Effects of repeated exposure to JP-8 jet fuel vapor on learning of simple and difficult operant tasks by rats

Groups of 16 Sprague-Dawley rats each were exposed by whole-body inhalation methods to JP-8 jet fuel at the highest vapor concentration without format ion of aerosol (1000 +/- 10% mg/m(3)); to 50% of this concentration (500 +/ - 10% mg/m(3)); or to treated room air (70 +/- 3 L/min) for 6 h/d, 5 d/wk, for 6 wk (180 h). Although two subjects died of apparent kidney complicatio ns during the study, no other change in the health status of exposed rats w as observed, including rate of weight gain. Following a 65-d period of rest , rats were evaluated for their capacity to learn and perform a series of o perant tasks. These tasks ranged in difficulty from learning of a simple fo od-reinforced lever pressing response, to learning a task in which subjects were required to emit up to four-response chains of pressing three differe nt levers (e.g., press levers C, R, L, then C). It was shown that repeated exposure to 1000 mg/m(3) JP-8 vapor induced significant deficits in acquisi tion or performance of moderately difficult or difficult tasks, but not sim ple learning tasks, as compared to those animals exposed to 500 mg/m(3). Le arning/performance of complex tasks by the 500-mg/m(3) exposure group gener ally exceeded the performance of control animals, while learning by the 100 0-mg/m(3) group was nearly always inferior to controls, indicating possible "neurobehavioral" hormesis. These findings appear consistent with some pre viously reported data for operant performance following acute exposure to c ertain hydrocarbon constituents of JP-8 (i.e., toluene, xylenes). There has , however, been little previously published research demonstrating long-ter m learning effects for repeated hydrocarbon fuel exposures. Examination of regional brain tissues from vapor-exposed rats indicated significant change s in levels of dopamine in the cerebral cortex and DOPAC in the brainstem, measured as long as 180 d postexposure, as compared to controls.