Slow AMPAR Synaptic Transmission Is Determined by Stargazin and Glutamate Transporters

Abstract

Summary AMPARs mediate the briefest synaptic currents in the brain by virtue of their rapid gating kinetics. However, at the mossy fiber-to-unipolar brush cell synapse in the cerebellum, AMPAR-mediated EPSCs last for hundreds of milliseconds, and it has been proposed that this time course reflects slow diffusion from a complex synaptic space. We show that upon release of glutamate, synaptic AMPARs were desensitized by transmitter by >90%. As glutamate levels subsequently fell, recovery of transmission occurred due to the presence of the AMPAR accessory protein stargazin that enhances the AMPAR response to low levels of transmitter. This gradual increase in receptor activity following desensitization accounted for the majority of synaptic transmission at this synapse. Moreover, the amplitude, duration and shape of the synaptic response was tightly controlled by plasma membrane glutamate transporters, indicating that clearance of synaptic glutamate during the slow EPSC is dictated by an uptake process.

δ subunit‐containing GABAA IPSCs are driven by both synaptic and diffusional GABA in mouse dentate granule neurons

“…Past work using dextran to probe diffusional contributions has led to different results, some of which qualitatively match our observations, and others that do not. For instance, dextran slows a glutamate EPSC in unipolar brush cells (Lu et al 2017) but speeds GABA IPSCs generated by neurogliaform cells in hippocampus (Szabadics et al 2007). The latter is similar to our results.…”

“…This is in contrast to some bona fide diffusion‐mediated PSCs (Lu et al . 2017) and raises the possibility that δ sIPSC decay time course is limited by slow deactivation kinetics of receptors rather than diffusion or transmitter rebinding. To explore this possibility, we employed rapid GABA application to nucleated outside‐out patches from dentate granule neurons.…”

“…For instance, dextran slows a glutamate EPSC in unipolar brush cells (Lu et al . 2017) but speeds GABA IPSCs generated by neurogliaform cells in hippocampus (Szabadics et al . 2007).…”

“…For WT dual-component current deactivation, comparisons of tau values below 30% of maximal peak amplitude (N = 9 patches from 2 WT animals) vs. those at maximal peak amplitude (N = 9 patches from 2 WT Exploring intrinsic channel deactivation kinetics of δ vs. γ2 receptors Figure 6 shows that slowing diffusion does not affect the decay of δ sIPSCs. This is in contrast to some bona fide diffusion-mediated PSCs (Lu et al 2017) and raises the possibility that δ sIPSC decay time course is limited by slow deactivation kinetics of receptors rather than diffusion or transmitter rebinding. To explore this possibility, we employed rapid GABA application to nucleated outside-out patches from dentate granule neurons.…”

Selective targeting of unipolar brush cell subtypes by cerebellar mossy fibers

“…ChR2 activation of primary afferents with blue light flashes caused bursts of action potentials in postsynaptic UBCs ( Figure 3B). Activation of primary afferents led to time-locked, depressing EPSCs, followed by a slow inward current; both responses were mediated by AMPA receptors, and are diagnostic of ON UBCs (Borges-Merjane and Trussell, 2015;Lu et al, 2017;Zampini et al, 2016).…”