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Abstract

Introduction The effect of the high magnetic field (MF) on biological processes can provide us a unique strategy to search for biological sources of MR contrast. Here we characterized the potential interaction of a high MF with inner ear formation in the zebrafish larvae. Otolith fusion in the larvae ear constitutes a good example of MF interference with biological processes. The mechanistic analysis of otolith fusion may lead us to certain biological targets sensing MF. Methods Zebrafish larvae were transferred to the bore of a 14T magnet for exposure times ranging from 3 to 62h in Petri dishes containing either Embryonic medium 3 (E3), agarose or E3 + Tricaine-S (fish anesthetic). After exposure, optical and confocal microscopes were used for observation of the inner ear. Ethovision (video tracking software) helped for balance assessment. To check for MF dependent effects, dishes containing the larvae were placed at different distances from the 14T magnet, being influenced by consequent different MF intensities. Results Wild-type larvae exhibit two otoliths (CaCO3 crystals that transmit acceleration forces and sound vibrations) in each ear (Fig.1A). Larvae exposed to a 14T MF in E3 were characterized by fused otoliths (Fig.1B). This anatomical observation was accompanied by aberrant swimming behavior. MF strength dependency was confirmed with a higher prevalence of altered phenotype in larvae kept at higher MF. The most vulnerable period for inner ear alteration upon high MF exposure occurs after 24hpf (once otoliths are formed) (Fig.2). 3h of exposure were sufficient to induce fusion of otoliths in larvae kept in E3, while in larvae embedded in agarose or under the effect of an anesthetic, this effect was barely noticeable. Conclusions High MF exposure led to otolith fusion in the zebrafish larvae. This scenario was largely hindered when larvae were treated by anesthetic or put into agarose medium, but resumed after the anesthetic was removed or with normal E3 medium in the same larvae. Both, agarose and the anesthetic, are expected to cause lower oxygen consumption rates and diminished metabolism, which may impede the interaction of the biological system with the strong MF. This result suggests that MF-induced otolith fusion may bear specific biological interactions. The mechanism behind our main finding might be related to the one underlying nystagmus and vertigo reported by human patients undergoing MRI.