A review of the bioeffects of low-intensity focused ultrasound and the benefits of a cellular approach

Abstract

This review article highlights the historical developments and current state of knowledge of an important neuromodulation technology: low-intensity focused ultrasound. Because compelling studies have shown that focused ultrasound can modulate neuronal activity non-invasively, especially in deep brain structures with high spatial specificity, there has been a renewed interest in attempting to understand the specific bioeffects of focused ultrasound at the cellular level. Such information is needed to facilitate the safe and effective use of focused ultrasound to treat a number of brain and nervous system disorders in humans. Unfortunately, to date, there appears to be no singular biological mechanism to account for the actions of focused ultrasound, and it is becoming increasingly clear that different types of nerve cells will respond to focused ultrasound differentially based on the complement of their ion channels, other membrane biophysical properties, and arrangement of synaptic connections. Furthermore, neurons are apparently not equally susceptible to the mechanical, thermal and cavitation-related consequences of focused ultrasound application—to complicate matters further, many studies often use distinctly different focused ultrasound stimulus parameters to achieve a reliable response in neural activity. In this review, we consider the benefits of studying more experimentally tractable invertebrate preparations, with an emphasis on the medicinal leech, where neurons can be studied as unique individual cells and be synaptically isolated from the indirect effects of focused ultrasound stimulation on mechanosensitive afferents. In the leech, we have concluded that heat is the primary effector of focused ultrasound neuromodulation, especially on motoneurons in which we observed a focused ultrasound-mediated blockade of action potentials. We discuss that the mechanical bioeffects of focused ultrasound, which are frequently described in the literature, are less reliably achieved as compared to thermal ones, and that observations ascribed to mechanical responses may be confounded by activation of synaptically-coupled sensory structures or artifacts associated with electrode resonance. Ultimately, both the mechanical and thermal components of focused ultrasound have significant potential to contribute to the sculpting of specific neural outcomes. Because focused ultrasound can generate significant modulation at a temperature <5°C, which is believed to be safe for moderate durations, we support the idea that focused ultrasound should be considered as a thermal neuromodulation technology for clinical use, especially targeting neural pathways in the peripheral nervous system.

Genetically encoded mediators for sonogenetics and their applications in neuromodulation

“…This trade-off explains why high-frequency and low-frequency FUS are typically used for superficial and deep tissue imaging, respectively. In addition to imaging, accumulating reports have shown that low-intensity FUS (averaging < 100 W/cm 2 acoustic pressure over the pulse train) can induce a wide range of bio-effects, including heating, mechanical forces, and cavitation (Figure 1) (Collins and Mesce, 2022). While the thermal effect is known to be triggered by continuous FUS at high frequencies (Pinton et al, 2010;Rossmanna and Haemmerich, 2014), the parameters of FUS that specifically cause other bio-effects are not fully understood (Chu et al, 2022).…”

“…Although many efforts have been made to evaluate the correlation between FUS parameters and induced bio-effects (Collins and Mesce, 2022), most of them focus on only one type of tissue or cell. Tissues with different acoustic properties could exhibit distinct bio-effects upon FUS stimulation.…”