Effects Of NR2B Overexpression On Acquisition Of Auditory Fear Memory And Synaptic Plasticity In Lateral Amygdala

The lateral nucleus of the amygdala (LA) is an essential component which accepts the thalamus auditory inputs information underlying auditory fear conditioning. A number of studies have assessed that NMDAR (N-methyl-D-aspatate receptor) is widely expressed on the lateral amygdala neurons, and the NMDAR-mediated synaptic plasticity is extremely important for auditory fear conditioning. NR2B subunit is a crucial component for NMDAR, and it regulates channel function of NMDAR directly. However, the effects of up-regulation of NR2B subunit in LA on acquisition of auditory fear memory and related thalamus-lateral amygdala (T-LA) circuit synaptic plasticity have not been reported.Therefore, using the forebrain specific overexpression of NR2B subunit transgenic mice, we combined molecular biology methods, electrophysiological techniques and behavioral tasks to study the effects of up-regulation of NR2B subunit on acquisition of auditory fear memory and its related T-LA circuit synaptic plasticity. Our main five conclusions are as below:1. The overexpression of NR2B subunit in amygdala in NR2B transgetic mice.By using the western blotting techniques, we identified that the expression of NR2B subunit in amygdala in transgenic mice was significantly increased compared with the wild-type mice. Besides, there was no significant change in the expression of NR2A subunit, and the NR1 subunit was slightly increased in the transgenic mice.2. NR2B overexpression did not affect anxiety-like and depressive-like behaviors.The elevated plus-maze test and the forced swim test showed that overexpression of NR2B subunit in amygdala did not influence anxiety-like and depressive-like behaviors in transgenic mice.3. NR2B overexpression significantly facilitated the ability of acquisition of auditory fear memory.Foot shock sensitivity threshold test and fear acquisition test indicated that NR2B overexpression did not affect the sensitivity to foot shock, while significantly enhanced the ability of acquisition of auditory fear memory in NR2B transgenic mice.4. NR2B overexpression did not affect basal synaptic transmission and LTD, but significantly enhanced LTP at T-LA synapses.By using in vitro field potential recording techniques, we investigated the input-output curve and paired-pulse depression curve at T-LA synapses and found no significant differences between two groups. By using in vitro patch clamp recording techniques, we examined the AMPAR-mediated miniature excitatory postsynaptic current and also found no distinctives between two groups. These results showed that NR2B overexpression had no effects on basal synaptic transmission at T-LA synapses in the transgenic mice.Then, we used in vitro field potential recording techniques to further examine the LTD and LTP at T-LA synapses in the transgenic mice, finding that NR2B overexpression had no influence on LTD, but significantly enhanced LTP at T-LA synapses in the transgenic mice.5. NR2B overexpression increased the amplitude and decay time of NMDAR-mediated evoked excitatory postsynaptic currents (eEPSC) at LA pyramidal neurons.To further explore the mechanism underlying the enhanced LTP at T-LA synapses in NR2B transgenic mice, we assessed NMDAR function of LA in NR2B transgenic mice. We utilized in vitro patch clamp recording techniques to examine the electrophysiological properties of LA pyramidal neurons such as resting membrane potential, input resistance and maximum firing rate, and found no significant differences between two groups. While recording NMDAR-mediated eEPSC, the amplitude and decay time of NMDAR-mediated eEPSC were significantly increased at LA pyramidal neurons in NR2B transgenic mice.In conclusion, our results demonstrated that forebrain NR2B overexpression facilitated the ability of acquisition of auditory fear memory and enhanced NMDAR-mediated LTP at T-LA synapses, which possiblly because that NR2B overexpression in LA pyramidal neurons increased amplitude and decay time of NMDAR-mediated eEPSC, resulting in a increase of Ca2+ influx at postsynaptic membrane. Furthermore, our results also provided the evidence that LTP at T-LA circuit is the synaptic mechanisms of acquisition of auditory fear memory.