GABABReceptor Modulation on the Antinociception of Intrathecal Sildenafil in the Rat Formalin Test

Comparison of sildenafil, fluoxetine and its co-administration against chronic constriction injury induced neuropathic pain in rats: An influential additive effect

Current drug treatment available for neuropathic pain (NP) provides meager and partial pain relief due to incomplete efficacy and dose-dependent adverse effect. Hence, combination therapy can provide prolongation in analgesic effect with milder side effects. The present investigation aimed at observing the effects of sildenafil (SD) on Fluoxetine (FLX) in attenuation of chronic constriction injury (CCI) induced NP in rats. CCI was achieved in rats by placing four loose ligations around the sciatic nerve and rats were received respective treatments on SD and FLX till 14 days further behaviors parameters like heat hyperalgesia and allodynia, pin prick and acetone drop test were executed in order to access thermal, mechanical and cold allodynia, respectively, on a predetermined time interval. On the 21st day the animals were sacrificed for determination of total protein, myeloperoxidase activity in the adjoining muscular tissues while glutathione and TNF-α in the sciatic nerve. Co-administration of SD + FLX + CCI gave the pronounced effect that was superior over individual responses of SD and FLX in all behavioral as well as biochemical parameters. It was observed that attenuation in the altered behavioral pattern of CCI induced rats was modified prominently from 3rd day only in a group of rats treated with SD + FLX + CCI. The whole study was finally supported by histopathological results. Finally, it was concluded that SD produces an additive effect when given with FLX in attenuation of NP may be due to elevation in the level of intracellular concentrations of cyclic guanosine monophosphate which further causes downregulation of calcium channel.

The role of adrenergic and cholinergic receptors on the antinociception of sildenafil in the spinal cord of rats

The role played by spinal adrenergic and cholinergic receptors in the antinociceptive effects of intrathecal sildenafil in formalin-induced nociception was examined. Intrathecal catheters were inserted into the subarachnoid space of male Sprague-Dawley rats, and nociception was assessed using the formalin test, consisting of a subcutaneous injection of 50μL of 5% formalin solution into the hind paw. We examined the effects of an alpha 1 adrenergic receptor antagonist (prazosin), an alpha 2 adrenergic receptor antagonist (yohimbine), a muscarinic acetylcholine receptor antagonist (atropine), and a nicotinic acetylcholine receptor antagonist (mecamylamine) on sildenafil-induced antinociception. Intrathecal sildenafil (3, 10, and 30μg) suppressed, in a dose-dependent manner, formalin-induced flinching during phases 1 and 2 of the test. Intrathecal sildenafil (30μg) could not show any effects against intrathecal prazosin (3μg), yohimbine (10μg), atropine (10μg), and mecamylamine (10μg) pretreatment during both phases of the formalin test. These results suggest that intrathecal sildenafil effectively attenuated the pain evoked by formalin injection. Additionally, spinal alpha 1, alpha 2, muscarinic and nicotinic receptors might play a role in sildenafil-induced antinociception.

Roles of adenosine and serotonin receptors on the antinociception of sildenafil in the spinal cord of rats

PURPOSE

The phosphodiesterase 5 inhibitor sildenafil has antinociceptive effects, mediated by an increase in cGMP. This study examined the role of spinal adenosine and serotonin receptors played in the antinociceptive effects of intrathecal sildenafil.

MATERIALS AND METHODS

Intrathecal catheters were inserted into the subarachnoid space of Sprague-Dawley male rats as a drug delivery device. Pain was induced by injecting formalin into the plantar surface of rats and observing nociceptive behavior (flinching response) for 60 minutes. Then, the effects of intrathecal adenosine and serotonin receptor antagonists on the antinociceptive activity of intrathecal sildenafil were examined.

RESULTS

Intrathecal sildenafil suppressed the flinching response in a dose-dependent manner during phases 1 and 2 in the formalin test. Both CGS 15943 and dihydroergocristine decreased the antinociceptive effects of sildenafil during phases 1 and 2 in the formalin test.

CONCLUSION

Intrathecal sildenafil effectively attenuated the pain evoked by formalin injection. Both adenosine and serotonin receptors may be involved in the antinociceptive action of sildenafil at the spinal level.

Effect of sildenafil on neuropathic pain and hemodynamics in rats

PURPOSE

The inhibition of phosphodiesterase 5 produces an antinociception through the increase of cyclic guanosine monophosphate (cGMP), and increasing cGMP levels enhance the release of gamma-aminobutyric acid (GABA). Furthermore, this phosphodiesterase 5 plays a pivotal role in the regulation of the vasodilatation associated to cGMP. In this work, we examined the contribution of GABA receptors to the effect of sildenafil, a phosphodiesterase 5 inhibitor, in a neuropathic pain rat, and assessed the hemodynamic effect of sildenafil in normal rats.

MATERIALS AND METHODS

Neuropathic pain was induced by ligation of L5/6 spinal nerves in Sprague-Dawley male rats. After observing the effect of intravenous sildenafil on neuropathic pain, GABAA receptor antagonist (bicuculline) and GABAB receptor antagonist (saclofen) were administered prior to delivery of sildenafil to determine the role of GABA receptors in the activity of sildenafil. For hemodynamic measurements, catheters were inserted into the tail artery. Mean arterial pressure (MAP) and heart rate (HR) were measured over 60 min following administration of sildenafil.

RESULTS

Intravenous sildenafil dose-dependently increased the withdrawal threshold to the von Frey filament application in the ligated paw. Intravenous bicuculline and saclofen reversed the antinociception of sildenafil. Intravenous sildenafil increased the magnitude of MAP reduction at the maximal dosage, but it did not affect HR response.

CONCLUSION

These results suggest that sildenafil is active in causing neuropathic pain. Both GABAA and GABAB receptors are involved in the antinociceptive effect of sildenafil. Additionally, intravenous sildenafil reduces MAP without affecting HR.