Depolarization of hippocampal neurons induces formation of nonsynaptic NMDA receptor islands resembling nascent postsynaptic densities

ABSTRACT

Depolarization of neurons in three week-old rat hippocampal cultures promotes a rapid increase in the density of surface NMDA receptors (NR), accompanied by transient formation of nonsynaptic NMDA receptor clusters, or NR islands. Islands exhibit cytoplasmic dense material resembling that at postsynaptic densities (PSD), and contain typical PSD components including MAGUKS, GKAP, Shank, Homer and CaMKII detected by pre-embedding immunogold electron microscopy. In contrast to mature PSDs, islands contain more NMDA than AMPA receptors, and more SAP102 than PSD95, features shared with nascent PSDs in developing synapses. Islands do not appear to be exocytosed or endocytosed directly as preformed packages because neurons lacked intracellular vacuoles containing island-like structures. Islands form and disassemble upon depolarization of neurons on a time scale of 2-3 min, perhaps representing an initial stage in synaptogenesis.

Significance Statement: Islands of extrasynaptic NMDA receptors populate the plasma membranes of hippocampal neurons. The receptor Islands also contain many typical postsynaptic density proteins and thus resemble nascent PSDs. NMDA receptors appear to be exocytosed only individually or in small groups rather than in concentrated clusters, so islands must form by clustering of individual NMDA receptors already in the neuronal plasma membrane. Additional islands rapidly form and resolve when neurons are depolarized during a 2-3 min window. These findings provide possible insight into one of the mechanisms of synapse formation.