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Objective: Focused ultrasound(FUS)canmodulateneuronalactivitybydepolarizationorhyperpolarization. Although FUS-evokeddepolarizationhasbeenstudiedextensively,themechanismsunderlyingFUS-evoked hyperpolarization (FUSH)havereceivedlittleattention.Inthestudydescribedhere,wedevelopedaprocedure using FUStoselectivelyhyperpolarizemotoraxonsincrayfish. Asapreviousstudyhadreportedthattheseaxons express mechano-andthermosensitivetwo-poredomainpotassium(K2P)channels,wetestedthehypothesisthat K2P channelsunderlieFUSH. Methods: Intracellular recordingsfromamotoraxonandamuscle fiber wereobtainedsimultaneouslyfromthe crayfish openerneuromuscularpreparation.FUSHwasexaminedwhileK2Pchannelactivitiesweremodulated by varyingtemperatureorbyK2Pchannelblockers. Results: FUSH intheaxonsdidnotexhibitacoherenttemperaturedependence,consistentwithpredictedK2P channel behavior,althoughchangesintherestingmembranepotentialofthesameaxonsindicatedwell-behaved K2P channeltemperaturedependence.Thesameconclusionwassupportedbypharmacologicaldata;namely, FUSH wasnotsuppressedbyK2Pchannelblockers.ComparisonbetweentheFUS-evokedresponsesrecordedin motor axonsandmuscle fibers revealedthatthelatterexhibitedverylittleFUSH,indicatingthattheFUSHwas specific totheaxons. Conclusion: It isnotlikelythatK2PchannelsaretheunderlyingmechanismforFUSHinmotoraxons.Alternative mechanisms suchassonophoreandaxon-specific potassiumchannelswereconsidered.Althoughthesonophore hypothesis couldaccountforelectrophysiologicalfeaturesofaxonalrecordings,itisnotconsistentwiththelack of FUSHinmuscle fibers. Anaxon-specific andmechanosensitivepotassiumchannelisalsoapossible explanation. 

Abstract—We have previously identified a novel non-selective membrane conductance (gUS) opened by focused ultrasound (FUS) in crayfish motor axons. In the work described here, we studied gUS properties further by comparing FUS-evoked depolarization (FUSD) in control and hypotonic saline with 75% of control osmolarity. The FUS was a train of 20 FUS bursts (2.1 MHz and 50 ms per burst) delivered at 1 kHz. The amplitude, onset latency, frequency of occurrence and duration of FUSD were compared in a 15-min time window before and after switching to hypotonic saline. Significant increases were observed for amplitude (p < 0.001) and frequency of occurrence (p < 0.01) while the onset latency exhibited a significant decrease (p < 0.001). FUSD duration did not significantly differ. These results support predictions based on our hypothesis that gUS is mediated by opening of nanopores in the lipid bilayer and that stretching of axonal membrane caused by swelling at low osmolarity should increase the probability of nanopore formation under FUS. The FUSD parameters, in addition, exhibited time-dependent trends when the window of observation was expanded to 45 min in each saline. The statistical significance of amplitude and duration differed between 15- and 45-min time windows, indicating the presence of adaptive responses of axonal membrane to osmotic manipulation. (E-mail: jenweilin@bu.edu) © 2022 Published by Elsevier Inc. on behalf of World Federation for Ultrasound in Medicine & Biology.