PTP and LTP at a hippocampal mossy fiber-interneuron synapse

Abstract

The mossy fiber-CA3 pyramidal neuron synapse is a main component of the hippocampal trisynaptic circuitry. Recent studies, however, suggested that inhibitory interneurons are the major targets of the mossy fiber system. To study the regulation of mossy fiber-interneuron excitation, we examined unitary and compound excitatory postsynaptic currents in dentate gyrus basket cells, evoked by paired recording between granule and basket cells or extracellular stimulation of mossy fiber collaterals. The application of an associative high-frequency stimulation paradigm induced posttetanic potentiation (PTP) followed by homosynaptic long-term potentiation (LTP). Analysis of numbers of failures, coefficient of variation, and paired-pulse modulation indicated that both PTP and LTP were expressed presynaptically. The Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) did not affect PTP or LTP at a concentration of 10 mM but attenuated LTP at a concentration of 30 mM. Both forskolin, an adenylyl cyclase activator, and phorbolester diacetate, a protein kinase C stimulator, lead to a long-lasting increase in excitatory postsynaptic current amplitude. H-89, a protein kinase A inhibitor, and bisindolylmaleimide, a protein kinase C antagonist, reduced PTP, whereas only bisindolylmaleimide reduced LTP. These results may suggest a differential contribution of protein kinase A and C pathways to mossy fiber-interneuron plasticity. Interneuron PTP and LTP may provide mechanisms to maintain the balance between synaptic excitation of interneurons and that of principal neurons in the dentate gyrus-CA3 network.

Figure 1

PTP and LTP of unitary EPSCs at the mossy fiber-basket cell synapse. (A) Schematic illustration of the paired recording configuration. GC, granule cell; BC, basket cell; GCL, granule cell layer; CA3, CA3 subfield; MF, mossy fiber. The figure was adapted from ref. . (B) An aHFS induces both PTP and LTP. Single presynaptic APs are shown on top, and average EPSCs (from 64, 10, and 100 single traces in the time intervals before the first aHFS, directly after the first aHFS, and 10–15 min after the third aHFS, respectively) are depicted below. Boxes indicate details of the aHFSs. (C and D) PTP and LTP of EPSCs induced by a single aHFS. Unitary EPSC peak amplitudes from paired recordings plotted against time. Data from a single paired recording are shown in C, and the summary graph for eight pairs is shown in D (with EPSC amplitudes normalized to the control value). The aHFS was applied at t = 0, as indicated by the arrow. (E and F) PTP and LTP of EPSCs induced by triple aHFS. Three single aHFS protocols were separated by 1.5-min intervals. Data from a single paired recording are shown in E, and the summary graph for three pairs is shown in F (●). For comparison, the effect of triple nHFS with the postsynaptic cell held in the voltage-clamp configuration at −70 mV during induction is depicted also (○, three pairs). The data shown in B and E were taken from the same experiment. Curves in D and F represent exponential functions fitted to the data points with time constants of 1.7 (D), 3.6 (F, aHFS), and 2.4 min (F, nHFS). The recording temperature was 34 ± 2°C in these and all subsequent experiments.

Figure 2

Presynaptic locus of PTP and LTP of unitary EPSCs at the mossy fiber-basket cell synapse. (A) Bar graph of the percentage of failures during BT, PTP, and LTP/LTD (with time intervals identical to those specified in Fig. 1). The mean effects of single aHFS (filled bars, left), triple aHFS (filled bars, center), and triple nHFS (open bars, right). (B) CV analysis of long-term changes. The CV⁻² of the EPSC amplitude in the late phase after induction was plotted against mean, both normalized to their respective control values before HFS. The effects of single aHFS (●), triple aHFS (♦), and triple nHFS (○) are shown. The location of the data points with respect to the identity line (dashed) was consistent with presynaptic changes in all cases. (C) Average unitary EPSCs (from 25, 15, and 25 single traces during BT, PTP, and LTP phases, respectively; single aHFS protocol) evoked by pairs of two APs, separated by 20-ms intervals. (D) Paired-pulse ratios for BT, PTP, and LTP/LTD periods. The mean effects of single aHFS (filled bars, left), triple aHFS (filled bars, center), and triple nHFS (open bars, right) are shown. The paired-pulse ratio was determined as A₂/A₁ from average trace.

Figure 3

The aHFS-induced LTP of compound EPSCs shows weak sensitivity to the Ca²⁺ chelator BAPTA. (A) Compound EPSCs are similar to unitary EPSCs with respect to PTP, LTP after aHFS, and DCG-4 sensitivity. Aa, average compound EPSCs in BT and LTP phases (stimulus artifacts blanked); Ab, corresponding EPSC peak amplitude values plotted against time. (B) Bar graph of compound EPSC amplitude (filled) and unitary EPSC amplitude (open) in the presence of 0.5–1 μM DCG-4 relative to respective control values. (C and D) Effects of single aHFS on compound EPSC amplitude with 0.1 mM intracellular EGTA (●; 11 cells), 10 mM intracellular BAPTA (C, ○; six cells), and 30 mM BAPTA (D, ○; eight cells). Amplitudes were normalized to the mean EPSC peak amplitude before aHFS. The aHFS was applied at t = 0, as indicated by the arrow. (E and F) Cumulative distributions of the extent of aHFS-induced PTP (E) and LTP (F) of compound EPSCs in individual experiments in 0.1 mM internal EGTA, 10 mM BAPTA, and 30 mM BAPTA. Paired recording data (with 0.2 mM internal EGTA) are shown superimposed.

Figure 4

Involvement of PKA and PKC pathways in aHFS-induced PTP and LTP of compound EPSCs. (A) Potentiation of compound EPSC amplitude (normalized to BT period) by bath application of 50 μM forskolin (○; six cells) and 1 μM PDA (●; five cells; horizontal bars indicate application times). (B) CV analysis of forskolin- (○) and PDA- (●) induced potentiation. The CV⁻² of the EPSC amplitude 10–15 min (forskolin, two cells), 15–20 min (forskolin, four cells; PDA, one cell), and 20–25 min (PDA, four cells) after the onset of application was plotted against mean, both normalized to their respective control values. Location of the data points with respect to the identity line (dashed) suggests presynaptic changes. (C and D) Effects of single aHFS on compound EPSC amplitude after preincubation with 10 μM H-89 (C, seven cells) and 1 μM bisindolylmaleimide (D, 11 cells). The aHFS was applied at t = 0, as indicated by the arrow. Curves in C and D represent exponential function fitted to the control data points (0.1 mM EGTA) in Fig. 3 C and D (●; time constant 1.7 min). (E and F) Cumulative distributions of the extent of aHFS-induced PTP (E) and LTP (F) of compound EPSCs in individual experiments in control conditions (same experiments as shown in Fig. 3C), after preincubation with 10 μM H-89 (C), and one after preincubation with 1 μM bisindolylmaleimide (D).