ABA renewal involves enhancements in both GluA2-lacking AMPA receptor activity and GluA1 phosphorylation in the lateral amygdala

Pre-extinction activation of hippocampal AMPK prevents fear renewal in mice

As a type of fear relapse, fear renewal compromises the efficacy of fear extinction, which serves as the laboratory analog of exposure therapy (a therapeutic strategy for anxiety disorders). Interventions aiming to prevent fear renewal would thus benefit exposure therapy. To date, it remains unknown whether central adenosine monophosphate (AMP)-activated protein kinase (AMPK) activation could produce inhibitory effects on fear renewal. Here, using pharmacological approach and virus-mediated gene overexpression technique, we demonstrated that activation of AMPK in dorsal hippocampus shortly before fear extinction training completely abolished subsequent fear renewal in male mice without affecting other types of fear relapse, including spontaneous recovery of fear and fear reinstatement. Furthermore, we also found that metformin, a first-line antidiabetic drug, was capable of preventing fear renewal in male mice by stimulating AMPK in dorsal hippocampus. Collectively, our study provides insight into the role of hippocampal AMPK in regulation of fear renewal and indicates that increasing activity of hippocampal AMPK can prevent fear renewal, thus enhancing the potency of exposure therapy.

Posterior parietal cortex mediates fear renewal in a novel context

The return of fear following extinction therapy is an important issue associated with the treatment of many fear-related disorders. Fear renewal is a suitable model, with which context-dependent modulation of the fear response can be examined. In this model, any context outside of an extinction context (e.g., novel or familiar contexts) could evoke relapse of the fear response. However, brain regions associated with context-dependent modulation are not fully understood. The posterior parietal cortex (PPC) is considered a center for integrating multisensory information and making decisions. To study its role in the contextual modulation of fear relapse, we reversibly inactivated the PPC in mice before they were exposed to various contexts after extinction training. When muscimol was infused into the PPC, fear renewal was impaired in a novel context, but not in a familiar context. Fear relapses were blocked during optogenetic inhibition of the PPC, only when animals were placed in a novel context. We propose that the neural activity of the PPC is necessary for the relapse of a precise response to an extinguished conditioned stimulus in a novel context.

Preferential generation of Ca2+-permeable AMPA receptors by AKAP79-anchored protein kinase C proceeds via GluA1 subunit phosphorylation at Ser-831

AMPA-type glutamate receptors (AMPARs) mediate fast excitatory neurotransmission in the mammalian central nervous system. Preferential AMPAR subunit assembly favors heteromeric GluA1/GluA2 complexes. The presence of the GluA2 subunit generates Ca²⁺-impermeable (CI) AMPARs that have linear current-voltage (I-V) relationships. However, diverse forms of synaptic plasticity and pathophysiological conditions are associated with shifts from CI to inwardly rectifying, GluA2-lacking, Ca²⁺-permeable (CP) AMPARs on time scales ranging from minutes to days. These shifts have been linked to GluA1 phosphorylation at Ser-845, a protein kinase A (PKA)-targeted site within its intracellular C-terminal tail, often in conjunction with protein kinase A anchoring protein 79 (AKAP79; AKAP150 in rodents), which targets PKA to GluA1. However, AKAP79 may impact GluA1 phosphorylation at other sites by interacting with other signaling enzymes. Here, we evaluated the ability of AKAP79, its signaling components, and GluA1 phosphorylation sites to induce CP-AMPARs under conditions in which CI-AMPARs normally predominate. We found that GluA1 phosphorylation at Ser-831 is sufficient for the appearance of CP-AMPARs and that AKAP79-anchored protein kinase C (PKC) primarily drives the appearance of these receptors via this site. In contrast, other AKAP79-signaling components and C-terminal tail GluA1 phosphorylation sites exhibited a permissive role, limiting the extent to which AKAP79 promotes CP-AMPARs. This may reflect the need for these sites to undergo active phosphorylation/dephosphorylation cycles that control their residency within distinct subcellular compartments. These findings suggest that AKAP79, by orchestrating phosphorylation, represents a key to a GluA1 phosphorylation passcode, which allows the GluA1 subunit to escape GluA2 dominance and promote the appearance of CP-AMPARs.

Regulation of Fear Extinction in the Basolateral Amygdala by Dopamine D2 Receptors Accompanied by Altered GluR1, GluR1-Ser845 and NR2B Levels

The amygdala, a critical structure for both Pavlovian fear conditioning and fear extinction, receives sparse but comprehensive dopamine innervation and contains dopamine D1 and D2 receptors. Fear extinction, which involves learning to suppress the expression of a previously learned fear, appears to require the dopaminergic system. The specific roles of D2 receptors in mediating associative learning underlying fear extinction require further study. Intra-basolateral amygdala (BLA) infusions of a D2 receptor agonist, quinpirole, and a D2 receptor antagonist, sulpiride, prior to fear extinction and extinction retention were tested 24 h after fear extinction training for long-term memory (LTM). LTM was facilitated by quinpirole and attenuated by sulpiride. In addition, A-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor glutamate receptor 1 (GluR1) subunit, GluR1 phospho-Ser845, and N-methyl-D-aspartic acid receptor NR2B subunit levels in the BLA were generally increased by quinpirole and down-regulated by sulpiride. The present study suggests that activation of D2 receptors facilitates fear extinction and that blockade of D2 receptors impairs fear extinction, accompanied by changes in GluR1, GluR1-Ser845 and NR2B levels in the amygdala.

mGluR long-term depression regulates GluA2 association with COPII vesicles and exit from the endoplasmic reticulum

mGluR long-term depression (mGluR-LTD) is a form of synaptic plasticity induced at excitatory synapses by metabotropic glutamate receptors (mGluRs). mGluR-LTD reduces synaptic strength and is relevant to learning and memory, autism, and sensitization to cocaine; however, the mechanism is not known. Here we show that activation of Group I mGluRs in medium spiny neurons induces trafficking of GluA2 from the endoplasmic reticulum (ER) to the synapse by enhancing GluA2 binding to essential COPII vesicle proteins, Sec23 and Sec13. GluA2 exit from the ER further depends on IP3 and Ryanodine receptor-controlled Ca²⁺ release as well as active translation. Synaptic insertion of GluA2 is coupled to removal of high-conducting Ca²⁺-permeable AMPA receptors from synapses, resulting in synaptic depression. This work demonstrates a novel mechanism in which mGluR signals release AMPA receptors rapidly from the ER and couple ER release to GluA2 synaptic insertion and GluA1 removal.

Brain region-specific effects of cGMP-dependent kinase II knockout on AMPA receptor trafficking and animal behavior

Phosphorylation of GluA1, a subunit of AMPA receptors (AMPARs), is critical for AMPAR synaptic trafficking and control of synaptic transmission. cGMP-dependent protein kinase II (cGKII) mediates this phosphorylation, and cGKII knockout (KO) affects GluA1 phosphorylation and alters animal behavior. Notably, GluA1 phosphorylation in the KO hippocampus is increased as a functional compensation for gene deletion, while such compensation is absent in the prefrontal cortex. Thus, there are brain region-specific effects of cGKII KO on AMPAR trafficking, which could affect animal behavior. Here, we show that GluA1 phosphorylation levels differ in various brain regions, and specific behaviors are altered according to region-specific changes in GluA1 phosphorylation. Moreover, we identified distinct regulations of phosphatases in different brain regions, leading to regional heterogeneity of GluA1 phosphorylation in the KO brain. Our work demonstrates region-specific changes in GluA1 phosphorylation in cGKII KO mice and corresponding effects on cognitive performance. We also reveal distinct regulation of phosphatases in different brain region in which region-specific effects of kinase gene KO arise and can selectively alter animal behavior.