Electroencephalographic effects of RVMM inhibition in free‐behaving rats

Abstract

Decrease in core temperature during torpor is concomitant with a left shift in the peak of the theta band of the EEG (5,5–9 Hz). It has been shown that inhibition of rostral ventromedial medulla (RVMM), produces a deep hypothermia in free behaving rats, caused by a massive peripheral vasodilation, and a concomitant inhibition of thermogenesis. The aim of our study was to investigate if the hypothermia obtained by RVMM inhibition in free behaving rat (that does not display spontaneous episode of torpor) also produced a shift in the theta frequency of the EEG. Eleven male CD Sprague‐Dawley rats were used. Animals were implanted under general anaesthesia with: electrodes for EEG recording, an hypothalamic thermistor to record deep brain temperature and a microcannula in RVMM for the drugs microinjection. Following 7 days of recovery, experiments were performed in a thermoregulated and sound‐attenuated box. After 2‐h baseline recording, animals were injected in RVMM with 100 nl of the following: a) 1 mM GABA‐A agonist muscimol b) isotonic saline (NaCl 0.9%). Muscimol evoked a rapid cutaneous vasodilation followed by a deep hypothermia and by a left shift in the theta band. These data shown that the theta band of the EEG is left‐shifted after centrally induced hypothermia, even in animals that don't display episodes of torpor. Supported by MUR, Italy, University of Bologna, and ESRS (Sanofi‐Aventis Research Grant).