Absence of Steroid-Dependent, Endogenous Opioid Peptide Suppression of Pulsatile Luteinizing Hormone Release between Diestrus 1 and Diestrus 2 in the Rat Estrous Cycle

The objective of this study was to determine whether the negative feedback action of ovarian steroids on pulsatile luteinizing hormone (LH) release in the diestrous 1 (Dl)-diestrous 2 (D2) interval of the rat estrous cycle is mediated by endogenous opioid peptides (EOPs), by examining the pulsatile LH release response to naloxone infusions in the presence or absence of D1-D2 levels of estradiol (E₂) and progesterone (P). As plasma E₂ and P levels increased between Dl and D2, mean blood LH levels decreased due solely to a decrease in LH pulse amplitude as frequency remained stable. However, ovariectomy increased both parameters of pulsatile LH release, indicating the effect of loss of ovarian steroid-negative feedback in this interval. Replacement of D1-D2 plasma levels of E₂ and P restored D2 values for both parameters of pulsatile LH release, and E₂ + P did not alter in vivo pituitary responsiveness to LH-releasing hormone (LHRH). In ovariectomized rats lacking the negative feedback provided by E₂ + P in this cycle interval, continuous infusion of naloxone caused a further dose-dependent augmentation in both LH pulse amplitude and frequency. This stimulatory action of naloxone was prevented by simultaneous infusion with morphine, and was not associated with any change in in vivo pituitary responsiveness to LHRH, indicating that this was an action exerted through centrally located EOP receptors. Naloxone also increased both parameters of pulsatile LH release in E₂ + P-treated rats. However, the magnitudes of the naloxone-induced increments in LH pulse amplitude and frequency in ovariectomized, steroid-treated rats were not greater than those seen in ovariectomized, nonsteroid-treated rats given naloxone versus saline. In addition, mean values for both parameters of pulsatile LH secretion during EOP receptor blockade in steroid-treated rats were reduced when compared to values in ovariectomized, nonsteroid-treated rats infused with naloxone. Thus the stimulatory effect of naloxone on pulsatile LH release was similar in the presence or absence of the negative feedback action of D1-D2 plasma levels of E₂ + P. This indicates that the negative feedback effect of E₂ + P on pulsatile LH release in this interval is not mediated by EOPs whose actions are blocked by naloxone.