Thiamethoxam Exposure Impairs Neuromuscular Function through Thyroid Hormone-Linked Mitochondrial Dysfunction in Pelophylax nigromaculatus.

Rice-paddy amphibians are routinely exposed to neonicotinoids, yet whether such exposure causes endocrine, mitochondrial, and neuromuscular toxicity remains poorly understood. Herein, we combined field surveillance across 39 rice-paddy sites in Zhejiang, China, with a 60-day exposure of adult Pelophylax nigromaculatus to thiamethoxam. Field monitoring in July detected all five neonicotinoids in 69.2% of rice-paddy water samples with summed thiamethoxam-equivalent concentrations ranging from 52.18 to 1282.69 μg/L, while concentrations in wild tadpoles ranged from 6.45 to 113.39 μg/kg. Laboratory thiamethoxam exposure (0.5 and 5 mg/L) resulted in predominant hepatic accumulation and detectable brain residues. Notably, thiamethoxam reduced serum T3, T4, FT3, and FT4, increased L-tyrosine, epinephrine, and dopamine, with dysregulation of HPT-axis-related genes. In skeletal muscle, thiamethoxam inhibited mitochondrial membrane potential, respiratory chain complexes I, II, III, V, as well as basal and maximal mitochondrial respiration (up to 55%). These defects coincided with fragmented AChR clusters, reduced receptor content, and impaired gastrocnemius force across 20-60 Hz in the 5 mg/L group. Some responses varied between sexes, with AChR fluorescence intensity significantly reduced only in females in 5 mg/L. Structural equation modeling supported a thyroid to mitochondrial pathway, in which thiamethoxam reduced integrated thyroid hormone status (β = -0.91), while thyroid hormone status positively predicted mitochondrial membrane potential (β = 0.79) and maximal respiration (β = 0.71). These findings identify thyroid hormone-linked mitochondrial dysfunction as a mechanistic bridge between thiamethoxam exposure and neuromuscular impairment.