Pain is the most common and debilitating sign of a medical problem, with nearly 15 million patients suffering from chronic pain, including neuropathic pain. Widely used therapies for treating neuropathic pain include tri-cyclic antidepressants, opioids, anticonvulsants, non-steroidal anti-inflammatory agents and combinations thereof. Despite the abundance of treatments, the management of chronic pain remains difficult due to an inability for many patients to achieve appropriate pain relief at doses which are tolerable over long periods of time.Opiates (natural products), or opioids (synthetic derivatives), are considered the gold standard of analgesic care, though with little efficacy for neuropathic pain. Opioids are associated with unwanted side effects, including paradoxical pain and abuse liability that may result from several nervous system adaptations within the pain modulating neural network. These dose related side effects become more prevalent as clinicians try to overcome analgesic tolerance.Molecular mechanisms underlying these unwanted side effects have been studied extensively, and the literature purports a variety of contributing factors and neurobiological adaptations. The studies herein describe additional molecular adaptations and novel pharmacological approaches to counteract these changes. First, the contributions of neurobiological remodeling within a single receptor system (the opioid system) were investigated in the spinal dorsal horn after peripheral nerve ligation and chronic exposure to an opioid agonist in combination with an ultra-low-dose of opioid antagonist. The effects of the ultra-low-dose opioid antagonist naltrexone on the efficacy of oxycodone for neuropathic pain were investigated after both central and systemic administration.Secondly, molecular remodeling occurs across different receptor systems in the pain network, including altered regulation of pronociceptive molecules (e.g. substance P; SP). Previous studies have reported that opioid-induced hyperalgesia, tolerance and reward can be prevented by a blockade or ablation of SP activity at the neurokinin 1 receptor (NK1). We have characterized single compounds, rationally designed to act as opioid agonists and an NK1 antagonist using in vitro assays and the efficacy in vivo using rodent models of pain, antinociceptive tolerance and reward. Collectively, these studies validate the concept of targeting multiple neurobiological adaptations as a therapeutic option for neuropathic pain and reducing opioid- mediated side effects.

L’utilisation des opioïdes est à l’origine de phénomènes de tolérance et d’hyperalgésie induite (HIO) aussi bien chez l’animal qu’en utilisation clinique. Ces phénomènes surviennent avec tous les opioïdes de manière dose-dépendante. Les mécanismes qui les sous-tendent sont complexes et imparfaitement connus. Le rémifentanil et le sufentanil sont les opioïdes les plus utilisés en France en anesthésie-réanimation. Leur utilisation s’accompagne d’une HIO qui majore la douleur postopératoire et peut être responsable de la persistance de la douleur à long terme. La perception des stimuli nociceptifs chez un patient sous anesthésie générale n’est pas aisée et repose encore sur des signes cliniques indirects d’activation du système sympathique. Ces signes peu sensibles et peu spécifiques conduisent à sous doser ou sur-doser les patients en opioïdes. Récemment, un nouvel outil de monitorage de la nociception est apparu, l’analgesia nociception index (ANI). L’ANI reflète le tonus parasympathique et de ce fait permettrait aux anesthésistes de mieux adapter le dosage des opioïdes. Dans cette thèse, nous avons d’abord évalué la sensibilité et la spécificité de l’ANI à détecter les stimuli nociceptifs, puis montré qu’elles étaient supérieures à celles des signes cliniques, et supérieures à d’autres indices de monitorage proposés. Ensuite nous avons validé la capacité de l’ANI à guider l’analgésie peropératoire du rémifentanil dans différentes situations.Sur le plan expérimental, nous avons exploré, après une exposition courte ou prolongée à différentes doses de rémifentanil et de sufentanil, les mécanismes associés à l’hyperalgésie thermique in vivo, chez la souris, et ex vivo, sur la voie des MAP kinases ERK1/2 et sur le trafic membranaire des récepteurs opioïdes de type µ (MOR) dans différentes cultures cellulaires. Chez la souris, nous avons mis en évidence une hyperalgésie précoce au saut sur plaque chaude, après exposition aux doses les plus élevées de rémifentanil, mais pas avec le sufentanil. De plus, nous n’avons pas observé d’HIO sur le léchage des pattes.Sur les cultures cellulaires, le rémifentanil comme le sufentanil activent la voie des MAPK ERK1/2 lors d’une exposition courte, avec apparition d’une désensibilisation lorsque l’exposition se prolonge. Le rémifentanil comme le sufentanil induisent une internalisation précoce et progressive des récepteurs MOR. / The use of opioids is associated with tolerance and induced hyperalgesia (OIH). Tolerance and OIH occur with all opioids and have been demonstrated both, in animals and in humans and are likely to be dose-dependent. The underlying mechanisms are complex and partially known. Remifentanil and sufentanil are the most used opioids in France in anesthesia and intensive care. Their use is associated with OIH that increases postoperative pain and may be responsible for persistent pain. In anesthetized patients, nociceptive stimuli are still detected according to clinical signs of sympathetic activation. These signs lack sensitivity and specificity and lead to underdosing or overdosing opioids. Recently, the analgesia nociception index (ANI), has been proposed as surrogate marker of nociception. The ANI reflects the parasympathetic tone and thus may allow anesthetists to better adapt the opioid dosage. In this thesis, we first evaluated the sensitivity and specificity of ANI to detect nociceptive stimuli, and showed that it better detects them than do clinical signs or than other currently available monitoring tools. Subsequently, we validated the ability of the ANI to adequately guide the intraoperative dosing of remifentanil in different clinical setting.After acute and sustained exposure to different doses of remifentanil and sufentanil we investigated, in vivo, the mechanisms associated with thermal hyperalgesia in mice, and ex vivo, the effect on the MAP kinase ERK1/2 pathway and the μ-type opioid receptor (MOR) membrane trafficking in human neuroblastoma and embryonic kidney cell cultures. In mice, high-dose remifentanil induced early hyperalgesia assessed by the jumping latency in a hot-plate test, but not the sufentanil. We did not observe OIH for the hind paw licking test. On cell cultures, after short exposure, both remifentanil and sufentanil produced activation of the MAP kinase ERK1/2 pathway, and rapid desensitization and internalization of the MOR.

BACKGROUND: Chronic low back pain (CLBP) is the leading cause of disability in the United States. People suffering from CLBP often have multiple comorbidities including depression, anxiety, and substance use disorder (SUD). Although the opioid epidemic has intensified the search for new treatment options, both pharmacological and other, opioids still remain the most common treatment for chronic pain. Long-term opioid therapy (LTOT) has been shown to lead to opioid-induced hyperalgesia (OIH), an increased sensitivity to painful stimuli. It remains unclear, however, the extent to which reductions in opioid dose impact OIH. METHODS: This is a longitudinal cohort study whose primary aim is to determine how changes in opioid doses are associated with changes in psychosocial and quantitative sensory testing (QST) variables. Participants were 24 adults with CLBP being treated with LTOT and visits were conducted on a monthly basis for six months. All 24 participants were included in the analysis of demographic and psychosocial variables (disability, anxiety, depression, opioid misuse, pain severity, pain interference, and catastrophizing). A subset of 13 participants were included in the analysis of QST variables. RESULTS: We found that pressure pain thresholds at the thumb and the trapezius, and heat pain threshold significantly (p < 0.05) improved between visit 1 and visit 6. We also found that a decrease in morphine equivalent doses (MED) is correlated (coefficient > 0.2) with improvements in punctuate probe rating, pain pressure at the thumb, and maximum cold ratings. DISCUSSION: Our results show that reductions in opioid dose are associated with reduced pain sensitivity, even while the psychosocial variables studied (including subjective pain score, depression, and anxiety) remain stable.

Background: The consequences of continued opioid abuse among patients treated with opioid substitution therapy (OST) are serious and can result in abnormal cardiovascular function, overdose, and mortality. Conflicting evidence exists that both implicates and refutes the role of chronic non-cancer pain (CNCP) as a major risk factor for continued opioid abuse within the addiction treatment setting. This thesis aims to 1) evaluate the impact of chronic pain on the treatment outcomes of patients with opioid addiction receiving OST, 2) determine whether a clinical or inflammatory profile exists to distinguish pain in this population, 3) explore the sources of heterogeneity in previous studies examining this question, 4) determine the best therapy for patients with chronic pain, and 5) evaluate the most effective treatment for opioid addiction. We anticipate chronic pain to be an important predictor of continued opioid abuse such that patients with comorbid pain will require careful consideration when managed on OST. Methods: We systematically reviewed the literature to determine the impact of pain in opioid addiction patients receiving methadone maintenance treatment (MMT). We determined the clinical and inflammatory profile of MMT patients using data from the Genetics of Opioid Addiction (GENOA) research collaborative between the Canadian Addiction Treatment Centres (CATC) and the Population Genomic Program. GENOA is a prospective cohort study aimed to determine the genetic, biological, and psychosocial determinants of treatment prognosis for opioid addiction patients receiving MMT. GENOA recruits patients ≥ 18 years of age meeting the DSM-IV criteria for opioid dependence. All GENOA participants are receiving MMT for the management of opioid addiction. Baseline data from the GENOA pilot study (n=235) were used to evaluate the impact of pain on illict opioid use behaviour and determine the clinical and inflammatory profile of patients with comorbid pain. We explored sources of heterogeneity in previous studies using data from the full-phase GENOA study (n=444), examining the prognostic value of different pain measures for predicting illicit opioid use. We then performed a multiple treatment comparison of all opioid substitution and antagonist therapies in efforts to determine the best intervention for improving treatment outcomes for patients with comorbid pain. We lastly determined the most effective treatment for opioid addiction by performing a network meta-analysis using data from a systematic review of opioid maintenance therapy trials. Results: Our initial systematic review confirmed a lack of consensus in the literature, whereby some studies suggest pain increases risk for illicit opioid use and other studies suggest pain has no effect on substance use behaviour. Findings from the analysis of GENOA pilot data confirmed chronic pain to be an important predictor of sustained opioid abuse and also showed patients with pain to have elevated Interferon-Gamma. Using data from the GENOA prospective cohort study we determined the Brief Pain Inventory (a commonly used pain measurement in pervious studies) to be highly sensitive with poor prognostic value. Our final reviews propose 1) there is limited evidence to suggest any OST is superior for managing patients with comorbid pain, and 2) heroin and high-dose methadone are the most effective treatments for improving treatment retention. The final systematic review and network meta-analysis in this thesis also highlights a major problem in the treatment of opioid use disorders, primarily the lack of consensus as to what outcomes matter for determining success in patients with addiction. Conclusion: Patients with comorbid pain and addiction are at high-risk for continued opioid abuse and should be managed closely by clinicians administering OST. Contention in the previous literature likely resulted from the use of pain measurements with poor prognostic value. No OST demonstrated superiority for managing patients with chronic pain. While our findings indicate heroin is the most effective treatment across multiple endpoints, we use this thesis to provide readers with 1) a sense of the feasibility issues associated with heroin administration, 2) a summary of the limitations of this evidence base, and 3) recommendations for how to improve the addiction trials’ design for future research. / Thesis / Doctor of Philosophy (PhD)

La douleur est un problème de santé publique majeur qui réduit la qualité de vie des patients et engendre un coût élevé pour la société. Malgré les efforts fournis pour développer de nouveaux analgésiques, les opiacés restent le moyen le plus efficace pour réduire la douleur moyenne à intense. Cependant, leur utilisation prolongée est responsable du développement d’une tolérance à leurs effets analgésiques et d’une hypersensibilité à la douleur (hyperalgésie). Il a été proposé que ces phénomènes pourraient résulter de l'activation d’un système anti-opioïde tel que celui des récepteurs RF-amide, mais leurs mécanismes d’action sont encore mal compris. L'objectif de ce projet a ainsi été d'étudier l'implication des récepteurs RF-amide NPFFR1, NPFFR2 et GPR103a dans le développement de l'hyperalgésie induite par les opiacés. Allant du niveau cellulaire, en s’intéressant à l’expression des ARNm, à la modulation de l’activité neuronale induite par le 26RFa, jusqu’à un niveau plus intégré via une approche in vivo et l’étude des seuils nociceptifs et de la douleur chez la souris ; ce travail a permis d’avoir une vision globale des effets du système RF-amide et principalement de GPR103a. En abordant la question des effets secondaires associés aux traitements chroniques opiacés, ce projet pourrait conduire à l'élaboration de stratégies de traitement de la douleur prometteuses. / Pain is a major health problem that reduces quality of life and imparts high social and economic costs. Despite efforts to develop new analgesics, opiates remain the most effective way to reduce severe pain. However, their prolonged use is associated with the development of analgesic tolerance and hypersensitivity to pain (hyperalgesia). It has been proposed that these phenomena would result from the activation of anti-opioid system like RF-amide receptors system, but their mechanism of action is still poorly understood. The objective of this project was to study the involvement of NPFFR1, NPFFR2 and GPR103a receptors in the development of opioidinduced hyperalgesia. This work gave an overall view of the effects of the RF-amide system and mainly of GPR103a, ranging from the cellular level, to the expression of mRNAs, to the modulation of the 26RFa-induced neuronal activity, up to an integrated level via an in vivo approach and the study of nociceptive thresholds and pain in mice. By addressing the issue of side effects associated with opioid chronic treatments, this project may lead to the development of promising strategies for pain treatment.