Thymoquinone activates MAPK pathway in hippocampus of streptozotocin-treated rat model

Streptozotocin (STZ), a glucosamine-nitrosourea compound, produces deficiencies in learning, memory, and cognitive functions when it was administered intracerebroventricularly (i.c.v). In molecular level, increase in neuroinflammation and oxidative stress in brain, and decrease in the number of surviving neurons are the outcomes of STZ administration. Herein, we aimed to investigate the effect of thymoquinone (TQ), an anti-inflammatory, immunomodulatory and neuroprotective agent, on STZ-induced neurodegeneration in rats. For this purpose, bilateral i.c.v. injection of STZ (3 mg/kg) was given to adult female rats on days 1 and 3. TQ (20 mg/kg/day in cornoil) was administered intragastrically to rats for 15 days starting from the 15th day of STZ injection. The Morris water maze test and passive avoidance test were applied to measure the learning and memory performance of animals. Following the behavioral tests, all of the rats were sacrificed for evaluation of molecular alterations. Rats in the STZ-TQ group showed higher performance in passive avoidance test than rats in the STZ group whose memory performance declined compared to control group. The worse memory performance in STZ group was correlated with low number of surviving neurons and high number of degenerating neurons. In addition, an increase in APOE expression and a decrease in NGF expression were observed with STZ injection. Administration of TQ reversed these STZ-triggered cognitive and molecular alterations. In the present study, we observed the neuroregenerative effects of TQ by activation of JNK protein, upregulation of mir-124, and downregulation of ERK1/2 and NOS enzymes. The same ameliorative effect of TQ was also observed in the pTau protein expression. To sum up, we can say that the healing effect of TQ on STZ induced neurodegeneration opens a new door for the development of Alzheimer's disease treatment using natural products as an adjuvant when their action mechanism was explained in detail.