Effects of intracerebroventricular histamine injection on circadian activity phase entrainment during rapid illumination changes

Histamine is reported to have different effects on shifting the circadian activity phase depending on its circadian administration time (CT). The delay-sensitive period is CT 12–15, and the advance-sensitive period is CT 0–3. The activity phase of rats was entrained by a new light-dark cycle within a week in groups treated with either saline or i.c.v. histamine at CT 12–15. However, on treatment at CT 0–3 the activity phase of the group treated with histamine was entrained by the new light-dark cycle in half the period required for entrainment in the control group.