Role Of AMPA Receptor Trafficking In NMDA Receptor-dependent Synaptic Plasticity In The Rat Lateral Amygdala

Learning and memory are important computational strategies of the brain. They are generally believed to result from changes in synaptic efficacy induced by afferent activity. Long-term potentiation (LTP) and long-term depression (LTD), the two most well characterized forms of synaptic plasticity, are thought to be important for learning and memory in behaving animals. Stimulated exocytosis and endocytosis of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype of glutamate receptors (AMPARs) have been proposed as primary mechanisms for the expression of hippocampal CA1 long-term potentiation (LTP) and long-term depression (LTD), respectively.In recent years, LTP and LTD are considered as two forms of synaptic plasticity that underlie the memory storage at sensory input synapses to the lateral amygdala (LA) during fear conditioning. However, the detailed underlying mechanisms especially the role of AMPAR trafficking in the expression of either LTP or LTD in this structure is still unknown. In the present study, we first observe the expression of LTP and LTD at thalamic input synapses induced by two different protocols in vitro and the effect of N-methy-D-aspartate receptors (NMDARs) blocked during LTP and LTD, to study the candidate mechanism for learning and memory during fear conditioning. Then we observe weather the induction protocols of LTP and LTD could increased or decreased the surface expression of AMPARs, thus NMDAR mediated AMPAR traffickings are responsible for the expression of LTP and LTD in the LA.1. NMDA receptors contribute to Long-term potentiation and long-term depression in the lateral amygdalaBoth LTP and LTD can also be induced in the lateral amygdala (LA), a critical structure involved in fear conditioning. Here we show that, using standard LTP or LTD stimulation protocols, N-methy-D-aspartate receptor (NMDAR)-dependent LTP and LTD can be reliably induced at the synapses of the auditory thalamic inputs to the LA in brain slices. High frequency stimulation (HFS: 3 episodes of 100 pulses at 100 Hz) could induce stable LTP, which could last for 40-60 min after the induction in fresh brain slices. While pairing protocol (200 pulses at 2Hz while depolarizing the cell to -5mv ) can induced more robust LTP in the LA. Stable and persistent LTD can be induced by low frequency stimulation (LFS: 900 pulses at 1 Hz) and paired protocol (delivering 480 pulses at 1Hz while holding potential was at -50 mv). Activation of NMDA receptors (NMDARs) is required for LTP and LTD of excitatory synaptic transmission at lateral amygdala synapses. Bath application of D-APV (50μM) during the recording can blocked the induction of LTP and LTD. Selectively blocking NMDARs that contain the NR2B subunit by the NR2B subunit-selective antagonist ifenprodil (3μM) or Ro25-6981 (0.5μM) can abolish the induction of LTD but not LTP. In contrast, preferential inhibition of NR2A-containing NMDARs by NVP-AAM077 (0.4μM) prevents the induction of LTP without affecting the induction of LTD.2. AMPA receptor trafficking is required for NMDA-dependent synaptic plasticity in the lateral amygdalaIt is generally agreed that postsynaptic changes in the number of surface AMPA receptors play an important role in synaptic plasticity. NMDAR mediated AMPAR trafficking is at least one of the mechanisms for the expression of LTP and LTD. Here we show that the expression of the bidirectional synaptic plasticity in the LA was also mediated by the trafficking of postsynaptic AMPARs. Postsynaptic application of a light chain of Clostridium tetanus neurotoxin (TeTx) which selectively cleaves vesicle-associated membrane proteins (VAMP) eliminated the expression of LTP, while postsynaptic application of a GluR2-derived interfering peptide which selectively blocks the stimulated clathrin-dependent regulated AMPAR endocytosis prevented the induction of LTD. The level of AMPAR subunits in the LA was examined following HFS or LFS stimulation by surface biotinylation assay, a quantification of the average density of individual band obtained by western blots. Biotinylation data also showed that the induction protocols of LTP or LTD respectively increased or decreased the surface expression of AMPARs.Results of the present study showed the induction of LTP and LTD at the synapses of the auditory thalamic inputs to the LA in brain slices were dependent upon NMDARs. Distinct NMDARs of different subunit composition have critical roles in determining the polarity of synaptic plasticity in lateral amygdala synapses of adult rats. The activation of NR2A-containing NMDARs leads to LTP formation, while the activation of NR2B-containing NMDARs produces LTD. The membrane fusion-dependent exocytosis plays an important role in the LTP expression and regulated AMPAR endocytosis is required for the NMDAR-dependent LTD. A rapid increase of AMPAR surface expression is responsible for the rise of transmission efficacy during LTP, while a rapid endocytotic-dependent decrease in the number of surface AMPARs accounts for LTD expression. This regulated change in the trafficking of postsynaptic AMPA receptors may serve as a common mechanism for synaptic strength alteration during synaptic plasticity in the central nerve system. The results of present study strongly suggest that NMDAR mediated AMPAR trafficking is at least one of the mechanisms for the expression of LTP and LTD in the LA.