Focused ultrasound excites neurons via mechanosensitive calcium accumulation and ion channel amplification

Ultrasonic neuromodulation has the unique potential to provide non-invasive control of neural activity in deep brain regions with high spatial precision and without chemical or genetic modification. However, the biomolecular and cellular mechanisms by which focused ultrasound excites mammalian neurons have remained unclear, posing significant challenges for the use of this technology in research and potential clinical applications. Here, we show that focused ultrasound excites neurons through a primarily mechanical mechanism mediated by specific calcium-selective mechanosensitive ion channels. The activation of these channels results in a gradual build-up of calcium, which is amplified by calcium- and voltage-gated channels, generating a burst firing response. Cavitation, temperature changes, large-scale deformation, and synaptic transmission are not required for this excitation to occur. Pharmacological and genetic inhibition of specific ion channels leads to reduced responses to ultrasound, while over-expressing these channels results in stronger ultrasonic stimulation. These findings provide a critical missing explanation for the effect of ultrasound on neurons and facilitate the further development of ultrasonic neuromodulation and sonogenetics as unique tools for neuroscience research

Computational modeling of a single-element transcranial focused ultrasound transducer for subthalamic nucleus stimulation

Objective. While transcranial focused ultrasound is a very promising neuromodulation technique for its non-invasiveness and high spatial resolution, its application to the human deep brain regions such as the subthalamic nucleus (STN) is relatively new. The objective of this study is to design a simple ultrasound transducer and study the transcranial wave propagation through a highly realistic human head model. The effects of skull morphology and skull and brain tissue properties on the focusing performance and energy deposition must therefore be known. Approach. A full-wave finite-difference time-domain simulation platform was used to design and simulate ultrasound radiation from a single-element focused transducer (SEFT) to the STN. Simulations were performed using the state-of-the-art Multimodal Imaging-based and highly Detailed Anatomical (MIDA) head model. In addition, the impact of changes in sound speed, density, and tissue attenuation coefficients were assessed through a sensitivity analysis. Main results. A SEFT model was designed to deliver an intensity of around 100 W m(-2) to the STN region; 20% of the STN volume was sonicated with at least half of the maximum of the peak intensity and it was predicted that 61.5% of the volume of the beam (above half of the peak intensity) falls inside the STN region. The sensitivity analysis showed that the skull's sound speed is the most influential acoustic parameter, which must be known with less than 1.2% error to obtain an acceptable accuracy in intracranial fields and focusing (for less than 5% error). Significance. Ultrasound intensity delivery at the STN by a simple single element transducer is possible and could be a promising alternative to complex multi-element phased arrays, or more general, to invasive or less focused (non-acoustic) neuromodulation techniques. Accurate acoustic skull and brain parameters, including detailed skull geometry, are needed to ensure proper targeting in the deep brain region

Structural, functional, and behavioral plasticity of sensorimotor integration

The general purpose of this dissertation was to analyze structural, functional, and behavioral changes of sensorimotor integration. Study 1 was designed to analyze the effects of altered stretch reflex sensitivity, via vibration, on motor unit behavior. We discovered that altering muscle spindle function resulted in altered MU behavior, with an increase in recruitment thresholds, decrease in firing rates, and reduced motor output; 9% reduction in maximal strength. Study 2's goal was to analyze structural and functional changes associated with aging, their relationships, and how they affect performance. Older adults had smaller CSARF (~5cm2), lower MQRF (~14 au), greater RMAG (~10% MVC), less STR (~101Nm), slower RTD (~554 Nm/s), and poorer balance (~ 0.5 sway index) when compared to the younger. The middle-age group had smaller CSARF (~2 cm2), less STR (~ 39 Nm), slower RTD (~ 415 Nm/s), and poorer performance (~0.29 sway index) when compared to the young group. When comparing the middle-age group to the older group, the older group had smaller CSARF (~3cm2), less STR (~63 Nm) and poorer balance performance in the balance conditions EOSS and ECSS (~0.55 sway index). Variance in STR and RTD were explained by CSARF and RMAG (STR: ~65%; RTD: ~13%). However, most of the variance in RTD was explained by MQRF (~32%). Regarding balance, RMAG was the functional variable explaining most of the variance in EOSS (~30%) condition and CSARF explained most of the variance in ECSS (~99%). Study 3 was designed to identify the determinants behind the age-related changes in antagonist muscle coactivation. The study is still in progress; so far it seems that the variables related to greater coactivation are: cortical agonist-antagonist representation areas overlap, the location of the muscles center of gravity and cortical inhibition. Therefore, age alters many dimensions in the control of voluntary movement; in these studies, we showed the relationship between spindle function and motor unit behavior and output; then we saw discovered significant relationships between sensory variables and motor variables and how they affect performance. In conclusion, to understand the aging process, it is important to analyze plasticity of sensorimotor integration as a whole

Development of a novel diffuse correlation spectroscopy platform for monitoring cerebral blood flow and oxygen metabolism: from novel concepts and devices to preclinical live animal studies

New optical technologies were developed to continuously measure cerebral blood flow (CBF) and oxygen metabolism (CMRO2) non-invasively through the skull. Methods and devices were created to improve the performance of near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) for use in experimental animals and humans. These were employed to investigate cerebral metabolism and cerebrovascular reactivity under different states of anesthesia and during models of pathological states. Burst suppression is a brain state arising naturally in pathological conditions or under deep general anesthesia, but its mechanism and consequences are not well understood. Electroencephalography (EEG) and cortical hemodynamics were simultaneously measured in rats to evaluate the coupling between cerebral oxygen metabolism and neuronal activity in the burst suppressed state. EEG bursts were used to deconvolve NIRS and DCS signals into the hemodynamic and metabolic response function for an individual burst. This response was found to be similar to the stereotypical functional hyperemia evoked by normal brain activation. Thus, spontaneous burst activity does not cause metabolic or hemodynamic dysfunction in the cortex. Furthermore, cortical metabolic activity was not associated with the initiation or termination of a burst. A novel technique, time-domain DCS (TD-DCS), was introduced to significantly increase the sensitivity of transcranial CBF measurements to the brain. A new time-correlated single photon counting (TCSPC) instrument with a custom high coherence pulsed laser source was engineered for the first-ever simultaneous measurement of photon time of flight and DCS autocorrelation decays. In this new approach, photon time tags are exploited to determine path-length-dependent autocorrelation functions. By correlating photons according to time of flight, CBF is distinguished from superficial blood flow. Experiments in phantoms and animals demonstrate TD-DCS has significantly greater sensitivity to the brain than existing transcranial techniques. Intracranial pressure (ICP) modulates both steady-state and pulsatile CBF, making CBF a potential marker for ICP. In particular, the critical closing pressure (CrCP) has been proposed as a surrogate measure of ICP. A new DCS device was developed to measure pulsatile CBF non-invasively. A novel method for estimating CrCP and ICP from DCS measurement of pulsatile microvascular blood flow in the cerebral cortex was demonstrated in rats.2018-03-08T00:00:00

Accurate Simulation of Low-Intensity Transcranial Ultrasound Propagation for Neurostimulation

Neural stimulation with low-intensity ultrasound is a potentially transformative technology with applications in therapy and research. To develop, it will require ultrasound to be tightly focused on brain structures with accurate spatial targeting and fine control over the ultrasound amplitude at the target. However, the skull is an impediment to the effective focusing of ultrasound. Simulations of ultrasound propagation through acoustic property maps derived from medical images can be used to derive focusing drive signals for multi-element arrays. Focusing effectiveness is dependent on the fidelity of the numerical simulations. In combination with MRI based treatment verification, model based focusing has been used to focus high-intensity ultrasound onto the brain for ablation. This thesis presents a thorough and systematic study of the simulation parameters required to achieve effective transcranial focusing. The literature on ultrasonic neurostimulation, transcranial ultrasonic focusing, and the derivation of property maps from medical images is reviewed. The sampling criteria required to ensure numerical accuracy for the k-space pseudospectral time domain simulation scheme is established through testing of individual sources of numerical error, and convergence testing of a simulated time-reversal protocol. With numerical accuracy assured, the importance of acoustic property maps is examined through simulations to determine the sensitivity of intracranial fields to the properties of the skull layer. These results are corroborated by matching experimental measurements of ultrasound propagation through skull bone phantoms with spatially registered simulations. Finally, the impact of image related homogenisation and loss of internal bone structure is determined using simulations through co-registered clinical CT and micro-CT data of the skull

Focused Ultrasound Mediated Blood-Brain Barrier Opening in Non-Human Primates: Safety, Efficacy and Drug Delivery

The blood-brain barrier (BBB) is physiologically essential for brain homeostasis. While it protects the brain from noxious agents, it prevents almost all currently available drugs from crossing to the parenchyma. This greatly hinders drug delivery for the treatment of neurological diseases and disorders such as Parkinson’s, Alzheimer’s and Huntington’s, as well as the development of drugs for the treatment of such diseases. Current drug delivery techniques to the brain are either invasive and target specific, or non-invasive with low special specificity. Neither group of techniques are optimal for long term treatment of patients with neurological diseases or disorders. Focused ultrasound coupled with intravenous administration of microbubbles (FUS) has been proven as an effective technique to selectively and noninvasively open the BBB in multiple in vivo models including non-human primates (NHP). Although this technique has promising potential for clinical outpatient procedures, as well as a powerful tool in the lab, the safety and potential neurological effects of this technique need to be further investigated. This thesis focuses on validating the safety and efficacy of using the FUS technique to open the BBB in NHP as well as the ability of the technique to facility drug delivery. First, a longitudinal study of repeatedly applying the FUS technique targeting the basal ganglia region in four NHP was conducted to determine any potential long-term adverse side effects over a duration of 4-20 months. The safety of the technique was evaluated using both MRI as well as behavioral testing. Results demonstrated that repeated application of the FUS technique to the basal ganglia in NHP did not generate permanent side effects, nor did it induce a permanent opening of the BBB in the targeted region. The second study investigated the potential of the FUS technique as a method to deliver drugs, such as a low dose of haloperidol, to the basal ganglia in NHP and mice to elicit pharmacodynamical effects on responses to behavioral tasks. After opening the BBB in the basal ganglia of mice and NHP, a low dose of haloperidol was successfully delivered generating significant changes in their baseline motor responses to behavioral tasks. Domperidone was also successfully delivered to the caudate of NHP after opening the BBB and induced transient hemilateral neglect. In the final section of this thesis, the safety and efficacy of the FUS technique was evaluated in fully alert NHP. The FUS technique was successful in generating BBB opening volumes larger on average to that of the BBB opening volumes in anesthetized experiments. Safety results through MRI verification as well as behavioral testing during application of the technique demonstrated that the FUS technique did not generate adverse neurological effects. Conversely, the FUS technique was found to induce slight positive effects on the response of the NHP to the behavioral task. Collectively, the work presented in this thesis demonstrates the safety and effectiveness of the FUS technique to open the BBB and deliver neuroactive drugs in the NHP

Endogenous and exogenous electric fields as modifiers of brain activity: rational design of noninvasive brain stimulation with transcranial alternating current stimulation

Synchronized neuronal activity in the cortex generates weak electric fields that are routinely measured in humans and animal models by electroencephalography and local field potential recordings. Traditionally, these endogenous electric fields have been considered to be an epiphenomenon of brain activity. Recent work has demonstrated that active cortical networks are surprisingly susceptible to weak perturbations of the membrane voltage of a large number of neurons by electric fields. Simultaneously, noninvasive brain stimulation with weak, exogenous electric fields (transcranial current stimulation, TCS) has undergone a renaissance due to the broad scope of its possible applications in modulating brain activity for cognitive enhancement and treatment of brain disorders. This review aims to interface the recent developments in the study of both endogenous and exogenous electric fields, with a particular focus on rhythmic stimulation for the modulation of cortical oscillations. The main goal is to provide a starting point for the use of rational design for the development of novel mechanism-based TCS therapeutics based on transcranial alternating current stimulation, for the treatment of psychiatric illnesses