Ultrasound Neurostimulation in Mice: Impact of Ultrasound Settings and Beam Properties

Ultrasound neurostimulation (USNS) is being investigated as a treatment approach for neuropsychiatric and neurodegenerative disorders. Indeed, unlike the existing methods that use electric or magnetic stimulation, it offers the possibility to modulate brain activity in a noninvasive way, with good spatial specificity and a high penetration capacity. However, there is no consensus yet on ultrasound parameters and beam properties required for efficient neurostimulation. In this context, this preclinical study aimed to elucidate the effect of frequency, peak negative pressure (PNP), pulse duration (PD), and focal spot diameter, on the USNS efficiency. This was done by targeting the motor cortex (M1) of 70 healthy mice and analyzing the elicited motor responses (visually and with electromyography). Also, a further investigation was performed by assessing the corresponding neuronal activity, using c-Fos immunostaining. The results showed that the success rate, a metric that depicts USNS efficacy, increased with PNP in a sigmoidal way, reaching up to 100%. This was verified at different frequencies (0.5, 1, 1.5, and 2.25 MHz) and PDs (53.3, 160, and 320 ms, at 1.5 MHz fixed frequency). Moreover, it was shown that higher PNP values were required to achieve a constant USNS efficacy not only when frequency increased, but also when the focal spot diameter decreased, emphasizing a close link between these acoustic parameters and USNS efficacy. These findings were confirmed with immunohistochemistry (IHC), which showed a strong relationship between neural activation, the applied PNP, and the focal spot diameter.