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.

Antinociceptive effect of (-)-epicatechin in inflammatory and neuropathic pain in rats

The aim of this study was to investigate the antinociceptive potential of (-)-epicatechin and the possible mechanisms of action involved in its antinociceptive effect. The carrageenan and formalin tests were used as inflammatory pain models. A plethysmometer was used to measure inflammation and L5/L6 spinal nerve ligation as a neuropathic pain model. Oral (-)-epicatechin reduced carrageenan-induced inflammation and nociception by about 59 and 73%, respectively, and reduced formalin- induced and nerve injury-induced nociception by about 86 and 43%, respectively. (-)-Epicatechin-induced antinociception in the formalin test was prevented by the intraperitoneal administration of antagonists: methiothepin (5-HT1/5 receptor), WAY-100635 (5-HT1A receptor), SB-224289 (5-HT1B receptor), BRL-15572 (5-HT1D receptor), SB-699551 (5-HT5A receptor), naloxone (opioid receptor), CTAP (μ opioid receptor), nor-binaltorphimine (κ opioid receptor), and 7-benzylidenenaltrexone (δ1 opioid receptor). The effect of (-)-epicatechin was also prevented by the intraperitoneal administration of L-NAME [nitric oxide (NO) synthase inhibitor], 7-nitroindazole (neuronal NO synthase inhibitor), ODQ (guanylyl cyclase inhibitor), glibenclamide (ATP-sensitive K channel blocker), 4-aminopyridine (voltage-dependent K channel blocker), and iberiotoxin (large-conductance Ca-activated K channel blocker), but not by amiloride (acid sensing ion channel blocker). The data suggest that (-)-epicatechin exerts its antinociceptive effects by activation of the NO-cyclic GMP-K channels pathway, 5-HT1A/1B/1D/5A serotonergic receptors, and μ/κ/δ opioid receptors.

Evidence for the participation of peripheral 5-HT₂A, 5-HT₂B, and 5-HT₂C receptors in formalin-induced secondary mechanical allodynia and hyperalgesia

The role of 5-HT₂A/₂B/₂C receptors in formalin-induced secondary allodynia and hyperalgesia in rats was assessed. Formalin produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term secondary mechanical allodynia and hyperalgesia. Pre-treatment for five consecutive days with compound 48/80 (1, 3, 10, 10, and 10 μg/paw) prevented formalin-induced secondary allodynia and hyperalgesia. Ipsilateral, but not contralateral, peripheral pre-treatment (nmol/paw) with the 5-HT₂ receptor agonist DOI (3-30), 5-HT (10-100) or fluoxetine (0.3-3) significantly increased 0.5% formalin-induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of DOI (10 nmol/paw), 5-HT (100 nmol/paw) and fluoxetine (1 nmol/paw) was blocked by selective 5-HT₂A (ketanserin), 5-HT₂B (RS-127445), and 5-HT₂C (RS-102221) receptor antagonists. Furthermore, ipsilateral pre-treatment (nmol/paw) with ketanserin (1, 10, and 100), RS-127445 (0.01, 0.1 and 1) or RS-102221 (1, 10 and 100) prevented while post-treatment reversed 1% formalin-induced secondary allodynia and hyperalgesia in both paws. In marked contrast, contralateral injection of the greatest tested dose of 5-HT₂A/₂B/₂C receptor antagonists did not modify long-lasting secondary allodynia and hyperalgesia. These results suggest that 5-HT released from mast cells after formalin injection sensitizes primary afferent neurons via 5-HT₂A/₂B/₂C receptors leading to the development and maintenance of secondary allodynia and hyperalgesia.