Protective Role of Apocynin via Suppression of Neuronal Autophagy and TLR4/NF-κB Signaling Pathway in a Rat Model of Traumatic Brain Injury

Neuronal autophagy and inflammatory responses are important in the pathogenesis of traumatic brain injury (TBI), and toll-like receptor 4 (TLR4) may play an important role in the related molecular cascade. The present study investigated the protective effect of apocynin, an inhibitor of NADPH oxidase, in a TBI rat model and further examined neuronal autophagy and the TLR4-mediated pathway. Adult male Sprague-Dawley rats were subjected to controlled cortical impact injury and intraperitoneally injected with apocynin (50 mg/kg) immediately after the trauma. In addition to motor and behavioral studies, brain water content and histology analyses were performed. Expression of autophagy-related proteins as well as TLR4/NF-kappa B signaling and inflammatory mediators was analyzed. The apocynin treatment significantly attenuated TBI-induced motor and behavioral impairment, brain edema and neuronal damage in rats. Immunohistochemical and Western blot analyses revealed that apocynin treatment significantly reduced the expression of NOX2, LC3 and Beclin1 in the hippocampus at 12-48 h after injury. Double immunolabeling demonstrated that apocynin decreased the co-localization of LC3 or TLR4-positive cells with hippocampal neurons at 24 h following TBI. In addition, CD11b (microglial marker) and GFAP (astrocyte marker)-immunopositive cells were also clearly decreased in hippocampal tissues. Meanwhile, protein levels of TLR4, NF-kappa B p65, TNF-alpha and IL-1 beta were found to be significantly downregulated by Western blot analysis. In conclusion, our findings indicate that the protective effects of apocynin may be related to modulation of neuronal autophagy and the TLR4/NF-kappa B signaling pathway.