Circular swimming in mice after exposure to a high magnetic field

Abstract

There is increasing evidence that exposure to high magnetic fields of 4 tesla and above perturbs the vestibular system of rodents and humans. Performance in a swim test is a sensitive test of vestibular function. In order to determine the effect of magnet field exposure on swimming in mice, mice were exposed for 30-min within a 14.1 tesla superconducting magnet and then tested at different times after exposure in a 2-min swim test. As previously observed in open field tests, mice swam in tight counter-clockwise circles when tested immediately after magnet exposure. The counter-clockwise orientation persisted throughout the 2-min swim test. The tendency to circle was transient, because no significant circling was observed when mice were tested at 3 min or later after magnet exposure. However, mice did show a decrease in total distance swum when tested between 3 and 40 min after magnet exposure. The decrease in swimming distance was accompanied by a pronounced postural change involving a counter-clockwise twist of the pelvis and hindlimbs that was particularly severe in the first 15 s of the swim test. Finally, no persistent difference from sham-exposed mice was seen in the swimming of magnet-exposed mice when tested 60 min, 24 h, or 96 h after magnet exposure. This suggests that there is no long-lasting effect of magnet exposure on the ability of mice to orient or swim. The transient deficits in swimming and posture seen shortly after magnet exposure are consistent with an acute perturbation of the vestibular system by the high magnetic field.

Behavioral effects on rats of motion within a high static magnetic field

“…Therefore, the absence of circling in the rats removed slowly from the magnet could be accounted for entirely by the additional restraint time prior to recording their post-exposure locomotor activity. This is consistent with our previous observations that magnet-induced circling decays away within 2–3 minutes after magnet exposure [15]. (Rearing, however, was suppressed in both groups, which is consistent with the prior observation that suppression of rearing persists for ~10 min after exposure [16].…”

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

“…In rodents, we have found that exposure to static magnetic fields of 7 T or greater induced circular locomotion (15,16), conditioned taste aversion (CTA) (16), and c-Fos induction in the vestibular nuclei of the brain stem (28). These effects are consistent with vestibular perturbation induced by more conventional stimuli such as whole-body rotation.…”

“…The circling is observed when animals walk in a test chamber but is more pronounced when activity is elicited by swimming. The circling and other postural deficits persist for less than 10 min after removal from the magnet (15). In this study, the induction of circular swimming varied with the angle of exposure, with a graded response from maximum at 0°to a minimum at 90°, back to maximal response at 180°.…”

“…After magnetic field exposure with the rostral-caudal body axis parallel to the magnetic field and with the head toward Bϩ, rats and mice walk or swim in counterclockwise circles (15,16). Conversely, when exposed with the head toward BϪ (achieved either by inverting the animal within a superconducting magnet or inverting the polarity of the magnetic field within a resistive magnet), rats walk in clockwise circles (16).…”

“…We measured circling immediately after exposure during a 2-min swimming test, which maximizes limb movements and locomotion while minimizing spatial and gravitational cues (15). We also measured c-Fos induction in vestibular and visceral nuclei of the brain stem to determine whether neural activation paralleled the behavioral response after exposure at three different angles (0°, 90°, and 180°).…”