Genetic Enhancement Of NR2B Subunit Facilitating Prefrontal Cortex Synaptic Plasticity And Enhancing Learning Memory Function In Mice

Learning and memory, as high function of brain, are critical to mammalian especially for human survival. Increasing evidences indicate that N- methyl- D-aspartate (NMDA) receptor, especially the NR2B subunit, plays a crucial role in synaptic plasticity, learning and memory. In 1999, Dr. Tsien and his colleagues at Princeton University (USA) breached the limitation of gene technique and established the transgenic mouse that overexpressed NR2B subunit in forebrain. This transgenic mouse showed the enhancement in activation of NMDA receptor, NMDA receptor channel open duration, and synaptic potentiation in response to high frequency stimulation in vitro. In addition, the mouse exhibited superior ability in learning and memory in various behavioral tasks (such as water maze, cued and contextual fear conditioning). These results indicated that NR2B subunit was a key switch for synaptic plasticity, learning and memory in hippocampus in mice. Furthermore, the function of NR2B subunit in prefrontal cortex synaptic plasticity and learning memory is much less characterized than in hippocampus. Thus, whether NR2B subunit represents a universal molecular switch in gating the level of synaptic plasticity and memory function in the adult brain is still unknown. In another word, is the NR2B subunit also critical for synaptic plasticity and learning memory in the different brain regions in mice? We used immunology, electrophysiological and behavioral techniques to systematically analyze the effect on synaptic plasticity of NR2B subunit overexpression and working memory. The main objectives are as follows:1. genotype analysis and the expression of functional NR2B receptorWestern-blot strategy was applied to identify the expression of NR2B gene. Data showed that the functional NR2B receptors were overexpressed in the synapse of prefrontal cortex in transgenic mice.2. Investigation of synaptic plasticity in transgenic mice2.1 The basic synaptic transmissionWe investigated the basic transmission function of the prefrontal layer V to layer II-III pyramidal neurons pathway in transgenic mice using the brain slice field potential recording technique. The results from the Input-Output curve and Paired pulse depression indicated that postsynaptic AMPA receptors and presynaptic functions in NR2B transgenic mice were normal.2.2 The synaptic plasticity mediated by NMDA receptor We used the same technique to investigate the effect of NR2B overexpression specific in forebrain on synaptic plasticity in the prefrontal layer V to layer II-III pyramidal neurons pathway. The results indicated that the NMDA receptor-dependent LTP was significantly enhanced. Furthermore, we used NMDA receptor NR2A subunit and NR2B subunit antagonists to study the role of NR2A and NR2B subunits in NMDA receptor mediated LTP. The results showed that LTPs from both the transgenic and wild-type mice were attenuated by NR2A or NR2B subunit antagonist. Especially, LTPs from the transgenic mice were significantly larger than that from wild type mice when NR2A subunit antagonist was delivered. These results indicated that both the NR2A and NR2B subunits participated in the formation of NMDA receptor mediated LTP. The overexpression of NR2B subunit could enhance the NMDA receptor-dependent LTP. On the other hand, there was no significant difference between NR2B transgenic and wild-type mice on formation of LTD (Long-term depression).3. Investigation of behavioral tasks in transgenic miceNumerous studies have demonstrated that the NMDA receptor dependent synaptic plasticity is associated with learning and memory. To investigate the relationship between overexpression of NR2B subunit in prefrontal cortex and prefrontal cortex dependent learning and memory function, we used T-maze (The delayed alternative T-maze, DAT) and modified water maze tasks to study the spatial working memory in mice. The results indicated that the transgenic mice had superior spatial working memory. Thus, the overexpression of NR2B subunits in forebrain also could improve the prefrontal cortex dependent learning and memory function in mice.In summary, our electrophysiological analyses show overexpression of NR2B subunit in forebrain of mice led to the enhancement of NMDAR-mediated LTP and better learning and memory. We conclude that the NR2B subunit may serve as a crucial gating switch for controlling learning and memory functions across different brain region other than hippocampus.