Role of peripheral group I and II metabotropic glutamate receptors in IL-1β-induced mechanical allodynia in the orofacial area of conscious rats

The present study investigated the role of peripheral group I and II metabotropic glutamate receptors (mGluRs) in interleukin-1β (IL-1β)-induced mechanical allodynia in the orofacial area. Experiments were carried out on Sprague-Dawley rats weighing between 230 and 280 g. After subcutaneous administration of 0.01, 0.1, 1, or 10 pg of IL-1β, we examined withdrawal behavioral responses produced by 10 successive trials of a ramp of air-puffs pressure applied ipsilaterally or contralaterally to the IL-1β injection site. The thresholds of air puffs were measured 10, 30, 60, 120, or 180 min after 25 μl of IL-1β was administered through an implanted tube. Subcutaneous injection of IL-1β produced bilateral mechanical allodynia. While the IL-1β-induced mechanical allodynia was blocked by pretreatment with an IL-1 receptor antagonist, the IL-1β-induced mirror-image mechanical allodynia was not blocked by an IL-1 receptor antagonist injected into the contralateral side. Subcutaneous administration of CPCCOEt or LY367385, an mGluR1 antagonist, or MPEP or SIB1893, an mGluR5 antagonist, 10 min prior to injection of IL-1β abolished IL-1β-induced mechanical allodynia. Pretreatment with APDC or DCG4, a group II mGluR agonist, blocked the IL-1β-induced mechanical allodynia. The anti-allodynic effect induced by APDC was inhibited by pretreatment with LY341495, a group II mGluR antagonist. These results suggest that peripheral group I and II mGluRs participate in IL-1β-induced mechanical allodynia in the orofacial area. Peripheral group I mGluR antagonists blocked the IL-1β-induced mechanical allodynia, while peripheral group II mGluR agonists produced anti-allodynic effects on IL-1β-induced mechanical allodynia in the orofacial area of rats.