Transcranial magnetic stimulation in the rat

Transcranial magnetic stimulation (TMS) allows for quantification of motor system excitability. While routinely used in humans, application in other s pecies 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 evaluatin g injury and recovery in animal models. This study was conducted to charact erize the rodent motor evoked potential to TMS (MEPTMS) and to develop a me thodology for reproducible assessment of motor excitability in the rat. MEP TMS were compared with responses evoked by electrical stimulation of cervic al spinal cord (MEPCES) and peripheral nerve. MEP were recorded by subcutan eous electrodes implanted bilaterally over the calf. Animals remained under propofol infusion and restrained in a stereotactic frame while TMS followe d 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. MEPTMS had onset la tencies of 6.7 +/-1.3 ms. Latencies decreased with higher stimulation inten sity (r=-0.7, P<0.05). Two morphologies, MEPTMS.1 and MEPTMS.2, were distin guished by latency of the first negative peak (N1), overall shape, and ampl itude. MEPTMS.2 were more frequent at higher stimulation intensity. While r ecruitment curves for MEPTMS.1 followed a sigmoid course, no supramaximal r esponse was reached for MEPTMS,2. Mid-cervical spinal transection completel y abolished any response to TMS. MEPCES showed a significantly shorter late ncy (5.29<plus/minus>0.24, P<0.0001). Two types of MEPCES resembling MEPTMS , 1 and 2 were observed. Neither MEPTMS nor MEPCES changed on repeat assess ment after 2 h. This study demonstrates the feasibility and reproducibility of TMS in the rat. Sigmoid recruitment curves for MEPTMS, 1 suggest input- output properties similar to those of the human corticospinal system. Laten cy 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 MEPTMS.