Histamine enhances inhibitory avoidance memory consolidation through a H2 receptor-dependent mechanism

Several evidences suggest that brain histamine is involved in memory consolidation but the actual contribution of the hippocampal histaminergic system to this process remains controversial. Here, we show that when infused into the CA1 region of the dorsal hippocampus immediately after training in an inhibitory avoidance task, but not later, histamine induced a dose-dependent promnesic effect without altering locomotor activity, exploratory behavior, anxiety state or retrieval of the avoidance response. The facilitatory effect of intra-CA1 histamine was mimicked by the histamine N-methyltransferase inhibitor SKF-91844 as well as by the H2 receptor agonist dimaprit and it was blocked completely by the H2 receptor antagonist ranitidine. Conversely, the promnesic action of histamine was unaffected by the H1 receptor antagonist pyrilamine, the H3 receptor antagonist, thioperamide, and the NMDAr polyamine-binding site antagonist ifenprodil. By themselves, ranitidine, pyrilamine, thioperamide, and ifenprodil did not affect IA memory consolidation. Our data indicate that, when given into CA1, histamine enhances memory consolidation through a mechanism that involves activation of H2 receptors; however, endogenous CA1 histamine does not seem to participate in the consolidation of IA memory at least at the post-training times analyzed.