Neurotoxic effects and osmoregulatory disbalance caused by the insecticide fipronil in the Amazon fish, Colossoma macropomum

Fipronil is an insecticide broadly used in agriculture and flowage and has been largely detected in aquatic ecosystems, threatening the health and development of fish. This present work investigated the neurotoxic and osmoregulatory potential of acute exposure to fipronil, emphasizing alterations of enzyme, histopathological, and morphophysiological activities in the brain and gills of Colossoma macropomum. The fish were exposed to the control (without fipronil) and exposed conditions, added fipronil in concentrations of 40 µg L⁻¹ (F40) and 160 µg L⁻¹ (F160) for 96 h, following the collection of plasma, white muscle, brain, and gills for analysis. The muscle of F160 presented an increased activity of acetylcholinesterase (AChE), whereas brain activity was constant, even with an intracellular edema shown by histopathology. The interference of the insecticide in the muscular AChE activity demonstrated a neurotoxic effect in the neuromuscular junction that caused behavior impairment, considering flaws in nervous impulse transmission. The immunohistochemical analysis did not show a difference in the density of MRC; however, an imbalance in plasmatic osmo-ionic regulation was observed. Plasmatic Na⁺ of F160 increased, whereas K⁺ decreased compared to control. On the other hand, in F40, the plasmatic K⁺ decreased, and the total osmolality increased. Histopathological indexes do not show morphological damage in gills; however, fipronil presented effects on physiological processes of gills that regulate ionic and osmotic homeostasis, which can affect animal survival in the long term.