On the confirmation of an effect of magnetic fields on the interictal firing rate of epileptic patients

A consensus panel review of central nervous system effects of the exposure to low-intensity extremely low-frequency magnetic fields

BACKGROUND

A large number of studies explored the biological effects of extremely low-frequency (0-300 Hz) magnetic fields (ELF-MFs) on nervous system both at cellular and at system level in the intact human brain reporting several functional changes. However, the results of different studies are quite variable and the mechanisms of action of ELF-MFs are still poorly defined. The aim of this paper is to provide a comprehensive review of the effects of ELF-MFs on nervous system.

METHODS

We convened a workgroup of researchers in the field to review and discuss the available data about the nervous system effects produced by the exposure to ELF-MFs.

MAIN FINDINGS/DISCUSSION

We reviewed several methodological, experimental and clinical studies and discussed the findings in five sections. The first section analyses the devices used for ELF-MF exposure. The second section reviews the contribution of the computational methods and models for investigating the interaction between ELF-MFs and neuronal systems. The third section analyses the experimental data at cellular and tissue level showing the effects on cell membrane receptors and intracellular signaling and their correlation with neural stem cell proliferation and differentiation. The fourth section reviews the studies performed in the intact human brain evaluating the changes produced by ELF-MFs using neurophysiological and neuropsychological methods. The last section shows the limits and shortcomings of the available data, evidences the key challenges in the field and tracks directions for future research.

Exposure to ELF magnetic and ELF-modulated radiofrequency fields: the time course of physiological and cognitive effects observed in recent studies (2001-2005)

In 2002, we published a review of the cognitive and physiological effects of extremely low frequency magnetic fields (ELF MFs) and ELF-modulated radiofrequency fields associated with mobile phones. Since the original preparation of that review, a significant number of studies have been published using techniques such as electroencephalography, event-related potentials and positron emission tomography to investigate electromagnetic field effects upon human physiology and various measures of performance (cognitive, perceptual, behavioral). We review these recent studies, and when effects were observed, we reference the time course of observed effects (immediate or delayed). In our concluding remarks, we discuss a number of variables that are not often considered in human bioelectromagnetics studies, such as personality, individual differences and the specific laterality of ELF MF and mobile phone exposure over the brain. We also consider the sensitivity of various physiological assays and performance measures in the study of biological effects of electromagnetic fields.

Evidence of a nonlinear human magnetic sense

Human subjects respond to low-intensity electric and magnetic fields. If the ability to do so were a form of sensory transduction, one would expect that fields could trigger evoked potentials, as do other sensory stimuli. We tested this hypothesis by examining electroencephalograms from 17 subjects for the presence of evoked potentials caused by the onset and by the offset of 2 G, 60 Hz (a field strength comparable to that in the general environment). Both linear (time averaging) and nonlinear (recurrence analysis) methods of data analysis were employed to permit an assessment of the dynamical nature of the stimulus/response relationship. Using the method of recurrence analysis, magnetosensory evoked potentials (MEPs) in the signals from occipital derivations were found in 16 of the subjects (P<0.05 for each subject). The potentials occurred 109-454 ms after stimulus application, depending on the subject, and were triggered by onset of the field, offset of the field, or both. Using the method of time averaging, no MEPs were detected. MEPs in the signals from the central and parietal electrodes were found in most subjects using recurrence analysis, but no MEPs were detected using time averaging. The occurrence of MEPs in response to a weak magnetic field suggested the existence of a human magnetic sense. In distinction to the evoked potentials ordinarily studied, MEPs were nonlinearly related to the stimulus as evidenced by the need to employ a nonlinear method to detect the responses.

Modification of the synaptic glutamate turnover in the hippocampal tissue exposed to low-frequency, pulsed magnetic fields

The influence of pulsed magnetic fields (PMF) on the release and uptake of glutamate was investigated. While the release was examined using hippocampal slices, synaptosomes were chosen to characterize the uptake process. (3)H-D-aspartate was used as a marker of glutamergic transmission. The pulsed magnetic fields (9-15 mT) applied according to the pattern which induced epileptic discharges in hippocampus amplified and attenuated the release and uptake of glutamate, respectively. However, the magnetic fields which induced an increase in neuronal excitability without concomitant seizures amplified both processes. These results confirm our previous reports and indicate that the glutamergic synapses are the target of magnetic fields action.

Amplification of evoked potentials recorded from mouse hippocampal slices by very low repetition rate pulsed magnetic fields

The influence of low repetition rate pulsed magnetic fields (LRMF) on the evoked potential (population spike) recorded from mouse hippocampal slices was investigated. LRMF were applied according to two protocols. In protocol A, LRMF applied with a constant strength (15 mT) and frequency ranging from 0.03 to 0.5 Hz resulted in an amplification of the potential. Although the frequency of 0.16 Hz was the most effective, enhancing the population spike by over 280%, it also caused an increase in spontaneous activity, seizures, and cessation of neuronal activity in 50% of the slices. In protocol B, LRMF were applied with a variable intensity (9-15 mT) and in cycles of different duration ranging from 5 to 20 min. While an increase in the amplitude of the population spike was observed in all slices exposed to LRMF applied according to protocol B, the longest exposure was the most effective. Neither seizures nor an increase in the spontaneous activity were observed in this group of the slices. These results support and extend our previous data and characterize further the relation between the pattern of applied magnetic fields and their influence on the nervous system.

An increase in cAMP concentration in mouse hippocampal slices exposed to low-frequency and pulsed magnetic fields

Our previous studies revealed that magnetic fields amplified evoked potentials recorded from mouse hippocampal slices. In search for the mechanism of this effect, we evaluated the concentration of cAMP in slices exposed to low-frequency and pulsed magnetic fields. Low-frequency magnetic fields of 15 mT applied at 0.16 Hz for 30 min enhanced the concentration of cAMP almost three-fold. The concentration of cAMP continued to rise through the first hour after turning magnetic fields off, reaching almost a four-fold increase, and then returned to control levels at the end of the second hour. Neither static magnetic fields nor magnetic fields applied with the frequency of 0.5 Hz had any effect on cAMP concentration. The increase in cAMP levels was dependent on the strength of the magnetic field and required the presence of extracellular calcium. A pulsed magnetic field applied with variable intensity (9-15 mT) and in cycles lasting from 5 to 20 min doubled the cAMP concentration. These results support our previous electrophysiological observations and provide biochemical correlates for their interpretation.