Chronic Morphine-Induced Changes in Signaling at the A3 Adenosine Receptor Contribute to Morphine-Induced Hyperalgesia, Tolerance, and Withdrawal.

Treating chronic pain by using opioids, such as morphine, is hampered by the development of opioid-induced hyperalgesia (OIH; increased pain sensitivity), antinociceptive tolerance, and withdrawal, which can contribute to dependence and abuse. In the central nervous system, the purine nucleoside adenosine has been implicated in beneficial and detrimental actions of morphine, but the extent of their interaction remains poorly understood. Here, we demonstrate that morphine-induced OIH and antinociceptive tolerance in rats is associated with a twofold increase in adenosine kinase (ADK) expression in the dorsal horn of the spinal cord. Blocking ADK activity in the spinal cord provided greater than 90% attenuation of OIH and antinociceptive tolerance through A3 adenosine receptor (A3AR) signaling. Supplementing adenosine signaling with selective A3AR agonists blocked OIH and antinociceptive tolerance in rodents of both sexes. Engagement of A3AR in the spinal cord with an ADK inhibitor or A3AR agonist was associated with reduced dorsal horn of the spinal cord expression of the NOD-like receptor pyrin domain-containing 3 (60%-75%), cleaved caspase 1 (40%-60%), interleukin (IL)-1ß (76%-80%), and tumor necrosis factor (50%-60%). In contrast, the neuroinhibitory and anti-inflammatory cytokine IL-10 increased twofold. In mice, A3AR agonists prevented the development of tolerance in a model of neuropathic pain and reduced naloxone-dependent withdrawal behaviors by greater than 50%. These findings suggest A3AR-dependent adenosine signaling is compromised during sustained morphine to allow the development of morphine-induced adverse effects. These findings raise the intriguing possibility that A3AR agonists may be useful adjunct to opioids to manage their unwanted effects. SIGNIFICANCE STATEMENT: The development of hyperalgesia and antinociceptive tolerance during prolonged opioid use are noteworthy opioid-induced adverse effects that reduce opioid efficacy for treating chronic pain and increase the risk of dependence and abuse. We report that in rodents, these adverse effects are due to reduced adenosine signaling at the A3AR, resulting in NOD-like receptor pyrin domain-containing 3-interleukin-1ß neuroinflammation in spinal cord. These effects are attenuated by A3AR agonists, suggesting that A3AR may be a target for therapeutic intervention with selective A3AR agonist as opioid adjuncts.

Imaging studies in Freund's complete adjuvant model of regional polyarthritis, a model suitable for the study of pain mechanisms, in the rat.

OBJECTIVE: In view of the extensive distribution in the spinal cord of primary afferent nociceptive nerve fibers from the rat hind paw, a model of regional inflammation would be useful in the study of arthritis pain. However, the subcutaneous intraplantar injection of a single low dose of Freund's complete adjuvant (CFA) has been used mostly as an inflammation model. We undertook this study to characterize this model by means of conventional radiography, microfocal computed tomography, and bone densitometry and by examining changes in pain-related behavior. METHODS: Male rats were injected unilaterally with CFA or saline subcutaneously in the hind paw and killed 7, 15, and 30 days after injection. Pain-related behavior was studied using the Hargreaves, von Frey, and acetone tests. RESULTS: CFA-injected animals developed soft tissue inflammation and polyarthritis restricted to the joints of the injected hind paw. No signs of joint involvement were observed 7 days after CFA injection. On day 15 after CFA injection, there was widening of joint space indicative of joint effusion. By day 30 after CFA injection, there was evidence of joint damage with joint space narrowing, erosions, osteophyte formation, and joint deformity. There were no changes contralaterally or in saline-injected rats. Mechanical hyperalgesia and cold allodynia were present in the affected hind paw from day 1 through day 30. CONCLUSION: Signs of arthritis were strictly unilateral and started only 2 weeks after injection. Since the affected area has a broad representation in the spinal cord, this model has advantages over monarthritis models for the study of plastic changes in the dorsal horn of the spinal cord.

Prescribing Time in Nature for Human Health and Well-Being: Study Protocol for Tailored Park Prescriptions.

Background: eHealth technologies offer an efficient method to integrate park prescriptions into clinical practice by primary health care (PHC) providers to help patients improve their health via tailored, nature-based health behavior interventions. This paper describes the protocol of the GoalRx Prescription Intervention (GPI) which was designed to leverage community resources to provide tailored park prescriptions for PHC patients. Methods: The GPI study was designed as a 3-arm, multi-site observational study. We enrolled low-income, rural adults either at-risk of or living with hypertension or diabetes (n = 75) from Federally Qualified Health Centers (FQHC) in two counties in North Carolina, USA into the 3-month intervention. Eligible participants self-selected to receive (1) a tailored park prescription intervention; (2) a tailored home/indoor PA prescription intervention; or (3) a healthy eating prescription (with no PA prescription beyond standard PA counseling advice that is already routinely provided in PHC) as the comparison group. The GPI app paired patient health data from the electronic health record with stated patient preferences and triggered app-integrated SMS motivation and compliance messaging directly to the patient. Patients were assessed at baseline and at a 3-month follow-up upon the completion of the intervention. The primary outcome (mean difference in weekly physical activity from baseline (T0) to post-intervention (T1) as measured by the Fitbit Flex 2) was assessed at 3 months. Secondary outcomes included assessment of the relationship between the intervention and biological markers of health, including body mass index (BMI), systolic and diastolic blood pressure, HbA1c or available glucose test (if applicable), and a depression screen score using the Patient Health Questionnaire 9. Secondary outcomes also included the total number of SMS messages sent, number of SMS messages responded to, number of SMS messages ignored, and opt-out rate. Discussion: The goal was to create a protocol utilizing eHealth technologies that addressed the specific needs of rural low-income communities and fit into the natural rhythms and processes of the selected FQHC clinics in North Carolina. This protocol offered a higher standard of health care by connecting patients to their PHC teams and increasing patient motivation to make longer-lasting health behavior changes.

Terminal arbor degeneration--a novel lesion produced by the antineoplastic agent paclitaxel.

The antineoplastic agent paclitaxel causes a dose-limiting distal, symmetrical, sensory peripheral neuropathy that is often accompanied by a neuropathic pain syndrome. In a low-dose model of paclitaxel-evoked painful peripheral neuropathy in the rat, we have shown that the drug causes degeneration of intraepidermal nerve fibers (IENFs), i.e. the fibers which give rise to the sensory afferent's terminal receptor arbor. However, we did not find any evidence for axonal degeneration in samples taken at the mid-nerve level. Here we aimed to determine whether the absence of degenerating peripheral nerve axons was due to sampling a level that was too proximal. We used electron microscopy to study the distal-most branches of the nerves innervating the hind paw glabrous skin of normal and paclitaxel-treated rats. We confirmed that we sampled at a time when IENF degeneration was prominent. Because degeneration might be easier to detect with higher paclitaxel doses, we examined a four-fold cumulative dose range (8-32 mg/kg). We found no evidence of degeneration in the superficial subepidermal axon bundles (sSAB) that are located just a few microns below the epidermal basal lamina. Specifically, for all three dose groups there was no change in the number of sSAB per millimeter of epidermal border, no change in the number of axons per sSAB and no change in the diameter of sSAB axons. We conclude that paclitaxel produces a novel type of lesion that is restricted to the afferent axon's terminal arbor; we name this lesion 'terminal arbor degeneration'.

Early diagnosis of peripheral nervous system involvement in Fabry disease and treatment of neuropathic pain: the report of an expert panel.

BACKGROUND: Fabry disease is an inherited metabolic disorder characterized by progressive lysosomal accumulation of lipids in a variety of cell types, including neural cells. Small, unmyelinated nerve fibers are particularly affected and small fiber peripheral neuropathy often clinically manifests at young age. Peripheral pain can be chronic and/or occur as provoked attacks of excruciating pain. Manifestations of dysfunction of small autonomic fibers may include, among others, impaired sweating, gastrointestinal dysmotility, and abnormal pain perception. Patients with Fabry disease often remain undiagnosed until severe complications involving the kidney, heart, peripheral nerves and/or brain have arisen. METHODS: An international expert panel convened with the goal to provide guidance to clinicians who may encounter unrecognized patients with Fabry disease on how to diagnose these patients early using simple diagnostic tests. A further aim was to offer recommendations to control neuropathic pain. RESULTS: We describe the neuropathy in Fabry disease, focusing on peripheral small fiber dysfunction - the hallmark of early neurologic involvement in this disorder. The clinical course of peripheral pain is summarized, and the importance of medical history-taking, including family history, is highlighted. A thorough physical examination (e.g., angiokeratoma, corneal opacities) and simple non-invasive sensory perception tests could provide clues to the diagnosis of Fabry disease. Reported early clinical benefits of enzyme replacement therapy include reduction of neuropathic pain, and adequate management of residual pain to a tolerable and functional level can substantially improve the quality of life for patients. CONCLUSIONS: Our recommendations can assist in diagnosing Fabry small fiber neuropathy early, and offer clinicians guidance in controlling peripheral pain. This is particularly important since management of pain in young patients with Fabry disease appears to be inadequate.

Hypolocomotion, asymmetrically directed behaviors (licking, lifting, flinching, and shaking) and dynamic weight bearing (gait) changes are not measures of neuropathic pain in mice.

BACKGROUND: Spontaneous (non-evoked) pain is a major clinical symptom of neuropathic syndromes, one that is understudied in basic pain research for practical reasons and because of a lack of consensus over precisely which behaviors reflect spontaneous pain in laboratory animals. It is commonly asserted that rodents experiencing pain in a hind limb exhibit hypolocomotion and decreased rearing, engage in both reflexive and organized limb directed behaviors, and avoid supporting their body weight on the affected side. Furthermore, it is assumed that the extent of these positive or negative behaviors can be used as a dependent measure of spontaneous chronic pain severity in such animals. In the present study, we tested these assumptions via blinded, systematic observation of digital video of mice with nerve injuries (chronic constriction or spared nerve injury), and automated assessment of locomotor behavior using photocell detection and dynamic weight bearing (i.e., gait) using the CatWalk system. RESULTS: We found no deficits in locomotor activity or rearing associated with neuropathic injury. The frequency of asymmetric (ipsilaterally directed) behaviors were too rare to be seriously considered as representing spontaneous pain, and in any case did not statistically exceed what was blindly observed on the contralateral hind paw and in control (sham operated and unoperated) mice. Changes in dynamic weight bearing, on the other hand, were robust and ipsilateral after spared nerve injury (but not chronic constriction injury). However, we observed timing, pharmacological, and genetic dissociation of mechanical allodynia and gait alterations. CONCLUSIONS: We conclude that spontaneous neuropathic pain in mice cannot be assessed using any of these measures, and thus caution is warranted in making such assertions.

Pathophysiology and animal models of cancer-related painful peripheral neuropathy.

There are undoubtedly several causes of painful peripheral neuropathy in cancer patients. Some mechanisms are directly attributable to the tumor; others lie with the therapy, be it surgery, radiation, or chemotherapy. Several animal models have been developed to study the pathophysiological mechanisms that contribute to neuropathic pain. These include inflammation-based models, nerve trauma-induced models, and chemotherapy-induced models of neuropathic pain. My colleagues and I recently identified abnormalities in mitochondrial structure and function in peripheral sensory fibers that are associated with neuropathic pain induced by common chemotherapeutic agents and that can be reversed by agents that enhance mitochondrial function. Our hope is that further identification and clarification of the pathophysiological mechanisms involved at the periphery will help us to develop new classes of medicines and treatment options.

Smoked cannabis for chronic neuropathic pain: a randomized controlled trial.

BACKGROUND: Chronic neuropathic pain affects 1%-2% of the adult population and is often refractory to standard pharmacologic treatment. Patients with chronic pain have reported using smoked cannabis to relieve pain, improve sleep and improve mood. METHODS: Adults with post-traumatic or postsurgical neuropathic pain were randomly assigned to receive cannabis at four potencies (0%, 2.5%, 6% and 9.4% tetrahydrocannabinol) over four 14-day periods in a crossover trial. Participants inhaled a single 25-mg dose through a pipe three times daily for the first five days in each cycle, followed by a nine-day washout period. Daily average pain intensity was measured using an 11-point numeric rating scale. We recorded effects on mood, sleep and quality of life, as well as adverse events. RESULTS: We recruited 23 participants (mean age 45.4 [standard deviation 12.3] years, 12 women [52%]), of whom 21 completed the trial. The average daily pain intensity, measured on the 11-point numeric rating scale, was lower on the prespecified primary contrast of 9.4% v. 0% tetrahydrocannabinol (5.4 v. 6.1, respectively; difference = 0.7, 95% confidence interval [CI] 0.02-1.4). Preparations with intermediate potency yielded intermediate but nonsignificant degrees of relief. Participants receiving 9.4% tetrahydrocannabinol reported improved ability to fall asleep (easier, p = 0.001; faster, p < 0.001; more drowsy, p = 0.003) and improved quality of sleep (less wakefulness, p = 0.01) relative to 0% tetrahydrocannabinol. We found no differences in mood or quality of life. The most common drug-related adverse events during the period when participants received 9.4% tetrahydrocannabinol were headache, dry eyes, burning sensation in areas of neuropathic pain, dizziness, numbness and cough. CONCLUSION: A single inhalation of 25 mg of 9.4% tetrahydrocannabinol herbal cannabis three times daily for five days reduced the intensity of pain, improved sleep and was well tolerated. Further long-term safety and efficacy studies are indicated. (International Standard Randomised Controlled Trial Register no. ISRCTN68314063).

A hypothesis for the cause of complex regional pain syndrome-type I (reflex sympathetic dystrophy): pain due to deep-tissue microvascular pathology.

Complex regional pain syndrome-type I (CRPS-I; reflex sympathetic dystrophy) is a chronic pain condition that usually follows a deep-tissue injury such as fracture or sprain. The cause of the pain is unknown. We have developed an animal model (chronic post-ischemia pain) that creates CRPS-I-like symptomatology. The model is produced by occluding the blood flow to one hind paw for 3 hours under general anesthesia. Following reperfusion, the treated hind paw exhibits an initial phase of hyperemia and edema. This is followed by mechano-hyperalgesia, mechano-allodynia, and cold-allodynia that lasted for at least 1 month. Light microscopic analyses and electron microscopic analyses of the nerves at the site of the tourniquet show that the majority of these animals have no sign of injury to myelinated or unmyelinated axons. However, electron microscopy shows that the ischemia-reperfusion injury produces a microvascular injury, slow-flow/no-reflow, in the capillaries of the hind paw muscle and digital nerves. We propose that the slow-flow/no-reflow phenomenon initiates and maintains deep-tissue ischemia and inflammation, leading to the activation of muscle nociceptors, and the ectopic activation of sensory afferent axons due to endoneurial ischemia and inflammation. These data, and a large body of clinical evidence, suggest that in at least a subset of CRPS-I patients, the fundamental cause of the abnormal pain sensations is ischemia and inflammation due to microvascular pathology in deep tissues, leading to a combination of inflammatory and neuropathic pain processes. Moreover, we suggest a unifying idea that relates the pathogenesis of CRPS-I to that of CRPS-II. Lastly, our hypothesis suggests that the role of the sympathetic nervous system in CRPS-I is a factor that is not fundamentally causative, but may have an important contributory role in early-stage disease.

CNS mastitis: nothing to worry about?

In this paper, we analyzed a very large field data set on intramammary infections (IMI) and the associated somatic cell count (SCC) in dairy cows. The objective of the study was to analyze the impact of coagulase-negative staphylococci (CNS) IMI on cow SCC, both mean and variability, and on the potential of these infections to have a major impact on the bulk milk SCC (BMSCC). Data and milk samples for bacterial culture were collected by Quality Milk Production Services (QMPS) between 1992 and March of 2007. The QMPS program services dairy farms in New York State and other states in the Northeastern USA and operates in conjunction with Cornell University. Only records from cows where SCC and milk production data were available, and where only one organism was isolated from bacterial cultures of milk samples (or where culture was negative) were used for this analysis. A total of 352,614 records from 4200 whole herd mastitis screening sampling qualified for this study. Within herds an average of 15% (S.D. 12%) of cows sampled were infected with CNS, ranging between 0 and 100%. Average within herd prevalence of cows with a CNS IMI and an SCC over 200,000 cells/ml was 2% (S.D. 4%) with a minimum of 0% and a maximum of 50%. Results of linear mixed models showed three distinct populations of IMI statuses: negative cultures with the lowest SCC; CNS and Corynebacterium bovis with a moderate increase in SCC, and Streptococcus agalactiae, Streptococcus spp. and Staphylococcus aureus showing an important increase in SCC. Surprisingly, milk production was slightly but significantly higher in CNS infected cows compared to culture-negative cows, whereas it was strongly reduced in cows with a major pathogen IMI. The percentage contribution of CNS infections to the BMSCC was 17.9% in herds with a BMSCC less than 200,000 cells/ml. This value decreased to 11.9 and 7.9% in herds with bulk milk SCC between 200,000 and 400,000 and over 400,000 cells/ml, respectively. We concluded that very few herds with milk quality problems would have an important increase in BMSCC that could be mostly attributed to CNS infections. On the other hand, in herds with low BMSCC, CNS infections may be an important contributor to the total number of somatic cells in the bulk milk.

Herpes zoster and postherpetic neuralgia: past, present and future.

OBJECTIVES: The history behind the current understanding of the varicella-zoster virus and its relationship to the pain conditions caused by shingles and postherpetic neuralgia are reviewed. The framework for the current conceptualization is Hope-Simpson's latency hypothesis. Data from recent work in virology, neuroanatomy and epidemiology are reviewed, as is work using varicella-zoster virus-infected animals. The recent data largely confirm Hope-Simpson's hypothesis and extend it significantly.

Cutaneous tactile allodynia associated with microvascular dysfunction in muscle.

BACKGROUND: Cutaneous tactile allodynia, or painful hypersensitivity to mechanical stimulation of the skin, is typically associated with neuropathic pain, although also present in chronic pain patients who do not have evidence of nerve injury. We examine whether deep tissue microvascular dysfunction, a feature common in chronic non-neuropathic pain, contributes to allodynia. RESULTS: Persistent cutaneous allodynia is produced in rats following a hind paw ischemia-reperfusion injury that induces microvascular dysfunction, including arterial vasospasms and capillary slow flow/no-reflow, in muscle. Microvascular dysfunction leads to persistent muscle ischemia, a reduction of intraepidermal nerve fibers, and allodynia correlated with muscle ischemia, but not with skin nerve loss. The affected hind paw muscle shows lipid peroxidation, an upregulation of nuclear factor kappa B, and enhanced pro-inflammatory cytokines, while allodynia is relieved by agents that inhibit these alterations. Allodynia is increased, along with hind paw muscle lactate, when these rats exercise, and is reduced by an acid sensing ion channel antagonist. CONCLUSION: Our results demonstrate how microvascular dysfunction and ischemia in muscle can play a critical role in the development of cutaneous allodynia, and encourage the study of how these mechanisms contribute to chronic pain. We anticipate that focus on the pain mechanisms associated with microvascular dysfunction in muscle will provide new effective treatments for chronic pain patients with cutaneous tactile allodynia.

Experimental studies of potential analgesics for the treatment of chemotherapy-evoked painful peripheral neuropathies.

OBJECTIVE: We investigated potential analgesics for chemotherapy-evoked neuropathic pain using rats treated with paclitaxel. DESIGN: Drugs were tested in a repeated dosing paradigm (four daily injections). Topiramate was tested with a long-term treatment paradigm (12 days). A literature search was performed to summarize prior data. MEASURES: Mechanical stimulation of the hind paw was used to assay antiallodynic and antihyperalgesic effects acutely and 24 hours after injection. RESULTS: Amitriptyline produced significant analgesia, but this was not apparent until after the second injection. Baclofen produced significant effects, but the response varied erratically. Mexiletine and NMED-126 (a mixed N- and T-type calcium channel blocker) produced consistent, significant analgesia when tested acutely, but the pain relief did not persist at 24 hours postinjection. Oxcarbazepine had no effect at any time. Tramadol produced consistent, near-complete analgesia when tested acutely, but the analgesia did not persist to 24 hours postinjection. Topiramate produced significant effects that were first evident after 6-8 days of dosing. CONCLUSIONS: The present data and data from the literature review suggest that there are several potential treatments for chemotherapy-evoked neuropathic pain. Nonsteroidal anti-inflammatory drugs have little or no efficacy. Opioids have an effect, but probably only with high doses. At least some antidepressants are analgesic in these conditions. Some, but clearly not all, anticonvulsants and sodium channel blockers have efficacy. Tramadol is a particularly promising candidate. Topiramate, acetyl-L-carnitine, carbamazepine, and venlafaxine may have protective or restorative effects. Clinical trials of these candidates are needed to advance the treatment of chemotherapy-evoked pain.

Dysregulation of sphingolipid metabolism contributes to bortezomib-induced neuropathic pain.

The development of chemotherapy-induced painful peripheral neuropathy is a major dose-limiting side effect of many chemotherapeutics, including bortezomib, but the mechanisms remain poorly understood. We now report that bortezomib causes the dysregulation of de novo sphingolipid metabolism in the spinal cord dorsal horn to increase the levels of sphingosine-1-phosphate (S1P) receptor 1 (S1PR1) ligands, S1P and dihydro-S1P. Accordingly, genetic and pharmacological disruption of S1PR1 with multiple S1PR1 antagonists, including FTY720, blocked and reversed neuropathic pain. Mice with astrocyte-specific alterations of S1pr1 did not develop neuropathic pain and lost their ability to respond to S1PR1 inhibition, strongly implicating astrocytes as a primary cellular substrate for S1PR1 activity. At the molecular level, S1PR1 engaged astrocyte-driven neuroinflammation and altered glutamatergic homeostasis, processes blocked by S1PR1 antagonism. Our findings establish S1PR1 as a target for therapeutic intervention and provide insight into cellular and molecular pathways. As FTY720 also shows promising anticancer potential and is FDA approved, rapid clinical translation of our findings is anticipated.

Sympathetic sprouting and changes in nociceptive sensory innervation in the glabrous skin of the rat hind paw following partial peripheral nerve injury.

Previous studies have suggested that sympathetic sprouting in the periphery may contribute to the development and persistence of sympathetically maintained pain in animal models of neuropathic pain. In the present study, we examined changes in the cutaneous innervation in rats with a chronic constriction injury to the sciatic nerve. At several periods postinjury, hind paw skin was harvested and processed by using a monoclonal antibody against dopamine-beta-hydroxylase to detect sympathetic fibers and a polyclonal antibody against calcitonin gene-related peptide to identify peptidergic sensory fibers. We observed migration and branching of sympathetic fibers into the upper dermis of the hind paw skin, where they were normally absent. This migration was first detected at 2 weeks, peaked at 4-6 weeks, and lasted for at least 20 weeks postlesion. At 8 weeks postlesion, there was a dramatic increase in the density of peptidergic fibers in the upper dermis. Quantification revealed that densities of peptidergic fibers 8 weeks postlesion were significantly above levels in sham animals. The ectopic sympathetic fibers did not innervate blood vessels but formed a novel association and wrapped around sprouted peptidergic nociceptive fibers. Our data show a long-term sympathetic and sensory innervation change in the rat hind paw skin after the chronic constriction injury. This novel fiber arrangement after nerve lesion may play an important role in the development and persistence of sympathetically maintained neuropathic pain after partial nerve lesions.

Unpleasant odors increase pain processing in a patient with neuropathic pain: psychophysical and fMRI investigation.

A 49-year old man with neuropathic pain in his right elbow, wrist and digits III-V of his hand reported that certain odors increased his pain by superimposing an electric shock-like pain to his already existing pain. Psychophysical testing revealed that the best predictor of pain exacerbation was odor unpleasantness. Functional magnetic resonance imaging (fMRI) showed increased activation following an unpleasant odor in pain related areas, including the thalamus, amygdala, insular and anterior cingulate cortices, with similar trends in primary somatosensory cortex hand/arm area. The increased pain and associated neural activations in response to unpleasant odors may be related to the phenomenon of synesthesia, to a rewiring of olfactory pathways onto pain pathways mimicking synesthesia or, to activation of the sympathetic nervous system.

Consensus guidelines: treatment planning and options. Diabetic peripheral neuropathic pain.

Despite the number of patients affected by diabetic peripheral neuropathic pain (DPNP), little consensus exists about the pathophysiology, best diagnostic tools, and primary treatment choices. Theories about the causes of DPNP are inextricably linked with the causes of diabetic neuropathles, yet most patients with such neuropathies do not experience pain. The factors that differentiate patients with pain from those without remain unknown and are the subject of much research. When choosing treatment for patients with DPNP, physicians are confronted with a myriad of choices, none of which has been shown to be effective for all patients. This article reviews the evidence for these treatments and attempts to guide physicians in choosing those treatments based on evidence from well-designed clinical trials to support their use. Two agents, duloxetine and pregabalin, are formally approved by the Food and Drug Administration for the treatment of DPNP. In addition, several other agents, including the tricyclic class of antidepressants, have been effective in clinical trials. Ultimately, treatment choice must also Include consideration of adverse effects, individual patient factors such as comorbidities, and often cost.

The role of sodium channels in chronic inflammatory and neuropathic pain.

UNLABELLED: Clinical and experimental data indicate that changes in the expression of voltage-gated sodium channels play a key role in the pathogenesis of neuropathic pain and that drugs that block these channels are potentially therapeutic. Clinical and experimental data also suggest that changes in voltage-gated sodium channels may play a role in inflammatory pain, and here too sodium-channel blockers may have therapeutic potential. The sodium-channel blockers of interest include local anesthetics, used at doses far below those that block nerve impulse propagation, and tricyclic antidepressants, whose analgesic effects may at least partly be due to blockade of sodium channels. Recent data show that local anesthetics may have pain-relieving actions via targets other than sodium channels, including neuronal G protein-coupled receptors and binding sites on immune cells. Some of these actions occur with nanomolar drug concentrations, and some are detected only with relatively long-term drug exposure. There are 9 isoforms of the voltage-gated sodium channel alpha-subunit, and several of the isoforms that are implicated in neuropathic and inflammatory pain states are expressed by somatosensory primary afferent neurons but not by skeletal or cardiovascular muscle. This restricted expression raises the possibility that isoform-specific drugs might be analgesic and lacking the cardiotoxicity and neurotoxicity that limit the use of current sodium-channel blockers. PERSPECTIVE: Changes in the expression of neuronal voltage-gated sodium channels may play a key role in the pathogenesis of both chronic neuropathic and chronic inflammatory pain conditions. Drugs that block these channels may have therapeutic efficacy with doses that are far below those that impair nerve impulse propagation or cardiovascular function.

Acetyl-L-carnitine prevents and reduces paclitaxel-induced painful peripheral neuropathy.

This study examines the potential efficacy of acetyl-L-carnitine (ALC) to prevent and treat paclitaxel-induced pain. Rats received four intraperitoneal (i.p.) injections of 2 mg/kg paclitaxel on alternate days which, following a short delay induced marked mechanical hypersensitivity. Daily administration of ALC (50 mg/kg and 100 mg/kg; p.o.; concurrently with paclitaxel and for 14 days afterwards) prevented the development of paclitaxel-induced pain. This effect was long lasting, for at least 3 weeks after the last dose of ALC. In a separate experiment, daily administration of ALC (100 mg/kg; p.o.; for 10 days) to rats with established paclitaxel-induced pain produced an analgesic effect. This effect dissipated shortly after ALC treatment was withdrawn. We conclude that ALC may be useful in the prevention and treatment of chemotherapy-induced painful peripheral neuropathy.

Dysregulation of cellular calcium homeostasis in chemotherapy-evoked painful peripheral neuropathy.

Paclitaxel and vincristine are chemotherapeutic drugs that often evoke a long-lasting painful peripheral neuropathy. Using drugs that reduce intracellular or extracellular calcium ions (Ca2+), we investigated the hypothesis that impaired Ca2+ regulation contributes to the chemotherapy-evoked neuropathic pain syndrome. For comparison, we also tested rats with painful peripheral neuropathy caused by nerve trauma and to the anti-human immunodeficiency virus nucleoside analog 2',3'-dideoxycytidine (ddC). Normal naïve (without neuropathy), paclitaxel-treated, and vincristine-treated rats received the following intrathecal injections: TMB-8 (46 nmol), Quin-2 (1.8 nmol), EGTA (0.1 micromol), EGTA-am (0.1 micromol), and their vehicle controls. Chronic constriction injury (CCI) rats were examined after TMB-8 and Quin-2 injections, and ddC-treated rats were examined after receiving TMB-8. Mechano-allodynia and mechano-hyperalgesia were evaluated after each injection. Drug effects on heat hyperalgesia were also tested in CCI rats. All four Ca2+-reducing drugs significantly inhibited mechano-allodynia and mechano-hyperalgesia in the rats treated with paclitaxel, vincristine, or ddC, but no effects were seen in the CCI or naïve rats. We conclude that a similar abnormality of cellular Ca2+ homeostasis contributes to the pain caused by paclitaxel, vincristine, and ddC, but not posttraumatic painful peripheral neuropathy.