Influence of the dopamine D2 receptor knockout on pain-related behavior in the mouse

We studied the role of the dopamine D2 receptor in physiological regulation of pain-related behavior. The experiments were performed in dopamine D2 receptor knockout mice and in their wild-type controls. Baseline sensitivity to thermal nociception was determined by measuring the response latency in the hot plate at three different stimulus temperatures and by determining the radiant-heat-induced paw withdrawal. Mechanical sensitivity was assessed by determining paw withdrawal responses to stimulation with a calibrated series of monofilaments. Intracolonic capsaicin was used to produce sustained pain-related behavior and referred hypersensitivity to mechanical stimulation. The hot plate response latencies were not significantly different between the dopamine D2 receptor knockout and wild-type animals, although the stimulus temperature-dependent decrease in the response latency was steeper in the wild-type group. The radiant-heat-induced paw withdrawal latency was slightly longer in the knockout animals. The number of capsaicin-induced behavioral responses or the latency to the occurrence of the first capsaicin-induced response was not different between the experimental groups. Dopamine D2 receptor knockout animals were more sensitive to mechanical stimulation of the hindpaws than wild-type animals both in the baseline condition and following development of capsaicin-induced referred hypersensitivity in the hindpaws. The results indicate that dopamine D2 receptors influence baseline nociception in the mouse, although this effect is weak and submodality selective. Additionally, dopamine D2 receptors may contribute to attenuation of referred hypersensitivity caused by sustained nociception.

Functional dyspepsia pathogenesis and therapeutic options--implications for management

Functional dyspepsia is far more common than dyspepsia due to organic disease, both in the community and general practice. Proposed aetiopathogenic factors include gastric acid, Helicobacter pylori infection, delayed emptying, hypersensitivity or impaired accommodation of the stomach, dysfunction of the duodenum or brain-gut axis, psychosocial morbidity and post-infective mucosal damage. More effective therapy will depend on the development of drugs targeted at these putative pathophysiological mechanisms. On current evidence tricyclic antidepressants appear to be more effective than either acid suppressants or H. pylori eradication.

Modulation of pain processing in hyperalgesia by cognitive demand

The relationship between pain and cognitive function is of theoretical and clinical interest, exemplified by observations that attention-demanding activities reduce pain in chronically afflicted patients. Previous studies have concentrated on phasic pain, which bears little correspondence to clinical pain conditions. Indeed, phasic pain is often associated with differential or opposing effects to tonic pain in behavioral, lesion, and pharmacological studies. To address how cognitive engagement interacts with tonic pain, we assessed the influence of an attention-demanding cognitive task on pain-evoked neural responses in an experimental model of chronic pain, the capsaicin-induced heat hyperalgesia model. Using functional magnetic resonance imaging (fMRI), we show that activity in the orbitofrontal and medial prefrontal cortices, insula, and cerebellum correlates with the intensity of tonic pain. This pain-related activity in medial prefrontal cortex and cerebellum was modulated by the demand level of the cognitive task. Our findings highlight a role for these structures in the integration of motivational and cognitive functions associated with a physiological state of injury. Within the limitations of an experimental model of pain, we suggest that the findings are relevant to understanding both the neurobiology and pathophysiology of chronic pain and its amelioration by cognitive strategies.

Dietary Omega-3 Fatty Acids May Be Associated with Increased Neuropathic Pain in Nerve-Injured Rats

Certain dietary proteins and oils are capable of decreasing chronic neuropathic pain levels in rats after partial sciatic nerve ligation injury. We tested, for the first time, the role of dietary polyunsaturated fatty acids in suppressing pain in partial sciatic nerve ligation-injured rats. Six groups of male Wistar rats were fed an identical casein-based, fat-free diet for 1 wk preceding partial sciatic nerve ligation injury and for 1 wk thereafter. In addition, rats received, via gavage, 1 mL/day of pure canola, corn, hemp, soy, or sunflower oil, differing significantly in their omega-3 and omega-6 polyunsaturated fatty acid content, or 1 mL of plain water. Responses to tactile and noxious heat stimuli were recorded before and after surgery and a difference score was calculated for each group by subtracting the preoperative from the post-partial sciatic nerve ligation values. Heat hyperalgesia, but not tactile allodynia, was significantly different among the dietary groups (P = 0.005). Heat hyperalgesia of rats fed hemp oil, developing the most robust response, was significantly larger compared with rats fed corn oil, developing the least pain model (difference score: 24.3 +/- 4.1 s versus 6.1 +/- 3.1 s, respectively; P < 0.001). These oils contain similar levels of omega-6 polyunsaturated fatty acids (hemp, 60%; corn, 58%) but their omega-3 levels are 28-fold different (20% versus 0.7%, respectively). A significant correlation was found among dietary levels of omega-3, but not omega-6 or the omega-3/omega-6 ratio, of the six dietary groups and heat hyperalgesia (P = 0.006). We conclude that dietary oil might predict levels of neuropathic pain in rats and that this effect may be associated with dietary omega-3 levels.

IMPLICATIONS

We found that certain commonly used oils can have a significant analgesic effect in rats with persistent pain after partial nerve injury. This effect may be associated with the amounts of omega-3 fatty acids consumed by rats.

Involvement of nitric oxide in long-term potentiation of spinal nociceptive responses in rats

Nitric oxide plays an important role in spinal nociception. The present study explored the effects of nitric oxide on the spinal long-term potentiation associated with nociception. (1) Nitric oxide synthase inhibitor L-NAME (1 mM, 20 microl) and the nitric oxide scavenger hemoglobin (2 mg/ml, 20 mul) strikingly blocked the induction of tetanic sciatic stimulation-induced spinal long-term potentiation of C-fiber-evoked field potentials. L-arginine, a substrate of nitric oxide synthase, completely reversed L-NAME-induced inhibition. However, D-NAME (1 mM, 20 microl), an inactive form of L-NAME, had little effect on the spinal LTP. (2) The same tetanic sciatic stimulation induced long-term thermal hyperalgesia, which was blocked by pre-application of L-NAME. These results suggest the involvement of nitric oxide in the spinal long-term potentiation of C-fiber-evoked field potentials and related behavior changes.

Irritable bowel syndrome: toward an understanding of severity

Irritable bowel syndrome (IBS) is a chronic disorder with symptoms that range in severity from mild and intermittent to severe and continuous. Although severity is a guiding factor in clinical decision making related to diagnosis and treatment, current guidelines related to IBS do not address the issue of severity. Recent data suggest that severity as a multidimensional concept, not fully explained by intensity of symptoms, has important clinical implications including health care utilization and health-related quality of life. Components of IBS severity include symptom intensity, time of assessment, whether the patient or physician makes the severity determination, the type of scale used to measure severity, and the degree of disability or impairment. Currently no consensus definition of IBS severity exists, although 2 validated scales of IBS severity have recently been published. Review of the literature suggests that the prevalence of severe or very severe IBS is higher than previously estimated with a range from 3%-69%. Individual IBS symptoms are important but are not sufficient to explain severity. Rather, severity has multiple components including health-related quality of life, psychosocial factors, health care utilization behaviors, and burden of illness. However, studies have not been adequately designed to determine the relative values of these factors in IBS severity.

Polysialic acid-induced plasticity reduces neuropathic insult to the central nervous system

Under chronic conditions of neuropathic pain, nociceptive C terminals are lost from their target region in spinal lamina II, leading to reduced thermal hyperalgesia. This region of the spinal cord expresses high levels of polysialic acid (PSA), a cell surface carbohydrate known to weaken cell-cell interactions and promote plasticity. Experimental removal of PSA from the spinal cord exacerbates hyperalgesia and results in retention of C terminals, whereas it has no effect on plasticity of touch Abeta fibers and allodynia. We propose that expression of PSA at this stress pathway relay point could serve to protect central circuitry from chronic sensory overload.

Sensory-motor responses to mechanical stimulation of the esophagus after sensitization with acid

AIM: Sensitization most likely plays an important role in chronic pain disorders, and such sensitization can be mimicked by experimental acid perfusion of the esophagus. The current study systematically investigated the sensory and motor responses of the esophagus to controlled mechanical stimuli before and after sensitization.

METHODS: Thirty healthy subjects were included. Distension of the distal esophagus with a balloon was performed before and after perfusion with 0.1 mol/L hydrochloric acid for 30 min. An impedance planimetry system was used to measure cross-sectional area, volume, pressure, and tension during the distensions. A new model allowed evaluation of the phasic contractions by the tension during contractions as a function of the initial muscle length before the contraction (comparable to the Frank-Starling law for the heart). Length-tension diagrams were used to evaluate the muscle tone before and after relaxation of the smooth muscle with butylscopolamine.

RESULTS: The sensitization resulted in allodynia and hyperalgesia to the distension volumes, and the degree of sensitization was related to the infused volume of acid. Furthermore, a nearly 50% increase in the evoked referred pain was seen after sensitization. The mechanical analysis demonstrated hyper-reactivity of the esophagus following acid perfusion, with an increased number and force of the phasic contractions, but the muscle tone did not change.

CONCLUSION: Acid perfusion of the esophagus sensitizes the sensory pathways and facilitates secondary contractions. The new model can be used to study abnormal sensory-motor mechanisms in visceral organs.

Acute painful neuropathy in thallium poisoning

Dysesthesia, allodynia, distal muscle weakness, and sensory impairment were noted in two patients with acute thallium intoxication. Two months later, nerve conduction studies showed an axonal degeneration. Sural nerve biopsy disclosed a decreased fiber density in the large myelinated fibers. Quantitative sensory testing also revealed an impairment of pinprick, temperature, and touch sensations. Cutaneous nerve biopsy confirmed a loss of epidermal nerves indicating an involvement of the small sensory nerves.

TRPV1 desensitisation and endogenous vanilloid involvement in the enhanced analgesia induced by capsaicin in inflamed tissues

The intra-plantar acute administration of 10 microg of capsaicin to mice which had received complete Freund's adjuvant (CFA) 1 week before inhibits the thermal inflammatory hyperalgesia it induces and even produces a long-lasting analgesia for at least 2 weeks. In this study, we show that the administration of capsaicin (10 microg) also reduces the immediate licking behavior evoked by the intra-plantar administration of a lower dose of capsaicin (0.1 microg), the duration of this inhibitory effect being greater in CFA-inflamed mice (at least 2 weeks) than in non-inflamed animals (less than 4 days). Since this reduction of capsaicin-induced licking behavior may be interpreted as a consequence of the transient receptor potential vanilloid 1 receptor (TRPV1) unresponsiveness, we conclude that the administration of 10 microg of capsaicin into inflamed tissues can render the TRPV1 desensitised. We next explored whether endogenous vanilloids released during inflammation contribute to maintain the analgesia triggered by exogenous capsaicin. The acute administration of capsazepine (10 microg; intra-plantarly (i.pl.)) abolished the analgesic effect induced by the injection of capsaicin 1 week before in inflamed mice. From these results, it may be proposed that the maintenance by endovanilloids of the TRPV1 desensitisation induced by capsaicin could contribute to prolonging the analgesic effect induced by this agonist in inflamed tissues.

Mechanical hyperalgesia in complex regional pain syndrome: A role for TNF- ?

Plasma concentrations of soluble tumor necrosis factor alpha (TNF-alpha) receptor type I (sTNF-RI) were assessed in two complex regional pain syndrome (CRPS) patient groups (n = 30 and n = 16) and healthy controls (n = 25). Patients with CRPS and mechanical hyperalgesia had higher levels of sTNF-RI (1,661.8 +/- 146.8 pg/mL) compared with those with CRPS with identical clinical appearance but without hyperalgesia (1,155.9 +/- 56.3 pg/mL) and controls (1,239.5 +/- 42.9 pg/mL). This study suggests involvement of TNF-alpha in mechanical hyperalgesia of CRPS.

Mechanical and heat sensitization of cutaneous nociceptors in rats with experimental peripheral neuropathy

This study examined whether or not the properties of cutaneous nociceptive fibers are altered in the neuropathic state by comparing lumbars 5 and 6 spinal nerve ligation (SNL) rats with sham-operated controls. The rats with the unilateral SNL developed mechanical allodynia in the ipsilateral hind limb, whereas the sham group did not. Two to 5 weeks after the neuropathic or sham surgery, rats were subjected to single fiber-recording experiments to examine the properties of afferent fibers in the sural and plantar nerves. A total of 224 afferents in the C- and Adelta-ranges were characterized in the neuropathic and sham groups. Spontaneous activity was observed in 16 of 155 fibers in the neuropathic group and one of 69 fibers in the sham group. The response threshold of both the C- and Adelta-fibers to mechanical stimuli was lower in the neuropathic group than the sham group. The afferent fibers responsive to heat stimuli were all C-fibers, and none were Adelta-fibers. The response threshold of the C-fibers to the heat stimuli was lower in the neuropathic group than the sham group. The magnitude of the responses of both C- and Adelta-fibers to the suprathreshold intensity of the mechanical stimulus was greater in the neuropathic group than the sham group. However, the magnitude of the responses of C-fibers to the suprathreshold intensity of the heat stimulus in the neuropathic group was not different from that in the sham group. These results suggest that after a partial peripheral nerve injury, the nociceptors on the skin supplied by an uninjured nerve become sensitized to both mechanical and heat stimuli. This nociceptor sensitization can contribute to neuropathic pain.

Developmental age influences the effect of epidural dexmedetomidine on inflammatory hyperalgesia in rat pups

BACKGROUND

Epidural alpha2-adrenergic agonists produce analgesic effects in children and adults, but efficacy and safety have not been established in neonates and infants. The aim of this study was to determine the effect of epidural dexmedetomidine on sensory processing, reversal of inflammatory hyperalgesia, and sedation during early development in rats.

METHODS

In rat pups aged 3, 10, and 21 postnatal days, mechanical withdrawal thresholds of the hind limbs were measured at baseline and after unilateral inflammation due to carrageenan. The effect of epidural dexmedetomidine on withdrawal thresholds was measured for 90 min after injection, and dose-response curves were constructed for each age group. The duration of the righting reflex was measured to assess sedation. The effects of epidural and systemic administration of dexmedetomidine were compared.

RESULTS

At all ages, carrageenan-induced hyperalgesia was reversed by doses of epidural dexmedetomidine that did not affect the threshold of the contralateral paw or prolong the righting reflex. Higher doses of epidural dexmedetomidine affected baseline nociception in the contralateral paw and produced sedation but had no effect when given systemically. Reversal of hyperalgesia and sedation were produced by lower doses of epidural dexmedetomidine in the youngest pups.

CONCLUSIONS

Spinally mediated selective reversal of inflammatory hyperalgesia by epidural dexmedetomidine can be achieved at all ages; relatively lower doses are effective in early life, but the therapeutic window is narrow. These data have implications for the use and dosing of epidural alpha2 agonists in neonates and infants.

Acid-induced pain and its modulation in humans

Despite the discovery of ion channels that are activated by protons, we still know relatively little about the signaling of acid pain. We used a novel technique, iontophoresis of protons, to investigate acid-induced pain in human volunteers. We found that transdermal iontophoresis of protons consistently caused moderate pain that was dose-dependent. A marked desensitization occurred with persistent stimulation, with a time constant of approximately 3 min. Recovery from desensitization occurred slowly, over many hours. Acid-induced pain was significantly augmented in skin sensitized by acute topical application of capsaicin. However, skin desensitized by repeated capsaicin application showed no significant reduction in acid-induced pain, suggesting that both capsaicin-sensitive and insensitive sensory neurons contribute to acid pain. Furthermore, topical application of non-steroidal anti-inflammatory drugs (NSAIDs) significantly attenuated acid-evoked pain but did not affect the heat pain threshold, suggesting a specific interaction between NSAIDs and peripheral acid sensors. Subcutaneous injection of amiloride (1 mm) also significantly inhibited the pain induced by iontophoresis of acid, suggesting an involvement of acid-sensing ion channel (ASIC) receptors. Conversely, iontophoresis of acid over a wide range of skin temperatures from 4 to 40 degrees C produced only minor changes in the induced pain. Together these data suggest a prominent role for ASIC channels and only a minor role for transient receptor potential vanilloid receptor-1 as mediators of cutaneous acid-induced pain.

Enhanced thermal avoidance in mice lacking the ATP receptor P2X3

P2X3 is an ATP-gated cation channel subtype expressed by a subpopulation of primary sensory neurons. In vivo spinal cord recordings in mice lacking P2X3 (P2X3-/-) have suggested that this protein may be important for the coding of peripheral warm stimuli. To explore this possibility more thoroughly, we examined behavioral and electrophysiological responses to thermal stimuli in P2X3-/- mice. As previously reported, recording from the spinal cord dorsal horn of anesthetized P2X3-/- mice revealed a blunted response of wide dynamic range neurons to hind paw heating. When placed in a thermal gradient, however, P2X3-/- mice exhibited an unexpectedly enhanced avoidance of both hot and cold temperatures, relative to controls. In the tail immersion test, mutant mice exhibited shorter withdrawal latencies at temperatures above and below thermoneutrality. Consistent with these changes, P2X3-/- mice exhibited enhanced induction of spinal cord c-FOS following hind paw heating to 45 degrees C. Thus, gain- and loss-of-function thermosensory phenotypes coexist in P2X3-/- mice. No changes in thermal preference were observed in wild-type mice injected subcutaneously with the P2X3 antagonist, A317491 or intrathecally with the P2X3 and P2X1 antagonist TNP-ATP. The reason for this apparent discrepancy is unclear, but we cannot exclude the possibility that compensatory events contribute, at least in part, to the P2X3-/- phenotype. Regardless, this study illustrates the utility of thermal preference assays as part of a comprehensive approach to the analysis of mouse thermosensation.

Dyspeptic patients with visceral hypersensitivity: sensitisation of pain specific or multimodal pathways?

BACKGROUND AND AIMS

Patients with functional dyspepsia who have hypersensitivity to gastric distension have more prevalent pain, suggesting the presence of hyperalgesia. It is unclear whether this reflects activation of pain specific afferent pathways or multimodal afferent pathways that also mediate non-painful sensations. In the former case, hyperalgesia should occur when intensity of non-painful sensations is still low. The aim of the study was to analyse whether the symptom profile during gastric dissentions in functional dyspepsia patients with hyperalgesia reflects sensitisation of pain specific or multimodal pathways.

METHODS

Forty eight consecutive dyspeptic patients (35 female) underwent gastric sensitivity testing with a barostat balloon using a double random staircase protocol. At the end of every distending step, patients scored perception of upper abdominal sensations on a graphic 0-6 rating scale and completed visual analogue scales (VAS 0-100 mm) for pain, nausea, satiety, and fullness. The end point was a rating scale of 5 or more.

RESULTS

Hypersensitivity was present in 20 patients (40%); gastric compliance did not differ between normo- and hypersensitive patients. At maximal distension (score 5 or more), hypersensitive patients had significantly lower distending pressures and intra-balloon volumes, but similar VAS scores for pain, nausea, satiety, and fullness compared with normosensitive patients. In both normosensitive and hypersensitive patients, elevation of pain VAS scores with increasing distending pressures paralleled the elevation in VAS scores for nausea, satiety, and fullness.

CONCLUSIONS

Hypersensitive dyspeptic patients reach the same intensity of painful and non-painful sensations as normosensitive patients but at lower distending pressures. Hyperalgesia occurs in hypersensitive dyspeptic patients at distending pressures that also induce intense non-painful sensations. These findings argue against isolated upregulation of pain specific afferents in functional dyspepsia patients with visceral hypersensitivity.

A rat model of unilateral hindpaw burn injury: Slowly developing rightwards shift of the morphine dose–response curve

Management of pain after burn injury is an unresolved clinical issue. In a rat model of hindpaw burn injury, we examined the effects of systemic morphine on nociceptive behaviors following injury. Injury was induced by immersing the dorsal part of one hindpaw into a hot water bath (85 degrees C) for 4, 7, or 12 s under pentobarbital anesthesia. Mechanical allodynia to von Frey filament stimulation and thermal hyperalgesia to radiant heat were assessed. Burn injury induced by the 12-s (but not 4-, or 7-s) hot water immersion resulted in reliable and lasting mechanical allodynia and thermal hyperalgesia evident by day 1. In addition, there was an upregulation of protein kinase Cgamma and a progressive downregulation of mu-opioid receptors within the spinal cord dorsal horn ipsilateral to injury as revealed by immunohistochemistry and Western blot. In both injured and sham rats, the anti-nociceptive effects of subcutaneous morphine were examined on post-injury days 7 and 14. While the morphine AD50 dose was comparable on day 7 between burn (1.61 mg/kg) and control (1.7 mg/kg) rats, the morphine dose-response curve was shifted to the right in burn-injured rats (4.6 mg/kg) on post-injury day 14 as compared with both the injured rats on post-injury day 7 and sham rats on day 14 (1.72 mg/kg). These data indicate that hindpaw burn injury reliably produces persistent mechanical allodynia and thermal hyperalgesia and that the reduced efficacy of morphine anti-nociception in chronic burn injury may be in part due to a downregulation of spinal mu-opioid receptors.

Repeated sound stress enhances inflammatory pain in the rat

While it is well established that acute stress can produce antinociception, a phenomenon referred to as stress-induced analgesia, repeated exposure to stress can have the opposite effect. Since, chronic pain syndromes, such as fibromyalgia and rheumatoid arthritis, may be triggered and/or exacerbated by chronic stress, we have evaluated the effect of repeated stress on mechanical nociceptive threshold and inflammatory hyperalgesia. Using the Randall-Selitto paw pressure test to quantify nociceptive threshold in the rat, we found that repeated non-habituating sound stress enhanced the mechanical hyperalgesia induced by the potent inflammatory mediator, bradykinin, which, in normal rats, produces hyperalgesia indirectly by stimulating the release of prostaglandin E2 from sympathetic nerve terminals. Hyperalgesia induced by the direct-acting inflammatory mediator, prostaglandin E2 as well as the baseline nociceptive threshold, were not affected. Adrenal medullectomy or denervation, reversed the effect of sound stress. In sound stressed animals, bradykinin-hyperalgesia had a more rapid latency to onset and was no longer inhibited by sympathectomy, compatible with a direct effect of bradykinin on primary afferent nociceptors. In addition, implants of epinephrine restored bradykinin-hyperalgesia in sympathectomized non-stressed rats, lending further support to the suggestion that increased plasma levels of epinephrine can sensitize primary afferents to bradykinin. These results suggest that stress-induced enhancement of inflammatory hyperalgesia is associated with a change in mechanism by which bradykinin induces hyperalgesia, from being sympathetically mediated to being sympathetically independent. This sympathetic-independent enhancement of mechanical hyperalgesia is mediated by the stress-induced release of epinephrine from the adrenal medulla.

Repeated exposure to water avoidance stress in rats: a new model for sustained visceral hyperalgesia

Chronic stress plays an important role in the development and exacerbation of symptoms in functional gastrointestinal disorders. To better understand the mechanisms underlying this relationship, we aimed to characterize changes in visceral and somatic nociception, colonic motility, anxiety-related behavior, and mucosal immune activation in rats exposed to 10 days of chronic psychological stress. Male Wistar rats were submitted daily to either 1-h water avoidance (WA) stress or sham WA for 10 consecutive days. The visceromotor response to colorectal distension, thermal somatic nociception, and behavioral responses to an open field test were measured at baseline and after chronic WA. Fecal pellets were counted after each WA stress or sham WA session as a measure of stress-induced colonic motility. Colonic samples were collected from both groups and evaluated for structural changes and neutrophil infiltration, mast cell number by immunohistochemistry, and cytokine expression by quantitative RT-PCR. Rats exposed to chronic WA (but not sham stress) developed persistent visceral hyperalgesia, whereas only transient changes in somatic nociception were observed. Chronically stressed rats also exhibited anxiety-like behaviors, enhanced fecal pellet excretion, and small but significant increases in the mast cell numbers and the expression of IL-1beta and IFN-gamma. Visceral hyperalgesia following chronic stress persisted for at least a month. Chronic psychological stress in rats results in a robust and long-lasting alteration of visceral, but not somatic nociception. Visceral hyperalgesia is associated with other behavioral manifestations of stress sensitization but was only associated with minor colonic immune activation arguing against a primary role of mucosal immune activation in the maintenance of this phenomenon.

Hyperpolarization-Activated, Cation-Nonselective, Cyclic Nucleotide-Modulated Channel Blockade Alleviates Mechanical Allodynia and Suppresses Ectopic Discharge in Spinal Nerve Ligated Rats

Abnormal spontaneous firing is well described in axotomized sensory neurons and likely contributes to nerve injury-induced pain. The hyperpolarization-activated current I(h) initiates spontaneous, rhythmic depolarization in the sinoatrial node and central neurons. This study was undertaken to investigate the possible contribution of I(h) to primary afferent ectopic discharge and pain behavior in nerve-injured rats. Nerve injury was produced by tight ligation of lumbar spinal nerves (L5/6). Two weeks later, rats showed marked mechanical allodynia. Withdrawal thresholds were measured before and after administration of saline or the specific I(h) antagonist ZD7288 (1, 3, or 10 mg/kg, intraperitoneally). ZD7288 dose-dependently reversed mechanical allodynia. In a second experiment, we performed both in vivo and in vitro extracellular single unit recordings from teased dorsal root fascicles. Intravenous infusion (2.5 or 5 mg/kg) of ZD7288 during a period of 10 minutes significantly blocked ectopic discharges in vivo. Perfusion (25 to 100 mumol/L) of ZD7288 for 5 minutes in vitro almost completely blocked ectopic discharges from large myelinated fibers (Abeta) while partially suppressing ectopic discharge from thinly myelinated fibers (Adelta). We conclude from these data that in axotomized sensory neurons, a ZD7288-sensitive current contributes to spontaneous discharges in myelinated fibers. Thus, I(h) might substantially contribute to the pathophysiology of nerve injury-related neuropathic pain.

PERSPECTIVE

The current study investigated the mechanism of abnormal spontaneous discharges (ectopic discharges) from axotomized sensory afferents. Ectopic discharges are a main driving source of nerve injury-induced neuropathic pain. Understanding the mechanism of ectopic discharges and identifying how to control them will be useful toward developing new therapies.

Peripheral inflammation selectively increases TRPV1 function in IB4-positive sensory neurons from adult mouse

C-fiber nociceptors can be divided into two groups based on growth factor dependency and isolectin B4 (IB4) binding. IB4-negative nociceptors have been proposed to contribute to inflammatory pain. Since the TRPV1 receptor is critical for inflammatory heat hyperalgesia, we hypothesized that inflammation would sensitize IB4 negative but not IB4-positive small-diameter neurons to TRPV1 stimuli. Two days after complete Freund's adjuvant (CFA)-induced inflammation in the hind paw of mice, lumbar 4/5 ganglia were dissociated and small-diameter (</=26 microm) neurons were quantified for responsiveness to the TRPV1 agonists, capsaicin and protons using patch clamp recordings. Surprisingly, inflammation did not alter the responsiveness of IB4-negative neurons to capsaicin or protons. Conversely, inflammation increased the percentage of IB4-positive neurons that responded to 1 microM capsaicin from 24 to 80% and increased the percentage that responded to pH 5.0 from 54 to 85%. In parallel, inflammation increased the percentage of IB4-positive neurons that was TRPV1-immunoreactive. The inflammation-induced increase in capsaicin- and proton-responsiveness was entirely mediated by TRPV1 because IB4-positive neurons from inflamed TRPV1-/- mice were capsaicin-insensitive and unaltered in proton-responsiveness. Interestingly, comparison of neurons from TRPV1+/+ and TRPV1-/- mice revealed that the sustained proton-evoked currents in IB4-positive neurons were independent of TRPV1 whereas the sustained-only proton currents in IB4-negative neurons were TRPV1-dependent. Together, these data indicate that TRPV1 function and expression are selectively increased in IB4-positive neurons during inflammation in mouse and suggest a novel role for IB4-positive C-fibers during inflammation.

G-Protein Activation by Neurokinin-1 Receptors Is Dynamically Regulated during Persistent Nociception

Previous work has demonstrated that persistent nociception evokes increased neurokinin-1 receptor (NK-1) gene expression in the spinal cord dorsal horn of the rat within 2 h but has failed to elucidate the relationship between increased NK-1 gene expression at later time points and functional regulation of NK-1 receptor signaling. This study was undertaken to assess changes in NK-1 receptor mRNA levels in models of persistent inflammatory hyperalgesia and to relate them to changes in the functional coupling of NK-1 receptors to G-protein activity in the dorsal horn of the rat. Thus, unilateral intraplantar formalin or complete Freund's adjuvant was used to alter mechanical and thermal withdrawal thresholds in the inflamed paw. One to 96 h later, NK-1 receptor mRNA levels were quantified using solution hybridization-nuclease protection assays. Formalin-evoked inflammation produced a 2-fold unilateral increase in NK-1 receptor mRNA levels apparent from 2 to 96 h postinjection. Histological sections of the lumbar cord from similarly treated rats were used to generate concentration-response curves using GTPgammaS35 functional binding assays stimulated by an NK-1 selective agonist. Results showed that formalin evoked a transient, bilateral decrease in the maximal functional response to 35% of control in the treated side at 24 h postinjection and as much as a 10-fold leftward shift in the EC50 of the agonist at 12 to 96 h postinjection. These results provide novel evidence that peripheral nociceptive activation promotes a central mechanism of hyperalgesia through increased functional sensitivity of NK-1 receptors in the spinal cord dorsal horn.

Hypersensitivity to cutaneous thermal nociceptive stimuli in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common intestinal ailment of which the pathophysiological mechanisms are not well understood. Most IBS patients demonstrate enhanced perception, visceral hypersensitivity, in response to distension of the gut lumen but there are conflicting results about changes in somatic sensitivity. This study focused on the possible contribution of abnormal pain sensitization due to positive feedback (vicious pain cycle) that affects somatic tissues due to viscero-somatic convergence. The specific objectives were to measure cutaneous thermal pain sensitivity along the segmental axis, including in dermatomes that are remote from the visceral pain focus. Pain sensitivity was probed with cutaneous thermal stimulation to the lower and upper extremities and the face in nine diarrhea-predominant IBS patients (diagnosed with ROME II criteria) and 12 healthy female controls. The stimuli were administered with a contact thermode, assuring that size of the stimulated area and stimulus duration were clearly defined and identical in all locations. Sensitization of IBS patients was not limited to symptomatic dermatomes (calf) but extended evenly across the body, including to the face (no sensitization gradient from foot to face). Also, the difference between IBS and control groups did not depend on the evoked pain intensity level, i.e. the degree of sensitization of IBS patients was similar near threshold (10% on the visual analog scale) and at higher intensities. Lastly, no correlation was found between IBS subjects' pain sensitivity of any of the three test sites and their ratings of spontaneous pain.