Acute Increase of Excitatory Activity in Pyramidal Neurons of Rat Motor Cortex under Static Magnetic Field

ObjectivesTranscranial application of a static magnetic field (SMF) was reported to result in subsequent modulation of neural excitability in the human motor cortex, but the acute mechanisms underlying this effect are unknown. We explore the mechanisms of this phenomenon with patch-clamp recording in rat brain slices during SMF exposure. Materials and MethodsPatch-clamp recording from layer II/III pyramidal neurons in motor cortex of acutely prepared brain slices were conducted during exposure to 0.20-0.35 T SMF or sham. ResultsDuring SMF exposure we observed an increase in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) as well as miniature excitatory post-synaptic currents (mEPSCs) recorded in the presence of TTX to suppress action potentials and thus also network effects. There was a significant acute increase in sEPSC frequency for SMF exposure duration of both 6 min and 10 min, but not for 10 min sham exposure. After SMF exposure, the sEPSC and mEPSC frequency returned to baseline. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was unaffected by SMF. The amplitude of the postsynaptic currents decreased with time for all recordings regardless of the condition, presumably due to the expected gradual deterioration of the patch clamp seal. ConclusionsThe acute effect of SMF exposure on sEPSC and mEPSC frequency, but not amplitude, is consistent with the assumption of a presynaptic or synaptic site mediating the effect. Furthermore, the consistency of the effect between sEPSCs and mEPSCs suggests that the effect is not related to action potential propagation in the presynaptic axon. The effect of SMF on EPSCs and not IPSCs may be related to the larger length of excitatory axons compared to inhibitory axons, or to effects on extracellular ionic gradients within the slice.