Melatonin ameliorates spatial memory and motor deficits via preserving the integrity of cortical and hippocampal dendritic spine morphology in mice with neurotrauma

We aimed to elucidate the role of cortical and hippocampal dendritic spines on neurological deficits associated with hippocampal microgliosis, hippocampal neurogenesis, and neuroinflammation in mice with cortical compact impact (CCI) injury. In the present study, we found that CCI reduced spatial memory mean latency (10 s. vs 50 s) and motor dysfunction (130 s. vs 150 s.) in mice, as determined by Morris water maze and rotarod test, respectively. Golgi staining of cortical pyramidal neurons revealed that, compared to the controls, the CCI group treated with vehicle solution had significantly lower values of dendritic order (or dendritic branch number) (4.0 vs 6.2), total spine length (400 μm vs 620 μm) and spine density (40 spines/μm vs 60 spines/μm), but had significantly higher values of dendritic beading (40 beadings/mm vs 20 beadings/mm). Additionally, Sholl analysis showed that, compared to controls, the CCI + NS group mice had significantly lower values of dendritic intersections (1.0 vs 2.0). Immunofluorescence assay also revealed that, compared to controls, the CCI + NS group mice had significantly higher values of the newly formed hippocampal cells (1250/mm² vs 1000/mm²) but significantly lower values of dendritic order (2.0 branch # vs 4.2 branch #), total spine length (180 μm vs 320 μm) and intersection (1.0 vs 3.0). The CCI + NS group mice further showed significantly higher numbers of microglia in the dentate gyrus of the hippocampus and higher concentrations of pro-inflammatory cytokines in the cerebrospinal fluids. All the CCI-induced spatial memory (40 s) and motor (150 s) dysfunction, deranged dendritic and spine morphology of cortical pyramidal neurons or hippocampal newly formed cells, hippocampal microgliosis, and central neuroinflammation were all significantly reduced by melatonin administration during post-CCI. Simultaneously, melatonin therapy caused an enhancement in the compensatory hippocampal neurogenesis and neurotrophic growth factors (e.g., doublecortin-1) and compensatory central anti-inflammatory cytokines. Our results indicate that melatonin attenuates the spatial memory and motor deficits via the modification of cortical and hippocampal dendritic spine morphology, hippocampal microgliosis and neurogenesis, and neuroinflammation in mice with traumatic brain injury.