The Inhibitory Thermal Effects of Focused Ultrasound on an Identified, Single Motoneuron
Morgan N. Collins, Wynn Legon, Karen A. Mesce
eneuro 2021, 8, ENEURO.0514-20.2021 · 10.1523/eneuro.0514-20.2021
Abstract
Focused ultrasound (US) is an emerging neuromodulation technology that has gained much attention because of its ability to modulate, noninvasively, neuronal activity in a variety of animals, including humans. However, there has been considerable debate about exactly which types of neurons can be influenced and what underlying mechanisms are in play. Are US-evoked motor changes driven indirectly by activated mechanosensory inputs, or more directly via central interneurons or motoneurons? Although it has been shown that US can mechanically depolarize mechanosensory neurons, there are no studies that have yet tested how identified motoneurons respond directly to US and what the underlying mechanism might be. Here, we examined the effects of US on a single, identified motoneuron within a well-studied and tractable invertebrate preparation, the medicinal leech, Hirudo verbana Our approach aimed to clarify single neuronal responses to US, which may be obscured in other studies whereby US is applied across a diverse population of cells. We found that US has the ability to inhibit tonic spiking activity through a predominately thermal mechanism. US-evoked effects persisted after blocking synaptic inputs, indicating that its actions were direct. Experiments also revealed that US-comparable heating blocked the axonal conduction of spontaneous action potentials. Finally, we found no evidence that US had significant mechanical effects on the neurons tested, a finding counter to prevailing views. We conclude that a non-sensory neuron can be directly inhibited via a thermal mechanism, a finding that holds promise for clinical neuromodulatory applications.
Abstract via europepmc.
Exposures
Exposure 1: 960-kHz pulsed US applied to the dorsal posterior (DP) nerve / DE-3 motoneuron axon
Target: leech neurons — “DE-3 motoneuron axon in the dorsal posterior (DP) nerve of the medicinal leech”
Device: Sonic Concepts · Sonic Concepts · H-102MR ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 960 | ✓✓✓ |
| Pulse duration (ms) | 0.3 | ✓✓✓⚑ |
| Pulse repetition frequency (Hz) | 1,000 | ✓✓✓⚑ |
| Duty cycle (%) | 30pulse duration × PRF gives 30% | ✓✓✓⚑ |
| Sonication duration (s) | 0.1, 0.316, 1, 3.16, 10, 30swept | ✓✓✓ |
| Free-field pressure (kPa) | 660, 1,300swept | ✓✓✓ |
|---|---|---|
| Free-field Isppa (W/cm²) | 14.52 | ✓✓✓ |
| Free-field Ispta (W/cm²) | 4.84 | ✓✓✓ |
| 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 |
US pulses of 290 cycles (~300 us) applied at a 1-kHz pulse repetition frequency (intrapulse duty cycle 30%) for a 30-s stimulus train, within a 90-s trial (30 s baseline, 30 s stimulus, 30 s recovery, with longer recovery for shorter-duration pilot trials). Five shorter application durations (100 ms-10 s) were piloted first and failed to reliably modulate firing before 30 s was adopted as the standard duration.
Flags from extraction
exposures[0].timing.pulse_duration_ms— Figure 2 legend states 'Each US pulse was 300 ms in duration', but the Methods text states pulses were '290 cycles and were 300 <micro>s [us] in duration' (consistent with 290 cycles at 960 kHz ~=302 us); the figure legend's 'ms' appears to be a units/typo error, so 300 us (0.3 ms) is used.exposures[0].timing.duty_cycle_pct— Figure 2 legend gives two different duty-cycle figures in the same sentence: '50% duty cycle' and 'intrapulse duty cycle of 30%'. 30% is consistent with the stated 300 us pulse duration and 1 kHz PRF (1 ms period); 50% is not explained and may refer to a different (unspecified) duty parameter.exposures[0].timing.pulse_repetition_frequency_hz— Methods text states 'We applied 500 pulses/s at a 1-kHz pulse repetition frequency', which is internally inconsistent (500 pulses/s implies 500 Hz, not 1 kHz); 1 kHz is used here as the explicitly labeled 'pulse repetition frequency'.exposures[0].unspecified_domain.pressure_kpa— Domain (free-field vs in-situ) is not stated by the paper; the leech nerve was immersed directly in a saline bath (no intervening skull/tissue), and pressure was characterized by hydrophone in that setting.n_subjects— 26 nerves were exposed to the main 30-s/960-kHz protocol (22 analysed after exclusions); additional, overlapping/non-overlapping subsets of nerves were also tested under modified conditions (dual-recording N=4, Ca2+-free saline N=10, low-heat latex-dish N=21, pressure-doubled N=4), which are not summed into a single total.