Radical-pair-based magnetoreception in birds: radio-frequency experiments and the role of cryptochrome

Abstract

The radical-pair hypothesis of magnetoreception has gained a lot of momentum, since the flavoprotein cryptochrome was postulated as a structural candidate to host magnetically sensitive chemical reactions. Here, we first discuss behavioral tests using radio-frequency magnetic fields (0.1–10 MHz) to specifically disturb a radical-pair-based avian magnetic compass sense. While disorienting effects of broadband RF magnetic fields have been replicated independently in two competing labs, the effects of monochromatic RF magnetic fields administered at the electronic Larmor frequency (~1.3 MHz) are disparate. We give technical recommendations for future RF experiments. We then focus on two candidate magnetoreceptor proteins in birds, Cry1a and Cry1b, two splice variants of the same gene (Cry1). Immunohistochemical studies have identified Cry1a in the outer segments of the ultraviolet/violet-sensitive cone photoreceptors and Cry1b in the cytosol of retinal ganglion cells. The identification of the host neurons of these cryptochromes and their subcellular expression patterns presents an important advance, but much work lies ahead to gain some functional understanding. In particular, interaction partners of cryptochrome Cry1a and Cry1b remain to be identified. A candidate partner for Cry4 was previously suggested, but awaits independent replication.

Electromagnetic 0.1–100 kHz noise does not disrupt orientation in a night-migrating songbird implying a spin coherence lifetime of less than 10 µs

“…Several previous studies of the effects of RF fields on magnetic compass orientation have resulted in seemingly contradictory findings ( [29,31,33], but see [35]). The possible reasons behind these discrepancies are debatable and could include differences in the sample sizes, as well as equipment and measurement quality [3,13,30,37]. In our study, we provide: (i) all the parameters of the static magnetic fields and their homogeneity (see §2.2); (ii) the measurements of the electric and magnetic field components in the range up to 100 MHz both for control and experimental conditions (figure 2); (iii) the amplitude of the RF field measured both in average and maxhold modes (figures 1 and 2); (iv) root-mean-square RF noise intensities to allow comparison with RF fields that have different frequency ranges (see §2.3); (v) the spectral amplitude in T (√Hz) −1 and not simply T. The latter is meaningless without also quoting the measurement bandwidth.…”

“…However, the reported all-or-nothing effect of monochromatic fields on magnetic orientation was recently challenged, when Schwarze et al [35] and Malkemper et al [36] did not observe an expected effect of the Larmor frequency field. The possible reasons behind these discrepancies in the results are debatable and could include differences in the sample sizes, as well as equipment and measurement quality [3,13,30,37].…”

Retinal photoreceptor and ganglion cell types and topographies in the red fox (Vulpes vulpes) and Arctic fox(Vulpes lagopus)

“…The magnetoreception may be light‐dependent and based on the flavoprotein cryptochrome 1 (Cry1) located in the eye, because, similar to the situation in birds (Denzau, Nießner, Rogers, & Wiltschko, ; Nießner et al, ), all S cones (the homologues of bird UV/V cones) of the red fox and Arctic fox contain Cry1 in their outer segments (Nießner et al, ). Cryptochromes are prime candidates for the receptor molecule of the light‐dependent, radical pair based magnetic compass in vertebrates, including mammals (Malkemper et al, ; Phillips et al, ), albeit with some reservations (for critical discussions see Hore & Mouritsen, ; Nießner & Winklhofer, ). In birds it has been hypothesized that the effect of the Earth's magnetic field on retinal cryptochromes leads to a visual perception of the magnetic field, for example, as a pattern superimposed on the normal visual surroundings (Ritz, Adem, & Schulten, ; Solov'yov, Mouritsen, & Schulten, ).…”

Quantum Biology: Does quantum physics hold the key to revolutionizing medicine?

“…At present, the exact nature of the radical pair that could be involved in this mechanism has yet to be determined, however, radical pair reaction is known to occur within the cryptochromes of the eye; and it is thought to be highly probable that this could be represent the means by which incoming light could be the trigger for both vision as well as navigation using the magnetic field [166] . Using simple models based on the these sort of radial pairs, based on pairs with highly anisotropic spin configurations, it has been demonstrated in principle that the reaction products of the radical-pair reaction described is sensitive to the direction of an external magnetic field; and is capable of reproducing the disruptive effect of time-dependent external magnetic fields at frequencies in the radiowave portion of the electromagnetic spectrum [163] .…”