Extrasynaptic Glutamate Diffusion in the Hippocampus: Ultrastructural Constraints, Uptake, and Receptor Activation

Abstract

Fast excitatory synapses are generally thought to act as private communication channels between presynaptic and postsynaptic neurons. Some recent findings, however, suggest that glutamate may diffuse out of the synaptic cleft and bind to several subtypes of receptors, either in the perisynaptic membrane or at neighboring synapses. It is not known whether activation of these receptors can occur in response to the release of a single vesicle of glutamate. Here we estimate the spatiotemporal profile of glutamate in the extrasynaptic space after vesicle exocytosis, guided by detailed ultrastructural measurements of the CA1 neuropil in the adult rat. We argue that the vicinity of the synapse can be treated as an isotropic porous medium, in which diffusion is determined by the extracellular volume fraction and the tortuosity factor, and develop novel stereological methods to estimate these parameters. We also estimate the spatial separation between synapses, to ask whether glutamate released at one synapse can activate NMDA and other high-affinity receptors at a neighboring synapse. Kinetic simulations of extrasynaptic glutamate uptake show that transporters rapidly reduce the free concentration of transmitter. Exocytosis of a single vesicle is, however, sufficient to bind to high-affinity receptors situated in the immediate perisynaptic space. The distance separating a typical synapse from its nearest neighbor is approximately 465 nm. Whether glutamate can reach a sufficient concentration to activate NMDA receptors at this distance depends critically on the diffusion coefficient in the extracellular space. If diffusion is much slower than in free aqueous solution, NMDA receptors could mediate crosstalk between neighboring synapses.

Dendritic spine geometry is critical for AMPA receptor expression in hippocampal CA1 pyramidal neurons

“…One can therefore surmise that spine structures are a key determinant of synaptic efficacy via control of the distribution of the AMPA receptors 9,10,43 . In addition, our study suggests that thin spines and filopodia may be postsynaptic components of silent synapses 28,32,34,44 . This might result from less effective transport of AMPA receptors into thinner spines with smaller volume-to-surface ratios.…”

“…A slight loss of resolution was due to the diffusion of glutamate before its binding to the receptors. Nevertheless, two-photon excitation of MNI-glutamate permits the functional mapping of glutamate sensitivity with a spatiotemporal resolution similar to that achieved by activation of presynaptic terminals (~0.4μm and ~0.1 ms) 28 . In the following experiments, uncaging was effected with low peak powers and longer irradiation times compared to the above (0.6 and 2 ms).…”

“…For instance, the peak glutamate concentration at the center of irradiation reaches 0.6 mM focally, at a laser power of 7 mW, MNI-glutamate concentration (C MNI ) of 8 mM, the duration of irradiation (t d ) of 50 μs. The glutamate concentration during mEPSCs (G m (x, t)) is obtained assuming a planar synaptic cleft with a width of 20 nm and a point glutamate source from which 5,000 glutamates are released according to m 2 t•;e − m•;t, m = 39 (ms −1 ) 28 . Using G(x, z, t) or G m (x, t), the opening probability of AMPA receptor can be obtained by the kinetic model of AMPA receptors 50 .…”

Contribution of extrasynapticN-methyl-d-aspartate and adenosine A1 receptors in the generation of dendritic glutamate-mediated plateau potentials

“…Extrasynaptic NMDA receptors are highly mobile and constantly in transit to and from synapses [31,32]. Glutamate can escape the restricted volume of the synaptic cleft and activate extrasynaptic NMDA receptors [17,33,34]. This scenario is more likely to occur during intense synaptic activity [35].…”

Hallucinogen-Induced UP States in the Brain Slice of Rat Prefrontal Cortex: Role of Glutamate Spillover and NR2B-NMDA Receptors

“…A growing body of literature suggests that activation of high-affinity extrasynaptic NMDA receptors by glutamatergic spillover may play a significant role in the brain function (Kullmann et al, 1996;Barbour and Hausser, 1997;Rusakov and Kullmann, 1998;Scimemi et al, 2004;Lozovaya et al, 2004). According to this view, the activation of any one synapse would not only activate its immediate postsynaptic partner, but may alsoFdepending on conditionsFbe able to activate high-affinity NMDA receptors at adjacent spines or presynaptic terminals.…”