Héctor I. Rocha-González

Resveratrol: a natural compound with pharmacological potential in neurodegenerative diseases.

Resveratrol is a phytoalexin structurally related to stilbenes, which is synthesized in considerable amounts in the skin of grapes, raspberries, mulberries, pistachios and peanuts, and by at least 72 medicinal and edible plant species in response to stress conditions. It was isolated in 1940 and did not maintain much interest for around five decades until its role in treatment of cardiovascular diseases was suggested. To date, resveratrol has been identified as an agent that may be useful to treat cancer, pain, inflammation, tissue injury, and other diseases. However, currently the attention is being focused in analyzing its properties against neurodegenerative diseases and as antiaging compound. It has been reported that resveratrol shows effects in in vitro models of epilepsy, Alzheimers disease, Parkinsons disease, Huntingtons disease, amyotrophic lateral sclerosis, and nerve injury. However, evidences in vivo as well as in human beings are still lacking. Thus, further investigations on the pharmacological effects of resveratrol in vivo are necessary before any conclusions on its effects on neurodegenerative diseases can be obtained.

The role of peripheral 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F serotonergic receptors in the reduction of nociception in rats

This study assessed the possible antinociceptive role of peripheral 5-HT(1) receptor subtypes in the rat formalin test. Rats were injected into the dorsum of the hind paw with 50 microl of diluted formalin (1%). Nociceptive behavior was quantified as the number of flinches of the injected paw. Reduction of flinching was considered as antinociception. Ipsilateral, but not contralateral, peripheral administration of the 5-HT(1) receptor agonists R(+)-UH-301 (5-HT(1A); 0.1-3 microg/paw), CGS-12066A (5-HT(1B); 0.01-0.3 microg/paw), GR46611 (5-HT(1B/1D); 0.3-10 microg/paw), BRL54443 (5-HT(1E/1F); 3-300 microg/paw) or LY344864 (5-HT(1F); 3-300 microg/paw) significantly reduced formalin-induced flinching. The corresponding vehicle was devoid of any effect by itself. The local antinociceptive effect of R(+)-UH-301 (0.3 microg/paw) was significantly reduced by WAY-100635 (30-100 microg/paw; a 5-HT(1A) receptor antagonist). Moreover, the antagonists GR55562 (30-100 microg/paw; 5-HT(1B/D)) or SB224289 (30-100 microg/paw; 5-HT(1B)) dose-dependently reduced the antinociceptive effect of CGS-12066A (0.3 microg/paw) whereas GR55562 (30-100 microg/paw) or BRL15572 (30-100 microg/paw, 5-HT(1D)) reduced the antinociceptive effect of GR46611 (0.3 microg/paw). Interestingly, the effects of BRL54443 and LY344864 (300 microg/paw each) were partially reduced by methiothepin, but not by the highest doses of WAY-100635, SB224289 or BRL15572. The above antagonists did not produce any effect by themselves. These results suggest that peripheral activation of the 5-HT(1A,) 5-HT(1B), 5-HT(1D), 5-HT(1F) and, probably, 5-HT(1E) receptor subtypes leads to antinociception in the rat formalin test. Thus, the use of selective 5-HT(1) receptor agonists could be a therapeutic strategy to reduce inflammatory pain.

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.

Acid increases inflammatory pain in rats: effect of local peripheral ASICs inhibitors.

The purpose of this study was to assess the possible antinociceptive effect of the acid sensing ion channels (ASICs) inhibitors amiloride and benzamil after local peripheral administration in three models of inflammatory pain in rats. Reduction of pH, from 7.4 to 5.8 units, significantly increased the flinching/licking behavior induced by either 0.1% serotonin, 0.1% capsaicin or 0.5% formalin. Local peripheral ipsilateral, but not contralateral, injection of amiloride or benzamil significantly reduced nociceptive behaviors (flinching and licking/lifting) induced by serotonin, capsaicin or formalin in acidic conditions (pH 6.2). Interestingly, benzamil produced antinociception at low doses (0.001-0.1 microM/paw) while higher doses (1 microM/paw) did not affect capsaicin- or formalin-induced licking/lifting. Our data suggest that local peripheral inhibition of ASICs play an important role in inflammatory pain.

Role of peripheral 5-HT4, 5-HT6, and 5-HT7 receptors in development and maintenance of secondary mechanical allodynia and hyperalgesia

&NA; The role of 5‐hydroxytryptamine (5‐HT)4, 5‐HT6, and 5‐HT7 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. Pretreatment (−10 min) with cromoglycate (195–1950 nmol/paw) partially inhibited acute nociceptive behaviors and completely prevented secondary allodynia and hyperalgesia on day 6 after injection. Ipsilateral peripheral pretreatment with the selective 5‐HT4 (ML‐10302, 1–100 nmol/paw), 5‐HT6 (EMD‐386088, 0.001–0.01 nmol/paw), and 5‐HT7 (LP‐12, 0.01–100 nmol/paw) receptor agonists significantly increased secondary allodynia and hyperalgesia in both paws. In contrast, ipsilateral peripheral pretreatment with the selective 5‐HT4 (GR‐125487, 1–100 nmol/paw), 5‐HT6 (SB‐258585, 0.00001–0.001 nmol/paw), and 5‐HT7 (SB‐269970, 0.1–10 nmol/paw) receptor antagonists significantly prevented formalin‐induced secondary allodynia and hyperalgesia in both paws. The pronociceptive effect of ML‐10302 (100 nmol/paw), EMD‐386088 (0.01 nmol/paw), and LP‐12 (100 nmol/paw) were completely prevented by GR‐125487 (5‐HT4 antagonist, 1 nmol/paw), SB‐258585 (5‐HT6 antagonist, 0.00001 nmol/paw), and SB‐269970 (5‐HT7, antagonist, 0.01 nmol/paw), respectively. Ipsilateral peripheral posttreatment with cromoglycate or GR‐125487 (1–100 nmol/paw), SB‐258585 (0.001–0.1 nmol/paw), and SB‐269970 (0.1–10 nmol/paw) reversed formalin‐induced secondary allodynia and hyperalgesia in both paws. Results suggest that a barrage of afferent input induced by 5‐HT at peripheral 5‐HT4, 5‐HT6, and 5‐HT7 receptors participate in the development and maintenance of formalin‐induced long‐term secondary allodynia and hyperalgesia in the rat. 5‐hydroxytryptamine (5‐HT) released in peripheral tissues after formalin injection sensitized primary afferent neurons via 5‐HT4, 5‐HT6, and 5‐HT7 receptors, leading to development and maintenance of secondary allodynia and hyperalgesia.

Role of opioid receptors in the reduction of formalin-induced secondary allodynia and hyperalgesia in rats

This study assesses the effects of peripheral or intrathecal pre-treatment or post-treatment with micro, delta, kappa and nociceptin/orphanin FQ (NOP) opioid receptor agonists (morphine, U-50488 [trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide hydrochloride], DADLE [D-Ala2-Leu5-enkephalin] and nociceptin, respectively) on formalin-induced secondary mechanical allodynia and hyperalgesia in rats. 1% Formalin injection produced acute nociceptive behaviors (flinching and licking/lifting) followed by long-term tactile secondary allodynia and hyperalgesia. Neither peripheral (into the formalin-injected paw) nor intrathecal morphine post-treatment reversed formalin-induced secondary allodynia and hyperalgesia. In contrast, morphine pre-treatment prevented the development of these pain behaviors. Intrathecal and peripheral post- but not pre-treatment with U-50488 or DADLE significantly reduced secondary allodynia and hyperalgesia. Interestingly, nociceptin reduced both pain behaviors regardless of the administration site or treatment time. Local antinociceptive effects of morphine, DADLE, U-50488 or nociceptin were blocked by naltrexone, naltrindole, 5-guanidinonaltrindole and [Nphe(1)]nociceptin(1-13)NH(2), respectively. These results suggest that the long-term nociceptive behaviors induced by formalin are differentially modulated by selective opioid receptor agonists. In addition, data suggest that peripheral and spinal delta and kappa opioid receptors are important when nociceptive behaviors are established. In contrast, micro opioid receptors are more important at the beginning of the injury when the sensory system has not changed. NOP receptors participate diminishing both the development and maintenance of nociceptive behaviors. Results suggest that a barrage of afferent input induced by formalin injection initiates a long-term differential change in peripheral and spinal processing that affect the efficacy of opioid receptor agonists.

Isobolographic Analyses of the Gabapentin-Metamizol Combination after Local Peripheral, Intrathecal and Oral Administration in the Rat

This study was designed to evaluate the possible antinociceptive interaction between gabapentin and metamizol on formalin-induced nociception. Gabapentin, metamizol or a fixed dose-ratio combination of both drugs were assessed after local peripheral, intrathecal and oral administration in rats. Isobolographic analyses were employed to define the nature of the interaction between drugs. Gabapentin, metamizol and gabapentin-metamizol combinations yielded a dose-dependent antinociceptive effect when administered by the three different routes. ED30 values were estimated for the individual drugs and isobolograms were constructed. Theoretical ED30 values for the combination estimated from the isobolograms were 21.11 ± 1.17 µg/paw, 104.6 ± 5.5 µg/rat and 78.8 ± 5.5 mg/kg for the local peripheral, intrathecal and oral administration routes, respectively. These values were significantly higher than the experimentally obtained ED30 values which were 11.3 ± 1.5 µg/paw, 36.8 ± 3.1 µg/rat and 15 ± 1.2 mg/kg indicating a synergistic interaction. Systemic administration resulted in the highest synergism. Data confirm that low doses of the gabapentin and metamizol can interact synergistically to reduce formalin-induced nociceptive behavior suggesting that this combination could be useful to treat inflammatory pain in humans.

Sildenafil and glyceryl trinitrate reduce tactile allodynia in streptozotocin-injected rats

The possible antiallodynic effect of phosphodiesterase 5 inhibitor sildenafil and nitric oxide donor glyceryl trinitrate as well as the changes in phosphodiesterase 5A2 mRNA expression in dorsal root ganglion and spinal cord of allodynic diabetic rats was assessed. Diabetes was induced by streptozotocin (50mg/kg, i.p.) in male Wistar rats. Streptozotocin injection produced hyperlglycemia, polydipsia, polyphagia and polyuria as well as long-term tactile allodynia (12 weeks) and a reduction of phosphodiesterase 5A2 mRNA expression in spinal cord of diabetic rats. Systemic administration of sildenafil (1-5.6 mg/kg, i.p.) reduced tactile allodynia in a dose-dependent manner in diabetic rats. Likewise, glyceryl trinitrate patches (0.2mg/h) also reduced tactile allodynia in diabetic rats. Moreover, both drugs reversed streptozotocin-induced phosphodiesterase 5A2 mRNA expression reduction. Our results indicate that glyceryl trinitrate and sildenafil reduce tactile allodynia in diabetic rats suggesting that nitric oxide and cyclic GMP supply is an important step in their mechanism of action of these drugs in diabetic animals. Data suggest that nitric oxide donors (as glyceryl trinitrate) and drugs which increase cyclic GMP levels (as sildenafil) could have a role in the pharmacotherapy of tactile allodynia in diabetic patients.

Role of NHE1 in Nociception

Intracellular pH is a fundamental parameter to cell function that requires tight homeostasis. In the absence of any regulation, excessive acidification of the cytosol would have the tendency to produce cellular damage. Mammalian Na+/H+ exchangers (NHEs) are electroneutral Na+-dependent proteins that exchange extracellular Na+ for intracellular H+. To date, there are 9 identified NHE isoforms where NHE1 is the most ubiquitous member, known as the housekeeping exchanger. NHE1 seems to have a protective role in the ischemia-reperfusion injury and other inflammatory diseases. In nociception, NHE1 is found in neurons along nociceptive pathways, and its pharmacological inhibition increases nociceptive behavior in acute pain models at peripheral and central levels. Electrophysiological studies also show that NHE modulates electrical activity of primary nociceptive terminals. However, its role in neuropathic pain still remains controversial. In humans, NHE1 may be responsible for inflammatory bowel diseases since its expression is reduced in Crohns disease and ulcerative colitis. The purpose of this work is to provide a review of the evidence about participation of NHE1 in the nociceptive processing.