Static magnetic field modulates excitatory activity in layer II/III pyramidal neurons of the rat motor cortex

Recent studies revealed that transcranial stimulation with static magnetic field (SMF) at 0.1-0.15 T results in reduction of neural excitability in the human motor cortex. We explore the mechanisms of this phenomenon with patch-clamp recording in rat brain slices. Recording from layer II/III pyramidal neurons in motor cortex, we observed an increase in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) during SMF exposure (n = 23). The amplitude of sEPSCs as well as the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs, n = 7) were unaffected by SMF. Sham stimulation did not result in significant changes in sEPSCs amplitude or frequency (n = 7). A minority of the neurons (9 of 23) from which sEPSCs where recorded did not exhibit frequency increase during SMF exposure, but showed significant frequency reduction after SMF exposure. Since SMF exposure did not cause significant changes in resting membrane potential or input impedance and since the sEPSC frequency, but not amplitude, was affected, our results are consistent with a presynaptic site of the neural effect of SMF. The presence of the effect on EPSCs and not IPSCs may be related to the larger length of excitatory axons compared to inhibitory axons.