A Modulatory Role for Luteinizing Hormone-Releasing Hormone in Nociceptive Responses of Female Rats

We have shown that responsiveness to noxious stimuli change after gonadal steroid treatment and during the estrous cycle. In the present study, we evaluate the role of LHRH in modulating nociceptive responses in female rats. In ovariectomized (OVX) rats, an LHRH agonist ([ Des-Gly10] LHRH ethyl amide; 1 ng/rat/microliters), given intraventricularly (icv) at either 90, 60, or 30 min before a hot-plate test caused a time-dependent, significant increase in sensitivity to the noxious thermal stimulus (hyperalgesia) vs. saline-treated controls. Further, the LHRH agonist (1 ng/rat/microliters; icv) attenuated morphine (5 mg/kg, sc)-induced antinociception. The injection of an LHRH antagonist, [D-Phe2,Pro3,D-Phe6] LHRH, to OVX rats in doses of 0.1, 1, or 10 ng/rat 30 min prior to morphine, enhanced and prolonged morphine-induced antinociception in a dose-dependent manner. Moreover, the hyperalgesia observed in OVX rats treated with naloxone (1 mg/kg, sc) was reversed by preinjection of either the LHRH antagonist (0.1 ng/rat, icv) or LHRH antiserum. OVX rats primed with estradiol benzoate (EB) and progesterone (P) were less sensitive to the antinociceptive effect of morphine than OVX rats. When EBP-treated rats received the LHRH antagonist prior to morphine, a twofold increase in morphine-induced antinociception was observed. A similar effect was observed in EBP-treated rats after the injection of LHRH antiserum. In conclusion, LHRH may interact with central opioid systems causing an increased sensitivity to nociceptive stimulation (hyperalgesia) and reduction of the antinociceptive effect of morphine.