New Insights into Melatonin’s Function on Thiacloprid-Induced Pyroptosis and Inflammatory Response in Head Kidney Lymphocytes of <i>Cyprinus carpio</i>: Implicating Mitochondrial Metabolic Imbalance and mtROS/cGAS-STING/NF-κB Axis

Thiacloprid (THI) is a synthetic insecticide, the misuse is targeted chiefly to control aphid pest species in orchards and vegetables. Melatonin (MET) is a hormone that plays crucial physiological roles in anti-inflammatory capacities of fish. We explored the function of MET (100 μM) to mitigate the toxicity induced by THI (20 μM) in lymphocytes. Our results indicate that THI led to a notable rise in lymphocyte mortality. Lymphocytes exposed to THI exhibited a heightened incidence of pyroptosis, accompanied by upregulation in expression associated with pyroptosis (NLRP3, GSDMEA, and IL-18). Meanwhile, THI exposure led to a decrease in lymphocyte mitochondrial membrane potential, an increase in mtROS levels, and a reduction in intracellular ATP, DNA, and NADPH/NADP⁺ levels, indicating an imbalance in the mitochondrial metabolism within the lymphocytes. Additionally, these effects were reversed by MET treatment, where MitoQ treatment showed that the suppression of mtROS reduced the lymphocyte pyroptosis caused by THI via the mtROS/cGAS-STING/NF-κB axis. Importantly, MET provided defense against the immunotoxic impacts of THI by ameliorating pyroptosis and enhancing anti-inflammatory capability via the mtROS/cGAS-STING/NF-κB axis. Our research potentiates the safeguarding of cultured fish from biological hazards caused by THI and highlights the valuable application of MET in common carp.