Behavioral effects of high-strength static magnetic fields on rats

Abstract

Advances in magnetic resonance imaging are driving the development of more powerful and higher-resolution machines with high-strength static magnetic fields. The behavioral effects of high-strength magnetic fields are largely uncharacterized, although restraint within a 9.4 T magnetic field is sufficient to induce a conditioned taste aversion (CTA) and induce brainstem expression of c-Fos in rats. To determine whether the behavioral effects of static magnetic fields are dependent on field strength, duration of exposure, and orientation with the field, rats were restrained within the bore of 7 or 14 T superconducting magnets for variable durations. Behavioral effects were assessed by scoring locomotor activity after release from the magnetic field and measuring CTA acquisition after pairing intake of a palatable glucose and saccharin (G+S) solution with magnetic field exposure. Magnetic field exposure at either 7 or 14 T suppressed rearing and induced tight circling. The direction of the circling was dependent on the rat's orientation within the magnetic field: if exposed head-up, rats circled counterclockwise; if exposed head-down, rats circled clockwise. CTA was induced after three pairings of taste and 30 min of 7 T exposure or after a single pairing of G+S and 1 min of 14 T exposure. These results suggest that magnetic field exposure has graded effects on rat behavior. We hypothesize that restraint with high-strength magnetic fields causes vestibular stimulation resulting in locomotor circling and CTA acquisition.

Fig. 1.

Tight-circling activity induced by magnetic field exposure. Rats were restrained for 30 min within the bore of a 14 T magnet in either head-up orientation (A) or head-down orientation (B). On release from restraint, rats oriented head-up circled counterclockwise, whereas rats oriented head-down circled clockwise.

Fig. 2.

CTAs induced by a single pairing of G+S intake with a 30 min restraint within magnetic fields of different strengths. For the 24 hr two bottle preference test after the pairing of G+S with magnetic exposure (A), a significant CTA against G+S was observed only after pairing with 14 T exposure. The CTA extinguished after 3 d of two bottle preference tests (B). *p < 0.05 versus 0 T (sham) exposure. Data for 9.4 T exposure are replotted from Nolte et al. (1998).

Fig. 3.

CTAs induced by a single pairing of G+S intake with restraint within 14 T magnetic fields for 0–30 min. Significant CTA was observed after ≥1 min of exposure to the magnetic field on the first 24 hr two bottle preference test (A); the CTAs persisted for several days of preference testing (B). *p < 0.05 versus 0 min exposure.

Fig. 4.

Acquisition of CTAs across three pairings of G+S with 30 min restraint within 7, 9.4, or 14 T magnetic fields. Intake during 10 min access to G+S was not different between sham- and 7 T-exposed rats across the 3 conditioning days (A). Rats exposed to 9.4 T decreased intake compared with sham-exposed rats on the third day of conditioning (i.e., after two pairings; B). Rats exposed to 14 T decreased intake on the second day of conditioning (i.e., after one pairing;C). *p < 0.05 versus sham-exposed rats. Data for 9.4 T exposure are replotted from Nolte et al. (1998).

Fig. 5.

CTAs induced by three pairings of G+S intake with restraint within 7, 9.4, or 14 T magnetic fields for 0–30 min. Significant CTAs were observed in all magnet-exposed rats on the first 24 hr two bottle preference test (A). Over subsequent 24 hr two bottle test days, the CTA of 7 T-exposed rats extinguished on the second day (B), whereas the CTAs of 9.4 T- and 14 T-exposed rats persisted for 8 d of preference testing (C, D). *p < 0.05 versus sham-exposed rats; ^†p < 0.05 versus 7 T-exposed rats. Data for 9.4 T exposure are replotted from Nolte et al. (1998).

Fig. 6.

CTAs induced by a single pairing of G+S with 14 T restraint in head-up or head-down orientations. After 10 min of access to G+S, rats were restrained in either a head-up orientation (black squares) or head-down orientation (black circles) for 30 min within the 14 T magnetic field. Although rats circled in opposite directions on release from restraint (with head-up rats circling counterclockwise and head-down rats circling clockwise), there was little difference in the CTA acquisition of the two groups. Control rats were sham-exposed while head-down for 30 min (white circles).