Physiological measures of neurotoxicity of diazinon and malathion to larval rainbow trout (Oncorhynchus mykiss) and their correlation with behavioral measures

Relations between neurotoxicants and changes in physiological parameters and behavior were investigated in larval rainbow trout (RBT; Oncorhynchus mykiss) exposed to sublethal concentrations of two organophosphate pesticides (OPs). Fish were exposed to diazinon and malathion in static‐renewal experiments. After exposures for 24, 96, or 96 h, followed by 48 h of recovery, individual RBT were videotaped to assess locomotory behaviors. Brain tissue from the same fish was assayed for the physiological endpoints, cholinesterase (ChE) activity, muscarinic cholinergic receptor (MChR) number (Bmax), and MChR affinity (KD). Cholinesterase activity decreased significantly with increasing concentrations of both diazinon and malathion and differed significantly among exposure durations, with 24‐ and 96‐h means less than 48‐h recovery means. Decreases in Bmax with OP concentration were not significant for either chemical, and KD was unaffected. Changes in swimming speed and distance were significantly correlated with changes in ChE activity for both chemicals; rate of turning was significantly correlated with ChE activity in malathion exposures. These results suggest that correlations between physiological and behavioral changes previously seen in mammals also occur in fish.