| Abstract: | Measurement of temperature within the cerebral cortex, hypothalamus, cerebellum and medulla of rats sham-, heat- or microwave-exposed revealed the presence of a thermal gradient within the brain. In all groups, cerebral cortex and the cerebellum were cooler than the deeper hypothalamus and medulla. Exposure to 2450 MHz CW microwaves or ambient heat (42 +/- 2 degrees C) resulted in measurable elevation of regional brain temperature, but without alteration of temperature gradients normally observed within the brain. Exposure to 20 mW/cm2 (SAR approximately equal to 4 W/kg) for 30, 90 or 180 min induced a small, but significantly (U = 0, P less than 0.05) increased temperature of the colon, and in each region of the brain studied. Exposure to an incident power density of 65 mW/cm2 (SAR approximately equal to 13.0 W/kg) for 30 or 90 min or to ambient heat (42 +/- 2 degrees C) for 90 min resulted in a substantially greater thermal response as indicated by higher colonic and brain temperatures. Comparison of regional brain temperature with individual colonic temperatures is expressed as delta T = t degrees Cbrain--t degrees Ccolon. In general delta T values for ambient heat or microwave-exposed rats did not differ significantly from those of sham-exposed animals. Exposure to microwaves or ambient heat did not alter the general relationships between regional brain and colonic temperatures, i.e., cortical and cerebellar temperatures were always below and hypothalamic and medullary temperatures always above corresponding colonic temperatures. The plotted temperature data (brain vs colonic temperature) indicate a linear relationship between brain and colonic temperatures. Levels of sodium fluorescein (NAFl), horseradish peroxidase (HRP) and [14C]sucrose (described in preceding papers) within the brain show a high correlation (P less than 0.05) with brain temperature. Suppression of blood-brain barrier permeability to hydrophilic tracers was most pronounced at brain temperatures exceeding approximately 40 degrees C and is demonstrated to be temperature dependent. |
