Peripheral diabetic neuropathy can be painful and its symptoms include hyperalgesia, allodynia and spontaneous pain. Hydrogen sulfide (H2S) is involved in diabetes-induced hyperalgesia and allodynia. However, the molecular target through which H2S induces hyperalgesia in diabetic animals is unclear. The aim of this study was to determine the possible involvement of transient receptor potential (TRP) channels in H2S-induced hyperalgesia in diabetic rats. Streptozotocin (STZ) injection produced hyperglycemia in rats. Intraplantar injection of NaHS (an exogenous donor of H2S, 3–100 µg/paw) induced hyperalgesia, in a time-dependent manner, in formalin-treated diabetic rats. NaHS-induced hyperalgesia was partially prevented by local intraplantar injection of capsazepine (0.3–3 µg/paw), HC-030031 (100–316 µg/paw) and SKF-96365 (10–30 µg/paw) blockers, at 21 days post-STZ injection. At the doses used, these blockers did not modify formalin-induced nociception. Moreover, capsazepine (0.3–30 µg/paw), HC-030031 (100–1000 µg/paw) and SKF-96365 (10–100 µg/paw) reduced formalin-induced nociception in diabetic rats. Contralateral injection of the highest doses used did not modify formalin-induced flinching behavior. Hyperglycemia, at 21 days, also increased protein expression of cystathionine-β-synthase enzyme (CBS) and TRPC6, but not TRPA1 nor TRPV1, channels in dorsal root ganglia (DRG). Repeated injection of NaHS enhanced CBS and TRPC6 expression, but hydroxylamine (HA) prevented the STZ-induced increase of CBS protein. In addition, daily administration of SKF-96365 diminished TRPC6 protein expression, whereas NaHS partially prevented the decrease of SKF-96365-induced TRPC6 expression. Concordantly, daily intraplantar injection of NaHS enhanced, and HA prevented STZ-induced intraepidermal fiber loss, respectively. CBS was expressed in small- and medium-sized cells of DRG and co-localized with TRPV1, TRPA1 and TRPC6 in IB4-positive neurons. Our data suggest that H2S leads to hyperalgesia in diabetic rats through activation of TRPV1, TRPA1 and TRPC channels and, subsequent intraepidermal fibers loss. CBS enzyme inhibitors or TRP-channel blockers could be useful for treatment of painful diabetic neuropathy.

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-HT_1/5 receptor), WAY-100635 (5-HT_1A receptor), SB-224289 (5-HT_1B receptor), BRL-15572 (5-HT_1D receptor), SB-699551 (5-HT_5A 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 Ca²⁺-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-HT_1A/1B/1D/5A serotonergic receptors, and μ/κ/δ opioid receptors.