Disruption of a putative working memory task and selective expression of brain c-fos following microwave-induced hyperthermia.

To discern the effects of hyperthermia on working memory, we recorded the ability of rats to discriminate between objects following microwave radiation exposure. Memory changes were evaluated by measuring relative exploration time of a familiar vs. a new stimulus object. A subject that extensively reexplores a stimulus with which it has previous experience is presumed to exhibit memory loss associated with that object. Between training and testing, rats were exposed to various doses of microwave radiation, were sham irradiated, or remained in their home cage. Brain (dural) and rectal temperatures were recorded. To discern brain regions activated or possibly damaged by microwave exposure, we also used immunocytochemistry techniques to identify sites of c-fos protein expression in the brains of several irradiated/sham-irradiated subjects. Rats exposed to > 5 W/kg exhibited hyperthermia when compared to nonirradiated controls. Normothermic control subjects (sham-irradiated rats and rats exposed to 0.1 W/kg) showed a distinct preference for the new object although other microwave-exposed rats (1, 5, 8.5, 9.3, 10 W/kg) did not. Microwave hyperthermia evoked prominent c-fos expression in periventricular strata, hypothalamic nuclei, amygdala, and several areas of the cortex. These data suggest that performance on a putative working memory task may be disrupted by a sufficiently intense microwave-induced hyperthermia. The pattern of expression of the early proto-oncogene c-fos may suggest candidate brain nuclei that mediate the behavioral changes we observed.