Primary Hyperalgesia Research Articles

Capsaicin-induced neurogenic inflammation in pig skin: A behavioural study

Topical capsaicin is a well-established model of experimental hyperalgesia. Its application to the study of animals has been limited to few species. The effect of topical capsaicin on hyperalgesia in porcine skin was evaluated as part of a study of inflammatory pain in the pig. Two experiments were carried out on pigs of 27 ± 5 kg (n = 8) and 57 ± 3 kg (n = 16). Thermal and mechanical noxious stimuli were provided (CO2 laser and Pressure Application Measurement device) to assess avoidance behaviours. Capsaicin induced significant thermal hyperalgesia in the smaller pigs (P < 0.05), while no mechanical hyperalgesia was observed in either animal group. The present model of topical capsaicin application may be useful to investigate the mechanisms of primary hyperalgesia in this species, although some experimental conditions, such as the administration route and cutaneous morphology, need to be evaluated.

Establishing a method for measuring primary knee hyperalgesia in the murine DMM model of osteoarthritis

Establishing a method for measuring primary knee hyperalgesia in the murine DMM model of osteoarthritis

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

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

Second Messengers Mediating the Expression of Neuroplasticity in a Model of Chronic Pain in the Rat

Hyperalgesic priming is a model of the transition from acute to chronic pain, in which previous activation of cell surface receptors or direct activation of protein kinase C epsilon markedly prolongs mechanical hyperalgesia induced by pronociceptive cytokines. We recently demonstrated a role of peripheral protein translation, alpha-calmodulin-dependent protein kinase II (αCaMKII) activation, and the ryanodine receptor in the induction of hyperalgesic priming. In the present study, we tested if they also mediate the prolonged phase of prostaglandin E2–induced hyperalgesia. We found that inhibition of αCaMKII and local protein translation eliminates the prolonged phase of prostaglandin E2 hyperalgesia. Although priming induced by receptor agonists or direct activation of protein kinase C epsilon occurs in male but not female rats, activation of αCaMKII and the ryanodine receptor also produces priming in females. As in males, the prolonged phase of prostaglandin E2–induced hyperalgesia in female rats is also protein kinase C epsilon–, αCaMKII-, and protein translation–dependent. In addition, in both male and female primed rats, the prolonged prostaglandin E2–induced hyperalgesia was significantly attenuated by inhibition of MEK/ERK. On the basis of these data, we suggest that the mechanisms previously shown to be involved in the induction of the neuroplastic state of hyperalgesic priming also mediate the prolongation of hyperalgesia. PerspectivesThe data provided by this study suggest that direct intervention on specific targets may help to alleviate the expression of chronic hyperalgesic conditions.

Local translation and retrograde axonal transport of CREB regulates IL-6-induced nociceptive plasticity.

Transcriptional regulation of genes by cyclic AMP response element binding protein (CREB) is essential for the maintenance of long-term memory. Moreover, retrograde axonal trafficking of CREB in response to nerve growth factor (NGF) is critical for the survival of developing primary sensory neurons. We have previously demonstrated that hindpaw injection of interleukin-6 (IL-6) induces mechanical hypersensitivity and hyperalgesic priming that is prevented by the local injection of protein synthesis inhibitors. However, proteins that are locally synthesized that might lead to this effect have not been identified. We hypothesized that retrograde axonal trafficking of nascently synthesized CREB might link local, activity-dependent translation to nociceptive plasticity. To test this hypothesis, we determined if IL-6 enhances the expression of CREB and if it subsequently undergoes retrograde axonal transport. IL-6 treatment of sensory neurons in vitro caused an increase in CREB protein and in vivo treatment evoked an increase in CREB in the sciatic nerve consistent with retrograde transport. Importantly, co-injection of IL-6 with the methionine analogue azido-homoalanine (AHA), to assess nascently synthesized proteins, revealed an increase in CREB containing AHA in the sciatic nerve 2 hrs post injection, indicating retrograde transport of nascently synthesized CREB. Behaviorally, blockade of retrograde transport by disruption of microtubules or inhibition of dynein or intrathecal injection of cAMP response element (CRE) consensus sequence DNA oligonucleotides, which act as decoys for CREB DNA binding, prevented the development of IL-6-induced mechanical hypersensitivity and hyperalgesic priming. Consistent with previous studies in inflammatory models, intraplantar IL-6 enhanced the expression of BDNF in dorsal root ganglion (DRG). This effect was blocked by inhibition of retrograde axonal transport and by intrathecal CRE oligonucleotides. Collectively, these findings point to a novel mechanism of axonal translation and retrograde trafficking linking locally-generated signals to long-term nociceptive sensitization.

The inhibitory effect of somatostatin receptor activation on bee venom-evoked nociceptive behavior and pCREB expression in rats.

The present study examined nociceptive behaviors and the expression of phosphorylated cAMP response element-binding protein (pCREB) in the dorsal horn of the lumbar spinal cord and the dorsal root ganglion (DRG) evoked by bee venom (BV). The effect of intraplantar preapplication of the somatostatin analog octreotide on nociceptive behaviors and pCREB expression was also examined. Subcutaneous injection of BV into the rat unilateral hindpaw pad induced significant spontaneous nociceptive behaviors, primary mechanical allodynia, primary thermal hyperalgesia, and mirror-thermal hyperalgesia, as well as an increase in pCREB expression in the lumbar spinal dorsal horn and DRG. Octreotide pretreatment significantly attenuated the BV-induced lifting/licking response and mechanical allodynia. Local injection of octreotide also significantly reduced pCREB expression in the lumbar spinal dorsal horn and DRG. Furthermore, pretreatment with cyclosomatostatin, a somatostatin receptor antagonist, reversed the octreotide-induced inhibition of the lifting/licking response, mechanical allodynia, and the expression of pCREB. These results suggest that BV can induce nociceptive responses and somatostatin receptors are involved in mediating the antinociception, which provides new evidence for peripheral analgesic action of somatostatin in an inflammatory pain state.

Analgesia to pressure–pain develops in the ipsilateral forehead after high- and low-frequency electrical stimulation of the forearm

In healthy participants, high-frequency electrical stimulation of the forearm not only evokes local hyperalgesia but also inhibits sensitivity to pressure-pain in the ipsilateral forehead, possibly due to activation of ipsilateral inhibitory pain modulation processes. The aim of this study was to compare the effects of high- and low-frequency electrical stimulation of the forearm on sensitivity to pressure-pain in the ipsilateral forehead, as inhibitory pain modulation may be stronger after low- than high-frequency electrical stimulation. Before and after high- and low-frequency electrical stimulation, sensitivity to heat and to blunt and sharp stimuli was assessed at and adjacent to the electrically conditioned site in the forearm. In addition, sensitivity to blunt pressure was measured bilaterally in the forehead. Pain was more intense after high- than low-frequency electrical stimulation and was followed by primary and secondary hyperalgesia to mechanical stimulation after high- but not low-frequency electrical stimulation. Nevertheless, sensitivity to pressure-pain decreased to the same extent in the ipsilateral forehead after both forms of electrical stimulation. This decrease was associated with heightened sensitivity to pressure-pain at the electrically conditioned forearm site and with diminished sensitivity to heat around this site. These findings suggest that sensitisation of pressure-sensitive nociceptive afferents at the site of electrical stimulation is associated with generation of an ipsilateral pain-inhibitory process. This ipsilateral pain-inhibitory process may decrease sensitivity to pressure-pain in the ipsilateral forehead and suppress secondary hyperalgesia to heat.

Balancing GRK2 and EPAC1 levels prevents and relieves chronic pain

Chronic pain is a major clinical problem, yet the mechanisms underlying the transition from acute to chronic pain remain poorly understood. In mice, reduced expression of GPCR kinase 2 (GRK2) in nociceptors promotes cAMP signaling to the guanine nucleotide exchange factor EPAC1 and prolongs the PGE2-induced increase in pain sensitivity (hyperalgesia). Here we hypothesized that reduction of GRK2 or increased EPAC1 in dorsal root ganglion (DRG) neurons would promote the transition to chronic pain. We used 2 mouse models of hyperalgesic priming in which the transition from acute to chronic PGE2-induced hyperalgesia occurs. Hyperalgesic priming with carrageenan induced a sustained decrease in nociceptor GRK2, whereas priming with the PKCε agonist ΨεRACK increased DRG EPAC1. When either GRK2 was increased in vivo by viral-based gene transfer or EPAC1 was decreased in vivo, as was the case for mice heterozygous for Epac1 or mice treated with Epac1 antisense oligodeoxynucleotides, chronic PGE2-induced hyperalgesia development was prevented in the 2 priming models. Using the CFA model of chronic inflammatory pain, we found that increasing GRK2 or decreasing EPAC1 inhibited chronic hyperalgesia. Our data suggest that therapies targeted at balancing nociceptor GRK2 and EPAC1 levels have promise for the prevention and treatment of chronic pain.

Analgesic and anti‐hyperalgesic effects of muscle injections with lidocaine or saline in patients with fibromyalgia syndrome

Patients with musculoskeletal pain syndrome including fibromyalgia (FM) complain of chronic pain from deep tissues including muscles. Previous research suggests the relevance of impulse input from deep tissues for clinical FM pain. We hypothesized that blocking abnormal impulse input with intramuscular lidocaine would decrease primary and secondary hyperalgesia and FM patients' clinical pain. We enrolled 62 female patients with FM into a double-blind controlled study of three groups who received 100 or 200 mg of lidocaine or saline injections into both trapezius and gluteal muscles. Study variables included pressure and heat hyperalgesia as well as clinical pain. In addition, placebo factors like patients' anxiety and expectation for pain relief were used as predictors of analgesia. Primary mechanical hyperalgesia at the shoulders and buttocks decreased significantly more after lidocaine than saline injections (p = 0.004). Similar results were obtained for secondary heat hyperalgesia at the arms (p = 0.04). After muscle injections, clinical FM pain significantly declined by 38% but was not statistically different between lidocaine and saline conditions. Placebo-related analgesic factors (e.g., patients' expectations of pain relief) accounted for 19.9% of the variance of clinical pain after the injections. Injection-related anxiety did not significantly contribute to patient analgesia. These results suggest that muscle injections can reliably reduce clinical FM pain, and that peripheral impulse input is required for the maintenance of mechanical and heat hyperalgesia of patients with FM. Whereas the effects of muscle injections on hyperalgesia were greater for lidocaine than saline, the effects on clinical pain were similar for both injectates.

Epigenetic Regulation of Spinal CXCR2 Signaling in Incisional Hypersensitivity in Mice

The regulation of gene expression in nociceptive pathways contributes to the induction and maintenance of pain sensitization. Histone acetylation is a key epigenetic mechanism controlling chromatin structure and gene expression. Chemokine CC motif receptor 2 (CXCR2) is a proinflammatory receptor implicated in neuropathic and inflammatory pain and is known to be regulated by histone acetylation in some settings. The authors sought to investigate the role of histone acetylation on spinal CXCR2 signaling after incision. Groups of 5-8 mice underwent hind paw incision. Suberoylanilide hydroxamic acid and anacardic acid were used to inhibit histone deacetylase and histone acetyltransferase, respectively. Behavioral measures of thermal and mechanical sensitization as well as hyperalgesic priming were used. Both message RNA quantification and chromatin immunoprecipitation analysis were used to study the regulation of CXCR2 and ligand expression. Finally, the selective CXCR2 antagonist SB225002 was administered intrathecally to reveal the function of spinal CXCR2 receptors after hind paw incision. Suberoylanilide hydroxamic acid significantly exacerbated mechanical sensitization after incision. Conversely, anacardic acid reduced incisional sensitization and also attenuated incision-induced hyperalgesic priming. Overall, acetylated histone H3 at lysine 9 was increased in spinal cord tissues after incision, and enhanced association of acetylated histone H3 at lysine 9 with the promoter regions of CXCR2 and keratinocyte-derived chemokine (CXCL1) was observed as well. Blocking CXCR2 reversed mechanical hypersensitivity after hind paw incision. Histone modification is an important epigenetic mechanism regulating incision-induced nociceptive sensitization. The spinal CXCR2 signaling pathway is one epigenetically regulated pathway controlling early and latent sensitization after incision.

Electrophysiological correlates of hyperalgesic priming in vitro and in vivo

We have modeled the transition from acute to chronic pain in the rat. In this model (termed hyperalgesic priming) a chronic state develops after a prior inflammatory process or exposure to an inflammatory mediator, in which response to subsequent exposure to prostaglandin E2 (PGE2) is characterized by a protein kinase Cε-dependent marked prolongation of mechanical hyperalgesia. To assess the effect of priming on the function of the nociceptor, we have performed in vitro patch clamp and in vivo single-fiber electrophysiology studies using tumor necrosis factor α to induce priming. In vitro, the only change observed in nociceptors cultured from primed animals was a marked hyperpolarization in resting membrane potential (RMP); prolonged sensitization, measured at 60minutes, could not be tested in vitro. However, complimentary with behavioral findings, in vivo baseline mechanical nociceptive threshold was significantly elevated compared to controls. Thirty minutes after injection of PGE2 into the peripheral receptive field, both primed and control nociceptors showed enhanced response to mechanical stimulation. However, 60minutes after PGE2 administration, the response to mechanical stimulation was further increased in primed but not in control nociceptors. Thus, at the level of the primary afferent nociceptor, it is possible to demonstrate both altered function at baseline and prolonged PGE2-induced sensitization. Intrathecal antisense (AS) to Kv7.2, which contributes to RMP in sensory neurons, reversibly prevented the expression of priming in both behavioral and single-fiber electrophysiology experiments, implicating these channels in the expression of hyperalgesic priming.

Role of Nociceptor CaMKII in Transition from Acute to Chronic Pain (Hyperalgesic Priming) in Male and Female Rats

We have previously shown that activation of protein kinase Cε (PKCε) in male rats induces a chronic, long-lasting change in nociceptors such that a subsequent exposure to proinflammatory mediators produces markedly prolonged mechanical hyperalgesia. This neuroplastic change, hyperalgesic priming, is dependent on activation of cytoplasmic polyadenylation element-binding protein (CPEB), downstream of PKCε, and consequent translation of mRNAs in the peripheral terminal of the nociceptor. Since α calmodulin-dependent protein kinase II (αCaMKII), a molecule implicated in neuroplasticity, is a target of CPEB and can also affect CPEB function, we investigated its role in the transition from acute to chronic pain. Priming induced by direct activation of PKCε can be prevented by inhibition of αCaMKII. In addition, direct activation of αCaMKII induces priming, which was not prevented by pretreatment with PKCε antisense, suggesting that αCaMKII is downstream of PKCε in the induction of priming. Activation of ryanodine receptors (RyRs), which can lead to activation of αCaMKII, also induced priming, in a calcium- and αCaMKII-dependent manner. Similarly, inhibition of the RyR and a calcium buffer prevented induction of priming by PKCε. Unlike activation of PKCε, ryanodine and αCaMKII induced priming in female as well as male rats. Our results demonstrate a contribution of αCaMKII to induction of hyperalgesic priming, a phenomenon implicated in the transition from acute to chronic pain.

Characterization of nociceptive behavioural responses in the awake pig following UV–B‐induced inflammation

Among the current translational inflammatory pain models, the ultraviolet (UV) irradiation is of rapidly growing interest. The development of primary thermal and mechanical hyperalgesia has been observed in humans and rodents. The pig as a translational animal model might be advantageous due to its great homology with humans. The skin in the flank of awake pigs was irradiated by a UV-B light source (1 J/cm(2) ) and changes in thermal and mechanical sensitivity 24 and 48 h following irradiation were measured via assessment of nociceptive behaviours. Thermal sensitivity increased significantly within the inflamed site 24 h after irradiation as indicated by the reduction of latency to respond to thermal stimulation from baseline to 24 h (P < 0.05). At 48 h, the response latency had not decreased any further (P = 0.414). Thermal sensitivity was also higher at the inflamed skin site than at the control site 24 and 48 h following irradiation (P < 0.05). An overall decrease of 50% of the baseline mechanical threshold was observed 24 and 48 h following UV-B irradiation (P = 0.092). Following the inflammatory challenge, the mechanical sensitivity was higher at the site of irradiation compared with the control skin at both 24 and 48 h (P < 0.05). Our study shows that behavioural recordings are a valid tool for the assessment of thermal hyperalgesia following UV-B inflammation in porcine skin, but they were not capable of providing a clear indication of the development of mechanical hyperalgesia.

Peripheral Administration of Translation Inhibitors Reverses Increased Hyperalgesia in a Model of Chronic Pain in the Rat

Chronic pain is extremely difficult to manage, in part due to lack of progress in reversing the underlying pathophysiology. Since translation of messenger ribonucleic acids (mRNAs) in the peripheral terminal of the nociceptor plays a role in the transition from acute to chronic pain, we tested the hypothesis that transient inhibition of translation in the peripheral terminal of the nociceptor could reverse hyperalgesic priming, a model of transition from acute to chronic pain. We report that injection of translation inhibitors rapamycin and cordycepin, which inhibit translation by different mechanisms, at the peripheral terminal of the primed nociceptor produces reversal of priming in the rat that outlasted the duration of action of these drugs to prevent the development of priming. These data support the suggestion that interruption of translation in the nociceptor can reverse a preclinical model of at least 1 form of chronic pain. PerspectiveThis study provides evidence that ongoing protein translation in the sensory neuron terminals is involved in pain chronification, and local treatment that transiently interrupts this translation may be a useful therapy to chronic pain.

Does hyperbaric oxygen therapy attenuate secondary hyperalgesia areas induced by a heat injury in humans?

Hyperbaric oxygen therapy (HBO) has, in animal experiments, been demonstrated to exert anti-inflammatory and mechanical anti-hypersensitivity effects in models of inflammation and neuropathy. Breathing 100% O2 at pressures greater than 1 ATA will increase the production of reactive oxygen and nitrogen species, and, impair β2-integrin function and chemokine production, explaining some of the anti-inflammatory effects of HBO, though the mechanisms are still incompletely understood. We hypothesized that the anti-inflammatory effect of HBO would attenuate secondary hyperalgesia areas (SHA) after a first degree heat injury in humans, using an open, randomized study design. Seventeen healthy subjects received two first degree heat injuries (HI) on the lower leg, with a minimum inter-session interval of 28 days. Induction of the HI by a contact thermode (12.5 cm2, 47.0°C, 7 min), was made after baseline assessments of mechanical primary and secondary hyperalgesia, erythema and SHA. Subjects were randomized in the first session to stay in normal ambient conditions (1 ATA, 21% O2) and in the second session to receive HBO (2.4 ATA, 100% O2) or vice versa. The assessments were repeated 30, 70, 100, 160 and 220 min after the HI. Subjects were randomized to, either, therapy in the first session with normal ambient conditions (1 ATA, 21% O2) and in the second session with 90 min of HBO (2.4 ATA, 100% O2), immediately after the HI, or, vice versa. A near-significant attenuation of the SHA, calculated as area-under-curve, was demonstrated from 886 cm2/min during ambient conditions compared to 569 cm2/min during HBO (P = 0.011). Our findings indicate that HBO may attenuate secondary hyperalgesia areas, compared to breathing under ambient conditions, in humans. The first degree heat injury is a sensitive model of anti-inflammatory effects of hyperbaric oxygen therapy in humans. Further research is needed. Supported by a grant from the “Direktør Ole Trock-Jansen og Hustrus Fond.”

Duration of action of a topical anaesthetic formulation for pain management of mulesing in sheep

To investigate the effect of topical anaesthesia on 'mothering up' of lambs after mulesing and marking, and for pain alleviation over a 24-h period. Two separate trials were performed on Merino lambs undergoing the mules procedure for flystrike prevention, to assess the efficacy of immediate postoperative topical anaesthetic wound dressing containing lignocaine (hydrochloride) 40.6 g/L, bupivacaine (hydrochloride) 4.5 g/L, adrenaline (tartrate) 24.8 mg/L and cetrimide 5.0 g/L in a gel base (Bayer Animal Health, Gordon, NSW, Australia). In both trials, lambs were assigned to one of three treatment regimens: control, mules procedure with topical anaesthetic (0.5 mL/kg) and mules procedure without topical anaesthetic treatment. Parameters measured included body weight, assessment of skin and wound sensitivity to light touch and pain stimulation, behavioural responses and time to mother up and to feed. In both trials there was rapid (1 min) and prolonged (up to 24 h) wound analgesia as shown by lower scores for light touch (P<0.001) and pain responses (P<0.001), with absent or significantly diminished primary and secondary hyperalgesia (P≤0.05) and significant reduction in pain-related behaviours (P<0.001) in treated versus untreated lambs. Significant pain alleviation and improved recovery can be achieved in lambs for at least 24 h after mulesing through the use of topical anaesthesia. It is suggested that the haemostatic action of adrenalin, together with inhibition of the inflammatory cascade and the barrier effect of the gel within the product, may explain the prolonged anaesthesia up to 24 h observed in the present study. These results suggest that topical anaesthesia has the capacity to dramatically improve the welfare of lambs undergoing mulesing.

Transition to Persistent Orofacial Pain after Nerve Injury Involves Supraspinal Serotonin Mechanisms

The orofacial region is a major focus of chronic neuropathic pain conditions characterized by primary hyperalgesia at the site of injury and secondary hyperalgesia outside the injured zone. We have used a rat model of injury to the maxillary branch (V2) of the trigeminal nerve to produce constant and long-lasting primary hyperalgesia in the V2 territory and secondary hyperalgesia in territories innervated by the mandibular branch (V3). Our findings indicate that the induction of primary and secondary hyperalgesia depended on peripheral input from the injured nerve. In contrast, the maintenance of secondary hyperalgesia depended on central mechanisms. The centralization of the secondary hyperalgesia involved descending 5-HT drive from the rostral ventromedial medulla and the contribution of 5-HT3 receptors in the trigeminal nucleus caudalis (Vc), the homolog of the spinal dorsal horn. Electrophysiological studies further indicate that after nerve injury spontaneous responses and enhanced poststimulus discharges in Vc nociresponsive neurons were time-dependent on descending 5-HT drive and peripheral input. The induction phase of secondary hyperalgesia involved central sensitization mechanisms in Vc neurons that were dependent on peripheral input, whereas the maintenance phase of secondary hyperalgesia involved central sensitization in Vc neurons conducted by a delayed descending 5-HT drive and a persistence of peripheral inputs. Our results are the first to show that the maintenance of secondary hyperalgesia and underlying central sensitization associated with persistent pain depend on a transition to supraspinal mechanisms involving the serotonin system in rostral ventromedial medulla-dorsal horn circuits.

Evaluation of the analgesic efficacy and psychoactive effects of AZD1940, a novel peripherally acting cannabinoid agonist, in human capsaicin‐induced pain and hyperalgesia

The aim of the present study was to investigate the effects of AZD1940, a novel peripherally acting cannabinoid CB(1) /CB(2) receptor agonist, on capsaicin-induced pain and hyperalgesia, as well as on biomarkers of cannabinoid central nervous system (CNS) effects. The present study was a randomized, double-blind, placebo-controlled, four-sequence, two-period, cross-over study in 44 male healthy volunteers aged 20-45 years. The effects of two single oral doses of AZD1940 (400 and 800 μg) were compared with placebo. Pain intensity after intradermal capsaicin injections in the forearm was assessed on a continuous visual analogue scale (VAS; 0-100 mm). Primary and secondary hyperalgesia induced by application of capsaicin cream on the calf were assessed by measuring heat pain thresholds and the area of mechanical allodynia, respectively. The CNS effects were assessed at baseline and up to 24 h after dosing using a visual analogue mood scales (VAMS) for feeling 'stimulated', 'high', 'anxious', 'sedated' or 'down'. AZD1940 did not significantly attenuate ongoing pain or primary or secondary hyperalgesia compared with placebo. Mild CNS effects for AZD1940were observed on the VAMS for 'high' and 'sedated'. Dose-dependent mild-to-moderate CNS-related and gastrointestinal adverse events were reported following treatment with AZD1940. No evidence of analgesic efficacy was found for a peripherally acting CB(1)/CB(2) receptor agonist in the human capsaicin pain model. The emergence of mild dose-dependent CNS effects suggests that the dose range predicted from preclinical data had been attained.

Role of a novel nociceptor autocrine mechanism in chronic pain

We have previously shown, in the rat, that neuropathic and inflammatory events produce a neuroplastic change in nociceptor function whereby a subsequent exposure to a proinflammatory mediator (e.g. prostaglandin E2 ; PGE2 ) produces markedly prolonged mechanical hyperalgesia. While the initial approximately 30min of this prolonged PGE2 hyperalgesia remains PKA-dependent, it subsequently switches to become dependent on protein kinase C epsilon (PKCε). In this study we tested the hypothesis that the delayed onset, PKCε-mediated, component of PGE2 hyperalgesia is generated by the active release of a nucleotide from the peripheral terminal of the primed nociceptor and this nucleotide is then metabolized to produce adenosine, which acts on a Gi-coupled A1 adenosine receptor on the nociceptor to generate PKCε-dependent hyperalgesia. We report that inhibitors of ATP-binding cassette transporters, of ecto-5'-phosphodiesterase and ecto-5'nucleotidase (enzymes involved in the metabolism of cyclic nucleotides to adenosine) and of A1 adenosine receptors each eliminated the late, but not the early, phase of PGE2 -induced hyperalgesia in primed animals. A second model of chronic pain induced by transient attenuation of G-protein-coupled receptor kinase2, in which the prolongation of PGE2 hyperalgesia is not PKCε-dependent, was not attenuated by inhibitors of any of these mechanisms. Based on these results we propose a contribution of an autocrine mechanism, in the peripheral terminal of the nociceptor, in the hyperalgesic priming model of chronic pain.

The pattern and time course of somatosensory changes in the human UVB sunburn model reveal the presence of peripheral and central sensitization

The ultraviolet B (UVB) sunburn model was characterized with a comprehensive battery of quantitative sensory testing (QST). Primary hyperalgesia in UVB-irradiated skin and secondary hyperalgesia in adjacent nonirradiated skin were studied in 22 healthy subjects 24h after irradiation with UVB at 3-fold minimal erythema dose of a skin area 5cm in diameter at the thigh and compared to mirror-image contralateral control areas. The time course of hyperalgesia over 96h was studied in a subgroup of 12 subjects. Within the sunburn area, cold hyperesthesia (P=.01), profound generalized hyperalgesia to heat (P<.001), cold (P<.05), pinprick and pressure (P<.001), and mild dynamic mechanical allodynia (P<.001) were present. The finding of cold hyperalgesia and cold hyperesthesia is new in this model. The sunburn was surrounded by large areas of pinprick hyperalgesia (mean±SEM, 218±32cm2) and a small rim of dynamic mechanical allodynia but no other sensory changes. Although of smaller magnitude, secondary hyperalgesia and dynamic mechanical allodynia adjacent to the UVB-irradiated area were statistically highly significant. Primary and secondary hyperalgesia developed in parallel within hours, peaked after 24–32h, and lasted for more than 96h. These data reveal that the UVB sunburn model activates a broad spectrum of peripheral and central sensitization mechanisms and hence is a useful human surrogate model to be used as a screening tool for target engagement in phases 1 and 2a of drug development.