IL-1beta in the trigeminal subnucleus caudalis contributes to extra-territorial allodynia/hyperalgesia following a trigeminal nerve injury

Abstract

It has been reported that the whisker pad (WP) area, which is innervated by the second branch of the trigeminal nerve, shows allodynia/hyperalgesia following transection of the mental nerve (MN: the third branch of the trigeminal nerve). However, the mechanisms of this extra-territorial pain induction still remain unclear. Glia and cytokines are known to facilitate perception of noxious input, raising a possibility that these non-neuronal elements are involved in the induction and spread of allodynia/hyperalgesia at non-injured skin territory. One day after MN transection, tactile allodynia/hyperalgesia developed on the ipsilateral WP area, which is in the non-injured skin territory. The tactile allodynia/hyperalgesia lasted for more than 56 days. In response to MN transection, astrocytes and microglia appeared to be in an activated state, and interleukin (IL)-1beta was up-regulated in astrocytes in the trigeminal subnucleus caudalis (Vc). Allodynia/hyperalgesia at WP area induced by MN transection was attenuated dose-dependently by IL-1 receptor antagonist IL-1ra (i.t., 0.05, 0.5, and 5 pg/rat). Fos-like immunoreactive (Fos-Li) neurons were observed in the Vc after non-noxious mechanical stimulation of the WP area in the rats with MN transection. Administration of IL-1ra also attenuated the number of Fos-Li neurons dose-dependently. Administration of a noncompetitive antagonist of NMDA receptors MK-801 (i.t., 5 μg/rat) reversed allodynia/hyperalgesia. IL-1 receptor type I (IL-1RI) was localized in Fos- and phospho NR1-immunoreactive neurons. These results suggest that IL-1beta in the Vc plays an important role in the development of extra-territorial tactile allodynia/hyperalgesia after MN transection.