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 formation 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 complications during the study, no other change in the health status of exposed rats was 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 operant tasks. These tasks ranged in difficulty from learning of a simple food-reinforced lever pressing response, to learning a task in which subjects were required to emit up to four-response chains of pressing three different 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 acquisition or performance of moderately difficult or difficult tasks, but not simple learning tasks, as compared to those animals exposed to 500 mg/m(3). Learning/performance of complex tasks by the 500-mg/m(3) exposure group generally exceeded the performance of control animals, while learning by the 1000-mg/m(3) group was nearly always inferior to controls, indicating possible neurobehavioral hormesis. These findings appear consistent with some previously reported data for operant performance following acute exposure to certain hydrocarbon constituents of JP-8 (i.e., toluene, xylenes). There has, however, been little previously published research demonstrating long-term learning effects for repeated hydrocarbon fuel exposures. Examination of regional brain tissues from vapor-exposed rats indicated significant changes in levels of dopamine in the cerebral cortex and DOPAC in the brainstem, measured as long as 180 d postexposure, as compared to controls.