Dynamic regulation of spinal pro-inflammatory cytokine release in the rat in vivo following peripheral nerve injury

Spinal release of cytokines may play a critical role in the maladapted nociceptive signaling underlying chronic pain states. In order to investigate this biology, we have developed a novel ‘high flux’ intrathecal microdialysis approach in combination with multiplex bead-based immunoassay technology to concurrently monitor the spinal release of interleukin (IL)-1β, IL-6 and tumour necrosis factor (TNF)α in rats with unilateral sciatic nerve chronic constriction injury (CCI). Intrathecal microdialysis was performed under isoflurane/N2O anaesthesia in rats with confirmed mechanical hypersensitivity. In a first study, C-fiber strength electrical stimulation of the operated nerve in neuropathic rats was found to evoke a dramatic increase in IL-1β efflux (∼15-fold) that was significantly greater than that observed in the sham-operated group. Spinal IL-6 efflux was also responsive to primary afferent stimulation, whereas TNFα was not. In a second study, treatment with the glial inhibitor propentofylline for 7 days normalized CCI-induced mechanical hypersensitivity. In the same animals, this treatment also significantly reduced intrathecal IL-1β, IL-6 and TNFα and prevented afferent stimulation-evoked cytokine release of both IL-1β and IL-6. These results provide support for glia as the source of the majority of intrathecal IL-1β, IL-6 and TNFα that accompanies mechanical hypersensitivity in the CCI rat. Moreover, our studies demonstrate the ability of a neurone-glia signaling mechanism to dynamically modulate this release and support a role of spinal IL-1β in the phasic transmission of abnormal pain signals.