Descending serotonergic facilitation mediated through rat spinal 5HT3 receptors is unaltered following carrageenan inflammation*1

Abstract

A descending facilitatory drive originating from superficial dorsal horn NK1-expressing neurones and relaying through parabrachial and rostroventral medial medulla to act on deep dorsal horn neurones, mediated through spinal 5HT3 receptors, was recently documented. To determine if this pathway plays a role in the pathophysiology of inflammation, we investigated the effects of spinally administered ondansetron (a selective 5HT3 receptor antagonist) on deep dorsal horn neuronal responses in carrageenan inflamed and naı̈ve animals using in vivo electrophysiology. The mechanical and thermal evoked responses of spinal neurones were dose dependently attenuated by ondansetron to a similar degree in both groups. In contrast, the electrically evoked responses (Aβ-, Aδ-, C-fibre evoked response and post-discharge) remained unaltered in both groups. Thus 5HT3 receptor mediated descending facilitation remains unaltered at this stage after tissue injury.