Neurological effects on startle response and escape from predation by medaka exposed to organic chemicals

Simultaneous electrophysiological and behavioral studies were performed on 21-32 day old juvenile medaka (Oryzias latipes) exposed at sublethal concen trations to organic chemicals representing various modes of action. Non-inv asive recordings were made of the electrical impulses generated within gian t neuronal Mauthner cells, associated interneurons and motoneurons, and axi al musculature, all of which initiate the startle or 'escape' response in f ish. Timing in ms between these electrical sequelae was measured for each f ish before and after 24 and 48 h exposure to a chemical. Carbaryl and pheno l affected Mauthner cell to motoneuron transmission while chlorpyrifos, car baryl, phenol and 2,4-dinitrophenol (DNP) showed neuromuscular effects. The variety of neurological effects detected at various concentrations of chem icals tested here suggest that different mechanisms may be responsible. Als o noted was the number of startle responses to number of stimuli ratio (RIS ); this ratio was affected by most chemicals. Medaka generally appeared to be more susceptible to predation after exposure to chlorpyrifos, carbaryl, fenvalerate, endosulfan, phenol, 1-octanol and DNP. The effects threshold f or many of the test compounds was found to be consistent for both the neuro physiological and behavioral endpoints. Consequently, electrophysiological responses of Mauthner cell-initiated startle responses provided a measure o f neurological injury that is also directly correlated to a definitive and ecologically relevant behavioral endpoint. Published by Elsevier Science B. V.