Effects of Cupric Acetate on Hypothalamic Gonadotropin-Releasing Hormone Release in Intact and Ovariectomized Rabbits

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The effects of intravenous injection of cupric acetate (CuAc) on release of hypothalamic gonadotropin-releasing hormone (GnRH) and pituitary luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL) were explored in conscious, ovarian intact does and in ovariectomized (OVEX) does that received subcutaneous implants, either blank Silastic capsules or Silastic capsules containing crystalline estradiol-17β (E2). Animals were subjected to push-pull perfusion of the posterior median eminence for 6 h, and CuAc was intravenously injected (2.5 mg/kg body weight) at the end of the 2nd h of push-pull perfusion. Perfusate samples were collected continuously and pooled for assay at 10-min intervals. Peripheral blood samples were obtained at 10- to 30-min intervals. Levels of GnRH in push-pull perfusate and LH, FSH, and PRL in plasma were measured by specific radioimmunoassays. In intact does (n = 4), intravenous injection of CuAc stimulated the release of hypothalamic GnRH and pituitary LH, FSH, and PRL. The initial increase and subsequent decrease in hypothalamic GnRH after CuAc injection preceded those in plasma LH, FSH, and PRL. Injection of saline into intact does (n = 4) had no effect on any of these hormone levels. In OVEX does that received blank Silastic capsules (n = 4), CuAc failed to stimulate either hypothalamic GnRH release or pituitary LH, FSH, and PRL release. In contrast, CuAc stimulated the release of all four hormones in OVEX does that received E2 containing Silastic capsules (n = 3). These results show that an increase in hypothalamic GnRH release precedes the increase in copper-induced preovulatory gonadotropin release and support the hypothesis that the neuropeptide GnRH mediates this gonadotropin release. The data also are compelling evidence that the action of CuAc requires the presence of the ovaries. One ovarian component in this neuroendocrine process probably is E2.

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