The Impact of Anesthesia on Ultrasonic Glymphatic Perturbation Protocol: Alertness, and Glymphatic Influx

We have recently discovered that transcranial low-intensity focused ultrasound can enhance glymphatic transport, which facilitates the removal of waste metabolites from the brain in a preclinical rat model [1]. The central hypothesis was that ultrasound, functioning as a pressure wave, has the potential to influence the convective forces generated by arterial pulsations, which are also pressure waves, serving as the primary mechanism for glymphatic transport. Importantly, our data revealed that (i) the ultrasound protocol (650 kHz at 0.2MPa for 10 minutes to the entire brain) is safe, as histological evaluations showed no parenchymal damage and no difference in levels of neuronal degeneration or astrocytic activation 72 hours after the intervention, and (ii) the required pressure is significantly low, 10 times below FDA-approved limits for diagnostic ultrasound, and can be achieved using existing FDA-approved clinical transcranial focused ultrasound systems, suggesting its ease of translation into clinical applications. However, the study was conducted under specific physiological states with approximately 2.5% isoflurane-induced anesthesia. There is a limitation in fully understanding the potential bioeffects of ultrasound within the glymphatic space across various physiological conditions, primarily because the glymphatic transports efficiency fluctuates with anesthetically induced physiological states. Hence, establishing standardized protocols of ultrasonic glymphatic transport at optimal physiological state is crucial for ensuring consistent, reliable results across research laboratories during the preclinical development of this technique. The primary objective of this study is to evaluate how ultrasonically manipulated glymphatic transport is influenced by different levels of isoflurane-induced anesthetic conditions, including 3% (just above 2.5%), 2% (slightly below 2.5%), and 1.5% (quasi-awake state, resembling the existing clinical practice of ultrasound treatment). Results show that the impact of ultrasound during glymphatic transport depends on the level of anesthesia. It increases alertness in lightly anesthetized animals and improves glymphatic transport, while it induces drowsiness in heavily anesthetized animals, leading to reduced glymphatic transport. The results suggest that lighter anesthesia is beneficial for efficient ultrasonic glymphatic transport, making it more in line with the awake state in current clinical ultrasound treatments, thus advancing the technology closer to clinical translation.