Functional MRI reveals regional changes of brain activity after five days of focal high-density theta burst stimulation (hdTBS) of the rat brain

BackgroundThe therapeutic effects of transcranial magnetic stimulation (TMS) likely stem from neuroplasticity induced by repeated sessions over time. While animal models offer insights into TMS-induced plasticity, a rodent model that faithfully replicates prolonged TMS conditions in humans is still lacking. Objective/HypothesisDevelop a rat model that mimics the spatial and temporal patterns of TMS in humans. MethodsExperiments were conducted on two cohorts of healthy adult rats (N=33). In cohort 1, rats underwent surgical implantation of microelectrodes for motor evoked potential (MEP) recording. With a rodent-specific coil and the high-density theta burst stimulation (hdTBS) paradigm, under awake condition, rats received daily TMS at 100% motor threshold for five days (days 1-5) to the hindlimb motor cortex. Cortical excitability was measured by input-output (I-O) curves on Day 0 (pre-hdTBS baseline) and Day 6 (post-hdTBS). The second cohort received identical TMS and underwent fMRI to map cerebral blood volume (CBV) on Days 0 and 6. ResultsDaily hdTBS session for 5 days significantly up-shifted I-O curves only in the TMS group (N=9), not in the sham group (N=7), indicating enhanced cortical excitability. fMRI data showed that, compared to sham group (N=9), rats receiving hdTBS (N=8) had increased basal CBV in several brain regions proximal and distal to the stimulation site, suggesting enhanced basal metabolism. Conclusion(s)Daily hdTBS session for 5 days focally delivered to the motor cortex of naive rats significantly altered basal brain activity in a network of brain regions, opening a novel platform for further investigating TMS-induced plasticity.