Spontaneous hypertensive rats: behavioral and corticosterone response depend on circadian phase.

Baseline and reactive motor activity and corticosterone levels of spontaneous hypertensive rats (SHR) and their normotensive controls (WKY) were compared with respect to circadian rhythmicity. In the open field, SHR showed higher ambulation and rearing and less defecation than WKY in both light and dark phases. There were significant interactions between strain and time of day in the rate of habituation. SHR showed elevated baseline motor activity, increased motor response and enhanced corticosterone response to "mild" stress only in the light phase. There may be a disorder of temporal organisation in SHR characterised by increased sensitivity to external stimuli during the sleeping state.