Histamine infused into basolateral amygdala enhances memory consolidation of inhibitory avoidance

The role of the basolateral amygdala (BLA) in the consolidation of aversive memory is well established. Here we investigate the involvement of the histaminergic system in BLA on this variable. Rats were chronically implanted with bilateral cannulae in the BLA and after recovery were trained in a one-trial step-down inhibitory avoidance task. Immediately after training histaminergic compounds either alone or in combination were infused through the cannulae. Memory was assessed in test sessions carried out 24 h after the training session. Post-training histamine (1-10 nmol; 0.5 mu l/side) enhanced consolidation and the histamine H-3 receptor antagonist thioperamide (50 nmol; 0.5 mu l/side) impaired memory consolidation. The effect was shared by the histamine N-methyltransferase inhibitor SKF-91844 (50 nmol; 0.5 mu l/side) as well as by the H-3 receptor agonist imetit (10 nmol; 0.5 mu l/side). The promnesic action of histamine was unaffected by the H-1 receptor antagonist pyrilamine (50 nmol; 0.5 mu l/side). The H-1 receptor agonist pyridylethylamine (10 nmol; 0.5 mu l/side), the H-2 agonist dimaprit (10 nmol; 0.5 mu l/side) and the H-2 antagonist ranitidine (50 nmol; 0.5 mu l/side) were ineffective. Histaminergic compounds infused into the BLA had no effect on open-field or elevated plus-maze behaviour. The data show that histamine induces a dose-dependent mnemonic effect in rats and indicate that this reflects a role of endogenous histamine in the BLA mediated by H-3 receptors.