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. Author manuscript; available in PMC: 2020 Apr 1.
Published in final edited form as: Curr Opin Behav Sci. 2018 Oct 30;26:69–74. doi: 10.1016/j.cobeha.2018.10.002

Pain And Opioid Systems, Implications In The Opioid Epidemic.

Nicolas Massaly 1,2,3, Jose A Morón 1,2,3,4
PMCID: PMC6457459  NIHMSID: NIHMS1509255  PMID: 30984806

Abstract

Pain has a useful protective role; through avoidance learning, it helps to decrease the probability of engaging in tissue-damaging, or otherwise dangerous experiences. In our modern society, the experience of acute post-surgical pain and the development of chronic pain states represent an unnecessary negative outcome. This has become an important health issue as more than 30% of the US population reports experiencing “unnecessary” pain at any given time. Opioid therapies are often efficacious treatments for severe and acute pain; however, in addition to their powerful analgesic properties, opioids produce potent reinforcing properties and their inappropriate use has led to the current opioid overdose epidemic in North America. Dissecting the allostatic changes occurring in nociceptors and neuronal pathways in response to pain are the first and most important steps in understanding the physiologic changes underlying the opioid epidemic. Full characterization of these adaptations will provide novel targets for the development of safer pharmacotherapies. In this review, we highlight the current efforts toward safer opioid treatments and describe our current knowledge of the interaction between pain and opioid systems.

INTRODUCTION

Acute and chronic pain are experienced by 30 to 40% of the US population at any given time (14). The high occurrence of these pain conditions highlights the need for efficacious therapeutic management, including opioid analgesics. In the late 1990s, the false belief that pain-experiencing patients were less likely to develop opioid addiction led to an increase in prescription opioid therapies (5, 6). Decades later, opioid analgesics represent the most prescribed class of therapeutics in the US (7). This high prescription prevalence, together with the limited number of therapeutic alternatives, is correlated with the apparition of opioid diversion, misuse, addiction and ultimately overdoses (CDC, 2018). Recent reports describe a 5-fold increase in opioid overdose in the United States in the last 15 years (8). In 2015, opioids contributed to more than 17,500 deaths were accounted to opioid pain relievers in addition to alarming nonfatal opioid overdoses that required medical care in a hospital or emergency department (8). Altogether, opioid use has reached a dire levels, with a daily 115 opioid-induced lethal overdoses reported (4). The occurrence of these dramatic events can be explained by several factors.

The development of tolerance after exposure to a few, or even a single dose of opioid analgesics (9, 10), represents a possible explanation for the increase in involuntary overdoses. Indeed, the analgesic and rewarding properties of opioid therapeutics are strongly decreased after a short term use (1114). In self-medicating patients, dose escalation in opioid medication to overcome the presence of analgesic tolerance could explain, at least in part, the observed increase in opioid-induced respiratory depression and subsequent accidental harm (8). Furthermore, exposure to early life stress episodes or undertreated pain are believed to increase both the risk or development of chronic pain and opioid misuse through allostatic changes (1518). Yet, dissecting the allostatic changes leading to opioid analgesic tolerance and analgesia (see (19)) may help to curtail substance abuse and avoid involuntary overdoses and the undertreatment of pain (20).

As the opioid epidemic continues to worsen and has reached unprecedented proportions, many state and federal level policies and strategies have been rolled out to address this national health issue (prescription drug monitoring programs, CDC prescription guidelines, novel compounds development). However these attempts are likely to remain ineffective at reducing overdose rates until our scientific and medical community better understand the neurobiology of the intersection between pain and opioid systems. This strategy may uncover new pharmacological targets to safely treat pain and OUD afflicted patients. This review will focus on our current knowledge on pain and opioid systems overlap in the reward circuitry leading to possible drug misuse liability.

The opioid system, a hub for pain and reward interaction

The endogenous opioid system has been studied for decades for its involvement in pain processes and currently represents the main target for analgesic treatment. However, the opioid system is also involved in numerous behavioral functions such as learning and memory, stress, mood, reward and addiction. On a cellular level, the opioid system is composed of four main subcategories: the Noceptin/orphanin-FQ (NOP), the delta-opioid receptor (DOR), the mu-opioid receptor (MOR) and the kappa-opioid receptor (KOR) systems. Those four systems can interact with one another and are all deeply involved in the modulation of pain and reward. The NOP system, expressed mainly in the brainstem, forebrain and spinal cord (2123), has a dual role in which its central stimulation blocks opioid- and stress-induced analgesia while intrathecal administration leads the analgesic properties (2325). Because the NOP system activation decreases the reinforcing properties and abuse liability of many drugs of abuse, it is currently considered as a possible molecular target for substance abuse treatment (2628). The DOR system is highly expressed in forebrain regions (29, 30) and modulates analgesia predominantly under in chronic pain conditions (31, 32). As a comparison the MOR and KOR systems, distributed throughout the brainstem, midbrain, and forebrain structures, are thoroughly involved in the integration of reinforcing and aversive stimuli, including severe and acute pain (20, 3344). Pain, composed of both a nociceptive and an emotional component, is detected by peripheral sensory neurons and processed though an interaction in between descending pain modulatory system and cortical networks. The MOR is expressed throughout this pain axis, and stimulation of MORs in both peripheral nociceptors and supraspinal structures alleviates the nociceptive component of pain. After injury met-enkephalins, endogenous MOR agonists, can be locally released at injury site and provide rapid anti-nociception (45). This pain relief represents a reinforcing experience, as alleviating painful stimuli improves general hedonic state, a phenomenon known as negative reinforcement (29, 30, 3537, 39). On the other hand, in non-painful conditions the activation of MOR in the mesolimbic reward pathway, from the ventral tegmental area (VTA) to the nucleus accumbens (NAc), leads to reinforcing effects through the release of dopamine, known as positive reinforcement (29, 48, 49). While acute activation of the MOR system, the main target of current opioid pharmacotherapies such as morphine and fentanyl, is correlated with analgesic properties and reward (44, 4951), activation of KOR system leads to dysphoric, anhedonic, and aversive behaviors (33, 34, 3941, 43). Due to their reinforcing and aversive properties the MOR and KOR, respectively, are often referred as opponent systems. Interestingly, some studies have demonstrated that KOR stimulation, in pain conditions, disrupts the reinforcing properties of MOR agonists through a dopamine release inhibition in the NAc (52, 53) while peripherally restricted KOR stimulation induces analgesia (54). A thorough dissection of the impact of pain to trigger allostatic changes in all the four opioid systems represents a necessary step to understand the interaction in between pain and opioid misuse liability (Figure 1).

Figure 1:

Figure 1:

Schematic representation of pain-induced allostatic changes in all four opioid systems driving the development of negative affective states. Ultimately, the presence of these negative affect together with the persistent/chronic nociceptive component of pain and the development of opioid treatment tolerance can lead to opioid prescription misuse and increased abuse liability.

In animal models, the occurrence of pain has been shown to strongly affect the reinforcing properties of rewards (5559). Numerous laboratories have shown a decrease in the reinforcing properties of morphine using a conditioned place paradigm in rodent models of neuropathic or chronic pain (52, 6062). Interestingly, Wu and collaborators found that animals exposed to chronic pain developed morphine-induced place preference when the dose of morphine was increased (63). This suggests a rightward shift in the dose-response for reinforcing properties of opioid in animals experiencing pain. Similarly, using opioid self-administration, the gold-standard methodology in the study of addiction, animals in pain demonstrate a decrease in low-dose (62, 6467) but an increase in high-dose opioid consumption when compared to control littermates (55). These alterations in opioid reward processing have been strongly correlated to impaired reinforcer-induced dopaminergic transmission and NAc function (6871). This dopaminergic release impairment in the NAc contributes, at least in part, to the negative affective states that accompany drug withdrawal (72), suggesting a possible common mechanism for pain to drive negative affective states. In that sense, recent preclinical studies have characterized significant allostatic changes in rodents NAc medium spiny neurons when animals are exposed to an inflammatory or neuropathic pain condition (57, 58). The presence of negative affective states, a consequence of the emotional component of pain, have been highly correlated with these neuronal adaptations (57, 58). However, to fully decipher how pain promotes the appearance of negative affective states it is important to acknowledge the role of other brain regions (besides the VTA and the NAc) that are critical in the regulation of pain, stress, and reward responses. The amygdala is very much involved in the processing of both positive and negative valence (see review (73)). Specifically, the BLA and the central nucleus of the amygdala, play major roles in the relationship in between pain and negative affective states (74, 75). The lateral hypothalamus (LH), a region critical to positive reinforcement through its direct connection to the mesolimbic pathway, is involved in pain responses, affect, and the rewarding properties of reinforcers (76, 77). This dual role in both pain and reward makes the lateral hypothalamus an ideal candidate to study interactions in between pain and the presence of negative affective states. Further studies dissecting the role of pain and opioid systems in these brain hubs, among others, will undoubtedly uncover the neuronal mechanisms responsible for the emotional component of pain.

Despite the promising outcomes of opioid analgesics with a low tolerance liability discussed earlier in the introduction, the rewarding properties of current opioid prescriptions remain a key factor in the North American opioid epidemic. Evidence from clinical studies depict a positive correlation between the increase in opioid prescription for pain treatment and the development of Opioid Use Disorders (OUD). According to recent reports, only 8% of pain patients go on to develop addiction (4, 78, 79). However, most interestingly, the rate of misuse and abuse behaviors occurs much more commonly, in 15 to 26% of patients (4, 78, 79). Thus, numerous groups of scientists have focused their efforts on developing novel opioid therapeutics which maintain the ability to relieve pain in the absence of abuse liability. For example, Spahn and collaborators recently developed a fentanyl derivate which acts strictly in painful, inflamed areas (80). Because pH is diminished at the site of painful inflammation, these authors were able to develop a fentanyl derivate with low pKA properties (NFEPP). This strategy allowed a specific action of the NFEPP in low pH milieu to provide analgesia in the absence of the central side effects (motor coordination, sedation, rewarding properties, constipation and respiratory depression) associated with fentanyl use (80). Earlier this year, Ding and collaborators developed a bifunctional NOP/MOR agonist that acts as a potent analgesic while lacking the generally observed side-effects of MOR agonists treatment, such as respiratory depression, development of tolerance and abuse liability (81). Another promising strategy can be found in the design of biased agonists. MOR agonist binding to their receptor can trigger the activation of several downstream pathways. Many laboratories have described selective role on these pathways activation to drive rewarding, tolerance, or analgesic properties of MOR agonists compounds. Embracing these numerous studies, Manglik and collaborators have recently uncovered, through a rigorous pharmacological compound screening, a novel biased MOR opioid agonist, PZM21, displaying analgesic properties while lacking rewarding properties in non-pain conditions (82). The same year, Brust and colleagues described a thorough assessment of a newly developed KOR biased agonist, triazole 1.1, which presents high analgesic properties without inducing any apparent sedation or dysphoric effects (83). While these elegant studies have only explored preclinical models of pain, biased agonists through which action on opioid receptors lead to selective activation of a certain pathway represent promising therapeutic candidates (10, 51, 84). Despite these encouraging results, the latency for novel therapeutics to emerge on the market will be substantial, given that these compounds have not yet made their way out of preclinical studies.

As for today, alternative therapies represent a possible way of decreasing opioid use and related morbidity and mortality. The rate of opioid related overdoses has been significantly decreased in the states where therapeutic cannabis has been legalized (85, 86). The analgesic and anxiolytic properties of cannabis may improve the treatment of both the nociceptive and emotional components of pain. However, further studies on the long-term effects of cannabis use would be prudent to ensure that this is a safe and sustainable means of ameliorating the opioid epidemic. In addition, acupuncture, meditation and other non-pharmacological approaches represents complementary and efficient ways to treat pain and its associated co-morbidities such as increased anxiety- and depression-like states (8791).

In conclusion we firmly believe that a combination of safer pharmacotherapies, better understanding of pain-induced allostatic changes in opioid systems and neurocircuitry and non-pharmacological approaches will undoubtedly help the medical community to improve patient suffering health care and quality of life.

ACKNOWLEDGMENTS

We would like to thank Dr. Adrianne Wilson-Poe and Dr. Nicholas Gregory for her help and suggestions in the preparation of the current manuscript. This work was supported by US National Institutes of Health (NIH) grant DA041781 (J.A.M.), DA042581 (J.A.M.), DA042499 (J.A.M.), DA041883 (J.A.M.), NARSAD Independent Investigator Award from the Brain and Behavior Research Foundation (J.A.M.), Philippe Foundation (N.M.).

Footnotes

DECLARATION OF INTERESTS

The authors declare no competing financial interests.

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REFERENCES

  • 1.Institute of Medicine (US) Committee on Advancing Pain Research, Care, and Education, Relieving Pain in America: A Blueprint for Transforming Prevention, Care, Education, and Research (National Academies Press (US), Washington (DC), 2011; http://www.ncbi.nlm.nih.gov/books/NBK91497/), The National Academies Collection: Reports funded by National Institutes of Health. [PubMed] [Google Scholar]
  • 2.Johannes CB, Le TK, Zhou X, Johnston JA, Dworkin RH, The Prevalence of Chronic Pain in United States Adults: Results of an Internet-Based Survey. J. Pain. 11, 1230–1239 (2010). [DOI] [PubMed] [Google Scholar]
  • 3.Tsang A et al. , Common chronic pain conditions in developed and developing countries: gender and age differences and comorbidity with depression-anxiety disorders. J. Pain Off. J. Am. Pain Soc 9, 883–891 (2008). [DOI] [PubMed] [Google Scholar]
  • 4.Volkow ND, McLellan AT, Opioid Abuse in Chronic Pain--Misconceptions and Mitigation Strategies. N. Engl. J. Med 374, 1253–1263 (2016). [DOI] [PubMed] [Google Scholar]; ** In this review Dr. Volkow and Dr. McLellan nicely and thoroughly describe the opioid prescription in misuse problems encountered in north America. A must read to understand this opioid epidemic health issue.
  • 5.Dowell D, Zhang K, Noonan RK, Hockenberry JM, Mandatory Provider Review And Pain Clinic Laws Reduce The Amounts Of Opioids Prescribed And Overdose Death Rates. Health Aff. Proj. Hope. 35, 1876–1883 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Kolodny A et al. , The prescription opioid and heroin crisis: a public health approach to an epidemic of addiction. Annu. Rev. Public Health. 36, 559–574 (2015). [DOI] [PubMed] [Google Scholar]
  • 7.Dowell D, Haegerich TM, Chou R, CDC Guideline for Prescribing Opioids for Chronic Pain-United States, 2016. JAMA 315, 1624–1645 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Frank JW, Binswanger IA, Calcaterra SL, Brenner LA, Levy C, Non-Medical Use of Prescription Pain Medications and Increased Emergency Department Utilization: Results of a National Survey. Drug Alcohol Depend. 157, 150–157 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Cahill CM, Walwyn W, Taylor AMW, Pradhan AAA, Evans CJ, Allostatic Mechanisms of Opioid Tolerance Beyond Desensitization and Downregulation. Trends Pharmacol. Sci 37, 963–976 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Williams JT, Christie MJ, Manzoni O, Cellular and synaptic adaptations mediating opioid dependence. Physiol. Rev 81, 299–343 (2001). [DOI] [PubMed] [Google Scholar]
  • 11.Al-Hasani R, Bruchas MR, Molecular Mechanisms of Opioid Receptor-Dependent Signaling and Behavior. Anesthesiology. 115, 1363–1381 (2011). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Kuo A, Wyse BD, Meutermans W, Smith MT, In vivo profiling of seven common opioids for antinociception, constipation and respiratory depression: no two opioids have the same profile. Br. J. Pharmacol. 172, 532–548 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 13.Pasternak GW, Pan Y-X, Mu Opioids and Their Receptors: Evolution of a Concept. Pharmacol. Rev 65, 1257–1317 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Williams JT et al. , Regulation of μ-Opioid Receptors: Desensitization, Phosphorylation, Internalization, and Tolerance. Pharmacol. Rev 65, 223–254 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Massaly N, Morón JA, Al-Hasani R, A Trigger for Opioid Misuse: Chronic Pain and Stress Dysregulate the Mesolimbic Pathway and Kappa Opioid System. Front. Neurosci 10 (2016), doi: 10.3389/fnins.2016.00480. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Sinatra R, Causes and consequences of inadequate management of acute pain. Pain Med. Malden Mass. 11, 1859–1871 (2010). [DOI] [PubMed] [Google Scholar]
  • 17.Tan M, Law LS-C, Gan TJ, Optimizing pain management to facilitate Enhanced Recovery After Surgery pathways. Can. J. Anaesth. J. Can. Anesth. 62, 203–218 (2015). [DOI] [PubMed] [Google Scholar]
  • 18.Clarke H et al. , The prevention of chronic postsurgical pain using gabapentin and pregabalin: a combined systematic review and meta-analysis. Anesth. Analg. 115, 428–442 (2012). [DOI] [PubMed] [Google Scholar]
  • 19.Corder G et al. , Loss of μ opioid receptor signaling in nociceptors, but not microglia, abrogates morphine tolerance without disrupting analgesia. Nat. Med 23, 164–173 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]; ** In this study Dr. Corder and collaborators determine the necessity of mu opioid receptor in peripheral nociceptors for the development of morphine tolerance. This elegant study provides important insights in the development of new therapeutics.
  • 20.Kieffer BL, Evans CJ, Opioid Tolerance–In Search of the Holy Grail. Cell. 108, 587–590 (2002). [DOI] [PubMed] [Google Scholar]
  • 21.Mollereau C, Mouledous L, Tissue distribution of the opioid receptor-like (ORL1) receptor. Peptides. 21, 907–917 (2000). [DOI] [PubMed] [Google Scholar]
  • 22.Mollereau C et al. , ORL1, a novel member of the opioid receptor family. Cloning, functional expression and localization. FEBS Lett 341, 33–38 (1994). [DOI] [PubMed] [Google Scholar]
  • 23.Ozawa A et al. , Analysis of the distribution of spinal NOP receptors in a chronic pain model using NOP-eGFP knock-in mice. Br. J. Pharmacol (2018), doi: 10.1111/bph.14225. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Toll L, Bruchas MR, Calo’ G, Cox BM Zaveri NT, Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems. Pharmacol. Rev 68, 419–457 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]; * A bright and complete review on the Nociceptin/Orphanin FQ receptor system and its involvement in pain processes.
  • 25.Zaveri NT, Nociceptin Opioid Receptor (NOP) as a Therapeutic Target: Progress in Translation from Preclinical Research to Clinical Utility. J. Med. Chem 59, 7011–7028 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.de Guglielmo G et al. , Cebranopadol Blocks the Escalation of Cocaine Intake and Conditioned Reinstatement of Cocaine Seeking in Rats. J. Pharmacol. Exp. Ther 362, 378–384 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Kallupi M et al. , Genetic Deletion of the Nociceptin/Orphanin FQ Receptor in the Rat Confers Resilience to the Development of Drug Addiction. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol. 42, 695–706 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Ubaldi M, Cannella N, Ciccocioppo R, Emerging Targets for Addiction Neuropharmacology; From Mechanisms to Therapeutics. Prog. Brain Res 224, 251–284 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Le Merrer J, Becker JAJ, Befort K, Kieffer BL, Reward processing by the opioid system in the brain. Physiol. Rev 89, 1379–1412 (2009). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Mansour A, Khachaturian H, Lewis ME, Akil H, Watson SJ, Anatomy of CNS opioid receptors. Trends Neurosci 11, 308–314 (1988). [DOI] [PubMed] [Google Scholar]
  • 31.Cahill CM, Holdridge SV, Morinville A, Trafficking of delta-opioid receptors and other G-protein-coupled receptors: implications for pain and analgesia. Trends Pharmacol. Sci 28, 23–31 (2007). [DOI] [PubMed] [Google Scholar]
  • 32.Vicente-Sanchez A, Pradhan AA, Ligand-Directed Signaling at the Delta Opioid Receptor. Handb. Exp. Pharmacol (2017), doi: 10.1007/164_2017_39. [DOI] [PubMed] [Google Scholar]
  • 33.Al-Hasani R et al. , Distinct Subpopulations of Nucleus Accumbens Dynorphin Neurons Drive Aversion and Reward. Neuron. 87, 1063–1077 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 34.Bruchas MR et al. , Stress-induced p38 mitogen-activated protein kinase activation mediates kappa-opioid-dependent dysphoria. J. Neurosci. Off. J. Soc. Neurosci 27, 11614–11623 (2007). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 35.Bruchas MR, Land BB, Chavkin C, The dynorphin/kappa opioid system as a modulator of stress-induced and pro-addictive behaviors. Brain Res 1314, 44–55 (2010). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Devine DP, Leone P, Pocock D, Wise RA, Differential involvement of ventral tegmental mu, delta and kappa opioid receptors in modulation of basal mesolimbic dopamine release: in vivo microdialysis studies. J. Pharmacol. Exp. Ther 266, 1236–1246 (1993). [PubMed] [Google Scholar]
  • 37.Knoll AT, Carlezon WA, Dynorphin, stress, and depression. Brain Res 1314, 56–73 (2010). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 38.Koob GF, Le Moal M, Drug addiction, dysregulation of reward, and allostasis. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol. 24, 97–129 (2001). [DOI] [PubMed] [Google Scholar]
  • 39.Land BB et al. , The dysphoric component of stress is encoded by activation of the dynorphin kappa-opioid system. J. Neurosci. Off. J. Soc. Neurosci 28, 407–414 (2008). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 40.Leitl MD et al. , Pain-related depression of the mesolimbic dopamine system in rats: expression, blockade by analgesics, and role of endogenous κ-opioids. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol. 39, 614–624 (2014). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Muschamp JW, Carlezon WA, Roles of nucleus accumbens CREB and dynorphin in dysregulation of motivation. Cold Spring Harb. Perspect. Med 3, a012005 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Muschamp JW et al. , Activation of CREB in the nucleus accumbens shell produces anhedonia and resistance to extinction of fear in rats. J. Neurosci. Off. J. Soc. Neurosci 31, 3095–3103 (2011). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.Shippenberg TS, Stein C, Huber A, Millan MJ, Herz A, Motivational effects of opioids in an animal model of prolonged inflammatory pain: alteration in the effects of kappa- but not of mu-receptor agonists. Pain. 35, 179–186 (1988). [DOI] [PubMed] [Google Scholar]
  • 44.Wade CL, Fairbanks CA, The Self-administration of Analgesic Drugs in Experimentally Induced Chronic Pain. Curr. Top. Behav. Neurosci 20, 217–232 (2014). [DOI] [PubMed] [Google Scholar]
  • 45.Stein C, Opioids, sensory systems and chronic pain. Eur. J. Pharmacol 716, 179–187 (2013). [DOI] [PubMed] [Google Scholar]
  • 46.Navratilova E et al. , Pain relief produces negative reinforcement through activation of mesolimbic reward-valuation circuitry. Proc. Natl. Acad. Sci. U. S. A 109, 20709–20713 (2012). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 47.Porreca F, Navratilova E, Reward, motivation and emotion of pain and its relief. Pain. 158, S43–S49 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]; * This review from Dr. Porreca and Dr. Navratilova provides general insight on our current knowledge on the emotional component of pain and negative reinforcement through its relief.
  • 48.Wise RA, Role of brain dopamine in food reward and reinforcement. Philos. Trans. R. Soc. B Biol. Sci 361, 1149–1158 (2006). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.Wise RA, Koob GF, The development and maintenance of drug addiction. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol. 39, 254–262 (2014). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 50.Matsui A, Jarvie BC, Robinson BG, Hentges ST, Williams JT, Separate GABA afferents to dopamine neurons mediate acute action of opioids, development of tolerance, and expression of withdrawal. Neuron. 82, 1346–1356 (2014). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 51.Siuda ER, Carr R, Rominger DH, Violin JD, Biased mu-opioid receptor ligands: a promising new generation of pain therapeutics. Curr. Opin. Pharmacol. 32, 77–84 (2017). [DOI] [PubMed] [Google Scholar]
  • 52.Narita M et al. , Direct evidence for the involvement of the mesolimbic kappa-opioid system in the morphine-induced rewarding effect under an inflammatory pain-like state. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol. 30, 111–118 (2005). [DOI] [PubMed] [Google Scholar]
  • 53.Niikura K, Narita M, Butelman ER, Kreek MJ, Suzuki T, Neuropathic and chronic pain stimuli downregulate central mu-opioid and dopaminergic transmission. Trends Pharmacol. Sci 31, 299–305 (2010). [DOI] [PubMed] [Google Scholar]
  • 54.Snyder LM et al. , Kappa Opioid Receptor Distribution and Function in Primary Afferents. Neuron. 99, 1274–1288.e6 (2018). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 55.Hipólito L et al. , Inflammatory Pain Promotes Increased Opioid Self-Administration: Role of Dysregulated Ventral Tegmental Area μ Opioid Receptors. J. Neurosci. Off. J. Soc. Neurosci 35, 12217–12231 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 56.Narita M et al. , Direct evidence for the involvement of the mesolimbic kappa-opioid system in the morphine-induced rewarding effect under an inflammatory pain-like state. Neuropsychopharmacol. Off. Publ. Am. Coll. Neuropsychopharmacol. 30, 111–118 (2005). [DOI] [PubMed] [Google Scholar]
  • 57.Ren W et al. , The indirect pathway of the nucleus accumbens shell amplifies neuropathic pain. Nat. Neurosci. 19, 220–222 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]; * In this study Ren et al., provides evidences of paininduced allostatic changes in D2-expressing MSNs EPSCs and its correlation with the development of negative affective states.
  • 58.Schwartz N et al. , Chronic pain. Decreased motivation during chronic pain requires long-term depression in the nucleus accumbens. Science. 345, 535–542 (2014). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 59.Taylor AMW et al. , Microglia disrupt mesolimbic reward circuitry in chronic pain. J. Neurosci. Off. J. Soc. Neurosci 35, 8442–8450 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 60.Cahill CM et al. , Changes in morphine reward in a model of neuropathic pain. Behav. Pharmacol. 24, 207–213 (2013). [DOI] [PubMed] [Google Scholar]
  • 61.Ozaki S et al. , Suppression of the morphine-induced rewarding effect in the rat with neuropathic pain: implication of the reduction in mu-opioid receptor functions in the ventral tegmental area. J. Neurochem. 82, 1192–1198 (2002). [DOI] [PubMed] [Google Scholar]
  • 62.Taylor AMW et al. , Microglia disrupt mesolimbic reward circuitry in chronic pain. J. Neurosci. Off. J. Soc. Neurosci 35, 8442–8450 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 63.Wu Y et al. , Upregulation of tumor necrosis factor-alpha in nucleus accumbens attenuates morphine-induced rewarding in a neuropathic pain model. Biochem. Biophys. Res. Commun. 449, 502–507 (2014). [DOI] [PubMed] [Google Scholar]
  • 64.Hipólito L et al. , Inflammatory Pain Promotes Increased Opioid Self-Administration: Role of Dysregulated Ventral Tegmental Area μ Opioid Receptors. J. Neurosci. Off. J. Soc. Neurosci 35, 12217–12231 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 65.Lyness WH, Smith FL, Heavner JE, Iacono CU, Garvin RD, Morphine self-administration in the rat during adjuvant-induced arthritis. Life Sci 45, 2217–2224 (1989). [DOI] [PubMed] [Google Scholar]
  • 66.Martin TJ, Ewan E, Chronic pain alters drug self-administration: implications for addiction and pain mechanisms. Exp. Clin. Psychopharmacol. 16, 357–366 (2008). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 67.Wade CL et al. , Effect of chronic pain on fentanyl self-administration in mice. PloS One. 8, e79239 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 68.Apkarian AV et al. , Chronic pain patients are impaired on an emotional decision-making task. Pain. 108, 129–136 (2004). [DOI] [PubMed] [Google Scholar]
  • 69.Baliki MN, Geha PY, Fields HL, Apkarian AV, Predicting value of pain and analgesia: nucleus accumbens response to noxious stimuli changes in the presence of chronic pain. Neuron. 66, 149–160 (2010). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 70.Becerra L, Borsook D, Signal valence in the nucleus accumbens to pain onset and offset. Eur. J. Pain Lond. Engl 12, 866–869 (2008). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 71.Borsook D et al. , Reward deficiency and anti-reward in pain chronification. Neurosci. Biobehav. Rev (2016), doi: 10.1016/j.neubiorev.2016.05.033. [DOI] [PubMed] [Google Scholar]
  • 72.Elman I, Borsook D, Volkow ND, Pain and suicidality: insights from reward and addiction neuroscience. Prog. Neurobiol. 109, 1–27 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 73.Janak PH, Tye KM, From circuits to behaviour in the amygdala. Nature. 517, 284–292 (2015). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 74.Veinante P, Yalcin I, Barrot M, The amygdala between sensation and affect: a role in pain. J. Mol. Psychiatry. 1, 9 (2013). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 75.Simons L et al. , The Human Amygdala and Pain: Evidence from Neuroimaging. Hum. Brain Mapp 35, 527–538 (2014). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 76.Barbano MF, Wang H-L, Morales M, Wise RA, Feeding and Reward Are Differentially Induced by Activating GABAergic Lateral Hypothalamic Projections to VTA. J. Neurosci 36, 2975–2985 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 77.Jahangirvand M, Yazdi F, Moradi M, Haghparast A, Intra-accumbal Orexin-1 Receptors are Involved in Antinociception Induced by Stimulation of the Lateral Hypothalamus in the Formalin Test as an Animal Model of Persistent Inflammatory Pain. Iran. J. Pharm. Res. IJPR 15, 851–859 (2016). [PMC free article] [PubMed] [Google Scholar]
  • 78.Fishbain DA, Cole B, Lewis J, Rosomoff HL, Rosomoff RS, What percentage of chronic nonmalignant pain patients exposed to chronic opioid analgesic therapy develop abuse/addiction and/or aberrant drug-related behaviors? A structured evidence-based review. Pain Med. Malden Mass. 9, 444–459 (2008). [DOI] [PubMed] [Google Scholar]
  • 79.Vowles KE et al. , Rates of opioid misuse, abuse, and addiction in chronic pain: a systematic review and data synthesis. Pain. 156, 569–576 (2015). [DOI] [PubMed] [Google Scholar]
  • 80.Spahn V et al. , A nontoxic pain killer designed by modeling of pathological receptor conformations. Science. 355, 966–969 (2017). [DOI] [PubMed] [Google Scholar]; ** Dr. Spahn and collaborators developed a fentanyl derivate with a pH-restricted site of action. This ne compound possesses analgesic properties while lacking centrally mediated side effects generally observed with mu opioid agonists analgesics.
  • 81.Ding H et al. , A bifunctional nociceptin and mu opioid receptor agonist is analgesic without opioid side effects in nonhuman primates. Sci. Transl. Med 10, eaar3483 (2018). [DOI] [PMC free article] [PubMed] [Google Scholar]; ** In this work Ding and collaborators developed a bifunctional NOP/MOR agonist with potent analgesic properties that does not induced respiratory depression or possesses abuse liability.
  • 82.Manglik A et al. , Structure-based discovery of opioid analgesics with reduced side effects. Nature. 537, 185–190 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]; >** In this work Manglik and collaborators exhaustively screened a bank mu opioid receptor agonists and selected a biased agonist which displays analgesic properties while lacking central side effects.
  • 83.Brust TF et al. , Biased agonists of the kappa opioid receptor suppress pain and itch without causing sedation or dysphoria. Sci. Signal. 9, ra117 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]; ** Through a thorough evaluation of a newly developed kappa opioid receptor biased agonist Burst et al., provide significant evidences on a therapeutical potential for Triazole 1.1. This compound exhibits analgesic properties while lacking the sedative and dysphoric effects generally observed with kappa agonists treatment.
  • 84.Schmid CL et al. , Bias Factor and Therapeutic Window Correlate to Predict Safer Opioid Analgesics. Cell. 171, 1165–1175.e13 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 85.Bachhuber MA, Saloner B, Cunningham CO, Barry CL, Medical Cannabis Laws and Opioid Analgesic Overdose Mortality in the United States, 1999–2010. JAMA Intern. Med 174, 1668–1673 (2014). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 86.Shi Y, Medical marijuana policies and hospitalizations related to marijuana and opioid pain reliever. Drug Alcohol Depend. 173, 144–150 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 87.Hilton L et al. , Mindfulness Meditation for Chronic Pain: Systematic Review and Meta-analysis. Ann. Behav. Med 51, 199–213 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 88.Li Y, Wang F, Feng C, Yang X, Sun Y, Massage Therapy for Fibromyalgia: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. PLoS ONE 9 (2014), doi: 10.1371/journal.pone.0089304. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 89.Vickers AJ et al. , Acupuncture for chronic pain: individual patient data meta-analysis. Arch. Intern. Med 172, 1444–1453 (2012). [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 90.Yuan Q et al. , Acupuncture for musculoskeletal pain: A meta-analysis and meta-regression of sham-controlled randomized clinical trials. Sci. Rep 6 (2016), doi: 10.1038/srep30675. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 91.Zeidan F, Vago D, Mindfulness meditation–based pain relief: a mechanistic account. Ann. N. Y. Acad. Sci 1373, 114–127 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]

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