Pannexin 1-Mediated ATP Signaling in the Trigeminal Spinal Subnucleus Caudalis Is Involved in Tongue Cancer Pain

Pain is one of the most severe concerns in tongue cancer patients. However, the underlying mechanisms of tongue cancer pain are not fully understood. We investigated the molecular mechanisms of tongue cancer-induced mechanical allodynia in the tongue by squamous cell carcinoma (SCC) inoculation in rats. The head-withdrawal threshold of mechanical stimulation (MHWT) to the tongue was reduced following SCC inoculation, which was inhibited by intracisternal administration of 10Panx, an inhibitory peptide for pannexin 1 (PANX1) channels. Immunohistochemical analyses revealed that the expression of PANX1 was upregulated in the trigeminal spinal subnucleus caudalis (Vc) following SCC inoculation. The majority of PANX1 immunofluorescence was merged with ionized calcium-binding adapter molecule 1 (Iba1) fluorescence and a part of it was merged with glial fibrillary acidic protein (GFAP) fluorescence. Spike frequencies of Vc nociceptive neurons to noxious mechanical stimulation were significantly enhanced in SCC-inoculated rats, which was suppressed by intracisternal 10Panx administration. Phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive (IR) neurons increased significantly in the Vc after SCC inoculation, which was inhibited by intracisternal 10Panx administration. SCC inoculation-induced MHWT reduction and increased pERK-IR Vc neuron numbers were inhibited by P2X7 purinoceptor (P2X7R) antagonism. Conversely, these effects were observed in the presence of P2X7R agonist in SCC-inoculated rats with PANX1 inhibition. SCC inoculation-induced MHWT reduction was significantly recovered by intracisternal interleukin-1 receptor antagonist administration. These observations suggest that SCC inoculation causes PANX1 upregulation in Vc microglia and adenosine triphosphate released through PANX1 sensitizes nociceptive neurons in the Vc, resulting in tongue cancer pain.