Transcranial magnetic stimulation in the rat

Area of Science:

• Neuroscience
• Motor Control
• Animal Models

Background:

• Transcranial magnetic stimulation (TMS) is a valuable tool for quantifying motor system excitability in humans.
• Its application in animal models is limited, with scarce data on rodent TMS characteristics.
• Non-invasive TMS is crucial for evaluating injury and recovery in animal models of neurological conditions.

Purpose of the Study:

• To characterize rodent motor evoked potentials to TMS (MEPTMS).
• To develop a reproducible methodology for assessing motor excitability in rats.
• To compare MEPTMS with responses from electrical stimulation of the cervical spinal cord (MEPCES) and peripheral nerve.

Main Methods:

• MEPTMS and MEPCES were recorded from rats using subcutaneous electrodes over the calf.
• Stimulation was delivered via a figure-of-eight coil for TMS and electrical stimulation for MEPCES.
• Measurements were taken before and after a 2-hour idle period under propofol infusion and stereotactic restraint.

Main Results:

• MEPTMS exhibited onset latencies of 6.7±1.3 ms, decreasing with higher stimulation intensity.
• Two distinct MEPTMS morphologies (MEPTMS, 1 and MEPTMS, 2) were identified, with MEPTMS, 2 more frequent at higher intensities.
• Spinal transection abolished TMS responses, while MEPCES showed shorter latencies (5.29±0.24 ms); neither response changed significantly after 2 hours.

Conclusions:

• This study demonstrates the feasibility and reproducibility of TMS in rats for motor excitability assessment.
• Sigmoid recruitment curves of MEPTMS, 1 suggest input-output properties analogous to the human corticospinal system.
• Latency differences and morphologies indicate supraspinal origins for MEPTMS, potentially reflecting diverse cortical or subcortical pathways.