Effects of vasopressin on histamine H₁ receptor antagonist-induced spatial memory deficits in rats

The effects of [Arg⁸] vasopressin on histamine H₁ receptor antagonist-induced memory deficits were investigated using the eight-arm radial maze performance test in rats. Pyrilamine and diphenhydramine as well as scopolamine induced memory deficits characterized by increases in the number of total errors, reference memory errors and working memory errors. [Arg⁸] vasopressin improved not only scopolamine-but also pyrilamine-and diphenhydramine-induced memory deficits, although a high dose of [Arg⁸] vasopressin was needed to antagonize pyrilamine-induced memory deficits. The effects of pyrilamine on the brain [Arg⁸] vasopressin content were studied, and the hippocampus [Arg⁸] vasopressin content was shown to be decreased after pyrilamine injection. From these observations, it seems likely that [Arg⁸] vasopressin participates in not only the cholinergic system but also the histaminergic system in spatial memory.