Article Figures & Data
Figures
- Figure 1.
Short-term depression of KAR–EPSCs at mossy fiber synapses. A, Representative traces illustrating short-term depression of mossy fiber KAR–EPSCs in CA3 pyramidal cells. A train of five stimulation pulses at 20 Hz was delivered to mossy fibers, followed by a test stimulation (Δt = 500 ms). The train was either given with the cell membrane potential being held at −80 mV [control train (ctr)] or at −40 mV [conditioning train (cond)]. The amplitudes of test KAR–EPSCs after a control (EPSCctr) or a conditioning (EPSCcond) train were compared (enlarged traces are superimposed in inset). B, Average time course of normalized mossy fiber KAR–EPSCs in response to the test stimulus. The KAR–EPSC value was normalized to the average of the amplitudes of KAR–EPSCs obtained in control conditions for the first four KAR–EPSCs in each recording. Arrows indicate the time when the conditioning train was applied. C, Time course of depression of KAR–EPSCs by the conditioning train of mossy fiber stimulation. Time between the end of the train and the control stimulus (Δt) was varied between 0.2 and 20 s.
- Figure 2.
Short-term depression of KAR–EPSCs involves NMDA activation, postsynaptic calcium rise, and calcineurin activation. A, Effects of the conditioning train on AMPAR–EPSCs. Enlarged traces for EPSCctr and EPSCcond are superimposed in inset. ctr, Control train; cond, conditioning train. B, Summary of the effects of the conditioning train in different experimental conditions. The conditioning train depressed KAR–EPSCs (Control), but not AMPAR–EPSCs, and was abolished in presence of the NMDAR antagonist d-AP5 (50 μm), or when BAPTA (20 mm) or the phosphatase calcineurin inhibitor deltamethrin (100 μm) were included in the patch pipette. **p < 0.01.
- Figure 3.
GluR6b subunit is necessary for NMDA-induced short-term depression of KAR–EPSCs. A, Representative traces illustrating short-term depression of mossy fiber KAR–EPSCs in CA3 pyramidal cells obtained when a GluR6b C-terminal peptide (pepR6b), which blocks the GluR6b interaction with calcineurin, was included in the patch pipette. B, Effects of the conditioning train (cond) in slices from transgenic mice expressing GluR6a but not GluR6b (R6a tg × R6−/−). ctr, Control train. C, Summary plot of the results obtained when interfering with the GluR6b subunit. Blocking GluR6b interaction with calcineurin by inclusion of pepR6b in the recording pipette, but not its scramble version, or working with slices from R6a tg × R6−/− mice greatly attenuated short-term depression of KAR–EPSCs induced by NMDAR activation. **p < 0.01. D, Summary plot of KAR–EPSCs amplitudes obtained in control conditions and in conditions in which the pepR6b was present in the patch pipette. E, Average time course of normalized mossy fiber KAR–EPSCs obtained when the pepR6b was present in the patch pipette. The KAR–EPSC value was normalized to the average of the amplitudes of KAR–EPSCs obtained in control conditions for the first four KAR–EPSCs in each recording. Arrows indicate the time when the conditioning train was applied. The presence of the pepR6b in the patch pipette did not affect the time course of control KAR–EPSCs, but it strongly reduced the depression induced by the conditioning train.
- Figure 4.
Short-term depression of KAR–EPSCs is homosynaptic. A, Representative traces of mossy fiber–EPSCs illustrating the protocol used to verify whether two independent mossy fibers were being stimulated. Two glass stimulating electrodes were positioned at different loci in the dentate gyrus [stimulation (stim) 1 and stim 2]. Prominent paired-pulse facilitation is observed for both mossy fiber synaptic inputs applied independently (stim 1 and stim 2). However, no potentiation to stim 1 is observed after stim 2 (and vice versa). The two fibers stimulated by stim 1 and stim 2 were considered to be independent. B, Homosynaptic: the conditioning protocol was applied to the same fiber as the test stimulation leading to short-term depression of KAR–EPSCs. Heterosynaptic: for two independent stimulations, applying the conditioning train to stim 1 was without any effect on KAR–EPSCs evoked in response to stim 2. The corresponding superimposed traces are enlarged in inset. ctr, Control train; cond, conditioning train. C, Summary plot of the results obtained when stimulating two different mossy fibers, testing whether applying the conditioning protocol to one mossy fiber could also induce the short-term depression of KAR–EPSCs recorded from stimulation of a second and independent mossy fiber. *p < 0.05.
- Figure 5.
Pairing mossy fiber stimulation with postsynaptic depolarization depresses KAR–EPSPs. A, Current-clamp recordings from CA3 pyramidal cells recorded in the current-clamp mode in the presence of 1.3 mm extracellular Mg+2. Evoking spikes with short depolarizing pulses (2 ms, 800 pA) 10 ms after mossy fiber stimulation (post depol) induced depression of KAR–EPSPs. B, If the depolarizing pulses were each applied 10 ms before mossy fiber stimulation (pre depol), no significant effect of the protocol on KAR–EPSPs was observed. ctr, Control train; cond, conditioning train. C, Summary of the effects of the protocol used in A on KAR–EPSPs in control conditions, in conditions in which 20 mm BAPTA was included in the patch pipette or after inhibition of NMDARs with 50 μm d-AP5, and with the conditions used in B. *p < 0.05
- Figure 6.
Depression of KAR–EPSPs can also be induced by a large train of action potentials and is important for summation properties of mossy fiber EPSPs. A, Applying a train of five spikes without pairing with synaptic stimulation does not change the amplitude of KAR–EPSPs. B, In contrast to what we observed with a train of five spikes, a train of 30 spikes induces a clear reduction in KAR–EPSP amplitude. C, Summary of the effects of trains of 5, 10, or 30 spikes on KAR–EPSPs. The decrease of KAR–EPSPs induced by the train of 30 spikes also occurred in the presence of 50 μm d-AP5. D, Representative traces of Mf–EPSPs obtained without blocking AMPA receptors in control conditions (ctr; black) and after applying a train of 30 spikes (cond; gray). In conditions in which KAR–EPSPs are depressed by ∼30%, the train of EPSPs displayed significantly less summation (measured as a) compared with the control train of EPSPs. E, The train of 30 spikes did not significantly change the amplitude of the first stimulus, suggesting that this protocol did not have any presynaptic effects, nor did it affect AMPA receptors. F, Summary plot of the effect of a train of 30 spikes in the summation (measured in A as indicated in D) of EPSPs obtained after stimulating mossy fiber with a train of five stimuli. *p < 0.05.
Additional Files
Supplemental Data
Files in this Data Supplement:
- supplemental material - Supplemental figure legend
- supplemental material - Figure 1
