Mechanisms of incisional pain

Postoperative, incisional pain is a unique but common form of acute pain. Because effective postoperative analgesia reduces morbidity following surgery, new treatments continue to be sought. It is through the development of investigational models and studies of the mechanisms that perioperative medicine can be advanced. This article reviews studies on a rat plantar hindpaw model for postoperative pain and proposes mechanisms for enhanced excitability of sensory neurons caused by incisions.

Amitriptyline reduces rectal pain related activation of the anterior cingulate cortex in patients with irritable bowel syndrome

BACKGROUND AND AIMS

Irritable bowel syndrome (IBS) is a disorder of intestinal hypersensitivity and altered motility, exacerbated by stress. Functional magnetic resonance imaging (fMRI) during painful rectal distension in IBS has demonstrated greater activation of the anterior cingulate cortex (ACC), an area relevant to pain and emotions. Tricyclic antidepressants are effective for IBS. The aim of this study was to determine if low dose amitriptyline reduces ACC activation during painful rectal distension in IBS to confer clinical benefits. Secondary aims were to identify other brain regions altered by amitriptyline, and to determine if reductions in cerebral activation are greater during mental stress.

METHODS

Nineteen women with painful IBS were randomised to amitriptyline 50 mg or placebo for one month and then crossed over to the alternate treatment after washout. Cerebral activation during rectal distension was compared between placebo and amitriptyline groups by fMRI. Distensions were performed alternately during auditory stress and relaxing music.

RESULTS

Rectal pain induced significant activation of the perigenual ACC, right insula, and right prefrontal cortex. Amitriptyline was associated with reduced pain related cerebral activations in the perigenual ACC and the left posterior parietal cortex, but only during stress.

CONCLUSIONS

The tricyclic antidepressant amitriptyline reduces brain activation during pain in the perigenual (limbic) anterior cingulated cortex and parietal association cortex. These reductions are only seen during stress. Amitriptyline is likely to work in the central nervous system rather than peripherally to blunt pain and other symptoms exacerbated by stress in IBS.

Behavioral and pharmacological characterization of a distal peripheral nerve injury in the rat

Previous rat neuropathic pain models have utilized peripheral nerve injuries that damage a significant proportion of large nerves such as the sciatic nerve or its divisions. Injuries that lead to neuropathic pain in humans may involve the peripheral extremities. The current study evaluated the behavioral effects of injury to the plantar nerves in the rat (distal nerve injury-DNI). A delayed onset of hypersensitivity to an innocuous mechanical stimulus was observed following cutting of the left plantar nerves, whereas mechanical hypersensitivity developed more rapidly in rats with either an injury near the sciatic nerve trunk (chronic constriction injury (CCI), spared nerve injury (SNI)) or a spinal nerve root (spinal nerve ligation (SNL). Similar to other nerve injury pain models, rats with injured plantar nerves also developed an early onset and persistent sensitivity to a cooling stimulus. The effects of morphine, gabapentin and imipramine on mechanical and cold hypersensitivity were evaluated in rats with a DNI, CCI and SNI. In all three models, morphine dose-dependently suppressed mechanical and cold hypersensitivity, whereas gabapentin only suppressed mechanical hypersensitivity. Imipramine had no effect on either cold or mechanical hypersensitivity in any of the nerve-injured rats. The pharmacological data suggest that the underlying basis of neuropathic pain may be similar irrespective of the site of nerve injury.

Dorsal column lesion reduces mechanical allodynia in the induction, but not the maintenance, phase in spinal hemisected rats

The dorsal column-medial lemniscal (DC-ML) system is known to be a route of ascending input signals for mechanical allodynia following peripheral nerve injury. We examined whether the pain signals after spinal hemisection were transmitted via the DC-ML system in the induction and maintenance phases of the neuropathic pain. Under enflurane anesthesia, rats were subjected to spinal hemisection at T13 level and bilateral DC lesion was made at T8 level 1 day or 3 weeks after the hemisection. The DC lesion 1 day after the hemisection significantly reduced the mechanical, but not cold, allodynia, whereas the DC lesion 3 weeks after the hemisection did not change both mechanical and cold allodynia. These results suggest that the signals for mechanical allodynia following spinal hemisection should be transmitted via the DC-ML system in the induction, but not maintenance, phase.

Functional chest pain: nociception and visceral hyperalgesia

Functional chest pain is a common, yet poorly understood entity. The focus of this review is to explore the evolving research and clinical approaches with a particular emphasis on the sensory or afferent neuronal dysfunction of the esophagus as a key player in the manifestation of this pain syndrome. Although once regarded as a psychologic or esophageal motility disorder, recent advances have shown that many of these patients have visceral hyperalgesia. Whether visceral hypersensitivity is a central or peripheral perturbation of the gut-brain axis remains debatable. Response to empirical therapy with high-dose proton pump inhibitors, upper endoscopy, or prolonged recording of esophageal pH may identify gastroesophageal reflux disease as a source of chest pain. Esophageal balloon distension study can serve as a useful test for identifying hypersensitivity. Newer techniques, including functional magnetic resonance imaging, magnetoencephalogram, and cortical evoked potentials, are being investigated. High doses of proton pump inhibitors and low doses of tricyclic antidepressants or trazadone remain the mainstay of therapy, although several new approaches including theophylline have been shown to be beneficial.

A??-afferents activate neurokinin-1 receptor in dorsal horn neurons after nerve injury

We provide new evidence demonstrating that peripheral nerve injury produces profound alterations in synaptic input to dorsal horn neurons mediated by non-nociceptive sensory neurons, and activation of neurokinin-1 receptor may be involved in the enhanced synaptic response and thus contribute to the tactile allodynia. Our results show that Abeta-fiber-evoked field potential significantly increased in the first postoperative week and decreased thereafter while maximal mechanical allodynia was exhibited. The neurokinin-1 receptor antagonist L703,606 significantly reduced Abeta-fiber-evoked field potential in nerve-injured but not in sham-operated animals. The non-N-methyl-D-aspartate receptor antagonist CNQX inhibited Abeta-fiber-evoked field potential in both nerve-injured and sham-operated rats, while the N-methyl-D-aspartate receptor antagonist MK-801 did not affect Abeta-fiber-evoked field potential in either CCI or sham-operated animals.

Hyperalgesia in non-obese diabetic (NOD) mice: A role for the inducible bradykinin B1 receptor

Most studies performed to investigate the role of the inducible bradykinin B(1) receptor in the pathology and complications of type 1 diabetes have been carried out using the model of streptozotocin (STZ)-induced diabetes. The model of spontaneous autoimmune diabetes in non-obese diabetic (NOD) mice involves a long-term inflammatory process that closely resembles the human type 1 diabetes. In the present study, we aimed at establishing the correlation between the progress of diabetic hyperalgesia and the incidence of diabetes, as a function of age, in NOD mice. We also evaluated the implication of the bradykinin B(1) receptor, a receptor up-regulated during the inflammatory progress of diabetes, in the development of diabetic hyperalgesia in NOD mice. Female NOD mice were followed up from the 4th to the 32nd week of age for the incidence of diabetes. Only NOD mice with plasma glucose concentration >20 mmol/l were considered diabetic. The nociception was assessed using the hot plate and the tail immersion pain tests and the effect of acute and chronic administration of the selective bradykinin B(1) receptor agonist, desArg(9)bradykinin and its selective antagonists, R-715 (Ac-Lys-[D-beta Nal(7), Ile(8)]desArg(9)bradykinin) and R-954 (Ac-Orn-[Oic(2), alpha-MePhe(5), D-beta Nal(7), Ile(8)]desArg(9)bradykinin), on the development of diabetic hyperalgesia was studied. Diabetic NOD mice developed a significant time-dependent hyperalgesia, as measured in both tests, starting from the 8th week of age with the maximum effect observed over 16 to 20 weeks, whereas the incidence of diabetes in the tested NOD mice was only 40.16% at the age of 16 weeks and reached a maximum of 73.23% at the age 24 weeks. Both acute and chronic administration of desArg(9)bradykinin (400 microg/kg) markedly increased the hyperalgesic activity in diabetic NOD mice in the hot plate and tail immersion nociceptive tests. The selective bradykinin B(1) receptor antagonist R-715 (400 microg/kg) and its more potent and long acting analogue R-954 (200 microg/kg), administered in acute or chronic manner, significantly attenuated diabetic hyperalgesia in NOD mice in both thermal pain tests and restored nociceptive responses to values observed in control non-diabetic siblings. Our results bring the first evidence that the development of hyperalgesia in NOD mice, a model of spontaneous type 1 diabetes, precedes the occurrence of hyperglycemia and is mediated by the bradykinin B(1) receptor. It is suggested that bradykinin B(1) receptor antagonism could become a novel therapeutic approach to the treatment of diabetic neuropathic complications.

Detection thresholds of capsaicin: a new test to assess facial skin neurosensitivity

The goal of this study was to assess the accuracy/reliability of a new test designed to measure cutaneous neurosensitivity. The test was carried out on a random population of 150 healthy adult women and was based on the determination of individual detection thresholds of topically applied capsaicin. Five capsaicin concentrations were used in 10% ethanol aqueous solution (3.16 x 10(-5)%; 1 x 10(-4)%; 3.16 x 10(-4)%; 1 x 10(-3)%; 3.16 x 10(-3)%). The methodology used to attain the detection threshold was capsaicin application in increasing concentration on the nasolabial folds. The vehicle was simultaneously applied following a split-face, single-blind plan. The test was stopped as soon as the subject reported a specific sensation lasting more than 30 seconds on the capsaicin side. The safety of the test was judged as excellent by the panelists since all the reported sensations were considered as slightly or moderately perceptible. The test allowed the classification of the test population according to six threshold levels corresponding to the sensitive reaction to one of the five capsaicin concentrations and to the absence of sensitivity to the highest concentration. Surprisingly, the distribution of the population was not unimodal and seemed to reveal the existence of two different sub-groups: individuals with a low capsaicin detection threshold and those with a high threshold. These two sub-populations strongly differed in their respective self-perception of sensitive skin. The higher the self-declared sensitive skin incidence was, the lower the detection threshold was. This new test of skin neurosensitivity is easy, quick, and truly painless. It appears to be a promising tool for the cosmetic diagnosis of sensitive skin.

Effects of the N-Methyl-D-Aspartate Receptor Antagonist Dextromethorphan on Temporal Summation of Pain are Similar in Fibromyalgia Patients and Normal Control Subjects

Temporal summation of second pain at least partly reflects temporal summation of dorsal horn neuronal responses, and both have been termed windup (WU), a form of nociception-dependent central sensitization. Animal and human experiments have shown that both forms of WU depend on N-methyl-D-aspartate (NMDA) and substance P receptor systems. WU of second pain (WU(SP)) in patients with fibromyalgia (FM) is enhanced compared with normal control (NC) subjects and is followed by exaggerated WU(SP) aftersensations and prolonged WU(SP) maintenance at low stimulus frequencies. Because the enhanced WU(SP) of FM patients could be related to abnormal endogenous modulation of NDMA receptors, we tested the effects of the NMDA receptor antagonist dextromethorphan (DEX) on WU(SP) in FM and NC subjects in a double-blind, placebo-controlled, crossover study. WU(SP) was elicited by trains of 0.7-second duration thermal pulses applied to the glabrous surface of the hands or by 1-second mechanical stimuli to the adductor pollicis muscle of the hands at a frequency of 0.33 Hz. In comparison to baseline and placebo conditions, single oral doses of DEX 60 and 90 mg reduced thermal and mechanical WU(SP) in NC and FM subjects, with DEX 90 mg being most effective. These effects did not differ for male and female NC subjects. FM subjects required less thermal and mechanical stimulus intensity than NC to achieve maximal WU(SP), but the extent of WU(SP) reduction by DEX did not statistically differ between NC and FM subjects for all study conditions. Thus, central pain processing of FM subjects is not different from NC in at least one important aspect, namely their NMDA receptor system responsiveness to pharmacologic inhibition by DEX.

PERSPECTIVE

Results of this study demonstrate that FM patients show abnormal WU(SP) during thermal and mechanical stimulation compared with NC. Because oral doses of the NMDA receptor antagonist DEX attenuated thermal and mechanical WU(SP) similarly in FM patients and NC, other mechanisms than WU(SP) need to be considered for the widespread pain of FM patients. These mechanisms might include tonic nociceptive input from peripheral tissues and/or enhanced descending facilitation.

Visceral hypersensitivity

Visceral hypersensitivity is considered one of the causes of functional gastrointestinal disorders. The objectives of this review are to provide a practical description of neuroanatomy and physiology of gut sensation, to describe the diverse tests of visceral sensation and the potential role of brain imaging to further our understanding of visceral sensitivity in health and disease. Changes in motor function in the gut may influence sensory levels, eg, during contractions or as a result of changes in viscus compliance. New insights on sensory end organs, such as intraganglionic laminar endings, and basic neurophysiologic studies showing afferent firing during changes in stretch rather than tension illustrate the importance of different types of stimuli, not just tension, to stimulate afferent sensation. These insights provide the basis for understanding visceral sensation in health and disease, which will be extensively discussed in subsequent articles.

Inhibition of thermal hyperalgesia and tactile allodynia by intrathecal administration of gamma-aminobutyric acid transporter-1 inhibitor NO-711 in rats with chronic constriction injury

The present study was undertaken to explore the role of gamma-aminobutyric acid transporters in the neuropathic pain. On the chronic constriction injury (CCI) rats 4 doses (5, 10, 20, 40 microg in group N5, N10, N20, N40, respectively) of specific gamma-aminobutyric acid transporter-1 inhibitor NO-711 or normal saline (in group NS) were intrathecally administered before sciatic nerve ligation (pre-treatment) or at the third day after ligation (post-treatment). The paw withdrawl latency (PWL) from a noxious thermal stimulus and paw withdrawl mechanical threshold (PWMT) of von Frey filament was used as measure of thermal hyperalgesia and tactile allodynia respectively. The results demonstrated that post-treatment of NO-711 significantly suppressed thermal hyperalgesia and allodynia in CCI rats (P<0.05, P<0.01), the inhibitory effect lasted for 2 h (N40 group) and 4 h (N20 group) respectively. NO-711 inhibited thermal hyperalgesia induced by CCI in a dose-dependent manner. Intrathecal pretreatment with different doses of NO-711 delayed the occurrence of thermal hyperalgesia, but could not delay the emergence of allodynia induced by CCI. This study indicates that gamma-aminobutyric acid transporter inhibitor has anti-thermal hyperalgesia and anti-tactile allodynia effects in neuropathic rats.

Characterization of N-methyl-D-aspartate receptor subunits responsible for postoperative pain

N-methyl-D-aspartate (NMDA) receptors have been suggested to be critical for the development of central sensitization, which may amplify postoperative pain. NMDA receptors are formed by GluRzeta (NR1) with any one of four GluRvarepsilon1-4 (NR2A-D) subunits. To clarify the involvement of NMDA receptors in postoperative pain, we examined the effect of the GluRepsilon2-selective antagonist (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenyl piperidino)-1-propanol (CP-101,606) on postoperative pain caused by plantar incision. We also applied the postoperative pain model to GluRepsilon1 and GluRepsilon4 knockout mice. CP-101,606 administered intrathecally 30 min prior to incision significantly increased mechanical withdrawal thresholds 2 h and 1-3 days after surgery and reduced postoperative pain dose-dependently. Neither GluRvarepsilon1 nor GluRepsilon4 knockout mice showed a difference in withdrawal thresholds as compared with wild-type mice. Pretreatment with CP-101,606 did not produce an additive analgesic effect in the mice. These results demonstrate that GluRepsilon2-containing NMDA receptors are involved in postoperative pain and that CP-101,606 may be effective in reducing it.

Painful stimuli induce in vivo phosphorylation and membrane mobilization of mouse spinal cord NKCC1 co-transporter

The Na+ --Cl- --K+ isoform 1 (NKCC1) is a co-transporter that increases the intracellular concentration of chloride. NKCC1 plays a critical role in neuronal excitability and it has been recently suggested that it can contribute to hyperalgesic states by modulating the chloride concentration inside nociceptive neurons. In the spinal cord, trafficking of neurotransmitter receptors from the cytosol to the plasma membrane has been demonstrated to contribute to the development of hyperalgesia. However, it is unknown if trafficking of co-transporters can also occur in the nervous system or if it can be induced by painful stimulation. In this study, we have induced referred mechanical hyperalgesia in vivo by intracolonic instillation of capsaicin in mice. Using subcellular fractionation of proteins and cross-linking of membrane proteins we have observed that intracolonic capsaicin induced a 50% increase in NKCC1 in the plasma membrane of lumbosacral spinal cord 90 and 180 min after instillation, in parallel with a similar decrease in the cytosolic fraction. These effects returned to basal levels 6 h after capsaicin treatment. Intracolonic capsaicin also evoked a rapid (10 min) and transient phosphorylation of NKCC1, however, intracolonic saline did not produce significant changes in either NKCC1 trafficking or phosphorylation and none of the treatments induced any alterations of NKCC1 in the thoracic spinal cord. These results suggest that phosphorylation and recruitment of NKCC1 might play a role in referred mechanical hyperalgesia evoked by a painful visceral stimulus. The time course of the effects observed suggests that phosphorylation could contribute to the initial generation of hyperalgesia whereas trafficking could participate in the maintenance of hyperalgesic states observed at longer time points.

Spinal p38beta isoform mediates tissue injury-induced hyperalgesia and spinal sensitization

Antagonist studies show that spinal p38 mitogen-activated protein kinase plays a crucial role in spinal sensitization. However, there are two p38 isoforms found in spinal cord and the relative contribution of these two to hyperalgesia is not known. Here we demonstrate that the isoforms are distinctly expressed in spinal dorsal horn: p38alpha in neurons and p38beta in microglia. In lieu of isoform selective inhibitors, we examined the functional role of these two individual isoforms in nociception by using intrathecal isoform-specific antisense oligonucleotides to selectively block the expression of the respective isoform. In these rats, down-regulation of spinal p38beta, but not p38alpha, prevented nocifensive flinching evoked by intraplantar injection of formalin and hyperalgesia induced by activation of spinal neurokinin-1 receptors through intrathecal injection of substance P. Both intraplantar formalin and intrathecal substance P produced an increase in spinal p38 phosphorylation and this phosphorylation (activation) was prevented when spinal p38beta, but not p38alpha, was down-regulated. Thus, spinal p38beta, probably in microglia, plays a significant role in spinal nociceptive processing and represents a potential target for pain therapy.

Anti-hyperalgesic effects of intrathecally administered neuropeptide W-23, and neuropeptide B, in tests of inflammatory pain in rats

Neuropeptide W-23 (NPW23) is an endogenous ligand of both GPR7 and GPR8, and neuropeptide B (NPB) is an endogenous ligand of GPR7. GPR7 mRNA has been detected in regions of the cortex, the hippocampus, the hypothalamus, and the spinal cord in the rat, but GPR8 has not been found in rodents. GPR7 and GPR8 receptors have structural features in common with both opioid and somatostatin receptors. The effects of intrathecal (i.t.) application of NPW23 and NPB were tested in two inflammatory pain models (plantar injection of formalin or carrageenan) and two thermal nociceptive tests (52.5 degrees C and 50.5 degrees C hot plates) and one mechanical nociceptive test in the rat. I.t. injection of either NPW23 or NPB decreased the number of agitation behaviors induced by paw formalin injection and attenuated the level of mechanical allodynia, but not the level of thermal hyperalgesia, induced by paw carrageenan injection in a dose-dependent manner at a dose between 0.1 and 10 microg, significantly. The effects of either 10 microg of NPW23 or 10 microg of NPB were not antagonized by 10 microg of naloxone. I.t. injection of either NPW23 or NPB had no effect in both the 52.5 degrees C hot plate test or in the 50.5 degrees C hot plate tests at a dose between 1 and 100 microg. I.t. injection of either 10 microg of NPW23 or 10 microg of NPB had no effect in the mechanical nociceptive test. I.t. injection of either 10 microg of NPW23 or 10 microg of NPB significantly suppressed the expression of Fos-like immunoreactivity of the L4-5 spinal dorsal horn induced by paw formalin injection. These data suggest that both spinally-applied NPW23 and NPB suppressed the input of nociceptive information to the spinal dorsal horn, produced an analgesic effect in the formalin test, and attenuated the level of mechanical allodynia in the carrageenan test, and that either spinally applied NPW23 or spinally applied NPB had no effect in the physiological condition.

Effects of acute and chronic restraint stress on nitroglycerin-induced hyperalgesia in rats

Nitric oxide (NO) plays an important role in initiation and maintenance of pain, and NO precursor nitroglycerin is able to activate spinal and brain structures involved in nociception. It is also known that acute and chronic stress induce biochemical changes affecting both pain threshold and behaviour, and that the biological pattern of depression can be mimicked in the laboratory using chronic unavoidable stress paradigms (learned helplessness). We, therefore, evaluated the effects of acute and chronic immobilization stress on pain response to nitroglycerin administration in the rat. Pain perception was expressed as the latency of response to a tail-flick test (hot stimulus). Measures were made 1, 2 and 4 h following nitroglycerin (10 mg/kg i.p.) or vehicle. Nitroglycerin caused hyperalgesia after 2 and 4 h (p < 0.05 versus baseline). Acute stress (90 min) induced a clear analgesic state (p < 0.01 versus non-stressed control animals), and nitroglycerin injection was unable to reverse stress-induced analgesia in this setting. By contrast, exposition to chronic immobilization stress (7 days) caused a significant increase in pain response (p < 0.05); in this case, hyperalgesia was shown to be further enhanced by nitroglycerin administration (p < 0.05 versus vehicle). These findings support the view that a condition of chronic stress used in the laboratory to reproduce the biological features of depression can enhance hyperalgesia induced by nitroglycerin administration. These observations may be relevant to pain disorders, and particularly to migraine, since nitroglycerin is able to induce spontaneous-like pain attacks in humans, and an unfavourable migraine outcome (transformation into a chronic daily headache) is associated with chronic stress and comorbid depression.

Chronic hyperalgesic priming in the rat involves a novel interaction between cAMP and PKCepsilon second messenger pathways

Toward the goal of defining new pharmacological targets for the treatment of chronic pain conditions, in previous studies we established a model, termed 'hyperalgesic priming,' in which an acute inflammatory stimulus causes a long-lasting latent susceptibility to hyperalgesia induced by subsequent exposures to the inflammatory mediator, prostaglandin E2 (PGE2). Those investigations suggested the hypothesis that priming induces a novel linkage between the PGE2-activated second messenger cascade and the epsilon isoform of protein kinase C (PKCepsilon). In the present study, comparison of dose-response relations for hyperalgesia produced by PGE2, forskolin, 8-Br-cAMP, or the protein kinase A (PKA) catalytic subunit, in primed versus normal animals, demonstrated that priming-induced enhancement of the PGE2-activated second messenger cascade occurs downstream to adenylate cyclase and upstream to PKA. Therefore, PGE2-induced hyperalgesia in the primed animal is enhanced by the recruitment of a novel cAMP/PKCepsilon signaling pathway in addition to the usual cAMP/PKA pathway. These observations suggest that pharmacological disruption of the novel interaction between cAMP and PKCepsilon might provide a route toward the development of highly specific methods to reverse cellular processes that underlie chronic pain states.

Expression of central glucocorticoid receptors after peripheral nerve injury contributes to neuropathic pain behaviors in rats

Peripheral glucocorticoid receptors (GRs) play a significant role in the anti-inflammatory effects of glucocorticoids; however, the role of central GRs in nociceptive behaviors after peripheral nerve injury (neuropathic pain behaviors) remains unknown. Here we show that the development of neuropathic pain behaviors (thermal hyperalgesia and mechanical allodynia) induced by chronic constriction nerve injury (CCI) in rats was attenuated by either the GR antagonist RU38486 (4 = 2 > 1 = 0.5 microg) or a GR antisense oligonucleotide administered intrathecally twice daily for postoperative days 1-6. The development of thermal hyperalgesia and mechanical allodynia after CCI also was prevented in adrenalectomized rats, whereas the GR agonist dexamethasone (100 microg/kg) given subcutaneously twice daily for postoperative day 1-6 restored CCI-induced neuropathic pain behaviors in the adrenalectomized rats. Mechanistically, CCI induced a time-dependent and region-specific expression of neuronal GRs primarily within the spinal cord dorsal horn ipsilateral to nerve injury, which showed a time course parallel to that of the development of neuropathic pain behaviors. Moreover, the expression of neuronal GR after CCI was mediated in part through an elevated spinal level of interleukin-6 (IL-6) and protein kinase Cgamma (PKCgamma), because intrathecal treatment with an IL-6 antiserum, a PKC inhibitor (cheryrithrine), or PKCgamma knock-out substantially reduced the expression of neuronal GRs as well as neuropathic pain behaviors after CCI. These findings indicate a central role of neuronal GRs in the mechanisms of neuropathic pain behaviors in rats and suggest a potential role for GR antagonists in clinical management of neuropathic pain.

Role of Transient Receptor Potential Vanilloid 1 Receptors in Adjuvant-Induced Chronic Arthritis: In Vivo Study Using Gene-Deficient Mice

The transient receptor potential vanilloid 1 (TRPV1) receptor is a nonselective cation channel localized on a subset of primary sensory neurons and can be activated by a wide range of stimuli. The present study investigated the role of this receptor in chronic arthritis evoked by complete Freund's adjuvant (CFA) using TRPV1 receptor gene-deleted (TRPV1-/-) mice and wild-type counterparts (TRPV1+/+). In TRPV1+/+ mice, CFA injected intraplantarly into the left hindpaw and the root of the tail induced swelling of the injected and contralateral paws up to 130 and 28%, respectively, measured by plethysmometry throughout 18 days. Mechanonociceptive threshold measured with dynamic plantar aesthesiometry was decreased by 50 and 18% on the injected and contralateral paws, respectively. Histological examination and scoring of the tibiotarsal joints revealed marked arthritic changes in wild-type mice. In TRPV1-/- animals edema, histological score and mechanical allodynia were significantly smaller. Daily treatment with the lipoxygenase inhibitor nordihydroguaretic acid (NDGA), the cyclooxygenase inhibitor indomethacin, the bradykinin B2 receptor antagonist HOE-140 [D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-thyenyl)-L-alanyl-L-seryl-D-1,2,2,4-tetrahydro-3-isoquinolinecarbonyl-L-(2a,3b,7ab)-octahydro-1H-indole-2-carbonyl-L-arginine], or the B1 receptor antagonist desArgHOE-140 [D-arginyl-L-arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-thyenyl)-L-alanyl-L-seryl-D-1,2,2,4-tetrahydro-3-isoquinolinecarbonyl-L-(2a,3b,7ab)-octahydro-1H-indole-2-carbonyl] was performed to reveal what mediators might activate TRPV1. NDGA significantly inhibited edema, hyperalgesia, and arthritis score in TRPV1+/+, but not in TRPV1-/- mice. The effect of indomethacin was markedly smaller in knockouts. In TRPV1+/+ animals, HOE-140, but not desArgHOE-140, inhibited arthritis, whereas in TRPV1-/- mice, HOE-140 produced limited effect. Thus, whereas bradykinin and lipoxygenase products seem to act exclusively via TRPV1 activation, prostanoids do not, or at least only partially, to enhance murine experimental arthritis and related hyperalgesia.

The Effects of Greater Occipital Nerve Block and Trigger Point Injection on Brush Allodynia and Pain in Migraine

OBJECTIVE

To evaluate the effect of GONB, with or without trigger point injection (TPI), on dynamic mechanical (brush) allodynia (BA) and on head pain in migraine. Background.-Patients with migraine often have cutaneous allodynia that is related to sensitization of central pain neurons. Greater occipital nerve block (GONB) is an effective treatment for migraine headache; however, its effect on cutaneous allodynia in migraine is unknown.

METHODS

We studied patients with migraine and BA who were treated with GONB with or without TPI. Demographic data, migraine history, and headache features were documented. Allodynia was evaluated using a structured questionnaire and by applying a 4 x 4-inch gauze pad to skin areas in the trigeminal and cervical dermatomes. Degree of allodynia (the allodynia score) was measured on a 100-mm visual analog scale (VAS) before treatment and 10 and 20 minutes thereafter. Headache levels were assessed using an 11-point verbal scale. Allodynia scores, as well as headache levels, before and after treatment were compared.

RESULTS

Nineteen patients were studied. Mean age was 43.6+/-11.8 years. Twenty minutes after treatment, headache was reduced in 17 patients (89.5%) and did not change in 2 (10.5%). The average headache level was 6.53 before treatment and 3.47, 20 minutes after it. The average allodynia score decreased after 20 minutes in all patients. Average allodynia score per site was reduced by 18.69 mm and 13.74 mm in the trigeminal and cervical areas, respectively. There was a positive correlation between allodynia index, obtained through the questionnaire, and allodynia score, obtained by examination.

CONCLUSION

GONB, with or without TPI, reduced both head pain and brush allodynia in this migraine patient group.

Does neonatal surgery lead to increased pain sensitivity in later childhood?

Does pain or tissue damage in early life lead to hyperalgesia persisting into childhood? We performed a cross-sectional study in 164 infants to investigate whether major surgery within the first 3 months of life increases pain sensitivity to subsequent surgery and to elucidate whether subsequent surgery in the same dermatome or in a different dermatome leads to differences in pain sensitivity. All infants received standard intraoperative and postoperative pain management, with rescue analgesia guided by a treatment algorithm. Differences in pain sensitivity during surgery were assessed by the intraoperative fentanyl intake and by (nor)epinephrine plasma concentrations. Differences in postoperative pain sensitivity were assessed by the observational pain measures COMFORT and VAS, and by morphine intake and (nor)epinephrine plasma concentrations. Infants previously operated upon in the same dermatome needed more intraoperative fentanyl, had higher COMFORT and VAS scores, had greater (nor)epinephrine plasma concentrations, and needed also more morphine than did infants with no prior surgery. In contrast, infants who previously underwent surgery in another dermatome had only significant higher postoperative analgesic requirements and norepinephrine plasma concentrations in comparison with infants with no prior surgery. These preliminary differences may indicate the occurrence of spinal and supraspinal changes following neonatal surgery. We conclude that the long-term consequences of surgery in early infancy are greater in areas of prior tissue damage and that these effects may portend limited clinical but important neurobiological differences.

Nociceptor and age specific effects of REM sleep deprivation induced hyperalgesia

REM sleep deprivation (REMSD) has been shown to increase rates of negatively reinforced operant behavior, but not operant responding maintained by positive reinforcement. The reason for this differential effect is currently unknown. We hypothesize that REMSD can increase sensitivity to noxious stimuli. In the present study, we sought to determine if REMSD was associated with a change in response to noxious heat (i.e., altered nociceptive sensitivity). Two groups of rats, aged 6 and 22 months, were subjected to hotplate algesia testing at two different temperatures (44 and 52 degrees C). Initially, baseline numbers of responses and total response time were obtained at 44 degrees C. Animals then were exposed to 48 h of REMSD or control conditions. The frequency and duration of hindpaw responses (licking and guarding) increased for young animals only after REMSD and none of the control conditions. Old rats showed increased duration of nocifensive responding after REMSD and tank control conditions without a change in the number of responses at 44 degrees C. Latency to first nocifensive response was significantly longer in the 44 degrees C hotplate tests, but decreased to levels observed throughout the 52 degrees C hotplate tests following REMSD and TC conditions. These findings suggest that REMSD increases nociceptive sensitivity under conditions of sustained, selective C nociceptor activation (42 degrees C), but not under conditions of phasic A-delta activation (52 degrees C). The findings also indicate that age can be a significant variable in REMSD studies.

Chronic inflammatory pain leads to increased blood-brain barrier permeability and tight junction protein alterations

The blood-brain barrier (BBB) maintains brain homeostasis by limiting entry of substances to the central nervous system through interaction of transmembrane and intracellular proteins that make up endothelial cell tight junctions (TJs). Recently it was shown that the BBB can be modulated by disease pathologies including inflammatory pain. This study examined the effects of chronic inflammatory pain on the functional and molecular integrity of the BBB. Inflammatory pain was induced by injection of complete Freund's adjuvant (CFA) into the right plantar hindpaw in female Sprague-Dawley rats under halothane anesthesia; control animals were injected with saline. Edema and hyperalgesia were assessed by plethysmography and infrared paw-withdrawal latency. At 72 h postinjection, significant edema formation and hyperalgesia were noted in the CFA-treated rats. Examination of permeability of the BBB by in situ perfusion of [14C]sucrose while rats were under pentobarbital anesthesia demonstrated that CFA treatment significantly increased brain sucrose uptake. Western blot analysis of BBB TJ proteins showed no change in expression of zonula occludens-1 (an accessory protein) or actin (a cytoskeletal protein) with CFA treatment. Expression of the transmembrane TJ proteins occludin and claudin-3 and -5 significantly changed with CFA treatment with a 60% decrease in occludin, a 450% increase in claudin-3, and a 615% increase in claudin-5 expression. This study demonstrates that during chronic inflammatory pain, alterations in BBB function are associated with changes in specific transmembrane TJ proteins.

Effects of selective tumor necrosis factor-alpha inhibition to pain-behavioral changes caused by nucleus pulposus-induced damage to the spinal nerve in rats

Application of nucleus pulposus to the spinal nerve and displacement of the adjacent nerve results in behavioral changes in rats. It has been reported that treatment with the tumor necrosis factor-alpha (TNFalpha) inhibitor, infliximab, significantly reduces spontaneous pain behavior in this animal model. However, there have been no reports of the effects of infliximab on mechanical or thermal hyperalgesia using this model. Disk incision and adjacent spinal nerve displacement were performed with (n = 6) or without (n = 6) injection of infliximab. A control group also underwent sham surgery (n = 6). The animals were tested for 3 days before and on days 1, 3, 7, 14, and 21 after surgery. Non-noxious mechanical thresholds were tested by determining the hind paw withdrawal response to von Frey hair stimulation of the plantar surface of the footpad with a touch stimulator. Thermal nociceptive thresholds were tested using a sensitive thermal testing device. While disk incision with displacement surgery rats showed mechanical and thermal hyperalgesia after surgery on the experimental side, neither rats treated with infliximab nor the sham operation controls showed these effects. Injection of infliximab seemed to prevent mechanical and thermal hyperalgesia caused by the combination of disk incision and nerve displacement.

Nociception and TRP Channels

Noxious thermal, mechanical, or chemical stimuli evoke pain through excitation of the peripheral terminals called nociceptor, and many kinds of ionotropic and metabotropic receptors are involved in this process. Capsaicin receptor TRPV1 is a nociceptor-specific ion channel that serves as the molecular target of capsaicin. TRPV1 can be activated not only by capsaicin but also by noxious heat (with a thermal threshold >43 degrees C) or protons (acidification), all of which are known to cause pain in vivo. Studies using TRPV1-deficient mice have shown that TRPV1 is essential for selective modalities of pain sensation and for thermal hyperalgesia. One mechanism underlying inflammatory pain which is initiated by tissue damage/inflammation and characterized by hypersensitivity is sensitization of TRPV1. In addition to TRPV1, there are five thermosensitive ion channels in mammals, all of which belong to the TRP (transient receptor potential) super family. These include TRPV2, TRPV3, TRPV4, TRPM8 and TRPA1. These channels exhibit distinct thermal activation thresholds (> 52 degrees C for TRPV2, > approximately 34-38 degrees C for TRPV3, > approximately 27-35 degrees C for TRPV4, < approximately 25-28 degrees C for TRPM8 and < 17 degrees C for TRPA1) and are expressed in primary sensory neurons as well as other tissues. Some of the thermosensitive TRP channels are likely to be involved in thermal nociception, since their activation thresholds are within the noxious range of temperatures.