Emotional modulation of hippocampus-dependent spatial learning

Abstract

Previous research has indicated that the amygdala exerts a modulatory influence on multiple memory systems. Evidence also indicates that emotional state can influence the use of multiple memory systems and that this effect is mediated by the amygdala. Anxiogenic drugs administered during acquisition in a task that can be acquired either through hippocampus-dependent “place” learning or caudate dependent “response” learning, resulted in the predominant use of response learning. It is not known whether inducing anxiety at other behavioral time points will also influence the relative use of multiple memory systems. In experiment 1, male Long-Evans rats were trained to swim from the same start point to an escape platform constantly located in a goal arm. Prior to memory retrieval rats were administered either alpha- two adrenoceptor antagonist RS 79948-197, peripherally (0.03, 0.01, 0.3 mg/kg) or into the basolateral amygdala (0.1 µg), or saline vehicle. Rats treated with RS 79948-197 prior to memory retrieval exhibited caudate-dependent response learning. Previous studies examining the effects of RS 77948-197 on memory were conducted with rats trained in an anxiogenic state and subsequently probed in a drug free state. Experiment 2 examined whether state dependency may account for those results. Animals received peripheral (0.1 mg/kg) or intra-amygdala (0.1 µg) administration of RS 79948-197, prior to both acquisition and memory retrieval. Rats treated with RS 79948- 197 predominantly exhibited response learning. Finally, experiments 3 and 4 examined whether the use of response learning produced by RS79948-197 was due to the impairing effect on hippocampus-dependent memory. Rats that were administered peripheral (0.03 mg/kg) or intra-amygdala (0.1 µg) injections of RS 79948-197 displayed impaired acquisition of the single solution place task relative to control animals. This indicates that place learning was impaired. Over, all the present findings indicate 1) peripheral and intra-amygdala anxiogenic drug administration results in the use of habit memory at both acquisition and retrieval, 2) state dependency does not play a role in the influence of RS 799948-197 on memory system use, 3) the use of response learning produced by peripheral and intra-amygdala injections of RS 79948-197 may result from an impairing effect of hippocampusdependent memory.