Properties of individual hippocampal synapses influencing NMDA-receptor activation by spontaneous neurotransmission

Abstract

NMDA receptor (NMDAR) activation is critical for maintenance and modification of synapse strength. Specifically, NMDAR activation by spontaneous glutamate release has been shown to mediate some forms of synaptic plasticity as well as synaptic development. Interestingly, there is evidence that within individual synapses each release mode may be segregated such that postsynaptically there are distinct pools of responsive receptors. In order to examine potential regulators of NMDAR activation due to spontaneous glutamate release in cultured hippocampal neurons, we utilized GCaMP6f imaging at single synapses in concert with confocal and super-resolution imaging. Using these single spine approaches, we found that Ca²⁺ entry activated by spontaneous release tends to be carried by GluN2B-NMDARs. Additionally, the amount of NMDAR activation varies greatly both between synapses and within synapses, and is unrelated to spine and synapse size, but does correlate loosely with synapse distance from the soma. Despite the critical role of spontaneous activation of NMDARs in maintaining synaptic function, their activation seems to be controlled factors other than synapse size or synapse distance from the soma. It is most likely that NMDAR activation by spontaneous release influenced variability in subsynaptic receptor position, release site position, vesicle content, and channel properties. Therefore, spontaneous activation of NMDARs appears to be regulated distinctly from other receptor types, notably AMPARs, within individual synapses.

Significance Statement Understanding the synaptic mechanisms of learning and memory is critically to the field of neuroscience and for human health. A key neurotransmitter receptor involved in learning is the NMDA receptor, and exploration of its regulation is vital. Here, we optimized optical tools to allow detailed characterization of NMDA receptor activity at single synapses, along with analysis of synaptic structural features. The amount of receptor activation is independent of synapse size, but weakly dependent on synapse position within the dendritic tree. Notably, we found that NMDA receptors activated following spontaneous neurotransmitter release tend be GluN2B-containing receptors. The unique mechanisms that regulate the number and positioning of these receptors within synapses will have important consequences for control of synaptic development and signaling.