Chronic stress-induced changes in REM sleep on θ oscillations in the rat hippocampus and amygdala

The present study investigated the effect of Chronic Immobilization Stress (CIS) on theta oscillations in the hippocampus and amygdala during Rapid Eye Movement (REM) sleep. Adult male Wistar rats were subjected to 2h of CIS daily for 10days. Polysomnographic recordings with electroencephalogram (EEG) from hippocampus (CA3 and CA1 subregion) and lateral nucleus of amygdala (LA) were carried out after termination of CIS protocol on the 7th, 14th and 21st day. The results showed a bimodal distribution on the total REM sleep duration in CIS rats: group of rats exhibited increased REM sleep duration considered as a stress-enhanced REM (SER) and rats with reduced REM sleep as stress-reduced REM sleep (SRR) group. The bimodal distribution in REM sleep was continued to exhibit even after 21 days of termination of stress, showing increased REM sleep in SER and reversible REM sleep in SRR rats. In addition to changes in sleep, increased REM sleep in SER rats was associated with attenuated theta activity in the hippocampus and amygdala, while the SRR rats did not show attenuated theta activities during the stress recovery period. Thus, the study demonstrates the dependence of synchronized amygdalo-hippocampal theta activity with the CIS-induced enhanced REM sleep duration. This raises the possibility that CIS-induced manifestations of the anxiety may be associated with synchronized theta oscillations in the hippocampus and amygdala.