The amygdala NT3-TrkC pathway underlies inter-individual differences in fear extinction and related synaptic plasticity.

Fear-related pathologies are among the most prevalent psychiatric conditions, having inappropriate learned fear and resistance to extinction as cardinal features. Exposure therapy represents a promising therapeutic approach, the efficiency of which depends on inter-individual variation in fear extinction learning, which neurobiological basis is unknown. We characterized a model of extinction learning, whereby fear-conditioned mice were categorized as extinction (EXT)-success or EXT-failure, according to their inherent ability to extinguish fear. In the lateral amygdala, GluN2A-containing NMDAR are required for LTP and stabilization of fear memories, while GluN2B-containing NMDAR are required for LTD and fear extinction. EXT-success mice showed attenuated LTP, strong LTD and higher levels of synaptic GluN2B, while EXT-failure mice showed strong LTP, no LTD and higher levels of synaptic GluN2A. Neurotrophin 3 (NT3) infusion in the lateral amygdala was sufficient to rescue extinction deficits in EXT-failure mice. Mechanistically, activation of tropomyosin receptor kinase C (TrkC) with NT3 in EXT-failure slices attenuated lateral amygdala LTP, in a GluN2B-dependent manner. Conversely, blocking endogenous NT3-TrkC signaling with TrkC-Fc chimera in EXT-success slices strengthened lateral amygdala LTP. Our data support a key role for the NT3-TrkC system in inter-individual differences in fear extinction in rodents, through modulation of amygdalar NMDAR composition and synaptic plasticity.

The role of post-translational modifications in synaptic AMPA receptor activity.

AMPA-type receptors for the neurotransmitter glutamate are very dynamic entities, and changes in their synaptic abundance underlie different forms of synaptic plasticity, including long-term synaptic potentiation (LTP), long-term depression (LTD) and homeostatic scaling. The different AMPA receptor subunits (GluA1-GluA4) share a common modular structure and membrane topology, and their intracellular C-terminus tail is responsible for the interaction with intracellular proteins important in receptor trafficking. The latter sequence differs between subunits and contains most sites for post-translational modifications of the receptors, including phosphorylation, O-GlcNAcylation, ubiquitination, acetylation, palmitoylation and nitrosylation, which affect differentially the various subunits. Considering that each single subunit may undergo modifications in multiple sites, and that AMPA receptors may be formed by the assembly of different subunits, this creates multiple layers of regulation of the receptors with impact in synaptic function and plasticity. This review discusses the diversity of mechanisms involved in the post-translational modification of AMPA receptor subunits, and their impact on the subcellular distribution and synaptic activity of the receptors.