific ovarian hormonal profiles. In addition, the posterior lobectomy-induced rise in plasma LH appears to be dependent on the presence of LHRH, but it remains to be determined whether a posterior pituitary substance(s) alters hypothalamic release of or anterior pituitary responsiveness to LHRH.