Activation of interleukin-1β receptor suppresses the voltage-gated potassium currents in the small-diameter trigeminal ganglion neurons following peripheral inflammation

The glial cytokine, interleukin-1 beta (IL-1 beta), potentiates the excitability of nociceptive trigeminal ganglion (TRG) neurons via membrane depolarization following peripheral inflammation. Perforated patch-clamp technique was used this study to show that the mechanism Underlying the excitability of small-diameter TRG neurons following inflammation is due to IL-1 beta. Inflammation was induced by injection of complete Freund's adjuvant (CFA) into the whisker pad. The TRG neurons innervating the site of inflammation were identified by fluorogold (FG) labeling. The threshold for escape from mechanical stimulation applied to the orofacial area in inflamed rats was significantly lower than observed for control rats. IL-1 beta at 1 nM suppressed total voltage-gated K+ currents in most TRG neurons (70%) under voltage-clamp conditions in control and inflamed rats. IL-1 beta significantly decreased the total, transient (I-A) and sustained (I-K) currents in FG-labeled small-diameter TRG neurons in both groups. The IL-1 beta-induced suppression of TRG neuron excitability was abolished by co-administration of ILra, an IL-1 beta P receptor blocker. The magnitude of inhibition of I-A and I-K currents by IL-1 beta was significantly greater in inflamed rats than in controls. IL-1 beta inhibited I-A to a significantly greater extent than I-K. These results suggest that the inhibitory effect of I-A and I-K currents by IL-1 beta in small-diameter TRG neurons potentiates neuronal excitability thereby contributing to trigeminal inflammatory hyperalgesia. These findings provide evidence for the development of voltage-gated K+ channel openers and IL-1 beta antagonists as therapeutic agents for the treatment of trigeminal inflammatory hyperalgesia. (C) 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.