Protein kinase Cδ mediates histamine-evoked itch and responses in pruriceptors.

BACKGROUND: Itch-producing compounds stimulate receptors expressed on small diameter fibers that innervate the skin. Many of the currently known pruritogen receptors are Gq Protein-Coupled Receptors (GqPCR), which activate Protein Kinase C (PKC). Specific isoforms of PKC have been previously shown to perform selective functions; however, the roles of PKC isoforms in regulating itch remain unclear. In this study, we investigated the novel PKC isoform PKCδ as an intracellular modulator of itch signaling in response to histamine and the non-histaminergic pruritogens chloroquine and ß-alanine. RESULTS: Behavioral experiments indicate that PKCδ knock-out (KO) mice have a 40% reduction in histamine-induced scratching when compared to their wild type littermates. On the other hand, there were no differences between the two groups in scratching induced by the MRGPR agonists chloroquine or ß-alanine. PKCδ was present in small diameter dorsal root ganglion (DRG) neurons. Of PKCδ-expressing neurons, 55% also stained for the non-peptidergic marker IB4, while a smaller percentage (15%) expressed the peptidergic marker CGRP. Twenty-nine percent of PKCδ-expressing neurons also expressed TRPV1. Calcium imaging studies of acutely dissociated DRG neurons from PKCδ-KO mice show a 40% reduction in the total number of neurons responsive to histamine. In contrast, there was no difference in the number of capsaicin-responsive neurons between KO and WT animals. Acute pharmacological inhibition of PKCδ with an isoform-specific peptide inhibitor (δV1-1) also significantly reduced the number of histamine-responsive sensory neurons. CONCLUSIONS: Our findings indicate that PKCδ plays a role in mediating histamine-induced itch, but may be dispensable for chloroquine- and ß-alanine-induced itch.

Isozyme-specific effects of protein kinase C in pain modulation.

BACKGROUND: Protein kinase C (PKC) is a family of serine/threonine kinases that contains more than 10 isozymes. Evidence suggests that PKC may play important roles in pain modulation, but the isozyme-specific effects of PKC on different aspects of pain modulation are not fully understood. We hypothesize that different PKC isozymes play different roles in different aspects of pain modulation. METHODS: The nociceptive behaviors of mice with deletion of PKCα, ß, γ, or δ in multiple pain models were compared with their respective wild-type littermates. Also, morphine analgesia and the development of morphine tolerance in mice with deletion of PKCγ were compared with their respective wild-type littermates. RESULTS: Thermal hyperalgesia induced by complete Freund's adjuvant injection was significantly attenuated by the deletion of PKCß, γ, or δ, but not PKCα. Deletion of PKCγ significantly attenuated neuropathic mechanical allodynia induced by spared nerve injury, whereas deletion of PKCα enhanced this allodynia. Baseline thermal and mechanical sensitivity, nociceptive behaviors induced by formalin, mechanical allodynia induced by complete Freund's adjuvant injection, were not altered by deletion of PKCα, ß, γ, or δ. Finally, morphine analgesia and the development of morphine tolerance were not altered in PKCγ-deficient mice. CONCLUSIONS: PKC has isozyme-specific effects in pain modulation.

A role for acid-sensing ion channel 3, but not acid-sensing ion channel 2, in sensing dynamic mechanical stimuli.

BACKGROUND: Acid-sensing ion channels 2 and 3 (ASIC2 and ASIC3, respectively) have been implicated as putative mechanotransducers. Because mechanical hyperalgesia is a prominent consequence of nerve injury, we tested whether male and female ASIC2 or ASIC3 knockout mice have altered responses to mechanical and heat stimuli at baseline and during the 5 weeks after spinal nerve ligation. METHODS: Age-matched, adult male and female ASIC2 knockout (n=21) and wild-type (WT; n=24) mice or ASIC3 knockout (n=20) and WT (n=19) mice were tested for sensitivity to natural stimuli before and after spinal nerve ligation surgery. All animals were first tested for baseline sensitivity to mechanical and heat stimuli and in a novel dynamic mechanical stimulation test. The same testing procedures were then repeated weekly after spinal nerve injury. RESULTS: Compared with their respective WT counterparts, ASIC2 and ASIC3 knockout mice had normal baseline sensitivity to standard mechanical and heat stimuli. However, when exposed to a novel stroking stimulus to test sensitivity to dynamic mechanical stimulation, ASIC3 knockout mice were significantly more sensitive than were WT mice. After spinal nerve ligation, ASIC2 and ASIC3 knockout mice developed mechanical and heat hyperalgesia comparable with that of their respective WT controls. In addition, in both experiments, female mice were more sensitive than male mice to heat at baseline and after the nerve injury. CONCLUSIONS: We conclude that ASIC2 and ASIC3 channels are not directly involved in the development or maintenance of neuropathic pain after spinal nerve ligation. However, the ASIC3 channel significantly modulates the sensing of dynamic mechanical stimuli in physiologic condition.

Genetic targeting of ERK1 suggests a predominant role for ERK2 in murine pain models.

The extracellular signal-regulated kinase (ERK) isoforms, ERK1 and ERK2, are believed to be key signaling molecules in nociception and nociceptive sensitization. Studies using inhibitors targeting the shared ERK1/2 upstream activator, mitogen-activated protein kinase kinase (MEK), and transgenic mice expressing a dominant-negative form of MEK have established the importance of ERK1/2 signaling. However, these techniques do not discriminate between ERK1 and ERK2. To dissect the function of each isoform in pain, we used mice with a targeted genetic deletion of ERK1 [ERK1 knock-out (KO)] to test the hypothesis that ERK1 is required for behavioral sensitization in rodent pain models. Despite activation (phosphorylation) of ERK1 after acute noxious stimulation and in models of chronic pain, we found that ERK1 was not required for formalin-induced spontaneous behaviors, complete Freund's adjuvant-induced heat and mechanical hypersensitivity, and spared nerve injury-induced mechanical hypersensitivity. However, ERK1 deletion did delay formalin-induced long-term heat hypersensitivity, without affecting formalin-induced mechanical hypersensitivity, suggesting that ERK1 partially shapes long-term responses to formalin. Interestingly, ERK1 deletion resulted in elevated basal ERK2 phosphorylation. However, this did not appear to influence nociceptive processing, since inflammation-induced ERK2 phosphorylation and pERK1/2 immunoreactivity in spinal cord were not elevated in ERK1 KO mice. Additionally, systemic MEK inhibition with SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile) attenuated formalin-induced spontaneous behaviors similarly in wild-type and ERK1 KO mice, indicating that unrelated signaling pathways do not functionally compensate for the loss of ERK1. Together, these results suggest that ERK1 plays a limited role in nociceptive sensitization and support a predominant role for ERK2 in these processes.

Effects of soy diet on inflammation-induced primary and secondary hyperalgesia in rat.

Soy consumption is said to prevent or treat atherosclerosis, cancer, pain, and memory deficits, but experimental and clinical evidence to support these claims are lacking. We used in vivo models of inflammation to determine whether a soy diet reduces primary or secondary hyperalgesia. In all three experiments, rats were fed either a soy- or casein-based diet for at least 2 weeks before induction of inflammation and for the duration of experiments. Mechanical and heat paw withdrawal thresholds and edema were measured before and several times after induction of inflammation. Primary hyperalgesia was assessed in two models: unilateral intraplantar injection with 0.1 ml of 25% complete Freund's adjuvant (CFA) or 0.1 ml of 1% carrageenan. Unilateral injection of the intra-articular knee space with 25% CFA (0.1 ml) was used to determine the effects of soy in a model of secondary hyperalgesia. Following intraplantar injection of CFA, soy-fed animals exhibited significantly less paw edema, mechanical allodynia, and heat hyperalgesia compared to casein-fed animals. In the carrageenan model of paw inflammation, soy-fed animals were also less allodynic to mechanical stimuli, than were casein-fed animals, but showed no diet based differences in paw edema or heat hyperalgesia. Soy diet did not affect any of the outcome measures after the intra-articular injection of CFA. Our results suggest that a soy diet significantly decreases aspects of inflammation-induced primary, but not secondary, hyperalgesia in rats.

Central serotonergic neurons are differentially required for opioid analgesia but not for morphine tolerance or morphine reward.

Opioids remain the most effective analgesics despite their potential adverse effects such as tolerance and addiction. Mechanisms underlying these opiate-mediated processes remain the subject of much debate. Here we describe opioid-induced behaviors of Lmx1b conditional knockout mice (Lmx1bf/f/p), which lack central serotonergic neurons, and we report that opioid analgesia is differentially dependent on the central serotonergic system. Analgesia induced by a kappa opioid receptor agonist administered at the supraspinal level was abolished in Lmx1bf/f/p mice compared with their wild-type littermates. Furthermore, compared with their wild-type littermates Lmx1bf/f/p mice exhibited significantly reduced analgesic effects of mu and delta opioid receptor agonists at both spinal and supraspinal sites. In contrast to the attenuation in opioid analgesia, Lmx1bf/f/p mice developed tolerance to morphine analgesia and displayed normal morphine reward behavior as measured by conditioned place preference. Our results provide genetic evidence supporting the view that the central serotonergic system is a key component of supraspinal pain modulatory circuitry mediating opioid analgesia. Furthermore, our data suggest that the mechanisms of morphine tolerance and morphine reward are independent of the central serotonergic system.

Lumbar sympathectomy attenuates cold allodynia but not mechanical allodynia and hyperalgesia in rats with spared nerve injury.

UNLABELLED: In certain patients with neuropathic pain, the pain is dependent on activity in the sympathetic nervous system. To investigate whether the spared nerve injury model (SNI) produced by injury to the tibial and common peroneal nerves and leaving the sural nerve intact is a model for sympathetically maintained pain, we measured the effects of surgical sympathectomy on the resulting mechanical allodynia, mechanical hyperalgesia, and cold allodynia. Decreases of paw withdrawal thresholds to von Frey filament stimuli and increases in duration of paw withdrawal to pinprick or acetone stimuli were observed in the ipsilateral paw after SNI, compared with their pre-SNI baselines. Compared with sham surgery, surgical lumbar sympathectomy had no effect on the mechanical allodynia and mechanical hyperalgesia induced by SNI. However, the sympathectomy significantly attenuated the cold allodynia induced by SNI. These results suggest that the allodynia and hyperalgesia to mechanical stimuli in the SNI model is not sympathetically maintained. However, the sympathetic nervous system may be involved, in part, in the mechanisms of cold allodynia in the SNI model. PERSPECTIVE: The results of our study suggest that the SNI model is not an appropriate model of sympathetically maintained mechanical allodynia and hyperalgesia but may be useful to study the mechanisms of cold allodynia associated with sympathetically maintained pain states.

Mice lacking central serotonergic neurons show enhanced inflammatory pain and an impaired analgesic response to antidepressant drugs.

A large body of literature has implicated serotonin [5-hydroxytryptamine (5-HT)] in descending modulation of nociceptive transmission. Here, we have studied the pain behavior of Lmx1b conditional knock-out mice (Lmx1b(f/f/p)), which lack 5-HT neurons in the CNS. Lmx1b(f/f/p) mutant mice showed normal thermal and visceral pain responses but were less sensitive to mechanical stimuli and exhibited enhanced inflammatory pain compared with their littermate control mice. Importantly, the analgesic effect of several antidepressant drugs, including selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), and tricyclic antidepressants, was either abolished or greatly attenuated in Lmx1b(f/f/p) mice. Moreover, in the acute versus persistent pain settings, the analgesic actions of the SNRI duloxetine and the SSRI fluoxetine were differentially affected. Together, our results provide in vivo genetic evidence demonstrating that although the predominant role of the central 5-HT system in inflammatory pain is inhibitory, its role in acute mechanical pain is facilitatory. The findings that the analgesic effects of various antidepressant drugs are differentially dependent on the central 5-HT system should help us to understand the mechanism of the analgesic action of different classes of antidepressants in the management of persistent 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.

Tart cherry anthocyanins suppress inflammation-induced pain behavior in rat.

The use of complementary and alternative medicine (CAM) has increased in the United States and more patients are seeking CAM therapies for control of pain. The present investigation tested the efficacy of orally administered anthocyanins extracted from tart cherries on inflammation-induced pain behavior in rats. Paw withdrawal latency to radiant heat and paw withdrawal threshold to von Frey probes were measured. The first set of experiments examined the effects of tart cherry anthocyanins (400 mg/kg) on the nociceptive behaviors and edema associated with inflammation induced by intraplantar injection of 1% carrageenan. These studies also included tests of motor coordination. The second set of experiments determined if tart cherry anthocyanins (15, 85, and 400 mg/kg) dose-dependently affected the inflammation induced by intraplantar injection of 25% complete Freund's adjuvant. We found that tart cherry extracts reduce inflammation-induced thermal hyperalgesia, mechanical hyperalgesia and paw edema. The suppression of thermal hyperalgesia was dose-dependent and the efficacy of highest dose (400 mg/kg) was similar to indomethacin (5 mg/kg). The highest dose anthocyanin (400 mg/kg) had no effects on motor function. These data suggest that tart cherry anthocyanins may have a beneficial role in the treatment of inflammatory pain. The antihyperalgesic effects may be related to the anti-inflammatory and antioxidant properties of anthocyanins. A better understanding of the modulatory role of dietary constituents and phytonutrients on pain will offer further therapeutic options for treating patients with persistent and chronic pain conditions.

Antiallodynic effects of systemic and intrathecal morphine in the spared nerve injury model of neuropathic pain in rats.

BACKGROUND: The efficacy of opioids for neuropathic pain remains controversial. The effects of morphine on pain behavior were investigated in two animal models of neuropathic pain: the spared nerve injury (SNI) model and the spinal nerve ligation (SNL) model. METHODS: Nerve injuries were created in rats either by tight ligation and section of the left tibial and common peroneal nerves (SNI) or by unilateral ligation of L5 and L6 spinal nerves (SNL). Paw withdrawal threshold to mechanical stimuli was measured using the up-down method in the hairy and glabrous skin territories of the sural nerve for SNI rats or in the mid-plantar paw of SNL rats. RESULTS: Before SNI, the median paw withdrawal thresholds in hairy and glabrous skin were similar (26 g [25%, 75% quartiles: 26, 26 g]). The paw withdrawal threshold decreased after SNI in both hairy and glabrous skin (P < 0.001). Thirty days after the SNI, the threshold in hairy skin (0.3 g) was significantly lower than in glabrous skin (1.9 g; P < 0.001). In blinded experiments, both subcutaneous and intrathecal morphine (0.1-10 microg) dose-dependently attenuated mechanical allodynia induced by SNI measured in the hairy skin, an effect that was naloxone reversible. The ED50 for the intrathecal morphine was 0.52 microg (95% confidence interval, 0.31-0.90 microg). Morphine (1 microg intrathecal) attenuated SNI-induced mechanical allodynia in glabrous skin with potency similar to that in hairy skin. In SNL rats, morphine (30 microg intrathecal) almost completely reversed the SNL-induced mechanical allodynia. CONCLUSIONS: (1) SNI-induced mechanical allodynia is characterized by a lower paw withdrawal threshold in hairy versus glabrous skin; (2) systemic and intrathecal morphine reverse SNI-induced mechanical allodynia in a dose-dependent fashion; and (3) intrathecal morphine also reverses SNL-induced mechanical allodynia. These results suggest that intrathecal opioids are likely to be effective in the treatment of neuropathic pain.

Nerve injury induces a tonic bilateral mu-opioid receptor-mediated inhibitory effect on mechanical allodynia in mice.

BACKGROUND: Mice lacking the mu-opioid receptor gene have been used to characterize the role of mu-opioid receptors in nociception and the analgesic actions of opioid agonists. In this study, the authors determined the role of mu-opioid receptors in neuropathic pain behaviors and the effectiveness of mu- and kappa-opioid receptor agonists on this behavior in mice. METHODS: The authors studied the behavioral responses of mu-opioid receptor knockout and wild-type mice to thermal and mechanical stimuli before and after neuropathic pain induced by unilateral ligation and section of the L5 spinal nerve. Response to mechanical stimuli was evaluated by determining the frequency of hind paw withdrawal to repetitive stimulation using a series of von Frey monofilaments. Thermal hyperalgesia was assessed by determining the paw withdrawal latencies to radiant heat and frequency of hind paw withdrawal to cooling stimuli. The effects of systemic morphine, the kappa-opioid agonist U50488H, and naloxone on responses to mechanical and thermal stimuli were also studied in spinal nerve-injured mice. RESULTS: After spinal nerve injury, wild-type mice developed increased responsiveness to mechanical, heat, and cooling stimuli ipsilateral to nerve injury. mu-Opioid receptor knockout mice not only had more prominent mechanical allodynia in the nerve-injured paw, but also expressed contralateral allodynia to mechanical stimuli. Hyperalgesia to thermal stimuli was similar between mu-opioid knockout and wild-type animals. Morphine decreased mechanical allodynia dose dependently (3-30 mg/kg subcutaneous) in wild-type mice--an effect that was attenuated in the heterozygous mice and absent in the homozygous mu-opioid knockout mice. The kappa-opioid agonist U50488H (3-10 mg/kg subcutaneous) attenuated mechanical allodynia in wild-type, heterozygous, and homozygous mu-opioid mice. Naloxone in wild-type mice resulted in enhanced ipsilateral and contralateral allodynia to mechanical stimuli that resembled the pain behavior observed in mu-opioid receptor knockout mice. CONCLUSIONS: The authors' observations indicate that (1) unilateral nerve injury induces a bilateral tonic activation of endogenous mu-opioid receptor-mediated inhibition that attenuates mechanical allodynia but not thermal hyperalgesia, (2) both mu- and kappa-opioid agonists attenuate neuropathic pain in mice, and (3) the antihyperalgesic actions of morphine are mediated primarily via mu-opioid receptors.

Analgesic effects of a soy-containing diet in three murine bone cancer pain models.

UNLABELLED: Bone is a common metastatic site for prostate and breast cancer, and bone cancer is usually associated with severe pain. Traditional treatments for cancer pain can sometimes be ineffective or associated with side effects. Thus an increasing number of patients seek alternative therapies. In this study we investigated the analgesic effects of a soy diet on 3 experimental models of bone cancer pain. Mice were fed a diet in which the protein source was either soy or casein. After 1 week on the diet, sarcoma cells (NCTC 2472) were injected into the medullary cavity of the humeri, femur, or calcaneus. Experimenters blinded to diet of the animal assessed the pain behavior in these animals, forelimb grip force in the humerus model and paw withdrawal frequency to mechanical stimuli in the calcaneus and femur models. The effect of morphine on cancer-induced pain behavior was investigated in calcaneus and femur models. In addition, in the femur model, the effects of soy on tumor size and bone destruction were studied. The soy diet reduced secondary mechanical hyperalgesia in the femur model but had no effect on primary mechanical hyperalgesia in the calcaneus model or on movement-related hyperalgesia in the humerus model. No dietary impact was discerned in measurements of tumor size, bone destruction, and body weight in the femur model, suggesting that the soy diet had no effect on cancer growth. Morphine dose-dependently reduced hyperalgesia with no diet-based difference. These results suggest that a soy diet might provide analgesia in certain forms of hyperalgesia associated with bone cancer. PERSPECTIVE: The study raises the possibility of dietary supplements influencing aspects of cancer pain. Further research will help determine if use of nutritional supplements, such as soy proteins, can reduce opioid analgesic use in chronic pain states and help minimize the side effects associated with long term use of opioids.

Impaired NMDA receptor-mediated postsynaptic function and blunted NMDA receptor-dependent persistent pain in mice lacking postsynaptic density-93 protein.

Modification of synaptic NMDA receptor (NMDAR) expression influences NMDAR-mediated synaptic function and associated persistent pain. NMDARs directly bind to a family of membrane-associated guanylate kinases (MAGUKs) that regulate surface and synaptic NMDAR trafficking in the CNS. We report here that postsynaptic density-93 protein (PSD-93), a postsynaptic neuronal MAGUK, is expressed abundantly in spinal dorsal horn and forebrain, where it colocalizes and interacts with NMDAR subunits NR2A and NR2B. Targeted disruption of the PSD-93 gene reduces not only surface NR2A and NR2B expression but also NMDAR-mediated excitatory postsynaptic currents and potentials, without affecting surface AMPA receptor expression or its synaptic function, in the regions mentioned above. Furthermore, mice lacking PSD-93 exhibit blunted NMDAR-dependent persistent pain induced by peripheral nerve injury or injection of Complete Freund's Adjuvant, although they display intact nociceptive responsiveness to acute pain. PSD-93 appears to be important for NMDAR synaptic targeting and function and to be a potential biochemical target for the treatment of persistent pain.

Role of micro-opioid receptors in formalin-induced pain behavior in mice.

Intraplantar formalin injection is widely used as an experimental model of tonic pain. We investigated the role of endogenous micro-opioid receptor mechanisms in formalin-induced nocifensive behavior in mice. The flinching response induced by formalin (2%, 20 microl) was studied in mice with normal (wild type, n = 8) and absent (homozygous micro-opioid receptor knockout, n = 8) micro-opioid receptor levels. The flinch responses were counted every 5 min for 60 min post-formalin injection. Lumbar spinal cord (L4, 5) was harvested 2 h post-formalin injection to examine c-Fos expression using immunohistochemistry. The effects of naloxone (5 mg/kg, sc) administered 30 min before the intraplantar formalin injection on the flinching response of wild-type mice (n = 7) were also recorded. The second-phase formalin response (10-60 min after formalin) was higher in homozygous micro-opioid receptor knockout mice compared to the wild-type mice (P < 0.01). Naloxone administration in wild-type mice before formalin injection resulted in pain behavior similar to that observed in homozygous micro-opioid receptor knockout mice (P > 0.05). The c-Fos expression induced by formalin injection in the knockout mice was not different from that observed in wild-type mice. Our results suggest that the endogenous micro-opioid system is activated by intraplantar formalin injection and exerts a tonic inhibitory effect on the pain behavior. These results suggest an important modulatory role of endogenous micro-opioid receptor mechanisms in tonic pain states.