Cellular and Molecular Changes to Cortical Neurons Following Low Intensity Repetitive Magnetic Stimulation at Different Frequencies

• Cellular effects in vitro of low intensity (LI) rTMS depend on stimulation frequency.
• LI-rTMS released calcium from intracellular stores in neurons.
• LI-rTMS induced regulation of genes related to apoptosis and neurite outgrowth.
• LI-rTMS modulated neuronal survival and morphology.
• We suggest a novel cell-intrinsic mechanism for plastic changes following LI-rTMS.

BackgroundRepetitive transcranial magnetic stimulation is increasingly used as a treatment for neurological dysfunction. Therapeutic effects have been reported for low intensity rTMS (LI-rTMS) although these remain poorly understood.ObjectiveOur study describes for the first time a systematic comparison of the cellular and molecular changes in neurons in vitro induced by low intensity magnetic stimulation at different frequencies.MethodsWe applied 5 different low intensity repetitive magnetic stimulation (LI-rMS) protocols to neuron-enriched primary cortical cultures for 4 days and assessed survival, and morphological and biochemical change.ResultsWe show pattern-specific effects of LI-rMS: simple frequency pulse trains (10 Hz and 100 Hz) impaired cell survival, while more complex stimulation patterns (theta-burst and a biomimetic frequency) did not. Moreover, only 1 Hz stimulation modified neuronal morphology, inhibiting neurite outgrowth. To understand mechanisms underlying these differential effects, we measured intracellular calcium concentration during LI-rMS and subsequent changes in gene expression. All LI-rMS frequencies increased intracellular calcium, but rather than influx from the extracellular milieu typical of depolarization, all frequencies induced calcium release from neuronal intracellular stores. Furthermore, we observed pattern-specific changes in expression of genes related to apoptosis and neurite outgrowth, consistent with our morphological data on cell survival and neurite branching.ConclusionsThus, in addition to the known effects on cortical excitability and synaptic plasticity, our data demonstrate that LI-rMS can change the survival and structural complexity of neurons. These findings provide a cellular and molecular framework for understanding what low intensity magnetic stimulation may contribute to human rTMS outcomes.