The role of P2X(3) receptors in bilateral masseter muscle allodynia in rats

AIM: To determine the relationship between bilateral allodynia induced by masseter muscle inflammation and P2X(3) receptor expression changes in trigeminal ganglia (TRG) and the influence of intramasseteric P2X(3) antagonist administration on bilateral masseter allodynia. METHODS: To induce bilateral allodynia, rats received a unilateral injection of complete Freund’s adjuvant (CFA) into the masseter muscle. Bilateral head withdrawal threshold (HWT) was measured 4 days later. Behavioral measurements were followed by bilateral masseter muscle and TRG dissection. Masseter tissue was evaluated histopathologically and TRG tissue was analyzed for P2X(3) receptor mRNA expression by using quantitative real-time polymerase chain reaction (PCR) analysis. To assess the P2X(3) receptor involvement in nocifensive behavior, two doses (6 and 60 μg/50 μL) of selective P2X(3) antagonist A-317491 were administrated into the inflamed masseter muscle 4 days after the CFA injection. Bilateral HWT was measured at 15-, 30-, 60-, and 120-minute time points after A-317491 administration. RESULTS: HWT was bilaterally reduced after the CFA injection (P < 0.001). Intramasseteric inflammation was confirmed ipsilaterally to the CFA injection. Quantitative real-time PCR analysis demonstrated enhanced P2X(3) expression in TRG ipsilaterally to CFA administration (P < 0.01). In comparison with controls, the dose of 6 μg of A-317491 significantly increased bilateral HWT at 15-, 30-, and 60-minute time points after the A-317491 administration (P < 0.001), whereas the dose of 60 μg of A-317491 was efficient at all time points ipsilaterally (P = 0.004) and at 15-, 30-, and 60-minute time points contralaterally (P < 0.001). CONCLUSION: Unilateral masseter inflammation can induce bilateral allodynia in rats. The study provided evidence that P2X(3) receptors can functionally influence masseter muscle allodynia and suggested that P2X(3) receptors expressed in TRG neurons are involved in masseter inflammatory pain conditions.