Neuroprotection via IGF-1 neuronal signaling activation by melatonin and edaravone synergy in methylmercury-induced ALS-like neurotoxicity: Comprehensive analysis of brain regions, spinal cord, CSF, and blood plasma

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disease characterized by motor neuron degeneration, oxidative stress, neuroinflammation, and neurotransmitter imbalances. This study explored the neuroprotective potential of melatonin (MLT), alone and in combination with edaravone (EDR), in a methylmercury (MEME)-induced ALS rat model. MEME exposure effectively replicated ALS pathology, causing behavioral deficits, oxidative stress, neuroinflammation, apoptosis, and widespread structural damage in critical brain regions and the spinal cord. MLT administration at 5 mg/kg (MLT5) and 10 mg/kg (MLT10) significantly mitigated MEME-induced neurotoxicity in a dose-dependent manner. MLT improved motor function, reduced depressive-like behavior, and restored body weight. Biochemically, MLT enhanced antioxidant defenses, including superoxide dismutase (SOD) and catalase (CAT), reduced pro-inflammatory cytokines, interleukin-1 beta (IL-1β), increased anti-inflammatory cytokines, interleukin-10 (IL-10), and restored neurotransmitter balance like dopamine and Gamma-Aminobutyric Acid (GABA). Mechanistically, MLT activated the IGF-1 signaling pathway, promoting neuronal survival and reducing apoptosis (Caspase-3 expression). Histopathological analyses confirmed that MLT preserved neuronal and glial integrity, reduced demyelination, and restored myelin basic protein (MBP) levels in brain and cerebrospinal fluid. The combination of MLT and EDR exhibited synergistic neuroprotective effects, surpassing the efficacy of individual treatments in reducing oxidative stress, inflammation, and neuronal damage. Behavioral and biochemical improvements were paralleled by systemic recovery, as evidenced by normalized hematological parameters and reduced methylmercury accumulation in brain tissues. These findings underscore MLT, particularly in combination with EDR, as a potent therapeutic agent for ALS, offering multi-targeted neuroprotection. Future studies should explore its translational potential in clinical settings for the treatment of neurodegenerative diseases.