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Gonadotropin-releasing hormone-induced desensitization may account for the decrease in pituitary responsiveness after
1Department of Physiology and Biophysics, University of Alabama, Birmingham 35294, USA.
Endocrinology
|March 1, 1996
Summary
The preovulatory surge of luteinizing hormone (LH) secretion in rats is preceded by increased estrogen and progesterone, which enhance gonadotrope responsiveness to gonadotropin-releasing hormone (GnRH). This study investigates GnRH-induced desensitization as the cause of the subsequent decline in GnRH responsiveness.
Area of Science:
- Reproductive Endocrinology
- Neuroendocrinology
- Hormonal Regulation of the Estrous Cycle
Background:
- Gonadotrope responsiveness to gonadotropin-releasing hormone (GnRH) fluctuates throughout the rat estrous cycle.
- Elevated estrogen and progesterone levels before and during the preovulatory surge enhance gonadotrope sensitivity to GnRH.
- The mechanism behind the significant decrease in GnRH responsiveness observed during or after the LH surge remains unclear.
Purpose of the Study:
- To investigate whether GnRH-induced desensitization of gonadotropes is responsible for the decline in GnRH responsiveness during the estrous cycle.
- To examine the effects of GnRH and progesterone preincubation on pituitary cell responsiveness to GnRH in rats.
Main Methods:
- Dispersed pituitary cells from proestrous and estrous rats were preincubated with GnRH, progesterone, or both.
- Cells were then subjected to the reverse hemolytic plaque assay to measure luteinizing hormone (LH) secretion in response to varying GnRH concentrations.
- This method allowed for the assessment of GnRH-induced desensitization and the role of progesterone.
Main Results:
- Estrous rat pituitary cells exhibited a significant decline (50-70%) in GnRH responsiveness compared to proestrous cells.
- Preincubation of proestrous cells with GnRH, mimicking physiological conditions, induced a similar decrease in responsiveness observed at estrus.
- Progesterone preincubation dose-dependently reduced pituitary responsiveness to GnRH, but without additive effects when combined with GnRH.
Conclusions:
- GnRH-induced desensitization of gonadotropes likely contributes to the marked decrease in pituitary responsiveness to GnRH from proestrus to estrus.
- The preovulatory surge of GnRH, while initiating the LH surge, may also induce a refractory state in gonadotropes.
- Progesterone also plays a role in modulating pituitary responsiveness to GnRH, independent of GnRH's desensitizing effects.