Repetitive Transcranial Magnetic Stimulation to the Unilateral Hemisphere of Rat Brain
October 22nd, 2016
We applied repetitive transcranial magnetic stimulation (rTMS) to the unilateral hemisphere of rat brain, by placing a 25-mm figure-8 coil 1 cm lateral to the vertex on the biauricular line and angulating the coil by 45°. An in-house water cooling system was used for rTMS for more than 20 min.
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.
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Overview
This study investigates the application of unilateral repetitive transcranial magnetic stimulation (rTMS) in a rat model to explore molecular mechanisms. The technique allows for the examination of interhemispheric differences in response to rTMS.
Key Study Components
Area of Science
- Neuroscience
- Neurostimulation
- Animal Models
Background
- rTMS is a non-invasive brain stimulation technique.
- It can help elucidate molecular responses in the brain.
- Unilateral stimulation allows for targeted investigation of hemispheric differences.
- Proper technique is crucial to avoid unintended stimulation of the entire brain.
Purpose of Study
- To conduct unilateral rTMS in a small animal model.
- To investigate molecular mechanisms affected by rTMS.
- To provide insights into interhemispheric differences in brain activity.
Methods Used
- Application of rTMS using a 25-mm figure-8 coil.
- Placement of the coil 1 cm lateral to the vertex on the biauricular line.
- Angulation of the coil at 45 degrees.
- Monitoring body temperature using a rectal probe and homeothermic blanket.
Main Results
- Successful application of unilateral rTMS in the rat model.
- Demonstration of the procedure by an experienced technician.
- Potential for exploring molecular events in response to rTMS.
- Insights into the challenges faced by new users of the technique.
Conclusions
- Unilateral rTMS is a valuable tool for neuroscience research.
- It enables the study of interhemispheric differences in brain function.
- Proper technique and monitoring are essential for successful outcomes.
Frequently Asked Questions
Executive Industry Relevance
Unilateral rTMS in rodent models addresses a critical gap in preclinical neuroscience by enabling hemisphere-specific stimulation that mirrors clinical protocols. This approach enhances target validation by isolating molecular responses in stimulated versus contralateral tissue, reducing confounding variables in mechanism-of-action studies. The model supports de-risking of neuromodulation targets through quantifiable interhemispheric biomarker changes, informing go/no-go decisions in CNS drug discovery.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of therapeutic hypotheses by measuring immediate early gene (Arc, Junb, Egr2) and bdnf expression changes in stimulated versus non-stimulated cortex.
- Operational Value: Provides a unilateral stimulation paradigm that reduces biological variability and improves assay sensitivity for target engagement.
- Predictive Value: Supports mechanistic de-risking by establishing dose-response relationships between stimulation parameters (1 Hz, 20 Hz) and molecular outputs.
Screening & Assay Development
- Assay Readiness: Generates quantitative mRNA expression readouts that can be normalized to contralateral controls for reproducible target modulation assessment.
- Scalability: Uses a water-cooled 25-mm figure-8 coil to enable repeated sessions (>20 min) at 20 Hz, supporting medium-throughput screening of stimulation protocols.
- Reproducibility: Standardized coil placement (1 cm lateral to vertex on biauricular line, 45° angulation) and motor threshold verification ensure consistent unilateral delivery across experiments.
Translational & Preclinical Research
- Translational Continuity: Mirrors clinical rTMS unilateral application, improving predictive validity for CNS therapeutics targeting cortical excitability or neuroplasticity pathways.
- Biomarker Alignment: Links stimulation to downstream effectors like bdnf, enabling correlation of target modulation with functional recovery models in neuropathology.
- Risk-Adjusted Advancement: Facilitates go/no-go decisions by demonstrating sustained molecular shifts (e.g., bdnf upregulation after 5 days of 1 Hz rTMS) indicative of target engagement durability.
Pipeline & Workflow Integration
The unilateral rTMS method fits within the discovery continuum from target hypothesis testing to preclinical validation, particularly for CNS modulators where cortical network engagement is a key mechanism.
- Discovery Biology: Tests target engagement by measuring immediate early gene and bdnf transcription as proximal biomarkers of neuronal activation.
- Screening: Enables protocol optimization (frequency, duration, intensity) through quantifiable hemispheric differentials in gene expression.
- Analytics: Provides normalized molecular readouts (stimulated vs. contralateral cortex) that support statistical comparison across treatment groups.
- Translational Research: Connects acute molecular responses to longer-term plasticity changes, supporting progression to disease-relevant preclinical models.
- Enterprise Reuse: Establishes a standardized unilateral stimulation platform applicable across multiple target classes in neuropsychiatric and neurodegenerative discovery.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence by isolating hemispheric-specific molecular effects, reducing false positives from global brain stimulation.
- Operational Value: Delivers standardized, reproducible stimulation with real-time temperature control and coil positioning verification.
- Strategic Value: Improves capital efficiency by enabling early de-risking of neuromodulation targets before costly in vivo efficacy studies.
- Portfolio Impact: Supports risk-adjusted prioritization of CNS targets based on measurable, mechanism-linked transcriptional responses.
Implementation Considerations
- Requires expertise in rodent stereotaxy, anesthesia management, and electrophysiological validation of unilateral motor responses.
- Depends on specialized hardware including a water-cooling system, figure-8 coil, and stimulation console capable of precise frequency and shot control.
- Necessitates cross-team standardization of coil placement, motor threshold determination, and sham control procedures to ensure data comparability.
- Involves adaptation considerations when translating protocols across rodent strains, ages, or disease models affecting cortical thickness or conductivity.
- Limited by the need for technical proficiency in motor threshold mapping, which can be time-intensive and may require dedicated personnel training.
