Enhanced excitability of nociceptive trigeminal ganglion neurons by satellite glial cytokine following peripheral inflammation

Peripheral nerve injury activates satellite cells to produce interleukin 1β (IL-1β) which mediates inflammation and hyperalgesia. This study investigated the hypothesis that activation of satellite glial cells modulates the excitability of trigeminal ganglion (TRG) neurons via IL-1β following inflammation. Inflammation was induced by injection of complete Freund’s adjuvant (CFA) into the whisker pad area. The threshold for escape from mechanical stimulation applied to the whisker pad in inflamed rats was significantly lower than that in control. Two days post-CFA injection, the mean percentage of TRG neurons encircled by glial fibrillary acidic protein (GFAP)-/IL-1β-immunoreactive cells was significantly increased compared to controls. GFAP and IL-1β immunoreactivities were coexpressed in the same cells. Fluorogold (FG) labeling identified the site of inflammation. The number of FG-labeled IL-receptor type I (IL-1RI) TRG neurons in inflamed rats was significantly greater than in controls. In FG-labeled small TRG neurons, the size of IL-1β (1 nM) induced-depolarization in inflamed rats was larger than in controls. IL-1β application significantly increased firing rates evoked by depolarizing pulses in the neurons of inflamed rats, compared to controls. The response to IL-1β was abolished by treatment with the IL-1RI antagonist. These results suggest that activation of satellite glial cells modulates the excitability of small-diameter TRG neurons via IL-1β following inflammation, and that the upregulation of IL-1RI in the soma may contribute to the mechanism underlying inflammatory hyperalgesia. Therefore IL-1β blockers are potential therapeutic agents for prevention of trigeminal hyperalgesia.