Trigeminal ganglion-trigeminal spinal subnucleus oralis pathway contributes to orofacial neuropathic pain

The aim of this study was to investigate the potential role of the trigeminal ganglion (TG)-trigeminal spinal subnucleus oralis (Vo) pathway in the orofacial neuropathic pain development, using a male rat model with partial ligation of the infraorbital nerve (PNL). Many TG-Vo neurons were found to have axon collaterals projecting to the trigeminal spinal subnucleus caudalis (Vc). The head-withdrawal threshold to mechanical stimulation of the whisker pad skin (MHWT) was significantly reduced in PNL rats compared with sham rats, and this reduction was reversed by Vo deactivation produced by quinolinic acid (QA) administration to Vo. Optogenetic stimulation of TG-Vo neurons in naïve rats induced a significantly increased frequency of freezing behavior and decreased MHWT. Vc or Vo deactivation by QA reduced the freezing frequency induced by optogenetic stimulation of TG-Vo neurons, and the MHWT reduction was significantly reversed after QA-Vc injection. Wide dynamic range (WDR) neurons in the Vo showed enhanced excitability to mechanical stimulation of the whisker pad skin in PNL rats compared with sham rats. The number of calcitonin gene-related peptide (CGRP)- and transient receptor potential Ankyrin 1 (TRPA1)-positive TG-Vo neurons was also significantly larger in PNL rats. Many CGRP-immunoreactive neurons also exhibited TRPA1 immunoreactivity. Ca2+ influx responding to TRPA1 agonist treatment was significantly higher in cultured TG-Vo neurons in PNL rats compared with sham rats. Vo WDR neuronal activity was attenuated by subcutaneous injection of the TRPA1 antagonist into the whisker pad in PNL rats. These findings suggest that TG-Vo nociceptive neuronal activity is significantly enhanced following trigeminal nerve injury, indicating that the TG-Vo pathway is involved in orofacial neuropathic pain development.