Neuropsychopharmacology Reviews | Published:

Neuronal and glial factors contributing to sex differences in opioid modulation of pain


Morphine remains one of the most widely prescribed opioids for alleviation of persistent and/or severe pain; however, multiple preclinical and clinical studies report that morphine is less efficacious in females compared to males. Morphine primarily binds to the mu opioid receptor, a prototypical G-protein coupled receptor densely localized in the midbrain periaqueductal gray. Anatomical and physiological studies conducted in the 1960s identified the periaqueductal gray, and its descending projections to the rostral ventromedial medulla and spinal cord, as an essential descending inhibitory circuit mediating opioid-based analgesia. Remarkably, the majority of studies published over the following 30 years were conducted in males with the implicit assumption that the anatomical and physiological characteristics of this descending inhibitory circuit were comparable in females; not surprisingly, this is not the case. Several factors have since been identified as contributing to the dimorphic effects of opioids, including sex differences in the neuroanatomical and neurophysiological characteristics of the descending inhibitory circuit and its modulation by gonadal steroids. Recent data also implicate sex differences in opioid metabolism and neuroimmune signaling as additional contributing factors. Here we cohesively present these lines of evidence demonstrating a neural basis for sex differences in opioid modulation of pain, with a focus on the PAG as a sexually dimorphic core of descending opioid-induced inhibition and argue for the development of sex-specific pain therapeutics.

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  1. 1.

    Martin PR. Headaches in health care: A behavioral approach. Sydney, Australia: Grune & Stratton; 1986.

  2. 2.

    Nahin RL. Estimates of pain prevalence and severity in adults: United States, 2012. J Pain. 2015;16:769–80.

  3. 3.

    Medicine Io. Relieving pain in America: A blueprint for transforming prevention, care, education, and research. Washington, DC: The National Academies Press; 2011.

  4. 4.

    Mogil JS. Sex differences in pain and pain inhibition: multiple explanations of a controversial phenomenon. Nat Rev Neurosci. 2012;13:859–66.

  5. 5.

    Ruau D, Liu LY, Clark JD, Angst MS, Butte AJ. Sex differences in reported pain across 11,000 patients captured in electronic medical records. J Pain. 2012;13:228–34.

  6. 6.

    Unruh AM. Gender variations in clinical pain experience. Pain. 1996;65:123–67.

  7. 7.

    Buse DC, Loder EW, Gorman JA, Stewart WF, Reed ML, Fanning KM, et al. Sex differences in the prevalence, symptoms, and associated features of migraine, probable migraine and other severe headache: results of the American Migraine Prevalence and Prevention (AMPP) Study. Headache. 2013;53:1278–99.

  8. 8.

    Fillingim RB, King CD, Ribeiro-Dasilva MC, Rahim-Williams B, Riley JL 3rd. Sex, gender, and pain: a review of recent clinical and experimental findings. J Pain. 2009;10:447–85.

  9. 9.

    Kennedy J, Roll JM, Schraudner T, Murphy S, McPherson S. Prevalence of persistent pain in the U.S. Adult population: new data from the 2010 national health interview survey. J Pain. 2014;15:979–84.

  10. 10.

    Brookoff D. Chronic pain: 1. A new disease? Hosp Pract (1995). 2000;35:45–52, 59.

  11. 11.

    Brookoff D. Chronic pain: 2. The case for opioids. Hosp Pract (1995). 2000;35:69–72, 75–66, 81–64.

  12. 12.

    Boudreau D, Von Korff M, Rutter CM, Saunders K, Ray GT, Sullivan MD, et al. Trends in long-term opioid therapy for chronic non-cancer pain. Pharmacoepidemiol Drug Saf. 2009;18:1166–75.

  13. 13.

    Roggeri D, Saramin C, Terrazzani G, Zusso M, Giusti P, Chinellato A. Resource consumption and costs of treating pain in patients affected by cancer in a district of northeast Italy. Pharmacol Res. 2007;56:329–34.

  14. 14.

    Trescot AM, Glaser SE, Hansen H, Benyamin R, Patel S, Manchikanti L. Effectiveness of opioids in the treatment of chronic non-cancer pain. Pain Physician. 2008;11:S181–200.

  15. 15.

    Al-Hasani R, Bruchas MR. Molecular mechanisms of opioid receptor-dependent signaling and behavior. Anesthesiology. 2011;115:1363–81.

  16. 16.

    Sarzi-Puttini P, Vellucci R, Zuccaro SM, Cherubino P, Labianca R, Fornasari D. The appropriate treatment of chronic pain. Clin Drug Investig. 2012;32:21–33.

  17. 17.

    Labianca R, Sarzi-Puttini P, Zuccaro SM, Cherubino P, Vellucci R, Fornasari D. Adverse effects associated with non-opioid and opioid treatment in patients with chronic pain. Clin Drug Investig. 2012;32:53–63.

  18. 18.

    Pasternak GW, Pan YX. Mu opioids and their receptors: evolution of a concept. Pharmacol Rev. 2013;65:1257–317.

  19. 19.

    Angst MS, Clark JD. Opioid-induced hyperalgesia: a qualitative systematic review. Anesthesiology. 2006;104:570–87.

  20. 20.

    Bray RM, Pemberton MR, Lane ME, Hourani LL, Mattiko MJ, Babeu LA. Substance use and mental health trends among U.S. military active duty personnel: key findings from the 2008 DoD Health Behavior Survey. Mil Med. 2010;175:390–9.

  21. 21.

    Basbaum AI, Clanton CH, Fields HL. Three bulbospinal pathways from the rostral medulla of the cat: an autoradiographic study of pain modulating systems. J Comp Neurol. 1978;178:209–24.

  22. 22.

    Basbaum AI, Fields HL. Endogenous pain control mechanisms: review and hypothesis. Ann Neurol. 1978;4:451–62.

  23. 23.

    Basbaum AI, Fields HL. Endogenous pain control systems: brainstem spinal pathways and endorphin circuitry. Annu Rev Neurosci. 1984;7:309–38.

  24. 24.

    Behbehani MM, Fields HL. Evidence that an excitatory connection between the periaqueductal gray and nucleus raphe magnus mediates stimulation produced analgesia. Brain Res. 1979;170:85–93.

  25. 25.

    Behbehani MM, Pomeroy SL. Effect of morphine injected in periadueductal gray on the activity of single units in nucleus raphe magnus of the rat. Brain Res. 1978;149:266–9.

  26. 26.

    Loyd DR, Wang X, Murphy AZ. Sex differences in micro-opioid receptor expression in the rat midbrain periaqueductal gray are essential for eliciting sex differences in morphine analgesia. J Neurosci. 2008;28:14007–17.

  27. 27.

    Morgan MM, Whittier KL, Hegarty DM, Aicher SA. Periaqueductal gray neurons project to spinally projecting GABAergic neurons in the rostral ventromedial medulla. Pain. 2008;140:376–86.

  28. 28.

    Thomas A, Miller A, Roughan J, Malik A, Haylor K, Sandersen C, et al. Efficacy of intrathecal morphine in a model of surgical pain in rats. PLoS ONE. 2016;11:e0163909.

  29. 29.

    Bernal SA, Morgan MM, Craft RM. PAG mu opioid receptor activation underlies sex differences in morphine antinociception. Behav Brain Res. 2007;177:126–33.

  30. 30.

    Boyer JS, Morgan MM, Craft RM. Microinjection of morphine into the rostral ventromedial medulla produces greater antinociception in male compared to female rats. Brain Res. 1998;796:315–8.

  31. 31.

    Jensen TS, Yaksh TL. Comparison of antinociceptive action of morphine in the periaqueductal gray, medial and paramedial medulla in rat. Brain Res. 1986;363:99–113.

  32. 32.

    Kumar A, Liu NJ, Madia PA, Gintzler AR. Contribution of endogenous spinal endomorphin 2 to intrathecal opioid antinociception in rats is agonist dependent and sexually dimorphic. J Pain. 2015;16:1200–10.

  33. 33.

    Zambotti F, Zonta N, Parenti M, Tommasi R, Vicentini L, Conci F, et al. Periaqueductal gray matter involvement in the muscimol-induced decrease of morphine antinociception. Naunyn Schmiede Arch Pharmacol. 1982;318:368–9.

  34. 34.

    Lane DA, Patel PA, Morgan MM. Evidence for an intrinsic mechanism of antinociceptive tolerance within the ventrolateral periaqueductal gray of rats. Neuroscience. 2005;135:227–34.

  35. 35.

    Morgan MM, Fossum EN, Levine CS, Ingram SL. Antinociceptive tolerance revealed by cumulative intracranial microinjections of morphine into the periaqueductal gray in the rat. Pharmacol Biochem Behav. 2006;85:214–9.

  36. 36.

    Siuciak JA, Advokat C. Tolerance to morphine microinjections in the periaqueductal gray (PAG) induces tolerance to systemic, but not intrathecal morphine. Brain Res. 1987;424:311–9.

  37. 37.

    Tortorici V, Morgan MM, Vanegas H. Tolerance to repeated microinjection of morphine into the periaqueductal gray is associated with changes in the behavior of off- and on-cells in the rostral ventromedial medulla of rats. Pain. 2001;89:237–44.

  38. 38.

    Tortorici V, Robbins CS, Morgan MM. Tolerance to the antinociceptive effect of morphine microinjections into the ventral but not lateral-dorsal periaqueductal gray of the rat. Behav Neurosci. 1999;113:833–9.

  39. 39.

    Jacquet YF, Lajtha A. The periaqueductal gray: site of morphine analgesia and tolerance as shown by 2-way cross tolerance between systemic and intracerebral injections. Brain Res. 1976;103:501–13.

  40. 40.

    Pereira EA, Lu G, Wang S, Schweder PM, Hyam JA, Stein JF, et al. Ventral periaqueductal grey stimulation alters heart rate variability in humans with chronic pain. Exp Neurol. 2010;223:574–81.

  41. 41.

    Dampney RA, Furlong TM, Horiuchi J, Iigaya K. Role of dorsolateral periaqueductal grey in the coordinated regulation of cardiovascular and respiratory function. Auton Neurosci. 2013;175:17–25.

  42. 42.

    Sorge RE, Totsch SK. Sex differences in pain. J Neurosci Res. 2017;95:1271–81.

  43. 43.

    Miller PL, Ernst AA. Sex differences in analgesia: a randomized trial of mu versus kappa opioid agonists. South Med J. 2004;97:35–41.

  44. 44.

    Mogil JS, Bailey AL. Sex and gender differences in pain and analgesia. Prog Brain Res. 2010;186:141–57.

  45. 45.

    Berkley KJ. Sex differences in pain. Behav Brain Sci. 1997;20:371–80. discussion 435–513.

  46. 46.

    Cepeda MS, Carr DB. Women experience more pain and require more morphine than men to achieve a similar degree of analgesia. Anesth Analg. 2003;97:1464–8.

  47. 47.

    Aubrun F, Salvi N, Coriat P, Riou B. Sex- and age-related differences in morphine requirements for postoperative pain relief. Anesthesiology. 2005;103:156–60.

  48. 48.

    Sarton E, Olofsen E, Romberg R, den Hartigh J, Kest B, Nieuwenhuijs D, et al. Sex differences in morphine analgesia: an experimental study in healthy volunteers. Anesthesiology. 2000;93:1245–54. discussion 1246A

  49. 49.

    Bijur PE, Esses D, Birnbaum A, Chang AK, Schechter C, Gallagher EJ. Response to morphine in male and female patients: analgesia and adverse events. Clin J Pain. 2008;24:192–8.

  50. 50.

    Fillingim RB, Ness TJ, Glover TL, Campbell CM, Hastie BA, Price DD, et al. Morphine responses and experimental pain: sex differences in side effects and cardiovascular responses but not analgesia. J Pain. 2005;6:116–24.

  51. 51.

    Glasson JC, Sawyer WT, Lindley CM, Ginsberg B. Patient-specific factors affecting patient-controlled analgesia dosing. J Pain Palliat Care Pharmacother. 2002;16:5–21.

  52. 52.

    Myles PS, Hunt JO, Moloney JT. Postoperative ‘minor’ complications. Comparison between men and women. Anaesthesia. 1997;52:300–6.

  53. 53.

    Cepeda MS, Farrar JT, Baumgarten M, Boston R, Carr DB, Strom BL. Side effects of opioids during short-term administration: effect of age, gender, and race. Clin Pharmacol Ther. 2003;74:102–12.

  54. 54.

    Comer SD, Cooper ZD, Kowalczyk WJ, Sullivan MA, Evans SM, Bisaga AM, et al. Evaluation of potential sex differences in the subjective and analgesic effects of morphine in normal, healthy volunteers. Psychopharmacology (Berl). 2010;208:45–55.

  55. 55.

    Pool GJ, Schwegler AF, Theodore BR, Fuchs PN. Role of gender norms and group identification on hypothetical and experimental pain tolerance. Pain. 2007;129:122–9.

  56. 56.

    Edwards R, Eccleston C, Keogh E. Observer influences on pain: an experimental series examining same-sex and opposite-sex friends, strangers, and romantic partners. Pain. 2017;158:846–55.

  57. 57.

    Belfer I, Segall SK, Lariviere WR, Smith SB, Dai F, Slade GD, et al. Pain modality- and sex-specific effects of COMT genetic functional variants. Pain. 2013;154:1368–76.

  58. 58.

    Gatchel RJ, Peng YB, Peters ML, Fuchs PN, Turk DC. The biopsychosocial approach to chronic pain: scientific advances and future directions. Psychol Bull. 2007;133:581–624.

  59. 59.

    Loyd DR, Murphy AZ. Sex differences in the anatomical and functional organization of the periaqueductal gray-rostral ventromedial medullary pathway in the rat: a potential circuit mediating the sexually dimorphic actions of morphine. J Comp Neurol. 2006;496:723–38.

  60. 60.

    Posillico CK, Terasaki LS, Bilbo SD, Schwarz JM. Examination of sex and minocycline treatment on acute morphine-induced analgesia and inflammatory gene expression along the pain pathway in Sprague-Dawley rats. Biol Sex Differ. 2015;6:33.

  61. 61.

    Wang X, Traub RJ, Murphy AZ. Persistent pain model reveals sex difference in morphine potency. Am J Physiol Regul Integr Comp Physiol. 2006;291:R300–6.

  62. 62.

    Cicero TJ, Nock B, O’Connor L, Meyer ER. Role of steroids in sex differences in morphine-induced analgesia: activational and organizational effects. J Pharmacol Exp Ther. 2002;300:695–701.

  63. 63.

    Craft RM, Stratmann JA, Bartok RE, Walpole TI, King SJ. Sex differences in development of morphine tolerance and dependence in the rat. Psychopharmacology (Berl). 1999;143:1–7.

  64. 64.

    Holtman JR Jr., Jing X, Wala EP. Sex-related differences in the enhancement of morphine antinociception by NMDA receptor antagonists in rats. Pharmacol Biochem Behav. 2003;76:285–93.

  65. 65.

    Ji Y, Murphy AZ, Traub RJ. Sex differences in morphine-induced analgesia of visceral pain are supraspinally and peripherally mediated. Am J Physiol Regul Integr Comp Physiol. 2006;291:R307–14.

  66. 66.

    Kepler KL, Kest B, Kiefel JM, Cooper ML, Bodnar RJ. Roles of gender, gonadectomy and estrous phase in the analgesic effects of intracerebroventricular morphine in rats. Pharmacol Biochem Behav. 1989;34:119–27.

  67. 67.

    Krzanowska EK, Ogawa S, Pfaff DW, Bodnar RJ. Reversal of sex differences in morphine analgesia elicited from the ventrolateral periaqueductal gray in rats by neonatal hormone manipulations. Brain Res. 2002;929:1–9.

  68. 68.

    Stoffel EC, Ulibarri CM, Craft RM. Gonadal steroid hormone modulation of nociception, morphine antinociception and reproductive indices in male and female rats. Pain. 2003;103:285–302.

  69. 69.

    Craft RM. Sex differences in opioid analgesia: “from mouse to man”. Clin J Pain. 2003;19:175–86.

  70. 70.

    Craft RM, Mogil JS, Aloisi AM. Sex differences in pain and analgesia: the role of gonadal hormones. Eur J Pain. 2004;8:397–411.

  71. 71.

    Doyle HH, Eidson LN, Sinkiewicz DM, Murphy AZ. Sex differences in microglia activity within the periaqueductal gray of the rat: a potential mechanism driving the dimorphic effects of morphine. J Neurosci. 2017;37:3202–14.

  72. 72.

    Doyle HH, Murphy AZ. Sex-dependent influences of morphine and its metabolites on pain sensitivity in the rat. Physiol Behav. 2018;187:32–41.

  73. 73.

    Krzanowska EK, Bodnar RJ. Morphine antinociception elicited from the ventrolateral periaqueductal gray is sensitive to sex and gonadectomy differences in rats. Brain Res. 1999;821:224–30.

  74. 74.

    Murphy AZ, Suckow SK, Johns M, Traub RJ. Sex differences in the activation of the spinoparabrachial circuit by visceral pain. Physiol Behav. 2009;97:205–12.

  75. 75.

    Ji Y, Murphy AZ, Traub RJ. Estrogen modulation of morphine analgesia of visceral pain in female rats is supraspinally and peripherally mediated. J Pain. 2007;8:494–502.

  76. 76.

    Ji Y, Tang B, Traub RJ. The visceromotor response to colorectal distention fluctuates with the estrous cycle in rats. Neuroscience. 2008;154:1562–7.

  77. 77.

    Larauche M, Mulak A, Kim YS, Labus J, Million M, Tache Y. Visceral analgesia induced by acute and repeated water avoidance stress in rats: sex difference in opioid involvement. Neurogastroenterol Motil. 2012;24:1031–547.

  78. 78.

    Holtman JR Jr, Wala EP. Characterization of morphine-induced hyperalgesia in male and female rats. Pain. 2005;114:62–70.

  79. 79.

    Juni A, Klein G, Kowalczyk B, Ragnauth A, Kest B. Sex differences in hyperalgesia during morphine infusion: effect of gonadectomy and estrogen treatment. Neuropharmacology. 2008;54:1264–70.

  80. 80.

    Barrett AC, Smith ES, Picker MJ. Sex-related differences in mechanical nociception and antinociception produced by mu- and kappa-opioid receptor agonists in rats. Eur J Pharmacol. 2002;452:163–73.

  81. 81.

    Peckham EM, Traynor JR. Comparison of the antinociceptive response to morphine and morphine-like compounds in male and female Sprague-Dawley rats. J Pharmacol Exp Ther. 2006;316:1195–201.

  82. 82.

    Stoffel EC, Ulibarri CM, Folk JE, Rice KC, Craft RM. Gonadal hormone modulation of mu, kappa, and delta opioid antinociception in male and female rats. J Pain. 2005;6:261–74.

  83. 83.

    Terner JM, Lomas LM, Smith ES, Barrett AC, Picker MJ. Pharmacogenetic analysis of sex differences in opioid antinociception in rats. Pain. 2003;106:381–91.

  84. 84.

    Bai X, Zhang X, Li Y, Lu L, Li B, He X. Sex differences in peripheral mu-opioid receptor mediated analgesia in rat orofacial persistent pain model. PLoS ONE. 2015;10:e0122924.

  85. 85.

    Islam AK, Beczkowska IW, Bodnar RJ. Interactions among aging, gender, and gonadectomy effects upon naloxone hypophagia in rats. Physiol Behav. 1993;54:981–92.

  86. 86.

    Kepler KL, Standifer KM, Paul D, Kest B, Pasternak GW, Bodnar RJ. Gender effects and central opioid analgesia. Pain. 1991;45:87–94.

  87. 87.

    Ali BH, Sharif SI, Elkadi A. Sex differences and the effect of gonadectomy on morphine-induced antinociception and dependence in rats and mice. Clin Exp Pharmacol Physiol. 1995;22:342–4.

  88. 88.

    Loyd DR, Murphy AZ. The role of the periaqueductal gray in the modulation of pain in males and females: are the anatomy and physiology really that different? Neural Plast. 2009;2009:462879.

  89. 89.

    Reynolds DV. Surgery in the rat during electrical analgesia induced by focal brain stimulation. Science. 1969;164:444–5.

  90. 90.

    Behbehani MM. Functional characteristics of the midbrain periaqueductal gray. Prog Neurobiol. 1995;46:575–605.

  91. 91.

    Budai D, Harasawa I, Fields HL. Midbrain periaqueductal gray (PAG) inhibits nociceptive inputs to sacral dorsal horn nociceptive neurons through alpha2-adrenergic receptors. J Neurophysiol. 1998;80:2244–54.

  92. 92.

    Heinricher MM, Cheng ZF, Fields HL. Evidence for two classes of nociceptive modulating neurons in the periaqueductal gray. J Neurosci. 1987;7:271–8.

  93. 93.

    McCarthy MM, Pfaff DW, Schwartz-Giblin S. Midbrain central gray GABAA receptor activation enhances, and blockade reduces, sexual behavior in the female rat. Exp Brain Res. 1991;86:108–16.

  94. 94.

    Murphy AZ, Hoffman GE. Distribution of gonadal steroid receptor-containing neurons in the preoptic-periaqueductal gray-brainstem pathway: a potential circuit for the initiation of male sexual behavior. J Comp Neurol. 2001;438:191–12.

  95. 95.

    Ogawa S, Kow L-M, McCarthy MM, Pfaff DW, Schwartz-Giblin S. Midbrain PAG control of female reproductive behavior: in vitro electrophysiological characterization of actions of lordosis-relevant substances. In: Depaulis A, Bandler R, editors. The midbrain periaqueductal gray matter. New York, NY: Plenum Press; 1991. p. 211–38.

  96. 96.

    Daniels D, Miselis RR, Flanagan-Cato LM. Central neuronal circuit innervating the lordosis-producing muscles defined by transneuronal transport of pseudorabies virus. J Neurosci. 1999;19:2823–33.

  97. 97.

    Kim JJ, Rison RA, Fanselow MS. Effects of amygdala, hippocampus, and periaqueductal gray lesions on short- and long-term contextual fear. Behav Neurosci. 1993;107:1093–8.

  98. 98.

    Scordalakes EM, Rissman EF. Aggression and arginine vasopressin immunoreactivity regulation by androgen receptor and estrogen receptor alpha. Genes Brain Behav. 2004;3:20–26.

  99. 99.

    Depaulis A, Keay KA, Bandler R. Longitudinal neuronal organization of defensive reactions in the midbrain periaqueductal gray region of the rat. Exp Brain Res. 1992;90:307–18.

  100. 100.

    Bandler R, Carrive P. Integrated defence reaction elicited by excitatory amino acid microinjection in the midbrain periaqueductal grey region of the unrestrained cat. Brain Res. 1988;439:95–106.

  101. 101.

    Bandler R, Depaulis A, Vergnes M. Identification of midbrain neurones mediating defensive behaviour in the rat by microinjections of excitatory amino acids. Behav Brain Res. 1985;15:107–19.

  102. 102.

    Zhang SP, Davis PJ, Bandler R, Carrive P. Brain stem integration of vocalization: role of the midbrain periaqueductal gray. J Neurophysiol. 1994;72:1337–56.

  103. 103.

    Davis PJ, Zhang SP, Bandler R. Pulmonary and upper airway afferent influences on the motor pattern of vocalization evoked by excitation of the midbrain periaqueductal gray of the cat. Brain Res. 1993;607:61–80.

  104. 104.

    Dogrul A, Yesilyurt O, Isimer A, Guzeldemir ME. L-type and T-type calcium channel blockade potentiate the analgesic effects of morphine and selective mu opioid agonist, but not to selective delta and kappa agonist at the level of the spinal cord in mice. Pain. 2001;93:61–8.

  105. 105.

    Vaughan CW, Christie MJ. Presynaptic inhibitory action of opioids on synaptic transmission in the rat periaqueductal grey in vitro. J Physiol. 1997;498:463–72.

  106. 106.

    Vaughan CW, Ingram SL, Connor MA, Christie MJ. How opioids inhibit GABA-mediated neurotransmission. Nature. 1997;390:611–4.

  107. 107.

    Bagley EE, Chieng BC, Christie MJ, Connor M. Opioid tolerance in periaqueductal gray neurons isolated from mice chronically treated with morphine. Br J Pharmacol. 2005;146:68–76.

  108. 108.

    Behbehani MM, Jiang MR, Chandler SD, Ennis M. The effect of GABA and its antagonists on midbrain periaqueductal gray neurons in the rat. Pain. 1990;40:195–204.

  109. 109.

    Wang H, Wessendorf MW. Mu- and delta-opioid receptor mRNAs are expressed in periaqueductal gray neurons projecting to the rostral ventromedial medulla. Neuroscience. 2002;109:619–34.

  110. 110.

    Commons KG, Aicher SA, Kow LM, Pfaff DW. Presynaptic and postsynaptic relations of mu-opioid receptors to gamma-aminobutyric acid-immunoreactive and medullary-projecting periaqueductal gray neurons. J Comp Neurol. 2000;419:532–42.

  111. 111.

    Commons KG, van Bockstaele EJ, Pfaff DW. Frequent colocalization of mu opioid and NMDA-type glutamate receptors at postsynaptic sites in periaqueductal gray neurons. J Comp Neurol. 1999;408:549–59.

  112. 112.

    Gutstein HB, Mansour A, Watson SJ, Akil H, Fields HL. Mu and kappa opioid receptors in periaqueductal gray and rostral ventromedial medulla. Neuroreport. 1998;9:1777–81.

  113. 113.

    Sandkuhler J, Willmann E, Fu QG. Blockade of GABAA receptors in the midbrain periaqueductal gray abolishes nociceptive spinal dorsal horn neuronal activity. Eur J Pharmacol. 1989;160:163–6.

  114. 114.

    Yeung JC, Yaksh TL, Rudy TA. Concurrent mapping of brain sites for sensitivity to the direct application of morphine and focal electrical stimulation in the production of antinociception in the rat. Pain. 1977;4:23–40.

  115. 115.

    Lau BK, Vaughan CW. Descending modulation of pain: the GABA disinhibition hypothesis of analgesia. Curr Opin Neurobiol. 2014;29:159–64.

  116. 116.

    Heinricher MM, Tavares I, Leith JL, Lumb BM. Descending control of nociception: Specificity, recruitment and plasticity. Brain Res Rev. 2009;60:214–25.

  117. 117.

    Vaughan CW, Bagley EE, Drew GM, Schuller A, Pintar JE, Hack SP, et al. Cellular actions of opioids on periaqueductal grey neurons from C57B16/J mice and mutant mice lacking MOR-1. Br J Pharmacol. 2003;139:362–7.

  118. 118.

    Moreau JL, Fields HL. Evidence for GABA involvement in midbrain control of medullary neurons that modulate nociceptive transmission. Brain Res. 1986;397:37–46.

  119. 119.

    Park C, Kim JH, Yoon BE, Choi EJ, Lee CJ, Shin HS. T-type channels control the opioidergic descending analgesia at the low threshold-spiking GABAergic neurons in the periaqueductal gray. Proc Natl Acad Sci USA. 2010;107:14857–62.

  120. 120.

    Osborne PB, Vaughan CW, Wilson HI, Christie MJ. Opioid inhibition of rat periaqueductal grey neurones with identified projections to rostral ventromedial medulla in vitro. J Physiol. 1996;490:383–9.

  121. 121.

    Samineni VK, Grajales-Reyes JG, Copits BA, O’Brien DE, Trigg SL, Gomez AM, et al. Divergent modulation of nociception by glutamatergic and gabaergic neuronal subpopulations in the periaqueductal gray. eNeuro. 2017;4:0129–16.

  122. 122.

    Loyd DR, Morgan MM, Murphy AZ. Morphine preferentially activates the periaqueductal gray-rostral ventromedial medullary pathway in the male rat: a potential mechanism for sex differences in antinociception. Neuroscience. 2007;147:456–68.

  123. 123.

    Shipley M, Ennis M, Rizvi TA. Topographical specificity of forebrain inputs to the midbrain periaqueductal gray: evidence for discrete longitudinally organized input columns. In: Depaulis A, Bandler R, editors. The midbrain periaqueductal gray matter. New York, NY: Plenum Press; 1991. p. 417–48.

  124. 124.

    Beitz AJ. The organization of afferent projections to the midbrain periaqueductal gray of the rat. Neuroscience. 1982;7:133–59.

  125. 125.

    Loyd DR, Murphy AZ. Forbrain modulation of the periaqueductal gray and its modulation by pain. In: Gebhart GF, Schmidt RF, editors. Encyclopedia of Pain. Vol. LXXVI. Berlin, Heidelberg: Springer-Verlag; 2013.

  126. 126.

    Loyd DR, Morgan MM, Murphy AZ. Sexually dimorphic activation of the periaqueductal gray-rostral ventromedial medullary circuit during the development of tolerance to morphine in the rat. Eur J Neurosci. 2008;27:1517–24.

  127. 127.

    Hack SP, Vaughan CW, Christie MJ. Modulation of GABA release during morphine withdrawal in midbrain neurons in vitro. Neuropharmacology. 2003;45:575–84.

  128. 128.

    Wang X, Loram LC, Ramos K, de Jesus AJ, Thomas J, Cheng K, et al. Morphine activates neuroinflammation in a manner parallel to endotoxin. Proc Natl Acad Sci USA. 2012;109:6325–30.

  129. 129.

    Eidson LN, Murphy AZ. Blockade of Toll-like receptor 4 attenuates morphine tolerance and facilitates the pain relieving properties of morphine. J Neurosci. 2013a;33:15952–63.

  130. 130.

    Lehnardt S, Massillon L, Follett P, Jensen FE, Ratan R, Rosenberg PA, et al. Activation of innate immunity in the CNS triggers neurodegeneration through a Toll-like receptor 4-dependent pathway. Proc Natl Acad Sci USA. 2003;100:8514–9.

  131. 131.

    Jou I, Lee JH, Park SY, Yoon HJ, Joe EH, Park EJ. Gangliosides trigger inflammatory responses via TLR4 in brain glia. Am J Pathol. 2006;168:1619–30.

  132. 132.

    Lewis SS, Hutchinson MR, Rezvani N, Loram LC, Zhang Y, Maier SF, et al. Evidence that intrathecal morphine-3-glucuronide may cause pain enhancement via toll-like receptor 4/MD-2 and interleukin-1beta. Neuroscience. 2010;165:569–83.

  133. 133.

    Hutchinson MR, Zhang Y, Shridhar M, Evans JH, Buchanan MM, Zhao TX, et al. Evidence that opioids may have toll-like receptor 4 and MD-2 effects. Brain Behav Immun. 2010;24:83–95.

  134. 134.

    Eidson LN, Murphy AZ. Persistent peripheral inflammation attenuates morphine-induced periaqueductal gray glial cell activation and analgesic tolerance in the male rat. J Pain. 2013b;14:393–404.

  135. 135.

    Eidson LN, Inoue K, Young LJ, Tansey MG, Murphy AZ. Toll-like receptor 4 mediates morphine-induced neuroinflammation and tolerance via soluble tumor necrosis factor signaling. Neuropsychopharmacology. 2017;42:661–70.

  136. 136.

    Ferrini F, Trang T, Mattioli TA, Laffray S, Del’Guidice T, Lorenzo LE, et al. Morphine hyperalgesia gated through microglia-mediated disruption of neuronal Cl(-) homeostasis. Nat Neurosci. 2013;16:183–92.

  137. 137.

    Corder G, Tawfik VL, Wang D, Sypek EI, Low SA, Dickinson JR, et al. Loss of mu opioid receptor signaling in nociceptors, but not microglia, abrogates morphine tolerance without disrupting analgesia. Nat Med. 2017;23:164–73.

  138. 138.

    Mattioli TA, Leduc-Pessah H, Skelhorne-Gross G, Nicol CJ, Milne B, Trang T, et al. Toll-like receptor 4 mutant and null mice retain morphine-induced tolerance, hyperalgesia, and physical dependence. PLoS ONE. 2014;9:e97361.

  139. 139.

    Fukagawa H, Koyama T, Kakuyama M, Fukuda K. Microglial activation involved in morphine tolerance is not mediated by toll-like receptor 4. J Anesth. 2013;27:93–7.

  140. 140.

    Ratka A, Simpkins JW. Effects of estradiol and progesterone on the sensitivity to pain and on morphine-induced antinociception in female rats. Horm Behav. 1991;25:217–28.

  141. 141.

    Terner JM, Barrett AC, Grossell E, Picker MJ. Influence of gonadectomy on the antinociceptive effects of opioids in male and female rats. Psychopharmacology (Berl). 2002;163:183–93.

  142. 142.

    Terner JM, Lomas LM, Picker MJ. Influence of estrous cycle and gonadal hormone depletion on nociception and opioid antinociception in female rats of four strains. J Pain. 2005;6:372–83.

  143. 143.

    Kiefel JM, Bodnar RJ. Roles of gender and gonadectomy in pilocarpine and clonidine analgesia in rats. Pharmacol Biochem Behav. 1992;41:153–8.

  144. 144.

    Okamoto K, Tashiro A, Hirata H, Bereiter DA. Differential modulation of TMJ neurons in superficial laminae of trigeminal subnucleus caudalis/upper cervical cord junction region of male and cycling female rats by morphine. Pain. 2005;114:203–1.

  145. 145.

    Murphy AZ, Hoffman GE. Distribution of androgen and estrogen receptor containing neurons in the male rat periaqueductal gray. Horm Behav. 1999;36:98–108.

  146. 146.

    VanderHorst VG, Schasfoort FC, Meijer E, van Leeuwen FW, Holstege G. Estrogen receptor-alpha-immunoreactive neurons in the periaqueductal gray of the adult ovariectomized female cat. Neurosci Lett. 1998;240:13–6.

  147. 147.

    Boers J, Gerrits PO, Meijer E, Holstege G. Estrogen receptor-alpha-immunoreactive neurons in the mesencephalon, pons and medulla oblongata of the female golden hamster. Neurosci Lett. 1999;267:17–20.

  148. 148.

    Turcotte JC, Blaustein JD. Immunocytochemical localization of midbrain estrogen receptor- and progestin receptor-containing cells in female guinea pigs. J Comp Neurol. 1993;328:76–87.

  149. 149.

    Vanderhorst VG, Terasawa E, Ralston HJ 3rd. Estrogen receptor-alpha immunoreactive neurons in the ventrolateral periaqueductal gray receive monosynaptic input from the lumbosacral cord in the rhesus monkey. J Comp Neurol. 2002;443:27–42.

  150. 150.

    VanderHorst VG, Terasawa E, Ralston HJ 3rd. Projections from estrogen receptor-alpha immunoreactive neurons in the periaqueductal gray to the lateral medulla oblongata in the rhesus monkey. Neuroscience. 2004;125:243–53.

  151. 151.

    Loyd DR, Murphy AZ. Androgen and estrogen (alpha) receptor localization on periaqueductal gray neurons projecting to the rostral ventromedial medulla in the male and female rat. J Chem Neuroanat. 2008;36:216–26.

  152. 152.

    Kelly MJ, Qiu J, Ronnekleiv OK. Estrogen modulation of G-protein-coupled receptor activation of potassium channels in the central nervous system. Ann N Y Acad Sci. 2003;1007:6–16.

  153. 153.

    Eckersell CB, Popper P, Micevych PE. Estrogen-induced alteration of mu-opioid receptor immunoreactivity in the medial preoptic nucleus and medial amygdala. J Neurosci. 1998;18:3967–76.

  154. 154.

    Micevych PE, Rissman EF, Gustafsson JA, Sinchak K. Estrogen receptor-alpha is required for estrogen-induced mu-opioid receptor internalization. J Neurosci Res. 2003;71:802–10.

  155. 155.

    Sun J, Liu S, Mata M, Fink DJ, Hao S. Transgene-mediated expression of tumor necrosis factor soluble receptor attenuates morphine tolerance in rats. Gene Ther. 2012;19:101–8.

  156. 156.

    Watkins LR, Hutchinson MR, Johnston IN, Maier SF. Glia: novel counter-regulators of opioid analgesia. Trends Neurosci. 2005;28:661–9.

  157. 157.

    Watkins LR, Hutchinson MR, Rice KC, Maier SF. The “toll” of opioid-induced glial activation: improving the clinical efficacy of opioids by targeting glia. Trends Pharmacol Sci. 2009;30:581–91.

  158. 158.

    Watkins LR, Milligan ED, Maier SF. Spinal cord glia: new players in pain. Pain. 2001;93:201–5.

  159. 159.

    Hao S, Liu S, Zheng X, Zheng W, Ouyang H, Mata M, et al. The role of TNFalpha in the periaqueductal gray during naloxone-precipitated morphine withdrawal in rats. Neuropsychopharmacology. 2011;36:664–76.

  160. 160.

    Hutchinson MR, Bland ST, Johnson KW, Rice KC, Maier SF, Watkins LR. Opioid-induced glial activation: mechanisms of activation and implications for opioid analgesia, dependence, and reward. ScientificWorldJournal. 2007;7:98–111.

  161. 161.

    Hutchinson MR, Coats BD, Lewis SS, Zhang Y, Sprunger DB, Rezvani N, et al. Proinflammatory cytokines oppose opioid-induced acute and chronic analgesia. Brain Behav Immun. 2008;22:1178–89.

  162. 162.

    Hutchinson MR, Northcutt AL, Chao LW, Kearney JJ, Zhang Y, Berkelhammer DL, et al. Minocycline suppresses morphine-induced respiratory depression, suppresses morphine-induced reward, and enhances systemic morphine-induced analgesia. Brain Behav Immun. 2008;22:1248–56.

  163. 163.

    Hutchinson MR, Zhang Y, Brown K, Coats BD, Shridhar M, Sholar PW, et al. Non-stereoselective reversal of neuropathic pain by naloxone and naltrexone: involvement of toll-like receptor 4 (TLR4). Eur J Neurosci. 2008;28:20–9.

  164. 164.

    Sorge RE, LaCroix-Fralish ML, Tuttle AH, Sotocinal SG, Austin JS, Ritchie J, et al. Spinal cord Toll-like receptor 4 mediates inflammatory and neuropathic hypersensitivity in male but not female mice. J Neurosci. 2011;31:15450–4.

  165. 165.

    Grace PM, Strand KA, Galer EL, Urban DJ, Wang X, Baratta MV, et al. Morphine paradoxically prolongs neuropathic pain in rats by amplifying spinal NLRP3 inflammasome activation. Proc Natl Acad Sci USA. 2016;113:E3441–50.

  166. 166.

    Carrigan KA, Lysle DT. Morphine-6 beta-glucuronide induces potent immunomodulation. Int Immunopharmacol. 2001;1:821–31.

  167. 167.

    Lotsch J, Geisslinger G. Morphine-6-glucuronide: an analgesic of the future? Clin Pharmacokinet. 2001;40:485–99.

  168. 168.

    Kilpatrick GJ, Smith TW. Morphine-6-glucuronide: actions and mechanisms. Med Res Rev. 2005;25:521–44.

  169. 169.

    Romberg R, Olofsen E, Sarton E, den Hartigh J, Taschner PE, Dahan A. Pharmacokinetic-pharmacodynamic modeling of morphine-6-glucuronide-induced analgesia in healthy volunteers: absence of sex differences. Anesthesiology. 2004;100:120–33.

  170. 170.

    Cann C, Curran J, Milner T, Ho B. Unwanted effects of morphine-6-glucoronide and morphine. Anaesthesia. 2002;57:1200–3.

  171. 171.

    Dahan A, van Dorp E, Smith T, Yassen A. Morphine-6-glucuronide (M6G) for postoperative pain relief. Eur J Pain. 2008;12:403–11.

  172. 172.

    Hanna MH, Elliott KM, Fung M. Randomized, double-blind study of the analgesic efficacy of morphine-6-glucuronide versus morphine sulfate for postoperative pain in major surgery. Anesthesiology. 2005;102:815–21.

  173. 173.

    Greenspan JD, Craft RM, LeResche L, Arendt-Nielsen L, Berkley KJ, Fillingim RB, et al. Studying sex and gender differences in pain and analgesia: a consensus report. Pain. 2007;132:S26–45.

  174. 174.

    Manubay J, Davidson J, Vosburg S, Jones J, Comer S, Sullivan M. Sex differences among opioid-abusing patients with chronic pain in a clinical trial. J Addict Med. 2015;9:46–52.

  175. 175.

    Campbell CI, Weisner C, Leresche L, Ray GT, Saunders K, Sullivan MD, et al. Age and gender trends in long-term opioid analgesic use for noncancer pain. Am J Public Health. 2010;100:2541–7.

  176. 176.

    Frenk SM, Porter KS, Paulozzi LJ. Prescription opioid analgesic use among adults: United States, 1999-2012. NCHS Data Brief. 2015;1–8.

  177. 177.

    Pain Management Task Force: providing a standardized DoD and VHA vision and approach to pain management to optimize the care for warriors and their families. Department of the Army, Office of the Surgeon General, USA; 2010.

  178. 178.

    Baamonde AI, Hidalgo A, Andrés-Trelles F. Sex-related differences in the effects of morphine and stress on visceral pain. Neuropharmacology. 1989;28:967–70.

  179. 179.

    Cicero TJ, Nock B, Meyer ER. Gender-related differences in the antinociceptive properties of morphine. J Pharmacol Exp Ther. 1996;279:767–73.

  180. 180.

    Barrett AC, Cook CD, Terner JM, Craft RM, Picker MJ. Importance of sex and relative efficacy at the mu opioid receptor in the development of tolerance and crosstolerance to the antinociceptive effects of opioids. Psychopharmacology (Berl). 2001;158:154–64.

  181. 181.

    Cook CD, Nickerson MD. Nociceptive sensitivity and opioid antinociception and antihyperalgesia in Freund's adjuvant-induced arthritic male and female rats. J Pharmacol Exp Ther. 2005;313:449--59.

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This work was supported by NIH grants DA16272 and DA041529 awarded to AZM.

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The authors declare no competing interests.

Correspondence to Anne Z. Murphy.

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