Focused ultrasound excites action potentials in mammalian peripheral neurons in part through the mechanically gated ion channel PIEZO2
Benjamin U. Hoffman, Yoshichika Baba, Stephen A. Lee, Chi-Kun Tong, Elisa E. Konofagou, Ellen A. Lumpkin
Proceedings of the National Academy of Sciences 2022, 119 · 10.1073/pnas.2115821119
Abstract
Neurons of the peripheral nervous system (PNS) are tasked with diverse roles, from encoding touch, pain, and itch to interoceptive control of inflammation and organ physiology. Thus, technologies that allow precise control of peripheral nerve activity have the potential to regulate a wide range of biological processes. Noninvasive modulation of neuronal activity is an important translational application of focused ultrasound (FUS). Recent studies have identified effective strategies to modulate brain circuits; however, reliable parameters to control the activity of the PNS are lacking. To develop robust noninvasive technologies for peripheral nerve modulation, we employed targeted FUS stimulation and electrophysiology in mouse ex vivo skin-saphenous nerve preparations to record the activity of individual mechanosensory neurons. Parameter space exploration showed that stimulating neuronal receptive fields with high-intensity, millisecond FUS pulses reliably and repeatedly evoked one-to-one action potentials in all peripheral neurons recorded. Interestingly, when neurons were classified based on neurophysiological properties, we identified a discrete range of FUS parameters capable of exciting all neuronal classes, including myelinated A fibers and unmyelinated C fibers. Peripheral neurons were excited by FUS stimulation targeted to either cutaneous receptive fields or peripheral nerves, a key finding that increases the therapeutic range of FUS-based peripheral neuromodulation. FUS elicited action potentials with millisecond latencies compared with electrical stimulation, suggesting ion channel–mediated mechanisms. Indeed, FUS thresholds were elevated in neurons lacking the mechanically gated channel PIEZO2. Together, these results demonstrate that transcutaneous FUS drives peripheral nerve activity by engaging intrinsic mechanotransduction mechanisms in neurons [B. U. Hoffman, PhD thesis, (2019)].
Abstract via europepmc.
Exposures
Exposure 1: FUS stimulation of cutaneous receptive fields (RF) of saphenous-nerve mechanosensory afferents
Target: saphenous nerve — “cutaneous receptive field (RF) of saphenous nerve afferents”
Device: Sonic Concepts · Sonic Concepts · SU-107 ✓
| Waveform | continuous | |
|---|---|---|
| Fundamental frequency (kHz) | 3,570 | ✓✓✓ |
| Pulse duration (ms) | not applicable | |
| Pulse repetition frequency (Hz) | not applicable | |
| Duty cycle (%) | not applicable | |
| Sonication duration (s) | 0.0001, 0.002swept | ✓✓✓ |
| Free-field pressure (kPa) | not reported | |
|---|---|---|
| Free-field Isppa (W/cm²) | 11, 743swept | ✓✓✓ |
| Free-field Ispta (W/cm²) | not reported | |
| In-situ estimate | not applicable | |
| In-situ pressure (kPa) | not applicable | |
| In-situ Isppa (W/cm²) | not applicable | |
| In-situ Ispta (W/cm²) | not applicable |
Each FUS stimulus was a single uninterrupted sinusoidal burst (0.1-2.0 ms duration, 0.1-0.5 ms steps) at 11-743 W/cm2 (25-60 W/cm2 steps), delivered to the receptive field via a laser-guided immersion cone, with a 5 s interstimulus interval; stimulus order was typically from short to long duration and low to high intensity (not randomized). Each parameter set was presented 4-10 times per neuron.
Exposure 2: FUS stimulation of the saphenous nerve trunk (NT)
Target: saphenous nerve — “saphenous nerve trunk (NT)”
Device: Sonic Concepts · Sonic Concepts · SU-107 ✓
| Waveform | continuous | |
|---|---|---|
| Fundamental frequency (kHz) | 3,570 | ✓✓✓ |
| Pulse duration (ms) | not applicable | |
| Pulse repetition frequency (Hz) | not applicable | |
| Duty cycle (%) | not applicable | |
| Sonication duration (s) | 0.0001, 0.002swept | ✓✓✓ |
| Free-field pressure (kPa) | not reported | |
|---|---|---|
| Free-field Isppa (W/cm²) | 11, 743swept | ✓✓✓ |
| Free-field Ispta (W/cm²) | not reported | |
| In-situ estimate | not applicable | |
| In-situ pressure (kPa) | not applicable | |
| In-situ Isppa (W/cm²) | not applicable | |
| In-situ Ispta (W/cm²) | not applicable |
The same FUS transducer/immersion-cone system used for RF stimulation was targeted to the saphenous nerve trunk (NT) instead of the receptive field; specific stimulus duration/intensity values for NT stimulation are not separately restated in the text. FUS-evoked compound action potentials were elicited from Ab, Ad, and C-fiber activity. The 50% sonication-energy threshold to activate action potentials was significantly higher at NTs than RFs (medians: Ab fibers, RF 175 nJ vs NT 764 nJ; Ad fibers, RF 203 nJ vs NT 627 nJ).
Flags from extraction
n_subjects— Paper reports neuron/fiber counts (e.g., n=172/172 neurons; class counts in Fig. 2) but does not state the total number of mice used for the main RF/NT excitation experiments; mouse counts are only given for the separate PIEZO2 genotype-comparison sub-study (control: 5 mice/88 fibers; Cdx2-Cre;Piezo2fl/fl: 6 mice/81 units).exposures[1]— Nerve-trunk (NT) stimulation intensity/duration parameters are not explicitly restated in the text; assumed to use the same FUS transducer/cone system described for RF stimulation, but exact applied values for NT trials are not given, so numeric fields are left not_reported.model_system— Classified as ex_vivo_tissue because FUS was applied to an excised, perfused mouse skin-saphenous nerve preparation rather than a live intact animal, even though the source animals were rodents.