Cellular Mechanisms of Action Associated with Transcranial Ultrasound for Modulation and its Acoustic Characterization through Skull

Recent in vivo modulation of region-specific brain activity suggests that Low Intensity Focused Ultrasound (LIFU) may be a non-invasive alternative therapy for drug-delivery applications and the treatment of neurological diseases, including epilepsy and Parkinson’s disease.Despite these recent successes, failure to reproduce published results continues to plague the field due to the limited understanding of cellular mechanisms that underlie neuromodulation and the impact of skull on targeting accuracy. The objective of this thesis is to help bridge these knowledge gaps and better understand non-invasive transcranial focused ultrasound modulation. While hypotheses exist explaining the mechanism underlying ultrasound modulation, they largely remain untested. It has been suggested that mechanical perturbation of cellular membranes with embedded protein channels using ultrasound has an impact on ion channel kinetics, resulting in depolarization, and ultimately, increased neural activity. In particular, this thesis investigates the hypothesis explaining the mechanical perturbation as a result of pure acoustic radiation forces using simplified in vitro models, including Large-Conductance Mechanosensitive Channels (MscL) and non-mechanically stimulated channels. The outcome revealed an increase in efflux through proteoliposomes regardless of the channel type except at the highest concentration of mechanosensitive channel (MS) model where a lowering efflux trend was noticed. These unexpected results suggest that focused ultrasound does not modulate the gating of ion channels, but instead effects the permeability of the membrane itself or protein-membrane interface. Also a dual effect of membrane stretch enhancement and pore formation is observed only at high MS channel concentration.In addition, to prepare for in vivo efficacy studies, the present dissertation characterizes the ultrasonic beam scatter and focal shifts that occur as ultrasound passes through a rat skull for a specific set of parameters. The results have shown significant beam shape deformation and target shift due to the skull. This suite of studies improved our understanding of the mechanism associated with LIFU-based stimulation at the molecular level, while also exploring how LIFU can be applied with greater accuracy and precision in vivo. In addition, insights gleaned from this approach are expected to promote new avenues of clinical applications for the treatment of drug delivery, gene therapy and neurological illnesses