High postovariectomy LH levels are not due to decreased opioid inhibition of GnRH

It has been proposed that endogenous opioid peptide (EOP) inhibition of hypothalamic GnRH secretion mediates and is dependent upon gonadal steroid feedback of LH secretion, although considerable conflicting data have been reported. Accordingly, a well-characterized replacement regimen was used to approximate physiological stimulation by estradiol (E2) and progesterone (P4) in adult rats 10 days after ovariectomy (OVX), followed by in vitro incubation of the isolated median eminence to evaluate the role of E2 and P4 in modifying GnRH release in response to the opiate receptor antagonist, naloxone (NAL). Basal (control) GnRH release from median eminences of OVX, OVX + E2, and OVX + E2 + P4 rats was similar, and NAL treatment elicited a comparable increase in GnRH release under all three gonadal steroid conditions. Thus, EOP suppression of median eminence GnRH secretion does not appear to mediate or be dependent upon negative feedback regulation of LH secretion by physiological concentrations of gonadal steroids.