Possible Role of Dopamine in the Enhancement of Intrahippocampal Arc Protein Expression Induced by Post-Learning Noradrenergic Stimulation of the Basolateral Amygdala

Extensive research in laboratory rodents has shown that novelty exposure enhances the consolidation of memories for preceding or following low-arousal events by elevating dopamine release in the dorsal hippocampus (dHipp). Additionally, numerous studies have demonstrated that post-encoding noradrenergic activation of the basolateral amygdala (BLA) can also enhance memory consolidation in dHipp. Since the BLA is most active during emotionally arousing or stress-related situations, it was suggested that this nuclear complex mediates the effects of emotional salience on memory consolidation. However, it is presently unknown whether the reinforcement of memory storage in dHipp induced by novel and arousing environmental conditions results from the interaction between these two modulatory systems. To test the hypothesis of a functional interaction between dopaminergic and noradrenergic systems, this study assessed their combined effects on a low-arousal object-location (OL) task. Rats received post-training intra-BLA infusions of vehicle or clenbuterol (Clen), a selective β-adrenoceptor agonist. Novelty-induced dopamine release in the dHipp was enhanced by omitting habituation prior to training, and the contribution of dopamine signaling was further evaluated using pre-infusion administration of the D1/D5 receptor antagonist SCH 23390. The distribution of two important proteins for memory processing, namely the activity-regulated cytoskeleton-associated protein (Arc) and the phosphorylated form of the transcription factor, cAMP-response element-binding protein (pCREB) in the dHipp, was explored in a subset of rats perfused 60 min after the training phase. Stimulation of the BLA significantly increased the number of Arc- and pCREB-positive cells in several dHipp areas. The preceding application of SCH 23390, however, substantially decreased these effects in the same areas, i.e., the dentate gyrus (DG), CA2, and CA1 subregions for pCREB, and the CA3b, CA3c, CA2, and CA1 subregions for Arc. This interaction is considered essential for the initial stages of memory consolidation. The obtained results indicate the presence of a region-specific interaction between BLA modulatory inputs and intrahippocampal dopaminergic transmission, the mechanisms of which remain to be elucidated.