Elsevier

Neuropharmacology

Volume 39, Issue 3, March 2000, Pages 427-439
Neuropharmacology

Regulation of γ-aminobutyric acid (GABA) release in cerebral cortex in the γ-hydroxybutyric acid (GHB) model of absence seizures in rat

https://doi.org/10.1016/S0028-3908(99)00152-5Get rights and content

Abstract

γ-Hydroxybutyric acid (GHB) has the ability to induce absence seizures. The precise way in which GHB causes seizures remains unclear, but GABAB- and/or GHB-mediated presynaptic mechanisms within thalamocortical circuitry may play a role. In the present study, we determined the basal and K+-evoked release of GABA and glutamate in the superficial laminae of frontal cortex during GHB-induced absence seizures. Our data indicate that both the basal and K+-evoked release of GABA were significantly decreased in laminae I–III of frontal cortex at the onset of GHB-induced absence seizures. The appearance and disappearance of the observed changes in basal and K+-evoked extracellular levels of GABA correlated with the onset and offset of absence seizures. In contrast, neither the basal nor the K+-evoked release of glutamate was altered in superficial laminae of cerebral cortex at any time during the absence seizures. Intracortical perfusion of the GABAB receptor antagonists, CGP 35348 and phaclofen as well as the GHB receptor antagonist, NCS 382 attenuated GHB-mediated changes in the basal and K+-evoked release of GABA. These data suggest that GHB induces a selective decrease in the basal and depolarization-induced release of GABA in cerebral cortex, and further, that this action of GHB may play a role in the mechanism by which GHB induces absence seizures.

Introduction

Generalized absence seizures occur most often in children and are characterized by rhythmic, bilaterally synchronous 3 Hz spike-and-wave discharges (SWD) associated with unresponsiveness of an abrupt onset, the timing of which coincides with the onset of the SWD. γ-Hydroxybutyric acid (GHB), is a naturally occurring metabolite of GABA and possesses the property of producing generalized absence seizures in experimental animals. GHB-induced seizures have behavioral, electrophysiologic and pharmacological characteristics which are similar to those seen in generalized absence seizures in humans (Snead, 1995).

During GHB-induced absence seizures the ventroposterolateral (VPL), ventroposteromedial (VPM), medial dorsal (MD), and nucleus reticularis thalami (nRT) nuclei of the thalamus discharge synchronously with layers I–III of cerebral cortex (Banerjee et al., 1993). Also, bilateral electrolytic lesions in the MD and intralaminar thalamic nuclei abolish GHB induced spike wave discharges from both cortex and thalamus (Banerjee and Snead, 1994). The precise mechanism by which GHB induces absence seizures is unknown, but is thought to be related to GABAB receptor- and/or GHB-mediated activity within thalamocortical circuitry.

The hypothesis that GABAB receptor-mediated mechanisms are operative in the pathogenesis of absence seizures, is supported by the observation that a specific GABAB receptor antagonist blocks absence seizures in four different experimental models of generalized absence seizures, including the GHB model (Snead, 1992, Snead, 1996a, Hosford et al., 1992, Liu et al., 1992). Conversely, the specific GABAB receptor agonist, (−)baclofen, causes a marked exacerbation of absence seizures in the GHB model, to the point of absence status epilepticus which lasts for hours (Snead, 1996a).

Although GABAB receptor antagonists block, and GABAB receptor agonists exacerbate GHB-induced absence seizures, there is little evidence that GHB acts directly at the GABAB receptor. GHB has been reported specifically to displace bound [3H]baclofen from GABAB sites in rat brain homogenates (Bernasconi et al., 1992); however, neither GHB nor specific GHB antagonists appear to compete for GABAB receptor binding, nor do (−)baclofen and postsynaptic GABAB receptor antagonists compete for binding at the GHB site (Snead et al., 1992, Snead, 1996b). In addition, there are significant differences in the regional anatomic distribution and the ontogeny of GHB and GABAB receptor binding sites in rat brain (Snead, 1994). Also, specific GHB antagonists fail to block the electrophysiologic effects of GHB upon the postsynaptic GABAB receptor (Xie and Smart, 1992, Williams et al., 1996).

These disparate findings have led to formulation of the hypothesis that there is a presynaptic GABAB receptor subtype operative in the mechanism of absence seizures in the GHB model which differs from other subtypes of the GABAB receptor by virtue of its affinity for GHB. This hypothesis is in keeping with evidence for heterogenity of pre- and postsynaptic GABAB receptors (Scherer et al., 1988, Bonanno and Raiteri, 1992, Bonanno and Raiteri, 1993) and is supported further by experiments which show that the most effective GABAB receptor ntagonists in the GHB model of absence are those which seem to have some specificity for presynaptic GABAB receptors and by the fact that seizures in the GHB model are also blocked by a specific GHB antagonist (Snead, 1996a).

In testing this hypothesis we have demonstrated a robust decrease in the basal presynaptic release of GABA in ventrobasal thalamus in the GHB absence model. This presynaptic inhibition of GABA release in the GHB model of absence seizures occurred at concentrations of GHB that are threshold for the occurrence of seizure in that model and had a time course similar to the time course of the absence seizures in the GHB model. The decrease in GABA release in thalamus in the GHB absence model was blocked by GHB antagonists and by specific presynaptic GABAB receptor antagonists, but not by postsynaptic GABAB receptor antagonists (Banerjee and Snead, 1995).

Because the respective roles of the cerebral cortex vs the thalamus in the genesis of absence seizures induced by GHB is not clearly understood, we sought to determine whether GHB-induced absence seizures might be related to perturbations of presynaptic GABAB/GHB receptors on interneurons in laminae I–III in cerebral cortex similar to those we have demonstrated in thalamus. Therefore, the present study was designed to address the question of the effect of GHB on presynaptic release of GABA and glutamate in the superficial laminae of cerebral cortex during absence seizures induced by GHB. This region was chosen because it is the area of cortex from whence GHB-induced absence seizures originate.

Section snippets

Drugs

The specific GABAB receptor antagonist, CGP 35348, was a gift of Dr R. Bernasconi (Novartis, Basel). The specific GHB receptor antagonist, NCS 382, was a gift of Dr J.J. Bourguignon (Centre de Neurochimie, Strasbourg). All other drugs were purchased from Sigma Chemical Co. (St Louis, MO). All analytical reagents were obtained from standard commercial sources and were of the highest available purity.

Surgery and EEG recordings

Adult male Sprague-Dawley rats (200–250 g, Charles River Lab) were used in all experiments.

Basal and K+-evoked extracellular release of GABA and glutamate in layers I–III of frontal cortex

Verification of placement of the dialysis probe within the layers I–III of frontal cortex of a rat is shown in Fig. 1. Within 1 h after the insertion of the dialysis probe, the basal extracellular levels of GABA and glutamate reached a steady state. Table 5 shows the basal release of GABA and glutamate in drug-naive behaving animal during the second hour of dialysis. There was no significant difference in the basal levels of either GABA or glutamate from 10 min epoch to 10 min epoch over a 60

Discussion

The superficial cortical laminae were chosen for scrutiny in this study because this is the region of cortex from whence spike-wave discharges (SWD) emanate in the GHB absence seizure model (Banerjee et al., 1993). The current study demonstrated that the basal and K+-evoked extracellular release of GABA in cortical laminae I–III of frontal cortex was significantly reduced following systemic administration of GBL. Further, the appearance and disappearance of the observed changes in basal

Acknowledgements

The authors wish to thank Dr Miguel Cortez for assisting in the EEG recordings. This work was supported in part by MRC Canada, The Bloorview Childrens Hospital Foundation, and the Bloorview Epilepsy Program.

References (35)

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