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Static, Low-Frequency, and Pulsed Magnetic Fields
3.3.1 In Vivo Studies
The use of animal models to determine the magnetic field effects has prompted a
variety of methodologies and provided important evidence, as shown in the following sections: Biological Sensing and Magnetite (3.3.1.1); Behavior and Recognition
(3.3.1.2); Occurance of Analgesia (3.3.1.3); Reproduction and Development (3.3.1.4);
Circulatory System Effects (3.3.1.5); Neuroendocrine, Visual, and Neurophysical
Systems (3.3.1.6); Musculoskeletal System (3.3.1.7); Skeletal System (3.3.1.8); In vivo
Genotoxicity (3.3.1.9).
3.3.1.1 Biological Sensing and Magnetite
The geomagnetic field has been used by some bacteria and animals for navigation
purposes, although it is a quasistatic and quasiuniform magnetic field ranging about
25–65 μT in unperturbed space. Ritz, Adem, and Schulten (2000) postulated the possibility that magnetoreception involves radical pair processes as a biophysical mechanism. They first considered a system of radical pairs as a model for the magnetic sensory
organ and evaluated the influence of the geomagnetic field on this radical pair system.
European robins (Erithacus rubecula) were used in this study, and the results showed
a disruption of magnetic orientation in robins when they were exposed to a vertically
aligned broadband field of 0.1–10.0 MHz and 0.085 mT or the single frequency of 7 MHz
and 0.47 mT together with the geomagnetic field (Ritz et al. 2004). The disorientation
observed was found to depend on the angle between the 7 MHz oscillating field and the
geomagnetic field. The robins oriented in the migratory direction when the oscillating
field was parallel to the geomagnetic field. The authors suggested a magnetic compass
based on a radical pair mechanism, due to the resonance effect on singlet–triplet (S–T)
transitions in oscillating fields.
It has been reported that chemical magnetoreception via the radical pair mechanism
requires in principle the following sequence of three events: (1) generation of a radical
pair species with correlated electron spins, either in an S or a T state; (2) coherent evolution of the radical pair between the near-degenerate S and T spin states; and (3) different
reaction pathways of the S and T radical pairs. The frequency of S–T interconversion in
the radical pair and hence the relative yields of reaction products and/or the lifetime of
the radical pair depend on the strength of any applied magnetic field. Magnetic field
effects in radical pairs are thus kinetic rather than thermodynamic in origin and may
be detected for magnetic fields whose electron Zeeman energies are much smaller than
the average thermal energy per molecule, k B T. The radical pair lifetime needs to be in
the microseconds range (Cintolesi et al. 2003) if a magnetic field of 50 μT is expected to
have a significant effect on it.
Henbest et al. (2008) demonstrated a magnetic field effect on the photochemical yield
of a flavin–tryptophan radical pair in Escherichia coli photolyase. This result provides
proof for the principle that photolyases, and most likely CRYs by extension, have the
fundamental properties required to form the basis of a magnetic compass. Gegear et al.
(2008, 2010) suggested that animal CRY mediates light-dependent magnetoreception
through an unconventional photochemical mechanism using Drosophila transgenesis,
and CRY-dependent magnetosensitivity does not require a functioning circadian clock.
Static, Low-Frequency, and Pulsed Magnetic Fields
3.3.1 In Vivo Studies
The use of animal models to determine the magnetic field effects has prompted a
variety of methodologies and provided important evidence, as shown in the following sections: Biological Sensing and Magnetite (3.3.1.1); Behavior and Recognition
(3.3.1.2); Occurance of Analgesia (3.3.1.3); Reproduction and Development (3.3.1.4);
Circulatory System Effects (3.3.1.5); Neuroendocrine, Visual, and Neurophysical
Systems (3.3.1.6); Musculoskeletal System (3.3.1.7); Skeletal System (3.3.1.8); In vivo
Genotoxicity (3.3.1.9).
3.3.1.1 Biological Sensing and Magnetite
The geomagnetic field has been used by some bacteria and animals for navigation
purposes, although it is a quasistatic and quasiuniform magnetic field ranging about
25–65 μT in unperturbed space. Ritz, Adem, and Schulten (2000) postulated the possibility that magnetoreception involves radical pair processes as a biophysical mechanism. They first considered a system of radical pairs as a model for the magnetic sensory
organ and evaluated the influence of the geomagnetic field on this radical pair system.
European robins (Erithacus rubecula) were used in this study, and the results showed
a disruption of magnetic orientation in robins when they were exposed to a vertically
aligned broadband field of 0.1–10.0 MHz and 0.085 mT or the single frequency of 7 MHz
and 0.47 mT together with the geomagnetic field (Ritz et al. 2004). The disorientation
observed was found to depend on the angle between the 7 MHz oscillating field and the
geomagnetic field. The robins oriented in the migratory direction when the oscillating
field was parallel to the geomagnetic field. The authors suggested a magnetic compass
based on a radical pair mechanism, due to the resonance effect on singlet–triplet (S–T)
transitions in oscillating fields.
It has been reported that chemical magnetoreception via the radical pair mechanism
requires in principle the following sequence of three events: (1) generation of a radical
pair species with correlated electron spins, either in an S or a T state; (2) coherent evolution of the radical pair between the near-degenerate S and T spin states; and (3) different
reaction pathways of the S and T radical pairs. The frequency of S–T interconversion in
the radical pair and hence the relative yields of reaction products and/or the lifetime of
the radical pair depend on the strength of any applied magnetic field. Magnetic field
effects in radical pairs are thus kinetic rather than thermodynamic in origin and may
be detected for magnetic fields whose electron Zeeman energies are much smaller than
the average thermal energy per molecule, k B T. The radical pair lifetime needs to be in
the microseconds range (Cintolesi et al. 2003) if a magnetic field of 50 μT is expected to
have a significant effect on it.
Henbest et al. (2008) demonstrated a magnetic field effect on the photochemical yield
of a flavin–tryptophan radical pair in Escherichia coli photolyase. This result provides
proof for the principle that photolyases, and most likely CRYs by extension, have the
fundamental properties required to form the basis of a magnetic compass. Gegear et al.
(2008, 2010) suggested that animal CRY mediates light-dependent magnetoreception
through an unconventional photochemical mechanism using Drosophila transgenesis,
and CRY-dependent magnetosensitivity does not require a functioning circadian clock.