Diversity of learning-induced synaptic plasticity at dorsal CA1 synapses: a possible underlying mechanism of contextual memory formation

Abstract

We expose various kinds of context in our everyday lives and hence contextual memory is critically important for us. Since hippocampus is a major area for processing contextual memory, it needs to encode different components of various contexts such as ‘what’, ‘where’ and ‘when’, etc. However, the underlying mechanism by which the hippocampus differentially encodes various kinds of contextual information is interesting but not yet revealed completely. Since long-term potentiation (LTP) had been discovered and believed to be mechanism of learning, many studies were conducted to investigate the underlying detailed mechanism of memory formation by using various LTP induction protocols. By combining HSV-mediated in vivo gene delivery with in vitro patch-clamp recordings, it was found that LTP can induce plastic changes such as delivery of GluA1-containing AMPA receptor to the dorsal CA1 synapses of the hippocampus and also the causal relationship between this AMPA receptor delivery and learning was discovered. Then it was found that learning induced synaptic plasticity with diverse pattern highlighting the importance of this diversity in contextual memory formation. Here, we mostly review the studies which used hippocampal-dependent learning paradigm (inhibitory avoidance task) for better comparison and mapping of the concept about how learning induced diversity of synaptic plasticity at dorsal hippocampal CA1 synapses and how this diversity may become responsible as an underlying mechanism of contextual memory formation. We found that contextual learning induces synaptic plasticity at CA1 neurons with synapse-specific diversity, input-specific diversity, subregion- or subfield-specific diversity, episode-specific diversity but only a few studies revealed their functional significance. For complete understanding of learning-induced diversity of synaptic plasticity, it demands further studies for investigation of their functional correlation to learning performance by using new highly promising approaches like optogenetic techniques and cell-specific self-entropy analysis, etc.