In vitrosingle-unit recordings reveal increased peripheral nerve conduction velocity by focused pulsed ultrasound

Abstract

Abstract. Ultrasound that is widely used in medical diagnosis has drawn growing interests as a noninvasive means of neuromodulation. Focused pulsed ultrasound (FPUS) effectively modulates neural encoding and transmission in the peripheral nervous system (PNS) with unclear mechanism of action, which is further confounded by contradictory experimental outcomes from recordings of compound action potentials (CAP). To address that, we developed a novel in vitro set up to achieve simultaneous single-unit recordings from individual mouse sciatic nerve axon and systematically studied the neuromodulation effects of FPUS on individual axon. Unlike previous CAP recordings, our single-unit recordings afford superior spatial and temporal resolution to reveal the subtle but consistent effects of ultrasonic neuromodulation. Our results indicate that, 1) FPUS did not evoke action potentials directly in mouse sciatic nerve at all the tested intensities (spatial peak temporal average intensity, ISPTA of 0.91 to 28.2 W/cm 2 ); 2) FPUS increases the nerve conduction velocity (CV) in both fast-conducting A-and slow-conducting C-type axons with effects more pronounced at increased stimulus duration and intensity; and 3) effects of increased CV is reversible and cannot be attributed to the change of local temperature. Our results support existing theories of non-thermal mechanisms underlying ultrasonic neuromodulation with low-intensity FPUS, including NICE, flexoelectricity, and solition models. This work also provides a solid experimental basis to further advance our mechanistic understandings of ultrasonic neuromodulation in the PNS. IntroductionUltrasound (US) has prevailed the field of medical diagnosis for long but yet to be established as a therapeutic paradigm. Neuromodulatory effects of US were first reported in 1929 when Harvey showed that innervated skeletal muscles from frogs and turtles responded to US stimulation in vitro (1). More recent researches focus on the US effects at the central nervous system (CNS), including the disruption of blood brain barrier, motor and sensory responses, and suppressed or evoked action potentials e.g., (2-12), which culminated in the approval of the Food and Drug Administration (FDA) to treat refractory patients with essential tremor using MRI-guided focused ultrasound (13). In addition, ex vivo studies on hippocampal slice cultures have shown that, low-intensity FPUS can elicit electrical activities as indicated by calcium imaging from mouse (14) and simultaneously enhance (at fiber volley) and suppress (at dendritic layers) compound action potentials (CAP) from rat hippocampal dentate gyrus (15).In contrast, the effects of US on the peripheral nervous system (PNS) is comparatively understudied with inconsistent reports. US appears to evoke or enhance the peripheral neural activities in frog sciatic nerve in vitro (16), sensitize neuron in C. elegans (17), cause deqi sensations (i.e., tingling, numbness, heaviness, and fullness) by stimulation of an acupuncture point (LI4, He Gu) (18), an...

A Review on Ultrasonic Neuromodulation of the Peripheral Nervous System: Enhanced or Suppressed Activities?

“…We, along with some other research groups, have demonstrated that US stimulation on the PNS, with I SPTA between 1 and 200 W/cm 2 , is unlikely to induce a sufficient temperature change in the target region to elicit temperature-driven neuromodulation [43,47]. This intermediate intensity range has been explored on the PNS by several neuromodulation studies, which have been systematically reviewed in the subsequent sections.…”

“…Several studies have reported neuromodulatory effects on the CNS, with US intensity <1 W/cm 2 [15,[20][21][22]28]. So far, there seem to be no studies reporting appreciable neuromodulatory effects on the PNS with US intensity <1 W/cm 2 [43]. On the other hand, I SPTA over 200 W/cm 2 is generally considered as high-intensity US and has been tested in a number of clinical trials [44,45], after being approved by the FDA, for the ablation of cancer cells in patients via local elevation of temperature up to 85 • C [44,46].…”

“…The single-unit recordings possess a much higher sensitivity and are able to detect neuromodulatory effects within individual nerve axons, i.e., at a "microscopic level". However, single-unit recordings are technically challenging and have only been implemented by a handful of studies [43,[60][61][62][63][64][65][66][67][68], whereas CAP recordings have been widely used. Among On the other hand, single-unit recording can record action potentials from individual nerve axons ( Figure 2B), capable of capturing relatively fine variations of neural responses to mechanical, chemical, and/or thermal stimuli [57][58][59].…”

“…However, single-unit recordings are technically challenging and have only been implemented by a handful of studies [43,[60][61][62][63][64][65][66][67][68], whereas CAP recordings have been widely used. Among the above single-unit studies, neuromodulation by ultrasound was assessed only by a recent study from us [43].…”

Optimal Multichannel Artifact Prediction and Removal for Neural Stimulation and Brain Machine Interfaces

“…All procedures were approved by the University of Connecticut Institutional Animal Care and Use Committee. Sciatic nerves of male C57BL/6 mice (6-8 weeks, Taconic, Germantown, NJ, United States) were harvested for extracellular recordings from teased nerve filaments as detailed previously (Chen et al, 2017;Ilham et al, 2018). Mice were anesthetized by isoflurane inhalation, euthanized by exsanguination from perforating the right atrium, and perfused through the left ventricle with oxygenated Krebs solution (in mM: 117.9 NaCl, 4.7 KCl, 25 NaHCO 3 , 1.3 NaH 2 PO 4 , 1.2 MgSO 4 , 2.5 CaCl 2 , and 11.1 D-glucose).…”