Photobiomodulation Activates Microglia/Astrocytes and Relieves Neuropathic Pain in Inferior Alveolar Nerve Injury

This study aimed to determine microglial/astrocyte changes and their associated analgesic effect in inferior alveolar nerve injury (IANI) model rats treated with photobiomodulation therapy (PBMT) using a 940-nm diode laser. Very few basic studies have investigated microglial/astrocyte dynamics following PBMT aimed at relieving neuropathic pain caused by IANI. Rats were divided into an IANI-PBM group, IANI+PBM group, and sham+PBM group. Observations were made on the day before IANI or the sham operation and on postoperative days 3, 5, 7, 14, and 28. PBMT was delivered for 7 consecutive days, with an energy density of 8 J/cm². Behavioral analysis was performed to determine pain thresholds, and immunohistological staining was performed for the microglia marker Iba1 and astrocyte marker glial fibrillary acidic protein, which are observed in the spinal trigeminal nucleus. Behavioral analysis showed that the pain threshold returned to the preoperative level on postoperative day 14 in the IANI+PBM group, but decreased starting from postoperative day 1 and did not improve thereafter in the IANI-PBM group (p ≤ 0.001). Immunological analysis showed that microglial and astrocyte cell counts were similar in the IANI+PBM group and IANI-PBM group shortly after IANI (day 3), but the expression area was larger (p ≤ 0.001) and hypertrophy of microglia and astrocyte cell bodies and end-feet extension (i.e., indicators of activation) were more prominent in the IANI+PBM group. PBMT after IANI prevented hyperalgesia and allodynia by promoting glial cell activation shortly after injury.