Induced ovulation and changes in pituitary responsiveness to continuous infusion of gonadotropin-releasing hormone during the ovarian cycle in the bullfrog, Rana catesbeiana

Chronic (2-4 days) constant-rate infusions of mammalian gonadotropin releasing hormone (GnRH) were performed in female bullfrogs, Rana catesbeiana. The magnitude and temporal relationship of profiles of plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH) and sex steroids testosterone (T), estradiol-17 beta (E2) and progesterone (P)] during GnRH infusion were dependent on ovarian stage. However, in all females, the same biphasic increase in plasma gonadotropins was apparent and initial elevations in gonadotropins were accompanied by correlated increments in plasma T and E2. Complete pituitary "desensitization" to chronic GnRH infusion was not observed. Females in early follicular stages were relatively unresponsive to infusions of 1.0-10.0 micrograms/h GnRH; elevations in plasma LH were marginal and FSH was unchanged. Females with fully developed (preovulatory) ovaries were more responsive: infusion of 1.0 micrograms/h GnRH produced significant elevations in plasma LH by 2 h followed by even larger increases ("surges") after 12 h. This LH "surge" was preceded by a decline in plasma T and E2 and was accompanied by abrupt elevations in plasma P and by ovulation. Postovulatory females showed a more gradual and smaller increase in plasma LH. Infusion of GnRH in the female bullfrog establishes a clear relationship between pituitary responsiveness and the ovarian cycle not evident from acute GnRH injection; GnRH was most effective immediately before ovulation. These data are also the first to detail periovulatory changes in plasma gonadotropins and ovarian steroids in an amphibian.