Novel mechanisms of DC and kilohertz electrical stimulation
Transcranial electrical stimulation is a promising technique where a weak electrical current is applied to the scalp with the goal of modulating brain activity. Understanding the cellular mechanism of direct current (DC) and kilohertz (kHz) electrical stimulation is of broad interest in neuromodulation. More specifically, there is a large mismatch between enthusiasm for clinical applications of the method and understanding of DC and kHz novel mechanisms of action. This dissertation is centered around two main fundamental aims: 1) systematic study of the acute and long-term effects of kilohertz electrical stimulation and amplitude-modulated waveform with kHz carrier frequency using a well-established animal model, hippocampal brain slice, 2) study the effect of tDCS on water exchange rate across the blood-brain barrier using an advanced MRI imaging technique in a healthy population to investigate effect of tDCS stimulation on neurovascular units. The neuronal membrane has a well-established low pass filtering characteristic. This feature attenuates the sensitivity of the nervous system to any waveforms with high-frequency components. On the contrary, kilohertz stimulation has recently revolutionized spinal cord stimulation and even generated promising results in transcranial electrical stimulation. Investigating the effect of low kilohertz stimulation for neuromodulation is of huge interest. In chapters 2 and 3, several experimental designs are used to systematically investigate the frequency and dose-response of neuronal activity to unmodulated and amplitude modulated waveforms in low kilohertz range. The results support the theory of membrane attenuation of high-frequency stimulation. This dissertation provides the first direct in vitro evidence on acute effects of kilohertz electrical stimulation on modulating gamma oscillation using both unmodulated and Amplitude-modulated waveforms. While supported by membrane characteristics of neurons, we uncovered that using low kilohertz stimulation diminishes the sensitivity of hippocampal neurons to electrical stimulation. Moreover, Amplitude-Modulated waveforms can generate a different pattern of modulation with even higher sensitivity to stimulation. However, the required electric field, in this case, is still significantly higher than low-frequency stimulation methods such as tACS. Effects of DC stimulation have been studied in neuronal depolarization/hyperpolarization, synaptic plasticity, and neuronal network modulation. Recent evidence suggests that DC stimulation can induce polarity-dependent water exchange rate across the blood-brain barrier (BBB) in cell culture experiments through a mechanism called electroosmosis. Modulating water exchange rate across BBB is of broad interest in neurological diseases such as dementia, Alzheimer’s, and stroke where the brain clearance system is disrupted. Investigating the effect of electrical stimulation on water exchange across BBB can potentially lead to complimentary treatment options. In chapter 4, an advanced MRI technique was used to investigate induced changes in cerebral blood flow (CBF) and water exchange rate across BBB during stimulation in areas under electrodes. Contrary to our hypothesis, we could not resolve an effect in the water exchange rate across BBB. In conclusion, in our efforts to investigate effects of high frequency stimulation we found that sensitivity of neuronal networks to oscillating electrical stimulation is governed by time constant of neuronal membrane. Moreover, neuronal networks are selective to different kilohertz waveforms (i.e., amplitude modulated) and this is governed by a nonlinear adaptive mechanism present in the network. For the effect of DC stimulation on neurovascular units, we hypothesized that stimulation affects water exchange rate across BBB through a mechanism known as electroosmosis which is a very small portion of a large water exchange across BBB in active transport. We believe that this may be the answer to our negative results in experiments.
Brain stimulation, kilohertz stimulation, Amplitude modulated stimulation, Biomedical Engineering and Bioengineering
City University of New York - City College
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