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

Abstract

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.

Spike frequency–dependent inhibition and excitation of neural activity by high-frequency ultrasound

“…The mechanically gated channel Piezo2 is expressed in a subset of CA1 pyramidal neurons ( Wang and Hamill, 2020 ). Piezo1 ( Qiu et al, 2019 ) and TRP (transient receptor potential) channels ( Oh et al, 2020 ; Yoo et al, 2020 ) have been experimentally linked to ultrasound neuromodulation effects. In addition, most ion channels and membrane proteins, while not functioning physiologically as mechanoreceptors, are sensitive to mechanical stimuli to some extent, either through the energetics of their interactions with the hydrophobic core of the lipid bilayer or through mechanical interactions with the cytoskeleton or extracellular matrix.…”

The Inhibitory Thermal Effects of Focused Ultrasound on an Identified, Single Motoneuron

“…An additional primary or complementary source of [K + ] O may be via two-pore potassium channels, which are thermosensitive ( Schneider et al, 2014 ), and whose conductance increases on exposure to US ( Kubanek et al, 2016 ) by a reportedly thermal mechanism ( Prieto et al, 2020 ). Importantly, both classes of potassium channels are expressed ubiquitously by neurons, and a mechanism targeting these channels would circumvent the need to limit US-based neuromodulation therapies to classes of cells that express ion channels susceptible to US mechanical activation, including Piezo ( Prieto et al, 2018 ), TRP ( Yoo et al, 2020 ), and DEG/ENaC/ASIC ( Kubanek et al, 2018 ), or to introduce non-endogenous mechanosensitive ion channels in desired target tissue a la sonogenetics ( Ibsen et al, 2015 ). Furthermore, given safety constraints associated with high pressures that may otherwise be required to modulate non-sensory neurons mechanically, thermal applications may be more practical and versatile than mechanical ones.…”

“…Population-level measurements can be difficult to interpret, as effects that appear to be direct on target tissues may result from mechanical activation of synaptically-coupled sensory neurons. Examples include auditory hair cells, which can produce widespread cortical activation following US brain application ( Guo et al, 2018 ; Sato et al, 2018 ), and cells expressing ion channels sensitive to US, including members of the Piezo ( Prieto et al, 2018 ), TRP ( Yoo et al, 2020 ), and DEG/ENaC/ASIC ( Kubanek et al, 2018 ) ion channel families, which are most commonly associated with sensory neurons.…”

Spike-frequency dependent inhibition and excitation of neural activity by high-frequency ultrasound

“…The mechanically gated channel Piezo2 is expressed in a subset of CA1 pyramidal neurons (Wang and Hamill, 2020). Piezo1 (Qiu et al, 2019) and TRP channels (Oh et al, 2020;Yoo et al, 2020) have been experimentally linked to ultrasound neuromodulation effects. In addition, most ion channels and membrane proteins, while not functioning physiologically as mechanoreceptors, are sensitive to mechanical stimuli to some extent, either through the energetics of their interactions with hydrophobic core of the lipid bilayer or through mechanical interactions with the cytoskeleton or extracellular matrix.…”