Research ArticlePHYSIOLOGY

Reduced MC4R signaling alters nociceptive thresholds associated with red hair

See allHide authors and affiliations

Science Advances  02 Apr 2021:
Vol. 7, no. 14, eabd1310
DOI: 10.1126/sciadv.abd1310
  • Fig. 1 Nociceptive thresholds and POMC production are affected by melanocyte status and MC1R signaling.

    Loss of function of MC1R increases mechanical and thermal nociceptive thresholds in C57BL/6J Tyrc/c albino mice (A and B) and in non-albino C57BL/6J mice (C and D). Nociceptive thresholds are decreased in K14-SCF strain compared with C57BL/6J mice measured by both von Frey assay (E) and hot plate assay (F). Mc1re/e;Mitf mi-wh/mi-wh double mutants had increased thresholds compared with C57BL/6J, but similar to Mitf mi-wh/mi-wh, by von Frey (G) and hot plate assays (H). (I) Levels of β-endorphin in blood plasma were assayed by radioimmunoassay, suggesting MC1R-dependent melanocytic regulation of POMC. (J) shMc1r in the mouse melanocytic Melan-C cell line decreases Pomc mRNA expression. ***P < 0.001 compared with all of the shControl clones and the parental line. (K) The MC1R ligand α-MSH (2 μM) increases Pomc production in Melan-C cells. (L) Pomc expression is decreased in the epidermal skin of red K14-SCF;Mc1re/e mice compared with black K14-SCF;Mc1rWT mice. Graphs show means ± SEM; numbers in parentheses = n per group. Data shown in (C) and (D) represent compilations of baseline thresholds from all black and red-haired male mice in the C57BL/6J background used in the study. Results in (E) and (F) are a compilation of two and three independent experiments, respectively. Each group in (J) and (K) consists of three biological replicates. N.S., not significant.

  • Fig. 2 Elevated nociceptive thresholds in red-haired mice depend on opioid signaling but not β-endorphin.

    β-Endorphin deletion has no effect on mechanical (A) or thermal (B) nociceptive thresholds in either black or red-haired mice (compilations of two independent experiments). Deletion of the μ-opioid receptor did not affect nociceptive thresholds in black mice but abolished the increased thresholds in red-haired mice in both mechanical (C) and thermal (D) nociception assays. (E) Naloxone treatment (10 mg/kg, intraperitoneally) abolished the increased nociceptive threshold of red mice. (F) Cyprodime treatment showed a dose-dependent effect on thermal nociception in red-haired, but not black, mice (for Mc1rWT, n = 11, except n = 10 for 0.0064 mg/kg; for Mc1re/e, n = 9, except n = 7 for 9.6 mg/kg). (G) Naloxone treatment also decreased nociceptive thresholds in Mitf mi-wh/mi-wh homozygous mice (n = 8 per group). For (A) to (D), the investigator performing the experiment was blinded to the β-endorphin or OPRM1 status of each mouse. Graphs show means ± SEM; numbers in parentheses = n per group.

  • Fig. 3 Low levels of β-endorphin are not compensated by other endogenous opioids.

    (A to C) Plasma levels of dynorphin, enkephalin, and endomorphin are unchanged in red versus black mice. (D and E) Loss of the dynorphin gene (Dyn−/−) has no effect on nociceptive thresholds in red or black mice. Graphs show means ± SEM; numbers in parentheses = n per group.

  • Fig. 4 MC4R signaling regulates nociception.

    α-MSH in blood plasma was found to be lowest in red-haired mice and Mitf mi-wh/mi-wh mice but elevated in K14-SCF mice (A), suggesting that melanocyte number and function are positively associated with plasma α-MSH level. Treatment with a melanocortin agonist, melanotan II (0.5 mg/kg), reduced thermal nociceptive thresholds in red-haired mice (B) (n = 10 per group, except n = 9 for Mc1re/e saline), while nociceptive thresholds are increased by the melanocortin antagonist SHU 9119 (1 μg/kg) in both black and K14-SCF red-haired mice (C). (D and E) Elevated redhead nociceptive thresholds are mimicked in Mc4r null mice in both mechanical and thermal nociception assays. These elevated thresholds are sensitive to naloxone (F) and naloxonazine (G) but, unlike in red-haired Mc1re/e mice (B), cannot be decreased by melanocortin rescue with melanotan II (H). (I) Elevated nociceptive thresholds are not restored by genetic deletion of Mc4r in Mc1re/e background (n = 10, 9, 20, and 6 per group). Graphs show means ± SEM; numbers in parentheses = n per group.

  • Fig. 5 Increased central opioid tone regulates basal nociception in red-haired mice.

    Blood-brain barrier (BBB)–impermeable methylnaltrexone (10 mg/kg) cannot reverse increased nociception in red Mc1re/e mice when injected peripherally (A) (n = 20 for all groups), whereas BBB-permeable naltrexone (10 mg/kg) can (B) (n = 20 per group), suggesting the involvement of CNS regions in the increased nociception. (C) In line with the coexpression of MC4R and OPRM1, melanocortin and opioid signaling can antagonistically modulate cAMP levels in primary rat hypothalamic neurons. DMSO, dimethyl sulfoxide. (D) Local administration of naloxone, naloxonazine, or melanotan to the periaqueductal gray area (PAG) reduces nociceptive thresholds in red Mc1re/e mice (n = 9 using the same mice for all treatments). (E) Summary of the antagonistic opioid-melanocortin model in black and red-haired contexts. Graphs show means ± SEM; numbers in parentheses = n per group.

Supplementary Materials

  • Supplementary Materials

    Reduced MC4R signaling alters nociceptive thresholds associated with red hair

    Kathleen C. Robinson, Lajos V. Kemény, Gillian L. Fell, Andrea L. Hermann, Jennifer Allouche, Weihua Ding, Ajay Yekkirala, Jennifer J. Hsiao, Mack Y. Su, Nicholas Theodosakis, Gabor Kozak, Yuichi Takeuchi, Shiqian Shen, Antal Berenyi, Jianren Mao, Clifford J. Woolf, David E. Fisher

    Download Supplement

    This PDF file includes:

    • Figs. S1 to S10
    • Table S1

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article