Effect of bucladesine, pentoxifylline, and H-89 as cyclic adenosine monophosphate analog, phosphodiesterase, and protein kinase A inhibitor on acute pain.

The aim of this study was to determine the effects of cyclic adenosine monophosphate (cAMP) and its dependent pathway on thermal nociception in a mouse model of acute pain. Here, we studied the effect of H-89 (protein kinase A inhibitor), bucladesine (Db-cAMP) (membrane-permeable analog of cAMP), and pentoxifylline (PTX; nonspecific phosphodiesterase (PDE) inhibitor) on pain sensation. Different doses of H-89 (0.05, 0.1, and 0.5 mg/100 g), PTX (5, 10, and 20 mg/100 g), and Db-cAMP (50, 100, and 300 nm/mouse) were administered intraperitoneally (I.p.) 15 min before a tail-flick test. In combination groups, we injected the first and the second compounds 30 and 15 min before the tail-flick test, respectively. I.p. administration of H-89 and PTX significantly decreased the thermal-induced pain sensation in their low applied doses. Db-cAMP, however, decreased the pain sensation in a dose-dependent manner. The highest applied dose of H-89 (0.5 mg/100 g) attenuated the antinociceptive effect of Db-cAMP in doses of 50 and 100 nm/mouse. Surprisingly, Db-cAMP decreased the antinociceptive effect of the lowest dose of H-89 (0.05 mg/100 g). All applied doses of PTX reduced the effect of 0.05 mg/100 g H-89 on pain sensation; however, the highest dose of H-89 compromised the antinociceptive effect of 20 mg/100 g dose of PTX. Co-administration of Db-cAMP and PTX increased the antinociceptive effect of each compound on thermal-induced pain. In conclusion, PTX, H-89, and Db-cAMP affect the thermal-induced pain by probably interacting with intracellular cAMP and cGMP signaling pathways and cyclic nucleotide-dependent protein kinases.

A proof-of-concept trial of protein kinase C iota inhibition with auranofin for the paclitaxel-induced acute pain syndrome.

PURPOSE: Paclitaxel causes the paclitaxel-induced acute pain (PIAP) syndrome. Based on preclinical data, we hypothesized that the protein kinase C (PKC) iota inhibitor, auranofin (a gold salt used for other pain conditions), palliates this pain. METHODS: In a randomized, double-blinded manner, patients who had suffered this syndrome were assigned a one-time dose of auranofin 6 mg orally on day #2 of the chemotherapy cycle (post-paclitaxel) versus placebo. Patients completed the Brief Pain Inventory and a pain diary on days 2 through 8 and at the end of the cycle. The primary endpoint was pain scores, as calculated by area under the curve, in response to "Please rate your pain by circling the one number that best describes your pain at its worse in the last 24 hours." RESULTS: Thirty patients were enrolled. For the primary endpoint, mean area under the curve of 55 units (standard deviation 19) and 61 units (standard deviation 22) were observed in auranofin-treated and placebo-exposed patients, respectively (p = 0.44). On day 8 and at the end of the cycle, pain scores in auranofin-treated patients were more favorable, although differences were not statistically significant. CONCLUSIONS: In the dose schedule studied, auranofin did not palliate the PIAP syndrome, but delayed beneficial trends suggest further study for this indication.

Aldehyde dehydrogenase-2 regulates nociception in rodent models of acute inflammatory pain.

Exogenous aldehydes can cause pain in animal models, suggesting that aldehyde dehydrogenase-2 (ALDH2), which metabolizes many aldehydes, may regulate nociception. To test this hypothesis, we generated a knock-in mouse with an inactivating point mutation in ALDH2 (ALDH2*2), which is also present in human ALDH2 of ~540 million East Asians. The ALDH2*1/*2 heterozygotic mice exhibited a larger response to painful stimuli than their wild-type littermates, and this heightened nociception was inhibited by an ALDH2-selective activator (Alda-1). No effect on inflammation per se was observed. Using a rat model, we then showed that nociception tightly correlated with ALDH activity (R(2) = 0.90) and that reduced nociception was associated with less early growth response protein 1 (EGR1) in the spinal cord and less reactive aldehyde accumulation at the insult site (including acetaldehyde and 4-hydroxynonenal). Further, acetaldehyde- and formalin-induced nociceptive behavior was greater in the ALDH2*1/*2 mice than in the wild-type mice. Finally, Alda-1 treatment was even beneficial when given after the inflammatory agent was administered. Our data in rodent models suggest that the mitochondrial enzyme ALDH2 regulates nociception and could serve as a molecular target for pain control, with ALDH2 activators, such as Alda-1, as potential non-narcotic, cardiac-safe analgesics. Furthermore, our results suggest a possible genetic basis for East Asians' apparent lower pain tolerance.

Differential distribution of PI3K isoforms in spinal cord and dorsal root ganglia: potential roles in acute inflammatory pain.

PI3-kinases (PI3Ks) participate in nociception within spinal cord, dorsal root ganglion (DRG), and peripheral nerves. To extend our knowledge, we immunohistochemically stained for each of the 4 class I PI3K isoforms along with several cell-specific markers within the lumbar spinal cord, DRG, and sciatic nerve of naive rats. Intrathecal and intraplantar isoform specific antagonists were given as pretreatments before intraplantar carrageenan; pain behavior was then assessed over time. The α-isoform was localized to central terminals of primary afferent fibers in spinal cord laminae IIi to IV as well as to neurons in ventral horn and DRG. The PI3Kß isoform was the only class I isoform seen in dorsal horn neurons; it was also observed in DRG, Schwann cells, and axonal paranodes. The δ-isoform was found in spinal cord white matter oligodendrocytes and radial astrocytes, and the γ-isoform was seen in a subpopulation of IB4-positive DRG neurons. No isoform co-localized with microglial markers or satellite cells in naive tissue. Only the PI3Kß antagonist, but none of the other antagonists, had anti-allodynic effects when administered intrathecally; coincident with reduced pain behavior, this agent completely blocked paw carrageenan-induced dorsal horn 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid (AMPA) receptor trafficking to plasma membranes. Intraplantar administration of the γ-antagonist prominently reduced pain behavior. These data suggest that each isoform displays specificity with regard to neuronal type as well as to specific tissues. Furthermore, each PI3K isoform has a unique role in development of nociception and tissue inflammation.

Spinal mitogen-activated protein kinase phosphatase-3 (MKP-3) is necessary for the normal resolution of mechanical allodynia in a mouse model of acute postoperative pain.

The mechanisms that drive the normal resolution of acute postoperative pain are not completely understood. We hypothesize a pivotal role of a major spinal mitogen-activated protein kinase (MAPKs) regulator, MAPK phosphatase (MKP)-3, in the resolution of postoperative pain. We used wild-type and MKP-3 knock-out (KO) mice, a paw incision model of acute postoperative pain, and behavioral and molecular biology experiments. We observed persistent mechanical allodynia in mice lacking MKP-3 (postoperative day 21), concurrently with persistent phosphorylation of spinal p38 and extracellular signal-regulated kinases (ERK)-1/2 on postoperative day 12, while both MAPK phosphorylation and allodynia resolved on postoperative day 7 in wild-type mice. Spinal p-ERK was expressed mainly in neurons and microglia, while spinal p-p38 was expressed mostly in microglia in MKP-3 KO mice, and their selective pharmacological inhibition reduced the persistent allodynia observed in these mice. Our findings strongly suggest that dysregulation of MKP-3 prevents spontaneous resolution of acute postoperative pain and drives its transition to persistent pain via persistent neuronal and microglial MAPK phosphorylation in the spinal cord.

Antinociceptive effect of Mirabilis jalapa on acute and chronic pain models in mice.

ETHNOPHARMACOLOGICAL RELEVANCE: The infusion or decoction of Mirabilis jalapa leaves is used in traditional medicine in Brazil to treat inflammatory and painful diseases. Thus, the present study was designed to investigate whether the leaf ethyl acetate (Eta) fraction from Mirabilis jalapa exhibits antinociceptive effect in clinically relevant pain models in mice. Furthermore, we have investigated the role of cholinergic system in the antinociceptive action produced by Eta in mice. MATERIALS AND METHODS: The effect of Eta administered orally (10mg/kg, p.o.) in mice was verified on the painful hypersensitivity (mechanical allodynia) in models of chronic inflammation (subcutaneous injection of complete Freund's Adjuvant-CFA in the plantar surface of the right hind paw), postoperative (paw surgical incision) and neuropathic (partial sciatic nerve ligation) pain. In the chronic inflammation model, we further verified the effect of Eta treatment on paw edema and interleukin-1ß (IL-1ß) levels. We also investigated the role of muscarinic and nicotinic receptors in the antiallodynic action produced by Eta as well as the possible action of Eta on in vitro and ex vivo acetylcholinesterase activity in CFA treated animals. Furthermore, we verified the effect of Eta treatment on the parameters of liver and kidney lesion (level of urea, and activity of aspartate aminotransferase and alanine aminotransferase). RESULTS: Eta produced marked reduction in the allodynia caused by CFA, surgical incision and partial sciatic nerve ligation. However, Eta did not alter the paw edema or the increase of IL-1ß levels produced by CFA. The antinociceptive effect of Eta was reversed by the pre-treatment of animals with the antagonists of muscarinic (atropine, 5mg/kg, s.c) or nicotinic (mecamylamine, 0.001mg/kg, s.c.) receptors. Eta did not alter in vitro acetylcholinesterase activity in blood or spinal cord samples, but it reversed the increase in the acetylcholinesterase activity observed in the spinal cord samples from mice injected with CFA. Moreover, Eta did not alter the indicators of liver or kidney lesion. CONCLUSIONS: Based on its use in traditional medicine, the results of the present study confirmed the antinociceptive properties of Eta in clinically relevant pain models. Also its effect on the CFA-induced chronic inflammation seems to be related to acetylcholinesterase inhibition and cholinergic system.

Effects of treatment with a carbon monoxide-releasing molecule and a heme oxygenase 1 inducer in the antinociceptive effects of morphine in different models of acute and chronic pain in mice.

RATIONALE: Treatment with a carbon monoxide-releasing molecule (tricarbonyldichlororuthenium(II) dimer, CORM-2) or a classical heme oxygenase 1 inducer (cobalt protoporphyrin IX, CoPP) has potent anti-inflammatory effects, but the role played by these treatments in the antinociceptive effects of morphine during acute and chronic pain was not evaluated. OBJECTIVES: In wild type (WT), neuronal (NOS1-KO), or inducible (NOS2-KO) nitric oxide synthases knockout mice, we evaluated the effects of CORM-2 and CoPP treatments in the antinociceptive actions of morphine and their interaction with nitric oxide during acute, visceral, and chronic inflammatory or neuropathic pain. METHODS: Acute and visceral pain was assessed through formalin and acid acetic writhing tests. Chronic inflammatory pain induced by the intra-articular administration of complete Freund's adjuvant and neuropathic pain by partial ligation of sciatic nerve were evaluated by measuring allodynia and hyperalgesia using the von Frey filaments, plantar, or cold plate tests. RESULTS: While nitric oxide, synthetized by NOS1 and/or NOS2, increased the local antinociceptive effects of morphine during acute and chronic pain, it decreased the inhibitory effects of morphine after visceral pain. Moreover, while CORM-2 or CoPP treatments did not alter or reduced the antinociceptive effects of morphine during acute and visceral pain, both treatments improved the local antiallodynic and antihyperalgesic effects of morphine after chronic inflammatory or neuropathic pain in WT, but not in KO mice. CONCLUSIONS: CORM-2 and CoPP treatments improved the local antinociceptive effects of morphine during chronic inflammatory and neuropathic pain by interaction with nitric oxide synthetized by NOS1 and NOS2 isoforms.

Novel animal models of acute and chronic cancer pain: a pivotal role for PAR2.

Targeted therapy to prevent the progression from acute to chronic pain in cancer patients remains elusive. We developed three novel cancer models in mice that together recapitulate the anatomical, temporal, and functional characteristics of acute and chronic head and neck cancer pain in humans. Using pharmacologic and genetic approaches in these novel cancer models, we identified the interaction between protease-activated receptor 2 (PAR2) and serine proteases to be of central importance. We show that serine proteases such as trypsin induce acute cancer pain in a PAR2-dependent manner. Chronic cancer pain is associated with elevated serine proteases in the cancer microenvironment and PAR2 upregulation in peripheral nerves. Serine protease inhibition greatly reduces the severity of persistent cancer pain in wild-type mice, but most strikingly, the development of chronic cancer pain is prevented in PAR2-deficient mice. Our results demonstrate a direct role for PAR2 in acute cancer pain and suggest that PAR2 upregulation may favor the development and maintenance of chronic cancer pain. Targeting the PAR2-serine protease interaction is a promising approach to the treatment of acute cancer pain and prevention of chronic cancer pain.

Resveratrol engages AMPK to attenuate ERK and mTOR signaling in sensory neurons and inhibits incision-induced acute and chronic pain.

BACKGROUND: Despite advances in our understanding of basic mechanisms driving post-surgical pain, treating incision-induced pain remains a major clinical challenge. Moreover, surgery has been implicated as a major cause of chronic pain conditions. Hence, more efficacious treatments are needed to inhibit incision-induced pain and prevent the transition to chronic pain following surgery. We reasoned that activators of AMP-activated protein kinase (AMPK) may represent a novel treatment avenue for the local treatment of incision-induced pain because AMPK activators inhibit ERK and mTOR signaling, two important pathways involved in the sensitization of peripheral nociceptors. RESULTS: To test this hypothesis we used a potent and efficacious activator of AMPK, resveratrol. Our results demonstrate that resveratrol profoundly inhibits ERK and mTOR signaling in sensory neurons in a time- and concentration-dependent fashion and that these effects are mediated by AMPK activation and independent of sirtuin activity. Interleukin-6 (IL-6) is thought to play an important role in incision-induced pain and resveratrol potently inhibited IL-6-mediated signaling to ERK in sensory neurons and blocked IL-6-mediated allodynia in vivo through a local mechanism of action. Using a model of incision-induced allodynia in mice, we further demonstrate that local injection of resveratrol around the surgical wound strongly attenuates incision-induced allodynia. Intraplantar IL-6 injection and plantar incision induces persistent nociceptive sensitization to PGE2 injection into the affected paw after the resolution of allodynia to the initial stimulus. We further show that resveratrol treatment at the time of IL-6 injection or plantar incision completely blocks the development of persistent nociceptive sensitization consistent with the blockade of a transition to a chronic pain state by resveratrol treatment. CONCLUSIONS: These results highlight the importance of signaling to translation control in peripheral sensitization of nociceptors and provide further evidence for activation of AMPK as a novel treatment avenue for acute and chronic pain states.

Novel p38α mitogen-activated protein kinase inhibitor shows analgesic efficacy in acute postsurgical dental pain.

SCIO-469 is a selective p38α mitogen-activated protein kinase (MAPK) inhibitor for preclinical models of acute pain. This prospective, double-blind, randomized clinical study compared efficacy and safety of oral SCIO-469, ibuprofen, and placebo in postsurgical dental pain. Subjects (n = 263) undergoing extraction of 1 or more impacted mandibular third molars received preoperative treatment with SCIO-469 (150, 210, or 300 mg), ibuprofen (400 mg), or placebo; the 210-mg group received 90 mg postoperatively. A 4-point categorical scale and a 100-mm visual analogue scale were used to measure pain intensity. The primary end point was median time from first incision to first rescue medication using the Kaplan-Meier product limit estimator. All SCIO-469 groups had significantly longer times to rescue medication compared with placebo; preoperative and postoperative treatment with 210 + 90 mg SCIO-469 resulted in 8.1 hours versus 4.1 hours to rescue for placebo (P = .003). Ibuprofen also increased time to rescue medication (6.6 hours) versus placebo (P = .04). Dizziness, headache, and nausea were the most frequently reported adverse events. This is the first clinical demonstration of antinociceptive effects in acute pain with preoperative administration of a p38α MAPK inhibitor.

Microglial GRK2: a novel regulator of transition from acute to chronic pain.

Pain is a hallmark of tissue damage and inflammation promoting tissue protection and thereby contributing to repair. Therefore, transient acute pain is an important feature of the adaptive response to damage. However, in a significant number of cases, pain persists for months to years after the problem that originally caused the pain has resolved. Such chronic pain is maladaptive as it no longer serves a protective aim. Chronic pain is debilitating, both physiologically and psychologically, and treatments to provide relief from chronic pain are often ineffective. The neurobiological mechanisms underlying the transition from adaptive acute pain to maladaptive chronic pain are only partially understood. In this review, we will summarize recent evidence that a kinase known as G protein-coupled receptor kinase (GRK2) is a key regulator of the transition from acute to chronic inflammatory pain. Our recent studies have shown that mice with a reduction in the cellular level of GRK2 develop chronic hyperalgesia in response to inflammatory mediators that induce only transient hyperalgesia in WT mice. This finding is clinically relevant because rodent models of chronic pain are associated with reduced cellular levels of GRK2. We propose that GRK2 is a newly discovered major player in the regulation of chronic pain. The pathways regulated by this kinase may open up new avenues for development of treatment strategies that target the cause, and not the symptoms of chronic pain.