Magnetoreception in birds: I. Immunohistochemical studies concerning the cryptochrome cycle.

Christine Nießner,Roswitha Wiltschko,Susanne Denzau,Wolfgang Wiltschko,Leo Peichl

doi:10.1242/jeb.110965

Christine Nießner, Roswitha Wiltschko + Show 3 more

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https://doi.org/10.1242/jeb.110965

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Abstract

Cryptochrome 1a, located in the UV/violet-sensitive cones in the avian retina, is discussed as receptor molecule for the magnetic compass of birds. Our previous immunohistochemical studies of chicken retinae with an antiserum that labelled only activated cryptochrome 1a had shown activation of cryptochrome 1a under 373 nm UV, 424 nm blue, 502 nm turquoise and 565 nm green light. Green light, however, does not allow the first step of photoreduction of oxidized cryptochromes to the semiquinone. As the chickens had been kept under ‘white’ light before, we suggested that there was a supply of the semiquinone present at the beginning of the exposure to green light, which could be further reduced and then re-oxidized. To test this hypothesis, we exposed chickens to various wavelengths (1) for 30 min after being kept in daylight, (2) for 30 min after a 30 min pre-exposure to total darkness, and (3) for 1 h after being kept in daylight. In the first case, we found activated cryptochrome 1a under UV, blue, turquoise and green light; in the second two cases we found activated cryptochrome 1a only under UV to turquoise light, where the complete redox cycle of cryptochrome can run, but not under green light. This observation is in agreement with the hypothesis that activated cryptochrome 1a is found as long as there is some of the semiquinone left, but not when the supply is depleted. It supports the idea that the crucial radical pair for magnetoreception is generated during re-oxidation.

Highlights

The Radical Pair Model (Ritz et al, 2000) proposes that the sensing of magnetic direction in birds is based on radical pair processes, with cryptochrome as receptor molecule
Applying radio frequency fields in the MHz range was suggested as a diagnostic tool to identify radical pair processes (Ritz, 2001; Henbest et al, 2004), and such fields disrupted magnetic orientation of European robins, Erithacus rubecula (Turdidae), domestic chickens, Gallus gallus (Phasianidae), and zebra finches, Taeniopygia guttata (Estrildidae) (Ritz et al, 2004; Ritz et al, 2009; Thalau et al, 2005; Wiltschko et al, 2007; Keary et al, 2009)
The birds had previously been kept in daylight where the entire cycle can run, and so we argued that when the exposure to green light began, there had been a certain supply of FADH● present, which could be photoreduced to the fully reduced form

Summary

Introduction

The Radical Pair Model (Ritz et al, 2000) proposes that the sensing of magnetic direction in birds is based on radical pair processes, with cryptochrome as receptor molecule. Received 14 July 2014; Accepted 26 August 2014 immunohistochemical study, a type of cryptochrome, Cry1a, was identified in the retina of robins and chickens, where it is located at the discs in the outer segment of the UV/violet cones (Nießner et al, 2011) These findings support the Radical Pair Model and the role of Cry1a as receptor molecule. In a subsequent study (Nießner et al, 2013), it became evident that this antiserum labelled only the light-activated form of Cry1a: in the non-activated state, the antigen site of the antiserum seems to be hidden; light activation appears to lead to a conformational change that exposes the C-terminus and allows our antiserum to bind This finding gave us the opportunity to study the activation characteristics of Cry1a in vivo under various wavelengths of narrow-band light, the same light we had used in previous behavioural tests. We found activated Cry1a under all wavelengths where birds had been shown to be able to use their magnetic compass (see Wiltschko et al, 2010): 373 nm UV, 424 nm blue, 502 nm turquoise and 565 nm green light (Nießner et al, 2013)

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