GABAB receptor- and metabotropic glutamate receptor-dependent cooperative long-term potentiation of rat hippocampal GABAA synaptic transmission

Abstract

Repetitive stimulation of Schaffer collaterals induces activity-dependent changes in the strength of polysynaptic inhibitory postsynaptic potentials (IPSPs) in hippocampal CA1 pyramidal neurons that are dependent on stimulation parameters. In the present study, we investigated the effects of two stimulation patterns, theta-burst stimulation (TBS) and 100 Hz tetani, on pharmacologically isolated monosynaptic GABAergic responses in adult CA1 pyramidal cells. Tetanization with 100 Hz trains transiently depressed both early and late IPSPs, whereas TBS induced long-term potentiation (LTP) of early IPSPs that lasted at least 30 min. Mechanisms mediating this TBS-induced potentiation were examined using whole-cell recordings. The paired-pulse ratio of monosynaptic inhibitory postsynaptic currents (IPSCs) was not affected during LTP, suggesting that presynaptic changes in GABA release are not involved in the potentiation. Bath application of the GABAB receptor antagonist CGP55845 or the group I/II metabotropic glutamate receptor antagonist E4-CPG inhibited IPSC potentiation. Preventing postsynaptic G-protein activation or Ca2+ rise by postsynaptic injection of GDP-beta-S or BAPTA, respectively, abolished LTP, indicating a G-protein- and Ca2+-dependent induction in this LTP. Finally during paired-recordings, activation of individual interneurons by intracellular TBS elicited solely short-term increases in average unitary IPSCs in pyramidal cells. These results indicate that a stimulation paradigm mimicking the endogenous theta rhythm activates cooperative postsynaptic mechanisms dependent on GABABR, mGluR, G-proteins and intracellular Ca2+, which lead to a sustained potentiation of GABAA synaptic transmission in pyramidal cells. GABAergic synapses may therefore contribute to functional synaptic plasticity in adult hippocampus.

Figure 2. LTP requires postsynaptic G-protein activation

A, examples of average IPSPs obtained before (1) and after (2) TBS in whole-cell current-clamp recordings when the patch solution contained GTP (a₁) or GDP-β-S (a₂). B, graph of IPSP amplitude in cells containing GTP (•) or GDP-β-S (□). LTP of IPSPs induced by TBS was abolished in cells in which GDP-β-S was present to block the activation of G-proteins. C, histogram of average IPSP amplitude 30 min after TBS showing the unchanged IPSPs in untetanized cells, the significant potentiation of IPSPs in cells receiving TBS, and the abolition of LTP in cells loaded with GDP-β-S (* P < 0.05). Numbers in parentheses indicate the number of cells recorded in each condition.

Figure 3. Paired-pulse ratio of IPSCs is not affected following LTP induction

Representative IPSCs from an untetanized cell (a₁) and a cell that received TBS (a₂). B, mean amplitude of IPSCs in untetanized cells (○; n = 11) and cells that received TBS (•; n = 12), showing LTP of IPSCs following delivery of TBS. C, paired-stimulation given at a 75 ms interval produced similar paired-pulse depression of IPSCs in an untetanized cell (c₁) and a cell that received TBS (c₂). D, histogram of the paired-pulse ratio (PPR) during the control period (open bars) and 30 min later (filled bars) in untetanized cells, or during LTP in cells that received TBS. No change in PPR was found in either group of cells. Numbers in parentheses indicate the number of cells tested.

Figure 6. Short-term potentiation of unitary IPSCs during paired interneuron-pyramidal cell recordings

A, superimposed average uIPSCs including (a1) and excluding (a2) failures recorded from a representative lacunosum-moleculare interneuron-pyramidal cell pair (LM-PYR) in control, 90 s and 180 s after TBS. The pyramidal cell soma was held at −40 mV (holding potential (V_h), top traces). uIPSCs were generated by a single spike (membrane potential (V_m), middle traces) evoked by somatic current injection (bottom traces) in the interneuron in current-clamp mode. B, histograms of average uIPSCs including failures (b1), average failure rate (b2) and average uIPSCs excluding failures (b3) for all pairs tested (n = 3). The transient increase in uIPSCs following TBS is accompanied by a decrease in the failure rate of synaptic transmission (* P < 0.05).

Figure 1. Long-term potentiation (LTP) of monosynaptic early IPSPs is induced by theta-burst stimulation (TBS) but not by high-frequency stimulation (HFS) in CA1 pyramidal cells

Cells recorded intracellularly received either TBS (A) or HFS (B). A, average monosynaptic IPSPs from a representative cell (a₁) and graphs of the early (a₂) and late (a₃) IPSP amplitude for all cells tested prior to (1) and after (2, 3) delivery of TBS in stratum radiatum. Fast GABA_A-mediated responses (•), but not late GABA_B-mediated IPSPs (▴), were gradually increased following TBS. B, results for cells that received HFS. Early (b₂) and late (b₃) IPSPs were depressed immediately after HFS and returned to control levels within 5 min. Arrows indicate the times that TBS or HFS were delivered. In this and subsequent figures, numbers 1–3 indicate times at which traces were taken. C, summary histogram of average IPSP amplitude in untetanized cells, following TBS and 100 Hz tetani (n = 10 per group), illustrating the selective potentiation of early IPSPs 30 min post-TBS.

Figure 4. TBS-induced LTP of IPSCs depends on GABA_B receptor activation

A, examples of IPSCs obtained from a cell in the presence of 2 µm CGP55845 (a₁) and 2 µm CGP55845 + 2 µm acetazolamide (actz) (a₂). B, graph of average IPSC amplitude for cell groups in CGP55845 (▵) and CGP55845 + acetazolamide (▾). Notice the similar changes in IPSCs in both conditions. IPSCs in normal ACSF (TBS alone; ○) are illustrated for comparison (same as in Fig. 3). C, summary histogram of average IPSC amplitudes showing that the LTP induced by TBS was inhibited by CGP55845 alone and by CGP55845 co-applied with acetazolamide. D, mean responses from three episodes of TBS obtained from a cell in control ACSF (left), in presence of CGP55845 (middle) and CGP55845 + acetazolamide (right). Note that the slow inward current generated during the application of the GABA_B receptor antagonist disappeared when acetazolamide was added. The dotted line corresponds to the baseline level before the delivery of TBS.

Figure 5. LTP of IPSCs is reduced by a group I/II mGluR antagonist and is abolished by postsynaptic Ca²⁺ chelation

A, representative IPSCs recorded before and after TBS during perfusion with 500 µm E4-CPG (a₁) and in a cell loaded with 10 mm BAPTA (a₂). B, the average IPSC amplitudes show a considerable decrease in the potentiation of IPSCs in E4-CPG (•) and the prevention of LTP when cells were loaded with BAPTA (⋄). Data for cells recorded with a standard patch solution (TBS alone ○; 0.5 mm EGTA; see Methods) has been added for comparison (same as in Fig. 3). C, summary histogram illustrating that TBS-induced potentiation of IPSCs is inhibited by 500 µm E4-CPG and blocked by 10 mm BAPTA.