Evidence for a cephalic site of action of high magnetic fields on the behavioral responses of rats

Abstract

Static high magnetic fields (MFs) from 7 T to 9 T can elicit behavioral responses in rodents such as suppression of rearing, locomotor circling, and acquisition of a conditioned taste aversion (CTA). MF exposure also induces c-Fos expression in the visceral and vestibular nuclei of the brainstem, suggesting the stimulation of some sensory pathways. It is not clear, however, if the effects of the MF are caused by exposure to the uniform maximal field at the center of the magnet, or by exposure to the steep field gradients along the bore of the magnet during the rat's placement. In addition, the site of action within the rat is unknown. In an attempt to limit MF exposure to rostral or caudal portions of the rats' body, we exposed male and female rats at different positions within the bore of a 14.1-T superconducting magnet ranging from 2 cm (1.6 T at the head) to 155 cm (0.05 T at the head), with the center of the bore at 65 cm (14.1 T across the whole body). This approach also allowed us to expose rats to the maximal field strength (14.1 T) vs. the maximal field gradients (54 T/m). To assess both immediate and delayed behavioral effects, locomotor and CTA responses were recorded. A small but significant CTA was seen after exposure of the head to the lowest MF tested (0.05 T at 155 cm). Graded effects were seen, however, with greater circling and CTA acquisition as the MF strength increased at the rostral end of the rat. This suggests a cephalic site of action. Furthermore, maximal circling and CTA were induced after exposure to the uniform center field, and not after exposure to high field gradients on either side of the center. This suggests that the behavioral responses seen after MF exposure are a consequence of the uniform static field at the center of the magnet, and are not caused by passage through, or exposure to, the vertical field gradients. Female rats responded similarly to male rats, although magnet-induced CTA appeared resistant to extinction in female rats.

Figure 1

A. Cross sectional schematic of the 14.1 T superconducting NMR magnet. B. The magnetic field measured in the center of the magnet’s bore. The opening to the magnet’s bore is set at 0 mm. The maximum central field (B₀) is 14.1 T. C. The field gradient along the vertical axis of the magnet (dB/dz) derived from the measured magnetic field. To expose rats to different MF strengths and gradients, rats were restrained in Plexiglas tubes and placed at different intervals with the vertical bore of the magnet. The positions of the rats and their alignment with the measured magentic field are shown for Experiments 1 (D), 2 (E), and 3 (F). All rats were the raised from the bottom of the bore into the magnetic field, except for the one group of rats in Experiment 3 that were lowered from the top of the bore.

Figure 2

Quantification (mean ± s.e.m.) of locomotor circling (A) and rearing (B) in male (white bars) and female rats (hatched bars) after 30-min exposure within the 14.1 T magnet at the given head positions (n=5–7/group). A. Significant circling was found by χ² in male rats after 35 and 65 cm exposure, and in female rats after exposure at 35, 65, and 95cm. B. Rearing was significantly suppressed in both male and fermale rats after exposure at 35, 65, and 95cm. nd, not determined for female rats. * p < 0.05 vs sham-exposed rats.

Figure 3

A. Initial magnitude of conditioned taste aversion (CTA) measured by saccharin preference (mean ± s.e.m.) on the first day of 2-bottle preference testing in male (white bars) and female rats (hatched bars.) B. Time course of CTA extinction across 14 days of 2-bottle preference tests in male rats. * p <0.05 35-cm group vs. sham-exposed rats; † p < 0.05 65-cm group vs. 35-cm group. C. Time course of CTA extinction in female rats. Saccharin preference in female rats exposed at 35 and 65 cm was significantly lower than sham-exposed rats on all 14 days. * p < 0.05 35-cm and 65-cm groups vs. sham-exposed rats. Other groups were not different from sham-exposed rats. nd, not determined for female rats. D. Saccharin preference on the last day of 2-bottle preference testing (day 14). The CTA of male rats (white bars) extinguished by day 14, while the saccharin preference of female rats (hatched bars) exposed at 35 cm and 65 cm were still significantly lower than sham-exposed rats. * p < 0.05 vs sham-exposed rats.

Figure 4

Quantification (mean ± s.e.m.) of locomotor circling (A) and rearing (B) in male rats after 30-min exposure within the 14.1 T magnet at intervals within the bore of the magnet ranging from 35 cm to 81 cm (n=6–8/group). A. Significant circling was found by χ² in all magnet-exposed groups. B. Rearing was significantly suppressed in all magnet-exposed groups. * p < 0.05 vs sham-exposed rats.

Figure 5

Initial magnitude (A) and extinction (B) of CTA measured by saccharin preference (mean ± s.e.m.) in male rats exposed at intervals within the bore of the magnet ranging from 35 cm to 81 cm. All magnet-exposed rats acquired a CTA. *p <0.05 vs sham-exposed rats. For clarity, significant differences are not indicated in panel B.

Figure 6

Initial magnitude (A) and extinction (B) of CTA measured by saccharin preference (mean ± s.e.m.) in males exposed at 155 cm (black bar and squares; n = 16) or in sham-exposed males (white bar and circles; n = 8). At this point the magnetic field was 0.05 T at the rats’ head, and a small but significant CTA was acquired. * p < 0.05 vs sham-exposed group.