Matthew Frerking

Synaptic activation of kainate receptors on hippocampal interneurons

Although kainate receptor activation has been known to evoke epileptiform activity, little is known about the role of kainate receptors in synaptic transmission. Here we report that kainate (KA) receptors are present on interneurons and, when activated, cause a large increase in the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) driven by action potentials. Stimulation of excitatory afferents generates a pharmacologically identifiable synaptic current mediated by KA receptors in interneurons. This synaptic current is similar to that mediated by AMPA receptors in its response to short stimulus trains, current–voltage relations and coefficient of variation, but it is much smaller in peak amplitude and much slower. KA application also considerably depresses evoked IPSCs. This depression seems to be in large part an indirect consequence of the repetitive firing evoked by the activation of the interneuronal somatic/dendritic KA receptors.

Variation in GABA mini amplitude is the consequence of variation in transmitter concentration

Miniature postsynaptic currents (minis) in cultured retinal amacrine cells, as in other central neurons, show large variations in amplitude. To understand the origin of this variability, we have exploited a novel form of synapse in which pre- and postsynaptic receptors sample the same quantum of transmitter. At these synapses, mini amplitudes measured simultaneously in the 2 cells show a strong correlation, accounting for, on average, more than half of the variance in amplitude. Two pieces of evidence support the conclusion that variations in the amount of transmitter in different quanta underlie this correlation. First, diazepam, which enhances GABA binding, increases mini amplitude, implying therefore that transmitter concentration is not saturating. Second, we show that amplitude distributions from all cells, even those with a small number of release sites, have the same shape, implying that most or all variance is intrinsic to each release site.

Synaptic Activation of Presynaptic Kainate Receptors on Hippocampal Mossy Fiber Synapses

Kainate receptors (KARs) are a poorly understood family of ionotropic glutamate receptors. A role for these receptors in the presynaptic control of transmitter release has been proposed but remains controversial. Here, KAR agonists are shown to enhance fiber excitability, and a number of experiments show that this is a direct effect of KARs on the presynaptic fibers. In addition, KAR activation inhibits evoked transmitter release from mossy fiber synapses. Synaptic release of glutamate from either neighboring mossy fiber synapses or associational/commisural (A/C) synapses results in the activation of these presynaptic ionotropic KARs. These results, along with previous studies, indicate that KARs, through the endogenous release of glutamate, mediate excitatory postsynaptic potentials (EPSPs), alter presynaptic excitability, and modulate transmitter release.

Synaptic kainate receptors.

Kainate receptors are a family of ionotropic glutamate receptors with poorly understood functions. Recent evidence firmly establishes kainate receptors as postsynaptic mediators of synaptic transmission. A second, presynaptic, modulatory role of kainate receptors has also been suggested, although the mechanism(s) involved remain controversial.

Saturation of postsynaptic receptors at central synapses

A fundamental issue in synaptic physiology is whether the postsynaptic response to a quantum of transmitter is limited by the number of receptors available. Fierce debate over the past few years has yielded no consensus. The majority of evidence suggests that the degree of receptor occupancy is likely to be sensitive to a number of factors, including the detailed anatomy of the synaptic cleft and the time course of transmitter clearance, and is probably different from one synapse to the next.

Presynaptic kainate receptors at hippocampal mossy fiber synapses

Hippocampal mossy fibers, which are the axons of dentate granule cells, form powerful excitatory synapses onto the proximal dendrites of CA3 pyramidal cells. It has long been known that high-affinity binding sites for kainate, a glutamate receptor agonist, are present on mossy fibers. Here we summarize recent experiments on the role of these presynaptic kainate receptors (KARs). Application of kainate has a direct effect on the amplitude of the extracellularly recorded fiber volley, with an enhancement by low concentrations and a depression by high concentrations. These effects are mediated by KARs, because they persist in the presence of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-selective antagonist GYKI 53655, but are blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/KAR antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and the KAR antagonist SYM2081. The effects on the fiber volley are most likely caused by a depolarization of the fibers via the known ionotropic actions of KARs, because application of potassium mimics the effects. In addition to these effects on fiber excitability, low concentrations of kainate enhance transmitter release, whereas high concentrations depress transmitter release. Importantly, the synaptic release of glutamate from mossy fibers also activates these presynaptic KARs, causing an enhancement of the fiber volley and a facilitation of release that lasts for many seconds. This positive feedback contributes to the dramatic frequency facilitation that is characteristic of mossy fiber synapses. It will be interesting to determine how widespread facilitatory presynaptic KARs are at other synapses in the central nervous system.

Effects of variance in mini amplitude on stimulus-evoked release: a comparison of two models

The strength of synaptic connections between two neurons is characterized by the number of release sites (N) on the presynaptic cell, the probability (p) of transmitter release at those sites in response to a stimulus, and the average size (A) of the postsynaptic response from each site. Quantal analysis can determine N, p, and A, but the large variance in the amplitudes of minis at central synapses is predicted to obscure quantal peaks and render quantal analysis unusable. Recently it has been suggested that the variance in mini amplitude is generated by differences between release sites, rather than by quantum-to-quantum fluctuations at identical sites, and that this form of variance in mini amplitude reduces the amount of variance expected in quantal peaks. Using simulations, we examine the possibility of resolving quantal peaks assuming either form of variance in mini amplitude. We find that individual quantal peaks are resolvable in neither case, provided that the uniquantal distribution is similar to the mini distribution. Because this lack of resolution compromises the utility of quantal analysis, we develop a general description that can solve N and p, given the statistical parameters of the mini distribution and the evoked distribution. We find that this description is relatively insensitive to the source of variance in mini amplitude.

Coupled Activity-dependent Trafficking of Synaptic SK2 Channels and AMPA Receptors

Small conductance Ca2+-activated K+ type 2 (SK2) channels are expressed in the postsynaptic density of CA1 neurons where they are activated by synaptically evoked Ca2+ influx to limit the size of EPSPs and spine Ca2+ transients. At Schaffer collateral synapses, the induction of long-term potentiation (LTP) increases the α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR)-mediated contribution to synaptic transmission and decreases the synaptic SK2 channel contribution through protein kinase A-dependent channel endocytosis. Using a combination of electrophysiology and immunoelectron microscopy in mice, the relationship between the dynamics of spine SK2 channels and AMPARs was investigated. Unlike AMPARs, synaptic SK2 channels under basal conditions do not rapidly recycle. Furthermore, SK2 channels occupy a distinct population of endosomes separate from AMPARs. However, blocking vesicular exocytosis or the delivery of synaptic GluA1-containing AMPARs during the induction of LTP blocks SK2 channel endocytosis. By ∼2 h after the induction of LTP, synaptic SK2 channel expression and function are restored. Thus, LTP-dependent endocytosis of SK2 is coupled to LTP-dependent AMPA exocytosis, and the ∼2 h window after the induction of LTP during which synaptic SK2 activity is absent may be important for consolidating the later phases of LTP.

Kainate receptors and synaptic plasticity.

Bortolotto et al. report that the kainate subtype of glutamate receptor is essential for the plasticity of certain types of synaptic transmission in the brain, which is of interest as these receptors were previously not thought to initiate plastic processes. In particular, a new antagonist (LY382884) was shown to act selectively against the GluR5 type of kainate receptor: in the presence of LY382884, which reduces kainate-receptor-mediated postsynaptic responses by ∼40%, long-term potentiation (LTP) at hippocampal mossy-fibre synapses could no longer be induced. Here we argue that the available evidence does not support a major role for kainate receptors in the induction of mossy-fibre LTP.

Subunit-Dependent Postsynaptic Expression of Kainate Receptors on Hippocampal Interneurons in Area CA1

Kainate receptors (KARs) contribute to postsynaptic excitation in only a select subset of neurons. To define the parameters that specify the postsynaptic expression of KARs, we examined the contribution of KARs to EPSCs on hippocampal interneurons in area CA1. Interneurons in stratum radiatum/lacunosum-moleculare express KARs both with and without the GluR5 subunit, but KAR-mediated EPSCs are generated mainly, if not entirely, by GluR5-containing KARs. Extrasynaptic glutamate spillover profoundly recruits AMPA receptors (AMPARs) with little effect on KARs, indicating that KARs are targeted at the synapse more precisely than AMPARs. However, spontaneous EPSCs with a conventional AMPAR component did not have a resolvable contribution of KARs, suggesting that the KARs that contribute to the evoked EPSCs are at a distinct set of synapses. GluR5-containing KARs on interneurons in stratum oriens do not contribute substantially to the EPSC. We conclude that KARs are localized to synapses by cell type-, synapse-, and subunit-selective mechanisms.