Enhanced neuronal activity in mouse motor cortex with microbubbles’ oscillations by transcranial focused ultrasound stimulation

Microbubbles (MBs) are known to serve as an amplifier of the mechanical effects of ultrasound, which combined with ultrasound are widely used in brain. The goal of this study is to investigate the effect of oscillating MBs on the neuronal activity in the central nervous system (CNS) of mammals. The motor cortex of mice brain was subjected to ultrasound stimulation with and without MBs, and evoked electromyogram signals were recorded. A c-fos immunofluorescence assay was performed to evaluate the neuronal activation in the region of ultrasound stimulation. BBB integrity during ultrasound stimulation with MBs was assessed in this study. Moreover, the safety of ultrasound stimulation with MBs was examined. Using ultrasound at 620 kHz, the injection of MBs significantly increased the success rate of motor response from 0.065 ± 0.06 to 0.28 ± 0.10 when stimulation was applied at 0.12 MPa and from 0.38 ± 0.09 to 0.77 ± 0.18 at 0.25 MPa (p < 0.001). The results of the c-fos immunofluorescence assay showed that the mean densities of c-fos⁺ cells were significantly increased from 15.67 ± 3.51 to 53.01 ± 9.54 at 0.12 MPa acoustic pressure. At 0.25 MPa, the mean density of c-fos + cells was 81 ± 10.97 without MBs and it significantly increased to 124.12 ± 25.71 with MBs (p < 0.05). Enhanced neuronal activities were observed with 0.12 MPa ultrasound stimulation with MBs, while the integrity of BBB was not compromised, but 0.25 MPa ultrasound stimulation with MBs resulted in BBB disruption. These findings reveal that the oscillations of MBs can enhance neuronal activity in the CNS of mammals, and may provide an insight into the application of MBs combined with ultrasound in brain.