Rebound Excitation of Epileptiform Activities by Transcranial Focused Ultrasound Stimulation

BackgroundConventional neurostimulations to treat epilepsy have adverse effects caused by post-inhibitory rebound excitations. Although ultrasound brain stimulation is feasible in inducing anticonvulsant effects, its association with paradoxical rebound excitations is unknown. ObjectiveThis study aimed at demonstrating rebound excitations with transcranial focused ultrasound. The modulations of epileptiform activities toward both suppressive and excitatory responses were investigated by changing ultrasonic transmit sequences. MethodsIn a pentylenetetrazol-injected acute epilepsy rat model, transcranial focused ultrasound stimulation was applied on the thalamus to modulate epileptiform activities. The parameters differentiated for pulse sequences were the pulse length, pulse pressure, and interval between the pulses. Sonication effects were assessed by electroencephalography (n=38), immuno-histochemical analysis (n=24), and optical measurement of cerebral blood volume changes (n=18). ResultsWhile ultrasonic patterns of stimuli at long intervals showed antiepileptic effects on electroencephalography, those at short intervals showed rebound excitatory responses followed by inhibitory activities. Further, suppressive states induced by inhibitory stimulations were transformed into excitatory states by applying a consecutive series of short bursts at higher acoustic pressure. Cerebral blood volume changes demonstrated consistent results with electroencephalography. Immunohistochemistry revealed that both inhibitory and excitatory neuronal cells were activated to generate rebound excitatory conditions, while inhibitory cells were activated for suppressive conditions. ConclusionsIn our study, variations in ultrasound stimulation patterns could modulate epileptiform activities in both upregulated and downregulated directions.