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OBJECTIVE To investigate the influence of persistently elevated corticosterone on sleep parameters in rat and to inspect the changes in sleep related nucleus in the brain, in order to elucidate how the dysfunction of HPA axis, always seen in mental disorders such as depression, affects sleep.METHODS Male SD rat was subcutaneously injected with 4 different dose of corticosterone for continuous 7 d.Sleep parameters were examined by EEG and EMG recording.C-Fos protein and specific neurotransmitter markers immunoreactivity in sleep-and wake-active nucleus were conducted with the method of immunohistochemistry.RESULTS Corticosterone administration at 10 mg·kg-1 , 20 mg·kg-1 and 40 mg·kg-1 for 7 d can significantly reduce the total sleep time and NREM sleep time without influence on SWS and REM sleep.The dose of 0.5 mg·kg-1 can only decrease light sleep time and NREM sleep time as a consequence.Noteworthingly, the ratio of REM sleep time was remarkably increased after the treatment of 40 mg· kg-1 corticosterone for consecutive 7 d.Thus immunohistochemical experiments were processed at 40 mg· kg-1 only.The results show a decline of c-Fos expression in GABAergic neurons of VLPO (Ventrolateral preoptic area), and an increase of c-Fos immunoreactivity in noradrenergic neurons of LC (Locus coeruleus).CONCLUSION These findings demonstrate that the reduction of total sleep time and augmentation of REM sleep ratio, which resulted from incessant addition of corticosterone, were due to the alteration of sleep-wake related nucleus, for which decreased activity of GABAergic neurons in VLPO and increased activity of noradrenergic neurons in LC are most likely to responsible.