Objective  To investigate the effect of photobiomodulation (PBM) on hippocampal neurogenesis, cognitive function, and inflammatory injury in rats with chronic cerebral hypoperfusion.

  Methods  Bilateral ovariectomy (OVX) was performed on female Sprague-Dawley (SD) rats. One week later, the rats were randomly assigned to three groups, Sham surgery (or Sham) group, bilateral common carotid artery occlusion (BCCAO) group, and PBM intervention (or BCCAO+PBM) group. There were 8 rats in each group. In the BCCAO group, chronic cerebral hyporeperfusion was induced by permanent ligation of bilateral common carotid arteries and no PBM was given. Rats in the Sham group underwent the same surgical procedure except for the occlusion of the two carotids arteries and no PBM was given. In addition to the BCCAO surgery, rats in the BCCAO+PBM group received 808 nm laser therapy (5 min each time at a laser dose of 20 mW/cm²) of the frontal cortex every other day for 1 month. Between 86 and 90 days after BCCAO, Morris water maze (MWM) was used to observe the spatial learning and memory function of the rats. The rats were sacrificed on day 90 and immunofluorescence staining and Western blot were performed thereafter. Immunofluorescence staining was used to determine the expression of 5-bromodeoxyuracil nucleoside (BrdU), a cell proliferation marker, glial fibrillary acidic protein (GFAP), an astrocyte marker, doublecortin (DCX), a specific marker of newborn neuron precursor cells, NeuN, a marker of mature neurons, and Iba1, a microglia marker, in the hippocampal dentate gyrus (DG) region. Western blot was performed to analyze the protein expressions of inflammasome components, NLRP3, ASC, cleaved caspase-1, and Iba1 in the hippocampus.

  Results  In the latency trial of MWM test, BCCAO+PBM rats spent shorter periods of time finding the underwater platform than the BCCAO rats did. In the probe trial, after the platform that was original placed in a quadrant was removed, the BCCAO+PBM rats spent longer periods of time exploring the quadrant than the BCCAO animals did (P<0.05). Compared with BCCAO rats, BCCAO+PBM rats showed significant decrease in the immunofluorescence intensities of GFAP and Iba1 (P<0.01). PBM intervention significantly increased the number of BrdU-positive cells in the hippocampal DG region compared with those of Sham and BCCAO groups (P<0.05). Furthermore, the number of NeuN positive cells showed no significant difference among the three groups, while in BCCAO+PBM group, the number of DCX-positive cells was significantly increased (P<0.001) and the number of DCX⁺/NeuN⁺ co-located cells was significantly increased compared to that of the BCCAO group (P<0.001). Compared with those of the BCCAO group, Western blot results showed that the protein expression levels of Iba1, NLRP3, and cleaved caspase-1 in the BCCAO+PBM group were significantly decreased (P<0.05), while the ASC protein expression level showed no significant difference.

  Conclusion  PBM can effectively improve the spatial learning and memory function in rats with chronic cerebral hypoperfusion, inhibit the activation of glial cells, reduce inflammatory damage mediated by NLRP3 inflammasome, and promote the regeneration of endogenous neural stem cells in the hippocampal DG region of rats.