Effects on sleep of the induction of a torpor-like state in the rat

Introduction: The periods of torpor are typically followed by the occurrence of high-Delta power NREM sleep (NREMS) (1). In the present study, this phenomenon was investigated in rats through the induction of a torpor like state by the pharmacological inhibition of the Rostral Ventromedial Medulla (RVMM), a key area in the central nervous control of metabolism, promoting thermogenesis (2). Methods: Eighteen male Sprague-Dawley rats (300–350 g) were surgically implanted, under general anaesthesia (Diazepam, 5 mg/ kg, i.m., ketamine, 100 mg/kg, i.p.), with electrodes for chronic EEG recording, a thermistor for the detection of the hypothalamic temperature (Thy), a catheter for arterial blood presssure recording, and a microcannula for drug delivery within the RVMM. Before the experiment, animals were exposed for three days to ambient conditions that are known to favor the occurrence of a natural torpor bout: ambient temperature (Ta) 15C, constant darkness and a highfat diet. Three groups of animals were microinjected within the RVMM (1 injection/h, for 6 h) with the GABAA agonist muscimol (1 mM, 100 nl; Group 1 (n = 6) and Group 2 (n = 6)) or with saline (0.9%, 100 nl, group 3 (n = 6)). One hour after the last injections, Ta was raised to 28C for groups 1 and 3, and to 37C for Group 2 to favour the return to normothermia. Results: In both Group 1 and 2, a deep hypothermia was observed during the 6-h microinjection period (Thy fell to 22.80 ± 0.8C) which was accompanied by a decrease of EEG activity and a progressive shift of the EEG spectral power towards the low-frequency region. After the injection period, EEG activity showed a progressive increase and the EEG spectral power shifted to the high frequency region. In particular, after the recovery of normothermia, a powerful intensification of Delta power in NREMS was observed, which lasted about 6 h. The peak in Delta power was significantly higher in Group 1 than in Group 2 (P < 0.01). No significant effects were observed in Group 3. Conclusion: The dynamics of EEG changes during and after the torpor-like state induction were similar to those described in natural torpor. The increase in Delta power in NREMS was attenuated under environmental conditions promoting the restoration of normothermia, suggesting a role for the degree of sympathetic activation during restoration in its determination. 1. Deboer T, Tobler I. Sleep. 2003; 26: 567–72. 2. Morrison S, Nakamura K. Front Biosci., 2011; 16: 74–104.