Postnatal Development of Conditioned Reflex Behavior: Comparison of the Times of Maturation of Plastic Processes in the Rat Hippocampus

Infant avoidance training alters cellular activation patterns in prefronto-limbic circuits during adult avoidance learning: II. Cellular imaging of neurons expressing the activity-regulated cytoskeleton-associated protein (Arc/Arg3.1)

Positive and negative feedback learning is essential to optimize behavioral performance. We used the two-way active avoidance (TWA) task as an experimental paradigm for negative feedback learning with the aim to test the hypothesis that neuronal ensembles activate the activity-regulated cytoskeletal (Arc/Arg3.1) protein during different phases of avoidance learning and during retrieval. A variety of studies in humans and other animals revealed that the ability of aversive feedback learning emerges postnatally. Our previous findings demonstrated that rats, which as infants are not capable to learn an active avoidance strategy, show improved avoidance learning as adults. Based on these findings, we further tested the hypothesis that specific neuronal ensembles are "tagged" during infant TWA training and then reactivated during adult re-exposure to the same learning task. Using cellular imaging by immunocytochemical detection of Arc/Arg3.1, we observed that, compared to the untrained control group, (1) only in the dentate gyrus the density of Arc/Arg3.1-expressing neurons was elevated during the acquisition phase of TWA learning, and (2) this increase in Arc/Arg3.1-expressing neurons was not specific for the TWA learning task. With respect to the effects of infant TWA training we found that compared to the naïve non-pretrained group (a) the infant pretraining group displayed a higher density of Arc/Arg3.1-expressing neurons in the anterior cingulate cortex during acquisition on training day 1, and (b) the infant pretraining group displayed elevated density of Arc/Arg3.1-expressing neurons in the dentate gyrus during retrieval on test day 5. Correlation analysis for the acquisition phase revealed for the ACd that the animals which showed the highest number of avoidances and the fastest escape latencies displayed the highest density of Arc/Arg3.1-expressing neurons. Taken together, we are the first to use the synaptic plasticity protein Arc/Arg3.1 to label neuronal ensembles which are involved in different phases of active avoidance learning and whose activity patterns are changing in response to previous learning experience during infancy. Our results indicate (1) that, despite the inability to learn an active avoidance response in infancy, lasting memory traces are formed encoding the subtasks that are learned in infancy (e.g., the association of the CS and UCS, escape strategy), which are encoded in the infant brain by neuronal ensembles, which alter their synaptic connectivity via activation of specific synaptic plasticity proteins such as Arc/Arg3.1 and Egr1, and (2) that during adult training these memories can be retrieved by reactivating these neuronal ensembles and their synaptic circuits and thereby accelerate learning.

Impaired active avoidance learning in infant rats appears to be related to insufficient metabolic recruitment of the lateral septum

The temporal dissociation between early information acquisition and output of complex behaviors is a common principle during development. Thus, although infant rats are not able to generate sufficient avoidance behavior during two-way active avoidance (TWA) training they obviously deposit a certain "memory trace" (Schäble, Poeggel, Braun, & Gruss, 2007). The ontogeny of learning is probably mirrored by the maturing functionality of different basal forebrain regions. Two of the basal forebrain regions involved in TWA learning are the medial septum/diagonal band of Broca (MS/DB), which is essential for the encoding and retrieval of memory and the lateral septum (LS) that plays a role in the generation of behavior. Mapping 2-fluoro-deoxy-glucose utilization in freely behaving animals, the aim of this study was to assess the functional recruitment of the MS/DB and LS in infant (P17-P21) and adolescent (P38-P42) rats during the first (acquisition) and fifth (retrieval) TWA training. Metabolic activity in the MS/DB was similar in both age groups during acquisition and retrieval indicating that this region is already mature in the infant rat. In contrast, metabolic activity in the LS was generally lower in the infant rats suggesting that this region is not yet fully functional during P17 and P21. This insufficient recruitment may be one reason for the poor TWA performance of infant rats. Finally, the LS displayed significantly higher activity during acquisition than during retrieval indicating that the highest amount of energy is consumed during the initial learning phase.