Modulating Cortical Excitability in a Rat Using Repetitive Transcranial Magnetic Stimulation

Place an anesthetized rat on a warming blanket to maintain body temperature, and apply eye ointment to prevent dryness.

Locate the target region of the cerebral cortex.

Position a magnetic coil over the primary motor cortex.

Apply the minimum stimulation intensity, termed the motor threshold or MT, which generates electrical activity in the cortical neurons and leads to muscle contraction in the contralateral forepaw.

Move the coil over the target area. Using an intensity equal to or greater than MT, apply repetitive transcranial magnetic stimulation, or rTMS, to induce electrical activity in the target cortical neurons.

The electrical activity opens voltage-gated calcium channels, leading to calcium influx, which triggers immediate early gene expression and promotes neurotrophic factor production.

The factors upregulate excitatory neurotransmitter receptors at the synapse and facilitate new synapse formation, which enhances cortical excitability.

After stimulation, allow the animal to recover to assess rTMS-induced changes in cortical excitability.

Place a rectal probe in an anesthetized adult male rat to monitor the body temperature, using a homeothermic blanket to maintain the temperature at 37 degrees Celsius. Apply ointment to the animal's eyes. Then, 10 minutes after the complete transition to IV anesthesia, mark a point 0.5 centimeters lateral to the vertex on the bi-auricular line, and fix the coil holder firmly to the apparatus.

Then locate the center of a 25-millimeter figure-of-eight coil at the point, and angulate the coil 45 degrees to the ground. Holding the coil surface flat against the calvaria, gradually increase the stimulation intensity to determine the minimum stimulus intensity required to evoke five or more palpable contralateral forepaw contractions by 10 consecutive biphasic stimuli through the coil. Verify that the stimulation is primarily causing the contralateral muscle contraction without any ipsilateral contraction to ensure a unilateral stimulation.

Use a water cooling system to cool the coil for 10 minutes. Maintaining the coil temperature with the cooling system, use the software console to set the stimulation intensity at 100 to 110% of the motor threshold. Then, to apply the rTMS at a 1 Hertz stimulation, first set the input to 1,200 shots for 20 minutes. Then move the center of the coil to the target site on one cerebral cortex, and tilt the coil to ensure the direct contact between the coil center and the surface of the skull at the point of stimulation.

For a 20-Hertz stimulation, set the input to 1,600 shots over 20 minutes, and conduct two seconds of stimulation, followed by 28 seconds of rest. For a sham stimulation, tilt the coil at a 90-degree rotation to the calvaria and place the edge of the coil 2 centimeters from the head surface. At the end of the stimulation, allow the animal to recover with full monitoring.