Ultrasound Stimulations Induce Prolonged Depolarization and Fast Action Potentials in Leech Neurons

Abstract

Objective: Ultrasound (US) stimulation carries the promise of a selective, reversible, and non-invasive modulation of neural activity without the need for genetic manipulation of neural structures. However, the mechanisms of US-induced generation of action potentials (APs) are still unclear. Methods: Here we address this issue by analyzing intracellularly recorded responses of leech nociceptive neurons to controlled delivery of US. Results: US induced a depolarization linearly accumulating in time and outlasting the duration of the stimulation. Spiking activity was reliably induced for an optimal US intensity range. Moreover, we found that APs induced by US differ in smaller amplitude and faster repolarization from those induced by electrical stimulation in the same cell but display the same repolarization rate. Conclusions: These results shed light on the mechanism by which spikes are induced by US and pave the way for designing more efficient US stimulation patterns.

Focused Ultrasound Neuromodulation and the Confounds of Intracellular Electrophysiological Investigation

“…We obtained these results by studying a synaptically-isolated identified motoneuron in the well-studied medicinal leech, Hirudo verbana. This work stands in contrast to some other single-cell reports whereby US was found to induce neuronal excitation via depolarization of the resting membrane potential (Tyler et al, 2008;Lin et al, 2019;Dedola et al, 2020). Because we used extracellular suction electrodes versus intracellular or patch electrodes to record action potentials from the axons of our identified neuron, we considered whether different recording methodologies might contribute to a phenomenon of excitation versus inhibition.…”

“…All data points are visible in Figure 4B. Despite our awareness of others achieving similar results with respect to US-induced depolarization (Dedola et al, 2020), several factors gave us pause with respect to the legitimacy of our data. First, we observed high variability in responses to our tested pressures, which was less expected in this system than others because of our use of the same identified neuron in all preparations.…”

“…The two Retzius neurons per segmental ganglion are electrotonically coupled and nearly isopotential (Hagiwara and Morita, 1962;Eckert, 1963). To compare our findings with a recent intracellular investigation of US on leech nociceptive (N) cells (Dedola et al, 2020), we performed additional experiments on this cell type.…”

“…5A). This cell was chosen because of its usage in a recent study in which US was reported to depolarize leech neurons in an intracellular paradigm (Dedola et al, 2020). We adjusted pulse parameters to mimic more closely those found to be effective in eliciting a response in N cells: we applied a single pulse of continuous US with a 300-ms pulse duration (Fig.…”

“…Efforts to elucidate how US modulates neural activity have been confounded by US activation of mechanosensory structures, including auditory hair cells (Guo et al, 2018;Sato et al, 2018). To circumvent these and other complicating factors, we and other groups have examined how US influences neurons on a foundational level in tractable invertebrate systems (Wright et al, 2015(Wright et al, , 2017Yoo et al, 2017;Kubanek et al, 2018;Dedola et al, 2020), mammalian cell culture (Muratore et al, 2009;Qiu et al, 2019), or slice (Rinaldi et al, 1991;Bachtold et al, 1998;Tyler et al, 2008;Prieto et al, 2018).…”