TRPV1 channels make major contributions to behavioral hypersensitivity and spontaneous activity in nociceptors after spinal cord injury

Chronic neuropathic pain is often a severe and inadequately treated consequence of spinal cord injury (SCI). Recent findings suggest that SCI pain is promoted by spontaneous activity (SA) generated chronically in cell bodies of primary nociceptors in dorsal root ganglia (DRG). Many nociceptors express transient receptor potential V1 (TRPV1) channels, and in a preceding study most dissociated DRG neurons exhibiting SA were excited by the TRPV1 activator, capsaicin. The present study investigated roles of TRPV1 channels in behavioral hypersensitivity and nociceptor SA after SCI. Contusive SCI at thoracic segment T10 increased expression of TRPV1 protein in lumbar DRG 1 month after injury and enhanced capsaicin-evoked ion currents and Ca2+ responses in dissociated small DRG neurons. A major role for TRPV1 channels in pain-related behavior was indicated by the ability of a specific TRPV1 antagonist, AMG9810, to reverse SCI-induced hypersensitivity of hind limb withdrawal responses to mechanical and thermal stimuli at a dose that did not block detection of noxious heat. Similar reversal of behavioral hypersensitivity was induced by intrathecal oligodeoxynucleotides antisense to TRPV1, which knocked down TRPV1 protein and reduced capsaicin-evoked currents. TRPV1 knockdown also decreased the incidence of SA in dissociated nociceptors after SCI. Prolonged application of very low concentrations of capsaicin produced nondesensitizing firing similar to SA, and this effect was enhanced by prior SCI. These results show that TRPV1 makes important contributions to pain-related hypersensitivity long after SCI, and suggest a role for TRPV1-dependent enhancement of nociceptor SA that offers a promising target for treating chronic pain after SCI.

Peripheral substance P and neurokinin-1 receptors have a role in inflammatory and neuropathic orofacial pain models

There is accumulating evidence that substance P released from peripheral sensory neurons participates in inflammatory and neuropathic pain. In this study it was investigated the ability of substance P to induce orofacial nociception and thermal and mechanical hyperalgesia, as well as the role of NK1 receptors on models of orofacial inflammatory and neuropathic pain. Substance P injected into the upper lip at 1, 10 and 100 μg/50 μL failed to induce nociceptive behavior. Also, substance P (0.1-10 μg/50 μL) injected into the upper lip did not evoke orofacial cold hyperalgesia and when injected at 1 μg/50 μL did not induce mechanical hyperalgesia. However, substance P at this latter dose induced orofacial heat hyperalgesia, which was reduced by the pre-treatment of rats with a non-peptide NK1 receptor antagonist (SR140333B, 3mg/kg). Systemic treatment with SR140333B (3 mg/kg) also reduced carrageenan-induced heat hyperalgesia, but did not exert any influence on carrageenan-induced cold hyperalgesia. Blockade of NK1 receptors with SR140333B also reduced by about 50% both phases of the formalin response evaluated in the orofacial region. Moreover, heat, but not cold or mechanical, hyperalgesia induced by constriction of the infraorbital nerve, a model of trigeminal neuropathic pain, was abolished by pretreatment with SR140333B. Considering that substance P was peripherally injected (i.e. upper lip) and the NK1 antagonist used lacks the ability to cross the blood-brain-barrier, our results demonstrate that the peripheral SP/NK1 system participates in the heat hyperalgesia associated with inflammation or nerve injury and in the persistent pain evoked by formalin in the orofacial region.

Does naloxone reinstate secondary hyperalgesia in humans after resolution of a burn injury? A placebo-controlled, double-blind, randomized, cross-over study

INTRODUCTION

Development of secondary hyperalgesia following a cutaneous injury is a centrally mediated, robust phenomenon. The pathophysiological role of endogenous opioid signalling to the development of hyperalgesia is unclear. Recent animal studies, carried out after the resolution of inflammatory pain, have demonstrated reinstatement of tactile hypersensitivity following administration of μ-opioid-receptor-antagonists. In the present study in humans, we analyzed the effect of naloxone when given after the resolution of secondary hyperalgesia following a first-degree burn injury.

METHODS

Twenty-two healthy volunteers were included in this placebo-controlled, randomized, double-blind, cross-over study. Following baseline assessment of thermal and mechanical thresholds, a first-degree burn injury (BI; 47°C, 7 minutes, thermode area 12.5 cm(2)) was induced on the lower leg. Secondary hyperalgesia areas around the BI-area, and separately produced by brief thermal sensitization on the contralateral thigh (BTS; 45°C, 3 minutes, area 12.5 cm(2)), were assessed using a polyamide monofilament at pre-BI and 1, 2, and 3 hours post-BI. At 72 hrs, BI and BTS secondary hyperalgesia areas were assessed prior to start of a 30 minutes intravenous infusion of naloxone (total dose 21 microg/kg) or placebo. Fifteen minutes after start of the infusion, BI and BTS secondary hyperalgesia areas were reassessed, along with mechanical and thermal thresholds.

RESULTS

Secondary hyperalgesia areas were demonstrable in all volunteers 1-3 hrs post-BI, but were not demonstrable at 72 hrs post-burn in 73-86% of the subjects. Neither magnitude of secondary hyperalgesia areas nor the mechanical and thermal thresholds were associated with naloxone-treated compared to placebo-treated subjects.

CONCLUSION

Naloxone (21 microg/kg) did not reinstate secondary hyperalgesia when administered 72 hours after a first-degree burn injury and did not increase BTS-generated hyperalgesia. The negative results may be due to the low dose of naloxone or insufficient tissue injury to generate latent sensitization.

How to erase memory traces of pain and fear

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Currently emerging concepts of maladaptive pain and fear suggest that they share basic neuronal circuits and cellular mechanisms of memory formation.

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Recent studies have revealed processes of erasing memory traces of pain and fear that may be promising targets for future therapies.

Pain and fear are both aversive experiences that strongly impact on behaviour and well being. They are considered protective when they lead to meaningful, adaptive behaviour such as the avoidance of situations that are potentially dangerous to the integrity of tissue (pain) or the individual (fear). Pain and fear may, however, become maladaptive if expressed under inappropriate conditions or at excessive intensities for extended durations. Currently emerging concepts of maladaptive pain and fear suggest that basic neuronal mechanisms of memory formation are relevant for the development of pathological forms of pain and fear. Thus, the processes of erasing memory traces of pain and fear may constitute promising targets for future therapies.

Ultralow dose fentanyl prevents development of chronic neuropathic pain in rats

BACKGROUND

Opioids may cause progressive enhancement of pain sensitivity (opioid-induced hyperalgesia [OIH]) and thus, exacerbate existing pain. Animal studies also demonstrate paradoxical OIH with an ultralow dose (ULD, subanalgesic) of opioid; eg, the μ-opioid, morphine. Repeated administration of ULD-morphine resulted in tolerance to ULD-OIH. Prior exposure to ULD-morphine prolonged subsequent morphine antinociception in intact rats (delay of tolerance) and blocked neuropathic pain in nerve-injured rats (no hyperalgesia). Hence, pre-emptive desensitization of the excitatory function of opioid receptors may reduce further activation of a pain facilitatory system exerted by opioid or nerve injury.

OBJECTIVE

We determined if ULD-fentanyl (μ-opioid) and U50488H (κ-opioid) also affect post-nerve-injury neuropathy (a rat model of chronic constriction nerve injury [CCI]).

DESIGN

Fentanyl (0.5-500 ng/kg) was administered acutely in noninjured rats. Chronic fentanyl (5 ng/kg/day) was initiated either immediately after CCI (day 1-28) or when neuropathy was established (day 7-14) in nerve-injured rats. U50488H (25 μg/kg/day) was given on day 1-28 post-CCI. Saline served as control. Responsiveness was assessed using tail-flick and paw-pressure tests, respectively, in intact and CCI Sprague-Dawley rats of both sexes.

RESULTS

ULD-fentanyl evoked pain sensitization in noninjured rats. ULD-OIH was related to dose (inversely), gender (female > male), and was reversed by ketamine. Neuropathy developed after CCI in control (saline) rats. This was not observed in rats of either sex exposed to ULD-fentanyl on day 1-28 post-CCI. Rats treated with ULD-fentanyl from day 7 after CCI exhibited hyperalgesia similar to control rats. U50488H did not block post-CCI neuropathy (regardless of gender).

CONCLUSIONS

Pre-emptive use of ULD μ-opioid (not κ-opiod) blocked initiation (not maintenance) of neuropathic pain after CCI in rats. These data may suggest a novel treatment approach in situations when the potential development of neuropathy can be anticipated.

Safety of epicenter versus intact parenchyma as a transplantation site for human neural stem cells for spinal cord injury therapy

Neural stem cell transplantation may have the potential to yield repair and recovery of function in central nervous system injury and disease, including spinal cord injury (SCI). Multiple pathological processes are initiated at the epicenter of a traumatic spinal cord injury; these are generally thought to make the epicenter a particularly hostile microenvironment. Conversely, the injury epicenter is an appealing potential site of therapeutic human central nervous system-derived neural stem cell (hCNS-SCns) transplantation because of both its surgical accessibility and the avoidance of spared spinal cord tissue. In this study, we compared hCNS-SCns transplantation into the SCI epicenter (EPI) versus intact rostral/caudal (R/C) parenchyma in contusion-injured athymic nude rats, and assessed the cell survival, differentiation, and migration. Regardless of transplantation site, hCNS-SCns survived and proliferated; however, the total number of hCNS-SCns quantified in the R/C transplant animals was twice that in the EPI animals, demonstrating increased overall engraftment. Migration and fate profile were unaffected by transplantation site. However, although transplantation site did not alter the proportion of human astrocytes, EPI transplantation shifted the localization of these cells and exhibited a correlation with calcitonin gene-related peptide fiber sprouting. Critically, no changes in mechanical allodynia or thermal hyperalgesia were observed. Taken together, these data suggest that the intact parenchyma may be a more favorable transplantation site than the injury epicenter in the subacute period post-SCI.

Interaction of somatostatin receptor-2 and neuropeptide Y receptor-1 in mice dorsal root ganglion neurons on the pinch-nerve injury model

The aim of this paper was to evaluate the interactions of Somatostatin receptor-2 (SST2) and Neuropeptide Y receptors-1 (Y1) by molecular, pharmacological and behavioral studies. Double-immunolabeling of SST2 and Y1 has shown the colocalization of these two receptors in the dorsal root ganglion (DRG) neurons. On the basis of the Pinch nerve injury model, the mechanical hyperalgesia and severely painful behavior (autotomy) were detected after the application of SST2 antibody (anti-SST2; 200μg/kg) on the pinch-injured nerve. The differential distribution of Y1 and up-regulation of PKC expression in DRGs were observed after anti-SST2 treatment. Our results indicated for the first time the interactions of SST2 and Y1 in DRGs, which have functional role in pain modulation and might give rise to explore possible novel therapeutic strategies against pain.

Low-dose vaporized cannabis significantly improves neuropathic pain

We conducted a double-blind, placebo-controlled, crossover study evaluating the analgesic efficacy of vaporized cannabis in subjects, the majority of whom were experiencing neuropathic pain despite traditional treatment. Thirty-nine patients with central and peripheral neuropathic pain underwent a standardized procedure for inhaling medium-dose (3.53%), low-dose (1.29%), or placebo cannabis with the primary outcome being visual analog scale pain intensity. Psychoactive side effects and neuropsychological performance were also evaluated. Mixed-effects regression models demonstrated an analgesic response to vaporized cannabis. There was no significant difference between the 2 active dose groups' results (P > .7). The number needed to treat (NNT) to achieve 30% pain reduction was 3.2 for placebo versus low-dose, 2.9 for placebo versus medium-dose, and 25 for medium- versus low-dose. As these NNTs are comparable to those of traditional neuropathic pain medications, cannabis has analgesic efficacy with the low dose being as effective a pain reliever as the medium dose. Psychoactive effects were minimal and well tolerated, and neuropsychological effects were of limited duration and readily reversible within 1 to 2 hours. Vaporized cannabis, even at low doses, may present an effective option for patients with treatment-resistant neuropathic pain.

PERSPECTIVE

The analgesia obtained from a low dose of delta-9-tetrahydrocannabinol (1.29%) in patients, most of whom were experiencing neuropathic pain despite conventional treatments, is a clinically significant outcome. In general, the effect sizes on cognitive testing were consistent with this minimal dose. As a result, one might not anticipate a significant impact on daily functioning.

Intrathecal administration of NTS1 agonists reverses nociceptive behaviors in a rat model of neuropathic pain

Chronic neuropathic pain arising from peripheral nerve damage is a severe clinical issue where there is a major unmet medical need. We previously demonstrated that both neurotensin (NT) receptor subtypes 1 (NTS1) and 2 (NTS2) are involved in mediating the naloxone-insensitive antinociceptive effects of neurotensin in different analgesic tests including hotplate, tail-flick, and tonic pain. However, the role of these receptors in neuropathic pain management has been poorly investigated. In the present study, we therefore examined whether intrathecal delivery of NTS1 agonists was effective in reducing neuropathic pain symptoms in rats. Neuropathy was induced by sciatic nerve constriction (CCI model), and the development of mechanical allodynia and thermal hyperalgesia on the ipsi- and contralateral hind paws was examined 3, 7, 14, 21, and 28 days post-surgery. CCI-operated rats exhibited significant increases in thermal and mechanical hypersensitivities over a 28-day testing period. Spinal injection of NT to CCI rats alleviated the behavioral responses to radiant heat and mechanical stimuli, with a maximal reversal of 91% of allodynia at 6 μg/kg. Intrathecal administration of the NTS1-selective agonist, PD149163 (30-90 μg/kg) also produced potent anti-allodynic and anti-hyperalgesic effects in nerve-injured rats. Likewise, heat hyperalgesia and tactile allodynia produced by CCI of the sciatic nerve were fully reversed by the NTS1 agonist, NT69L (5-25 μg/kg). Altogether, these results support the idea that the NTS1 receptor subtype is involved in pain modulation, and the potential use of NTS1 agonists for the treatment of painful neuropathies.

The effect of venlafaxine on ongoing and experimentally induced pain in neuropathic pain patients: a double blind, placebo controlled study

BACKGROUND AND AIM

The aim of this randomized double blind placebo controlled study was to investigate the effectiveness and the safety of venlafaxine XR 75 and 150 mg on ongoing pain and on quantitative sensory tests in 60 patients with neuropathic pain for 8 weeks.

METHODS

Evaluation parameters consisted of ongoing pain intensity (VAS), patient satisfaction, side effects, global efficacy and tolerance. Quantitative sensory measurements taken from the affected area before and after the drug treatment included pin-prick hyperalgesia, allodynia, detection and pain thresholds to electrical and heat stimuli, temporal summation of repetitive electrical and heat stimuli.

RESULTS

A total of 55 patients completed the study. VAS scores decreased significantly compared to the baseline measurements in all groups. There was no significant difference between the groups regarding pain intensity and escape medication. The areas of allodynia and pin-prick hyperalgesia decreased significantly in venlafaxine groups compared to the placebo. There was no significant difference between the groups regarding the detection thresholds (electrical and heat). The pain threshold and the summation threshold to electrical stimuli and the summation threshold to heat stimuli increased significantly following treatment in both venlafaxine groups. In addition, the degree of the temporal summation to electrical and heat stimuli decreased significantly following treatment in both venlafaxine groups compared to the placebo.

CONCLUSION

The study showed significant effect of venlafaxine in the manifestations of hyperalgesia and temporal summation, but not on the ongoing pain intensity. Furthermore, the quantitative sensory tests provided complementing information to the clinical measures.

Indwelling supradural catheters for induction of facial allodynia: surgical procedures, application of inflammatory stimuli, and behavioral testing

Migraine headaches are debilitatingly painful and poorly managed. Facial allodynia is often associated with migraine, and clinical evidence indicates that it is a critical point in migraine progression. That is, if the migraine can be treated prior to the onset of facial allodynia, the migraine can be halted using triptans, whereas if treatment is administered after facial allodynia has begun, the treatment is ineffective. The meninges and the immune cells therein have been implicated in migraine facial pain. Indeed, application of inflammatory mediators over the meninges has been used to study changes in pain responsive neurons in trigeminal complex, and changes in their receptive fields. Much of this research has been carried out in anesthetized rats, which limits the clinical application. Our indwelling supradural catheter model, in which inflammatory mediators can be administered to the meninges in awake and freely moving rats, allows for the assessment of behavioral changes shortly after injection. Following administration of inflammatory soup (histamine, serotonin, bradykinin, and prostaglandin E2) or the immunogenic HIV-1 coat protein gp120 results in reliable periorbital mechanical allodynia. This model provides an additional means to study the neurocircuitry and neuropharmacology of facial allodynia. Here, we describe detailed methods for the placement of the catheter, injection procedures, and assessment of facial allodynia.

Effect of restraint stress on nociceptive responses in rats: role of the histaminergic system

Stress induced analgesia (SIA) is well known, but the reverse phenomenon, hyperalgesia is poorly documented. This study investigated the role of the histaminergic system in restraint stress hyperalgesia in rats, using thermal stimulation method (hot plate and tail flick tests). Paw licking and tail withdrawal latencies were taken before and after restraint for about one hour. Significant decreases (p<0.05) were obtained in these latencies after the restraint in both tests. Administration of H1 and H2 receptor blockers, chlorpheniramine and cimetidine respectively 30 mins before the restraint still resulted in significant reductions (p<0.05) in these latencies, connoting the persistence of hyperalgesia, showing that histamine H1 and H2 receptors did not participate in the mechanism of restraint stress hyperalgesia. We therefore suggest a histaminergic independent mechanism for restraint stress induced hyperalgesia.

Sexual dysfunction is correlated with tenderness in female fibromyalgia patients

OBJECTIVES

The purpose of the current study was to evaluate sexual dysfunction among female fibromyalgia syndrome (FMS) patients.

METHODS

Fifty female subjects were recruited and were asked to complete a questionnaire regarding sexual functioning. The control group included fifty-five healthy age-matched volunteers. The participants underwent a physical examination and tender point assessment was performed using manual palpation. All participants filled out the Arizona Sexual Experience Scale, which evaluates five areas of sexual functioning: sexual drive, sexual arousal, vaginal wetting, orgasm and sexual satisfaction.

RESULTS

FMS patients had significantly lower scores on all five aspects of sexual function assessed. A positive correlation was observed between the sexual drive score, signifying a decrease in sexual drive, and the number of tender points documented on examination. Similarly, a positive correlation was observed between the sexual satisfaction scale (indicating decreasing levels of sexual satisfaction) and the number of tender points documented. A positive correlation was demonstrated between the sexual arousal and orgasmatic scales and between the tender point counts, indicating a decrease in sexual arousal and in orgasmatic function in correlation with an increasing number of tender points.

CONCLUSIONS

The results of the current study indicate a multi-factorial sexual dysfunction among female FMS patients. All stages of sexual functioning, evaluated were significantly disturbed in comparison with the healthy controls. Physicians treating FMS patients should be aware of, and actively inquire about, sexual dysfunction as part of a multi-disciplinary evaluation of such patients.

Fibromyalgia in patients with rheumatoid arthritis: driven by depression or joint damage?

OBJECTIVES

Studies have shown an increased incidence of fibromyalgia (FMS) in RA patients. The aims of this study were to explore the effect of mood and disease damage on the prevalence of FMS.

METHODS

RA patients underwent a standardised clinical assessment, including disease activity (DAS-28), disease damage (mechanical joint score, MJS), fibromyalgia tender point assessment and the Hospital Anxiety and Depression Scale (HADS) and Health Assessment Questionnaire (HAQ). Patients were classified with FMS using two criteria a) tender-swollen joint count was ≥7 or b) tender point score of ≥11/18.

RESULTS

44/285 (15%) patients were classified as having FMS using the joint count difference of ≥7, compared to 18/285 (6%) using the tender point score of >11. Using the joint count difference to classify patients as having FMS, those with FMS had higher HAQ scores than those without FMS (2.12 vs. 1.5, p<0.0001). Although the DAS-28 was higher in this group (5.4 vs. 3.82, p<0.0001), the MJS was similar (8 vs. 7, p=0.19), suggesting similar levels of joint damage. Those classified as having FMS were more likely to have HAD-D scores of >11 (25% vs. 6%, p=0.0001).

CONCLUSIONS

Coexistent FMS was common in our cohort, although using the tender point count to define FMS classified fewer patients with FMS. Within this group those with FMS had higher levels of depression but similar scores for joint damage indicating that in this cohort FMS and poorer physical functioning is mediated by low mood rather than joint damage.

Effectiveness of multidisciplinary therapy on symptomatology and quality of life in women with fibromyalgia

OBJECTIVES

To study the effects of a 3-month multidisciplinary intervention based on exercise and psychological therapy on symptomatology and quality of life in women with fibromyalgia.

METHODS

Seventy-five women with fibromyalgia volunteered to participate and were allocated to a 3-month (3-times/week) multidisciplinary (pool, land-based and psychological session based on the Acceptance and Commitment Therapy) intervention (n=41), or to a usual care group (n=34). Sixty-five women with fibromyalgia completed the study protocol (n=33 multidisciplinary intervention, aged 51.4±7.4 years and n=32 usual care group, aged 50.0±7.3 years). The outcomes variables were Fibromyalgia Impact Questionnaire (FIQ), Short Form Health Survey 36 (SF-36), Hospital Anxiety and Depression Scale, Vanderbilt Pain Management Inventory and Rosenberg Self-Esteem Scale.

RESULTS

We observed a significant interaction effect (group*time) for the FIQ total score, the subscales fatigue, stiffness, anxiety and depression, and the subscales of SF-36 physical role, bodily pain, vitality and social functioning. Post-hoc analysis revealed significant improvements in total score of FIQ (p<0.001), fatigue (p=0.001), stiffness (p<0.001), anxiety (p=0.011), depression (p=0.008), physical role (p=0.002), bodily pain (p<0.001), vitality (p<0.001) and social functioning (p<0.001) in the intervention group, whereas in the control group, there was a significant worsening in the subscale depression (p=0.006) and social functioning (p=0.019).

CONCLUSIONS

A 3-month low-moderate intensity multidisciplinary intervention improved fibromyalgia symptomatology and quality of life in women with fibromyalgia.

Synergistic anti-allodynic effects of nociceptin/orphanin FQ and cannabinoid systems in neuropathic mice

Combinations of analgesics from different classes are commonly used in the management of chronic pain. The goal is to enhance pain relief together with the reduction of side effects. The present study was undertaken to examine the anti-allodynic synergy resulting from the combination of WIN 55,212-2, a cannabinoid CB1 receptor agonist, and JTC-801, a nociceptin/orphanin FQ receptor antagonist, on neuropathic pain. Mice were tested for behavioral effects before and 2-4 weeks after the surgery, in which a partial tight ligation of the sciatic nerve was made. Nerve injury-induced mechanical allodynia was assessed with Dynamic Plantar Aesthesiometer, and a hot/cold plate was used to assess cold allodynia. Both WIN 55,212-2 and JTC-801 produced dose-dependent mechanical and cold anti-allodynic effects. As shown by isobolographic analysis, WIN 55,212-2/JTC-801 combinations interacted synergistically at all three ratios studied in the mechanical allodynia assay. In conclusion, co-administration of a cannabinoid with a nociceptin/orphanin FQ receptor antagonist resulted in a synergistic interaction, which may have utility in the pharmacological treatment of neuropathic pain.

Negative affect heightens opiate withdrawal-induced hyperalgesia in heroin dependent individuals

This study examined the effect of emotion on opiate withdrawal induced hyperalgesia to determine whether emotional states modulate the magnitude of hyperalgesia. One hundred Hispanic men were recruited into one of three groups: heroin withdrawal, long-term heroin abstinence, and control. Participants were presented with pictures to induce neutral, positive, and negative emotional states. Affective valence, arousal, pain threshold, and tolerance to ischemic pain were measured. When pain threshold and tolerance were compared, the withdrawal group displayed significant heightened pain sensitivity when negative affect was induced. The authors also found that former heroin addicts showed heightened pain sensitivity following months of abstinence.

Involvement of the lateral amygdala in the antiallodynic and reinforcing effects of heroin in rats after peripheral nerve injury

BACKGROUND

Neuropathic pain alters opioid self-administration in rats. The brain regions altered in the presence of neuropathic pain mediating these differences have not been identified, but likely involve ascending pain pathways interacting with the limbic system. The amygdala is a brain region that integrates noxious stimulation with limbic activity.

METHODS

μ-Opioid receptors were blocked in the amygdala using the irreversible antagonist, β-funaltrexamine, and the antiallodynic and reinforcing effects of heroin were determined in spinal nerve-ligated rats. In addition, the effect of β-funaltrexamine was determined on heroin self-administration in sham-operated rats.

RESULTS

β-Funaltrexamine decreased functional activity of μ-opioid receptors by 60 ± 5% (mean ± SD). Irreversible inhibition of μ-opioid receptors in the amygdala significantly attenuated the ability of doses of heroin up to 100 μg/kg to reverse hypersensitivity after spinal nerve ligation. Heroin intake by self-administration in spinal nerve-ligated rats was increased from 5.0 ± 0.3 to 9.9 ± 2.1 infusions/h after administration of 2.5 nmol of β-funaltrexamine in the lateral amygdala, while having no effect in sham-operated animals (5.8 ± 1.6 before, 6.7 ± 0.9 after). The antiallodynic effects of 60 μg/kg heroin were decreased up to 4 days, but self-administration was affected for up to 14 days.

CONCLUSIONS

μ-Opioid receptors in the lateral amygdala partially meditate heroin's antiallodynic effects and self-administration after peripheral nerve injury. The lack of effect of β-funaltrexamine on heroin self-administration in sham-operated subjects suggests that opioids maintain self-administration through a distinct mechanism in the presence of pain.

Haemodialysis course is associated to changes in pain threshold and in the relations between arterial pressure and pain

ANTECEDENTS

Arterial pressure is negatively associated to pain perception.

OBJECTIVES

In this study, pain and the relations between arterial pressure and pain threshold were compared at the beginning and end of the haemodialysis.

METHODS

14 patients with chronic renal disease participated in the study. Pain thresholds were evaluated with pressure algometry bilaterally at two tender points: the second rib and the knee. Arterial pressure and pain thresholds were assessed twice: 1) 15 min alter dialysis onset and 2) 30 min before dialysis ended.

RESULTS

Arterial pressure remains unchanged through the dialysis. The course of dialysis was associated to a decrease in pain threshold in the second left rib and left and right knees. At the beginning of dialysis arterial pressure were uncorrelated with pain, while at the end of the dialysis both systolic and diastolic arterial pressure were strongly associated to pain thresholds (rs between 0.552 and 0.806): increased arterial pressure was associated to lower pain in terms of increased threshold.

CONCLUSIONS

Haemodialysis is associated to changes in pain sensitivity and in the relationships between arterial pressure and pain, suggesting a modification in the ascending pain inhibition system arising from the cardiovascular system. Possible explanations of this effect include the changes produced by haemodialysis in cognitive-perceptive functions, in autonomic cardiovascular regulation, and in the habituation of stress-related variables.

Prevention of oxaliplatin-induced mechanical allodynia and neurodegeneration by neurotropin in the rat model

Oxaliplatin is a key drug for colorectal cancer, but it causes acute peripheral neuropathy (triggered by cold) and chronic neuropathy (sensory and motor neuropathy) in patients. Neurotropin, a non-protein extract from the inflamed rabbit skin inoculated with vaccinia virus, has been used to treat various chronic pains. In the present study, we investigated the effect of neurotropin on the oxaliplatin-induced neuropathy in rats. Repeated administration of oxaliplatin caused cold hyperalgesia from Day 5 to Day 29 and mechanical allodynia from Day 15 to Day 47. Repeated administration of neurotropin relieved the oxaliplatin-induced mechanical allodynia but not cold hyperalgesia, and inhibited the oxaliplatin-induced axonal degeneration in rat sciatic nerve. Neurotropin also inhibited the oxaliplatin-induced neurite degeneration in cultured pheochromocytoma 12 (PC12) and rat dorsal root ganglion (DRG) cells. On the other hand, neurotropin did not affect the oxaliplatin-induced cell injury in rat DRG cells. These results suggest that repeated administration of neurotropin relieves the oxaliplatin-induced mechanical allodynia by inhibiting the axonal degeneration and it is useful for the treatment of oxaliplatin-induced neuropathy clinically.

Repeated psychological stress-induced alterations of visceral sensitivity and colonic motor functions in mice: influence of surgery and postoperative single housing on visceromotor responses

Visceral pain modulation by chronic stress in mice has been little studied. Electromyography (EMG) recording of abdominal muscle contractions, as a proxy to the visceromotor response (VMR), requires electrode implantation and post-surgical single housing (SH) which could affect the VMR to stress. To test this hypothesis, male mice had electrode implantation surgery (S) plus SH, or no surgery and were group housed (NS-GH) or single housed (NS-SH) and exposed to either water avoidance stress (WAS, 1 h/day) or left undisturbed in their home cages for 10 days. The VMR to phasic ascending colorectal distension (CRD) was assessed before (basal) and 24 h after 10 days of WAS or no stress using a surgery-free method of intraluminal colonic pressure (ICP) recording (solid-state manometry). WAS heightened significantly the VMR to CRD at 30, 45, and 60 mmHg in S-SH vs. NS-GH, but not compared to NS-SH conscious mice. Compared to basal CRD, WAS increased VMR at 60 mmHg in the S-SH group and decreased it at 30-60 mmHg in NS-GH mice, while having no effect in NS-SH mice. The average defecation during the hour of repeated WAS over 10 days was 1.9 and 2.4 fold greater in S-SH vs. NS-GH and NS-SH mice, respectively. These data indicate that the combination of S-SH required for VMR monitoring with EMG is an important component of repeated WAS-induced post-stress visceral hypersensitivity and defecation in mice.

A rat model of temporomandibular joint pain with histopathologic modifications

AIMS

To develop a rat model of temporomandibular joint (TMJ) pain and to characterize in it the development and temporal response of behavioral hypersensitivity as well as to evaluate if and to what extent a loading protocol is associated with histological changes in the TMJ consistent with osteoarthritic pathology.

METHODS

A novel rat model of TMJ pain was developed using a noninvasive, mechanical loading protocol. Rats were exposed to steady mouth-opening for 7 days (2 N force, 1 hour/day), and mechanical hyperalgesia (increased pain response) was measured during the loading period and for 14 days thereafter. Histological modifications in the joint cartilage were also evaluated. Outcomes for the mouth-opening exposure were compared to age-matched controls. Thresholds for evoking responses were compared using a ranked ANOVA with repeated measures.

RESULTS

Increased mechanical hypersensitivity in the temporomandibular region developed during daily loading and persisted even after the termination of the loading protocol. Histologic characterization revealed thinning of the cartilaginous structures of the joint and irregular zonal cellular arrangements in the condylar cartilage of rats subjected to the daily loading protocol.

CONCLUSION

The injury model presented here is the first to demonstrate mechanically-induced behavioral hypersensitivity accompanied by osteoarthritic pathology in the TMJ.

Neonatal maternal separation elevates thalamic corticotrophin releasing factor type 1 receptor expression response to colonic distension in rat

OBJECTIVES

Early life psychological stress is an essential factor contributing to the development of irritable bowel syndrome (IBS), with corticotrophin releasing factor (CRF) having been implicated in this common gastrointestinal disorder. The aim of our study is to examine the effect of neonatal maternal separation (NMS), an early life stress model, on the brain CRF expression following visceral pain induced by colorectal distension (CRD) stimuli in male rats.

METHODS

Male neonatal Sprague-Dawley rats were subjected to 3-hr daily maternal separation on postnatal day 2-21, with unseparated normal (N) rats serving as controls. Electromyogram signals (EMG) in response to phasic CRD were measured. The results demonstrated an increased pain response and EMG magnitudes in NMS rats as compared to N rats in response to CRD stimulation. The mRNA and protein expressions of CRF in hippocampus, cortex and thalamus of NMS and N group following the CRD stress were determined by real-time quantitative PCR and western-blotting studies respectively.

RESULTS

There was an increased mRNA and protein level of CRF in thalamus of NMS rats but no apparent change in CRF expression in hippocampus and cortex of both groups. Furthermore, an increased expression of CRF type 1 receptor (CRF-R1) was observed in the thalamus of NMS rats.

CONCLUSION

These results suggested an up-regulation of thalamus CRF-R1 is associated with visceral hyperalgesia in the rat model of NMS.

Evidence that opioids may have toll-like receptor 4 and MD-2 effects

Opioid-induced proinflammatory glial activation modulates wide-ranging aspects of opioid pharmacology including: opposition of acute and chronic opioid analgesia, opioid analgesic tolerance, opioid-induced hyperalgesia, development of opioid dependence, opioid reward, and opioid respiratory depression. However, the mechanism(s) contributing to opioid-induced proinflammatory actions remains unresolved. The potential involvement of toll-like receptor 4 (TLR4) was examined using in vitro, in vivo, and in silico techniques. Morphine non-stereoselectively induced TLR4 signaling in vitro, blocked by a classical TLR4 antagonist and non-stereoselectively by naloxone. Pharmacological blockade of TLR4 signaling in vivo potentiated acute intrathecal morphine analgesia, attenuated development of analgesic tolerance, hyperalgesia, and opioid withdrawal behaviors. TLR4 opposition to opioid actions was supported by morphine treatment of TLR4 knockout mice, which revealed a significant threefold leftward shift in the analgesia dose response function, versus wildtype mice. A range of structurally diverse clinically-employed opioid analgesics was found to be capable of activating TLR4 signaling in vitro. Selectivity in the response was identified since morphine-3-glucuronide, a morphine metabolite with no opioid receptor activity, displayed significant TLR4 activity, whilst the opioid receptor active metabolite, morphine-6-glucuronide, was devoid of such properties. In silico docking simulations revealed ligands bound preferentially to the LPS binding pocket of MD-2 rather than TLR4. An in silico to in vitro prediction model was built and tested with substantial accuracy. These data provide evidence that select opioids may non-stereoselectively influence TLR4 signaling and have behavioral consequences resulting, in part, via TLR4 signaling.