Activation of Opioid Receptor Like-1 Receptor in the Spinal Cord Produces Sex-Specific Antinociception in the Rat: Estrogen Attenuates Antinociception in the Female, whereas Testosterone Is Required for the Expression of Antinociception in the Male

J. Claiborne, S. Nag, S. S. Mokha
2006 Journal of Neuroscience  
Sex-related differences in the perception and modulation of pain have been reported. The present study is the first to investigate systematically whether activation of opioid receptor-like 1 receptor (ORL 1 ) by orphanin FQ (OFQ) produces sex-specific modulation of spinal nociception and whether estrogen or testosterone contributes to these differences using the rat as an experimental animal. Two behavioral models, the NMDA and heat-induced nociceptive tests, were used to examine sex-specific
more » ... mine sex-specific modulation of spinal nociception. Intrathecal microinjection of OFQ in male, ovariectomized (OVX), and diestrous rats produced a significant antinociceptive effect on both tests. However, OFQ failed to produce antinociception in proestrous rats, the phase of the estrous cycle with the highest levels of circulating estradiol, and produced a dose-dependent effect in OVX females treated with 1 ng to 100 g of estradiol. The antinociceptive effects of OFQ were dose dependent in male and OVX animals and were reversibly antagonized by UFP-101 ([Nphe 1 ,Arg 14 ,Lys 15 ]N/ OFQ(1-13)-NH 2 ), an ORL 1 receptor-selective antagonist. Interestingly, OFQ was ineffective in gonadectomized (GDX) males, whereas testosterone replacement restored the antinociceptive effect of OFQ in GDX males. We conclude that OFQ produces sex-specific modulation of spinal nociception; estrogen attenuates antinociception in the female in parallel with normal cycling of estrogen levels, and testosterone is required for the expression of antinociception in the male; thus, the sensitivity of the male to the antinociceptive effects of OFQ is not simply attributable to the intrinsically low estrogen levels in these animals.
doi:10.1523/jneurosci.4783-06.2006 pmid:17167094 fatcat:d6dqvdtenrbqhmjft3s2ijzpme