Role of hydrogen sulfide in the pain processing of non-diabetic and diabetic rats.

Hydrogen sulfide (H2S) is a gasotransmitter endogenously generated from the metabolism of L-cysteine by action of two main enzymes called cystathionine ß-synthase (CBS) and cystathionine γ-lyase (CSE). This gas has been involved in the pain processing and insulin resistance produced during diabetes development. However, there is no evidence about its participation in the peripheral neuropathy induced by this metabolic disorder. Experimental diabetes was induced by streptozotocin (50mg/kg, i.p.) in female Wistar rats. Streptozotocin injection increased formalin-evoked flinching in diabetic rats as compared to non-diabetic rats after 2 weeks. Peripheral administration of NaHS (an exogenous donor of H2S) and L-cysteine (an endogenous donor of H2S) dose-dependently increased flinching behavior in diabetic and non-diabetic rats. Contrariwise, hydroxylamine (HA, a CBS inhibitor) and DL-propargylglycine (PPG, a CSE inhibitor) decreased formalin-induced nociceptive behavior in both experimental groups. In addition, an ineffective dose of HA and PPG partially prevented the L-cysteine-induced hyperalgesia in diabetic and non-diabetic rats. Interestingly, HA and PPG were three order of magnitude more potent in diabetic rats respect to non-diabetic rats, whereas NaHS was ten times more potent in the streptozotocin-diabetic group. Nine to 11 weeks after diabetes induction, tactile allodynia was observed in the streptozotocin-injected rats. On this condition, subcutaneous administration of PPG or HA reduced tactile allodynia in diabetic rats. Paradoxically, H2S levels were decreased in nerve sciatic, dorsal root ganglion and spinal cord, but not paw nor blood plasma, during diabetes-associated peripheral neuropathy development. Collectively, results suggest that H2S synthesized by CBS and CSE participate in formalin-induced nociception in diabetic and non-diabetic rats, as well as; in tactile allodynia in streptozotocin-injected rats. In addition, data seems to indicate that diabetic rats are more sensible to H2S-induced hyperalgesia than normoglycemic rats.

Role of peripheral and spinal 5-HT6 receptors according to the rat formalin test.

The present study assessed the possible pronociceptive role of peripheral and spinal 5-HT(6) receptors in the formalin test. For this, local peripheral administration of selective 5-HT(6) receptor antagonists N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)-benzenesulphonamide (SB-399885) (0.01-1 nmol/paw) and 4-iodo-N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]benzene-sulfonamide hydrochloride (SB-258585) (0.001-0.1 nmol/paw) significantly reduced formalin-induced flinching. Local peripheral serotonin (5-HT) (10-100 nmol/paw) or 5-chloro-2-methyl-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole hydrochloride (EMD-386088) (0.01-0.1 nmol/paw; a selective 5-HT(6) receptor agonist) augmented 0.5% formalin-induced nociceptive behavior. The local pronociceptive effect of 5-HT (100 nmol/paw) or EMD-386088 (0.1 nmol/paw) was significantly reduced by SB-399885 or SB-258585 (0.1 nmol/paw). In contrast to peripheral administration, intrathecal injection of 5-HT(6) receptor antagonists SB-399885 and SB-258585 (0.1-10 nmol/rat) did not modify 1% formalin-induced nociceptive behavior. Spinal 5-HT (50-200 nmol/rat) significantly reduced formalin-induced flinching behavior during phases 1 and 2. Contrariwise, intrathecal EMD-386088 (0.1-10 nmol/rat) dose-dependently increased flinching during phase 2. The spinal pronociceptive effect of EMD-386088 (1 nmol/rat) was reduced by SB-399885 (1 nmol/rat) and SB-258585 (0.1 nmol/rat). Our results suggest that 5-HT(6) receptors play a pronociceptive role in peripheral as well as spinal sites in the rat formalin test. Thus, 5-HT(6) receptors could be a target to develop analgesic drugs.

Identification of the Na+/H+ exchanger 1 in dorsal root ganglion and spinal cord: its possible role in inflammatory nociception.

mRNA and protein presence of Na+/H+ exchanger (NHE) 1 (NHE1) and 5 (NHE5) in dorsal root ganglion (DRG) and dorsal spinal cord as well as its possible role in three inflammatory nociception tests were determined. Local peripheral ipsilateral, but not contralateral, administration of NHE inhibitors 5-(N,N-dimethyl)amiloride hydrochloride (DMA, 0.3-30 microM/paw), 5-(N-ethyl-N-isopropyl)amiloride (EIPA, 0.3-30 microM/paw) and amiloride (0.1-10 microM/paw) significantly increased flinching but not licking behavior in the capsaicin and 5-HT tests. Moreover, DMA and EIPA (0.03-30 microM/paw) as well as amiloride (0.1-1 microM/paw) augmented, in a dose-dependent manner, 0.5% formalin-induced flinching behavior during phase II but not during phase I. Reverse transcription-polymerase chain reaction showed the expression of NHE1 and NHE5 in DRG and dorsal spinal cord. Western blot analysis confirmed the presence of NHE1 in DRG and spinal cord. Moreover, NHE5 was expressed in dorsal spinal cord, but not in DRG where a 45 kDa truncated isoform of NHE5 was identified. Collectively, these data suggest that NHE1, but not NHE5, plays an important role reducing inflammatory pain in rats.