Activation of diffuse noxious inhibitory controls (DNIC) in rats with an experimental peripheral mononeuropathy

Descending facilitation from rostral ventromedial medulla mu opioid receptor-expressing neurons is necessary for maintenance of sensory and affective dimensions of chronic neuropathic pain

ABSTRACT

Pharmacological ablation of rostral ventromedial medulla (RVM) mu opioid receptor-expressing cells before peripheral nerve injury prevents the development of neuropathic pain. However, whether these neurons are required for the expression of established neuropathic pain is not known. Male Oprm1Cre heterozygous (MORCre) or wild-type (MORWT) mice received AAV8-hSyn-DIO-hM4D(Gi)-mCherry in the RVM. After partial sciatic nerve ligation (PSNL), we evaluated pain behaviors and descending control of nociception in response to acute or sustained chemogenetic inhibition of RVM-MOR cells expressing hM4D(Gi). A single systemic administration of hM4D(Gi) agonist clozapine-N-oxide (CNO) reversibly inhibited hind paw tactile allodynia and produced conditioned place preference only in MORCre mice with PSNL. Intrathecal CNO also reversibly inhibited PSNL-induced hind paw allodynia, suggesting that the spinal projections from these RVM-MOR cells are critical for manifestation of pain behaviors. Consistent with enhanced descending facilitation from RVM-MOR cells, MORCre-hM4D(Gi) mice with PSNL showed diminished descending control of nociception that was restored by systemic CNO. Sustained CNO in drinking water before PSNL prevented expression of chronic pain without affecting acute surgical pain; however, relief of chronic pain required sustained CNO treatment. Thus, in male mice, activity of spinally projecting RVM-MOR cells is required (1) for expression and manifestation of both sensory and affective dimensions of established neuropathic pain and (2) to promote descending facilitation that overcomes apparently intact descending inhibition to maintain chronic pain. Enhanced descending facilitation likely regulates the output signal from the spinal cord to the brain to shape the pain experience and may provide a mechanism for nonopioid management of pain.

Disrupted stress-induced analgesia in a neuropathic pain state is rescued by the endocannabinoid degradation inhibitor JZL195

Acute stress normally engages descending brain pathways to produce an antinociceptive response, known as stress-induced analgesia. Paradoxically, these descending pain modulatory pathways are also involved in the maintenance of the abnormal pain associated with chronic neuropathic pain. It remains unclear how stress-induced analgesia is affected by neuropathic pain states. We therefore examined the impact of a chronic constriction nerve-injury (CCI) model of neuropathic pain on restraint stress-induced analgesia in C57BL/6 mice. Thirty minutes of restraint stress produced analgesia in the hotplate thermal nociceptive assay that was less in CCI compared to control mice who underwent a sham-surgery. In sham but not CCI mice, stress-induced analgesia was reduced by the opioid receptor antagonist naltrexone. The cannabinoid CB1 receptor antagonist AM281 did not affect stress-induced analgesia in either sham or CCI mice. Low-dose pre-treatment with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195 increased stress-induced analgesia in CCI but not sham mice. The JZL195 enhancement of stress-induced analgesia in CCI mice was abolished by AM281 but was unaffected by naltrexone. These findings indicate that the acute opioid-mediated analgesic response to a psychological stressor is disrupted in a nerve-injury model of neuropathic pain. Importantly, this impairment of stress-induced analgesia was rescued by blockade of endocannabinoid breakdown via a cannabinoid CB1 receptor dependent mechanism. These findings suggest that subthreshold treatment with endocannabinoid degradation blockers could be used to alleviate the disruption of endogenous pain control systems in a neuropathic pain state.

Intrinsic brain connectivity alterations despite intact pain inhibition in subjects with neuropathic pain after spinal cord injury: a pilot study

Endogenous pain modulation in humans is frequently investigated with conditioned pain modulation (CPM). Deficient pain inhibition is a proposed mechanism that contributes to neuropathic pain (NP) after spinal cord injury (SCI). Recent studies have combined CPM testing and neuroimaging to reveal neural correlates of CPM efficiency in chronic pain. This study investigated differences in CPM efficiency in relation to resting-state functional connectivity (rsFC) between 12 SCI-NP subjects and 13 age- and sex-matched healthy controls (HC). Twelve and 11 SCI-NP subjects were included in psychophysical and rsFC analyses, respectively. All HC were included in the final analyses. Psychophysical readouts were analysed to determine CPM efficiency within and between cohorts. Group differences of rsFC, in relation to CPM efficiency, were explored with seed-to-voxel rsFC analyses with pain modulatory regions, e.g. ventrolateral periaqueductal gray (vlPAG) and amygdala. Overall, pain inhibition was not deficient in SCI-NP subjects and was greater in those with more intense NP. Greater pain inhibition was associated with weaker rsFC between the vlPAG and amygdala with the visual and frontal cortex, respectively, in SCI-NP subjects but with stronger rsFC in HC. Taken together, SCI-NP subjects present with intact pain inhibition, but can be differentiated from HC by an inverse relationship between CPM efficiency and intrinsic connectivity of supraspinal regions. Future studies with larger cohorts are necessary to consolidate the findings in this study.

Conditioned pain modulation is more efficient in patients with painful diabetic polyneuropathy than those with nonpainful diabetic polyneuropathy

ABSTRACT

Endogenous pain modulation, as tested by the conditioned pain modulation (CPM) protocol, is typically less efficient in patients with chronic pain compared with healthy controls. We aimed to assess whether CPM is less efficient in patients with painful diabetic polyneuropathy (DPN) compared with those with nonpainful DPN. Characterization of the differences in central pain processing between these 2 groups might provide a central nervous system explanation to the presence or absence of pain in diabetic neuropathy in addition to the peripheral one. Two hundred seventy-one patients with DPN underwent CPM testing and clinical assessment, including quantitative sensory testing. Two modalities of the test stimuli (heat and pressure) conditioned to cold noxious water were assessed and compared between patients with painful and nonpainful DPN. No significant difference was found between the groups for pressure pain CPM; however, patients with painful DPN demonstrated unexpectedly more efficient CPMHEAT (-7.4 ± 1.0 vs -2.3 ± 1.6; P = 0.008). Efficient CPMHEAT was associated with higher clinical pain experienced in the 24 hours before testing (r = -0.15; P = 0.029) and greater loss of mechanical sensation (r = -0.135; P = 0.042). Moreover, patients who had mechanical hypoesthesia demonstrated more efficient CPMHEAT (P = 0.005). More efficient CPM among patients with painful DPN might result from not only central changes in pain modulation but also from altered sensory messages coming from tested affected body sites. This calls for the use of intact sites for proper assessment of pain modulation in patients with neuropathy.

Advancing our understanding of neuropathic pain in diabetes mellitus using conditioned pain modulation: further considerations for age and testing site

ABSTRACT

Neuropathy is common among individuals with diabetes mellitus, and is associated with decreased quality of life, greater comorbidity, and substantial economic burden. However, the mechanisms underlying painful diabetic polyneuropathy has yet to be fully elucidated. While it is recognized that diabetic polyneuropathy places patients at a greater risk for developing neuropathic pain, it is still not clear why some individuals develop pain and others do not. Similar to other chronic pain conditions, painful diabetic neuropathy is likely driven by alterations in both the peripheral and central nervous system. Experimental conditioned pain modulation paradigms have contributed substantially to our current understanding of chronic pain across various disease states. In a new study, researchers have extended this work by examining the efficiency of conditioned pain modulation in patients with painful and non-painful diabetic polyneuropathy. Surprisingly, the results indicate individuals with painful neuropathy experience greater endogenous pain inhibition, which may seem counterintuitive at first blush. Here, we discuss potential explanations and directions for future research, including consideration for age effects, testing site, and disease type, with the goal of further advancing this important line of research.

Sexually dimorphic response of colorectal motility to noxious stimuli in the colorectum in rats

KEY POINTS

This study showed a remarkable sex difference in responses of colorectal motility to noxious stimuli in the colorectum in rats: colorectal motility was enhanced in response to intracolonic administration of a noxious stimulant, capsaicin, in male rats but not in female rats. The difference in descending neurons from the brain to spinal cord operating after noxious stimulation could be responsible for the sex difference. In male rats, serotoninergic and dopaminergic neurons are dominantly activated, both of which activate the spinal defaecation centre. In female rats, GABAergic neurons in addition to serotoninergic neurons are activated. GABA may compete for facilitative action of 5-HT in the spinal defaecation centre, and thereby colorectal motility is not enhanced in response to intracolonic administration of capsaicin. The findings provide a novel insight into pathophysiological mechanisms of sex differences in functional defaecation disorders such as irritable bowel syndrome.

ABSTRACT

We previously demonstrated that noxious stimuli in the colorectum enhance colorectal motility through activation of descending pain inhibitory pathways in male rats. It can be expected that the regulatory mechanisms of colorectal motility differ in males and females owing to remarkable sex differences in descending pain inhibitory pathways. Thus, we aimed to clarify sex differences in responses of colorectal motility to noxious stimuli in rats. Colorectal motility was measured in vivo in anaesthetized rats. Administration of a noxious stimulant, capsaicin, into the colorectal lumen enhanced colorectal motility in male rats but not in female rats. Quantitative PCR and immunohistochemistry showed that TRPV1 expression levels in the dorsal root ganglia and in the colorectal mucosa were comparable in male and female rats. When a GABA_A receptor inhibitor was intrathecally administered to the L6-S1 level of the spinal cord, colorectal motility was facilitated in response to intracolonic capsaicin even in female rats. The capsaicin-induced response in the presence of the GABA blocker in female rats was inhibited by intrathecal administration of 5-HT2 and -3 receptor antagonists but not by a D2-like dopamine receptor antagonist. Our findings demonstrate that intracolonic noxious stimulation activates GABAergic and serotoninergic descending neurons in female rats, whereas serotoninergic and dopaminergic neurons are dominantly activated in male rats. Thus, the difference in the descending neurons operating after noxious stimulation would be responsible for the sexually dimorphic responses of colorectal motility. Our findings provide a novel insight into pathophysiological mechanisms of sex differences in functional defaecation disorders such as irritable bowel syndrome.

Effects of intensity of electroacupuncture on chronic pain in patients with knee osteoarthritis: a randomized controlled trial

BACKGROUND

Conditioned pain modulation (CPM) is impaired in people with chronic pain such as knee osteoarthritis (KOA). The purpose of this randomized, controlled clinical trial was to investigate whether strong electroacupuncture (EA) was more effective on chronic pain by strengthening the CPM function than weak EA or sham EA in patients with KOA.

METHODS

In this multicenter, three-arm parallel, single-blind randomized controlled trial, 301 patients with KOA were randomly assigned. Patients were randomized into three groups based on EA current intensity: strong EA (> 2 mA), weak EA (< 0.5 mA), and sham EA (non-acupoint). Treatments consisted of five sessions per week, for 2 weeks. Primary outcome measures were visual analog scale (VAS), CPM function, and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC).

RESULTS

Three hundred one patients with KOA were randomly assigned, among which 271 (90.0%) completed the study (mean age 63.93 years old). One week of EA had a clinically important improvement in VAS and WOMAC but not in CPM function. After 2 weeks treatment, EA improved VAS, CPM, and WOMAC compared with baseline. Compared with sham EA, weak EA (3.8; 95% CI 3.45, 4.15; P < .01) and strong EA (13.54; 95% CI 13.23, 13.85; P < .01) were better in improving CPM function. Compared with weak EA, strong EA was better in enhancing CPM function (9.73; 95% CI 9.44, 10.02; P < .01), as well as in reducing VAS and total WOMAC score.

CONCLUSION

EA should be administered for at least 2 weeks to exert a clinically important effect on improving CPM function of KOA patients. Strong EA is better than weak or sham EA in alleviating pain intensity and inhibiting chronic pain.

TRIAL REGISTRATION

This study was registered with the Chinese Clinical Trial Registry ( ChiCTR-ICR-14005411 ), registered on 31 October 2014.

Descending Control Mechanisms and Chronic Pain

PURPOSE OF REVIEW

The goal of the review was to highlight recent advances in our understanding of descending pain-modulating systems and how these contribute to persistent pain states, with an emphasis on the current state of knowledge around "bottom-up" (sensory) and "top-down" (higher structures mediating cognitive and emotional processing) influences on pain-modulating circuits.

RECENT FINDINGS

The connectivity, physiology, and function of these systems have been characterized extensively over the last 30 years. The field is now beginning to ask how and when these systems are engaged to modulate pain. A recent focus is on the parabrachial complex, now recognized as the major relay of nociceptive information to pain-modulating circuits, and plasticity in this circuit and its connections to the RVM is marked in persistent inflammatory pain. Top-down influences from higher structures, including hypothalamus, amygdala, and medial prefrontal areas, are also considered. The challenge will be to tease out mechanisms through which a particular behavioral context engages distinct circuits to enhance or suppress pain, and to understand how these mechanisms contribute to chronic pain.

Electroacupuncture Potentiates Cannabinoid Receptor-Mediated Descending Inhibitory Control in a Mouse Model of Knee Osteoarthritis

Knee osteoarthritis (KOA) is a highly prevalent, chronic joint disorder, which can lead to chronic pain. Although electroacupuncture (EA) is effective in relieving chronic pain in the clinic, the involved mechanisms remain unclear. Reduced diffuse noxius inhibitory controls (DNIC) function is associated with chronic pain and may be related to the action of endocannabinoids. In the present study, we determined whether EA may potentiate cannabinoid receptor-mediated descending inhibitory control and inhibit chronic pain in a mouse model of KOA. We found that the optimized parameters of EA inhibiting chronic pain were the low frequency and high intensity (2 Hz + 1 mA). EA reversed the reduced expression of CB1 receptors and the 2-arachidonoylglycerol (2-AG) level in the midbrain in chronic pain. Microinjection of the CB1 receptor antagonist AM251 into the ventrolateral periaqueductal gray (vlPAG) can reversed the EA effect on pain hypersensitivity and DNIC function. In addition, CB1 receptors on GABAergic but not glutamatergic neurons are involved in the EA effect on DNIC function and descending inhibitory control of 5-HT in the medulla, thus inhibiting chronic pain. Our data suggest that endocannabinoid (2-AG)-CB1R-GABA-5-HT may be a novel signaling pathway involved in the effect of EA improving DNIC function and inhibiting chronic pain.

Orofacial Pain in Cancer: Part I—Mechanisms

The mechanisms involved, and possible treatment targets, in orofacial pain due to cancer are poorly understood. The aim of the first of this two-part series is to review the involved pathophysiological mechanisms and explore their possible roles in the orofacial region. However, there is a lack of relevant research in the trigeminal region, and we have therefore applied data accumulated from experiments on cancer pain mechanisms in rodent spinal models. In the second part, we review the clinical presentation of cancer-associated orofacial pain at various stages: initial diagnosis, during therapy (chemo-, radiotherapy, surgery), and in the post-therapy period. In the present article, we provide a brief outline of trigeminal functional neuro-anatomy and pain-modulatory pathways. Tissue destruction by invasive tumors (or metastases) induces inflammation and nerve damage, with attendant acute pain. In some cases, chronic pain, involving inflammatory and neuropathic mechanisms, may ensue. Distant, painful effects of tumors include paraneoplastic neuropathic syndromes and effects secondary to the release of factors by the tumor (growth factors, cytokines, and enzymes). Additionally, pain is frequent in cancer management protocols (surgery, chemotherapy, and radiotherapy). Understanding the mechanisms involved in cancer-related orofacial pain will enhance patient management.

Diffuse noxious inhibitory controls and nerve injury: restoring an imbalance between descending monoamine inhibitions and facilitations

Diffuse noxious inhibitory controls (DNICs) utilize descending inhibitory controls through poorly understood brain stem pathways. The human counterpart, conditioned pain modulation, is reduced in patients with neuropathy aligned with animal data showing a loss of descending inhibitory noradrenaline controls together with a gain of 5-HT3 receptor-mediated facilitations after neuropathy. We investigated the pharmacological basis of DNIC and whether it can be restored after neuropathy. Deep dorsal horn neurons were activated by von Frey filaments applied to the hind paw, and DNIC was induced by a pinch applied to the ear in isoflurane-anaesthetized animals. Spinal nerve ligation was the model of neuropathy. Diffuse noxious inhibitory control was present in control rats but abolished after neuropathy. α2 adrenoceptor mechanisms underlie DNIC because the antagonists, yohimbine and atipamezole, markedly attenuated this descending inhibition. We restored DNIC in spinal nerve ligated animals by blocking 5-HT3 descending facilitations with the antagonist ondansetron or by enhancing norepinephrine modulation through the use of reboxetine (a norepinephrine reuptake inhibitor, NRI) or tapentadol (μ-opioid receptor agonist and NRI). Additionally, ondansetron enhanced DNIC in normal animals. Diffuse noxious inhibitory controls are reduced after peripheral nerve injury illustrating the central impact of neuropathy, leading to an imbalance in descending excitations and inhibitions. Underlying noradrenergic mechanisms explain the relationship between conditioned pain modulation and the use of tapentadol and duloxetine (a serotonin, NRI) in patients. We suggest that pharmacological strategies through manipulation of the monoamine system could be used to enhance DNIC in patients by blocking descending facilitations with ondansetron or enhancing norepinephrine inhibitions, so possibly reducing chronic pain.

Electroacupuncture inhibits chronification of the acute pain of knee osteoarthritis: study protocol for a randomized controlled trial

Background

Previous studies have shown that electroacupuncture (EA) has a significant effect on acute pain, but it has not solved the clinical problem of the chronification of acute pain. Diffuse noxious inhibitory controls (DNIC) function as a reliable indicator to predict the risk of chronic pain events. DNIC function in knee osteoarthritis (KOA) patients has been demonstrated to gradually decrease during the development of chronic pain. The purpose of this study is to conduct a randomized, controlled clinical trial to determine if EA can repair impaired DNIC function and thus prevent chronification of the acute pain of KOA.

Methods/Design

This is a multicenter, single blind, randomized, controlled, three-arm, large-scale clinical trial. A total of 450 KOA patients will be randomly assigned to three groups. The strong EA group will receive EA with high-intensity current (2 mA < current < 5 mA) at the ipsilateral ‘Neixiyan’ (EX-LE5), ‘Dubi’(ST35), ‘Liangqiu’(ST34) and ‘Xuehai’ (SP10). The weak EA group will receive EA with low-intensity current (0 mA < current < 0.5 mA) on the same acupoints. The sham EA group will receive EA with low-intensity current (0 mA < current < 0.5 mA) with fine needles inserted superficially into the sites 2 cm lateral to the above acupoints. The patients will be treated with EA once a day, 30 minutes per session, in 5 sessions per week, for 2 weeks. In order to determine the best stage of KOA for effective EA intervention, patients within the treatment groups also will be divided into four stages. The primary outcomes are Visual Analog Scale (VAS), DNIC function and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Clinical assessments will be evaluated at baseline (before treatment) and after 5 to 10 sessions of treatment.

Discussion

This trial will be helpful in identifying whether strong EA is more effective than weak EA in reversing chronification of acute pain through repairing the impaired DNIC function and in screening for the best stage of KOA for effective EA intervention.

Trial registration

Chinese Clinical Trial Registry Number: ChiCTR-ICR-14005411. The date of registration is 31 October 2014.

Spinal p38 activity and analgesic effect after low- and high-intensity electroacupuncture stimulation in a plantar incision rat model

AIMS

Postoperative pain is a major problem. Electroacupuncture (EA) has been accepted as a useful and low-risk complementary therapy for post-operative pain. Animal studies indicate that surgical incision activates p38 MAPK in the spinal microglia, which critically contributes to post-incisional nociceptive development. How EA affects incision-induced p38 activation is important but yet to be fully elucidated.

METHODS

Male adult rats received plantar incision (PI) at the right hind paw followed by 30-min EA of 4-Hz, one of two intensities (3 and 10mA), and at right ST36 (Zusanli) acupoint immediately after PI and for 3 successive days. EA analgesia was evaluated by von Frey fibers and Hargreaves' tests. Spinal p38 activation was examined by immunostaining. In separate groups, SB203580, a p38 inhibitor, was intrathecally injected alone or with EA to test the combining effect on nociception and spinal phospho-p38.

KEY FINDINGS

EA of 10-mA significantly ameliorated mechanical allodynia, but 3-mA did not. None of them altered thermal hyperalgesia. Repeated EA could not inhibit phospho-p38 in the PI rats, contrarily, EA per se significantly induced phospho-p38 in the normal rats. Intrathecal SB203580 injection dose-dependently prevented PI-induced allodynia. Combination of low-dose SB203580 and 3-mA EA, which were ineffective individually, profoundly reduce post-PI allodynia.

SIGNIFICANCE

We demonstrated that 10-mA EA exerts a significant inhibition against post-PI mechanical hypersensitivity via a p38-independent pathway. Importantly, co-treatment with low-dose p38 inhibitor and 3-mA EA can counteract spinal phospho-p38 to exert strong analgesic effect. Our finding suggests a novel strategy to improve EA analgesic quality.

Conditioned pain modulation: a predictor for development and treatment of neuropathic pain

Psychophysical evaluation of endogenous pain inhibition via conditioned pain modulation (CPM) represents a new generation of laboratory tests for pain assessment. In this review we discuss recent findings on CPM in neuropathic pain and refer to psychophysical, neurophysiological, and methodological aspects of its clinical implications. Typically, chronic neuropathic pain patients express less efficient CPM, to the extent that incidence of acquiring neuropathic pain (e.g. post-surgery) and its intensity can be predicted by a pre-surgery CPM assessment. Moreover, pre-treatment CPM evaluation may assist in the correct choice of serotonin-noradrenalin reuptake inhibitor analgesic agents for individual patients. Evaluation of pain modulation capabilities can serve as a step forward in individualizing pain medicine.

On the g-protein-coupled receptor heteromers and their allosteric receptor-receptor interactions in the central nervous system: focus on their role in pain modulation

The modulatory role of allosteric receptor-receptor interactions in the pain pathways of the Central Nervous System and the peripheral nociceptors has become of increasing interest. As integrators of nociceptive and antinociceptive wiring and volume transmission signals, with a major role for the opioid receptor heteromers, they likely have an important role in the pain circuits and may be involved in acupuncture. The delta opioid receptor (DOR) exerts an antagonistic allosteric influence on the mu opioid receptor (MOR) function in a MOR-DOR heteromer. This heteromer contributes to morphine-induced tolerance and dependence, since it becomes abundant and develops a reduced G-protein-coupling with reduced signaling mainly operating via β-arrestin2 upon chronic morphine treatment. A DOR antagonist causes a return of the Gi/o binding and coupling to the heteromer and the biological actions of morphine. The gender- and ovarian steroid-dependent recruitment of spinal cord MOR/kappa opioid receptor (KOR) heterodimers enhances antinociceptive functions and if impaired could contribute to chronic pain states in women. MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, mediating morphine induced itch. Other mechanism for the antinociceptive actions of acupuncture along meridians may be that it enhances the cross-desensitization of the TRPA1 (chemical nociceptor)-TRPV1 (capsaicin receptor) heteromeric channel complexes within the nociceptor terminals located along these meridians. Selective ionotropic cannabinoids may also produce cross-desensitization of the TRPA1-TRPV1 heteromeric nociceptor channels by being negative allosteric modulators of these channels leading to antinociception and antihyperalgesia.

Blockade of opioid receptors in the medullary reticularis nucleus dorsalis, but not the rostral ventromedial medulla, prevents analgesia produced by diffuse noxious inhibitory control in rats with muscle inflammation

Diffuse Noxious Inhibitory Controls (DNIC) involves application of a noxious stimulus outside the testing site to produce analgesia. In human subjects with a variety of chronic pain conditions, DNIC is less effective; however, in animal studies, DNIC is more effective after tissue injury. While opioids are involved in DNIC analgesia, the pathways involved in this opioid-induced analgesia are not clear. The aim of the present study was to test the effectiveness of DNIC in inflammatory muscle pain, and to study which brainstem sites mediate DNIC- analgesia. Rats were injected with 3% carrageenan into their gastrocnemius muscle and responses to cutaneous and muscle stimuli were assessed before and after inflammation, and before and after DNIC induced by noxious heat applied to the tail (45 °C and 47 °C). Naloxone was administered systemically, into rostral ventromedial medulla (RVM), or bilaterally into the medullary reticularis nucleus dorsalis (MdD) prior to the DNIC-conditioning stimuli. DNIC produced a similar analgesic effect in both acute and the chronic phases of inflammation reducing both cutaneous and muscle sensitivity in a dose-dependent manner. Naloxone systemically or microinjected into the MdD prevented DNIC-analgesia, while naloxone into the RVM had no effect on DNIC analgesia. Thus, DNIC analgesia involves activation of opioid receptors in the MdD.

PERSPECTIVE

The current study shows that DNIC activates opioid receptors in the MdD, but not the RVM, to produce analgesia. These data are important for understanding clinical studies on DNIC as well as for potential treatment of chronic pain patients.

A preconditioning nerve lesion inhibits mechanical pain hypersensitivity following subsequent neuropathic injury

BACKGROUND

A preconditioning stimulus can trigger a neuroprotective phenotype in the nervous system - a preconditioning nerve lesion causes a significant increase in axonal regeneration, and cerebral preconditioning protects against subsequent ischemia. We hypothesized that a preconditioning nerve lesion induces gene/protein modifications, neuronal changes, and immune activation that may affect pain sensation following subsequent nerve injury. We examined whether a preconditioning lesion affects neuropathic pain and neuroinflammation after peripheral nerve injury.

RESULTS

We found that a preconditioning crush injury to a terminal branch of the sciatic nerve seven days before partial ligation of the sciatic nerve (PSNL; a model of neuropathic pain) induced a significant attenuation of pain hypersensitivity, particularly mechanical allodynia. A preconditioning lesion of the tibial nerve induced a long-term significant increase in paw-withdrawal threshold to mechanical stimuli and paw-withdrawal latency to thermal stimuli, after PSNL. A preconditioning lesion of the common peroneal induced a smaller but significant short-term increase in paw-withdrawal threshold to mechanical stimuli, after PSNL. There was no difference between preconditioned and unconditioned animals in neuronal damage and macrophage and T-cell infiltration into the dorsal root ganglia (DRGs) or in astrocyte and microglia activation in the spinal dorsal and ventral horns.

CONCLUSIONS

These results suggest that prior exposure to a mild nerve lesion protects against adverse effects of subsequent neuropathic injury, and that this conditioning-induced inhibition of pain hypersensitivity is not dependent on neuroinflammation in DRGs and spinal cord. Identifying the underlying mechanisms may have important implications for the understanding of neuropathic pain due to nerve injury.

DNIC-mediated analgesia produced by a supramaximal electrical or a high-dose formalin conditioning stimulus: roles of opioid and alpha2-adrenergic receptors

Background

Diffuse noxious inhibitory controls (DNIC) can be produced by different types of conditioning stimuli, but the analgesic properties and underlying mechanisms remain unclear. The aim of this study was to differentiate the induction of DNIC analgesia between noxious electrical and inflammatory conditioning stimuli.

Methods

First, rats subjected to either a supramaximal electrical stimulation or an injection of high-dose formalin in the hind limb were identified to have pain responses with behavioral evidence and spinal Fos-immunoreactive profiles. Second, suppression of tail-flick latencies by the two noxious stimuli was assessed to confirm the presence of DNIC. Third, an opioid receptor antagonist (naloxone) and an α2-adrenoreceptor antagonist (yohimbine) were injected, intraperitoneally and intrathecally respectively, before conditioning noxious stimuli to test the involvement of descending inhibitory pathways in DNIC-mediated analgesia.

Results

An intramuscular injection of 100 μl of 5% formalin produced noxious behaviors with cumulative pain scores similar to those of 50 μl of 2% formalin in the paw. Both electrical and chemical stimulation significantly increased Fos expression in the superficial dorsal horns, but possessed characteristic distribution patterns individually. Both conditioning stimuli prolonged the tail-flick latencies indicating a DNIC response. However, the electrical stimulation-induced DNIC was reversed by yohimbine, but not by naloxone; whereas noxious formalin-induced analgesia was both naloxone- and yohimbine-reversible.

Conclusions

It is demonstrated that DNIC produced by different types of conditioning stimuli can be mediated by different descending inhibitory controls, indicating the organization within the central nervous circuit is complex and possibly exhibits particular clinical manifestations.

Noxious counterirritation in patients with advanced osteoarthritis of the knee reduces MCC but not SII pain generators: A combined use of MEG and EEG

Chronic pain is mainly a result of two processes: peripheral and central sensitization, which can result in neuroplastic changes. Previous psychophysical studies suggested a decrease of the so-called pain-inhibiting-pain effect (DNIC) in chronic pain patients. We aimed to study the DNIC effect on the neuronal level using magnetoencephalography and electroencephalography in 12 patients suffering from advanced unilateral knee osteoarthritis (OA). DNIC was induced in patients by provoking the typical OA pain by a slightly hyperextended joint position, while they received short electrical pain stimuli. Although the patients did not report a reduction of electrical pain perception, the cingulate gyrus showed a decrease of activation during provoked OA pain, while activity in the secondary somatosensory cortex did not change. Based on much stronger DNIC induction at comparable intensities of an acute counterirritant pain in healthy subjects this result suggests a deficit of DNIC in OA patients. We suggest that the strength of DNIC is subject to neuronal plasticity of descending inhibitory pain systems and diminishes during the development of a chronic pain condition.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Spinal pathways involved in supraspinal modulation of neuropathic manifestations in rats

Controversial results have been recently reported on the role of supraspinal centers in the modulation of nociceptive behavior in animal models of mononeuropathy. Our aim was to investigate the role of the various spinal pathways in the modulation of the neuropathic manifestations. Several groups of rats were subjected to selective spinal-tract lesions, either 2-3 weeks before or 2-3 weeks after the induction of mononeuropathy following the chronic constriction injury (CCI) or the spared nerve injury (SNI) models. Tactile and cold allodynias were assessed by Von Frey filaments and the acetone drops test, respectively. Thermal hyperalgesia was assessed by the paw withdrawal and the hot plate tests. The effects of unilateral and bilateral lesions of the dorso-lateral funiculus (DLF), the anterolateral column (ALC) or hemisection were tested over a period of 4-8 weeks. All spinal tract lesions produced reversible, but significant decrease of allodynia and hyperalgesia over a period of 1-3 weeks. The most pronounced effects were observed with bilateral lesions. The stronger attenuation was observed on thermal hyperalgesia, assessed by the paw withdrawal test, while cold allodynia was the least affected. Spinal lesions performed before the induction of neuropathy did not produce significant alterations in the temporal development of neuropathic manifestations. The present results allow the conclusion that all spinal tracts can be involved in the rostral transmission and the descending modulation of neuropathic manifestations. The recovery of symptoms following spinal lesions provides illustration on the plasticity of the neural network involved in the processing of the neuropathic syndromes.

Chronic pancreatitis patients show hyperalgesia of central origin: a pilot study

BACKGROUND

The pain of chronic pancreatitis remains challenging to manage, with treatment all too often being unsuccessful. A main reason for this is lacking understanding of underlying mechanisms of chronic pain in these patients.

AIM

To document, using somatic quantitative sensory testing, changes in central nervous system processing (neuroplasticity) associated with chronic pancreatitis pain and thus gain insight into underlying pain mechanisms.

PATIENTS AND METHODS

We studied 10 chronic pancreatitis patients on stable opioid analgesic medication. Ten matched surgical patients without pain served as controls. Pain verbal numeric rating scores (NRS) and thresholds to electric skin stimulation and pressure pain were measured in dermatomes T10 (pancreatic area), C5, T4, L1 and L4.

RESULTS

The pancreatitis patients had a median NRS pain score of 5 (range 3-8). Electric sensation and pain thresholds were significantly increased in the pancreatic region, tending to be more so in female pancreatitis patients. Pressure pain thresholds were significantly lower in pancreatitis patients than in controls, with men tending towards greater generalised relative hyperalgesia than women.

CONCLUSIONS

Chronic pancreatitis patients show pronounced generalised deep hyperalgesia that is present despite opioid therapy. These signs, consistent with central sensitisation, appear relatively more prominent in men than women. There is also evidence suggesting that women may have a better segmental inhibitory response than men, possibly explaining their relatively less prominent generalised deep tissue hyperalgesia compared to men.

Descending facilitatory control of mechanically evoked responses is enhanced in deep dorsal horn neurones following peripheral nerve injury

Pain resulting from peripheral nerve injury, characterised by ongoing pain, hyperalgesia and allodynia arises from peripheral and central processes. Here, we studied the potential role of central facilitations in nerve injury by investigating the effect of blocking the excitatory 5HT3 receptor with ondansetron. 5HT3 receptors play a pronociceptive role in the spinal cord and ondansetron has previously been shown to produce antinociception in behavioural studies. We investigated the effects of spinally administered ondansetron (10, 50 and 100 microg) on the responses of deep dorsal horn neurones, evoked by peripheral electrical stimuli and a range of natural (mechanical punctate and heat) stimuli, 2 weeks after nerve injury induced through tight ligation of L5/6 spinal nerves (SNL). Comparisons were made between SNL rats and a sham-operated group. Ondansetron produced little effect on the electrically evoked responses (Abeta-, Adelta- and C-fibre-evoked responses, postdischarge); however, responses to mechanical punctate stimuli (von Frey filaments 1-75 g) were markedly reduced in both SNL and control groups. Furthermore, the drug effect was significantly enhanced after SNL (p<0.05). In particular, the lowest dose (10 microg) now became effective after SNL. Ondansetron produced less marked effects on thermal responses. Our results demonstrate that neuropathic pain states are associated with an enhanced descending facilitatory control of mechanical responses of spinal neurones, mediated through the activation of spinal 5HT3 receptors. These excitatory influences are likely to contribute to the development and maintenance of central sensitisation in the spinal cord, and furthermore, to the behavioural manifestation of tactile allodynia.

Colorectal distension-induced suppression of a nociceptive somatic reflex response in the rat: modulation by tissue injury or inflammation

Inhibition of somatic nociception by conditioning noxious visceral stimulation was studied under pathophysiological conditions in rats. Viscero-somatic inhibition was enhanced following visceral inflammation and reduced by a somatic heat injury. The enhancement was reversed by an N-methyl-D-aspartate (NMDA) receptor antagonist. These changes in viscero-somatic inhibition may be explained by corresponding changes in excitatory drives evoked by conditioning and test stimulation, although disinhibition may contribute to reduction of inhibition following somatic injury.

[Neuropathic pain: experimental advances and clinical applications]

Neuropathic pain is a clinical entity designating the different types of pain associated with a lesion of the nervous system including a wide range of pathological conditions from painful peripheral lesions (for example diabetic neuropathy, post-zoster pain, trauma-induced nerve injury) and central pain (particularly stroke-induced pain, spinal lesions, and multiple sclerosis). Despite this wide range of etiologies, neuropathic pain has well characterized clinical features which generally allow distinction from other types of pain: continuous often burn-like pain, paroxysmal pain (electrical discharge, knife stab), evoked pain, highly invalidating pain (allodynia, hyperalgesia), and associated dysethesia and/or paresthesia. Over the last ten Years, very little work has been published on neuropathic pain, which is now becoming a very active domain of research in neurobiology. Advances to date have not been spectacular although better tolerated agents have been recently marketed. Future progress should enable an appropriate response to the therapeutic challenge of neuropathic pain.

Territorial and extra-territorial distribution of Fos protein in the lumbar spinal dorsal horn neurons in rats with chronic constriction nerve injuries

This study aimed to examine the relationship between temporal and spatial expression patterns of Fos protein in the spinal dorsal horn neurons and thermal hyperalgesia behaviors in rats with chronic constriction injury (CCI) to the sciatic nerve. Our results demonstrated that Fos protein expression in the spinal dorsal horn neurons at L5 segment ipsilateral and contralateral to CCI of the sciatic nerve was significantly greater than in sham rats from days 10 to 30 postoperatively (PO 10d to 30d), and was concentrated on the injury (ipsilateral) side. Unlike the short-lived expression after tissue inflammation, laminae I to VI (especially laminae III/IV) displayed a persistent greater number of Fos-like immunoreactive (Fos-LI) neurons for at least 30 days after CCI of the sciatic nerve. After the increase in laminae III/IV, Fos-LI neurons tended to gradually increase in laminae I/II and V/VI at L5 segment from PO 2d to 30d, which were correlated with the heat hyperalgesia (48 degrees C) behaviors measured by paw withdrawal latency in CCI rats but not in sham rats. Interestingly, a persistent increase of Fos-LI neurons in laminae I to VI at L5 segment of the ipsilateral and contralateral sides and at the L1 segment that was out of the normal central terminations of the sciatic nerve suggested the probable presence of territorial and extra-territorial central sensitization or inadequate central nervous system (CNS) adaptive mechanisms. These findings may partly explain why abnormal pain sensations are sometimes distributed in a pattern that does not coincide with the territories of nerves or with the posterior roots of the peripheral nerve after injury.

A role for peripheral somatostatin receptors in counter-irritation-induced analgesia

Our hypothesis is that peripheral somatostatin (SRIF) has a role in counter-irritation-induced analgesia. Our paradigm involves the reduction of nociceptive behaviors produced by primary noxious stimuli (formalin or complete Freund's adjuvant [CFA] in the rat hind paw) by a counter-irritating stimulus (capsaicin [CAP] in the tail or muzzle). Activation of peripheral SRIF receptors is key since an SRIF receptor antagonist cyclo-somatostatin (c-SOM) and SRIF antibodies in the hind paw attenuate the counter-irritation-induced analgesia of both formalin and more persistent CFA nociception. Specificity of c-SOM is shown by reversal of its effects with octreotide, a SRIF analog. Injection of formalin in one hind paw and c-SOM in the other does not reduce the counter-irritation analgesia demonstrating local action of the c-SOM. Approximately 33% of peripheral sensory axons contain SRIF, which could release the peptide to activate SRIF receptors on cutaneous axons. Intraplantar naloxone has no effect on the counter-irritation analgesia indicating that SRIF is not activating opioid receptors. These results indicate that in addition to the classic central descending noxious inhibitory control systems that underlie counter-irritation-induced analgesia, there is a peripheral contribution arising from activation of SRIF receptors. Identifying a peripheral contribution of SRIF to mechanisms of counter-irritation analgesia offers opportunities for peripheral therapy.

Neurophysiologic evidence for a central sensitization in patients with fibromyalgia

OBJECTIVE

To determine whether abnormalities of peripheral and central nociceptive sensory input processing exist outside areas of spontaneous pain in patients with fibromyalgia (FM) as compared with controls, by using quantitative sensory testing (QST) and a neurophysiologic paradigm independent from subjective reports.

METHODS

A total of 164 outpatients with FM who were attending a self-management program were invited to participate in the study. Data for 85 patients were available and were compared with those for 40 non-FM controls matched for age and sex. QST was performed using thermal, mechanical, and electrical stimuli at locations of nonspontaneous pain. Pain assessment was 2-fold and included use of subjective scales and the spinal nociceptive flexion reflex (NFR), a specific physiologic correlate for the objective evaluation of central nociceptive pathways. Questionnaires regarding quality of life and the impact of FM were available.

RESULTS

Participants were mainly middle-aged women, with a mean disease duration of 8 years. Between-group differences were significant for neurophysiologic, clinical, and quality of life measures. In patients with FM, peripheral QST showed significantly altered cold and heat pain thresholds, and tolerance to cold pain was radically reduced. The median NFR threshold in patients with FM (22.7 mA [range 17.5-31.7]) was significantly decreased compared with that in controls (33 mA [range 28.1-41]). A cutoff value of <27.6 mA for NFR provided sensitivity of 73% and specificity of 80% for detecting central allodynia in the setting of FM.

CONCLUSION

Our results strongly, although indirectly, point to a state of central hyperexcitability of the nociceptive system in patients with FM. The NFR can be used to assess central allodynia in FM. It may also help discriminate patients who may benefit from use of centrally acting analgesics.

Comparison of the pain suppressive effects of clinical and experimental painful conditioning stimuli

Studies in healthy volunteers suggested that the classical counterirritation phenomenon (i.e. pain inhibits pain effect) might depend on diffuse noxious inhibitory controls (DNIC), which modulate the spinal transmission of nociceptive signals. In the present study, we sought to determine whether similar mechanisms were at play in patients with different subtypes of neuropathic pain. Ten patients presenting with a traumatic peripheral nerve injury associated with dynamic mechano-allodynia (i.e. pain triggered by brushing) or static mechano-allodynia (i.e. pain triggered by light pressure stimuli) were included in this study. To investigate counterirritation mechanisms in these patients, we analysed the RIII nociceptive flexion reflex and concomitant painful sensation elicited by electrical stimulation of the sural nerve. We compared the effects of heterotopic 'clinical' conditioning stimuli (i.e. pain evoked by brushing or pressure within the allodynic area located in the upper limb or chest) to those of experimental heterotopic noxious stimuli (HNCS) consisting of a cold pressor test or tourniquet test applied to the normal upper limb. Static mechano-allodynia induced inhibitions of both the RIII reflex and the concomitant painful sensation. These effects were similar to those induced by HNCS and were probably due to an increased activation of DNIC. In contrast, in patients with dynamic allodynia, brushing within the allodynic area reduced the pain sensation at the foot, but did not inhibit the electrophysiological responses, suggesting that in this case the counterirritation effect may take place at the supraspinal level. Thus, the mechanisms of counterirritation are not univocal, but depend on the pathophysiological mechanisms of clinical pain.

Acetic acid conditioning stimulus induces long-lasting antinociception of somatic inflammatory pain

A wide variety of noxious stimuli are known to induce a powerful inhibition of pain sensation evoked at a remote region of the body. Here we show that an intraperitoneal acetic acid (AA) conditioning stimulus produces long-lasting inhibition of formalin-evoked somatic inflammatory pain behavior in mice. This novel long-lasting antinociception was completely blocked by the 5-hydroxytryptamine type 2A/2C (5-HT(2A/2C)) receptor antagonists, ketanserin and ritanserin, but not by the opioid receptor antagonist, naloxone, and alpha-adrenergic receptor antagonists, phentolamine and yohimbine. In contrast, the 5-HT(3/4) receptor antagonist, tropisetron, significantly potentiated this long-lasting antinociception. The conditioning stimulus significantly upregulated the levels of both tryptophan hydroxylase immunoreactivity in the medulla oblongata and the 5-HT(2A/2C) receptor mRNA level in the spinal cord. These results suggested that the visceral noxious stimulus caused a long-lasting augmentation of the serotonergic inhibitory system and downregulated the somatic inflammatory nociceptive transmission.

The whole body receptive field of dorsal horn multireceptive neurones

Multireceptive neurones are found in the spinal dorsal horn and may be projection neurones and/or interneurones for polysynaptic reflexes. The cutaneous receptive field of a multireceptive neurone exhibits a gradient of sensitivity with the centre responding to any mechanical stimulus, including hair movements and light touch, while the periphery responds only to noxious stimuli. These neurones also receive signals from viscera, muscles and joints. This convergence of inputs means that multireceptive neurones are continuously capturing all the information from both the interface with the external environment (the skin) and the internal milieu (the viscera, muscles, etc.). This information constitutes a 'basic somaesthetic activity' that could help the somatosensory system build a 'global representation of the body'. In addition to be seen as a global entity, the output of multireceptive neurones should be understood in dynamic terms since the size of the peripheral fields of the individual neurones may change, as a result of the plasticity of both excitatory and inhibitory segmental processes. Furthermore, the activity of these neurones can be inhibited from most of the remaining parts of the body via supraspinal mechanisms. These diffuse noxious inhibitory controls (DNIC) are triggered by peripheral A delta- and C-fibres, involve brain structures confined to the caudal-most part of the medulla including the subnucleus reticularis dorsalis (SRD) and are mediated by descending pathways in the dorsolateral funiculi. A painful focus that both activates a segmental subset of neurones and inhibits the remaining population can seriously disrupt this basic activity, resulting in the distortion of the body representation in favour of the painful focus, which becomes pre-eminent and (relatively) oversized.

Neuropathy reduces viscero-somatic inhibition via segmental mechanisms in rats

The effect of an experimental neuropathy on the viscero-somatic inhibition was studied in lightly anesthetized rats. In controls, colo-rectal distension at noxious intensities produced a multisegmental prolongation of the withdrawal response induced by noxious stimulation of the skin. In rats with a spinal nerve-ligation induced neuropathy this viscero-somatic inhibition was significantly reduced within the neuropathic segment (the hindlimb) but not outside of it (the tail). Naloxone, an opioid antagonist, attenuated this viscero-somatic inhibition in controls and it did not restore the inhibition in neuropathic rats. The results indicate that somatic neuropathy produces a segmental attenuation of viscero-somatic inhibition and this attenuation cannot be explained by a nerve injury-induced release of endogenous opioids. The decreased inhibition of somatic signals may contribute to the hypersensitivity observed in neuropathic conditions.

Strain-dependent modification of neuropathic pain behaviour in the rat hindpaw by a priming painful trigeminal nerve injury

The aim of the present study was to test the behavioural effect of infraorbital (IO) chronic constriction injury (CCI) on the development of neuropathic pain in the rat hindpaw following sciatic nerve CCI performed 7 days later. Control groups consisted of rats that underwent sham infraorbital surgery followed by sciatic CCI at identical time points. Sensory testing of the rat's face and hindpaw was performed at baseline and at 4, 11, 14, 17 and 21 days postoperative (dpo) relative to the IOCCI, at which time all rats were euthanized. To test for strain differences the experiment was performed on Sprague-Dawley, Sabra and Lewis rats. In Lewis rats the trigeminal nerve injury significantly accelerated the development of hindpaw mechanoallodynia (11th, 14th and 17th dpo, unpaired t-test, P<0.05) and mechanohyperalgesia (14th and 17th dpo, unpaired t-test, P<0.05), following a second sciatic nerve CCI relative to the control group. This effect was not observed in Sprague-Dawley or Sabra rats.