A study on fear memory retrieval and REM sleep in maternal separation and isolation stressed rats.

As rapid brain development occurs during the neonatal period, environmental manipulation during this period may have a significant impact on sleep and memory functions. Moreover, rapid eye movement (REM) sleep plays an important role in integrating new information with the previously stored emotional experience. Hence, the impact of early maternal separation and isolation stress (MS) during the stress hyporesponsive period (SHRP) on fear memory retention and sleep in rats were studied. The neonatal rats were subjected to maternal separation and isolation stress during postnatal days 5-7 (6h daily/3d). Polysomnographic recordings and differential fear conditioning was carried out in two different sets of rats aged 2 months. The neuronal replay during REM sleep was analyzed using different parameters. MS rats showed increased time in REM stage and total sleep period also increased. MS rats showed fear generalization with increased fear memory retention than normal control (NC). The detailed analysis of the local field potentials across different time periods of REM sleep showed increased theta oscillations in the hippocampus, amygdala and cortical circuits. Our findings suggest that stress during SHRP has sensitized the hippocampus-amygdala-cortical loops which could be due to increased release of corticosterone that generally occurs during REM sleep. These rats when subjected to fear conditioning exhibit increased fear memory and increased fear generalization. The development of helplessness, anxiety and sleep changes in human patients, thus, could be related to the reduced thermal, tactile and social stimulation during SHRP on brain plasticity and fear memory functions.

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.