Spatio-temporal characterization of causal electrophysiological activity stimulated by single pulse focused ultrasound: an ex vivo study on hippocampal brain slices
Ivan M Suarez-Castellanos, Elena Dossi, Jérémy Vion-Bailly, Léa Salette, Jean-Yves Chapelon, Alexandre Carpentier, Gilles Huberfeld, William Apoutou N’Djin
Journal of Neural Engineering 2021 · 10.1088/1741-2552/abdfb1
Abstract
Objective. The brain operates via generation, transmission and integration of neuronal signals and most neurological disorders are related to perturbation of these processes. Neurostimulation by focused ultrasound (FUS) is a promising technology with potential to rival other clinically used techniques for the investigation of brain function and treatment of numerous neurological diseases. The purpose of this study was to characterize spatial and temporal aspects of causal electrophysiological signals directly stimulated by short, single pulses of FUS on ex vivo mouse hippocampal brain slices. Approach. Microelectrode arrays (MEAs) are used to study the spatio-temporal dynamics of extracellular neuronal activities both at the single neuron and neural networks scales. Hence, MEAs provide an excellent platform for characterization of electrical activity generated, modulated and transmitted in response to FUS exposure. In this study, a novel mixed FUS/MEA platform was designed for the spatio-temporal description of the causal responses generated by single 1.78 MHz FUS pulses in ex vivo mouse hippocampal brain slices. Main results. Our results show that FUS pulses can generate local field potentials (LFPs), sustained by synchronized neuronal post-synaptic potentials, and reproducing network activities. LFPs induced by FUS stimulation were found to be repeatable to consecutive FUS pulses though exhibiting a wide range of amplitudes (50-600 μ V), durations (20-200 ms), and response delays (10-60 ms). Moreover, LFPs were spread across the hippocampal slice following single FUS pulses thus demonstrating that FUS may be capable of stimulating different neural structures within the hippocampus. Significance. Current knowledge on neurostimulation by ultrasound describes neuronal activity generated by trains of repetitive ultrasound pulses. This novel study details the causal neural responses produced by single-pulse FUS neurostimulation while illustrating the distribution and propagation properties of this neural activity along major neural pathways of the hippocampus.
Abstract via europepmc.
Exposures
Exposure 1: Single-pulse FUS stimulation of hippocampal brain slice (CA3/Schaffer collaterals)
Target: CA3 — “CA3 / Schaffer collaterals (hippocampal formation)”
Device: custom-built · Meggitt A/S, Kvistgaard, Denmark · custom single-element focused transducer, PZ26 element ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 1,780 | ✓✓✓ |
| Pulse duration (ms) | 0.16, 0.2swept | ✓✓✓ |
| Pulse repetition frequency (Hz) | 0.2, 1swept | ✓✓✓⚑ |
| Duty cycle (%) | not reported | |
| Sonication duration (s) | 0.00016, 0.0002swept | ✓✓✓ |
| Free-field pressure (kPa) | 2,500, 8,000swept | ✓✓✓ |
|---|---|---|
| Free-field Isppa (W/cm²) | not reported | |
| 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 'sonication' was a single short FUS tone burst (pulse duration 160-200 microseconds across most experiments) delivered repeatedly with several seconds between pulses (PRF stated as 0.2-1 Hz across the main experiments, e.g. 1 Hz or 0.1 Hz depending on the sub-study), analogous to repeated single-pulse trials rather than a duty-cycled train within one sonication; this inter-pulse repetition rate is therefore not entered in the numeric PRF field. In a separate TTX-control experiment, the pulse duration was lengthened; the paper states this value inconsistently as 3.5 ms in one place and 5.5 ms in another (same experiment: PRF 4 Hz, 6.3 MPa, n=100), and a stable baseline response was established over ~90 s before TTX perfusion for 600 s, followed by 200 s of post-TTX FUS.
Flags from extraction
exposures[0].target— Focal spot was preferentially aimed at CA3/Schaffer collaterals but electrodes across the whole hippocampal slice (including CA1) recorded responses; treated as one target (CA3) since that is the paper's stated aim point.exposures[0].timing.pulse_repetition_frequency_hz— Paper reports a 'PRF' (0.2-1 Hz for main experiments, 4 Hz for the TTX experiment) describing the interval between separate single-pulse trials, not periodic on/off gating within one sonication; classified as waveform=continuous per schema rule and PRF left null, with the repetition rate described in protocol_description instead.exposures[0].timing.protocol_description— The TTX-experiment pulse duration is stated as '3.5 ms' in the main Methods text and as '5.5 ms' in the later summary of slice divisions, for what appears to be the same experiment (6.3 MPa, PRF 4 Hz); this discrepancy could not be resolved from the text so both values are reported and the number is not entered in a numeric field.exposures[0].in_situ.reported_as— The in_situ pressures are five COMSOL-simulated mean±SD values at different depths through the brain slice thickness (a spatial profile), not a mean or range across subjects; 'mean_or_range_across_subjects' is used here only because it is the closest available vocabulary term for a reported mean±SD.sham_type— No conventional sham arm is described; classified as active_control_site based on the control experiment where FUS pulses were applied away from the hippocampal structure (same slice, off-target) to confirm responses were not electromagnetic artifact.exposures[0].timing.waveform— set to pulsed by the tie-break: the ruled timing has a pulse repetition or duty cycle