Abstract
Implantable magnetic stimulation is an emerging type of neuromodulation using coils that are small enough to be implanted in the brain. A major advantage of this method is that stimulation performance could be sustained even though the coil is encapsulated by gliosis due to foreign body reactions. Magnetic fields can induce indirect electric fields and currents in neurons. Compared to transcranial magnetic stimulation, the coil size used in implantable magnetic stimulation can be greatly reduced. However, the size reduction is accompanied by an increase in coil resistance. Hence, the coil could potentially damage neurons from the excess heat generated. Therefore, it is necessary to study the stimulation performance and possible thermal damage by implantable magnetic stimulation. Here, we devised contact-mode magnetic stimulation (CMS), wherein magnetic stimulation was applied to hippocampal slices through a customized planar-type coil underneath the slice in the contact mode. With acute hippocampal slices, we investigated the synaptic responses to examine the field excitatory postsynaptic responses of CMS and the temperature rise during CMS. A long-lasting synaptic depression was exhibited in the CA1 stratum radiatum after CMS, while the temperature remained in a safe range so as not to seriously affect the neural responses.
Highlights
Implantable magnetic stimulation (IMS) has recently been suggested as a new form of brain stimulation[1,2,3]
Neuromodulation effects using coils developed by each research team have been extensively studied, there has been little research on the heat generated around the coil during magnetic stimulation
We found that more long-lasting synaptic depression was induced as the stimulation intensity of contact-mode magnetic stimulation (CMS) increased
Summary
Implantable magnetic stimulation (IMS) has recently been suggested as a new form of brain stimulation[1,2,3]. Unlike transcranial magnetic stimulation (TMS), which is widely used as one of the non-invasive brain stimulation methods for diagnostic and therapeutic purposes[4], IMS uses small implantable coils to deliver neuronal stimulation via electromagnetic induction. A significantly reduced coil size enables more localized stimulation than conventional TMS18,19 With these advantages, the feasibility of IMS has been studied using in vivo[20] and in vitro models[18]. Excess heat generation within the coils by high currents, required to evoke neural responses, may affect normal cellular physiology when the coils are implanted within the body. Based on recent experimental evidences showing that the stimulation frequency is a crucial parameter for altering inhibitory or excitatory synaptic responses, QPS is an interesting stimulus pattern that contains both low and high frequencies
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