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Bioelectromagnetism

 

magnetoreception (Marley et al., 2014). Tey inferred that this study paves the way for assessing the

infuence of the amplitude and orientation of the GMF intensity in the μT range on seizure duration in

Drosophila larvae (Marley et al., 2014).

Tus, both the “magnetite theory” and the “CRY theory” (CRY-based RPM theory) are powerful theo­

ries, and there is evidence that some organisms may use a combination of these two mechanisms (reviewed

by Roberts, 2016; Wiltschko and Wiltschko, 2021), but there are many unclear points. Recently, protein

complex, so-called “MagR/CRY complex,” which is present in retinal cells such as pigeons, has been found

to behave like a magnetic compass (Qin et al., 2016). Briefy, the complex composed of the above-mentioned

blue-light photoreceptor protein CRY and the protein-coding by the CG8198 gene, called the magnetic

receptive protein (MagR), has an extremely rare property of responding and orienting to weak MFs similar

to that of the GMF (Qin et al., 2016). Due to this property, the MagR/CRY complex has been noted as a

potential causative agent of animal magnetic sensitivity. Qin et al. (2016) showed through biochemical and

biophysical methods that the MagR/CRY complex is stable in the retina of pigeons and can also form in

butterfies, rats, whales, and human retinal cells. Tey note that it is unclear how the MagR/CRY complex

can sense MFs and whether the MagR/CRY complex is involved in animal magnetic sensing (Qin et al.,

2016). However, the discovery of protein complexes for magnetic compasses has the potential to create a

wide range of new approaches to MF-induced macromolecular manipulation, as well as cell behavior.

Pang et al. (2017) investigated that MagR expression alone could achieve cellular activation by SMFs

up to 1.2 mT. Despite systematically testing diferent ways of measuring intracellular calcium and dif­

ferent SMF exposure protocols, it was not possible to detect any cellular or neuronal responses to SMF

exposure in MagR-expressing HEK cells or primary neurons from the dorsal root ganglion and the

hippocampus (Pang et al., 2017). By contrast, in neurons, co-expressing MagR and channel rhodopsin,

artifcial visible light but not SMF exposure increased calcium infux ([Ca²⁺]i) in hippocampal neurons

(Pang et al., 2017). Te discovery that MagR/CRY is a putative magneto-responsive protein complex

does not directly imply that MagR itself may induce a neuronal response in transfected cells (Pang

et al., 2017). While the possibility exists that MagR, when associated with other proteins such as CRY

or linked to other channels such as TRV4 may be used for magnetogenetics, these results suggested that

more factors seem necessary, in addition to the expression of MagR alone, for MagR to be used as a tool

for neuronal modulation via MFs (Pang et al., 2017). Moreover, regarding other prospects, the search

for the molecular principle of the signaling mechanism from the receptor molecule or structure to the

activated neural cell remains the cardinal question of current magnetoreception research (Vácha, 2017).

Nordmann et al. (2017) assumed that magnetoreception relies on a single receptor, or indeed a single

mechanism. At conferences, CRY ofen faces magnetite on the scientifc battlefeld, but this confict is

illusionary (Nordmann et al., 2017). Selective pressure in diverse environments, from the oceans to the

air, may have facilitated the evolution of a multiplicity of magnetoreceptors (Nordmann et al., 2017).

Tis mystery might therefore have more than one solution (Nordmann et al., 2017).

In another aspect, in the case of elasmobranch magnetoreception in sharks, skates, and rays, “EM

induction theory” has been proposed as a feasible mechanism by which elasmobranchs could perceive

the GMF through the electroreceptor system, so-called “ampullae of Lorenzini” (frst described by

Murray, 1960). Elasmobranchs, which are evolutionarily much older than teleosts, have the ampullae of

Lorenzini, and perceive the electric current induced by the fsh itself and the electric current induced by

the water movement when the fsh or the water moves in a constant MF or SMF (Brown and Ilyinsky,

1978). However, it is not completely understood if the electroreceptor system receives only electric infor­

mation, or if it can receive magnetic information as well (Formicki et al., 2019).

According to the principles of the active mode of EM induction (Kalmijn, 1981, 1982; Paulin, 1995;

Molteno and Kennedy, 2009), it is the horizontal polarity component of the GMF that induces vertical

electric felds (EFs) that could convey information regarding MF directionality. “Te EM induction-

based magnetoreceptor system” is used to gain a compass heading regarding the direction of travel.

More recently, Keller et al. (2021) conducted magnetic displacement experiments on wild-caught bon­

nethead sharks (Sphyrna tiburo) and show that magnetic map cues can elicit homeward orientation.