Muscle pain in models of chemotherapy-induced and alcohol-induced peripheral neuropathy

Sex-specific differences in alcohol-induced pain sensitization

Pain sensitization is a phenomenon that occurs to protect tissues from damage and recent studies have shown how a variety of non-noxious stimuli included in our everyday lives can lead to pain sensitization. Consumption of large amounts of alcohol over a long period of time invokes alcohol use disorder (AUD), a complex pathological state that has many manifestations, including alcohol peripheral neuropathy (neuropathic pain). We asked if 'non-pathological' alcohol consumption can cause pain sensitization in the absence of other pathology? Studies have pointed to glia and other immune cells and their role in pain sensitization that results in cell and sex-specific responses. Using a low-dose and short-term ethanol exposure model, we investigated whether this exposure would sensitize mice to a subthreshold dose of an inflammatory mediator that normally does not induce pain. We observed female mice exhibited specific mechanical and higher thermal sensitivity than males. We also observed an increase in CD68⁺ macrophages in the ipsilateral dorsal root ganglia (DRG) and Iba1⁺ microglia in the ipsilateral spinal dorsal horn of animals that were exposed to ethanol and injected with subthreshold inflammatory prostaglandin E2. Our findings suggest that short-term ethanol exposure stimulates peripheral and central, immune and glial activation, respectively to induce pain sensitization. This work begins to reveal a possible mechanism behind the development of alcoholic peripheral neuropathy.

Probiotics attenuate alcohol-induced muscle mechanical hyperalgesia: Preliminary observations

Alcohol use disorder (AUD) is a major health problem that causes millions of deaths annually world-wide. AUD is considered to be a chronic pain disorder, that is exacerbated by alcohol withdrawal, contributing to a high (∼80%) relapse rate. Chronic alcohol consumption has a marked impact on the gut microbiome, recognized to have a significant effect on chronic pain. We tested the hypothesis that modulating gut microbiota through feeding rats with probiotics can attenuate alcohol-induced muscle mechanical hyperalgesia. To test this hypothesis, rats were fed alcohol (6.5%, 4 days on 3 days off) for 3 weeks, which induced skeletal muscle mechanical hyperalgesia. Following alcohol feeding, at which time nociceptive thresholds were ∼37% below pre-alcohol levels, rats received probiotics in their drinking water, either Lactobacillus Rhamnosus GG (Culturelle) or De Simone Formulation (a mixture of 8 bacterial species) for 8 days; control rats received plain water to drink. When muscle mechanical nociceptive threshold was evaluated 1 day after beginning probiotic feeding, nociceptive thresholds were significantly higher than rats not receiving probiotics. Mechanical nociceptive thresholds continued to increase during probiotic feeding, with thresholds approaching pre-alcohol levels 5 days after starting probiotics; nociceptive threshold in rats not receiving probiotics remained low. After probiotics were removed from the drinking water, nociceptive thresholds gradually decreased in these two groups, although they remained higher than the group not treated with probiotic (21 days after ending alcohol feeding). These observations suggest that modification of gut microbiota through probiotic feeding has a marked effect on chronic alcohol-induced muscle mechanical hyperalgesia. Our results suggest that administration of probiotics to individuals with AUD may reduce pain associated with alcohol consumption and withdrawal, and may be a novel therapeutic intervention to reduce the high rate of relapse seen in individuals with AUD attempting to abstain from alcohol.

Neuro-immune interactions in paclitaxel-induced peripheral neuropathy

BACKGROUND

Paclitaxel is a taxane-based chemotherapeutic agent used as a treatment in breast cancer. There is no effective prevention or treatment strategy for the most common side effect of peripheral neuropathy. In this manuscript, we reviewed the molecular mechanisms that contribute to paclitaxel-induced peripheral neuropathy (PIPN) with an emphasis on immune-related processes.

METHODS

A systematic search of the literature was conducted in PubMed, EMBASE and Cochrane Library. The SYRCLE's risk of bias tool was used to assess internal validity.

RESULTS

156 studies conducted with rodent models were included. The risk of bias was high due to unclear methodology. Paclitaxel induces changes in myelinated axons, mitochondrial dysfunction, and mechanical hypersensitivity by affecting ion channels expression and function and facilitating spinal transmission. Paclitaxel-induced inflammatory responses are important contributors to PIPN.

CONCLUSION

Immune-related processes are an important mechanism contributing to PIPN. Studies in humans that validate these mechanistic data are highly needed to facilitate the development of therapeutic strategies.

Marked sexual dimorphism in neuroendocrine mechanisms for the exacerbation of paclitaxel-induced painful peripheral neuropathy by stress

Chemotherapy-induced neuropathic pain is a serious adverse effect of chemotherapeutic agents. Clinical evidence suggests that stress is a risk factor for development and/or worsening of chemotherapy-induced peripheral neuropathy (CIPN). We evaluated the impact of stress and stress axis mediators on paclitaxel CIPN in male and female rats. Paclitaxel produced mechanical hyperalgesia, over the 4-day course of administration, peaking by day 7, and still present by day 28, with no significant difference between male and female rats. Paclitaxel hyperalgesia was enhanced in male and female rats previously exposed to unpredictable sound stress, but not in rats that were exposed to sound stress after developing paclitaxel CIPN. We evaluated the role of the neuroendocrine stress axes: in adrenalectomized rats, paclitaxel did not produce hyperalgesia. Intrathecal administration of antisense oligodeoxynucleotides (ODN) reduced expression of β2-adrenergic receptors on nociceptors, and paclitaxel-induced hyperalgesia was slightly attenuated in males, but markedly attenuated in females. By contrast, after intrathecal administration of antisense ODN to decrease expression of glucocorticoid receptors, hyperalgesia was markedly attenuated in males, but unaffected in females. Both ODNs together markedly attenuated paclitaxel-induced hyperalgesia in both males and females. We evaluated paclitaxel-induced CIPN in stress-resilient (produced by neonatal handling) and stress-sensitive (produced by neonatal limited bedding). Neonatal handling significantly attenuated paclitaxel-induced CIPN in adult male, but not in adult female rats. Neonatal limited bedding did not affect the magnitude of paclitaxel-induced CIPN in either male or female. This study provides evidence that neuroendocrine stress axis activity has a marked, sexually dimorphic, effect on paclitaxel-induced painful CIPN.

Behavior and electrophysiology studies of the peripheral neuropathy induced by individual and co-administration of paclitaxel and oxaliplatin in rat

AIMS

Antitumor agents, as taxanes and platinum compounds, induce peripheral neuropathies which can hamper their use for cancer treatment. The study of chemotherapy-induced neuropathies in humans is difficult because of ethical reasons, differences among administration protocols and intrinsic characteristics of patients. The aim of the present study is to compare the neuropathic signs induced by individual or combined administration of paclitaxel and oxaliplatin.

MAIN METHODS

Oxaliplatin and paclitaxel were administered individually and combined to induce peripheral neuropathy in rats, sensory neuropathic signs were assessed in the hind limbs and orofacial area. The in vitro skin-saphenous nerve preparation was used to record the axonal activity of Aδ sensory neurons.

KEY FINDINGS

Animals treated with the combination developed mechanical allodynia in the paws and muscular hyperalgesia in the orofacial area, which was similar to that in animals treated with monotherapy, the latter also developed cold allodynia in the paws. Aδ-fibers of the rats treated with the combination were hyperexcited and presented hypersensitivity to pressure stimulation of the innervated skin, also similar to that recorded in the fibers of the animals treated with monotherapy.

SIGNIFICANCE

Our work objectively demonstrates that the combination of a platinum compound with a taxane does not worsen the development of sensorial neuropathies in rats, which is an interesting data to take into account when the combination of antitumor drugs is necessary. Co-administration of antitumor drugs is more effective in cancer treatment without increasing the risk of the disabling neuropathic side effects.

Involvement of TACAN, a Mechanotransducing Ion Channel, in Inflammatory But Not Neuropathic Hyperalgesia in the Rat

TACAN (Tmem120A), a mechanotransducing ion channel highly expressed in a subset of nociceptors, has recently been shown to contribute to detection of noxious mechanical stimulation. In the present study we evaluated its role in sensitization to mechanical stimuli associated with preclinical models of inflammatory and chemotherapy-induced neuropathic pain (CIPN). Intrathecal administration of an oligodeoxynucleotide antisense (AS-ODN) to TACAN mRNA attenuated TACAN protein expression in rat dorsal root ganglia (DRG). While TACAN AS-ODN produced only a modest increase in mechanical nociceptive threshold, it markedly reduced mechanical hyperalgesia produced by intradermal administration of prostaglandin E₂, tumor necrosis factor alpha, and low molecular weight hyaluronan, and systemic administration of lipopolysaccharide, compatible with a prominent role of TACAN in mechanical hyperalgesia produced by inflammation. In contrast, TACAN AS-ODN had no effect on mechanical hyperalgesia associated with CIPN produced by oxaliplatin or paclitaxel. Our results provide evidence that TACAN plays a role in mechanical hyperalgesia induced by pronociceptive inflammatory mediators, but not CIPN, compatible with multiple mechanisms mediating mechanical nociception, and sensitization to mechanical stimuli in preclinical models of inflammatory versus CIPN. PERSPECTIVE: We evaluated the role of TACAN, a mechanotransducing ion channel in nociceptors, in preclinical models of inflammatory and CIPN. Attenuation of TACAN expression reduced hyperalgesia produced by inflammatory mediators but had not chemotherapeutic agents. Our findings support the presence of multiple mechanotransducers in nociceptors.

Prevention of paclitaxel-induced neuropathy by formulation approach

Chemotherapy-induced peripheral neuropathy (CIPN) is a major adverse effect of paclitaxel. Several liposome-based products have been approved and demonstrated superior efficacy and safety profiles for other drugs. The first objective of this work was to evaluate the effect of liposome formulation of paclitaxel (L-PTX) on neurotoxicity in-vitro and in-vivo in comparison to the standard Taxol® formulation. The second aim was to investigate the effect of formulation on paclitaxel biodistribution following intravenous administration in an animal model. Free paclitaxel was toxic to cell of neuronal origin (IC50 = 18.4 μg/mL) at a lower concentration than to lung cancer cells (IC50 = 59.1 μg/mL), and L-PTX demonstrated a comparable toxicity in both cell lines (IC50 = 31.8 and 33.7 μg/mL). Administration of L-PTX at 2 mg/kg per dose for a total of 4 doses on day 0, 2, 4, and 6 to rats did not result in increased sensitivity in response to mechanical or thermal stimulation of hind paws, in comparison to Taxol® administration at the same dose level that resulted in neuropathy. Paclitaxel biodisposition was evaluated for two formulations in plasma, liver, lung, brain, spinal cord, skin and muscle of rats after single intravenous dose at 6 mg/kg. The exposure to paclitaxel in brain, spinal cord, muscle, and skin was lower in the L-PTX group compared to Taxol® group. PEGylated liposomes containing paclitaxel were successfully developed and demonstrated reduced neurotoxicity in-vitro in neuronal cells and prevented development of peripheral neuropathy in-vivo. This proof of concept study showed that formulation in nanoparticles is a promising approach for reducing (or preventing) neurotoxicity caused by cancer drugs.

Activating transcription factor 3 modulates protein kinase C epsilon activation in diabetic peripheral neuropathy

Background

Skin denervation that develops in patients with diabetes mellitus as a neuropathic manifestation is known as diabetic peripheral neuropathy (DPN). Skin denervation is parallel to neuronal injuries that alter intracellular signaling. To date, the correlation between nerve injury and the activation of intracellular responses to neuropathic manifestations has not been elucidated; specifically, whether activating transcription factor 3 (ATF3) is responsible for neuronal injury and a critical molecule that modulates the activation of intracellular protein kinase C epsilon (p-PKCε) and pain development in DPN is a crucial question.

Methods

To address, ATF3 knockout (atf3^−/− group, C57/B6 genetic background) and wild-type mice (atf3^+/+ group) received a single dose of streptozotocin (200 mg/kg) to generate a mouse model of DPN.

Results

Both atf3^+/+ and atf3^−/− mice exhibited hyperglycemia and the same pathology of skin denervation at posttreatment month 2, but only atf3^+/+ mice developed thermal hyperalgesia (P<0.001) and mechanical allodynia (P=0.002). The atf3^+/+ group, but not the atf3^−/− group, had preferential ATF3 upregulation on p-PKCε(+) neurons with a ratio of 37.7%±6.1% in p-PKCε(+):ATF3(+) neurons (P<0.001). In addition, B-cell lymphoma-extra large (Bcl-_XL), an antiapoptotic Bcl2 family protein, exhibited parallel patterns to p-PKCε (ie, Bcl-_XL upregulation was reversed in atf3^−/− mice). These two molecules were colocalized and increased by approximately two-fold in the atf3^+/+ group compared with the atf3^−/− group (30.0%±3.4% vs 13.7% ± 6.2%, P=0.003). Furthermore, linear analysis results showed that the densities of p-PKCε and Bcl-_XL had a reverse linear relationship with the degrees of thermal hyperalgesia and mechanical allodynia.

Conclusion

Collectively, this report suggested that ATF3 is a critical upstream molecule that modulates p-PKCε and Bcl-_XL expression, which consequently mediated the development of neuropathic manifestation in DPN.

Nociceptor interleukin 10 receptor 1 is critical for muscle analgesia induced by repeated bouts of eccentric exercise in the rat

Delayed-onset muscle soreness is typically observed after strenuous or unaccustomed eccentric exercise. Soon after recovery, blunted muscle soreness is observed on repeated eccentric exercise, a phenomenon known as repeated bout effect (RBE). Although regular physical activity decreases muscle hyperalgesia, likely because of increased production of the anti-inflammatory cytokine interleukin-10 (IL-10) in the skeletal muscle, whether IL-10 also contributes to the antinociceptive effect of RBE is unknown. Furthermore, whether IL-10 attenuates muscle hyperalgesia by acting on muscle nociceptors remains to be established. Here, we explored the hypothesis that blunted muscle nociception observed in RBE depends on a local effect of IL-10, acting on IL-10 receptor 1 (IL-10R1) expressed by muscle nociceptors. Results show that after a second bout of eccentric exercise, rats exhibited decreased muscle hyperalgesia, indicative of RBE, and increased expression of IL-10 in the exercised gastrocnemius muscle. Although knockdown of IL-10R1 protein in nociceptors innervating the gastrocnemius muscle by intrathecal antisense oligodeoxynucleotide did not change nociceptive threshold in naive rats, it unveiled latent muscle hyperalgesia in rats submitted to eccentric exercise 12 days ago. Furthermore, antisense also prevented the reduction of muscle hyperalgesia observed after a second bout of eccentric exercise. These data indicate that recovery of nociceptive threshold after eccentric exercise and RBE-induced analgesia depend on a local effect of IL-10, acting on its canonical receptor in muscle nociceptors.

CD44 Signaling Mediates High Molecular Weight Hyaluronan-Induced Antihyperalgesia

We studied, in male Sprague Dawley rats, the role of the cognate hyaluronan receptor, CD44 signaling in the antihyperalgesia induced by high molecular weight hyaluronan (HMWH). Low molecular weight hyaluronan (LMWH) acts at both peptidergic and nonpeptidergic nociceptors to induce mechanical hyperalgesia that is prevented by intrathecal oligodeoxynucleotide antisense to CD44 mRNA, which also prevents hyperalgesia induced by a CD44 receptor agonist, A6. Ongoing LMWH and A6 hyperalgesia are reversed by HMWH. HMWH also reverses the hyperalgesia induced by diverse pronociceptive mediators, prostaglandin E₂, epinephrine, TNFα, and interleukin-6, and the neuropathic pain induced by the cancer chemotherapy paclitaxel. Although CD44 antisense has no effect on the hyperalgesia induced by inflammatory mediators or paclitaxel, it eliminates the antihyperalgesic effect of HMWH. HMWH also reverses the hyperalgesia induced by activation of intracellular second messengers, PKA and PKC_ε, indicating that HMWH-induced antihyperalgesia, although dependent on CD44, is mediated by an intracellular signaling pathway rather than as a competitive receptor antagonist. Sensitization of cultured small-diameter DRG neurons by prostaglandin E₂ is also prevented and reversed by HMWH. These results demonstrate the central role of CD44 signaling in HMWH-induced antihyperalgesia, and establish it as a therapeutic target against inflammatory and neuropathic pain.SIGNIFICANCE STATEMENT We demonstrate that hyaluronan (HA) with different molecular weights produces opposing nociceptive effects. While low molecular weight HA increases sensitivity to mechanical stimulation, high molecular weight HA reduces sensitization, attenuating inflammatory and neuropathic hyperalgesia. Both pronociceptive and antinociceptive effects of HA are mediated by activation of signaling pathways downstream CD44, the cognate HA receptor, in nociceptors. These results contribute to our understanding of the role of the extracellular matrix in pain, and indicate CD44 as a potential therapeutic target to alleviate inflammatory and neuropathic pain.

Antihyperalgesic effect of tetrodotoxin in rat models of persistent muscle pain

Persistent muscle pain is a common and disabling symptom for which available treatments have limited efficacy. Since tetrodotoxin (TTX) displays a marked antinociceptive effect in models of persistent cutaneous pain, we tested its local antinociceptive effect in rat models of muscle pain induced by inflammation, ergonomic injury and chemotherapy-induced neuropathy. While local injection of TTX (0.03-1 μg) into the gastrocnemius muscle did not affect the mechanical nociceptive threshold in naïve rats, exposure to the inflammogen carrageenan produced a marked muscle mechanical hyperalgesia, which was dose-dependently inhibited by TTX. This antihyperalgesic effect was still significant at 24h. TTX also displayed a robust antinociceptive effect on eccentric exercise-induced mechanical hyperalgesia in the gastrocnemius muscle, a model of ergonomic pain. Finally, TTX produced a small but significant inhibition of neuropathic muscle pain induced by systemic administration of the cancer chemotherapeutic agent oxaliplatin. These results indicate that TTX-sensitive sodium currents in nociceptors play a central role in diverse states of skeletal muscle nociceptive sensitization, supporting the suggestion that therapeutic interventions based on TTX may prove useful in the treatment of muscle pain.

Roles of ASIC3, TRPV1, and NaV1.8 in the transition from acute to chronic pain in a mouse model of fibromyalgia

Background

Tissue acidosis is effective in causing chronic muscle pain. However, how muscle nociceptors contribute to the transition from acute to chronic pain is largely unknown.

Results

Here we showed that a single intramuscular acid injection induced a priming effect on muscle nociceptors of mice. The primed muscle nociceptors were plastic and permitted the development of long-lasting chronic hyperalgesia induced by a second acid insult. The plastic changes of muscle nociceptors were modality-specific and required the activation of acid-sensing ion channel 3 (ASIC3) or transient receptor potential cation channel V1 (TRPV1). Activation of ASIC3 was associated with increased activity of tetrodotoxin (TTX)-sensitive voltage-gated sodium channels but not protein kinase Cϵ (PKCϵ) in isolectin B4 (IB4)-negative muscle nociceptors. In contrast, increased activity of TTX-resistant voltage-gated sodium channels with ASIC3 or TRPV1 activation in Na_V1.8-positive muscle nociceptors was required for the development of chronic hyperalgesia. Accordingly, compared to wild type mice, Na_V1.8-null mice showed briefer acid-induced hyperalgesia (5 days vs. >27 days).

Conclusion

ASIC3 activation may manifest a new type of nociceptor priming in IB4-negative muscle nociceptors. The activation of ASIC3 and TRPV1 as well as enhanced Na_V1.8 activity are essential for the development of long-lasting hyperalgesia in acid-induced, chronic, widespread muscle pain.

Pain in cancer survivors

Pain is a common problem in cancer survivors, especially in the first few years after treatment. In the longer term, approximately 5% to 10% of survivors have chronic severe pain that interferes with functioning. The prevalence is much higher in certain subpopulations, such as breast cancer survivors. All cancer treatment modalities have the potential to cause pain. Currently, the approach to managing pain in cancer survivors is similar to that for chronic cancer-related pain, pharmacotherapy being the principal treatment modality. Although it may be appropriate to continue strong opioids in survivors with moderate to severe pain, most pain problems in cancer survivors will not require them. Moreover, because more than 40% of cancer survivors now live longer than 10 years, there is growing concern about the long-term adverse effects of opioids and the risks of misuse, abuse, and overdose in the nonpatient population. As with chronic nonmalignant pain, multimodal interventions that incorporate nonpharmacologic therapies should be part of the treatment strategy for pain in cancer survivors, prescribed with the aim of restoring functionality, not just providing comfort. For patients with complex pain issues, multidisciplinary programs should be used, if available. New or worsening pain in a cancer survivor must be evaluated to determine whether the cause is recurrent disease or a second malignancy. This article focuses on patients with a history of cancer who are beyond the acute diagnosis and treatment phase and on common treatment-related pain etiologies. The benefits and harms of the various pharmacologic and nonpharmacologic options for pain management in this setting are reviewed.

Role for monocyte chemoattractant protein-1 in the induction of chronic muscle pain in the rat

While raised levels of monocyte chemoattractant protein 1 (MCP-1) have been observed in patients with chronic muscle pain, direct evidence for its role as an algogen in skeletal muscle is still lacking. In the rat, MCP-1 induces a dose-dependent mechanical hyperalgesia lasting for up to 6weeks. Following recovery, rats exhibited a markedly prolonged hyperalgesia to an intramuscular injection of prostaglandin E2, hyperalgesic priming. Intrathecal pretreatment with isolectin B4 (IB4)-saporin, which selectively destroys IB4-positive (IB4+) nociceptors, markedly decreased MCP-1-induced hyperalgesia and prevented the subsequent development of priming. To evaluate the involvement of MCP-1 in stress-induced chronic pain we administered, intrathecally, antisense (AS) or mismatch oligodeoxynucleotides directed against CCR2 (the canonical receptor for MCP-1) mRNA, during the exposure to water-avoidance stress, a model of stress-induced persistent muscle pain. The AS treatment attenuated this hyperalgesia, whereas IB4-saporin abolished water-avoidance stress-induced muscle hyperalgesia and prevented stress-induced hyperalgesic priming. These results indicate that MCP-1 induces persistent muscle hyperalgesia and a state of latent chronic sensitization to other algogens, by action on its cognate receptor on IB4+ nociceptors. Because MCP-1 also contributes to stress-induced widespread chronic muscle pain, it should be considered as a player in chronic musculoskeletal pain syndromes.

The human amygdala and pain: evidence from neuroimaging

The amygdala, a small deep brain structure involved in behavioral processing through interactions with other brain regions, has garnered increased attention in recent years in relation to pain processing. As pain is a multidimensional experience that encompasses physical sensation, affect, and cognition, the amygdala is well suited to play a part in this process. Multiple neuroimaging studies of pain in humans have reported activation in the amygdala. Here, we summarize these studies by performing a coordinate-based meta-analysis within experimentally induced and clinical pain studies using an activation likelihood estimate analysis. The results are presented in relation to locations of peak activation within and outside of amygdala subregions. The majority of studies identified coordinates consistent with human amygdala cytoarchitecture indicating reproducibility in neuroanatomical labeling across labs, analysis methods, and imaging modalities. Differences were noted between healthy and clinical pain studies: in clinical pain studies, peak activation was located in the laterobasal region, suggestive of the cognitive-affective overlay present among individuals suffering from chronic pain; while the less understood superficial region of the amygdala was prominent among experimental pain studies. Taken together, these findings suggest several important directions for further research exploring the amygdala's role in pain processing.

Factors associated with the pressure pain threshold in healthy Chinese men

OBJECTIVE

The goal of present study was to examine the influence of demographic characteristics, lifestyle, and metabolic factors on pressure-induced pain threshold.

DESIGN AND SETTING

The study population comprised 2,517 healthy men at second-phase recruitment of a population-based cohort in China. The pressure pain threshold (PPT) at two locations, triceps and inguinal lines, was obtained using a digital pressure algometer. Education and occupation, as well as lifestyle factors, were self-reported in a face-to-face interview. Blood lipid and fasting glucose were measured as a routine healthy check-up program.

RESULTS

A lower PPT was found among men with younger age, higher level of education, and non-manual occupation. In addition, men with central obesity and moderate leisure time physical exercise were more sensitive to muscle mechanical stimuli. PPT was significantly correlated with high-density lipoprotein cholesterol (β for triceps = 0.064, and β for inguinal lines = 0.052) in a multivariate linear regression model, after controlling for multiple variables. Elevated PPT was also found among subjects with hyperglycemia (both P for triceps and inguinal lines <0.001) and excess drinking (P for triceps = 0.005).

CONCLUSIONS

It is important for physicians and researchers to consider these variables when evaluating pain sensitivity in clinic and in research. The underling mechanisms between these factors and pressure pain perception are worthy of further exploration.

Surgically induced neuropathic pain: understanding the perioperative process

Nerve damage takes place during surgery. As a consequence, significant numbers (10%-40%) of patients experience chronic neuropathic pain termed surgically induced neuropathic pain (SNPP). The initiating surgery and nerve damage set off a cascade of events that includes both pain and an inflammatory response, resulting in "peripheral and central sensitization," with the latter resulting from repeated barrages of neural activity from nociceptors. In affected patients, these initial events produce chemical, structural, and functional changes in the peripheral and central nervous systems (CNS). The maladaptive changes in damaged nerves lead to peripheral manifestations of the neuropathic state-allodynia, sensory loss, shooting pains, etc, that can manifest long after the effects of the surgical injury have resolved. The CNS manifestations that occur are termed "centralization of pain" and affect sensory, emotional, and other (eg, cognitive) systems as well as contributing to some of the manifestations of the chronic pain syndrome (eg, depression). Currently there are no objective measures of nociception and pain in the perioperative period. As such, intermittent or continuous pain may take place during and after surgery. New technologies including direct measures of specific brain function of nociception and new insights into preoperative evaluation of patients including genetic predisposition, appear to provide initial opportunities for decreasing the burden of SNPP, until treatments with high efficacy and low adverse effects that either prevent or treat pain are discovered.

The human amygdala and pain: evidence from neuroimaging

The amygdala, a small deep brain structure involved in behavioral processing through interactions with other brain regions, has garnered increased attention in recent years in relation to pain processing. As pain is a multidimensional experience that encompasses physical sensation, affect, and cognition, the amygdala is well suited to play a part in this process. Multiple neuroimaging studies of pain in humans have reported activation in the amygdala. Here, we summarize these studies by performing a coordinate-based meta-analysis within experimentally induced and clinical pain studies using an activation likelihood estimate analysis. The results are presented in relation to locations of peak activation within and outside of amygdala subregions. The majority of studies identified coordinates consistent with human amygdala cytoarchitecture indicating reproducibility in neuroanatomical labeling across labs, analysis methods, and imaging modalities. Differences were noted between healthy and clinical pain studies: in clinical pain studies, peak activation was located in the laterobasal region, suggestive of the cognitive-affective overlay present among individuals suffering from chronic pain; while the less understood superficial region of the amygdala was prominent among experimental pain studies. Taken together, these findings suggest several important directions for further research exploring the amygdala's role in pain processing.

GDNF induces mechanical hyperalgesia in muscle by reducing I(BK) in isolectin B4-positive nociceptors

We have assessed the mechanism underlying glial cell-derived neurotrophic factor (GDNF)-induced mechanical hyperalgesia in the gastrocnemius muscle, using patch clamp electrophysiology, in vivo electrophysiology and behavioral studies. Cultured isolectin B4-positive (IB4+) dorsal root ganglion neurons that innervated this muscle were held under current clamp; the majority developed an increase in action potential duration (a factor of increase of 2.29±0.24, compared to 1.13±0.17 in control, P<0.01) in response to GDNF (200 ng/ml) by 15 min after application. They also demonstrated a depolarization of resting membrane potential, but without significant changes in rheobase, action potential peak, or after-hyperpolarization. Large-conductance voltage- and calcium-activated potassium (BK) channels, which have recently been shown to play a role in the repolarization of IB4+ nociceptors, were inhibited under voltage clamp, as indicated by a significant reduction in the iberiotoxin-sensitive current. In vivo single-fiber recording from muscle afferents revealed that injection of iberiotoxin into their peripheral nociceptive field caused an increase in nociceptor firing in response to a 60s suprathreshold stimulus (an increase from 392.2±119.8 spikes to 596.1±170.8 spikes, P<0.05). This was observed in the absence of changes in the mechanical threshold. Finally, injection of iberiotoxin into the gastrocnemius muscle produced dose-dependent mechanical hyperalgesia. These data support the suggestion that GDNF induces nociceptor sensitization and mechanical hyperalgesia, at least in part, by inhibiting BK current in IB4+ nociceptors.

Characterization of oxaliplatin-induced chronic painful peripheral neuropathy in the rat and comparison with the neuropathy induced by paclitaxel

Anti-neoplastic agents in the platinum-complex, taxane, vinca alkaloid, and proteasome-inhibitor classes induce a dose-limiting, chronic, distal, symmetrical, sensory peripheral neuropathy that is often accompanied by neuropathic pain. Clinical descriptions suggest that these conditions are very similar, but clinical data are insufficient to determine the degree of similarity and to determine if they share common pathophysiological mechanisms. Animal models do not have the limitations of clinical studies and so we have characterized a rat model of chronic painful peripheral neuropathy induced by a platinum-complex agent, oxaliplatin, in order to compare it with a previously characterized model of chronic painful peripheral neuropathy induced by a taxane agent, paclitaxel. The oxaliplatin model evokes mechano-allodynia, mechano-hyperalgesia, and cold-allodynia that have a delayed onset, gradually increasing severity, a distinct delay to peak severity, and duration of about 2.5 months. There is no effect on heat sensitivity. Electron microscopy (EM) analyses found no evidence for axonal degeneration in peripheral nerve, and there is no upregulation of activating transcription factor-3 in the lumbar dorsal root ganglia. There is a statistically significant loss of intraepidermal nerve fibers in the plantar hind paw skin. Oxaliplatin treatment causes a significant increase in the incidence of swollen and vacuolated mitochondria in peripheral nerve axons, but not in their Schwann cells. Nerve conduction studies found significant slowing of sensory axons, but no change in motor axons. Single fiber recordings found an abnormal incidence of A- and C-fibers with irregular, low-frequency spontaneous discharge. Prophylactic dosing with two drugs that are known to protect mitochondria, acetyl-l-carnitine and olesoxime, significantly reduced the development of pain hypersensitivity. Our results are very similar to those obtained previously with paclitaxel, and support the hypothesis that these two agents, and perhaps other chemotherapeutics, produce very similar conditions because they have a mitotoxic effect on primary afferent neurons.

Abnormal muscle afferent function in a model of Taxol chemotherapy-induced painful neuropathy

Despite muscle pain being a well-described symptom in patients with diverse forms of peripheral neuropathy, the role of neuropathic mechanisms in muscle pain have received remarkably little attention. We have recently demonstrated in a well-established model of chemotherapy-induced painful neuropathy (CIPN) that the anti-tumor drug paclitaxel (Taxol) produces mechanical hyperalgesia in skeletal muscle, of similar time course to and with shared mechanism with cutaneous symptoms. In the present study, we evaluated muscle afferent neuron function in this rat model of CIPN. The mechanical threshold of muscle afferents in rats exposed to paclitaxel was not significantly different from the mechanical threshold of muscle afferents in control animals (P = 0.07). However, paclitaxel did produce a marked increase in the number of action potentials elicited by prolonged suprathreshold fixed intensity mechanical stimulation and a marked increase in the conduction velocity. In addition, the interspike interval (ISI) analysis (to evaluate the temporal characteristics of the response of afferents to sustained mechanical stimulation) showed a significant difference in rats treated with paclitaxel; there was a significantly greater ISI percentage of paclitaxel-treated muscle afferents with 0.01- and 0.02-s ISI. In contrast, an analysis of variability of neuronal firing over time (CV2 analysis) showed no effect of paclitaxel administration. These effects of paclitaxel on muscle afferent function contrast with the previously reported effects of paclitaxel on the function of cutaneous nociceptors.