Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells.

Abstract

1. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under voltage-clamp conditions. Consistent with activation of non-NMDA-type glutamate receptors, the sEPSCs reversed at potentials above 0 mV, were blocked by 1 UM CNQX and prolonged by 2 mm aniracetam. 2. The peak conductance of the averaged sEPSCs (n = 70-400) was 130 + 60 pS (mean + S.D.; 17 cells, ranging from 70 to 290 pS). Amplitude distributions were skewed towards larger amplitudes. 3. The decay of individual and mean sEPSCs was exponential with a mean time constant (Td) of 3.75 + 0-84 ms (n = 13), which was voltage independent. The 10-90% rise time of the sEPSCs was 1-30 + 0 44 ms (n = 13). There was no correlation between sEPSC rise time and Td suggesting that dendritic filtering alone did not shape the time course of sEPSCs. Light-evoked EPSCs in these retinal ganglion cells are mediated by concomitant activation ofNMDA and non-NMDA receptors; however, no NMDA component was discerned in the sEPSCs, even when recording at -96 mV in Mg2+-free solutions. The decay time course was not altered by 20 uM AP7, an NMDA antagonist, nor was an NMDA component unmasked by adding glycine or D-serine. These results suggest that NMDA and non-NMDA receptors are not coactivated by a single vesicle of transmitter during spontaneous release, and thus are probably not colocalized in the postsynaptic membrane at the sites of spontaneous release. 5. The sEPSCs were an order of magnitude faster than the non-NMDA receptor-mediated EPSCs evoked by light stimuli, and it is proposed that the EPSC time course is determined largely by the extended time course of release of synaptic vesicles from bipolar cells. The quantal content of a light-evoked non-NMDA receptor-mediated EPSC in an on-off cell is about 200 quanta.Bipolar cells provide the major presynaptic excitatory inputs to retinal ganglion cells (Wong-Riley, 1974 (Finch, Fischer & Jackson, 1990;LoTurco, Mody & Kriegstein, 1990). Although the time course of sEPSCs can directly reflect the kinetics of activated channels, it may also be affected by the membrane properties of the neurone. In some neurones, correlation between rise and decay times indicates that electrotonic decay along the dendrites filters sEPSCs recorded at the cell soma (McBain & Dingledine, 1993;Ulrich & Luscher, 1993). In this study, using the whole-cell patch-clamp technique to study sEPSCs in retinal ganglion cells of tiger salamander, we investigated whether sEPSCs are solely glutamatergic and whether there are dual NMDA and non-NMDA components in the sEPSCs consistent with a colocalization of receptor subtypes. In addition, we characterized the amplitude distribution and time course of sEPSCs and computed the approximate number of glutamate channels activated by a typical sEPSC. We also tested the possibility that sEPSC time courses were shaped primarily by filtering due to conduction along the dendritic process. Some of the results have been presented in abstract form (Taylor, Chen & Copenhagen, 1991). This is the first study to ch...

Prolonged Reciprocal Signaling via NMDA and GABA Receptors at a Retinal Ribbon Synapse

“…This is similar to what was reported in retinal ganglion cells (Mittman et al, 1990) and in granule cells in the olfactory bulb (Isaacson and Strowbridge, 1998). NMDARs do not contribute to the spontaneous EPSCs in ganglion cells, although they are activated during light stimuli (Taylor et al, 1995;Chen and Diamond, 2002). Furthermore, in the mouse retina, NMDARs were activated during the light response of transient ON-type ACs only when glutamate release was increased by blocking GABA C receptors on BCs (Matsui et al, 2001).…”

Distinct perisynaptic and synaptic localization of NMDA and AMPA receptors on ganglion cells in rat retina

“…NMDARs are not activated by glutamate released from a single vesicle (Taylor et al, 1995;Matsui et al, 1998;Chen and Diamond, 2002), unless glutamate uptake is reduced (Chen and Diamond, 2002), and NMDARs encounter a lower synaptic glutamate concentration during evoked responses than do AMPARs (Chen and Diamond, 2002). Taken together, these previous physiological data and the morphological results presented here suggest that transmitter released from a single vesicle is not sufficient to activate NMDARs located perisynaptically, several hundred nm from the release site.…”

“…These studies did not determine the identity of the immunopositive postsynaptic process, however, and the preembedding immunoperoxidase method, which allows reaction product to diffuse throughout the immunopositive process, precludes distinction between localization to the PSD versus the immediately surrounding extrasynaptic membrane (Ottersen and Landsend, 1997;Nusser, 2000;Bredt and Nicoll, 2003). In the case of AMPARs, physiological and anatomical data are consistent with a synaptic localization on RGCs (Mittman et al, 1990;Taylor et al, 1995;Qin and Pourcho, 1996;Lukasiewicz et al, 1997;Qin and Pourcho, 1999;Jacoby and Wu, 2001;Chen and Diamond, 2002), although subsynaptic localization of AMPAR and the identity of immunopositive processes have not been examined with immunogold EM.…”

State-Dependent AMPA Receptor Trafficking in the Mammalian Retina

“…Consistent with this, we rarely observed any excitatory synaptic input detected as spontaneous EPSCs (sEPSCs) in neurons from these cultures. It has been found that sEPSCs in retinal ganglion and amacrine cells are mediated purely by AMPARs (Taylor et al, 1995;Matsui et al, 1998). We therefore attempted to increase excitatory input by adding a low concentration of AMPA (10 M) to the cultures together with 50 M APV.…”