Ondansetron Reverses Antihypersensitivity from Clonidine in Rats after Peripheral Nerve Injury: Role of gamma -Aminobutyric Acid in alpha 2-Adrenoceptor and 5-HT3 Serotonin Receptor Analgesia

Introduction: Monoaminergic pathways, impinging an alpha 2-adrenoceptors and 5-HT3 serotonin receptors, modulate nociceptive transmission, but their mechanisms and interactions after neuropathic injury are unknown. Here we examine these interactions in rodents after nerve injury. Methods: Male Sprague-Dawley rats following L5-L6 spinal nerve ligation (SNL) were used for either behavioral testing, in vivo microdialysis for gamma -aminobutyric acid (GABA) and acetylcholine release, or synaptosome preparation for GABA release. Results: Intrathecal administration of the alpha 2-adrenoceptor agonist (clonidine) and 5-HT3 receptor agonist (chlorophenylbiguanide) reduced hypersensitivity in SNL rats via GABA receptor-mediated mechanisms. Clonidine increased GABA and acetylcholine release in vivo in the spinal cord of SNL rats but not in normal rats. Clonidine-induced spinal GABA release in SNL rats was blocked by alpha 2-adrenergic and nicotinic cholinergic antagonists. The 5-HT3 receptor antagonist ondansetron decreased and chlorophenylbiguanide increased spinal GABA release in both normal and SNL rats. In synaptosomes from the spinal dorsal horn of SNL rats, presynaptic GABA release was increased by nicotinic agonists and decreased by muscarinic and alpha 2-adrenergic agonists. Spinally administered ondansetron significantly reduced clonidine-induced antihypersensitivity and spinal GABA release in SNL rats. Conclusion: These results suggest that spinal GABA contributes to antihypersensitivity from intrathecal alpha 2-adrenergic and 5-HT3 receptor agonists in the neuropathic pain state, that cholinergic neuroplasticity after nerve injury is critical for alpha 2-adrenoceptor-mediated GABA release, and that blockade of spinal 5-HT3 receptors reduces alpha 2-adrenoceptor-mediated antihypersensitivity via reducing total GABA release.