Circular swimming in mice after exposure to a high magnetic field

Abstract

There is increasing evidence that exposure to high magnetic fields of 4T 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 14.1T 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 15s 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, 24h, 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.

Figure 1

Traces of paths taken by mice during 2-min swim test after 30-min sham exposure (left), immediately after 30-min exposure to 14.1 T (center), or 10 min after the end of 30-min magnet exposure (right). Mice were placed in the center of the 1-m diameter swimming pool. The solid line shows swimming for the first 15-s of each swim test, and the dotted line shows swimming for the remaining 105 s. The sham-exposed mouse swam in straight lines or broad curves with only occasional circles. The mouse tested immediately after magnet exposure showed intense counter-clockwise circling during the first 15 s, and the swim path continued to be interrupted with bouts of tight circling. In contrast, the mouse placed in the swimming pool 10 min after magnet exposure was nearly motionless for the first 15 s in the water, and showed no circling when swimming for the remainder of the test.

Figure 2

Examples of angular velocity (degrees/s) calculated from successive bearings of mice during the 2-min swim test, from a sham-exposed mouse (A) or a mouse tested immediately after 30-min magnet exposure (B). Positive values indicated counter-clockwise turning, and 0 indicates straight swimming (dashed line). Although sham-exposed mice showed occasional turns in either counter-clockwise or clockwise directions, their mean angular velocity stayed close to 0. Magnet-exposed mice, when tested immediately after exposure, showed persistent bouts of counter-clockwise turning throughout the 2-min test.

Figure 3

Photograph of a mouse placed in the swimming pool at 10 min after a 30-min exposure to the high magnetic field, show characteristic floating with turning of the pelvis to the right and extension of the rear right leg out of the water. After ~ 15 s, mice typically recovered from this posture and swam without much circling.

Figure 4

Quantification of swimming in mice tested at 0–60 min after magnet exposure, or after sham-exposure. Values are shown for the first 15 s (left panels), or the entire 2 min of the swim test (right panels). (A,B) Angular velocity in degrees/s calculated from successive bearings of mice. (C,D) Total number of circles was calculated from the cumulative angular velocity. Positive values of angular velocity and circling are in the counter-clockwise direction. Mice tested at 0 min after magnet exposure showed significant more counter-clockwise swimming and circles than sham-exposed mice or mice tested at later time points. (E,F) Cumulative distance swum in cm. Mice tested at 3 – 40 min after magnet exposure swam significantly shorter distance in the first 15 s of the swim test. * p < 0.05 vs sham-exposed; † p < 0.05 vs. 0 min.

Figure 5

Quantification of swimming in mice tested at 24 h or 96 h after sham exposure (white bars) or magnet exposure (black bars). Values are shown for the first 15 s (left panels), or the entire 2 min of the swim test (right panels). (A,B) Angular velocity in degrees/s calculated from successive bearings of mice. (C,D) Total number of circles was calculated from the cumulative angular velocity. Positive values of angular velocity and circling are in the counter-clockwise direction. (E,F) Cumulative distance swum in cm. With the exception of one comparison (E), there was no significant difference in swimming between sham- and magnet-exposed mice tested days after exposure. * p < 0.05 vs sham-exposed mice.