Neuronal Activity and Synaptic Plasticity Underlying Hippocampal Memory Engram

Activity dependent synaptic plasticity is considered to be the physiological mechanism underlying learning and memory. Since the discovery of long-term potentiation (LTP) in 1973 by Bliss and Lomo, the popularity of synaptic plasticity has grown immensely. By October 2016, more than 51,000 research articles and reviews were indexed by PubMed. Recent emergence of optogenetics and chemogenetics has enabled researchers to generate neuronal activity patterns with precise temporal and spatial control in awake animals and brain slices. The relative ease of combining these tools with traditional electrophysiology will further expand the scope of synaptic plasticity research in freely behaving animals while they engage in cognition and memory tasks. This chapter aims to provide an overview of recent advances in the field of synaptic plasticity and learning and memory with a focus on cognitive impairments caused by stress-induced maladaptive synaptic plasticity. We summarize the latest memory engram data that claims to specifically manipulate the activity of hippocampal memory cells by employing optogenetics and DREADD chemogenetics. We end by discussing the potential consequences of long term usage of circuit genetic tools on neuronal network dynamics.