Transcranial magnetic stimulation in the rat

Abstract.

Transcranial magnetic stimulation (TMS) allows for quantification of motor system excitability. While routinely used in humans, application in other species is rare and little is known about the characteristics of animal TMS. The unique features of TMS, i.e., predominantly interneuronal stimulation at low intensity and non-invasiveness, are particularly useful in evaluating injury and recovery in animal models. This study was conducted to characterize the rodent motor evoked potential to TMS (MEP_TMS) and to develop a methodology for reproducible assessment of motor excitability in the rat. MEP_TMS were compared with responses evoked by electrical stimulation of cervical spinal cord (MEP_CES) and peripheral nerve. MEP were recorded by subcutaneous electrodes implanted bilaterally over the calf. Animals remained under propofol infusion and restrained in a stereotactic frame while TMS followed by CES measurements were obtained before and after 2 h of idle time. TMS was applied using a 5-cm-diameter figure-of-eight coil. MEP_TMS had onset latencies of 6.7±1.3 ms. Latencies decreased with higher stimulation intensity (r=–0.7, P<0.05). Two morphologies, MEP_TMS, 1 and MEP_TMS, 2, were distinguished by latency of the first negative peak (N1), overall shape, and amplitude. MEP_{TMS, 2} were more frequent at higher stimulation intensity. While recruitment curves for MEP_TMS, 1 followed a sigmoid course, no supramaximal response was reached for MEP_TMS, 2. Mid-cervical spinal transection completely abolished any response to TMS. MEP_CES showed a significantly shorter latency (5.29±0.24, P<0.0001). Two types of MEP_CES resembling MEP_{TMS, 1 and 2} were observed. Neither MEP_TMS nor MEP_CES changed on repeat assessment after 2 h. This study demonstrates the feasibility and reproducibility of TMS in the rat. Sigmoid recruitment curves for MEP_TMS, 1 suggest input-output properties similar to those of the human corticospinal system. Latency differences between CES and TMS point to a supraspinal origin of the MEP_TMS. The two morphologies likely reflect different cortical or subcortical origins of MEP_TMS.