Shikonin ameliorates oxidative stress and neuroinflammation via the Akt/ERK/JNK/NF‐κB signalling pathways in a model of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder. Shikonin plays protective roles in age-associated diseases. Therefore, we investigate the biological functions of shikonin and its mechanisms involved in PD pathogenesis. The neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was used to mimic PD-like conditions in animal models. The learning and memory capacities were assessed by Morris water-maze test, pole test, locomotor activity test and rotarod test. Neuroinflammation was determined by measuring the levels of tumour necrosis factor α (TNF-α), interleukin (IL)-1β, IL-6, inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). The quantification of superoxide dismutase, malondialdehyde and glutathione in substantia nigra was performed to estimate oxidative damage. Histopathologic changes were examined by haematoxylin and eosin staining. Immunofluorescence staining was conducted to determine the activation of astrocytes, tyrosine hydroxylase (TH)-positive neurons, and nuclear translocation of p65. Immunohistochemistry was performed to evaluate dopamine transporter (DAT)-positive neurons. Protein levels were measured by western blotting. Shikonin alleviates the cognitive and behavioural impairments. The death of dopaminergic neurons in nigra was attenuated by shikonin. The MPTP-induced neuroinflammation and oxidative stress in substantia nigra were alleviated by shikonin administration. Shikonin ameliorated the neuronal damage in nigra and inhibited the activation of astrocyte. Shikonin modulated the protein kinase B (Akt)/extracellular regulated kinase (ERK)/c-Jun N-terminal kinase (JNK)/nuclear factor κB (NF-κB) pathways. Shikonin ameliorates dopaminergic neuronal apoptosis by inhibiting oxidative stress and neuroinflammation via the Akt/ERK/JNK/NF-κB pathways in PD. The study has several limitations. First, in a previous study, levels of phosphorylated ERK were increased by MPTP. In our current study, we observed decreased p-ERK in nigra following MPTP treatment. Therefore, further investigation in the mechanisms of shikonin against PD progression is required. Second, the biological functions of shikonin need more exploration, including mitochondrial function and autophagy. Moreover, specific molecular targets for shikonin remain uncertain.