Prolonged Synaptic Currents and Glutamate Spillover at the Parallel Fiber to Stellate Cell Synapse

Abstract

Although neurons often fire in bursts, most of what is known about glutamate signaling and postsynaptic receptor activation is based on experiments using single stimuli. Here we examine the activation of ionotropic glutamate receptors by bursts at the parallel fiber to stellate cell synapse. We show that brief stimulus trains generate prolonged AMPA receptor (AMPAR)- and NMDA receptor (NMDAR)-mediated EPSCs recorded in whole-cell voltage clamp. These EPSCs contrast with the rapid AMPAR-mediated EPSC evoked by a single stimulus. The prolonged AMPAR-mediated EPSC is promoted by high-frequency and high-intensity trains and can persist for hundreds of milliseconds. This EPSC is also increased by l-trans-2,4-PDC, an inhibitor of glutamate transporters, suggesting that these transporters usually limit the synaptic response to trains. These prolonged EPSCs reflect both receptor properties and a long-lasting glutamate signal. In addition, several experiments demonstrate that glutamate spillover can contribute to receptor activation. First, imaging stimulus-evoked changes in presynaptic calcium establishes that distinct parallel fiber bands can be activated. Second, activation of parallel fibers that do not directly synapse onto a given stellate cell can evoke indirect AMPAR- and NMDAR-mediated EPSCs in that cell. Third, experiments using the use-dependent NMDAR blocker MK-801 show that these indirect EPSCs reflect glutamate spillover in response to trains. Together, these findings indicate that stimulus trains can generate a sustained and widespread glutamate signal that can in turn evoke large and prolonged EPSCs mediated by ionotropic glutamate receptors. These synaptic properties may have important functional consequences for stellate cell firing.

Group I Metabotropic Glutamate Receptors on Second-Order Baroreceptor Neurons Are Tonically Activated and Induce a Na+–Ca2+Exchange Current

“…In pilot studies, PDC efficacy in increasing glutamate in the cleft was verified on secondorder baroreceptor NTS neurons by the following: 1) PDC in the perfusate increased the decay time constant for TS-evoked glutamatergic excitatory postsynaptic currents by 34% (2.7 Ϯ 0.3 to 3.7 Ϯ 0.6 ms, n ϭ 3); and 2) increased the peak amplitude of the inward current evoked by application of exogenous glutamate (300 M) with NBQX, AP5, bicuculline, and TTX in the perfusate by 146% (146.3 Ϯ 39.8 pA, n ϭ 4 vs. 59.5 Ϯ 7.2 pA, n ϭ 5; one-tail t-test, P ϭ 0.034), consistent with previous findings (Carter and Regehr 2000). The neurons were initially held at their own resting membrane potentials (Ϫ50.5 Ϯ 1.3 mV) and then recorded in current-clamp mode.…”

Glial Glutamate Transporters Maintain One-to-One Relationship at the Climbing Fiber-Purkinje Cell Synapse by Preventing Glutamate Spillover

“…It has been demonstrated in many synapses that the inhibition of uptake of synaptically released transmitter causes spillover of transmitter to neighboring synapses (Rossi and Hamann, 1998;Carter and Regehr, 2000;Diamond, 2001;Clark and Cull-Candy, 2002;DiGregorio et al, 2002;Takayasu et al, 2004). The results shown in this study can also be reasonably explained by the hypothesis that glutamate spillover causes both pharmacologically induced slow-rising and mutant atypical CF-EPSCs.…”

Imaging extrasynaptic glutamate dynamics in the brain

“…K716A-EOS signal amplitude increased with the time-integral of the EPSC, showing the dependence of extrasynaptic glutamate dynamics on the density of stimulated PFs (Fig. 3C) (8,11,30). It is thus strongly suggested that glutamate released from neighboring synapses can be integrated in the extrasynaptic space.…”