Synaptic Expression of Glutamate Receptor after Encoding of Fear Memory in the Rat Amygdala

Abstract

Fear conditioning has been ascribed to presynaptic mechanisms, particularly presynaptic facilitation of transmission at thalamo-and cortico-amygdala synapses. Here, by labeling surface receptors with biotin or using membrane fractionation approaches, we report that fear conditioning resulted in an increase in surface expression of GluR1 subunit of ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in the amygdala, whereas total GluR1 mRNA and protein levels were unchanged. The control group that received conditioned stimulus (CS) and unconditioned stimulus in an unpaired fashion did not present any increase, indicating that GluR1 increase was specific to the learning component of the task. Conditioninginduced increase in surface expression of GluR1 depended on the activation of N-methyl-D-aspartate receptors and protein kinases and required the synthesis of new proteins. CS-alone trials applied 24 h before training attenuated fear-potentiated startle and prevented conditioning-induced increase in surface expression of GluR1. Increase in GluR1 was also observed in the amygdala slices after delivery of tetanic stimulation that elicited long-term potentiation of synaptic transmission. Proteasome inhibitor increased surface expression of GluR1 in a time-and dose-dependent manner. Furthermore, pretraining administration of proteasome inhibitor into the amygdala facilitated the fear-potentiated startle. These results suggest that long-term memory formation is correlated with the change in synaptic expression of GluR1, and trafficking of GluR1 to the synaptic sites contributes at least in part to the expression of fear memory.Fear conditioning, an animal model of emotional learning and post-traumatic stress disorder, is initiated by a cue (conditioned stimulus, CS) that is previously paired with an aversive stimulus (unconditioned stimulus, US) such as footshock. Neuronal changes mediating the association between the CS and US occur in the lateral (LA) and basolateral (BLA) amygdala (McKernan and Shinnick-Gallagher, 1997;Rogan et al., 1997;Pare, 2003). It is generally recognized that consolidation of long-term memory in vertebrate and invertebrate brains requires transcription and translation of new proteins (Dudai, 1996;McGaugh, 2000). Newly synthesized proteins are believed to deposit at the synapses that encode the persistent changes in synaptic strength. Despite the signal cascades that have been identified to be responsible for the consolidation of fear memory, little is known about the downstream effectors leading to the expression of memory. Previous study has showed that conditioned fear is associated with a reduction in paired-pulse facilitation and an enhancement in the probability of transmitter release in the thalamo-amygdala pathway (McKernan and Shinnick-Gallagher, 1997). Quantal analysis of unitary synaptic responses in cortico-amygdala pathway after induction of long-term potentiation (LTP) revealed a marked decrease in the fraction of failures, with no change in potency of CV of the...

The Membrane Proximal Region of AMPA Receptors in Lateral Amygdala is Essential for Fear Memory Formation

“…Our observation is thus consistent with the findings showing that fear conditioning drives GluA1-containing AMPA receptors into synapses in LA neurons (Rumpel et al, 2005;Yeh et al, 2006) and that short-and long-term fear conditioning memory are impaired if GluA1-containing AMPAR insertion is blocked (Rumpel et al, 2005). Moreover, similarly to our findings, GluA1-AMPAR trafficking inhibition in LA has no effect on freezing during training (Rumpel et al, 2005).…”

“…GluA1-4 subunits proteins are detected in the LA of adult rats (Farb et al, 1995;Radley et al, 2007). It has been shown that fear conditioning drives GluA1-containing AMPARs into synapses in LA neurons and increases their level (Rumpel et al, 2005;Yeh et al, 2006;Nedelescu et al, 2010;Ota et al, 2010). Moreover, fear conditioning is impaired if GluA1-AMPAR insertion is blocked (Rumpel et al, 2005).…”

Augmentation of Fear Extinction by D-Cycloserine is Blocked by Proteasome Inhibitors

“…When degradation of an inhibitory protein is necessary for memory consolidation, proteasome inhibitors blocked the formation of longterm memory (Chain et al, 1999;Lopez-Salon et al, 2001;Upadhya et al, 2004). Conversely, when UPS functioned as an inhibitory constraint, removal of constraint by proteasome inhibitor enhanced synaptic transmission and facilitated memory formation (Zhao et al, 2003;Yeh et al, 2006). In the present study, we found that proteasome inhibitors blocked DCS-induced GluR1 internalization.…”

“…Local circuit inhibitory interneurons, upon their activation, release g-aminobutyric acid (GABA) and exert powerful control on the pyramidal cells. Repetitively pairing animals a cue with an aversive stimulus increases LTP of synapses from auditory thalamus and cortex to the LA (McKernan and Shinnick-Gallagher, 1997;Rogan et al, 1997) and synaptic GluR1 subunit of AMPA receptors (Rumpel et al, 2005;Yeh et al, 2006). Extinction training, on the other hand, increases function of GABA A receptors in the BLA (Chhatwal et al, 2005) without affecting surface GluR1 level .…”

Disruption of AMPA Receptor Endocytosis Impairs the Extinction, but not Acquisition of Learned Fear

“…Manipulations that block these processes also disrupt the formation of LTD (Ahmadian et al, 2004). In this regard, it is interesting to note that AMPA receptors show an increased surface expression following fear conditioning (Rumpel et al, 2005;Yeh et al, 2006). Therefore, it is reasonable to propose that a reduction in the expression of learned fear during extinction may be mediated in part by a decrease in the surface expression of AMPA receptors.…”