Ultrasound pulse repetition frequency preferentially activates different neuron populations independent of cell type.

Transcranial ultrasound stimulation serves as an external input to a neuron, and thus the evoked response relies on neurons' intrinsic properties. Neural activity is limited to a couple hundred hertz and often exhibits preference to input frequencies. Accordingly, ultrasound pulsed at specific physiologic pulse repetition frequencies (PRFs) may selectively engage neurons with the corresponding input frequency preference. However, most ultrasound parametric studies examine the effects of supraphysiologic PRFs. It remains unclear whether pulsing ultrasound at different physiologic PRFs could activate distinct neurons in the awake mammalian brain. 

Approach: We recorded cellular calcium responses of individual motor cortex neurons to ultrasound pulsed at PRFs of 10, 40, and 140 Hz in awake mice. We compared the evoked responses across these PRFs in the same neurons. To further understand the cell-type dependent effects, we categorized the recorded neurons as parvalbumin positive fast spiking interneurons or putative excitatory neurons and analyzed single-cell mechanosensitive channel expression in mice and humans using the Allen Brain Institute's RNA-sequencing databases.

Main results: We discovered that many neurons were preferentially activated by only one PRF and different PRFs selectively engaged distinct neuronal populations. Ultrasound-evoked cellular calcium responses exhibited the same characteristics as those naturally occurring during spiking, suggesting that ultrasound increases intrinsic neuronal activity. Furthermore, evoked responses were similar between fast-spiking inhibitory neurons and putative excitatory neurons. Thus, variation in individual neuron's cellular properties dominates ultrasound-evoked response heterogeneity, consistent with our observed cell-type independent expression patterns of mechanosensitive channels across individual neurons in mice and humans. Finally, ultrasound transiently increased network synchrony without producing prolonged over-synchronization that could be detrimental to neural circuit functions.

Significance: These results highlight the feasibility of activating distinct neuronal subgroups by varying PRF and the potential to improve neuromodulation effects by combining physiologic PRFs.&#xD.