Mechanism of low-frequency, low-intensity ultrasound modulation of the mouse retina.

Objective.Ultrasound has been shown to modulate the activity of retinal ganglion cells (RGCs) in mice, but the mechanism remains poorly understood. This study aims to address this question.Approach.Multi-electrode recordings together with pharmacological methods were used to investigate the possible cellular/circuitry mechanism(s) underlying the neuronal modulation induced by low-frequency (1 MHz), low-intensity (ISPTA0.5 W cm-2) ultrasound stimulation.Main results.We found that ultrasound activated mechanosensitive channels (transient receptor potential vanilloid 4 (TRPV4) channels are involved) in Müller cells, causing the release of glutamate, which acts on the extrasynapticN-methyl-D-aspartate receptors of RGCs, thus leading to the modulation of neuronal activity.Significance.Our results reveal a novel mechanism of low-frequency, low-intensity ultrasound modulation, involving TRPV4 as a mechanosensitive target for ultrasound and glutamate as an essential mediator of neuron-glia communication. These findings also demonstrate that the mechanical-force-mediated pathway is important for retinal signal modulation during visual processes, such as visual accommodation.

Low-frequency, low-intensity ultrasound modulates light responsiveness of mouse retinal ganglion cells.

Objective. Ultrasound modulates the firing activity of retinal ganglion cells (RGCs), but the effects of lower-frequency, lower-intensity ultrasound on RGCs and underlying mechanism(s) remain poorly understood. This study aims to address these questions.Approach. Multi-electrode recordings were used in this study to record the firing sequences of RGCs in isolated mouse retinas. RGCs' background firing activities as well as their light responses were recorded with or without ultrasound stimulation. Cross-correlation analyses were performed to investigate the possible cellular/circuitry mechanism(s) underlying ultrasound modulation.Main results. It was found that ultrasound stimulation of isolated mouse retina enhanced the background activity of ON-RGCs and OFF-RGCs. In addition, background ultrasound stimulation shortened the light response latency of both ON-RGCs and OFF-RGCs, while enhancing part of the RGCs' (both ON- and OFF-subtypes) light response and decreasing that of the others. In some ON-OFF RGCs, the ON- and OFF-responses of an individual cell were oppositely modulated by the ultrasound stimulation, which suggests that ultrasound stimulation does not necessarily exert its effect directly on RGCs, but rather via its influence on other type(s) of cells. By analyzing the cross-correlation between the firing sequences of RGC pairs, it was found that concerted activity occurred during ultrasound stimulation differed from that occurred during light stimulation, in both spatial and temporal aspects. These results suggest that the cellular circuits involved in ultrasound- and light-induced concerted activities are different and glial cells may be involved in the circuit in response to ultrasound.Significance. These findings demonstrate that ultrasound affects neuronal background activity and light responsiveness, which are critical for visual information processing. These results may also imply a hitherto unrecognized role of glial cell activation in the bidirectional modulation effects of RGCs and may be critical for the nervous system.

Improved Anatomical Specificity of Non-invasive Neuro-stimulation by High Frequency (5 MHz) Ultrasound.

Low frequency ultrasound (<1 MHz) has been demonstrated to be a promising approach for non-invasive neuro-stimulation. However, the focal width is limited to be half centimeter scale. Minimizing the stimulation region with higher frequency ultrasound will provide a great opportunity to expand its application. This study first time examines the feasibility of using high frequency (5 MHz) ultrasound to achieve neuro-stimulation in brain, and verifies the anatomical specificity of neuro-stimulation in vivo. 1 MHz and 5 MHz ultrasound stimulation were evaluated in the same group of mice. Electromyography (EMG) collected from tail muscles together with the motion response videos were analyzed for evaluating the stimulation effects. Our results indicate that 5 MHz ultrasound can successfully achieve neuro-stimulation. The equivalent diameter (ED) of the stimulation region with 5 MHz ultrasound (0.29 ± 0.08 mm) is significantly smaller than that with 1 MHz (0.83 ± 0.11 mm). The response latency of 5 MHz ultrasound (45 ± 31 ms) is also shorter than that of 1 MHz ultrasound (208 ± 111 ms). Consequently, high frequency (5 MHz) ultrasound can successfully activate the brain circuits in mice. It provides a smaller stimulation region, which offers improved anatomical specificity for neuro-stimulation in a non-invasive manner.