Memory facilitation by histamine.

The immediate posttraining intracerebroventricular administration of histamine (1 or 10, but not 0.1 or 100, ng/rat) facilitated retention test performance of step-down inhibitory avoidance behavior measured 24 hr later, in rats. The effect was antagonized by the simultaneous administration of both promethazine (1000 ng/rat) and cimetidine (1000 ng/rat), but not by either promethazine or cimetidine given alone. The antagonists had no effect of their own on behavior. The findings show that histamine has an extraordinarily powerful effect on memory processes, but do not necessarily suggest that this substance has a physiological role in memory modulation.