The melanocortin-1 receptor gene mediates female-specific mechanisms of analgesia in mice and humans

Abstract

Sex specificity of neural mechanisms modulating nociceptive information has been demonstrated in rodents, and these qualitative sex differences appear to be relevant to analgesia from kappa-opioid receptor agonists, a drug class reported to be clinically effective only in women. Via quantitative trait locus mapping followed by a candidate gene strategy using both mutant mice and pharmacological tools, we now demonstrate that the melanocortin-1 receptor (Mc1r) gene mediates kappa-opioid analgesia in female mice only. This finding suggested that individuals with variants of the human MC1R gene, associated in our species with red hair and fair skin, might also display altered kappa-opioid analgesia. We found that women with two variant MC1R alleles displayed significantly greater analgesia from the kappa-opioid, pentazocine, than all other groups. This study demonstrates an unexpected role for the MC1R gene, verifies that pain modulation in the two sexes involves neurochemically distinct substrates, and represents an example of a direct translation of a pharmacogenetic finding from mouse to human.

Figure 1

Linkage of distal mouse chromosome 8 to U50,488 (50 mg/kg, i.p.) analgesia in female but not male B6D2F2 mice. (a) Male and female LOD score curves, showing the logarithm of the likelihood for a presence of a QTL at that location. A free genetic model yielded a peak LOD score of 2.7 in females; additive and recessive models also yielded significant LOD scores in this sex (2.3 and 2.4, respectively). The approximate locations of microsatellite markers used are shown below the ordinate. (b) Impact on U50,488 analgesia of inheritance of the three possible genotypes (B6/B6, homozygous for B6 allele; B6/D2, heterozygous; D2/D2, homozygous for D2 allele) at D8Mit56. The time course of U50,488 analgesia was identical in all groups (data not shown), peaking at 15 min postinjection. Bars represent mean ± SEM percent total analgesia (% analgesia) over the 60-min testing period. *, Significantly different from B6/B6 group, P < 0.05.

Figure 2

Sex and genotype specificity of U50,488 analgesia dose–response relationships (10–70 mg/kg, i.p.), and the antagonistic effect of the NMDA antagonist, MK-801 (0.075 mg/kg, s.c.), in male and female mice with functional (B6, a and b) or nonfunctional (e/e, c and d) MC1Rs. Symbols (●, saline-treated; ○, MK-801-treated) represent mean ± SEM percent total analgesia (% analgesia) over the 60-min testing period. No differences were observed in baseline nociceptive sensitivity among genotypes (P = 0.12); males of both genotypes had modestly but significantly increased latencies relative to females (P < 0.005). n = 4–11 mice per genotype per sex per dose; these data have been replicated with equivalent sample sizes (not shown). See Table 1 for AD₅₀ estimates calculated from these data.

Figure 3

Effects of pharmacological blockade of MC1Rs by the peptide antagonist, Ac-Nle-Asp-Trp-DPhe-Nle-Trp-Lys-NH₂ (ANT; 20 μg per mouse, i.c.v.), on the magnitude and MK-801 sensitivity of U50,488 analgesia in mice of both sexes. Bars represent mean ± SEM percent total analgesia (% analgesia) over the 60-min testing period. The outbred Crl:CD-1 mice used in this experiment were even less sensitive to U50,488 than C57BL/6 mice (see Fig. 2), but also displayed a quantitative sex difference in analgesic magnitude (P < 0.001). There was no evidence of analgesia from the ANT alone in either sex (data not shown). n = 4–6 mice per condition. These data have been replicated in a separate laboratory (not shown). *, Significantly lower than corresponding saline group, P < 0.001. †, Significantly higher than corresponding vehicle group, P < 0.001.

Figure 4

Pentazocine analgesia by sex and MC1R genotype (two variant alleles vs. zero or one variant alleles) in humans. Bars represent mean (± SEM) difference scores for summed pain intensity ratings during each pain procedure (thermal pain, a; ischemic pain, b) were computed by subtracting postdrug from predrug ratings. No differences were observed in baseline pain responses or pain responses to saline administration across sex or genotype. *, Significantly different from other genotype within sex, P < 0.05.