Role of Postsynaptic Density Protein 95 (PSD95) and Neuronal Nitric Oxide Synthase (NNOS) Interaction in the Regulation of Conditioned Fear

Abstract

Stimulation of N-­methyl-­D-­aspartic acid receptors (NMDARs) and the resulting activation of neuronal nitric oxide synthase (nNOS) are critical for fear memory formation. A variety of previously studied NMDAR antagonists and NOS inhibitors can disrupt fear memory, but they also affect many other CNS functions. Following NMDAR stimulation, efficient activation of nNOS requires linking nNOS to a scaffolding protein, the postsynaptic density protein 95 (PSD95). We hypothesized that PSD95-­nNOS interaction in critical limbic regions (such as amygdala and hippocampus) during fear conditioning is important in regulating fear memory formation, and disruption of this protein-­protein binding may cause impairments in conditioned fear memory. Utilizing co-­immunoprecipitation, electrophysiology and behavioral paradigms, we first showed that fear conditioning results in significant increases in PSD95-­nNOS binding within the basolateral amygdala (BLA) and the ventral hippocampus (vHP) in a time-­dependent manner, but not in the medial prefrontal cortex (mPFC). Secondly, by using ZL006, a small molecule disruptor of PSD95-­ nNOS interaction, it was found that systemic and intra-­BLA disruption of PSD95-­ nNOS interaction by ZL006 impaired the consolidation of cue-­induced fear. In contrast, disruption of PSD95-­nNOS interaction within the vHP did not affect the consolidation of cue-­induced fear, but significantly impaired the consolidation of context-­induced fear. At the cellular level, disruption of PSD95-­nNOS interaction with ZL006 was found to impair long-­term potentiation (LTP) in the BLA neurons. Finally, unlike NMDAR antagonist MK-­801, ZL006 is devoid of adverse effects on many other CNS functions, such as motor function, social activity, cognitive functions in tasks of object recognition memory and spatial memory. These findings collectively demonstrated that PSD95-­nNOS interaction within the conditioned fear network appears to be a key molecular step in regulating synaptic plasticity and the consolidation of conditioned fear. Disruption of PSD95-­nNOS interaction holds promise as a novel treatment strategy for fear-­ motivated disorders, such as post-­traumatic stress disorder and phobias.