The overall goal of this experiment is to conduct a unilateral repetitive transcranial magnetic stimulation, or rTMS, in a small animal model. This method can help answer key questions regarding molecular mechanisms of rTMS, by enabling unilateral hemispheric stimulation to the rat brain. The main advantage of this technique is that it provides an opportunity to explore the interhemispheric differences of molecular events, in response to unilateral rTMS.
Individuals new to this method may struggle because the rTMS coil can be so large that they stimulate the whole brain, and because the timing of the motor threshold is time consuming and laborious. An experienced technician from our laboratory, Jin-Joo Lee, will be demonstrating the procedure. 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 biauricular 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 ten 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 one hertz stimulation, first set the input to 1200 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 1600 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 two centimeters from the head surface. At the end of the stimulation, allow the animal to recover with full monitoring.
In this representative experiment, six eight-week-old rats were imaged by micro-positron emission tomography to evaluate the uptake of a radio isotope in the cerebral cortices regions of interest. The radioactivity in the contralateral area was used as a reference to normalize the data obtained in the ipsilateral area. The mean differential uptake ratios obtained from three consecutive transverse images, were then averaged to calculate the differential uptake ratio for the rats, demonstrating a focal increase in the glucose metabolism in the stimulated left cortical area, in the one hertz group.
Supporting the unilaterality of the rTMS. In this mRNA microarray study, the expression levels of several immediate early genes were observed to be significantly higher in the rTMS animals than in the sham group. In addition, after a one hertz stimulation, brain-derived neurotrophic factor mRNA expression was also significantly higher in the stimulated cortex, revealing differential rTMS-induced changes in the stimulated versus the contralateral cerberal cortices.
Once administered, the rTMS technique can be completed in one hour, if it's performed properly. While attempting this procedure, it's important to take care to properly position the rTMS coil, and to prepare the contra and ipsilateral forepaw contractions to confirm the unilateral stimulation of the hemispheres. This technique will be useful in the field of translational neurorehabilitation research, for exploring the molecular mechanisms of rTMS, through the analysis of interhemispheric differences, and the various pathologic conditions.
After watching this video, you should have a good understanding of how to conduct unilateral rTMS, in a small animal model.
