Receptor Actions of Synaptically Released Glutamate: The Role of Transporters on the Scale from Nanometers to Microns

Abstract

Actions of the excitatory neurotransmitter glutamate inside and outside the synaptic cleft determine the activity of neural circuits in the brain. However, to what degree local glutamate transporters affect these actions on a submicron scale remains poorly understood. Here we focus on hippocampal area CA1, a common subject of synaptic physiology studies. First, we use a two-photon excitation technique to obtain an estimate of the apparent (macroscopic) extracellular diffusion coefficient for glutamate, approximately 0.32 mum(2)/ms. Second, we incorporate this measurement into a Monte Carlo model of the typical excitatory synapse and examine the influence of distributed glutamate transporter molecules on signal transmission. Combined with the results of whole-cell recordings, such simulations argue that, although glutamate transporters have little effect on the activation of synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, this does not rule out the occurrence of up to several dozens of transporters inside the cleft. We further evaluate how the expression pattern of transporter molecules (on the 10-100 nm scale) affects the activation of N-methyl-D-aspartic acid or metabotropic glutamate receptors in the synaptic vicinity. Finally, we extend our simulations to the macroscopic scale, estimating that synaptic activity sufficient to excite principal neurons could intermittently raise extracellular glutamate to approximately 1 muM only at sparse (microns apart) hotspots. Greater rises of glutamate occur only when <5% of transporters are available (for instance, when an astrocyte fails). The results provide a quantitative framework for a better understanding of the relationship between glutamate transporters and glutamate receptor signaling.

The mGluR2/3 Agonist LY379268 Blocks the Effects of GLT-1 Upregulation on Prepulse Inhibition of the Startle Reflex in Adult Rats

“…mGluR2/3 activation depends on the spread of glutamate from release sites to the perisynaptic zone, where these receptors are predominantly located (Scanziani et al, 1997;Min et al, 1998;Mitchell and Silver, 2000;Ferraguti and Shigemoto, 2006); and GLT-1 controls extracellular levels of glutamate and it is mainly localized perisynaptically (Melone et al, 2009a). Thus, by modulating glutamate spillover, GLT-1 influences activation of mGluR2/3 Barbour, 2001;Diamond, 2002;Tzingounis and Wadiche, 2007;Zheng et al, 2008;Omrani et al, 2009). Our western blotting and immunocytochemical studies are consistent with this view.…”

Morphological plasticity of astroglia: Understanding synaptic microenvironment

“…The PAP remodeling, with or without local rearrangement of glutamate transporter proteins, is capable of altering the efficiency of extrasynaptic glutamate clearance or escape (Benediktsson et al, 2012; Danbolt, 2001; Panatier et al, 2006; Zheng et al, 2008) (although (Diamond et al, 1998)) and hence affecting the modus operandi for local excitatory circuitry. Altered glial coverage could also affect astrocyte‐neuron lactate transport contributing to synaptic efficacy (Suzuki et al, 2011) and local water homeostasis (Simard and Nedergaard, 2004).…”

“…Detailed modeling of release events in the synaptic environment suggests that covering half of the synaptic perimeter by PAPs can reduce extra‐cleft glutamate transients several‐fold, and 95% coverage enhances this phenomenon by an order of magnitude (Rusakov, 2001; Zheng et al, 2008). Clearly, changes in PAP coverage have important consequences for glutamate escape and subsequent activation of perisynaptic glutamate receptors.…”

“…Interestingly, high‐affinity receptors, such as NMDARs or mGluRs, which are located just outside the synaptic cleft, appear most sensitive to changes in extrasynaptic glutamate escape. This is because the glutamate concentration time course at that locus corresponds to the highest dynamic range of receptor responses (Sylantyev et al, 2013; Zheng et al, 2008). Thus, such receptors could be “strategically” positioned to respond strongly to any small changes in astroglial coverage of the synapse.…”

Input-Specific Intrasynaptic Arrangements of Ionotropic Glutamate Receptors and Their Impact on Postsynaptic Responses

“…Time steps for AMPAR and NMDAR simulations were set as 0.5 and 5 s, respectively, and the glutamate concentration profile at each receptor location was calculated using the Excel software. Unless otherwise noted, the number of glutamate molecules released ( N) was set at 2000 molecules, and the diffusion coefficient of glutamate ( D) was set at 0.3 m 2 /ms (Barbour, 2001;Rusakov, 2001;Franks et al, 2003, Nielsen et al, 2004, Zheng et al, 2008. Validity of the selection of these parameters is discussed in detail in the supplemental discussion.…”