Effect of cAMP on GnRH stimulated LH secretion from individual pituitary gonadotropes

Enhanced responsiveness of the pituitary gland to GnRH is a fundamental step required for secretion of the proestrous LH surge, and is achieved primarily by the actions of ovarian steroid hormones on the gonadotropes. The mechanisms involved are still unclear but the cAMP second messenger pathway can mediate some of these activities. The present study was undertaken to determine the effects of in vitro cholera toxin (CTX; increases cAMP) pretreatment of pituitary cells from proestrous and diestrous 1 rats relative to their LH secretory response to GnRH using a reverse hemolytic plaque assay. The number of gonadotropes that secrete LH in the group treated with CTX increased at low doses of GnRH and also in the absence of the peptide, but decreased at high doses, showing a dual effect: stimulation of some gonadotropes and inhibition of others. The inhibition was achieved within 3 h of pretreatment in proestrous cells but it was not seen until 20 h in diestrous 1 cells. This suggests the existence of at least two subpopulations of gonadotropes, one of which is stimulated by cAMP and another which is inhibited.