The Effects of a Brief Light Pulse and Intracerebroventricular Injection on Sleep-Wake Activity in Rats

Abstract

Objectives The light/dark (L/D) cycle is an important regulator of the circadian rhythm of sleep-wake activity and neuroendocrine fluctuation. We previously reported that the intracerebroventricular (ICV) injection of corticotropin-releasing hormone (CRH) antisense oligodeoxynucleotide during the middle of the dark period altered spontaneous waking; however, it is believed that either the light pulse during the ICV injection or the injection per se in the middle of the L/D period may disturb circadian rhythm of rats. The purpose of this study was to further elucidate the effects of ICV injection and the light pulse given during the dark period and during the light period on sleep-wake activity. Methods. Male Sprague-Dawley rats were implanted with electroencephalogram (EEG) electrodes and an ICV guide cannula. Rats were accustomed to a 12/12h L/D cycle. Na rats received a 5-mm period of light flash either in the middle of the light period or in the middle of the dark period, whereas the well-habituated rats received a light pulse and ICV injection of pyrogen-free saline (PFS) either in the middle of the light period or in the middle of the dark period. Results In naive rats, the 5-mm period of light pulse given during the dark period of the L/D cycle significantly enhanced slow wave sleep (SWS) and decreased rapid eye movement sleep (REMS) during the hour when the light flashed. Sleep was not altered when the light pulse occurred during the light period. There was no change in brain temperature. In well-habituated rats, the light pulse with ICV injection of PFS decreased SWS and REMS during the hour when the manipulation was given; during the subsequent 12-h period, the SWS and REMS significantly rebounded. Brain temperature increased after ICV injection. Conclusions. Our results indicate that injection and handling, which are considered to be stressors to rats, either during the middle of light period or during the middle of the dark period, mask the effect of light pulse on sleep-wake activities.