17-β Estradiol Rescued Immature Rat Brain against Glutamate-Induced Oxidative Stress and Neurodegeneration via Regulating Nrf2/HO-1 and MAP-Kinase Signaling Pathway

Dysregulated glutamate signaling, leading to neuronal excitotoxicity and death, has been associated with neurodegenerative pathologies. 17β-estradiol (E2) is a human steroid hormone having a role in reproduction, sexual maturation, brain health and biological activities. The study aimed to explain the neuroprotective role of E2 against glutamate-induced ROS production, MAP kinase-dependent neuroinflammation, synaptic dysfunction and neurodegeneration in the cortex and hippocampus of postnatal day 7 rat brain. Biochemical and immunofluorescence analyses were applied. Our results showed that a single subcutaneous injection of glutamate (10 mg/kg) induced brain oxidative stress after 4 h by disturbing the homeostasis of glutathione (GSH) and revealed an upsurge in ROS and LPO levels and downregulated the expression of Nrf2 and HO-1 antioxidant protein. The glutamate-exposed P7 pups illustrated increased phosphorylation of stress-activated c-Jun N-terminal kinase (JNK) and p38 kinase (p38) and downregulated expression of P-Erk1/2. This was accompanied by pathological neuroinflammation as revealed by enhanced gliosis with upregulated expression of GFAP and Iba-1, and the activation of proinflammatory cytokines (TNF-α) in glutamate-injected P7 pups. Moreover, exogenous glutamate also reduced the expression of synaptic markers (PSD-95, SYP) and induced apoptotic neurodegeneration in the cortical and hippocampal regions by dysregulating the expression of Bax, Bcl-2 and caspase-3 in the developing rat brain. On the contrary, co-treatment of E2 (10 mg/kg) with glutamate significantly abrogated brain neuroinflammation, neurodegeneration and synapse loss by alleviating brain oxidative stress by upregulating the Nrf2/HO-1 antioxidant pathway and by deactivating pro-apoptotic P-JNK/P-p38 and activation of pro-survival P-Erk1/2 MAP kinase pathways. In brief, the data demonstrate the neuroprotective role of E2 against glutamate excitotoxicity-induced neurodegeneration. The study also encourages future studies investigating if E2 may be a potent neuroprotective and neurotherapeutic agent in different neurodegenerative diseases.