The detection threshold for extremely low frequency magnetic fields may be below 1000 nT‐Hz in mice

Abstract

Previous experiments with mice have shown that a repeated 1 h daily exposure to an ambient magnetic field shielded environment induces analgesia (anti-nociception). This shielding reduces ambient static and extremely low frequency magnetic fields (ELF-MF) by approximately 100 times for frequencies below 120 Hz. To determine the threshold of ELF-MF amplitude that would attenuate or abolish this effect, 30 and 120 Hz magnetic fields were introduced into the shielded environment at peak amplitudes of 25, 50, 100 and 500 nT. At 30 Hz, peak amplitudes of 50, 100, and 500 nT attenuated this effect in proportion to the amplitude magnitude. At 120 Hz, significant attenuation was observed at all amplitudes. Exposures at 10, 60, 100, and 240 Hz with peak amplitudes of 500, 300, 500, and 300 nT, respectively, also attenuated the induced analgesia. No exposure abolished this effect except perhaps at 120 Hz, 500 nT. If the peak amplitude frequency product was kept constant at 6000 nT-Hz for frequencies of 12.5, 25, 50, and 100 Hz, the extent of attenuation was constant, indicating that the detection mechanism is dependent on the nT-Hz product. A plot of effect versus the induced current metric nT-Hz suggests a threshold of ELF-MF detection in mice at or below 1000 nT-Hz.

Magnetoreception in laboratory mice: sensitivity to extremely low-frequency fields exceeds 33 nT at 30 Hz

“…This is consistent with fractional attenuation values of 0.44 for 1500 nT-Hz (at 30 Hz) and 0.60 for 3000 nT-Hz (at 30 Hz) previously reported by our group [19]. Results from experiment 2 (figure 2) indicate an attenuation in induced analgesia of 60 per cent for an exposure of 990 nT-Hz (table 4), which seems high based on previous work.…”

“…The magnitude of the magnetic field as a function of frequency in the animal housing room and for the testing room has been previously reported [19]. The static field was approximately 45 µT in magnitude, and the ELF spectrum shows peaks at 60, 120, 180 and 240 Hz, which combined have an amplitude of approximately 0.15 µT.…”

“…Here we show, using a laboratory mouse model in which magnetic sensitivity may be driven by the same mechanism as that for bird orientation and homing [19,20], that sensitivity to tens of nanotesla does occur. This is consistent with the concept that animals capable of using the geomagnetic field for orientation and homing perceive the spatial variation in the static field as an induced wELFMF as the animal traverses that spatial variation.…”

“…This response is probably opioid-mediated and is sensitive to the intensity and wavelength of light [20], similar to some aspects of bird navigation [22]. Using this experimental protocol, we estimated in a preliminary pilot experiment in 20 animals that nociceptive behaviour effects may be detected for 30 Hz fields possibly as low as 30 nT [19]. Here, we show that (i) the effect at 30 Hz, 30 nT is robust, because it has been confirmed in 120 animals, (ii) the induction of analgesia is definitely not related to the elimination of ambient electric fields, and (iii) the induction of analgesia may also be partially attenuated by the introduction of a static field of geomagnetic amplitude (44 µT ± spatial variability of 7%).…”

“…Here, we show that (i) the effect at 30 Hz, 30 nT is robust, because it has been confirmed in 120 animals, (ii) the induction of analgesia is definitely not related to the elimination of ambient electric fields, and (iii) the induction of analgesia may also be partially attenuated by the introduction of a static field of geomagnetic amplitude (44 µT ± spatial variability of 7%). Earlier work [19] suggested an induced current mechanism based on the observation that the size of the effect of the introduced ELF magnetic fields was dependent on the product of the ELF frequency and the ELF magnitude. However, how such weak fields could induce sufficient current remains a mystery [23].…”