(K-436) Induction of a torpor-like hypothermic and hypometabolic state by ultrasound

Introduction::

Torpor, like hibernation, is an energy-conserving state in which animals dramatically decrease their metabolic rate and body temperature to survive fatal conditions. Artificially inducing a torpor-like state has great promise to improve patient survival rates in fatal conditions and enable long-duration human spaceflight. However, noninvasive and safe induction of a torpor-like state is still considered science fiction. This study introduces a novel method using focused ultrasound to induce a hypothermic and hypometabolic (UIH) state by activating hypothalamic preoptic area (POA) neurons.

Materials and Methods::

Natural torpor entails hypothermia, hypometabolism, and hypoactivity. We measured surface temperature, core body temperature, and metabolic rate in mice receiving ultrasound (US) stimulation at POA via a wearable US transducer (Fig. A) using a thermal camera, telemetry body temperature sensor, and metabolic cage. We developed a closed-loop feedback-controlled US stimulation system to maintain a prolonged torpor-like state by controlling US output based on the difference between the actual body temperature and the targeted temperature (34 °C). To uncover the neural mechanisms, we invented a unique system integrating US stimulation with the fiber-photometry recording of neural calcium activity. To further elucidate the molecular mechanisms, we conducted single-nucleus RNA-sequencing of POA neurons post-stimulation to identify potential ultrasound-sensitive ion channels associated with UIH.

Results, Conclusions, and Discussions::

Through the development and use of a wearable ultrasound transducer (Fig A), we targeted the preoptic area (POA) of freely moving mice and observed a significant decrease in body temperature (by -3.26 ± 0.19 °C, Fig A), particularly in brown adipose tissue (BAT), along with a reduction in locomotor activity. Metabolic analysis revealed a decrease in oxygen consumption rates (VO2, by 36.61 ± 1.74%) and a shift in energy substrate utilization from carbohydrates to fat oxidations. These reversible changes closely resembled natural torpor. Furthermore, we developed a closed-loop feedback control system that allowed to maintain the UIH state for more than 24 hours (24.90 ± 0.63 h) (Fig B). Investigating the neuronal mechanism, we found that ultrasound activation of POA neurons led to increased neuronal activity (Fig C), and resulted in the activation of downstream DMH neurons. To investigate the molecular mechanism, we used Single-nucleus RNA sequencing and identified the expression of TRP and PIEZO ion channels in torpor-associated neurons, with TRPM2 showing high expression level within the ultrasound-activated neuronal cluster (Fig D). Subsequent experiments confirmed TRPM2 as an ultrasound-sensitive ion channel involved in the induction of the torpor-like state (Fig D). We also successfully demonstrated UIH feasibility in a non-torpid animal, the rat. These findings suggest that UIH is a promising approach for noninvasive and safe induction of a torpor-like state.

Acknowledgements (Optional): : This work was supported by the NIH R01MH116981, UG3MH126861, R01EB027223 and R01EB030102

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