Orientation within a high magnetic field determines swimming direction and laterality of c-Fos induction in mice

High-strength static magnetic fields (>7 tesla) perturb the vestibular system causing dizziness, nystagmus, and nausea in humans; and head motion, locomotor circling, conditioned taste aversion, and c-Fos induction in brain stem vestibular nuclei in rodents. To determine the role of head orientation, mice were exposed for 15 min within a 14.1-tesla magnet at six different angles (mice oriented parallel to the field with the head toward B+ at 0 degrees; or pitched rostrally down at 45 degrees, 90 degrees, 90 degrees sideways, 135 degrees, and 180 degrees), followed by a 2-min swimming test. Additional mice were exposed at 0 degrees, 90 degrees, and 180 degrees and processed for c-Fos immunohistochemistry. Magnetic field exposure induced circular swimming that was maximal at 0 degrees and 180 degrees but attenuated at 45 degrees and 135 degrees. Mice exposed at 0 degrees and 45 degrees swam counterclockwise, whereas mice exposed at 135 degrees and 180 degrees swam clockwise. Mice exposed at 90 degrees (with their rostral-caudal axis perpendicular to the magnetic field) did not swim differently than controls. In parallel, exposure at 0 degrees and 180 degrees induced c-Fos in vestibular nuclei with left-right asymmetries that were reversed at 0 degrees vs. 180 degrees. No significant c-Fos was induced after 90 degrees exposure. Thus, the optimal orientation for magnetic field effects is the rostral-caudal axis parallel to the field, such that the horizontal canal and utricle are also parallel to the field. These results have mechanistic implications for modeling magnetic field interactions with the vestibular apparatus of the inner ear (e. g., the model of Roberts et al. of an induced Lorenz force causing horizontal canal cupula deflection).