Melatonin mitigates oxidative stress and metabolic dysfunction induced by interleukin-6 and dopamine in SH-SY5Y cells

Melatonin has emerged as a promising pharmacological candidate for bipolar disorder (BD), though its mechanisms of action remain incompletely understood. Its antioxidant, anti-inflammatory, and anti-dopaminergic properties suggest potential relevance to BD pathophysiology. This study investigated melatonin’s effects on dopamine signalling, metabolism, and oxidative stress under inflammatory and hyperdopaminergic conditions in differentiated SH-SY5Y neuronal cells. Cells were pretreated with 100nM melatonin or vehicle for 2 hours, then exposed to vehicle, IL-6 (20ng/mL), dopamine (5µM or 500µM), or dopamine (500µM) with ascorbic acid (1mM) for 12 or 24 hours. Dopaminergic markers were assessed via real-time PCR and HPLC; metabolic outcomes were measured using Seahorse assay, central carbon metabolomics, in-cell Western assay, and glucose uptake assay; and oxidative stress was evaluated via reactive oxygen species (ROS), superoxide (SOX), and total antioxidant capacity (TAC) assays. IL-6 increased dopamine levels, p-Erk1/2/Erk1/2, p-AMPK/AMPK, nucleotide pools, and TAC, while reducing dopamine turnover, SV2C expression, and spare respiratory capacity. Melatonin alone increased nucleotides and NADH, while reducing dopamine turnover, ROS, and glucose-1-phosphate. In IL-6 conditions, melatonin pretreatment enhanced spare respiratory capacity, glucose uptake, and NADH, while reducing dopamine, TAC, p-AMPK/AMPK, p-GSK3β/GSK3β, and non-mitochondrial oxygen consumption. High-dose dopamine (500µM) elevated SOX, p-Erk1/2/Erk1/2, insulin receptor-α, GLUT1, glycolytic ATP (glycoATP), and non-mitochondrial oxygen consumption. Melatonin pretreatment attenuated p-Erk1/2/Erk1/2 and GLUT1 elevations. Combined dopamine and ascorbic acid further increased glycolytic intermediates, ROS, p-AMPK/AMPK, and TAC, while reducing p-Erk1/2/Erk1/2, p-mTOR, GLUT1, glucose uptake, and glycoATP. Overall, melatonin mitigated IL-6-induced dopaminergic, oxidative, and metabolic alterations, and partially protected against dopamine-induced metabolic shifts. These findings suggest melatonin may alleviate manic symptoms in BD via both direct dopaminergic modulation and indirect antioxidant and metabolic regulatory effects.