Torpor and hibernation are hypothermic/hypometabolic (HH) states characterized by a deep drop in core temperature and EEG amplitude, and a left shift of the EEG frequency bands. The capability to induce a HH state in non-hibernators would be extremely beneficial in clinical practice. Here we show that the inhibition of the Rostral Ventromedial Medulla (RVMM), a key area in thermoregulatory cold-defence, produces a reversible HH state in free-behaving rats.

Rats were implanted under general anesthesia with: electrodes for EEG and EMG recording; a thermistor for hypothalamic temperature (Thy) recording; a catheter into the femoral artery to record arterial pressure (AP) and to derive heart rate (HR); a cannula for microinjections positioned within the RVMM. Preliminary results (n=4) show that GABAA receptors activation in the RVMM by repeated (1/h, for 6h) microinjections of muscimol (100 nl, 1 mM) induced a deep drop in Thy (°C, from 37.4 ± 0,2 to 21.2 ± 0,8) and HR (bpm, from 460 ± 7 to 159 ± 14), but almost no changes in AP. EEG theta power was strongly reduced (6.8 ± 1.8 % of baseline) and the peak within the theta band was left-shifted (Hz, from 6.9 ± 0.2 to 1.7 ± 0.1). After the treatment, all physiological variables returned within the baseline range in few hours and a large increase in EEG Delta power during non-REM sleep was observed. Present data show that specific changes in CNS activity can promote a fully reversible torpor-like state in a non-hibernator.