Effects of weak static magnetic fields on the gene expression of seedlings of Arabidopsis thaliana

Magnetic-field reception of animals and plants is currently discussed in the framework of a cryptochrome-based radical-pair mechanism. Efforts to unravel magnetoreception in plants suffered historically from several shortcomings, most prominently, the conspicuous absence of detailed stimulus-response relationships. To determine the sensitivity of seedlings of Arabidopsis thaliana to weak static magnetic fields we generated stimulus-response curves between near zero and 188 μT for the transcript levels of the genes rbcl, cab4, pal4 and ef1. The moderate magneto-responsiveness of dark-grown seedlings was greatly enhanced under blue light, and for rbcl and pal4 also under red light. The stimulus-response curves obtained under blue light of constant photon-fluence rate displayed multiple maxima and thus a pattern fundamentally different from that prevalent in plant and animal physiology. A double mutant lacking cryptochromes 1 and 2 displayed altered stimulus-response curves without losing, however, magneto-responsiveness completely. A reversal of the magnetic field direction substantially affected the gene expression and the quantity of CAB-protein (chlorophyll a,b-binding protein). The majority of our results are at variance with the notion of cryptochromes acting as the only magnetic-field sensors. They do not, however, exclude the possibility that cryptochromes participate in the magnetic field reception of Arabidopsis. The findings have the unexpected implication that cryptochrome- and phytochrome-mediated plant responses can be modulated by the strength and the orientation of the local geomagnetic field.

Differential magnetic field effects on heart rate and nociception in anosmic pigeons

Several studies have shown that exposure to altered magnetic fields affects nociception by suppressing stress-induced hypoalgesia, and that this effect is reduced or abolished if the treatment is performed in the absence of light. This raises the question as to whether other sources of sensory stimuli may also modulate these magnetic effects. We investigated the possible role of olfaction in the magnetically induced effects on sensitivity to nociceptive stimuli and heart rate (HR) in restraint-stressed homing pigeons exposed to an Earth-strength, irregularly varying (<1 Hz) magnetic field. The magnetic treatment decreased the nociceptive threshold in normally smelling birds and an opposite effect was observed in birds made anosmic by nostril plugging. Conversely, no differential effect of olfactory deprivation was observed on HR, which was reduced by the magnetic treatment both in smelling and anosmic pigeons. The findings highlight an important role of olfactory environmental information in the mediation of magnetic effects on nociception, although the data cannot be interpreted unambiguously because of the lack of an additional control group of olfactory-deprived, non-magnetically exposed pigeons. The differential effects on a pigeon's sensitivity to nociceptive stimulus and HR additionally indicate that the magnetic stimuli affect nociception and the cardiovascular system in different ways.

Light alters nociceptive effects of magnetic field shielding in mice: intensity and wavelength considerations

Previous experiments with mice have shown that repeated 1 hour daily exposure to an ambient magnetic field-shielded environment induces analgesia (antinociception). The exposures were carried out in the dark (less than 2.0x1016 photonss-1m-2) during the mid-light phase of the diurnal cycle. However, if the mice were exposed in the presence of visible light (2.0x1018 photonss-1m-2, 400-750 nm), then the analgesic effects of shielding were eliminated. Here, we show that this effect of light is intensity and wavelength dependent. Introduction of red light (peak at 635 nm) had little or no effect, presumably because mice do not have photoreceptors sensitive to red light above 600 nm in their eyes. By contrast, introduction of ultraviolet light (peak at 405 nm) abolished the effect, presumably because mice do have ultraviolet A receptors. Blue light exposures (peak at 465 nm) of different intensities demonstrate that the effect has an intensity threshold of approximately 12% of the blue light in the housing facility, corresponding to 5x1016 photonss-1m-2 (integral). This intensity is similar to that associated with photoreceptor-based magnetoreception in birds and in mice stimulates photopic/cone vision. Could the detection mechanism that senses ambient magnetic fields in mice be similar to that in bird navigation?

Pain perception and electromagnetic fields

A substantial body of evidence has accumulated showing that exposure to electromagnetic fields (EMFs) affects pain sensitivity (nociception) and pain inhibition (analgesia). Consistent inhibitory effects of acute exposures to various EMFs on analgesia have been demonstrated in most studies. This renders examinations of changes in the expression of analgesia and nociception a particularly valuable means of addressing the biological effects of and mechanisms underlying the actions of EMFs. Here we provide an overview of the effects of various EMFs on nociceptive sensitivity and analgesia, with particular emphasis on opioid-mediated responses. We also describe the analgesic effects of particular specific EMFs, the effects of repeated exposures to EMFs and magnetic shielding, along with the dependence of EMF effects on lighting conditions. We further consider some of the underlying cellular and biophysical mechanisms along with the clinical implications of these effects of various EMFs.

Daily repeated magnetic field shielding induces analgesia in CD-1 mice

We have recently observed that a single exposure of mice to a magnetically shielded environment can attenuate opioid induced analgesia. Here, we report the effect of repeated exposures to the same magnetically shielded environment. Adult male Swiss CD-1 mice were placed in a Mu-metal lined box or an opaque Plexiglas box (sham condition) for 1 h per day for 10 consecutive days. Nociception was measured as the latency time to a foot lift/lick in response to an aversive thermal stimulus (hotplate analgesiometer, 50 +/- 1 degrees C) before and immediately after exposure. Multiple experiments were conducted in which thermal latency was tested on each of the 10 days or on days 1, 5, and 10, with some utilizing post-exposure testing only. It was shown that mice can detect and will respond to the repeated absence of the ambient magnetic field, with a maximum analgesic response occurring over days 4-6 of exposure and returning to baseline thereafter. The effect was robust, independent of pre-exposure and intermittent testing, and seems to be opioid related, since the results obtained on day 5 were similar to those from a 5 mg/kg dose of morphine and were abolished with the opioid antagonist, naloxone.

Light alters nociceptive effects of magnetic field shielding

Orientation and nociception (pain sensitivity) are affected by exposure to geomagnetic or low frequency (<1,000 Hz) magnetic fields of approximately the earth's field strength, i.e., 50 microT. However, these effects are often dependent on the simultaneous presence of visible light. Recently, it was shown that nociception was affected in mice acutely exposed to an electromagnetic-shielded environment in the dark (<0.05 W/m(2)) during the mid-light phase of the diurnal cycle. Here, we report for the first time that if mice are exposed to magnetic shielding in the presence of visible light (0.6 W/m(2), 400-750 nm) that most of the effects of shielding are eliminated. This simple experimental protocol may be useful in investigating the role that light plays in the detection of ambient electromagnetic fields.

Extremely low frequency magnetic fields can either increase or decrease analgaesia in the land snail depending on field and light conditions

Results of prior investigations with opioid peptide mediated antinociception or analgaesia have suggested that these extremely low frequency (ELF) magnetic field effects are described by a resonance mechanism rather than mechanisms based on either induced currents or magnetite. Here we show that ELF magnetic fields (141-414 microT peak) can, in a manner consistent with the predictions of Lednev's parametric resonance model (PRM) for the calcium ion, either (i) reduce, (ii) have no effect on, or (iii) increase endogenous opioid mediated analgaesia in the land snail, Cepaea nemoralis. When the magnetic fields were set to parameters for the predictions of the PRM for the potassium ion, opioid-peptide mediated analgaesia increased and there was evidence of antagonism by the K(+) channel blocker, glibenclamide. Furthermore, these effects were dependent on the presence of light; the effects were absent in the absence of light. These observed increases and decreases in opioid analgaesia are largely consistent with the predictions of Lednev's PRM.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

The case for light-dependent magnetic orientation in animals

Light-dependent models of magnetoreception have been proposed which involve an interaction between the magnetic field and either magnetite particles located within a photoreceptor or excited states of photopigment molecules. Consistent with a photoreceptor-based magnetic compass mechanism, magnetic orientation responses in salamanders, flies and birds have been shown to be affected by the wavelength of light. In birds and flies, it is unclear whether the effects of light on magnetic orientation are due to a direct effect on a magnetoreception system or to a nonspecific (e.g. motivational) effect of light on orientation behavior. Evidence from shoreward-orienting salamanders, however, demonstrates that salamanders perceive a 90 degrees counterclockwise shift in the direction of the magnetic field under long-wavelength (&gt;=500 nm) light. A simple physiological model based on the antagonistic interaction between two magnetically sensitive spectral mechanisms suggests one possible way in which the wavelength-dependent effects of light on the salamander's magnetic compass response might arise. Assuming that the wavelength-dependent characteristics of the avian magnetic response can be attributed to an underlying magnetoreception system, we discuss several hypotheses attempting to resolve the differences observed in the wavelength-dependent effects of light on magnetic orientation in birds and salamanders. By considering the evidence in the context of photoreceptor- and non-photoreceptor-based mechanisms for magnetoreception, we hope to encourage future studies designed to distinguish between alternative hypotheses concerning the influence of light on magnetoreception.