Dose-Dependence of Changes in Cortical Protein Expression Induced with Repeated Transcranial Magnetic Theta-Burst Stimulation in the Rat

BackgroundTheta Burst stimulation (TBS) applied via transcranial magnetic stimulation (TMS) effectively modulates human neocortical excitability but repeated applications of the same TBS protocol at short intervals may be not simply accumulative.ObjectiveOur aim was to investigate the impact of multiple blocks of either intermittent (iTBS) or continuous TBS (cTBS) on the expression of neuronal activity marker proteins in rat cortex.MethodsUp to four iTBS- or cTBS-blocks of 600 stimuli were applied to urethane-anesthetized rats followed by immunohistochemical and Western blot analyses.ResultsThe effects of iTBS and cTBS were similar but slightly differed with regard to the number of stimuli applied. The expression of the 65-kD isoform of glutamic acid decarboxylase (GAD65) increased with each stimulation block, while that of the 67-kD isoform (GAD67), and that of the calcium-binding proteins (CaBP) Parvalbumin (PV) and Calbindin (CB) and that of the immediate early gene c-Fos progressively decreased. Both TBS protocols increased the expression of the vesicular glutamate transporter 1 (VGLUT1) with 1200–1800 stimuli but then decreased them after the 4th block.ConclusionOur findings indicate that repeated TBS elicits no simple accumulative dose-dependent effect for all activity-markers but distinct profiles with threshold characteristics and a waxing-and-waning effect especially for the markers of inhibitory activity CB and GAD67. Interestingly, somatic activity markers, such as c-Fos for mainly excitatory and GAD67, CB and PV for inhibitory neurons, decreased with repeated stimulation while synaptic activity markers mainly increased which could be a result of the artificial stimulation of axons.