Interaction of NHE1 and TRPA1 Activity in DRG Neurons Isolated from Adult Rats and its Role in Inflammatory Nociception.

Transient receptor potential ankyrin 1 (TRPA1) channel is expressed in a subset of nociceptive neurons. This channel integrates several nociceptive signals. Particularly, it is modulated by intracellular pH (pHi). Na+/H+ exchanger 1 (NHE1) contributes to the maintenance of pHi in nociceptors. However, it is currently unknown whether the interaction between TRPA1 and NHE1 contributes to the nociceptive processing. Thus, the purpose of this study was to assess the functional interaction between NHE1 and TRPA1 in small dorsal root ganglion (DRG) neurons from primary culture obtained from adult rats. Moreover, we also evaluated their possible interaction in acute and inflammatory pain. Zoniporide (selective NHE1 inhibitor) reduced pHi and increased intracellular calcium in a concentration-dependent fashion in DRG neurons. Zoniporide and allyl isothiocyanate (AITC, TRPA1 agonist) increased calcium transients in the same DRG neuron, whereas that A-967079 (TRPA1 antagonist) prevented the effect of zoniporide in DRG neurons. Repeated AITC induced TRPA1 desensitization and this effect was prevented by zoniporide. Both NHE1 and TRPA1 were localized at the membrane surface of DRG neurons in culture. Local peripheral zoniporide enhanced AITC-induced pronociception and this effect was prevented by A-967079. Likewise, zoniporide potentiated Complete Freund's Adjuvant (CFA)-induced hypersensitivity, effect which was prevented by A-967079 in vivo. CFA paw injection increased TRPA1 and decresed NHE1 protein expression in DRG. These results suggest a functional interaction between NHE1 and TRPA1 in DRG neurons in vitro. Moreover, data suggest that this interaction participates in acute and inflamatory pain conditions in vivo.

Pharmacological Analysis of the Anti-inflammatory and Antiallodynic Effects of Zinagrandinolide E from Zinnia grandiflora in Mice.

Zinagrandinolide E (1, ZGE) is an elemanolide with antinociceptive action isolated from Zinnia grandiflora (Asteraceae), valued in North México and southwestern United States for pain relief. Herein, we report the anti-inflammatory and antiallodynic action of ZGE (1) in carrageenan-induced inflammation and tactile allodynia in mice and in a neuropathic pain model in hyperglycemic mice. Local peripheral administration of ZGE (1-30 µg/paw) induced dose-dependent acute anti-inflammatory and antiallodynic effects. The anti-inflammatory effect was comparable to diclofenac (30 µg/paw). Intrathecal (i.t.) administration of ZGE (30 µg) in acute experiments did not affect carrageenan-induced inflammation but significantly reduced tactile allodynia in a dose-dependent fashion. In long-term experiments (15 or 6 days), using two different scheme treatments (pretreatment or post-treatment), ZGE (3-30 µg/paw) showed antiallodynic but not anti-inflammatory action. Local peripheral (3-30 µg/paw) or intrathecal (3-30 µg) administration of ZGE partially reversed tactile allodynia in hyperglycemic mice, better or comparable, respectively, with those of pregabalin (30 µg/paw or 30 µg i.t.). The effects were dose-dependent. According to the pharmacological tools employed, the anti-inflammatory and antiallodynic activities of ZGE are multitarget; these involve the opioidergic, serotoninergic, and GABAergic systems, as well as the NO-cGMP-ATP-sensitive K+ channel signaling pathway.

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55.

Background: Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment. Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB1 and CB2) have been deeply studied and classified. Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB1/CB2 receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors. Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors. According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration. Methods: This article reviews select relevant information about the potential role of GPR18 and GPR55 in the pathophysiology of pain. Results: This work summarized novel data supporting that, besides cannabinoid CB1 and CB2 receptors, GPR18 and GPR55 may be useful for pain treatment. Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.

Selective melatonin MT2 receptor ligands relieve neuropathic pain through modulation of brainstem descending antinociceptive pathways.

Neuropathic pain is an important public health problem for which only a few treatments are available. Preclinical studies show that melatonin (MLT), a neurohormone acting on MT1 and MT2 receptors, has analgesic properties, likely through MT2 receptors. Here, we determined the effects of the novel selective MLT MT2 receptor partial agonist N-{2-([3-bromophenyl]-4-fluorophenylamino)ethyl}acetamide (UCM924) in 2 neuropathic pain models in rats and examined its supraspinal mechanism of action. In rat L5-L6 spinal nerve ligation and spared nerve injury models, UCM924 (20-40 mg/kg, subcutaneously) produced a prolonged antinociceptive effect that is : (1) dose-dependent and blocked by the selective MT2 receptor antagonist 4-phenyl-2-propionamidotetralin, (2) superior to a high dose of MLT (150 mg/kg) and comparable with gabapentin (100 mg/kg), but (3) without noticeable motor coordination impairments in the rotarod test. Using double staining immunohistochemistry, we found that MT2 receptors are expressed by glutamatergic neurons in the rostral ventrolateral periaqueductal gray. Using in vivo electrophysiology combined with tail flick, we observed that microinjection of UCM924 into the ventrolateral periaqueductal gray decreased tail flick responses, depressed the firing activity of ON cells, and activated the firing of OFF cells; all effects were MT2 receptor-dependent. Altogether, these data demonstrate that selective MT2 receptor partial agonists have analgesic properties through modulation of brainstem descending antinociceptive pathways, and MT2 receptors may represent a novel target in the treatment of neuropathic pain.

Antineuropathic effect of 7-hydroxy-3,4-dihydrocadalin in streptozotocin-induced diabetic rodents.

BACKGROUND: Painful neuropathy is the most common and debilitating complication of diabetes and results in hyperalgesia and allodynia. Hyperglycemia clearly plays a key role in the development and progression of diabetic neuropathy. Current therapeutic approaches are only partially successful and they are only thought to reduce the pain associated with peripheral neuropathy. Some natural products offer combined antioxidant, anti-inflammatory and antinociceptive properties that may help to treat in a more integrative manner this condition. In this regard, the purpose of this study was to investigate the antineuropathic effect of 7-hydroxy-3,4-dihydrocadalin in streptozotocin-induced diabetic rats and mice without glucose control as well as the possible mechanism of action involved in this effect. METHODS: Rats and mice were injected with 50 or 200 mg/kg streptozotocin, respectively, to produce hyperglycemia. The formalin test and von Frey filaments were used to assess the nociceptive activity. Rota-rod was utilized to measure motor activity and malondialdehyde assay to determine anti-oxidative properties. RESULTS: After 3 weeks of diabetes induction, chemical hyperalgesia was observed in streptozotocin-injected rats. Oral acute administration of 7-hydroxy-3,4-dihydrocadalin (0.3-30 mg/kg) decreased in a dose-dependent manner formalin-evoked hyperalgesia in diabetic rats. In addition, methiothepin (non-selective 5-HT receptor antagonist, 1 mg/kg, i.p.) and ODQ (guanylyl cyclase inhibitor, 2 mg/kg, i.p.), but not naltrexone (opioid receptor antagonist, 1 mg/kg, s.c.), prevented 7-hydroxy-3,4-dihydrocadalin-induced antihyperalgesic effect. The anti-hyperalgesic effect of 7-hydroxy-3,4-dihydrocadalin was similar to that produced by pregabalin (10 mg/kg, p.o.). Furthermore, oral acute administration of 7-hydroxy-3,4-dihydrocadalin (30 mg/kg) reduced streptozotocin-induced changes in malondialdehyde concentration from plasma samples. Unlike pregabalin, 7-hydroxy-3,4-dihydrocadalin did not affect motor activity. Six weeks after diabetes induction, tactile allodynia was observed in the streptozotocin-injected rats. At this time, oral administration of 7-hydroxy-3,4-dihydrocadalin (30 mg/kg) or pregabalin (10 mg/kg) reduced in a similar way tactile allodynia in diabetic rats. Finally, chronic oral administration of 7-hydroxy-3,4-dihydrocadalin (30-300 mg/kg, 3 times/week, during 6 weeks), significantly prevented the development of mechanical hyperalgesia and allodynia in streptozotocin-induced diabetic mice. CONCLUSIONS: Data suggests that 7-hydroxy-3,4-dihydrocadalin has acute and chronic effects in painful diabetic neuropathy. This effect seems to involve antioxidant properties as well as activation of 5-HT receptors and inhibition of guanylyl cyclase enzyme.

Antinociceptive effect of 7-hydroxy-3,4-dihydrocadalin isolated from Heterotheca inuloides: role of peripheral 5-HT1 serotonergic receptors.

The purpose of this study was to investigate the possible antinociceptive effect of Heterotheca inuloides in inflammatory pain and to identify the main compounds involved in this effect. Dose-response curves were obtained for hexane, dichlorometane, ethyl acetate and methanol extracts from Heterotheca inuloides inflorescences in the formalin test. Hexane extract was more potent and effective than other extracts. Bio-guided fractionation was performed to determine the main antinociceptive compounds of the plant. Gas chromatography-mass spectrometry technique demonstrated the composition of the most active fraction from hexane extract revealing the presence of caryophyllene oxide, cedrene, 7-hydroxy-3,4-dihydrocadalin, 7-hydroxycadalene and a compound not identified. The isolated compounds were individually evaluated in the formalin test in a preliminary dose of 100 µg/paw and only 7-hydroxy-3,4-dihydrocadalin showed a significant antinociceptive effect. Dose-response curves were then obtained for 7-hydroxy-3,4-dihydrocadalin and diclofenac, a prototypical analgesic drug. Both drugs were equieffective and equipotent in the second phase of the formalin test, but 7-hydroxy-3,4-dihydrocadalin was more effective and potent in the first phase than diclofenac. In addition, 7-hydroxy-3,4-dihydrocadalin reduced carrageenan-induced mechanical hyperalgesia and inflammation in a dose-dependent manner. Finally, in mechanistic studies, the antinociceptive effect of 7-hydroxy-3,4-dihydrocadalin in the formalin test was prevented by methiothepin, WAY100635, SB224289 and BRL15572 but not by naltrexone. Results support the use of H. inuloides inflorescences as analgesic in the Mexican traditional medicine. Moreover, data indicate that 7-hydroxy-3,4-dihydrocadalin is partly responsible of this pharmacological activity, and suggest that 5-HT(1A), 5-HT(1B), and 5-HT(1D) serotonergic, but not opioid, receptors participate in the antinociceptive effect of this drug.

Benfotiamine relieves inflammatory and neuropathic pain in rats.

Benfotiamine has shown therapeutic efficacy in the treatment of painful diabetic neuropathy in human beings. However, so far there is no evidence about the efficacy of this drug in preclinical models of pain. The purpose of this study was to assess the possible antinociceptive and antiallodynic effect of benfotiamine in inflammatory and neuropathic pain models in the rat. Inflammatory pain was induced by injection of formalin in non-diabetic and diabetic (2 weeks) rats. Reduction of flinching behavior was considered as antinociception. Neuropathic pain was induced by either ligation of left L5/L6 spinal nerves or administration of streptozotocin (50 mg/kg, i.p.) in Wistar rats. Benfotiamine significantly reduced inflammatory (10-300 mg/kg) and neuropathic (75-300 mg/kg) nociception in non-diabetic and diabetic rats. Results indicate that oral administration of benfotiamine is able to reduce tactile allodynia from different origin in the rat and they suggest the use of this drug to reduce inflammatory and neuropathic pain in humans.