Synaptically released glutamate reduces gamma-aminobutyric acid (GABA)ergic inhibition in the hippocampus via kainate receptors

Abstract

Exogenous application of agonists at the kainate subtype of glutamate receptors has been shown to depress evoked monosynaptic inhibition by gamma-aminobutyric acid (GABA)ergic interneurons in the hippocampus. This observation has led to the hypothesis that synaptic release of endogenous glutamate might have a disinhibitory effect on neuronal circuits, in addition to depolarizing neurons via postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate, and N-methyl-D-aspartic acid (NMDA) receptors. It is not known, however, if glutamate released from excitatory neurons has the same kainate receptor-mediated effect on monosynaptic inhibitory transmission as exogenous agonist application. Indeed, the recent demonstration that excitatory synaptic signals elicited in interneurons are partly mediated by kainate receptors suggests that these receptors may have a pro- rather than disinhibitory role. Here, we examine the effect of synaptically released glutamate on monosynaptic inhibitory signaling. In the presence of antagonists to AMPA and NMDA receptors, brief bursts of activity in glutamatergic afferent fibers reduce GABAergic transmission. This depression of inhibition is reversibly abolished by blocking kainate receptors. It persists when GABA(B) receptors are blocked and is enhanced by blocking metabotropic glutamate receptors, possibly explained by presynaptic regulation of glutamate release from excitatory afferents by metabotropic autoreceptors. We conclude that the net kainate receptor-mediated effect of synaptically released glutamate is to reduce monosynaptic inhibition. Since this form of disinhibition may contribute to seizure initiation, kainate receptors may constitute an important target for anticonvulsant drug development.

Figure 1

Experimental design. (A) Schematic illustration of the hippocampal slice showing the positioning of the recording pipette and stimulating electrodes. Schaffer collateral axons are shown in black, and a local inhibitory interneuron is shown in gray. (B) Effect of blocking AMPA and NMDA receptors with GYKI52466 and APV, respectively. The response to the distal stimulus (●) was completely blocked. The proximal stimulus (○), however, continued to elicit a monosynaptic response. Insets show averages of five successive traces obtained at the times indicated.

Figure 2

Depression of GABAergic transmission. (A) Upper, Superimposed traces showing the monosynaptic GABAergic PSC elicited on its own (thin line) or after a high-frequency burst delivered via the distal electrode (thick line). The depression (1) is abolished by addition of DNQX to the perfusion solution (2). Lower, Proximal PSC amplitude plotted against time for the same experiment. APV and GYKI52466 were present throughout, to block NMDA and AMPA receptors. The traces in A were obtained by averaging 10 records under each condition, at the times indicated. Horizontal lines indicate the average amplitudes of the test and conditioned PSC before and during DNQX perfusion. Picrotoxin (100 μM) abolished the PSC, indicating that it was mediated by GABA_A receptors. (B) Average of 11 experiments. The PSC amplitudes were normalized to set the average amplitude of the unconditioned response to 100%. Each point shows the average of 2–4 successive trials. DNQX completely abolished the depression of inhibition.

Figure 3

Blockade of kainate receptors reversibly reduces disinhibition. (A) Effect of kynurenic acid (2.5 mM) in one cell. Upper, superimposed averages of conditioned (thick lines) and unconditioned PSCs (thin lines) recorded before (1), during (2), and after (3) perfusion with kynurenic acid. Lower, proximal PSC amplitude plotted against time. The averaged traces were obtained at the times indicated (numbers in brackets indicate % decrease in PSC amplitude produced by the conditioning train). The depression of inhibition was abolished by kynurenic acid and partially recovered following washout. (B) Effect of gadolinium studied in another cell. The averaged PSCs were obtained before (1), during (2), and after (3) perfusion with 10 μM Gd³⁺. (C) Averaged results obtained in 11 cells (washout data was obtained in 8 cells). Each point shows the average of three successive trials. The depression of the PSC by the conditioning train disappeared in the presence of Gd³⁺ and recovered after washout. APV and GYKI52466 were present throughout all of the illustrated experiments.

Figure 4

Summary of results indicating that kainate receptors mediate the depression of monosynaptic inhibition. The depression was abolished by 50 μM DNQX (A) and reversibly reduced by 2.5–10 mM kynurenic acid (B) or 10 μM Gd³⁺ (C). The depression was nonsignificantly reduced by 100 μM CGP35348 (D) but in the same neurons was abolished by the subsequent addition of DNQX. Blockade of metabotropic receptors with 250 μM MCPG and 200 μM MSOP increased the depression (E). PDC (100–200 μM) had no significant effect on the depression (F). The distal tetanus was 5 pulses at 100 Hz except for the experiments in D, where it consisted of 10 pulses at 200 Hz.