Repetitive transcranial magnetic stimulation (rTMS) is increasingly used as a treatment for neurological dysfunction and therapeutic effects have also been reported for low intensity rTMS (LI-rTMS) although these remain poorly understood. Our 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. We applied 5 different low intensity repetitive magnetic

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... (LI-rMS) protocols to neuron-enriched primary cortical cultures for 4 days and assessed survival, and morphological and biochemical change. We 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 depolarisation, 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, which were consistent with our morphological data on cell survival and neurite branching. Thus, 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.