PURPOSE: More and more researches are exploring the neuromodulation effect of ultrasound (US) on the central nervous system (e.g. brain and retina) and the peripheral nervous system (such as skin). US stimulation has been regarded as a new noninvasive neurostimulation approach by many researchers. Our previous studies had shown that the temporal response patterns of RGCs could be modulated by US in vitro. In this article, we studied US stimulation to the retina in vivo. This study attempted to use low-frequency (2.25 MHz) focused US to stimulate the rat eyes and investigate the effect on the primary visual cortex.
METHODS: Experiments were conducted on adult male and female Sprague Dawley rats (250–300 g). A 2.25 MHz focused US transducer (D = 0.75 in., SF = 2.0 in., Olympus, Waltham, MA, USA) was used to stimulate the rat eyes in vivo. Rats were anaesthetized with urethane (5 ml/kg, 20% aqueous solution, intraperitoneally; Sigma-Aldrich, Munich, Germany). Next, the rat was laid prone on an automatic heating pad (69002, RWD Life Science Co.) at 37℃ and with its head gently immobilized using a stereotaxic frame (68028, RWD Life Science Co.). The skin on the head was swabbed with iodine and then a local anesthetic (lidocaine hydrochloride, Lidocaine 0.5%, 1mL) was injected subcutaneously along the incision line. The skull was exposed and trephined in an area (4x4 mm2) overlaying the monocular visual cortex: 6.0 mm posterior to bregma, and 3.0 mm lateral from the midline, the depth of multi-electrode arrays implantation were 300-500µm below the pia surface. US stimulation was modulated in the pulsed mode, which parameters included: pulse repetition frequency (PRF) = 1 kHz, tone burst duration (TBD) = 0.5 ms, sonication duration (SD) = 300 ms, and inter-stimulus interval (ISI) = 3 s. In each experiment, 40 trials (stimulus trains) were delivered, and there were 2-min intervals between each experiment. The neural activities from the primary visual cortex was amplified, filtered and digitized by Cerebus 64-Channel system (Animal Use).
RESULTS: As shown in Fig. 1a, the low-frequency focused US (2.25 MHz) transducer was used to stimulate the rat left eye and a 4x4 multi-electrode array was used to record neural activities from the right primary visual cortex. Some preliminary experimental results showed that the local field potentials and the single neuron spikes recorded from the primary visual cortex were both changed by US stimulation. Fig. 1b showed the responses of local field potentials that were recorded by 13 electrodes to US stimulation. The average latency of these responses was about 250 ms, which was consistent with the previous study. The peri-stimulus time histogram (PSTH) of a single spiking unit that was sorted by Offline Sorter showed that the neuron responded to US stimulation at the offset Fig. 1c.
CONCLUSIONS: Such influence on the neural activities in brain demonstrated that the low-frequency focused US was capable of stimulating retinas in vivo, which might become a novel therapy tool for ophthalmic diseases.
Figure 1. US influenced on the neural activities of visual cortex. (a) The experimental paradigm. US stimulation to the rat left eye and multi-electrode arrays recording from the right primary visual cortex. (b) The local field potentials responded to US stimulation. X-axis is time (ms) and Y-axis is recording channels. (c) The PSTH of a single spiking unit that was sorted by Offline Sorter. X-axis is time (ms) and Y-axis is firing rate (Hz) .