Since the reported alterations of permeability of the blood-brain barrier by microwave radiation have implications for safety considerations in man, studies were conducted to replicate some of the initial investigations. No transfer of parenterally-administered fluorescein across the blood-brain barrier of rats after 30 min of 1.2-GHz radiation at power densities from 2--75 mW/cm2 was noted. Increased fluorescein uptake was seen only when the rats were made hyperthermic in a warm-air environment. Similarly, no increase of brain uptake of 14C-mannitol using the Oldendorf dual isotope technique was seen as a result of exposure to pulsed 1.3-GHz radiation at peak power densities up to 20 mW/cm2, or in the continuous wave mode from 0.1--50 mW/cm2. An attempt to alter the permeability of the blood-brain barrier for serotonin with microwave radiation was unsuccessful. From these studies it would appear that the brain must be made hyperthermic for changes in permeability of the barrier induced by microwave radiation to occur.

Rat brain tissue, loaded with 45Ca2+ by intraventricular injection was exposed in vitro to pulsemodulated 1‐GHz (SAR of 0.29 or 2.9 W/kg) or 2.45‐GHz radiation (SAR = 0.3 W/kg), and in vivo to 2.06‐GHz radiation (SAR of 0.12 to 2.4 W/kg). There were no significant differences in efflux of 45Ca2+ between the microwave‐ and sham‐irradiated groups.

This paper describes the effect of field orientation on the hyperthermia produced by microwave irradiation. Rats exposed in the E-orientation (long-axis of animal parallel to E-field) to 1600 tmhz radiation become significantly hotter than those exposed in the H-field at the same power density. Norepinephrine and dopamine, but not serotonin content of the basal hypothalamus is reduced as a concomitant of microwave-induced brain hyperthermia.

Rats were exposed to a hyperthermal environment or 1600 MHz radiation for 10 minutes to achieve equivalent rectal temperature increase (4 degrees C). Brain areas were quantitatively assayed for changes in magnesium, calcium, copper, zinc, iron, sodium, and potassium. Iron was increased in several brain regions of both hyperthermal and irradiated animals. Copper content of the cortex was also increased in both experimental groups, whereas zinc was decreased in the hypothalamus. Magnesium was increased in the cortex and medulla of the irradiated rats.