Ornithine decarboxylase activity of L929 cells after exposure to continuous wave or 50 Hz modulated radiofrequency radiation--a replication study

A replication study with some extensions was made to confirm enhancement of ornithine decarboxylase (ODC) activity in murine L929 fibroblasts after radiofrequency (RF) field exposure reported in earlier studies. L929 cells purchased from two cell banks were exposed for 2, 8, or 24 h to continuous wave or DAMPS (burst modulated at 50 Hz, with 33% duty cycle) signals at specific absorption rate (SAR) levels of 2.5 or 6.0 W/kg. Exposures were carried out in Crawford and waveguide chambers, at frequencies 835 and 872 MHz, respectively. The results did not confirm findings of previous studies reporting increased ODC activity in RF-exposed cells. When Crawford cell exposure system was used, ODC activity was either not affected (in the case of 8 or 24 h exposures) or decreased after 2 h exposure at the highest SAR level (6 W/kg). The decrease was most pronounced when cooling with air flow was not used, and is most likely related to increased temperature. The minor methodological differences (use of antibiotics, increased sensitivity of ODC assay) are not likely to explain the inconsistency of the findings of the present and previous studies. Different results were obtained in experiments with the waveguide system that involves more efficient temperature control. In this exposure system, ODC activity was increased after 8 h exposure at 6 W/kg. Further studies are warranted to explore whether this finding reflects a true non-thermal effect. The present study did not provide evidence for modulation-specific effects reported in earlier studies.     

Effects of continuous and intermittent exposure to RF fields with a wide range of SARs on cell growth, survival, and cell cycle distribution

To examine the biological effects of radio frequency (RF) electromagnetic fields in vitro, we have examined the fundamental cellular responses, such as cell growth, survival, and cell cycle distribution, following exposure to a wide range of specific absorption rates (SAR). Furthermore, we compared the effects of continuous and intermittent exposure at high SARs. An RF electromagnetic field exposure unit operating at a frequency of 2.45 GHz was used to expose cells to SARs from 0.05 to 1500 W/kg. When cells were exposed to a continuous RF field at SARs from 0.05 to 100 W/kg for 2 h, cellular growth rate, survival, and cell cycle distribution were not affected. At 200 W/kg, the cell growth rate was suppressed and cell survival decreased. When the cells were exposed to an intermittent RF field at 300 W/kg(pk), 900 W/kg(pk) and 1500 W/kg(pk) (100 W/kg(mean)), no significant differences were observed between these conditions and intermittent wave exposure at 100 W/kg. When cells were exposed to a SAR of 50 W/kg for 2 h, the temperature of the medium around cells rose to 39.1 degrees C, 100 W/kg exposure increased the temperature to 41.0 degrees C, and 200 W/kg exposure increased the temperature to 44.1 degrees C. Exposure to RF radiation results in heating of the medium, and the thermal effect depends on the mean SAR. Hence, these results suggest that the proliferation disorder is caused by the thermal effect.     

THE EFFECT OF LOW LEVEL CONTINUOUS 2.45 GHz WAVES ON ENZYMES OF DEVELOPING RAT BRAIN

The present work describes the effect of low level continuous microwaves (2.45 GHz) on developing rat brain. Some 35-day-old Wistar rats were used for this study. The animals were exposed 2 hr/day for 35 days at a power density of 0.34 mW/cm² [specific absorption rate (SAR), 0.1 W/kg] in a specially made anechoic chamber. After the exposure, the rats were sacrificed and the brain tissue was dissected out and used for various biochemical assays. A significant increase in calcium ion efflux and ornithine decarboxylase (ODC) activity was observed in the exposed group as compared to the control. Correspondingly, a significant decrease in the calcium-dependent protein kinase activity was observed. These results indicate that this type of radiation affects the membrane bound enzymes, which are associated with cell proliferation and differentiation, thereby pointing out its possible role as a tumor promoter.     

Biochemical Changes in Rat Brain Exposed to Low Intensity 9.9 GHz Microwave Radiation

Present study concerns with various biochemical changes in the developing rat brain exposed to 9.9 GHz (square wave modulated, 1 kHz) at power density 0.125 mW/cm² (specific absorption rate 1.0 W/kg) for 2 h/day for 35 days. Thirty days old male wistar rats were used for this present study. Each group consists of eight animals. After the exposure, biochemical assays such as calcium ion efflux, calcium-dependent protein kinase (PKC), and ornithine decarboxylase (ODC) were performed on the brain tissue. Results of this study reveal that chronic exposure of rat to microwave radiation alter the activity of certain enzymes. There was a significant increase in calcium ion efflux and the activity of ODC. On the other hand, there is a significant decrease in PKC activity. Since these enzymes are related to growth, any alteration may lead to affect functioning of the brain and its development.     

Effects of a 2450 MHz high-frequency electromagnetic field with a wide range of SARs on the induction of heat-shock proteins in A172 cells

In this study, we investigated whether exposure to 2450 MHz high-frequency electromagnetic fields (HFEMFs) could act as an environmental insult to evoke a stress response in A172 cells, using HSP70 and HSP27 as stress markers. The cells were exposed to a 2450 MHz HFEMF with a wide range of specific absorption rates (SARs: 5-200 W/kg) or sham conditions. Because exposure to 2450 MHz HFEMF at 50-200 W/kg SAR causes temperature increases in culture medium, appropriate heat control groups (38-44 degrees C) were also included. The expression of HSP 70 and HSP 27, as well as the level of phosphorylated HSP 27 ((78)Ser) (p-HSP27), was determined by Western blotting. Our results showed that the expression of HSP 70 increased in a time and dose-dependent manner at >50 W/kg SAR for 1-3 h. A similar effect was also observed in corresponding heat controls. There was no significant change in HSP 27 expression caused by HFEMF at 5-200 W/kg or by comparable heating for 1-3 h. However, HSP 27 phosphorylation increased transiently at 100 and 200 W/kg to a greater extent than at 40-44 degrees C. Phosphorylation of HSP 27 reached a maximum after 1 h exposure at 100 W/kg HFEMF. Our results suggest that exposure to a 2450 MHz HFEMF has little or no apparent effect on HSP70 and HSP27 expression, but it may induce a transient increase in HSP27 Phosphorylation in A172 cells at very high SAR (>100 W/kg).     

Genetic damage in mammalian somatic cells exposed to radiofrequency radiation: a meta-analysis of data from 63 publications (1990-2005)

During the last several decades, numerous researchers have examined the potential of in vitro and /or in vivo exposure of radiofrequency( RF) radiation to damage the genetic material in mammalian somatic cells. A meta-analysis of reported data was conducted to obtain a quantitative estimate ( with 95% confidence intervals) of genotoxicity in RF-radiation-exposed cells compared with sham-exposed/unexposed control cells. The extent of genotoxicity was assessed for various end points, including single- and double-strand breaks in the DNA, incidence of chromosomal aberrations, micronuclei and sister chromatid exchanges. Among the several variables in the experimental protocols used in individual investigations, the influence of three specific variables related to RF-radiation exposure characteristics was examined in the meta-analysis: frequency, specific absorption rate, and exposure as continuous-wave, pulsed-wave and occupationally exposed/cell phone users. The overall data indicated that (1) the difference between RF-radiation exposure was small with few exceptions; (2) at certain RF radiation exposure conditions, there were statistically significant increases in genotoxicity for some end points; and (3) the mean indices for chromosomal aberrations and micronuclei in RF-radiation -exposed and sham-/unexposed controls were within the spontaneous levels reported in the historical database. Considerable evidence for publication bias was found in the meta-analysis.     

Modest increase in temperature affects ODC activity in L929 cells: low-level radiofrequency radiation does not

The effects of low-level radiofrequency (RF) radiation and elevated temperature on ornithine decarboxylase (ODC) activity were investigated in murine L929 fibroblasts. The cells were exposed at 900 MHz either to a pulse-modulated (pulse frequency 217 Hz; GSM-type modulation) or a continuous wave signal at specific absorption rate (SAR) levels of 0.2 W kg⁻¹ (0.1–0.3 W kg⁻¹) and 0.4 W kg⁻¹ (0.3–0.5 W kg⁻¹) for 2, 8, or 24 h. RF radiation did not affect cellular ODC activity. However, a slight increase in temperature (0.8–0.9°C) in the exposure system lead to decreased ODC activity in cell cultures. This was verified by tests in which cells were exposed to different temperatures in incubators. The results show that ODC activity is sensitive to small temperature differences in cell cultures. Hence, a precise temperature control in cellular ODC activity studies is needed.     

Characteristics of microwave evoked body movements in mice

Microwave evoked body movements were studied in mice. A resonant cavity was used to provide head and neck exposure of the mouse to pulsed and gated continuous wave (CW) 1.25 GHz microwaves. No difference in response to pulsed and gated CW stimuli of equal average power was found. The incidence of the microwave evoked body movements increased proportionally with specific absorption (dose) when the whole-body average specific absorption rate was at a constant level (7300 W/kg). Under a constant average specific absorption rate, the response incidence reached a plateau at 0.9 kJ/kg. For doses higher than 0.9 kJ/kg, response incidence was proportional to the specific absorption rate and reached a plateau at 900 W/kg. Body movements could be evoked by a single microwave pulse. The lowest whole-body specific absorption (SA) tested was 0.18 kJ/kg, and the corresponding brain SA was 0.29 kJ/kg. Bulk heating potentials of these SAs were less than 0.1 °C. For doses higher than 0.9 kJ/kg, the response incidence was also proportional to subcutaneous temperature increment and subcutaneous heating rate. The extrapolated absolute thresholds (0% incidence) were 1.21 °C temperature increment and 0.24 °C/s heating rate. Due to high subcutaneous heating rates, these microwaves must be perceived by the mouse as an intense thermal sensation but not a pain sensation because the temperature increment was well below the threshold for thermal pain. Results of the present study should be considered in promulgation of personnel protection guideline against high peak power but low average power microwaves. © 1994 Wiley-Liss, Inc.     

Cell physiological effects of radiofrequency electromagnetic fields

Ovarian and body cavity eggs from R. temporaria were exposed to radiofrequency (rf) electromagnetic fields in the frequency range 10-27 MHz with specific absorption rates (SAR) up to 800 W/kg. The effect of the exposure was investigated by measurement of the water-related cell physiological parameters, isotopic and osmotic water membrane permeability and density of the egg cells. Only the osmotic water permeability, Pf, of ovarian eggs was significantly altered. A decrease of about 30% was seen for SARs of 50 W/kg and exposure times up to 2 h. Tests ruled out that the effect was due to temperature increase during the exposure. The observed decrease of Pf was most likely due to cloudy swelling of the egg cytoplasm resulting from the rf irradiation.     

Role of modulation on the effect of microwaves on ornithine decarboxylase activity in L929 cells

The effect of 835 MHz microwaves on the activity of ornithine decarboxylase (ODC) in L929 murine cells was investigated at an SAR of ∼2.5 W/kg. The results depended upon the type of modulation employed. AM frequencies of 16 Hz and 60 Hz produced a transient increase in ODC activity that reached a peak at 8 h of exposure and returned to control levels after 24 h of exposure. In this case, ODC was increased by a maximum of 90% relative to control levels. A 40% increase in ODC activity was also observed after 8 h of exposure with a typical signal from a TDMA digital cellular telephone operating in the middle of its transmission frequency range (∼840 MHz). This signal was burst modulated at 50 Hz, with approximately 30% duty cycle. By contrast, 8 h exposure with 835 MHz microwaves amplitude modulated with speech produced no significant change in ODC activity. Further investigations, with 8 h of exposure to AM microwaves, as a function of modulation frequency, revealed that the response is frequency dependent, decreasing sharply at 6 Hz and 600 Hz. Exposure with 835 MHz microwaves, frequency modulated with a 60 Hz sinusoid, yielded no significant enhancement in ODC activity for exposure times ranging between 2 and 24 h. Similarly, exposure with a typical signal from an AMPS analog cellular telephone, which uses a form of frequency modulation, produced no significant enhancement in ODC activity. Exposure with 835 MHz continuous wave microwaves produced no effects for exposure times between 2 and 24 h, except for a small but statistically significant enhancement in ODC activity after 6 h of exposure. Comparison of these results suggests that effects are much more robust when the modulation causes low-frequency periodic changes in the amplitude of the microwave carrier. Bioelectromagnetics 18:132–141, 1997. © 1997 Wiley-Liss, Inc.     

Mobile phone usage and male infertility in Wistar rats

A significant decrease in protein kinase C and total sperm count along with increased apoptosis were observed in male Wistar rats exposed to mobile phone frequencies (2 h/day x 35 days at 0.9 W/kg specific absorption rate). The results suggest that a reduction in protein kinase activity may be related to overproduction of reactive oxygen species (ROS) under microwave field exposure. Decrease in sperm count and an increase in apoptosis may be causative factor due to mobile radiation exposure leading to infertility.     

No effects of radiofrequency radiation on 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone-induced tumorigenesis in female Wistar rats

This study evaluated possible effects of radiofrequency (RF) radiation on tumorigenesis induced by the mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) given in drinking water. Female Wistar rats aged 7 weeks at the beginning of the experiments were randomly divided into four groups of 72 animals: a cage-control group and three MX-exposed groups (a daily average dose of 1.7 mg MX/kg body weight for 104 weeks), of which two were exposed to 900 MHz pulsed RF radiation and the third served as a sham-RF-radiation group. The RF-radiation groups were exposed 2 h per day, 5 days per week for 104 weeks at nominal whole-body average SARs of 0.3 W/kg and 0.9 W/kg. Complete histopathology was performed on the rats of the three MX-exposed groups. The tumor types and incidences observed in the MX-exposed animals were similar to those reported earlier in MX-exposed female Wistar rats. RF radiation did not statistically significantly affect mortality or organ-specific incidence of any tumor type. The only statistically significant difference was an increase in the combined frequency of vascular tumors of the mesenteric lymph nodes in the high-RF-radiation group compared to the sham-RF-radiation group. However, additional histopathological analysis of the cage-control animals suggested that this difference was due to unusually low frequency of this type of tumor in the sham-RF-radiation group rather than a high frequency in the high-RF-radiation group. With respect to non-neoplastic findings, statistically significant differences between the RF-radiation groups and the sham-RF-radiation group were observed only for single findings in the lacrimal glands, lungs, liver and skin. Such changes are commonly seen in aged rats and were considered to be unrelated to RF radiation. The results of the present study do not support co-carcinogenic effects of low-level long-term RF-radiation exposure in rats.     

Measurement of DNA damage and apoptosis in Molt-4 cells after in vitro exposure to radiofrequency radiation

To determine whether exposure to radiofrequency (RF) radiation can induce DNA damage or apoptosis, Molt-4 T lymphoblastoid cells were exposed with RF fields at frequencies and modulations of the type used by wireless communication devices. Four types of frequency/modulation forms were studied: 847.74 MHz code-division multiple-access (CDMA), 835.62 MHz frequency-division multiple-access (FDMA), 813.56 MHz iDEN(R) (iDEN), and 836.55 MHz time-division multiple-access (TDMA). Exponentially growing cells were exposed to RF radiation for periods up to 24 h using a radial transmission line (RTL) exposure system. The specific absorption rates used were 3.2 W/kg for CDMA and FDMA, 2.4 or 24 mW/kg for iDEN, and 2.6 or 26 mW/kg for TDMA. The temperature in the RTLs was maintained at 37 degrees C +/- 0.3 degrees C. DNA damage was measured using the single-cell gel electrophoresis assay. The annexin V affinity assay was used to detect apoptosis. No statistically significant difference in the level of DNA damage or apoptosis was observed between sham-treated cells and cells exposed to RF radiation for any frequency, modulation or exposure time. Our results show that exposure of Molt-4 cells to CDMA, FDMA, iDEN or TDMA modulated RF radiation does not induce alterations in level of DNA damage or induce apoptosis.     

Effect of high-frequency electromagnetic fields with a wide range of SARs on chromosomal aberrations in murine m5S cells

To investigate the induction of chromosomal aberrations in mouse m5S cells after exposure to high-frequency electromagnetic fields (HFEMFs) at 2.45 GHz, cells were exposed for 2 h at average specific absorption rates (SARs) of 5, 10, 20, 50 and 100 W/kg with continuous wave-form (CW), or at a mean SAR of 100 W/kg (with a maximum of 900 W/kg) with pulse wave-form (PW). The effects of HFEMF exposure were compared with those in sham-exposed controls and with mitomycin C (MMC) or X-ray treatment as positive controls. We examined all structural, chromatid-type and chromosome-type changes after HFEMF exposures and treatments with MMC and X-rays. No significant differences were observed following exposure to HFEMFs at SARs from 5 to 100 W/kg CW and at a mean SAR of 100 W/kg PW (a maximum SAR of 900 W/kg) compared with sham-exposed controls, whereas treatments with MMC and X-rays increased the frequency of chromatid-type and chromosome-type aberrations. In summary, HFEMF exposures at 2.45 GHz for 2 h with up to 100 W/kg SAR CW and an average 100 W/kg PW (a maximum SAR of 900 W/kg) do not induce chromosomal aberrations in m5S cells. Furthermore, there was no difference between exposures to CW and PW HFEMFs.     

Bioeffects induced by exposure to microwaves are mitigated by superposition of ELF noise

We have previously demonstrated that microwave fields, amplitude modulated (AM) by an extremely low-frequency (ELF) sine wave, can induce a nearly twofold enhancement in the activity of ornithine decarboxylase (ODC) in L929 cells at SAR levels of the order of 2.5 W/kg. Similar, although less pronounced, effects were also observed from exposure to a typical digital cellular phone test signal of the same power level, burst modulated at 50 Hz. We have also shown that ODC enhancement in L929 cells produced by exposure to ELF fields can be inhibited by superposition of ELF noise. In the present study, we explore the possibility that similar inhibition techniques can be used to suppress the microwave response. We concurrently exposed L929 cells to 60 Hz AM microwave fields or a 50 Hz burst-modulated DAMPS (Digital Advanced Mobile Phone System) digital cellular phone field at levels known to produce ODC enhancement, together with band-limited 30–100 Hz ELF noise with root mean square amplitude of up to 10 μT. All exposures were carried out for 8 h, which was previously found to yield the peak microwave response. In both cases, the ODC enhancement was found to decrease exponentially as a function of the noise root mean square amplitude. With 60 Hz AM microwaves, complete inhibition was obtained with noise levels at or above 2 μT. With the DAMPS digital cellular phone signal, complete inhibition occurred with noise levels at or above 5 μT. These results suggest a possible practical means to inhibit biological effects from exposure to both ELF and microwave fields. Bioelectromagnetics 18:422–430, 1997. © 1997 Wiley-Liss, Inc.     

Chromosome damage and micronucleus formation in human blood lymphocytes exposed in vitro to radiofrequency radiation at a cellular telephone frequency (847.74 MHz, CDMA)

Peripheral blood samples collected from four healthy nonsmoking human volunteers were diluted with tissue culture medium and exposed in vitro for 24 h to 847.74 MHz radiofrequency (RF) radiation (continuous wave), a frequency employed for cellular telephone communications. A code division multiple access (CDMA) technology was used with a nominal net forward power of 75 W and a nominal power density of 950 W/m(2) (95 mW/cm(2)). The mean specific absorption rate (SAR) was 4.9 or 5.5 W/kg. Blood aliquots that were sham-exposed or exposed in vitro to an acute dose of 1.5 Gy of gamma radiation were included in the study as controls. The temperatures of the medium during RF-radiation and sham exposures in the Radial Transmission Line facility were controlled at 37 +/- 0.3 degrees C. Immediately after the exposures, lymphocytes were cultured at 37 +/- 1 degrees C for 48 or 72 h. The extent of genetic damage was assessed from the incidence of chromosome aberrations and micronuclei. The kinetics of cell proliferation was determined from the mitotic indices in 48-h cultures and from the incidence of binucleate cells in 72-h cultures. The data indicated no significant differences between RF-radiation-exposed and sham-exposed lymphocytes with respect to mitotic indices, frequencies of exchange aberrations, excess fragments, binucleate cells, and micronuclei. The response of gamma-irradiated lymphocytes was significantly different from that of both RF-radiation-exposed and sham-exposed cells for all of these indices. Thus there was no evidence for induction of chromosome aberrations and micronuclei in human blood lymphocytes exposed in vitro for 24 h to 847.74 MHz RF radiation (CDMA) at SARs of 4.9 or 5.5 W/kg.     

Effect of cell phone radiofrequency radiation on body temperature in rodents: Pilot studies of the National Toxicology Program's reverberation chamber exposure system

Radiofrequency radiation (RFR) causes heating, which can lead to detrimental biological effects. To characterize the effects of RFR exposure on body temperature in relation to animal size and pregnancy, a series of short‐term toxicity studies was conducted in a unique RFR exposure system. Young and old B6C3F1 mice and young, old, and pregnant Harlan Sprague‐Dawley rats were exposed to Global System for Mobile Communication (GSM) or Code Division Multiple Access (CDMA) RFR (rats = 900 MHz, mice = 1,900 MHz) at specific absorption rates (SARs) up to 12 W/kg for approximately 9 h a day for 5 days. In general, fewer and less severe increases in body temperature were observed in young than in older rats. SAR‐dependent increases in subcutaneous body temperatures were observed at exposures ≥6 W/kg in both modulations. Exposures of ≥10 W/kg GSM or CDMA RFR induced excessive increases in body temperature, leading to mortality. There was also a significant increase in the number of resorptions in pregnant rats at 12 W/kg GSM RFR. In mice, only sporadic increases in body temperature were observed regardless of sex or age when exposed to GSM or CDMA RFR up to 12 W/kg. These results identified SARs at which measurable RFR‐mediated thermal effects occur, and were used in the selection of exposures for subsequent toxicology and carcinogenicity studies. Bioelectromagnetics. 39:190–199, 2018. © 2018 The Authors. Bioelectromagnetics Published by Wiley Periodicals, Inc.     

Acute low-intensity microwave exposure increases DNA single-strand breaks in rat brain cells

Levels of DNA single-strand break were assayed in brain cells from rats acutely exposed to low-intensity 2450 MHz microwaves using an alkaline microgel electrophoresis method. Immediately after 2 h of exposure to pulsed (2 μs width, 500 pulses/s) microwaves, no significant effect was observed, whereas a dose rate-dependent [0.6 and 1.2 W/kg whole body specific absorption rate (SAR)] increase in DNA single-strand breaks was found in brain cells of rats at 4 h postexposure. Furthermore, in rats exposed for 2 h to continuous-wave 2450 MHz microwaves (SAR 1.2 W/kg), increases in brain cell DNA single-strand breaks were observed immediately as well as at 4 h postexposure. © 1995 Wiley-Liss, Inc.     

Development of a rat head exposure system for simulating human exposure to RF fields from handheld wireless telephones

The aim of this project was to develop an animal exposure system for the biological effect studies of radio frequency fields from handheld wireless telephones, with energy deposition in animal brains comparable to those in humans. The finite‐difference time‐domain (FDTD) method was initially used to compute specific absorption rate (SAR) in an ellipsoidal rat model exposed with various size loop antennas at different distances from the model. A 3 × 1 cm rectangular loop produced acceptable SAR patterns. A numerical rat model based on CT images was developed by curve‐fitting Hounsfield Units of CT image pixels to tissue dielectric properties and densities. To design a loop for operating at high power levels, energy coupling and impedance matching were optimized using capacitively coupled feed lines embedded in a Teflon rod. Sprague Dawley rats were exposed with the 3 × 1 cm loop antennas, tuned to 837 or 1957 MHz for thermographically determined SAR distributions. Point SARs in brains of restrained rats were also determined thermometrically using fiberoptic probes. Calculated and measured SAR patterns and results from the various exposure configurations are in general agreement. The FDTD computed average brain SAR and ratio of head to whole body absorption were 23.8 W/kg/W and 62% at 837 MHz, and 22.6 W/kg/W and 89% at 1957 MHz. The average brain to whole body SAR ratio was 20 to 1 for both frequencies. At 837 MHz, the maximum measured SAR in the restrained rat brains was 51 W/kg/W in the cerebellum and 40 W/kg/W at the top of the cerebrum. An exposure system operating at 837 MHz is ready for in vivo biological effect studies of radio frequency fields from portable cellular telephones. Two‐tenths of a watt input power to the loop antenna will produce 10 W/kg maximum SAR, and an estimated 4.8 W/kg average brain SAR in a 300 g medium size rat. Bioelectromagnetics 20:75–92, 1999. © 1999 Wiley‐Liss, Inc.     

Low-level microwave irradiation and central cholinergic activity: a dose-response study

Rats were irradiated with circularly polarized, 2,450-MHz pulsed microwaves (2-microseconds pulses, 500 pulses per second [pps]) for 45 min in the cylindrical waveguide system of Guy et al:(Radio Sci 14:63-74, 1979). Immediately after exposure, sodium-dependent high-affinity choline uptake, an indicator of cholinergic activity in neural tissue, was measured in the striatum, frontal cortex, hippocampus, and hypothalamus. The power density was set to give average whole-body specific absorption rates (SAR) of 0.3, 0.45, 0.6, 0.75, 0.9, or 1.2 W/kg to study the dose-response relationship between the rate of microwave energy absorption and cholinergic activity in the different areas of the brain. Decrease in choline uptake was observed in the striatum at a SAR of 0.75 W/kg and above, whereas for the frontal cortex and hippocampus, decreases in choline uptake were observed at a SAR of 0.45 W/kg and above. No significant effect was observed in the hypothalamus at the irradiation power densities studied. The probit analysis was used to determine the SAR50 in each brain area, i.e., the SAR at which 50% of maximum response was elicited. SAR50 values for the striatum, frontal cortex, and hippocampus were 0.65, 0.38, and 0.44 W/kg, respectively.