Searching for magnetic compass mechanism in pigeon retinal photoreceptors

Abstract

Migratory birds can detect the direction of the Earth's magnetic field using the magnetic compass sense. However, the sensory basis of the magnetic compass still remains a puzzle. A large body of indirect evidence suggests that magnetic compass in birds is localized in the retina. To confirm this point, an evidence of visual signals modulation by magnetic field (MF) should be obtained. In a previous study we showed that MF inclination impacts the amplitude of ex vivo electroretinogram (ERG) recorded from isolated pigeon retina. Here we present the results of an analysis of putative MF effect on one component of ERG, the photoreceptor's response, isolated from the total ERG by adding sodium aspartate and barium chloride to the perfusion solution. Photoresponses were recorded from isolated retinae of domestic pigeons Columba livia. The retinal samples were placed in MF that was modulated by three pairs of orthogonal Helmholtz coils. Light stimuli (blue and red) were applied under two inclinations of MF, 0° and 90°. In all the experiments, preparations from two parts of retina were used, red field (with dominant red-sensitive cones) and yellow field (with relatively uniform distribution of cone color types). In contrast to the whole retinal ERG, we did not observe any effect of MF inclination on either amplitude or kinetics of pharmacologically isolated photoreceptor responses to blue or red half-saturating flashes. A possible explanations of these results could be that magnetic compass sense is localized in retinal cells other than photoreceptors, or that photoreceptors do participate in magnetoreception, but require some processing of compass information in other retinal layers, so that only whole retina signal can reflect the response to changing MF.

Fig 1. Scheme of fundus of the pigeon eye according to [16].

Positions of the red and yellow fields are shown.

Fig 2. Typical set of photoreceptor responses of isolated pigeon retina to blue flashes of increasing intensities.

Flash duration is 10 ms. The intensities of flashes were 3.7×10⁶, 4.67×10⁷, 1.17×10⁸, 4.17×10⁸, 1.32×10⁹, 4.36×10⁹ photons/mm². Red curve indicates approx. half-saturated response (flash 1.17×10⁸) that was selected for further testing with modulated MF inclination.

Fig 3. Effect of the MF inclination on photoreceptor responses from the isolated pigeon retina.

(A) Series of four average photoreceptor responses to half-saturating blue flashes (10 ms, intensity 2.1×10⁸ photons/mm²) recorded one after another with changing of MF inclination (90°→0°→90°→0°). (B) Two average photoreceptor responses to half-saturating blue flashes (10 ms, intensity 2.1×10⁸ photons/mm²) at different MF inclinations. (C) Same for two average responses to half-saturating red flashes (10 ms, intensity 1.8×10¹⁰ photons/mm²).

Fig 4. Analysis of gradual changes in the response maximum.

(A) Differences in the amplitude maximum of the average responses to blue flashes recorded one after another with certain time intervals (normalized by response amplitude, achieved for a given preparation). For all retinal preparations (n = 31) responses show significant decreasing of the amplitude maximum. Data presented as medians (black horizontal lines) and quartiles (boxes and bars).*—statistically significant changes for one-way repeated measures ANOVA with post hoc Bonferroni correction. (B) Pair of responses recorded under the same 0° inclination of MF was used for building the linear trend in the amplitude of photoreceptor responses over time, and then such calculated trend was used for calculation of corrected response amplitude at the same time point as the amplitude for inclination 90°. The value of amplitude used for subsequent statistical analysis is marked as “corrected data”.

Fig 5. Analysis of potential effect of MF inclination change on the response maximum (amplitude).

Y-axis shows the ratio of the response maximum recorded under magnetic inclination 0° to the response maximum for inclination 90°. (A) Results for yellow field preparations. n = 16 for responses both to blue and red flashes. (B) Results for red field preparations. n = 15 for responses both to blue and red flashes. For both types of retinal fields no significant changes in response maximum’s ratio were detected by Student’s one sample t-test. (C) Results for total retinal preparations from a previous study [15]. n = 20 for responses both to blue and red flashes. Ratio of amplitudes recorded under 0°/90° inclinations under blue flashes was significantly different from 1 (Student’s one sample t-test, t = 2.192, p = 0.041). For red flashes, no significant changes from 1 for response maximum’s ratio were detected. Data presented as medians (black horizontal lines) and quartiles (boxes and bars).

Fig 6. Analysis of potential effect of MF inclination change on the response kinetics.

Y-axis shows average sums of point-by-point differences between normalized responses recorded under the same (0°) or under two different (0° and 90°) MF inclinations. Results presented for responses both to blue and red flashes. “Different inclination-1” and “Different inclination-2” refer to the sums of point-by-point differences between the responses recorded under the MF inclination 90° and 1st or 2nd set of responses recorded under inclination 0°, respectively. (A) Results for yellow field preparations. n = 16 for responses both to blue and red flashes. (B) Results for red field preparations. n = 15 for responses both to blue and red flashes. For both types of retinal fields no significant changes in response maximum were detected by one-way repeated measures ANOVA. Data presented as medians (black horizontal lines) and quartiles (boxes and bars).