Image-guided focused ultrasound modulates electrically evoked motor neuronal activity in the mouse peripheral nervous system in vivo

Min Gon Kim, Hermes A S Kamimura, Stephen A Lee, Christian Aurup, Nancy Kwon, Elisa E Konofagou

Journal of Neural Engineering 2020 · 10.1088/1741-2552/ab6be6

rodenthealthyemg mephistology molecular

Abstract

Objective Focused ultrasound (FUS) has recently been demonstrated capable of exciting motor neuronal activity. However, comprehensive understanding of elucidated excitatory and inhibitory effects is required to better assess FUS-mediated modulation. In this study, we demonstrate that image-guided FUS can selectively modulate motor neuron activity in the mouse sciatic nerve in vivo and attribute motor responses to thermal effects. Approach FUS was applied on the sciatic nerve of anesthetized mice in vivo through the intact skin and muscle using ultrasound imaging for targeting. Both excitatory and inhibitory effects were recorded using electromyography (EMG) along with muscle response of the hind limb. The effects of FUS modulation versus heating by invasive alternative heating source (AHS) on electrically evoked EMG responses in the sciatic nerve in vivo were also investigated. The safety and reversibility of the technique were validated using histology and EMG recovery. Main results The FUS was capable of eliciting motor neuronal activity comparable to electrical stimulation ES, and facilitating motor neuronal response on electrically evoked potentials with temperature elevation up to 11.5 °C ± 0.3 °C (PRF ⩽ 40 Hz). On the other hand, FUS-induced temperature elevations above 15.1 °C ± 1.6 °C (PRF ⩾ 100 Hz) resulted in the suppression of electrically-evoked motor neuronal activity along with a decrease in EMG latency and area under the curve (AUC), which was validated against the invasive AHS with temperature elevation of 18.1 °C ± 8.5 °C. Histological findings along with EMG responses after FUS modulation demonstrated a reversible or irreversible modulation. Significance The findings reported herein indicate that image-guided FUS (PRF ⩽ 100 Hz) induces safe and controllable modulation of involuntarily evoked motor neuron activity in vivo.

Abstract via europepmc.

Speciesmouse (C57BL/6J)

Subjectsnot reported animals

Sessions per subjectnot reported

Randomisednot reported

Blindingsingle

Sham / controlnone

Auditory controlnot reported

Readout timingboth

Anaesthesiaanaesthetised

Readoutsemg mep, histology molecular — electromyography (EMG) of gastrocnemius muscle: onset/peak latency and area under the curve (AUC); H&E and TUNEL histology of sciatic nerve and muscle

Direction of effectbidirectionalLow PRF FUS (1-40 Hz, temperature rise <=11.5C) facilitated electrically evoked motor neuronal responses (decreased latency without loss of AUC), while high PRF FUS (>=100 Hz, temperature rise >=15.1C) suppressed electrically evoked EMG responses (decreased latency and AUC), matching thermally-mediated inhibition produced by an alternative heating source.

Adverse eventsobservedPRF <=100 Hz (temperature elevation 3.7-15.1C) was histologically safe (no damage, no TUNEL-positive cells); PRF of 250-500 Hz produced macrophage/neutrophil infiltration and early vacuolation of nerve fibers, and PRF of 500 Hz produced irreversible EMG changes considered unsafe modulation.

Exposures

Exposure 1: FUS to distal sciatic nerve, varying PRF

Target: sciatic nerve — “distal sciatic nerve (mouse hind limb)”
Device: Sonic Concepts · Sonic Concepts, Inc. · H-108 ✓

Pulse timing
Waveform	pulsed
Fundamental frequency (kHz)	3,100	✓✓✓
Pulse duration (ms)	1	✓✓✓
Pulse repetition frequency (Hz)	1, 10, 40, 100, 250, 500swept	✓✓✓
Duty cycle (%)	not reported	⚑
Sonication duration (s)	2	✓✓✓

Pressure and intensity, by domain
Free-field pressure (kPa)	not reported
Free-field Isppa (W/cm²)	not reported
Free-field Ispta (W/cm²)	not reported
In-situ estimate	not reported
In-situ pressure (kPa)	not reported
In-situ Isppa (W/cm²)	not reported
In-situ Ispta (W/cm²)	not reported
Pressure, domain unspecified (kPa)	11,800	✓✓✓⚑

Protocol, in the paper’s words

To control thermal effects, six different PRF values of 1 Hz, 10 Hz, 40 Hz, 100 Hz, 250 Hz, and 500 Hz were used with peak negative pressure of 11.8 MPa, PD of 1 ms, and total sonication time of 2 s, so that PRF alone determined the cooling duration between pulses and the resulting temperature elevation.

Flags from extraction

n_subjects — Paper reports n in terms of 'mice legs' used per PRF group (n=35 total used mice legs across the 6 PRF conditions), not a distinct count of animals; some animals may have contributed more than one leg.
blinding — Only the histological readers were reported as blinded ('read by pathologists who were blinded between the negative and positive control group'); the main EMG/PRF experiment itself was not described as randomised or blinded.
exposures[0].unspecified_domain.pressure_kpa — Domain (free-field water tank vs in situ) is not explicitly stated for the 11.8 MPa peak negative pressure value, though the transducer's focal dimensions were characterised via hydrophone in a water tank; placed in unspecified_domain rather than assumed free_field.
exposures[0].timing.duty_cycle_pct — Not stated directly by the paper; could be computed from pulse duration and PRF but this would be arithmetic not stated by the authors.