Effect of 2450 MHz microwave energy on the blood-brain barrier to hydrophilic molecules. D. Brain temperature and blood-brain barrier permeability to hydrophilic tracers

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 °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 4 W/kg) for 30, 90 or 180 min induced a small, but significantly (U = 0, ) increased temperature of thcolon, and in each region of the brain studied. Exposure to an incident power density of 65 mW/cm² (SAR 13.0 W/kg) for 30 or 90 min or to ambient heat (42 +- 2 °C) for 90 min resulted in a substantially greater thermal response as indicated by higher colonie and brain temperatures. Comparison of regional brain temperature with individual colonie temperatures is expressed as ΔT = t °C_brain − t °C_colon. In general ΔT values for ambient heat or microwaveexposed 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 colonie temperatures, i.e., cortical and cerebellar temperatures were always below and hypothalamic and medullary temperatures always above corresponding colonie temperatures.The plotted temperature data (brain vs colonie temperature) indicate a linear relationship between brain and colonie temperatures. Levels of sodium fluorescein (NAFl), horseradish peroxidase (HRP) and [¹⁴C]sucrose (described in preceding papers) within the brain show a high correlation (P < 0.05) with brain temperature. Suppression of blood-brain barrier permeability to hydrophilic tracers was most pronounced at brain temperatures exceeding ~40 °C and is demonstrated to be temperature dependent.