Protective effect of low-intensity transcranial ultrasound stimulation after differing delay following an acute ischemic stroke

Background: Ischemic stroke is a clinically common disease requiring early treatment in the acute period of stroke onset. Such early treatment reduces the level of brain injury, promotes functional recovery, and improves long-term prognosis. Previous studies have demonstrated that, in the event of an acute ischemic stroke, low-intensity transcranial ultrasound stimulation (LITUS) can provide neuroprotection. However, from existing studies, the protective effect of LITUS with differing delay after an acute ischemic stroke remains unclear. Objective: The aim of the present study is to investigate whether the protective effect of LITUS in the event of an acute ischemic stroke differs with the time delay between the stroke and the application of LITUS. Methods: In the present study, 60 Sprague Dawley rats (250 +/- 10 g) were divided randomly into five groups (each group including LITUS group (6 rats) and control group (CTRL group: 6 rats)) with different treatment delays (0.5 h, 1 h, 3 h, 6 h, and 9 h). The rats were made to have an ischemic stroke by means of a distal middle cerebral artery occlusion. Next, magnetic response (MR) imaging was performed 15 min before LITUS in each group (marked as Pre-Stim). Once the MR imaging was finished, the rats were stimulated with ultrasound for 10 min (I-sppa = 2.6 W/cm(2), fundamental frequency: 500 kHz, duty cycle: 50%, stimulation duration: 400 ms) in the LITUS groups and went without ultrasound stimulation in the control (CTRL) group. Then, MR imaging was performed once every hour for three hours after LITUS (marked as Post-Stim (1 h), Post-Stim (2 h), and Post-Stim (3 h)). Finally, triphenyltetrazolium chloride staining of the rat brain tissues was performed. We analyzed the apparent diffusion coefficient (ADC) of the lesion area in Pre-Stim and Post-Stim and evaluated the differences between the LITUS groups and the CTRL group for the different treatment delays (0.5 h, 1 h, 3 h, 6 h, and 9 h). Results: Compared to the CTRL group, LITUS significantly inhibited the ADC decrease (i) at the Post-Stim (1 h), Post-Stim (2 h), and Post-Stim (3 h) stages when the rats were stimulated at 0.5 h, (ii) at Post-Stim (1 h) and Post-Stim (2 h) when the rats were stimulated at 1 h and 3 h, and (iii) at Post-Stim (1 h) when the rats were stimulated at 6 h. There was no significant change in the ADC at Post-Stim (1 h), Post-Stim (2 h), and Post-Stim (3 h) when the rats were stimulated at 9 h. Conclusion: In the event of an acute ischemic stroke, LITUS can inhibit the decrease of ADC and the effect is closely related to the delay in treatment. The earlier the ultrasound intervention, the better the protective effect.