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

Relations between neurotoxicants and changes in physiological parameters an d behavior were investigated in larval rainbow trout (RBT; Oncorhynchus myk iss) exposed to sublethal concentrations of two organophosphate pesticides (OPs). Fish were exposed to diazinon and malathion in static-renewal experi ments. After exposures for 24, 96, or 96 h, followed by 48 h of recovery, i ndividual RBT were videotaped to assess locomotory behaviors. Brain tissue from the same fish was assayed for the physiological endpoints, cholinester ase (ChE) activity, muscarinic cholinergic receptor (MChR) number (B-max), and MChR affinity (K-D). Cholinesterase activity decreased significantly wi th increasing concentrations of both diazinon and malathion and differed si gnificantly among exposure durations, with 24- and 96-h means less than 48- h recovery means. Decreases in B-max with OP concentration were not signifi cant for either chemical, and K-D was unaffected. Changes in swimming speed and distance were significantly correlated with changes in ChE activity fo r both chemicals; rate of turning was significantly correlated with ChE act ivity in malathion exposures. These results suggest that correlations betwe en physiological and behavioral changes previously seen in mammals also occ ur in fish.