Intracerebroventricular injection of interleukin-6 induces thermal hyperalgesia in rats

We assessed the effect of interleukin-6 (IL-6) in the brain on nociception by using the hot-plate test in rats. Recombinant human IL-6 (rhIL-6, 30 pg-300 ng) was microinjected into the lateral cerebroventricle (LCV) and the paw-withdrawal latency was then measured for 60 min after injection. RhIL-6 at 300 pg reduced the paw-withdrawal latency at 15 min after injection. Further increase of rhIL-6 doses to 3, 30 and 300 ng resulted in the decreased paw-withdrawal latency at 15 and 30 min. Although the peak responses observed at 3–300 ng did not differ significantly, the time taken for recovery tended to be longer with increasing doses. The rhIL-6 (30 ng)-induced reduction of the paw-withdrawal latency was completely blocked by the co-injection of either Na salicylate (30 ng, LCV) or α-melanocyte stimulating hormone (30 ng, LCV), an anti-cytokine substance. However, it was not affected by the co-injection of IL-1 receptor antagonist (30 ng, LCV) which had been previously shown to be able to block IL-1β-induced hyperalgesia. These findings indicate that IL-6 in the brain induces hyperalgesia by its prostanoids-dependent action in rats. The hyperalgesic action of central IL-6 thus does not appear to depend on the action of IL-1.