Depression of glutamate and GABA release by presynaptic GABAB receptors in the entorhinal cortex in normal and chronically epileptic rats

Presynaptic GABAB receptors (GABABR) control glutamate and GABA release at many synapses in the nervous system. In the present study we used whole-cell patch-clamp recordings of spontaneous excitatory and inhibitory synaptic currents in the presence of TTX to monitor glutamate and GABA release from synapses in layer II and V of the rat entorhinal cortex (EC)in vitro. In both layers the release of both transmitters was reduced by application of GABABR agonists. Quantitatively, the depression of GABA release in layer II and layer V, and of glutamate release in layer V was similar, but glutamate release in layer II was depressed to a greater extent. The data suggest that the same GABABR may be present on both GABA and glutamate terminals in the EC, but that the heteroreceptor may show a greater level of expression in layer II. Studies with GABABR antagonists suggested that neither the auto- nor the heteroreceptor was consistently tonically activated by ambient GABA in the presence of TTX. Studies in EC slices from rats made chronically epileptic using a pilocarpine model of temporal lobe epilepsy revealed a reduced effectiveness of both auto- and heteroreceptor function in both layers. This could suggest that enhanced glutamate and GABA release in the EC may be associated with the development of the epileptic condition. Copyright © 2006 S. Karger AG

GABA(B) receptor activation promotes seizure activity in the juvenile rat hippocampus

We analyzed how the GABA(B) receptor agonist baclofen (10-50 microM) influences the activity induced by 4-aminopyridine (4-AP, 50 microM) in the CA3 area of hippocampal slices obtained from 12- to 25-day-old rats. Interictal and ictal discharges along with synchronous GABA-mediated potentials occurred spontaneously in the presence of 4-AP. Baclofen abolished interictal activity (n = 29 slices) and either disclosed (n = 21/29) or prolonged ictal discharges (n = 8/29), whereas GABA-mediated potentials occurred at a decreased rate. The N-methyl-D-aspartate (NMDA) receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphate (CPP, 10 microM, n = 8) did not modify the GABA-mediated potentials or the ictal events recorded in 4-AP + baclofen. In contrast ictal, activity, but not GABA-mediated potentials, was blocked by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM, n = 5). Most baclofen effects were reversed by the GABA(B) receptor antagonist CGP 35348 (1 mM; n = 4). Baseline and transient increases in [K(+)](o) associated with the 4-AP-induced synchronous activity were unaffected by baclofen. Baclofen hyperpolarized CA3 pyramids (n = 8) recorded with K-acetate-filled electrodes by 4.8 +/- 1.3 mV and made spontaneous, asynchronous hyperpolarizing and depolarizing potentials disappear along with interictal depolarizations. GABA-mediated synchronous long-lasting depolarizations (LLDs) and asynchronous depolarizations were also studied with KCl-filled electrodes in 4-AP + CPP + CNQX (n = 6); under these conditions baclofen did not reduce LLD amplitude but abolished the asynchronous events. Dentate hilus stimulation at 0. 2-0.8 Hz suppressed the ictal activity recorded in 4-AP + baclofen (n = 8). Our data indicate that GABA(B) receptor activation by baclofen decreases transmitter release leading to disappearance of interictal activity along with asynchronous excitatory and inhibitory potentials. By contrast, GABA-mediated LLDs and ictal events, which reflect intense action potential firing invading presynaptic inhibitory and excitatory terminals respectively, are not abolished. We propose that the proconvulsant action of baclofen results from 1) block of asynchronous GABA-mediated potentials causing disinhibition and 2) activity-dependent changes in hippocampal network excitability

Epileptiform synchronization and GABA(B) receptor antagonism in the juvenile rat hippocampus

The GABA(B) receptor agonist baclofen enhances the epileptiform activity induced by 4-aminopyridine (4AP) in juvenile rat hippocampal slices. In this study, we used a similar experimental approach (i.e., field potential, intracellular, and [K+]o recordings in the CA3 area of slices obtained from 15-23-day-old rats) to establish whether antagonizing GABA(B) receptors could exert opposite (presumably anticonvulsant) effects. Bath application of 4AP (50 microM) induced spontaneous interictal and ictal discharges along with synchronous GABA receptor-mediated potentials. All types of 4AP-induced synchronous activity occurred more frequently during application of the GABA(B) receptor antagonist p3-amino-propyl,p-diethoxymethylphosphonic acid (CGP 35348) (0.1-1 mM; EC50 = 0.25 mM). Moreover, CGP 35348 augmented the frequency and, to a lesser extent, the duration of the asynchronous synaptic activity recorded intracellularly from CA3 pyramids (n = 19). In medium containing 4AP + ionotropic glutamatergic antagonists (which abolished interictal and ictal activity), CGP 35348 prolonged both GABA-receptor-mediated field potentials and the accompanying intracellular long-lasting depolarizations without modifying their rate (n = 12). The transient elevations in [K+]o associated with GABA receptor-mediated potentials in 4AP-containing medium (n = 7 slices) became larger during CGP 35348 application. Similar findings were obtained when CGP 35348 was applied to medium containing 4AP + ionotropic glutamatergic antagonists (n = 6). Thus, the effect of CGP 35348 on 4AP-induced epileptiform activity is not anticonvulsant and to some extent similar to what was reported in this model during GABA(B) receptor activation. We propose that the facilitation of ictal activity by CGP 35348 is mainly caused by the blockade of presynaptic GABA(B) receptor, leading to an increase in GABA release and subsequent larger [K+]o elevations

Thalamocortical connectivity in a rat brain slice preparation: participation of the ventrobasal complex to synchronous activities.

We studied the synchronous cortical and thalamic activities induced by low (0.5–1 mM) and high (50–100 mM) concentrations of theK+ channel blocker 4-aminopyridine (4AP) in a rat thalamocortical preparation. The presence of reciprocal thalamocortical connectivitywas documented by diffusion of the fluorescent tracer Di-IC18 between the somatosensory cortex and the ventrobasal complex (VB) of thethalamus in vitro. Functional reciprocal connectivity was also demonstrated by stimulating the cortical middle-deep layers (which elicitedorthodromic responses in VB) or the VB (which induced orthodromic and antidromic responses in the cortex). Spontaneous field potentialswere not recorded in either the thalamus or cortex in control medium. Low concentrations of 4AP produced local spindle-like rhythmicoscillations in cortex and VB (duration = 0.4–3.5 s; frequency = 9–16 Hz). In contrast, high concentrations of 4AP induced widespreadictal-like epileptiform discharges (duration = 8–45 s) characterised by a ‘tonic’ component followed by a period of ‘clonic’ discharges inboth cortex and VB. Spindle-like activity was abolished in cortex and thalamus by applying the excitatory amino acid receptor antagonistkynurenic acid in VB. In contrast, the same procedure exacerbated ictal-like discharges, while depressing VB activity. Our results indicatethat thalamus and cortex can produce synchronous activities in this in vitro thalamocortical network: spindle-like rhythmic oscillationsare generated at the thalamic level and imposed upon the cortical network whereas ictal-like discharges have a cortical origin and aremodulated by the thalamic network activity. In addition, we have shown that it is possible to preserve reciprocal projections between cortexand thalamus in an in vitro rat slice preparation that could be a valuable tool to study epileptic-prone rat strains

Masking Synchronous GABA‐mediated Potentials Controls Limbic Seizures

Summary:  Purpose: We determined how CA3‐driven interictal discharges block ictal activity generated in the entorhinal cortex during bath application of 4‐aminopyridine (4AP, 50 μM).Methods: Field potential and [K+]o recordings were obtained from mouse combined hippocampus–entorhinal cortex slices maintained in vitro.Results: 4AP induced N‐methyl‐d‐aspartate (NMDA) receptor–dependent ictal discharges that originated in the entorhinal cortex, disappeared over time, but were reestablished by cutting the Schaffer collateral (n = 20) or by depressing CA3 network excitability with local application of glutamatergic receptor antagonists (n = 5). In addition, two types of interictal activity occurred throughout the experiment. The first type was CA3 driven and was abolished by a non‐NMDA glutamatergic receptor antagonist. The second type was largely contributed by γ‐aminobutyric acid type A (GABAA) receptor–mediated conductances and persisted during blockade of glutamatergic transmission. The absence of CA3‐driven interictal discharges in the entorhinal cortex after Schaffer collateral cut facilitated the GABA‐mediated interictal potentials that corresponded to large [K+]o elevations and played a role in ictal discharge initiation. Accordingly, ictal discharges along with GABA‐mediated interictal potentials disappeared during GABAA‐receptor blockade (n = 7) or activation of μ‐opioid receptors that inhibit GABA release (n = 4).Conclusions: Our findings suggest that CA3‐driven interictal events restrain ictal discharge generation in the entorhinal cortex by modulating the size of interictal GABA‐mediated potentials that lead to large [K+]o elevations capable of initiating ictal discharges in this structure