Dissociation of nNOS from PSD‐95 promotes functional recovery after cerebral ischaemia in mice through reducing excessive tonic GABA release from reactive astrocytes

Mechanisms underlying functional recovery after stroke are little known, and effective drug intervention during the delayed stage is desirable. One potential drug target, the protein–protein interaction between neuronal nitric oxide synthase (nNOS) and postsynaptic density protein 95 (PSD‐95), is critical to acute ischaemic damage and neurogenesis. We show that nNOS–PSD‐95 dissociation induced by microinjection of a recombinant fusion protein, Tat‐nNOS‐N1–133, or systemic administration of a small‐molecule, ZL006, from day 4 to day 10 after photothrombotic ischaemia in mice reduced excessive tonic inhibition in the peri‐infarct cortex and ameliorated motor functional outcome. We also demonstrated improved neuroplasticity including increased dendrite spine density and synaptogenesis after reducing excessive tonic inhibition by nNOS–PSD‐95 dissociation. Levels of gamma‐aminobutyric acid (GABA) and GABA transporter‐3/4 (GAT‐3/4) are increased in the reactive astrocytes in the peri‐infarct cortex. The GAT‐3/4‐selective antagonist SNAP‐5114 reduced tonic inhibition and promoted function recovery, suggesting that increased tonic inhibition in the peri‐infarct cortex was due to GABA release from reversed GAT‐3/4 in reactive astrocytes. Treatments with Tat‐nNOS‐N1–133 or ZL006 after ischaemia inhibited astrocyte activation and GABA production, prevented the reversal of GAT‐3/4, and consequently decreased excessive tonic inhibition and ameliorated functional outcome. The underlying molecular mechanisms were associated with epigenetic inhibition of glutamic acid decarboxylase 67 and monoamine oxidase B expression through reduced NO production. The nNOS–PSD‐95 interaction is thus a potential target for functional restoration after stroke and ZL006, a small molecule inhibitor of this interaction, is a promising pharmacological lead compound. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.