In vivo characterization of focused ultrasound neuromodulation in peripheral nerves

Ultrasound, long used in a consolidated and widespread way as a diagnostic means, has emerged as a promising therapeutic method for procedures requiring non-invasiveness, target specificity, and temporal efficiency. In recent years, it has been demonstrated that delivering low-intensity focused ultrasound (LIFUS) to nervous tissues can induce inhibitory and/or excitatory effects in the central nervous system (CNS) and peripheral nervous system (PNS). FUS stimulation of the CNS has been widely developed, and several in vivo studies have demonstrated that it can elicit responses in motor and sensory neurons. On the other hand, ultrasonic neuromodulation of the PNS is little explored in vivo, and its mechanisms of action are still uncertain. Studies have shown that LIFUS stimulation of peripheral nerves is able to induce the generation of de novo action potentials in ex vivo settings. However, further in vivo investigations are required to evaluate ultrasound-mediated modulation and the impact of the acoustic stimulation parameters on the neural response. Indeed, a characterization of in vivo modulation of PNS would offer a crucial supplementary tool for targeted, non-invasive treatment of peripheral neuropathy. In this context, the objective of the current study is to characterize peripheral focused ultrasound neuromodulation in vivo by investigating the variety of FUS parameters capable of inducing neural responses in the rat sciatic nerve. The experimental procedure employed to achieve this goal involved FUS stimulation of the sciatic nerve of an anesthetized rat, while nerve compound action potentials (CAPs) were recorded through a cuff electrode. The ultrasonic stimulation protocol consisted of the delivery of single pulses with different amplitude and width. In addition, subsequent procedures were conducted to characterize and optimize the experimental setup. Stimulation of the rat spinal cord was performed to assess the recording sensitivity of different types of electrodes. The neural signals were processed and analyzed to evaluate the ultrasonic parameters that can ensure effective stimulation. As a result, short pulses (less than 5 ms) are not able to induce any response to stimulation. For longer pulses, the success rate increases with increasing pulse amplitudes. Ultrasonic pulses with amplitudes of 6 and 8 MPa and widths of 10 ms achieve a maximum success rate of about 80%. By computing the area under the curve, the presence of increased neural activity in the FUS excitation period is quantified. The amplitude of the CAPs exhibits a growing trend with the amplitude of stimulation, while there is no discernible trend in the latency of the CAPs with respect to the intensity of stimulation. The findings reported herein indicate that FUS stimulation can modulate neural activity in peripheral nerves in vivo. However, more experimental acquisitions are needed to standardize the procedure, and more data is required for a comprehensive analysis.