Local Efficacy of Glutamate Uptake Decreases with Synapse Size
Authors: Stefan Passlick
Colin J Jackson
Dirk Dietrich
Kirsten Bohmbach
Christian Henneberger
Natascha Vana
Martin K Schwarz
Cátia Domingos
Michel K Herde
Joanna A Komorowska-Müller
Inna Schwarz
Journal: Cell Reports (2020)
Discipline: MOLECULAR BIOLOGY & GENETICS
Keywords: astrocyte morphology; astrocytes; expansion microscopy; glutamate uptake; hippocampus
Affiliations: Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany., Department for Neurosurgery, University Hospital Bonn, Bonn, Germany., Institute of Epileptology, Medical Faculty, University of Bonn, Bonn, Germany., Research School of Chemistry, Australian National University, Canberra, Australia., Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany; Institute of Neurology, University College London, London, UK; German Centre for Neurodegenerative Diseases (DZNE), Bonn, Germany. Electronic address: .

Abstract:  Synaptically released glutamate is largely cleared by glutamate transporters localized on perisynaptic astrocyte processes. Therefore, the substantial variability of astrocyte coverage of individual hippocampal synapses implies that the efficacy of local glutamate uptake and thus the spatial fidelity of synaptic transmission is synapse dependent. By visualization of sub-diffraction-limit perisynaptic astrocytic processes and adjacent postsynaptic spines, we show that, relative to their size, small spines display a stronger coverage by astroglial transporters than bigger neighboring spines. Similarly, glutamate transients evoked by synaptic stimulation are more sensitive to pharmacological inhibition of glutamate uptake at smaller spines, whose high-affinity N-methyl-D-aspartate receptors (NMDARs) are better shielded from remotely released glutamate. At small spines, glutamate-induced and NMDAR-dependent Ca2+ entry is also more strongly increased by uptake inhibition. These findings indicate that spine size inversely correlates with the efficacy of local glutamate uptake and thereby likely determines the probability of synaptic crosstalk. 

Retracted: No

PubPeer comments:
 In Fig. 4 (reproduced below) the authors show NMDA currents arising from focal uncaging of glutamate. There are several aspects of the responses that I do not understand. Firstly, I find the decay of the NMDA component, especially in panel 4c, to be extraordinarily slow. The time constant appears to be greater than half a second. Surely synaptic NMDAR-mediated currents decay much more quickly? A second issue is that releasing glutamate 500nm from the synapse evokes a response that is barely smaller (in fact identical in Fig. 4c) than that to release at the synapse. This is unexpected, to say the least. I suspect two technical issues not considered by the authors explain these strong deviations from expected behaviour. Firstly, it is likely that something of a distributed cloud of glutamate is produced by the 0.6 ms illumination throughout the two-photon spot. Secondly, caged glutamate is only present in the extracellular space. I suspect that large synapses occlude more of the extracellular space than small ones, thereby reducing release at the synapse. There may also be a contribution from the greater extent of the PSD at large synapses. How did the authors ensure that the uncaging spot for the distal stimulation did not fall within an unlabelled dendrite or cell body? In the presence of these unexplained and unaddressed issues, I think it is difficult to draw firm conclusions from the experiments in Fig. 4. ![file] ,