Tyrosine phosphorylation of the GluR2 subunit is required for long‐term depression of synaptic efficacy in young animals in vivo

Abstract

The study of the intracellular mechanics that underlay changes in synaptic efficacy is a rapidly evolving field of research. It is currently believed that NMDA receptors play a significant role in the induction of synaptic plasticity, whereas AMPA receptors play a significant role in its expression. For AMPA receptors, it has been shown that tyrosine phosphorylation of the GluR2 carboxyl termini is required for the expression of long-term depression of synaptic efficacy (LTD) in vitro (Ahmadian et al. (2004) EMBO J 23:1040-1050). In the present study, we sought to determine whether similar mechanisms are involved in vivo, where different stimulation parameters are required for the induction of LTD. We initially used a paired-burst (PB) paradigm that reliably induces LTD in vivo. In these animals we were able to prevent the induction and expression of PB-LTD by administering a peptide (GluR-3Y) that acted as a competitive inhibitor of tyrosine phosphorylation. In a separate set of animals, we exposed animals to brief periods of stress (S) before using low-frequency stimuli to induce LTD (S-LTD). Again, GluR2-3Y blocked both the induction and expression of S-LTD. In contrast, an inert version of the peptide, with alanine replacing the three tyrosine residues, did not inhibit LTD induction. In addition, we demonstrated that GluR2-3Y did not affect the induction of long-term potentiation in vivo. These findings support the hypothesis that tyrosine phosphorylation and AMPA receptor endocytosis are necessary steps for the induction and maintenance of two forms of LTD in the CA1 region.

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

“…Indeed, recent studies have demonstrated that systemic administration of Ro 25-6981 at the same dose used in the present study blocked selectively the induction of LTD while having no effect on LTP in the hippocampus in vivo (Fox et al, 2006;Wong et al, 2007). The Tat-GluR2 3Y peptide used in these experiments has also been shown to block the expression of LTD specifically through the blockade of regulated AMPA receptor endocytosis (Ahmadian et al, 2004;Brebner et al, 2005;Fox et al, 2007). Our observations that these two manipulations that disrupt LTD formation (inhibition of AMPA receptor endocytosis or NMDA NR2B receptor antagonism) also disrupt the initial extinction of a fear response during training further support the notion that LTD plays a central role in mediating certain components of fear extinction.…”

“…Of particular relevance to the present study is the recent report that i.v. administration of the Tat-GluR2 3Y peptide blocks the induction and expression of LTD in the hippocampus in vivo (Fox et al, 2007). Taken together, these studies provide compelling evidence that i.v.…”

“…This lack of effect is not particularly surprising, given (1) the array of evidence that LTP is the primary mechanism underlying the formation of learned fear, and (2) that Tat-GluR2 3Y does not disrupt LTP formation (Fox et al, 2007). LTP-like changes in neural activity associated with learned fear have been observed in the lateral amygdala (Quirk et al, 1995;Hobin et al, 2003), and treatments that block LTP such as NMDA receptor antagonists, calcium channel blockers, and protein synthesis inhibitors prevent the acquisition of conditioned fear (Maren et al, 1996;Gewirtz and Davis, 1997).…”

“…Nevertheless, the studies discussed above strongly suggest that the lateral nuclei of the amygdala may be an important neuroanatomical locus where blockade of AMPA receptor endocytosis or NMDA NR2B receptors would affect this form of learning. However, it is equally plausible that these effects may be due to alterations in synaptic mechanisms in the hippocampus, given that (1) both Tat-GluR2 3Y and Ro 25-6981 disrupt LTD formation in the hippocampus (Fox et al, 2006(Fox et al, , 2007Duffy et al, 2007) and (2) inactivation of this region causes a similar impairment in extinction learning (Corcoran et al, 2005). The effects of local administration of these drugs in the hippocampus and amygdala on fear extinction are the topic of current investigation in our laboratory.…”

“…It follows that disruption of AMPA receptor endocytosis (which selectively blocks the formation of LTD) may exert a selective effect on the extinction of a previously established fear memory, without affecting the acquisition of learned fear itself (formation of a new memory). To test this hypothesis directly, we employed the use of the Tat-GluR2 3Y peptide, which we have shown previously to block the GluR2-dependent, regulated AMPA receptor endocytosis associated with the formation of LTD in structures such as the nucleus accumbens and dorsal hippocampus (Brebner et al, 2005;Fox et al, 2007;Wong et al, 2007).…”