Estrogen-dependent, sex-specific modulation of mustard oil-induced secondary thermal hyperalgesia by orphanin FQ in the rat

Abstract

Activation of opioid receptor-like 1 receptor (ORL(1)) by intrathecal administration of orphanin FQ (OFQ), an endogenous ligand for the ORL(1) receptor, has been shown to produce antinociception. In addition, we have recently shown gonadal hormone-dependent, sex-specific modulation of acute spinal nociception such that estrogen attenuated OFQ-induced antinociception in the female whereas testosterone was required for the expression of antinociception in the male. However, sex-related differences in the role of OFQ under hyperalgesic conditions are unknown. Hence, we investigated whether OFQ produces sex-specific modulation of mustard oil-induced secondary thermal hyperalgesia in the rat. Mustard oil application to the hind limb significantly reduced the tail-flick latencies (TFL) in male, and ovariectomized (OVX), estradiol treated ovariectomized (OVX+E), proestrous (ProE) and diestrous (DiE) females. Intrathecal administration of OFQ not only attenuated mustard oil-induced decrease in TFLs, i.e. reversed hyperalgesia, but also led to a significant increase in TFLs above the baseline, i.e. produced antinociception in male, OVX, and diestrous rats. However, OFQ failed to alter TFLs in proestrous or OVX+E females, thus these two groups with elevated estrogen levels remained hyperalgesic following mustard oil treatment. These findings demonstrate that OFQ modulates mustard oil-induced secondary hyperalgesia in an estrogen-dependent, sex-specific manner.

Figure 1

A. OFQ reverses mustard oil (MO)-induced secondary thermal hyperalgesia and produces antinociception in male, and OVX animals but not in OVX+E animals. A repeated measures ANOVA conducted on tail flick latencies yielded significant main effects of Group (F_(2,18= 59.56; p < 0.001), Time (F_(32–576)= 29.73; p < 0.001), Drug (F_(1,18)= 239.19; p < 0.001), and interactions between Group × Drug (F_(2,18)= 60.67; p < 0.001) and Group × Time ×Drug (F_(64,576)= 11.18; p < 0.001). Post hoc comparisons indicated that OFQ (given 5 min. prior to mustard oil application) produced significant increases in tail-flick latencies in males (n=6) and OVX females (n=3; beginning at time 5 min) as compared to baseline latencies and mustard oil control latencies with respect to each group (all p < 0.05). Also, mustard oil alone (control) produces robust secondary thermal hyperalgesia in male, OVX, and OVX+E animals (n=4/gp) as evident by the significant decrease in tail-flick latencies from baseline of between 6 and 8 seconds (all p < 0.05). The antihyperalgesic/antinociceptive effects of OFQ persisted for up to 30 min. However, OFQ failed to modulate mustard oil-induced secondary thermal hyperalgesia in OVX+E females (n=4). B. OFQ is a substantially effective antihyperalgesic/antinociceptive agent, inhibiting mustard oil induced secondary thermal hyperalgesia and producing antinociception in male and OVX rats but not in OVX+E rats. ANOVA of the area under the curve yielded significant main effect of Group (F_(5,23) = 89.73; p < 0.001). Post hoc comparisons revealed that OFQ produced significant antihyperalgesic effects in males and OVX females against mustard oil-induced secondary thermal hyperalgesia as compared to control groups (all p < 0.05). In contrast, the antihyperalgesic effects of OFQ were not observed in the estradiol-treated OVX+E group. *p<0.05.

Figure 2

A. OFQ reverses mustard oil-induced secondary thermal hyperalgesia and produces antinociception in normally cycling females at the diestrous (low levels of circulating estrogen) but not the proestrous (high levels of circulating estrogen) stage of the estrous cycle. A repeated measures ANOVA conducted on tail flick latencies yielded significant main effects of Group (F_(1,11)= 2188; p < 0.001), Time (F_(32–352)= 13.56; p < 0.001), Drug (F_(1,11)= 1807; p < 0.001), and interactions between Group × Drug (F_(1,11)= 1650; p < 0.001) and Group × Time × Drug (F_(32,352)= 31.85; p < 0.001). Post hoc analysis comparisons revealed that OFQ (given 5 min prior to MO) significantly increased tail-flick latencies starting at time 5 minutes in DiE females (p < 0.05), but not in ProE females (n=4/gp). Post hoc analysis also indicated that mustard oil alone significantly facilitated tail-flick latencies in proestrous (n=4) and diestrous (n=3) groups (all p < 0.05). B. OFQ proves to be substantially effective antihyperalgesic/antinociceptive agent in DiE, but not in ProE rats. ANOVA of the area under the curve yielded significant main effect of Group (F_(3,14) = 481; p < 0.001). Post hoc comparisons revealed that OFQ produced significant antihyperalgesic and antinociceptive effects in DiE females against mustard oil-induced secondary thermal hyperalgesia as compared to control and ProE groups (all p < 0.05). In contrast, the antihyperalgesic effects of OFQ were not observed in ProE females. * p < 0.05.