Effect of histamine on memory retrieval in old rats.

Effects of intracerebroventricular (i.c.v) injection of histamine (Hi) and its related compounds on prolongation of the response latency ensuing after a long interruption of learning were studied by testing the active avoidance response in old rats. After Hi application at doses of 50 and 100 ng, the response latency became significantly shorter than that determined in the pre-injection periods, suggesting that Hi facilitates memory retrieval in old rats. H1-agonists, 2-methylHi and 2-thiazolylethylamine, effected a dose-related shortening of response latency as seen after Hi application, whereas H2-agonists, 4-methylHi and impromidine, failed to prompt the response latency. Simultaneous i.c.v. injection of pyrilamine, a H1-antagonist and Hi abolished the Hi-induced shortening of response latency. Furthermore, intraperitoneal administration of histidine at doses of 200 and 500 mg/kg significantly shortened the response latency. Neither (R)-alpha-methylHi nor thioperamide caused a significant effect indicating that H3-receptor may not be involved in Hi-induced facilitation of memory retrieval. Based on these findings, it may be concluded that Hi takes an active part in facilitating memory recall via H1-receptor in old rats.