Effects of histamine H(1) receptor antagonists on delayed spatial win-shift task in rats.

We investigated the effects of histamine H(1) receptor antagonists in 3 different stages of spatial working memory in comparison with scopolamine. The effects of drugs were measured using an eight-arm radial maze with a delayed spatial win-shift task in rats. Drugs were injected 10 min before the training phase (timing 1), immediately after the training phase (timing 2), or 10 min before the test phase (timing 3). Intrahippocampal injection of pyrilamine and diphenhydramine resulted in significant effects on the number of correct choices and across-phase errors in all 3 timings. Scopolamine impaired the number of correct choices and across-phase errors in timing 1 and 3; however, no significant effect was observed in timing 2. These results indicate that pyrilamine and diphenhydramine impaired encoding, retention and retrieval processes of spatial working memory, whereas scopolamine did not impair the retention process.