Magnetic Fields Modulate Blue-Light-Dependent Regulation of Neuronal Firing by Cryptochrome

Abstract

Many animals are able to sense the Earth's geomagnetic field to enable behaviors such as migration. It is proposed that the magnitude and direction of the geomagnetic field modulates the activity of cryptochrome (CRY) by influencing photochemical radical pair intermediates within the protein. However, this proposal will remain theoretical until a CRY-dependent effect on a receptor neuron is shown to be modified by an external magnetic field (MF). It is established that blue-light (BL) photoactivation of CRY is sufficient to depolarize and activate Drosophila neurons. Here, we show that this CRY-dependent effect is significantly potentiated in the presence of an applied MF (100 mT). We use electrophysiological recordings from larval identified motoneurons, in which CRY is ectopically expressed, to show that BL-dependent depolarization of membrane potential and increased input resistance are markedly potentiated by an MF. Analysis of membrane excitability shows that these effects of MF exposure evoke increased action potential firing. Almost nothing is known about the mechanism by which a magnetically induced change in CRY activity might produce a behavioral response. We further report that specific structural changes to the protein alter the impact of the MF in ways that are strikingly similar to those from recent behavioral studies into the magnetic sense of Drosophila These observations provide the first direct experimental evidence to support the hypothesis that MF modulation of CRY activity is capable of influencing neuron activity to allow animal magnetoreception.

Significance statement: The biophysical mechanism of animal magnetoreception is still unclear. The photoreceptor protein cryptochrome has risen to prominence as a candidate magnetoreceptor molecule based on multiple reports derived from behavioral studies. However, the role of cryptochrome as a magnetoreceptor remains controversial primarily because of a lack of direct experimental evidence linking magnetic field (MF) exposure to a change in neuronal activity. Here, we show that exposure to an MF (100 mT) is sufficient to potentiate the ability of light-activated cryptochrome to increase neuronal action potential firing. Our results provide critical missing evidence to show that the activity of cryptochrome is sensitive to an external MF that is capable of modifying animal behavior.

Keywords: Drosophila; action potential; cryptochrome; depolarization; magnetic field; radical pair.

Figure 1.

MF exposure potentiates the BL-evoked depolarization of the membrane potential. A, Representative electrophysiological recordings from L3 aCC/RP2 ectopically expressing wild-type cry. Reversible changes in V_m were observed after BL exposure in presence or absence of a 100 mT MF. No detectable increase in V_m was exhibited by the control group (Gal4 ElaV^C155). B, Quantitative analysis of MF effect on the BL-induced V_m changes to control cells and cells expressing CRY. C, No difference in RMP, which was measured before light stimulation, was observed in any experimental group. All values shown are means ± SEM and n is shown in each bar. **p < 0.01.

Figure 2.

MF exposure potentiates the BL-evoked increase in input resistance. A, Series of current steps (4 pA/1 s) ranging from −12 to +8 pA applied before (gray traces) and during (black traces) BL exposure in the absence (−MF) and presence (+MF) of a 100 mT MF. B, C, Effect of BL exposure on input resistance in the absence or presence of the MF.

Figure 3.

MF exposure enhances action potential firing. A, Input–output relationship of L3 aCC/RP2 motoneurons determined by successively greater depolarizing current injections (4 pA steps, 0–60 pA/1 s) in the presence (dark blue) or absence (pale blue) of an MF (100 mT). All values shown are means ± SEM. B, Representative traces showing the firing of action potentials evoked by injection of a 48 pA current pulse.

Figure 4.

CRY variants show divergent sensitivity to MF exposure. A, Representative electrophysiological recordings from L3 aCC/RP2-expressing CRY variants: CRY^W342F and a C-terminal truncation (CRY^Δ14.6). Mutagenesis of W342 within the triad responsible for the RPM reveals that this residue is not essential for the MF response. Conversely, deletion of the CRY C-terminus dramatically affects the MF-dependent potentiation without compromising photoactivation by BL. B, Quantitative analysis of MF effect on the BL-induced V_m changes to cells expressing CRY variants. C, RMP measured before light stimulation. All values shown are means ± SEM and n is shown in each bar. **p < 0.01.