Construction and Evaluation of Rodent-Specific rTMS Coils

Abstract

Rodent models of transcranial magnetic stimulation (TMS) play a crucial role in aiding the understanding of the cellular and molecular mechanisms underlying TMS induced plasticity. Rodent-specific TMS have previously been used to deliver focal stimulation at the cost of stimulus intensity (12 mT). Here we describe two novel TMS coils designed to deliver repetitive TMS (rTMS) at greater stimulation intensities whilst maintaining spatial resolution. Two circular coils (8 mm outer diameter) were constructed with either an air or pure iron-core. Peak magnetic field strength for the air and iron-cores were 90 and 120 mT, respectively, with the iron-core coil exhibiting less focality. Coil temperature and magnetic field stability for the two coils undergoing rTMS, were similar at 1 Hz but varied at 10 Hz. Finite element modeling of 10 Hz rTMS with the iron-core in a simplified rat brain model suggests a peak electric field of 85 and 12.7 V/m, within the skull and the brain, respectively. Delivering 10 Hz rTMS to the motor cortex of anaesthetized rats with the iron-core coil significantly increased motor evoked potential amplitudes immediately after stimulation (n = 4). Our results suggest these novel coils generate modest magnetic and electric fields, capable of altering cortical excitability and provide an alternative method to investigate the mechanisms underlying rTMS-induced plasticity in an experimental setting.

The effects of repetitive transcranial magnetic stimulation in an animal model of tinnitus

“…In a recent study, the same intensity of LI-rTMS modulated motor cortex excitability in rats39. In addition, the small reduction of increased spontaneous activity in the IC, though not statistically significant, may have been the result of activation of descending pathways originating in the auditory cortex.…”

“…Intensity at the base of the coil was 90 mT and the half-maximum field occurred at 2 mm z axis , 3.5 mm xy axis , defining a focal stimulation area that was restricted to the auditory cortex of one hemisphere. These intensities have previously been shown to modulate motor cortical excitability in rats39.…”

“…Here, we use our model to investigate whether rTMS can reduce behavioural evidence of tinnitus and the associated hyperactivity in inferior colliculus (IC). Upon behavioural evidence of tinnitus, we commenced treatment (10 minutes of 1 Hz low intensity rTMS (LI-rTMS) or sham stimulation over auditory cortex contralateral to the affected cochlea for 10 daily sessions over 14 days) using a custom-built coil3839. This coil approximates the coil to brain-size ratio used in humans providing improved focality40.…”

Low‐intensity transcranial magnetic stimulation promotes the survival and maturation of newborn oligodendrocytes in the adult mouse brain

“…As the intensity of the magnetic field and induced current diminishes with increasing distance from the TMS coil (Tang, Lowe, et al, ), more ventral brain regions may be less responsive to stimulation. To determine whether iTBS can increase the number of new oligodendrocytes detected in each cortical layer, we performed a laminar analysis, quantifying the proportion of YFP + OLIG2 + cells that were new oligodendrocytes (YFP + OLIG2 + PDGFRα‐neg / YFP + OLIG2 + x 100; Figure ).…”

“…3.2 | LI-rTMS can increase new oligodendrocyte number in the superficial and deep layers of the cortex As the intensity of the magnetic field and induced current diminishes with increasing distance from the TMS coil (Tang, Lowe, et al, 2016), more ventral brain regions may be less responsive to stimulation. To (Figure 2a,b), but had no effect on layers II/III, IV or the CC (Figure 2b and Figure S5), suggesting that these layers either accommodate fewer new oligodendrocytes or are refractory to stimulation (Koudelka et al, 2016).…”