Mexican Plants and Derivates Compounds as Alternative for Inflammatory and Neuropathic Pain Treatment-A Review.

Despite the availability of many anti-pain drugs, in the form of NSAIDs, steroids, gabapentinoids, opioids, and antidepressants, in this study we address the natural compounds belonging to the group of Mexican medicinal plants or "Mexican folk medicine", used for pain management in Mexico. Our interest in this subject is due to the growing idea that "natural is harmless" and to the large number of side effects exhibited in pharmacotherapy. The objective of this review was to document the scientific evidence about Mexican medicinal plants and their derivatives used for inflammatory and neuropathic pain treatment, as well as the mechanisms of action implicated in their antinociceptive effects, their possible adverse effects, and the main pharmacological aspects of each plant or compound. Our data review suggested that most studies on Mexican medicinal plants have used inflammatory experimental models for testing. The anti-pain properties exerted by medicinal plants lack adverse effects, and their toxicological assays report that they are safe to consume; therefore, more studies should be performed on preclinical neuropathic pain models. Moreover, there is no convincing evidence about the possible mechanisms of action involved in the anti-pain properties exerted by Mexican plants. Therefore, the isolation and pharmacological characterization of these plant derivatives' compounds will be important in the design of future preclinical studies.

Spinal 5-HT5A receptors mediate 5-HT-induced antinociception in several pain models in rats.

The antinociceptive role of spinal 5-HT5A receptors in rat models of pain along with their expression was evaluated in the spinal cord and dorsal root ganglion (DRG). Nociception was assessed in the formalin, capsaicin, and acetic acid writhing tests. The expression of 5-HT5A receptors was determined by Western blot analysis. Intrathecal treatment with serotonin (5-HT, 10-100 nmol) or 5-carboxamidotryptamine (5-CT, 0.03-0.3 nmol) dose-dependently prevented 1% formalin-induced nociception. Furthermore, 5-HT reduced capsaicin- and acetic acid-induced nociception. 5-HT- or 5-CT-induced antinociception in the formalin test was diminished by the selective 5-HT5A receptor antagonist N-[2-(dimethylamino)ethyl]-N-[[4'-[[(2-phenylethyl)amino] methyl][1,1'-biphenyl]-4-yl]methyl]cyclopentanepropanamide dihydrochloride (SB-699551; 3 and 10 nmol). In addition, 5-HT-induced spinal antinociception in the capsaicin and acetic acid tests was blocked by SB-699551 (10 nmol). Given alone, intrathecal injection of SB-699551 did not affect nociception induced by any irritant. 5-HT5A receptors were expressed in the dorsal spinal cord and DRG, even though formalin injection increased after 24h 5-HT5A receptor expression only in the spinal cord. Data suggest that 5-HT and 5-CT produce antinociception by activation of spinal 5-HT5A receptors in both the spinal cord and DRG. Furthermore, our results suggest that spinal 5-HT5A receptors play an antinociceptive role in several pain models in rats. 5-HT5A receptors may provide a therapeutic target to develop analgesic drugs.

Synergistic antiallodynic interaction between gabapentin or carbamazepine and either benfotiamine or cyanocobalamin in neuropathic rats.

Anticonvulsants, including gabapentin and carbamazepine, have shown activity against several types of neuropathic pain; however, they have limiting side effects that may minimize their use. In this study the possible synergistic interaction between anticonvulsants and benfotiamine or cyanocobalamin on spinal nerve ligation-induced tactile allodynia was assessed. Oral administration of gabapentin (15-300 mg/kg), carbamazepine (10-300 mg/kg), benfotiamine (30-600 mg/kg) or cyanocobalamin (0.3-6.0 mg/kg) significantly reduced tactile allodynia in rats. Maximal antiallodynic effects were reached with gabapentin 300 mg/kg (approximately 70%), carbamazepine 300 mg/kg (approximately 66%), benfotiamine 600 mg/kg (approximately 51%) and cyanocobalamin 6 mg/kg (approximately 59%). At the highest tested doses, gabapentin, but not carbamazepine, benfotiamine or cyanocobalamin, significantly reduced motor coordination. Coadministration of gabapentin or carbamazepine with benfotiamine or cyanocobalamin in a fixed ratio markedly reduced spinal nerve ligation-induced tactile allodynia, showing a synergistic interaction between anticonvulsants and B vitamins. Data indicate that combinations of anticonvulsants with benfotiamine or cyanocobalamin are able to reduce tactile allodynia without affecting motor coordination in rats, and suggest the possible clinical use of these combinations in the treatment of neuropathic pain in humans.