Effect of electric foot shocks, immobilization, and corticosterone administration on the sleep-wake pattern in the...

Knowledge concerning the impact of stressful situations on the sleep-wake pattern has been growing rapidly in the last decade. Immobilization (IMB) in rats elicits a significant increase of rapid eye movement (REM) sleep during the following 10 h. Participation of the adrenergic system has been clearly shown in this effect. On the other hand, it is well known that the time of the circadian cycle in which the stressor is applied could influence the results. It is also well known that the activation of the hypothalamic-pituitary-adrenal (HPA) axis, the release of corticosterone (COR), and the activation of the adrenergic and of the opioidergic systems are the most evident effects of stress. In the present study, we analyzed the effects of two stressors, IMB and electric foot shocks (EFS), on 24 h of continuous sleep recordings. These stressors were applied immediately before the onset of the light period. COR was also administered in an attempt to replicate the stressor-induced effects. Adult, male Wistar rats were chronically implanted for sleep recording, and after a recovery period and a 24-h basal sleep recording, they were submitted to EFS, COR, and IMB. A 10-day period elapsed between each treatment, and all of them were applied during the last moments of the dark phase of the light cycle. Results showed that IMB increased the percentage of REM sleep (83.7%) and slow-wave sleep II (SWS II; 17.3%) mainly during the dark phase (i.e., after 12 h), while EFS and COR administration elicited only slight and transient changes in the sleep-wake pattern. These data suggest that IMB applied to rats at the end of the dark cycle is effective in producing a sleep-elevating response, although this effect is enhanced during the dark phase. It seems, however, that not all the stressful situations are capable of eliciting this sleep-promoting effect, and also that COR release does not mediate this response.