Agomelatine restores a physiological response to stress in the aged rat.

Short duration immobilization stress (IS) in younger rats is followed by a sleep rebound involving slow wave sleep (SWS) and, more particularly, rapid eye movement (REM) sleep. This rebound, expressing the ability of the brain to confront a stress challenge, is now accepted as a marker of the homeostasis. In older rats (24-25 months), however, an IS of 1h is not followed by a sleep rebound. To determine whether this impairment is reversible, we analyzed the effects of the antidepressant agomelatine, on stress-related sleep rebound in older animals. Older and younger (3-5 months) rats were equipped with electroencephalographic (EEG) and electromyographic (EMG) electrodes and polygraphic recordings were achieved under basal conditions with a digitized set-up. Older rats were pretreated with agomelatine (40mg/kg/day) for 3 days, with IS applied on the third day, whereas younger rats were only subjected to IS. Polygraphic recordings achieved under basal conditions confirmed the conventional impairments of the sleep/wake architecture in older animals, including decreased delta power, shortened REM sleep bouts, and modified sleep/wake circadian rhythms. Older rats pretreated with agomelatine for 3 days showed a reversal of the deficit observed in the beta-1, but not in the delta, EEG power band. Application of an IS to older rats after agomelatine pretreatment resulted in a REM sleep rebound in response to stress. These findings indicate that agomelatine, by improving beta-1 EEG power band and by inducing stress-related sleep rebound in older animals, contributes to the homeostasis maintenance.