Calcium-permeable presynaptic kainate receptors involved in excitatory short-term facilitation onto somatostatin interneurons during natural stimulus patterns

Abstract

Schaffer collateral synapses in hippocampus show target-cell specific short-term plasticity. Using GFP-expressing Inhibitory Neuron (GIN) transgenic mice that express enhanced green fluorescent protein (EGFP) in a subset of somatostatin-containing interneurons (SOM interneurons), we previously showed that Schaffer collateral synapses onto SOM interneurons in stratum (S.) radiatum have unusually large (up to 6-fold) paired-pulse facilitation. This results from a low initial release probability and the enhancement of facilitation by synaptic activation of presynaptic kainate receptors. Here we further investigate the properties of these kainate receptors and examine their effects on short-term facilitation during physiologically derived stimulation patterns, using excitatory postsynaptic currents recorded in S. radiatum interneurons during Schaffer collateral stimulation in acute slices from juvenile GIN mice. We find that GluR5 and GluR6 antagonists decrease short-term facilitation at Schaffer collateral synapses onto SOM interneurons with no additive effects, suggesting that the presynaptic kainate receptors are heteromers containing both GluR5 and GluR6 subunits. The calcium-permeable receptor antagonist 1-napthyl acetyl spermine (NASPM) both mimics and occludes the effect of the kainate receptor antagonists, indicating that the presynaptic kainate receptors are calcium permeable. Furthermore, Schaffer collateral synapses onto SOM interneurons show up to 11-fold short-term facilitation during physiologically derived stimulus patterns, in contrast to other interneurons that have less than 1.5-fold facilitation. Blocking the kainate receptors reduces facilitation in SOM interneurons by approximately 50% during the physiologically derived patterns and reduces the dynamic range. Activation of calcium-permeable kainate receptors containing GluR5/GluR6 causes a dramatic increase in short-term facilitation during physiologically derived stimulus patterns, a mechanism likely to be important in regulating the strength of Schaffer collateral synapses onto SOM interneurons in vivo.

FIG. 1.

GluR5 is involved in Schaffer collateral synapse facilitation onto SOM interneurons. A: Example of EPSCs recorded from an s. radiatum EGFP-expressing SOM interneuron before and during application of the GluR5-specific kainate receptor antagonist UBP302 (15 μM). Scale bars: 20 ms, 60 pA. B: Group results for paired-pulse ratios versus interval from EGFP-expressing SOM interneurons before (black circles) and during application of UBP302 (gray triangles) (n = 13). UBP302 caused a significant decrease in paired-pulse facilitation for intervals from 20 to 40 ms. C: Average amplitude of first EPSC for experiments in B was not different in UBP302. D: Example of EPSCs recorded from an s. radiatum non-EGFP interneuron before and during application of 15 μM UBP302. Scale bars: 20 ms, 35 pA. E: Group results for paired-pulse ratios versus interval from non-EGFP interneurons before (black circles) and during application of UBP302 (gray triangles) (n = 7) show no change in paired-pulse facilitation. F: Average amplitude of first EPSC for experiments in E. G: Percent decrease in paired-pulse facilitation with UBP302 at 20 ms and 40 ms paired-pulse intervals (combined) for each cell type. * = significant, P < 0.05.

FIG. 2.

Kainate receptors at Schaffer collateral synapses onto SOM interneurons are heteromeric and contain GluR5/GluR6. A: Example of EPSCs recorded from an s. radiatum EGFP-expressing SOM interneuron before and during application of the GluR5-specific homomeric kainate receptor antagonist NS3763 (10 μM). Scale bars: 20 ms, 50 pA. B: Group results for paired-pulse ratios versus interval from EGFP-expressing SOM interneurons before (black circles) and during application of NS3763 (gray triangles) (n = 7). Blockade of homomeric GluR5 receptors had no effect on paired-pulse facilitation at Schaffer collateral synapses onto SOM interneurons. C: Average amplitude of first EPSC for experiments in B was not different in NS3763. D: Example traces of EPSCs recorded from an s. radiatum EGFP-expressing SOM interneuron before and during application of UBP302 (15 μM) and NS102 (20 μM), to block all kainate receptors containing GluR5 and GluR6. Scale bars: 20 ms, 20 pA. E: Group results for paired-pulse ratios showing that blockade of GluR5 and GluR6 containing kainate receptors with co-application of UBP302 and NS102 reduces paired-pulse facilitation in SOM interneurons (n = 12). The combination of UBP302 and NS102 causes a significant decrease in the paired-pulse facilitation for short intervals. F: Average amplitude of first EPSC for experiments in E. G: Percent decrease in paired-pulse facilitation in SOM interneurons (combining the 20 ms and 40 ms intervals) for NS3763 (n = 7), UPB302 (n = 13), NS102 (n = 6), and UBP302 plus NS102 (n = 12). No additive effect is seen for UBP302+NS102. H, Group results for paired-pulse facilitation with UBP302 versus UBP302+NS102 (n = 9). Order of application: Control (no drug, black circles), UBP302 alone (gray triangles), co-application of UBP302 and NS102 (black open triangles). While both UBP302 and UBP302+NS102 have less paired-pulse facilitation compared with Control at 20 and 40 ms, they are not different from each other. I, Percent decrease in paired-pulse facilitation in SOM interneurons (combining 20 ms and 40 ms intervals) for UBP302 versus UBP302+NS102 (from experiments in H) are not different. * = significant, P < 0.05.

FIG. 3.

Presynaptic kainate receptors are calcium permeable at Schaffer collateral synapses onto SOM interneurons. A: Example of EPSCs recorded from an s. radiatum EGFP-expressing SOM interneuron before and during application of NASPM (200 μM), an antagonist of calcium-permeable AMPA and kainate receptors. Scale bars: 20 ms, 30 pA. B: Group results for paired-pulse ratios showing that blockade of calcium-permeable kainate receptors with application of NASPM reduces paired-pulse facilitation in SOM interneurons at short intervals (n = 12). C: Average amplitude of first EPSC for experiments in B. D: Example of EPSCs recorded from an s. radiatum non-EGFP-expressing interneuron before and during application of NASPM (200 μM). Scale bars: 20 ms, 50 pA. E: Group results for paired-pulse ratios versus interval from non-EGFP-expressing interneurons before and during application of NASPM (n = 10). NASPM caused no change in the paired-pulse facilitation. F: Average amplitude of first EPSC for experiments in E. G: Percent decrease in paired-pulse facilitation with NASPM at 20 ms and 40 ms paired-pulse intervals (combined) for both cell types. H, Group results for paired-pulse ratios versus interval from SOM interneurons for NASPM versus NASPM+UBP302+NS102. Order of application: Control (no drug, black circles), NASPM alone (gray triangles), co-application of NASPM with UBP302 and NS102 (black open triangles). While both NASPM and NASPM+UBP302+NS102 have less paired pulse facilitation compared with Control at 20 and 40 ms, they are not different from each other. I, Percent decrease in paired-pulse facilitation in SOM interneurons (combining 20 ms and 40 ms intervals) for NASPM versus NASPM+UBP302+NS102 (from experiments in H) are not different. * = significant, P < 0.05.

FIG. 4.

Natural pattern stimulation induces large short-term facilitation in SOM interneurons. A: Timing of NSP is highly variable. Top panel: timing of whole NSP (46 stimuli). Bottom panel: timing of one cluster of stimuli shown on an expanded time scale. B: Example trace of EPSCs recorded in response to a region (stimuli 16–31) of the NSP applied to Schaffer collateral synapses onto an s. radiatum EGFP-expressing SOM interneuron. Scale bars: 500 ms, 35 pA. C: Example trace of EPSCs recorded in response to stimuli 16 through 31 of the NSP applied to Schaffer collateral synapses onto a non-EGFP-expressing interneuron. Scale bars: 500 ms, 25 pA. D: Group results show very large short-term facilitation at Schaffer collateral synapses onto EGFP-expressing SOM interneurons (gray triangles, n = 8) in contrast to non-EGFP interneurons (black circles, n = 7) which show little facilitation. Stimuli 47–60 are control period at 0.1 Hz for normalization. E: The dynamic range over which the Schaffer collateral synapses operate (maximum-minimum, from experiments in D) is larger for EGFP-expressing SOM interneurons compared with other s. radiatum interneurons. F: The average amplitude of the entire NSP (the first 46 points, from experiments in D) is significantly different between the two different interneuronal groups. G: Responses to natural pattern stimulation plotted versus interstimulus interval shows that large facilitation occurs at shorter intervals in EGFP-expressing SOM interneurons. * = significant, P < 0.05.

FIG. 5.

Large short-term facilitation during natural pattern stimulation is mediated in part through calcium-permeable kainate receptors. A: Group results showing that blocking calcium-permeable kainate receptors with 200 μM NASPM (gray triangles) reduces short-term facilitation at Schaffer collateral synapses onto SOM interneurons in response to NSP (n = 6). B: Average range of responses (maximum –minimum, from experiments in A) is reduced by NASPM. C: Average amplitude of the responses to the NSP is reduced by NASPM. D: Responses to NSP plotted versus interstimulus interval show that the effect of NASPM is greatest at shorter intervals. E: Difference between control responses and responses in NASPM for different ranges of interstimulus intervals. Numbers on the bars indicate the number of stimuli (out of 46) that were in each range. F: Percent reduction of short-term facilitation with NASPM is greater during natural stimulus pattern (NSP, from experiments in A) than during paired-pulse stimulation (PPF, from experiments in 3B) for interstimulus intervals (ISIs) of ≤80 ms. G: Percent reduction with NASPM is greater during NSP than PPF for intervals of 200 –500 ms. * = significant, P < 0.05.