Induction of pain facilitation by sustained opioid exposure: relationship to opioid antinociceptive tolerance

HSP90 inhibition in the mouse spinal cord enhances opioid signaling by suppressing an AMPK-mediated negative feedback loop

Opioids and other agonists of the μ-opioid receptor are effective at managing acute pain, but their chronic use can lead to tolerance that limits their efficacy. We previously reported that inhibiting the chaperone protein HSP90 in the spinal cords of mice promotes the antinociceptive effects of opioids in a manner that involved increased activation of the kinase ERK. Here, we found that the underlying mechanism involves the relief of a negative feedback loop mediated by the kinase AMPK. Intrathecal treatment of male and female mice with the HSP90 inhibitor 17-AAG decreased the abundance of the β1 subunit of AMPK in the spinal cord. The antinociceptive effects of 17-AAG with morphine were suppressed by intrathecal administration of AMPK activators and enhanced by an AMPK inhibitor. Opioid treatment increased the abundance of phosphorylated AMPK in the dorsal horn of the spinal cord, where it colocalized with a neuronal marker and the neuropeptide CGRP. Knocking down AMPK in CGRP-positive neurons enhanced the antinociceptive effects of morphine and demonstrated that AMPK mediated the signal transduction between HSP90 inhibition and ERK activation. These data suggest that AMPK mediates an opioid-induced negative feedback loop in CGRP neurons of the spinal cord and that this loop can be disabled by HSP90 inhibition to enhance the efficacy of opioids.

Toll-Like Receptor 4 in Pain: Bridging Molecules-to-Cells-to-Systems

Pain impacts the lives of billions of people around the world - both directly and indirectly. It is complex and transcends beyond an unpleasant sensory experience to encompass emotional experiences. To date, there are no successful treatments for sufferers of chronic pain. Although opioids do not provide any benefit to chronic pain sufferers, they are still prescribed, often resulting in more complications such as hyperalgesia and dependence. In order to develop effective and safe medications to manage, and perhaps even treat pain, it is important to evaluate novel contributors to pain pathologies. As such, in this chapter we review the role of Toll-like receptor 4, a receptor of the innate immune system, that continues to gain substantial attention in the field of pain research. Positioned in the nexus of the neuro and immune systems, TLR4 may provide one of the missing pieces in understanding the complexities of pain. Here we consider how TLR4 enables a mechanistical understanding of pain as a multidimensional biopsychosocial state from molecules to cells to systems and back again.

Fentanyl-induced hyperalgesia and analgesic tolerance in male rats: common underlying mechanisms and prevention by a polyamine deficient diet

Opioids are a mainstay of pain management but can induce unwanted effects, including analgesic tolerance and paradoxical hyperalgesia, either of which leads to increased pain. Clinically, however, the relationship between these two phenomena remains elusive. By evaluating changes in mechanical nociceptive threshold in male rats, we found that in contrast to a purely analgesic control response to a single subcutaneous administration of fentanyl (25 μg/kg), in rats subjected to inflammatory pain 2 weeks previously (Day₀), the same test dose (D₁₃) induced a bi-phasic response: initial decreased analgesia (tolerance) followed by hyperalgesia lasting several hours. Both the tolerance and hyperalgesia were further enhanced in rats that had additionally received fentanyl on D₀. The dose-response profiles (5 fg to 50 μg/kg) of pain- and opioid-experienced rats were very different from pain/drug-naive rats. At ultra-low fentanyl doses (<5 ng/kg and <500 ng/kg for naïve control and pain/drug-experienced rats, respectively), solely hyperalgesia was observed in all cases. At higher doses, which now produced analgesia alone in naive rats, reduced analgesia (tolerance) coupled with hyperalgesia occurred in pain/fentanyl-experienced rats, with both phases increasing with dose. Transcriptomic and pharmacological data revealed that an overactivation of the spinal N-methyl-_D-aspartate receptor-inducible NO synthase cascade plays a critical role in both acute tolerance and hyperalgesia, and together with the finding that the magnitudes of analgesia and associated hyperalgesia are negatively correlated, is indicative of closely related phenomena. Finally, a polyamine deficient diet prevented inducible NO synthase transcript upregulation, restored fentanyl's analgesic efficacy and suppressed the emergence of hyperalgesia.

The cellular mechanism by which the rostral ventromedial medulla acts on the spinal cord during chronic pain

Clinical therapies for chronic pain are limited. While targeted drugs are promising therapies for chronic pain, they exhibit insufficient efficacy and poor targeting. The occurrence of chronic pain partly results from central changes caused by alterations in neurons in the rostral ventromedial medulla (RVM) in the brainstem regulatory pathway. The RVM, which plays a key role in the descending pain control pathway, greatly contributes to the development and maintenance of pain. However, the exact roles of the RVM in chronic pain remain unclear, making it difficult to develop new drugs targeting the RVM and related pathways. Here, we first discuss the roles of the RVM and related circuits in chronic pain. Then, we analyze synaptic transmission between RVM neurons and spinal cord neurons, specifically focusing on the release of neurotransmitters, to explore the cellular mechanisms by which the RVM regulates chronic pain. Finally, we propose some ideas for the development of drugs targeting the RVM.

Remote buprenorphine-naloxone initiation as an essential service for people with chronic pain and opioid dependence during the COVID-19 pandemic: Case reports, clinical pathways, and implications for the future

Many health care professions have reacted swiftly to the COVID-19 pandemic. In-person care has been ramped down and telemedicine/telehealth has been thrust to the forefront of clinical care. For people living with chronic pain and often concomitantly dealing with opioid-related issues, this is a time of great stress. With population-wide movements to shelter in place, people living with pain are more isolated, more stressed, and more vulnerable to mental health concerns like depression and anxiety that can increase pain-related suffering. This article presents two case reports of patients struggling with chronic pain and opioid dependence in which a telemedicine-based buprenorphine-naloxone conversion was chosen as a treatment option by two Canadian programs: The Transitional Pain Service at the Toronto General Hospital in Toronto, Ontario, and The Opioid Deprescribing Program in Calgary, Alberta. Both cases presented highlight the use of telemedicine during the COVID-19 pandemic and suggest that there will be substantial need for these services well beyond the apex of the crisis. A buprenorphine-naloxone home induction protocol is presented and we provide insight into important lessons learned regarding the appropriate selection of patients with chronic pain struggling with opioid use disorder for buprenorphine-naloxone conversion. The provision of health care during the COVID-19 pandemic has rapidly forced practitioners to evolve novel health care practices, and these changes will have long-term implications.

Effect of fentanyl and its three novel analogues on biochemical, oxidative, histological, and neuroadaptive markers after sub-acute exposure in mice

AIMS

Comparative sub-acute toxicity, including tolerance and dependence potential of fentanyl and its three novel and potent analogues was determined in mice.

MAIN METHODS

Comparative sub-acute (21 d, intraperitoneal; 1/10 LD₅₀) toxicity of fentanyl and its three novel analogues viz., N-(1-(2-phenoxyethyl)-4-piperidinyl) propionanilide (2), N-isopropyl-3-(4-(N-phenylpropionamido)piperidin-1-yl)propanamide (5), and N-t-butyl-3-(4-(N-phenylpropionamido)piperidin-1-yl)propanamide (6) was determined in mice. Animals were observed for additional seven days to assess the recovery. The brain, liver and kidney toxicity was assessed on the basis of various biochemical, oxidative, histological, and neuroadaptive markers. The expression levels of key neuronal markers associated with drug tolerance and dependence were investigated by western blot and immunohistochemistry.

KEY FINDINGS

Fentanyl and its analogues caused abnormal levels of liver and kidney specific biomarkers in plasma. Brain malondialdehyde (MDA) levels were raised by all the treatments and kidney MDA level by analogue 6 (21 d). Reduced glutathione levels in brain, liver, and kidney were diminished by all the treatments (21 & 28 d) and a significant change in the levels of antioxidant enzymes was also produced mainly after 21 d. The deleterious effects of fentanyl and its analogues were further substantiated by corresponding histopathological changes in brain, liver and kidney (21 & 28 d). These compounds were also found to produce neuroadaptive changes as evidenced by the increased expression levels of c-Fos, glucocorticoid receptor, N-methyl-d-aspartate receptor1 and μ-opioid receptor (21 & 28 d).

SIGNIFICANCE

Three novel analogues of fentanyl were envisaged to have alternative therapeutic potentials. However, their comparative sub-acute toxicity revealed undesirable side effects, thereby masking their therapeutic ability.

Regulatory mechanisms and therapeutic potential of microglial inhibitors in neuropathic pain and morphine tolerance

Microglia are important cells involved in the regulation of neuropathic pain (NPP) and morphine tolerance. Information on their plasticity and polarity has been elucidated after determining their physiological structure, but there is still much to learn about the role of this type of cell in NPP and morphine tolerance. Microglia mediate multiple functions in health and disease by controlling damage in the central nervous system (CNS) and endogenous immune responses to disease. Microglial activation can result in altered opioid system activity, and NPP is characterized by resistance to morphine. Here we investigate the regulatory mechanisms of microglia and review the potential of microglial inhibitors for modulating NPP and morphine tolerance. Targeted inhibition of glial activation is a clinically promising approach to the treatment of NPP and the prevention of morphine tolerance. Finally, we suggest directions for future research on microglial inhibitors.

Evolution of Analgesic Tolerance and Opioid-Induced Hyperalgesia Over 6 Months: Double-Blind Randomized Trial Incorporating Experimental Pain Models

Contributors to the ongoing epidemic of prescription opioid abuse, addiction, and death include opioid tolerance, withdrawal symptoms, and possibly opioid-induced hyperalgesia (OIH). Thirty stable chronic nonmalignant pain patients entered a 6-month long, randomized, double-blind, dose-response, 2-center trial of the potent opioid levorphanol, conducted over a decade ago during an era of permissive opioid prescribing. Eleven were taking no opioids at study entry and eleven were taking between 35 and 122 morphine equivalents. Five weeks titration preceded twenty weeks stable dosing. Tolerance and OIH were inferred individually based on chronic pain ratings, brief pain inventory scores, and results of the brief thermal sensitization model at 5 opioid dosing sessions. Seventeen patients completed. The average final daily opioid dose was 132; range 14 to 300; average addition 105 morphine equivalents. After observed dosing, the brief thermal sensitization area of hyperalgesia changed minimally but the painfulness of skin heating was reduced. Weekly 0 to 100 visual analog scale pain ratings (average 64 at study entry, 48 at end titration, 45 at end stable dosing) decreased a median 19%, but 8 completed with higher visual analog scale ratings. Three completers had evidence of both tolerance and hyperalgesia. A fully-powered trial similar to this feasibility study is ethically questionable. A large-scale pragmatic trial is more realistic. TRIAL REGISTRATION: NCT00275249 Evolution of Analgesic Tolerance With Opioids PERSPECTIVE: A double-blind, 6-month, high-dose opioid feasibility trial, completed years ago, provides critically important data for clinically defining analgesic tolerance and OIH. Overall benefit was small, and 18% of patients had evidence of both tolerance and OIH. Future work requires a different approach than a classic randomized controlled trial design.

Neurobiology of Opioid Addiction: Opponent Process, Hyperkatifeia, and Negative Reinforcement

Opioids are powerful drugs that usurp and overpower the reward function of endogenous opioids and engage dramatic tolerance and withdrawal via molecular and neurocircuitry neuroadaptations within the same reward system. However, they also engage the brain systems for stress and pain (somatic and emotional) while producing hyperalgesia and hyperkatifeia, which drive pronounced drug-seeking behavior via processes of negative reinforcement. Hyperkatifeia (derived from the Greek "katifeia" for dejection or negative emotional state) is defined as an increase in intensity of the constellation of negative emotional or motivational signs and symptoms of withdrawal from drugs of abuse. In animal models, repeated extended access to drugs or opioids results in negative emotion-like states, reflected by the elevation of reward thresholds, lower pain thresholds, anxiety-like behavior, and dysphoric-like responses. Such negative emotional states that drive negative reinforcement are hypothesized to derive from the within-system dysregulation of key neurochemical circuits that mediate incentive-salience and/or reward systems (dopamine, opioid peptides) in the ventral striatum and from the between-system recruitment of brain stress systems (corticotropin-releasing factor, dynorphin, norepinephrine, hypocretin, vasopressin, glucocorticoids, and neuroimmune factors) in the extended amygdala. Hyperkatifeia can extend into protracted abstinence and interact with learning processes in the form of conditioned withdrawal to facilitate relapse to compulsive-like drug seeking. Compelling evidence indicates that plasticity in the brain pain emotional systems is triggered by acute excessive drug intake and becomes sensitized during the development of compulsive drug taking with repeated withdrawal. It then persists into protracted abstinence and contributes to the development and persistence of compulsive opioid-seeking behavior.

Opioid-Induced Tolerance and Hyperalgesia

Opioids are very potent and efficacious drugs, traditionally used for both acute and chronic pain conditions. However, the use of opioids is frequently associated with the occurrence of adverse effects or clinical problems. Other than adverse effects and dependence, the development of tolerance is a significant problem, as it requires increased opioid drug doses to achieve the same effect. Mechanisms of opioid tolerance include drug-induced adaptations or allostatic changes at the cellular, circuitry, and system levels. Dose escalation in long-term opioid therapy might cause opioid-induced hyperalgesia (OIH), which is a state of hypersensitivity to painful stimuli associated with opioid therapy, resulting in exacerbation of pain sensation rather than relief of pain. Various strategies may provide extra-opioid analgesia. There are drugs that may produce independent analgesic effects. A tailored treatment provided by skilled personnel, in accordance with the individual condition, is mandatory. Any treatment aimed at reducing opioid consumption may be indicated in these circumstances. Interventional techniques able to decrease the pain input may allow a decrease in the opioid dose, thus reverting the mechanisms producing tolerance of OIH. Intrathecal therapy with local anesthetics and a sympathetic block are the most common techniques utilized in these circumstances.

Electroacupuncture alleviates morphine‑induced hyperalgesia by regulating spinal CB1 receptors and ERK1/2 activity

Electroacupuncture (EA), a traditional Chinese therapeutic technique, is considered an effective method for treating certain painful neuropathies induced by various neuropathological damage. The current study investigated the effect of EA on intrathecal (IT) morphine-induced hyperalgesia (MIH) and examined the hypothesis that activation of cannabinoid receptor 1 (CB1) could enhance the antinociceptive effect of EA on MIH via regulation of the extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway. Using a rat model of IT MIH, mechanical and thermal hyperalgesia were evaluated by an electronic von Frey filament and hotplate at baseline (1 day before IT administration) and at days 1, 3, 5 and 7 after IT administration. Rats received IT normal saline, IT morphine or IT morphine + EA at ST36-GB34. The protein levels of ERK1/2, phosphorylated (p)-ERK1/2 and CB1 in the spinal cord were assayed by western blotting. Furthermore, the effect of IT injection of the CB1 agonist WIN 55,212-2 and the CB1 antagonist SR141716 on the antinociceptive effect of EA in rats with MIH was investigated. Nociceptive behavior and ERK1/2, phosphorylated (p)-ERK1/2 and CB1 protein levels were evaluated as mentioned above. The results revealed that chronic IT injections of morphine induced a significant decrease in mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) accompanied with remarkable upregulation of p-ERK1/2 in the spinal cord, which could be attenuated by EA at the ST36-GB34 acupoints. In the rat model of MIH, IT injection of WIN 55,212-2 combined with EA induced a significant increase in MWT and TWL accompanied with a significant decrease in p-ERK1/2 and a significant increase in CB1 protein level compared with EA alone, while SR141716 induced the opposite results. The present study suggests that EA alleviates hyperalgesia induced by IT injection of morphine partially through the inhibition of ERK1/2 activation. Activation of the CB1 receptor enhances the antinociceptive effect of EA in rats with MIH partly through the regulation of the spinal CB1-ERK1/2 signaling pathway.

Altered Signaling in the Descending Pain-modulatory System after Short-Term Infusion of the μ-Opioid Agonist Remifentanil

μ-Opioid receptor agonists are widely used within the contemporary treatment of pain, but abrupt opioid suspension, even after short-term infusion, can paradoxically increase the sensitivity to noxious stimuli, a phenomenon that has been, for example, reported after application of the fast-acting μ-opioid receptor agonist remifentanil. To investigate the mechanisms underlying the effects of discontinuation of remifentanil application on pain processing in the human CNS, we analyzed neuronal responses to thermal stimuli before and after a short-term infusion of remifentanil (30 min 0.1 μg/kg body weight/min) compared with control in the brain, brainstem, and spinal cord in drug-naive male volunteers using fMRI. Subsequent to remifentanil suspension, we observed reduced heat pain thresholds and increased neuronal responses in pain-encoding as well as in key regions of the descending pain-modulatory system, such as the periaqueductal gray matter, the nucleus cuneiformis, and the rostral ventromedial medulla. Moreover, the spinal pain-related multivoxel activity pattern showed an opioid-specific change after drug suspension. Importantly, remifentanil suspension increased the functional coupling between the nucleus cuneiformis and the rostral anterior cingulate cortex, and the coupling strength between the rostral anterior cingulate cortex and the nucleus cuneiformis correlated negatively with the individual pain threshold after opioid suspension. These findings demonstrate that, already subsequent to a short-term infusion of the μ-opioid receptor agonist remifentanil, signaling in the descending pain-modulatory system is fundamentally altered and that these changes are directly related to the behavioral sensitivity to pain.SIGNIFICANCE STATEMENT Opioids are widely used in modern medicine, but, in addition to their known side effects, it is increasingly recognized that opioids can also increase sensitivity to pain subsequent to their use. Using the fast-acting μ-opioid receptor agonist remifentanil and fMRI in healthy male volunteers, this study demonstrates how signaling changes occur along the entire descending pain-modulatory pathway after opioid discontinuation and how these alterations are closely linked to increased behavioral pain sensitivity. Particularly by revealing modified responses in pain-modulatory brainstem regions that have been previously demonstrated to be causally involved in acute opioid withdrawal effects in rodents, the data provide a plausible neuronal mechanism by which the increased sensitivity to pain after opioid suspension is mediated in humans.

Opioid-induced hyperalgesia: Cellular and molecular mechanisms

Opioids produce strong analgesia but their use is limited by a paradoxical hypersensitivity named opioid-induced hyperalgesia (OIH) that may be associated to analgesic tolerance. In the last decades, a significant number of preclinical studies have investigated the factors that modulate OIH development as well as the cellular and molecular mechanisms underlying OIH. Several factors have been shown to influence OIH including the genetic background and sex differences of experimental animals as well as the opioid regimen. Mu opioid receptor (MOR) variants and interactions of MOR with different proteins were shown important. Furthermore, at the cellular level, both neurons and glia play a major role in OIH development. Several neuronal processes contribute to OIH, like activation of neuroexcitatory mechanisms, long-term potentiation (LTP) and descending pain facilitation. Increased nociception is also mediated by neuroinflammation induced by the activation of microglia and astrocytes. Neurons and glial cells exert synergistic effects, which contribute to OIH. The molecular actors identified include the Toll-like receptor 4 and the anti-opioid systems as well as some other excitatory molecules, receptors, channels, chemokines, pro-inflammatory cytokines or lipids. This review summarizes the intracellular and intercellular pathways involved in OIH and highlights some mechanisms that may be challenged to limit OIH in the future.

Targeting Opioid-Induced Hyperalgesia in Clinical Treatment: Neurobiological Considerations

Opioid analgesics have become a cornerstone in the treatment of moderate to severe pain, resulting in a steady rise of opioid prescriptions. Subsequently, there has been a striking increase in the number of opioid-dependent individuals, opioid-related overdoses, and fatalities. Clinical use of opioids is further complicated by an increasingly deleterious profile of side effects beyond addiction, including tolerance and opioid-induced hyperalgesia (OIH), where OIH is defined as an increased sensitivity to already painful stimuli. This paradoxical state of increased nociception results from acute and long-term exposure to opioids, and appears to develop in a substantial subset of patients using opioids. Recently, there has been considerable interest in developing an efficacious treatment regimen for acute and chronic pain. However, there are currently no well-established treatments for OIH. Several substrates have emerged as potential modulators of OIH, including the N-methyl-D-aspartate and γ-aminobutyric acid receptors, and most notably, the innate neuroimmune system. This review summarizes the neurobiology of OIH in the context of clinical treatment; specifically, we review evidence for several pathways that show promise for the treatment of pain going forward, as prospective adjuvants to opioid analgesics. Overall, we suggest that this paradoxical state be considered an additional target of clinical treatment for chronic pain.

Development of a peptidomimetic antagonist of neuropeptide FF receptors for the prevention of opioid-induced hyperalgesia

Through the development of a new class of unnatural ornithine derivatives as bioisosteres of arginine, we have designed an orally active peptidomimetic antagonist of neuropeptide FF receptors (NPFFR). Systemic low-dose administration of this compound to rats blocked opioid-induced hyperalgesia, without any apparent side-effects. Interestingly, we also observed that this compound potentiated opioid-induced analgesia. This unnatural ornithine derivative provides a novel therapeutic approach for both improving analgesia and reducing hyperalgesia induced by opioids in patients being treated for chronic pain.

Role of opioid receptor heterodimerization in pain modulation and tolerance development

Protein to protein interactions leading to homo/heteromerization of receptor is well documented in literature. These interactions leading to dimeric/oligomers formation of receptors are known to modulate their function, particularly in case of G-protein coupled receptors. The opioid receptor heteromers having changed pharmacological properties than the constituent protomers provides preferences for novel drug targets that could lead to potential analgesic activity devoid of tolerance and physical dependence. Heterodimerization of opioid receptors appears to generate novel binding properties with improved specificity and lack of side effects. Further the molecules which can interact simultaneously to both the protomers of the heteromer, or to both the binding sites (orthosteric and allosteric) of a receptor protein could be potential therapeutic molecules. This review highlights the recent advancements in exploring the plausible role of heteromerization of opioid receptors in induction of tolerance free antinociception.

The relationship between opioids and immune signalling in the spinal cord

Opioids are considered the gold standard for the treatment of moderate to severe pain. However, heterogeneity in analgesic efficacy, poor potency and side effects are associated with opioid use, resulting in dose limitations and suboptimal pain management. Traditionally thought to exhibit their analgesic actions via the activation of the neuronal G-protein-coupled opioid receptors, it is now widely accepted that neuronal activity of opioids cannot fully explain the initiation and maintenance of opioid tolerance, hyperalgesia and allodynia. In this review we will highlight the evidence supporting the role of non-neuronal mechanisms in opioid signalling, paying particular attention to the relationship of opioids and immune signalling.

Hydrogen sulfide inhibits opioid withdrawal-induced pain sensitization in rats by down-regulation of spinal calcitonin gene-related peptide expression in the spine

Hyperalgesia often occurs in opioid-induced withdrawal syndrome. In the present study, we found that three hourly injections of DAMGO (a μ-opioid receptor agonist) followed by naloxone administration at the fourth hour significantly decreased rat paw nociceptive threshold, indicating the induction of withdrawal hyperalgesia. Application of NaHS (a hydrogen sulfide donor) together with each injection of DAMGO attenuated naloxone-precipitated withdrawal hyperalgesia. RT-PCR and Western blot analysis showed that NaHS significantly reversed the gene and protein expression of up-regulated spinal calcitonin gene-related peptide (CGRP) in naloxone-treated animals. NaHS also inhibited naloxone-induced cAMP rebound and cAMP response element-binding protein (CREB) phosphorylation in rat spinal cord. In SH-SY5Y neuronal cells, NaHS inhibited forskolin-stimulated cAMP production and adenylate cyclase (AC) activity. Moreover, NaHS pre-treatment suppressed naloxone-stimulated activation of protein kinase C (PKC) α, Raf-1, and extracellular signal-regulated kinase (ERK) 1/2 in rat spinal cord. Our data suggest that H2S prevents the development of opioid withdrawal-induced hyperalgesia via suppression of synthesis of CGRP in spine through inhibition of AC/cAMP and PKC/Raf-1/ERK pathways.

The Netrin-1 receptor DCC is a regulator of maladaptive responses to chronic morphine administration

BACKGROUND

Opioids are the cornerstone of treatment for moderate to severe pain, but chronic use leads to maladaptations that include: tolerance, dependence and opioid-induced hyperalgesia (OIH). These responses limit the utility of opioids, as well as our ability to control chronic pain. Despite decades of research, we have no therapies or proven strategies to overcome this problem. However, murine haplotype based computational genetic mapping and a SNP data base generated from analysis of whole-genome sequence data (whole-genome HBCGM), provides a hypothesis-free method for discovering novel genes affecting opioid maladaptive responses.

RESULTS

Whole genome-HBCGM was used to analyze phenotypic data on morphine-induced tolerance, dependence and hyperalgesia obtained from 23 inbred strains. The robustness of the genetic mapping results was analyzed using strain subsets. In addition, the results of analyzing all of the opioid-related traits together were examined. To characterize the functional role of the leading candidate gene, we analyzed transgenic animals, mRNA and protein expression in behaviorally divergent mouse strains, and immunohistochemistry in spinal cord tissue. Our mapping procedure identified the allelic pattern within the netrin-1 receptor gene (Dcc) as most robustly associated with OIH, and it was also strongly associated with the combination of the other maladaptive opioid traits analyzed. Adult mice heterozygous for the Dcc gene had significantly less tendency to develop OIH, become tolerant or show evidence of dependence after chronic exposure to morphine. The difference in opiate responses was shown not to be due to basal or morphine-stimulated differences in the level of Dcc expression in spinal cord tissue, and was not associated with nociceptive neurochemical or anatomical alterations in the spinal cord or dorsal root ganglia in adult animals.

CONCLUSIONS

Whole-genome HBCGM is a powerful tool for identifying genes affecting biomedical traits such as opioid maladaptations. We demonstrate that Dcc affects tolerance, dependence and OIH after chronic opioid exposure, though not through simple differences in expression in the adult spinal cord.

The anterior cingulate cortex and pain processing

The neural network that contributes to the suffering which accompanies persistent pain states involves a number of brain regions. Of primary interest is the contribution of the cingulate cortex in processing the affective component of pain. The purpose of this review is to summarize recent data obtained using novel behavioral paradigms in animals based on measuring escape and/or avoidance of a noxious stimulus. These paradigms have successfully been used to study the nature of the neuroanatomical and neurochemical contributions of the anterior cingulate cortex (ACC) to higher order pain processing in rodents.

Src family kinases involved in CXCL12-induced loss of acute morphine analgesia

Functional interactions between the chemokine receptor CXCR4 and opioid receptors have been reported in the brain, leading to a decreased morphine analgesic activity. However the cellular mechanisms responsible for this loss of opioid analgesia are largely unknown. Here we examined whether Src family-kinases (SFK)-linked mechanisms induced by CXCR4 contributed to the loss of acute morphine analgesia and could represent a new physiological anti-opioid signaling pathway. In this way, we showed by immunohistochemistry and western blot that CXCL12 rapidly activated SFK phosphorylation in vitro in primary cultured lumbar rat dorsal root ganglia (DRG) but also in vivo in the DRG and the spinal cord. We showed that SFK activation occurred in a sub population of sensory neurons, in spinal microglia but also in spinal nerve terminals expressing mu-(MOR) and delta-opioid (DOR) receptor. In addition we described that CXCR4 is detected in MOR- and DOR-immunoreactive neurons in the DRG and spinal cord. In vivo, we demonstrated that an intrathecal administration of CXCL12 (1μg) significantly attenuated the subcutaneous morphine (4mg/kg) analgesia. Conversely, pretreatment with a potent CXCR4 antagonist (5μg) significantly enhanced morphine analgesia. Similar effects were obtained after an intrathecal injection of a specific SFK inhibitor, PP2 (10μg). Furthermore, PP2 abrogated CXCL12-induced decrease in morphine analgesia by suppressing SFK activation in the spinal cord. In conclusion, our data highlight that CXCL12-induced loss of acute morphine analgesia is linked to Src family kinases activation.

Clarifying the roles of homeostasis and allostasis in physiological regulation

Homeostasis, the dominant explanatory framework for physiological regulation, has undergone significant revision in recent years, with contemporary models differing significantly from the original formulation. Allostasis, an alternative view of physiological regulation, goes beyond its homeostatic roots, offering novel insights relevant to our understanding and treatment of several chronic health conditions. Despite growing enthusiasm for allostasis, the concept remains diffuse, due in part to ambiguity as to how the term is understood and used, impeding meaningful translational and clinical research on allostasis. Here, we provide a more focused understanding of homeostasis and allostasis by explaining how both play a role in physiological regulation, and a critical analysis of regulation suggests how homeostasis and allostasis can be distinguished. Rather than focusing on changes in the value of a regulated variable (e.g., body temperature, body adiposity, or reward), research investigating the activity and relationship among the multiple regulatory loops that influence the value of these regulated variables may be the key to distinguishing homeostasis and allostasis. The mechanisms underlying physiological regulation and dysregulation are likely to have important implications for health and disease.

Opioid-induced hyperalgesia and tolerance: understanding opioid side effects

Opioid-induced pain or opioid tolerance should be considered when opioid therapy fails to provide expected analgesic effects or when there is unexplainable pain exacerbation following opioid treatment. As a result, an increase in the opioid dosage may not be the solution to ineffective opioid therapy for chronic pain management. A decrease in the opioid mass may actually provide pain relief in many instances. At one time, it was anticipated that opioid-induced pain was related to upregulation of NMDA receptors with a downregulation of mu receptors. However, there is growing evidence to suggest the opioid receptor-based hyperalgesic mechanism may be directly modulated by the NMDA receptor. Furthermore, the mechanism that causes opioid tolerance may be the same mechanism that causes opioid-induced pain. Current evidence suggests that opioid-induced pain sensitivity could be prevented by interrupting the cellular and molecular changes associated with the development of opioid tolerance. Continued research may lead the way to a new period in which patients prone to opioid-induced pain could be identified, allowing one to tailor pharmacologic pain therapy to each patient.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

Involvement of the opioid and cannabinoid systems in pain control: new insights from knockout studies

The endogenous opioid and cannabinoid systems are involved in the physiological inhibitory control of pain and are of particular interest for the development of therapeutic approaches for pain management. The involvement of these endogenous systems in pain control has been studied from decades by the use of compounds with different affinities for each cannabinoid and opioid receptor or for the different enzymes involved in endocannabinoid and endogenous opioid metabolism. However, the selectivity of these pharmacological tools in vivo has represented an important limitation for these studies. The generation of genetically modified mice with selective mutations in specific components of the endocannabinoid and endogenous opioid system has provided important advances in the identification of the specific contribution of each component of these endogenous systems in the perception of noxious stimuli and the development of pathological pain states. Different lines of constitutive and conditional knockout mice deficient in specific cannabinoid and opioid receptors, specific precursors of the endogenous opioid peptides and the main enzymes involved in endocannabinoid and endogenous opioid degradation are now available. These knockout mice have also been used to evaluate the contribution of each component of the endocannabinoid and opioid system in the antinociceptive effects of cannabinoid and opioid agonists, including those currently used to treat pain in humans. This review summarizes the main advances provided in the last 15 years by the use of these genetic tools in the knowledge of the physiological control of pain and the pharmacology of cannabinoid and opioid compounds for pain management.

NADPH-oxidase 2 activation promotes opioid-induced antinociceptive tolerance in mice

The analgesic effectiveness of long-term opioid therapies is compromised by the development of antinociceptive tolerance linked to the overt production of peroxynitrite (ONOO(-), PN), the product of the interaction between superoxide (O2(-), SO) and nitric oxide (NO), and to neuroinflammatory processes. We have recently reported that in addition to post-translational nitration and inactivation of mitochondrial manganese superoxide dismutase (MnSOD), activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase holoenzyme (NOX) in the spinal cord is a major source for the overt production of superoxide-derived PN during the development of morphine-induced antinociceptive tolerance. However, the NOX complex involved in these processes is not known. The objective of these studies is to identify a potential role for the NOX2 complex, an enzyme involved in inflammation. Mice lacking the catalytic subunit of NOX2 (Nox2(-/-)) or its regulatory subunit, p47(phox) (p47(phox)(-/-)), developed antinociceptive tolerance similar to wildtype (wt) mice after 3 days of continuous morphine. However, while wt mice continue to develop tolerance by day six, morphine analgesia was restored in both Nox2(-/-) and p47(phox)(-/-) mice. Moreover, the loss of Nox2 or p47 did not affect acute morphine analgesia in naïve mice. In wt mice, antinociceptive tolerance was associated with increased activation of NOX, nitration of MnSOD, and proinflammatory cytokines production in the spinal cord. These events were markedly attenuated in Nox2(-/-) and p47(phox)(-/-) mice and instead, there was enhanced formation of antiinflammatory cytokine (IL4 and IL10) production. These results suggest that NOX2 activity provides a significant source of superoxide-derived PN to undertake post-translational modifications of mitochondrial MnSOD and to engage neuroinflammatory signaling in the spinal cord associated with opioid-induced antinociceptive tolerance. Thus, NOX2 may provide a potential target for adjuvant therapy to protect opioid analgesia.

How Frequently are "Classic" Drug-Seeking Behaviors Used by Drug-Seeking Patients in the Emergency Department?

Introduction:

Drug-seeking behavior (DSB) in the emergency department (ED) is a very common problem, yet there has been little quantitative study to date of such behavior. The goal of this study was to assess the frequency with which drug seeking patients in the ED use classic drug seeking behaviors to obtain prescription medication.

Methods:

We performed a retrospective chart review on patients in an ED case management program for DSB. We reviewed all visits by patients in the program that occurred during a 1-year period, and recorded the frequency of the following behaviors: complaining of headache, complaining of back pain, complaining of dental pain, requesting medication by name, requesting a refill of medication, reporting medications as having been lost or stolen, reporting 10/10 pain, reporting greater than 10/10 pain, reporting being out of medication, and requesting medication parenterally. These behaviors were chosen because they are described as “classic” for DSB in the existing literature.

Results:

We studied 178 patients from the case management program, who made 2,486 visits in 1 year. The frequency of each behavior was: headache 21.7%, back pain 20.8%, dental pain 1.8%, medication by name 15.2%, requesting refill 7.0%, lost or stolen medication 0.6%, pain 10/10 29.1%, pain greater than 10/10 1.8%, out of medication 9.5%, and requesting parenteral medication 4.3%. Patients averaged 1.1 behaviors per visit.

Conclusion:

Drug-seeking patients appear to exhibit “classically” described drug-seeking behaviors with only low to moderate frequency. Reliance on historical features may be inadequate when trying to assess whether or not a patient is drug-seeking.

Opioids and central sensitisation: II. Induction and reversal of hyperalgesia

Opioids are powerful analgesics when used to treat acute pain and some forms of chronic pain. In addition, opioids can preempt some forms of central sensitization. Here we review evidence that opioids may also induce and perhaps reverse some forms of central sensitization.

Exploring the neuroimmunopharmacology of opioids: an integrative review of mechanisms of central immune signaling and their implications for opioid analgesia

Vastly stimulated by the discovery of opioid receptors in the early 1970s, preclinical and clinical research was directed at the study of stereoselective neuronal actions of opioids, especially those played in their crucial analgesic role. However, during the past decade, a new appreciation of the non-neuronal actions of opioids has emerged from preclinical research, with specific appreciation for the nonclassic and nonstereoselective sites of action. Opioid activity at Toll-like receptors, newly recognized innate immune pattern recognition receptors, adds substantially to this unfolding story. It is now apparent from molecular and rodent data that these newly identified signaling events significantly modify the pharmacodynamics of opioids by eliciting proinflammatory reactivity from glia, the immunocompetent cells of the central nervous system. These central immune signaling events, including the release of cytokines and chemokines and the associated disruption of glutamate homeostasis, cause elevated neuronal excitability, which subsequently decreases opioid analgesic efficacy and leads to heightened pain states. This review will examine the current preclinical literature of opioid-induced central immune signaling mediated by classic and nonclassic opioid receptors. A unification of the preclinical pharmacology, neuroscience, and immunology of opioids now provides new insights into common mechanisms of chronic pain, naive tolerance, analgesic tolerance, opioid-induced hyperalgesia, and allodynia. Novel pharmacological targets for future drug development are discussed in the hope that disease-modifying chronic pain treatments arising from the appreciation of opioid-induced central immune signaling may become practical.

Intrathecal PKA-selective siRNA treatment blocks sustained morphine-mediated pain sensitization and antinociceptive tolerance in rats

Sustained morphine treatment has been shown to produce paradoxical pain sensitization (opioid-induced hyperalgesia) and also causes increase in spinal pain neurotransmitter, such as calcitonin gene related peptide (CGRP), concentration in experimental animals. Studies have also shown that cyclic adenosine-monophosphate (cAMP)-dependent protein kinase (PKA) plays a major role in the regulation of presynaptic neurotransmitter (such as CGRP and substance P) synthesis and release. We have previously shown that in cultured primary sensory dorsal root ganglion (DRG) neurons sustained in vitro opioid agonist treatment upregulates cAMP levels (adenylyl cyclase (AC) superactivation) and augments basal and capsaicin evoked CGRP release in a PKA dependent manner. In the present study, we investigated the in vivo role of PKA in sustained morphine-mediated pain sensitization. Our data indicate that selective knock-down of spinal PKA activity by intrathecal (i.th.) pretreatment of rats with a PKA-selective small interference RNA (siRNA) mixture significantly attenuates sustained morphine-mediated augmentation of spinal CGRP immunoreactivity, thermal hyperalgesia, mechanical allodynia and antinociceptive tolerance. The present findings indicate that sustained morphine-mediated activation of spinal cAMP/PKA-dependent signaling may play an important role in opioid induced hyperalgesia.

Pathophysiology of medication overuse headache: insights and hypotheses from preclinical studies

INTRODUCTION

Medication overuse headache (MOH) is a clinical concern in the management of migraine headache. MOH arises from the frequent use of medications used for the treatment of a primary headache. Medications that can cause MOH include opioid analgesics as well as formulations designed for the treatment of migraine, such as triptans, ergot alkaloids, or drug combinations that include caffeine and barbiturates.

LITERATURE REVIEW

Gathering evidence indicates that migraine patients are more susceptible to development of MOH, and that prolonged use of these medications increases the prognosis for development of chronic migraine, leading to the suggestion that similar underlying mechanisms may drive both migraine headache and MOH. In this review, we examine the link between several mechanisms that have been linked to migraine headache and a potential role in MOH. For example, cortical spreading depression (CSD), associated with migraine development, is increased in frequency with prolonged use of topiramate or paracetamol.

CONCLUSIONS

Increased CGRP levels in the blood have been linked to migraine and elevated CGRP can be casued by prolonged sumatriptan exposure. Possible mechanisms that may be common to both migraine and MOH include increased endogenous facilitation of pain and/or diminished diminished endogenous pain inhibition. Neuroanatomical pathways mediating these effects are examined.

Opioid-induced hyperalgesia and tolerance

We have all encountered the following postanesthesia care unit dilemma a myriad of times. As the attending covering the postanesthesia care unit, the anesthesiologist will be confronted not infrequently with the following clinical scenario: "He needed 500 μg fentanyl in the operating room for a toe amputation and has received 20 mg morphine, and he's still complaining of severe pain…. Do you think he may need more morphine?" Opiates do prevail as first-line therapy for moderate to severe surgical and chronic pain states. However, their use may actually confound the clinical picture postoperatively, because opiate exposure counterintuitively may actually trigger exaggerated pain sensation. When assessing a patient experiencing exaggerated postoperative or chronic pain, several questions should come to mind. First, is this patient experiencing tolerance or hyperalgesia induced by opiate therapy? Second, does the management differ for the two etiologies? Third, what underlying mechanisms, both at the neuroanatomic and molecular/chemical levels, underlie the two processes? Fourth, how does the recent literature on opiate-induced hyperalgesia influence previously accepted views of pre-emptive analgesia? Fifth, what treatment modalities exist for opiate-induced hyperalgesia? Most importantly, sixth, how can opiate-induced hyperalgesia be prevented? In this literature review, we aim to address these questions and to hopefully change the current perception and management of perioperative and chronic pain states with opiates.

Counter-regulation of opioid analgesia by glial-derived bioactive sphingolipids

The clinical efficacy of opiates for pain control is severely limited by analgesic tolerance and hyperalgesia. Herein we show that chronic morphine upregulates both the sphingolipid ceramide in spinal astrocytes and microglia, but not neurons, and spinal sphingosine-1-phosphate (S1P), the end-product of ceramide metabolism. Coadministering morphine with intrathecal administration of pharmacological inhibitors of ceramide and S1P blocked formation of spinal S1P and development of hyperalgesia and tolerance in rats. Our results show that spinally formed S1P signals at least in part by (1) modulating glial function because inhibiting S1P formation blocked increased formation of glial-related proinflammatory cytokines, in particular tumor necrosis factor-α, interleukin-1βα, and interleukin-6, which are known modulators of neuronal excitability, and (2) peroxynitrite-mediated posttranslational nitration and inactivation of glial-related enzymes (glutamine synthetase and the glutamate transporter) known to play critical roles in glutamate neurotransmission. Inhibitors of the ceramide metabolic pathway may have therapeutic potential as adjuncts to opiates in relieving suffering from chronic pain.

Increased spinal prodynorphin gene expression in reinflammation-associated hyperalgesia after neonatal inflammatory insult

Background

Neuroplasticity induced by neonatal inflammation is the consequence of a combination of activity-dependent changes in neurons. We investigated neuronal sensitivity to a noxious stimulus in a rat model of neonatal hind-paw peripheral inflammation and assessed changes in pain behaviour at the physiological and molecular levels after peripheral reinflammation in adulthood.

Results

A decrease in paw withdrawal latency (PWL) after a heat stimulus was documented in rats that received inflammatory injections in their left hind paws on postnatal day one (P1) and a reinflammation stimulus at postnatal 6-8 weeks of age, compared with normal rats. An increase in the expression of the prodynorphin (proDYN) gene was noted after reinflammation in the spinal cord ipsilateral to the afferents of the neonatally treated hind paw. The involvement of the activation of extracellular signal-regulated kinases (ERK) in peripheral inflammatory pain hypersensitivity was evidenced evident by the increase in phospho-ERK (pERK) activity after reinflammation.

Conclusions

Our results indicate that peripheral inflammation in neonates can permanently alter the pain processing pathway during the subsequent sensory stimulation of the region. Elucidation of the mechanism underlying the developing pain circuitry will provide new insights into the understanding of the early pain behaviours and the subsequent adaptation to pain.

Opioid-induced latent sensitization in a model of non-inflammatory viscerosomatic hypersensitivity

Exposure to opioids can induce a state of "latent sensitization" characterized by long-lasting enhanced responses to subsequent cutaneous injury. Here, we explored the possibility that prior treatment with morphine could induce a state of latent sensitization to visceral pain conditions. Following butyrate enemas to induce non-inflammatory visceral pain, acute morphine administration produced dose-related inhibition of referred viscerosomatic hypersensitivity. Treatment with morphine for a period of six days resulted in a persistent hyperalgesia that resolved many days after termination of drug administration. In morphine pre-exposed rats, butyrate-induced referred hypersensitivity was enhanced and extended in duration. No differences were observed in the morphine dose-response curve in suppression of acute nociception (i.e., the hot-plate assay) when morphine pre-exposed rats were compared to naïve rats indicating that opioid antinociceptive tolerance was not present. However, the morphine dose-response curve to suppress evoked viscerosomatic hypersensitivity was displaced to the right by approximately 4-fold in morphine pre-exposed rats. Induction of viscerosomatic hypersensitivity resulted in an increased labeling of CGRP-, but not substance P-positive cells in the lumbar dorsal root ganglia; increased labeling was not affected by prior exposure to morphine. The data indicate that a period of morphine exposure can induce a state of "latent sensitization" to subsequent visceral pain characterized by extended duration of hypersensitivity. This condition likely reflects enhanced visceral "pain" intensity as a consequence of persistent pronociceptive adaptive changes.

Negligible analgesic tolerance seen with extended release oxymorphone: a post hoc analysis of open-label longitudinal data

OBJECTIVE

To examine the development of analgesic tolerance in patients on oxymorphone extended-release (OxymER).

DESIGN

Post hoc analysis of data from a previously conducted prospective 1 year multi-center open-label extension study in which patients were able to titrate as needed.

PATIENTS

Sample of 153 hip and knee osteoarthritis (OA) subjects on OxymER. Primary analyses were limited to study completers (n = 62) due to the large amount of missing data for the noncompleters (n = 91).

OUTCOME MEASURES

Main outcome measures included OxymER doses (pill counts) and pain intensity ratings using a visual analog scale at monthly visits.

RESULTS

There were significant dose increases from weeks 1 to 2 and 2 to 6 (P < 0.05). Doses stabilized around week 6, suggesting the completion of what we defined as "titration." Both doses and pain ratings were stable when this titration phase was excluded from the analysis (P = 0.751; P = 0.056, respectively). Only 28% of the patients had any dose changes following this titration. While there was a significantly greater dose at week 52 compared with week 10 (P = 0.010), the increase in dose became insignificant after excluding four subjects who required two dose increases (P = 0.103).

CONCLUSIONS

The results showed that most of the titration/dose stabilization changes occurred within the first 10 weeks. A minority (28%) of subjects required dosage increases after this (defined) titration period. Pain reports stabilized statistically after 2 weeks. The findings of this post hoc analysis suggest a lack of opioid tolerance in the majority (72%) of these OA patients who completed this study following a defined titration period on OxymER.

SUMMARY

This post hoc analysis of oxymorphone ER consumption in osteoarthritis pain vs pain report showed that most dose changes occurred during an initial "titration period" as defined. Following this titration few subjects increased dose and analgesia remained stable. These findings suggest a lack of longitudinal opioid tolerance in the majority of those OA subjects who completed the trial.

Chronic stress induces transient spinal neuroinflammation, triggering sensory hypersensitivity and long-lasting anxiety-induced hyperalgesia

Chronic stressful events induce biochemical, physiological and psychological changes, resulting in stress-related neuropsychiatric disorders, such as anxiety or depression. Using repeated social defeat as a stressful event model, we show that this preclinical paradigm induces a transient increase in the expression of the genes encoding the pro-inflammatory molecules iNOS and COX-2. We provide the first demonstration that chronic stress affects spinal plasticity through a mechanism involving local neuroinflammation. The functional consequences of such neuroinflammation are associated with a transient decrease in the mechanical nociceptive threshold. Administration of the cholecystokinin(CCK)-2 receptor antagonist, CI-988, directly into the Rostral Ventromedial Medulla reverses the chronic stress-induced decrease in the nociceptive threshold. These data strongly suggest that chronic stress induces a spinal neuroinflammation associated with transient sensory hypersensitivity involving the activation of CCK-dependent nociceptive descending facilitatory pathways. Pharmacological data show that chronic social stress-induced long-lasting state of anxiety is not responsible for maintaining the spinal neuroinflammation and, therefore, for the associated sensory hypersensitivity. Conversely, an evaluation of pain-related behavior in the formalin model indicates that anxiety is directly related to prolonged hyperalgesia prevented by systemic benzodiazepine or CCK-2 receptor antagonist treatments. The present study highlights the adverse effects of chronic stress on spinal neuroinflammation triggering sensory hypersensitivity. Exploration of this phenomenon points out the divergence between pain sensitivity and anxiety-induced hyperalgesia, which is in agreement with clinical observations. Altogether, these data open up new perspectives for clinical research devoted to the evaluation and treatment of pain in anxio-depressive patients.

Triptan-induced latent sensitization: a possible basis for medication overuse headache

OBJECTIVE

Identification of the neural mechanisms underlying medication overuse headache resulting from triptans.

METHODS

Triptans were administered systemically to rats by repeated intermittent injections or by continuous infusion over 6 days. Periorbital and hind paw sensory thresholds were measured to detect cutaneous allodynia. Immunofluorescent histochemistry was employed to detect changes in peptidic neurotransmitter expression in identified dural afferents. Enzyme-linked immunoabsorbent assay was used to measure calcitonin gene-related peptide (CGRP) levels in blood.

RESULTS

Sustained or repeated administration of triptans to rats elicited time-dependent and reversible cutaneous tactile allodynia that was maintained throughout and transiently after drug delivery. Triptan administration increased labeling for CGRP in identified trigeminal dural afferents that persisted long after discontinuation of triptan exposure. Two weeks after triptan exposure, when sensory thresholds returned to baseline levels, rats showed enhanced cutaneous allodynia and increased CGRP in the blood following challenge with a nitric oxide donor. Triptan treatment thus induces a state of latent sensitization characterized by persistent pronociceptive neural adaptations in dural afferents and enhanced responses to an established trigger of migraine headache in humans.

INTERPRETATION

Triptans represent the treatment of choice for moderate and severe migraine headaches. However, triptan overuse can lead to an increased frequency of migraine headache. Overuse of these medications could induce neural adaptations that result in a state of latent sensitization, which might increase sensitivity to migraine triggers. The latent sensitization could provide a mechanistic basis for the transformation of migraine to medication overuse headache.

Sustained morphine-induced sensitization and loss of diffuse noxious inhibitory controls in dura-sensitive medullary dorsal horn neurons

Overuse of medications used to treat migraine headache can produce a chronic daily headache, termed medication overuse headache (MOH). Although "overuse" of opioids, triptans, and over-the-counter analgesics can all produce MOH, the neuronal mechanisms remain unknown. Headache pain is likely to be produced by stimulation of primary afferent neurons that innervate the intracranial vasculature and the resulting activation of medullary dorsal horn (MDH) neurons. The present study compared the receptive field properties of MDH dura-sensitive neurons in rats treated with morphine to those given vehicle. Animals were implanted with osmotic minipumps or pellets for sustained subcutaneous administration of morphine or vehicle 6-7 d before recording from dura-sensitive neurons. Electrical and mechanical activation thresholds from the dura were significantly lower in chronic morphine-treated animals when compared to vehicle controls. In addition, sustained morphine increased the cutaneous receptive field sizes. The presence of diffuse noxious inhibitory controls (DNICs) was examined by placing the tail in 55 degrees C water during concomitant noxious thermal stimulation of the cutaneous receptive field, usually located in the ophthalmic region. The DNIC stimulus produced significant inhibition of heat-evoked activity in vehicle- but not chronic morphine-treated animals. Inactivation of the rostral ventromedial medulla with 4% lidocaine reinstated DNICs in chronic morphine-treated animals. These results are consistent with studies demonstrating a loss of DNICs in patients that suffer from chronic daily headache and may partially explain why overuse of medication used to treat migraine can induce headaches.

The Role of Opioids in Managing Chronic Non-cancer Pain

The use of opioids for the treatment of chronic non-cancer pain has become more widespread recently. Available data support the short-term use of opioids in clearly defined nociceptive and neuropathic pain states. Their use in ‘pathological’ pain states without a clear diagnosis, such as chronic low back pain, is more contentious. A decision to initiate opioid treatment in these conditions requires careful consideration of benefits and risks; the latter include not only com- monly considered adverse effects such as constipation, but also opioid-induced hyperalgesia, abuse, addiction and diversion. Ideally, treatment goals should not only be relief of pain, but also improvement of function. Opioid treatment of chronic non-cancer pain requires informed consent by, and preferably a treatment contract with, the patient. Treatment should be initiated by a trial period with defined endpoints using slow-release or transdermal opioids. Ongoing management of the patient requires ideally a multi-disciplinary setting. Treatment should not be regarded as life-long and can be discontinued by tapering the dose. Key words: Neuropathic pain, Opioid-induced hyperalgesia, Pain management, Prescription drug abuse

Buprenorphine for Neuropathic Pain—Targeting Hyperalgesia

Opioids are well known to relieve severe, acute, and chronic nociceptive pain, but neuropathic pain shows a relatively poor response to opioids. Buprenorphine, a partial mu and ORL-1-receptor agonist, kappa-delta receptor antagonist, interacts with different G proteins than potent mu agonists and hence is not cross-tolerant to standard opioids. Buprenorphine blocks central sensitization (hyperalgesia) that is commonly found with neuropathic pain. We present a patient with neuropathic pain and tactile allodynia in which buprenorphine alleviated the hyperalgesia to a greater extent than pain severity. We found buprenorphine to be effective in reducing hypersensitivity in neuropathic pain when pure mu agonists fail to produce a response or in individuals who are intolerant to pure mu agonists.