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

Abstract

Magnetoreception in the animal kingdom has focused primarily on behavioural responses to the static geomagnetic field and the slow changes in its magnitude and direction as animals navigate/migrate. There has been relatively little attention given to the possibility that weak extremely low-frequency magnetic fields (wELFMF) may affect animal behaviour. Previously, we showed that changes in nociception under an ambient magnetic field-shielded environment may be a good alternative biological endpoint to orientation measurements for investigations into magnetoreception. Here we show that nociception in mice is altered by a 30 Hz field with a peak amplitude more than 1000 times weaker than the static component of the geomagnetic field. When mice are exposed to an ambient magnetic field-shielded environment 1 h a day for five consecutive days, a strong analgesic (i.e. antinociception) response is induced by day 5. Introduction of a static field with an average magnitude of 44 µT (spatial variability of ±3 µT) marginally affects this response, whereas introduction of a 30 Hz time-varying field as weak as 33 nT has a strong effect, reducing the analgesic effect by 60 per cent. Such sensitivity is surprisingly high. Any purported detection mechanisms being considered will need to explain effects at such wELFMF.

Figure 1.

Pre- (open circles) and post-exposure (filled circles) latencies for 30 Hz, 65 nT as well as fibreglass sham, stainless steel control and 0 nT (mu-metal positive control). The five entries for each of the eight conditions refer to the five experimental days. Error bars correspond to ±standard error of the mean (s.e.m.). Where s.e.m. bars are not evident, they fall within the symbols. The ANOVA analysis showed a significant three-way interaction between day (5, repeated) by pre–post (2, repeated) by condition (8, independent) (F_2,28 = 5.30, p < 0.001, η² = 0.14).

Figure 2.

Pre- (open circles) and post-exposure (filled circles) latencies for 30 Hz, 65 nT and four 30 Hz, 33 nT exposures as well as a fibreglass sham, a stainless steel control and a 0 nT mu-metal positive control. The five entries for each of the eight conditions refer to the five experimental days. Error bars correspond to±standard error of the mean (s.e.m.). Where s.e.m. bars are not evident, they fall within the symbols. The ANOVA analysis showed a significant three-way interaction between day (5, repeated) by pre–post (2, repeated) by condition (8, independent) (F_2,28 = 4.00, p < 0.001, η² = 0.11).

Figure 3.

Pre- (open circles) and post-exposure (filled circles) latencies for a 44 µT static field exposure as well as a fibreglass sham, a stainless steel control and a 0 nT mu-metal positive control. The five entries for each of the four conditions refer to the five experimental days. Error bars correspond to ±standard error of the mean (s.e.m.). Where SEM bars are not evident, they fall within the symbols. The ANOVA analysis showed a significant three-way interaction between day (5, repeated) by pre–post (2, repeated) by condition (4, independent) (F_2,38 = 5.98, p < 0.001, η² = 0.13).