Effects of pentobarbital tolerance and dependence on convulsant and GABAA receptor antagonist binding

Hyperexcitability induced by GABA withdrawal facilitates hippocampal long-term potentiation

In some mammals, epileptic seizures have been induced in the cerebral cortex, hippocampus and other limbic structures after the sudden suppression of chronically infused GABA. This hyperexcitability state induced by the endogenous neurotransmitter resembles the withdrawal seizure-responses to other GABA(A) receptor agonists such as benzodiazepines, barbiturates and alcohol. Hyperexcitability induced by GABA withdrawal also persists in in vitro preparation. Hippocampal slices, obtained from rats with seizures induced by GABA-withdrawal showed field potential oscillations and paroxysmal activity in the Ammon's horn region 1. During GABA-withdrawal hyperexcitability the threshold of hippocampal long-term potentiation (LTP) decreased to a point in which a brief frequency stimulation that normally failed to produce long lasting changes in synaptic strength, was now able to induce LTP. Facilitation of the LTP induction was associated with a decreased GABA(A)-mediated inhibitory activity, because the effect of the GABA(A) receptor antagonist, bicuculline, was occluded during hyperexcitability and the dose-response curve for bicuculline showed a 50% efficacy reduction with a shift in the effective concentration required for half-maximal activation from 4.5-1.1 microM relative to controls. Nevertheless, the dissociation constant of the antagonist did not change significantly. Our results support the idea that changes in hippocampal plasticity under altered inhibitory neurotransmission states, like those induced by withdrawal syndromes to anxiolytic, sedative or anticonvulsant drugs may be engaged during seizures.

Role of NMDA receptors in pentobarbital tolerance/dependence

Effects of continuous pentobarbital administration on binding characteristics of [3H]MK-801 in the rat brain were examined by autoradiography. Animals were rendered tolerant to pentobarbital using i.c.v. infusion of pentobarbital (300 micrograms/10 microliters/hr for 7 days) by osmotic minipumps and dependent by abrupt withdrawal from pentobarbital. The levels of [3H]MK-801 binding were elevated in rats 24-hr after withdrawal from pentobarbital while there were no changes except in septum and anterior ventral nuclei in tolerant rats. For assessing the role of NMDA receptor in barbiturate action, an NMDA receptor antagonist (MK-801, 2.7 femto g/10 microliters/hr) was co-infused with pentobarbital. The pentobarbital-infused group had a shorter duration of pentobarbital-induced loss of righting reflex (sleeping time) than that of the control group, and MK-801 alone did not affect the righting reflex. However, co-infusion of MK-801 blocked hyperthermia, and prolonged the onset of convulsions induced by t-butylbicyclophosphorothionate (TBPS) in pentobarbital withdrawal rats. In addition, elevated [35S]TBPS binding was significantly attenuated by co-infusion with MK-801. These results suggest the involvement of NMDA receptor up-regulation in pentobarbital withdrawal and that the development of dependence can be attenuated by the treatment of subtoxic dose of MK-801.

Pharmacology of barbiturate tolerance/dependence: GABAA receptors and molecular aspects

Barbiturates are central nervous system depressants that are used as sedatives, hypnotics, anesthetics and anticonvulsants. However, prolonged use of the drugs produces physical dependence, and the drugs have a high abuse liability. The gamma-aminobutyric acidA (GABAA) receptor is one of barbiturates' main sites of action, and therefore it is thought to play a pivotal role in the development of tolerance to and dependence on barbiturates. Recent advances in the study of the GABAA receptor/chloride channel complex allow us to examine possible mechanisms that underlie barbiturate tolerance/dependence in a new light. In this minireview, we mainly focus on molecular and cellular aspects of the action of barbiturates and the possible mechanisms that contribute to development of tolerance to and dependence on barbiturates.

GABA(A) Receptor-Related Chloride Flux of Rats Receiving Continuous Intracerebroventricular Infusion of Pentobarbital

The in vitro receptor-stimulated synaptoneurosomal chloride uptake induced by pentobarbital or muscimol was studied in the brains of rats which had been rendered tolerant to or dependent on pentobarbital. In the chronic study, rats received continuous administration of sodium pentobarbital by intracerebroventricular (i.c.v.) infusion. In rats continuously infused with pentobarbital for 6 days or in rats which had the infusion terminated for 24 h, the basal synaptoneurosomal chloride uptake was not altered in either the cerebral cortex or the cerebellum. On the other hand, pentobarbital (500 &mgr;M)-stimulated chloride uptake was significantly decreased in both cerebral cortices and cerebella of rats which received pentobarbital for 6 days as compared with the saline control groups. Twenty-four hours after termination of pentobarbital infusion, this high concentration of pentobarbital-stimulated chloride uptake remained low in both cerebral cortices and cerebella of rats which had been infused with pentobarbital for 6 days. In addition, similar results were also obtained in muscimol (2.5 &mgr;M)-stimulated chloride uptake in the cerebral cortices of pentobarbital-infused animals. However, pentobarbital infusion failed to alter muscimol-stimulated chloride uptake in the cerebellum. These results suggest that the GABA(A) receptor regulated chloride uptake is downregulated after chronic pentobarbital administration. The results also suggest that down-regulation of the GABA(A) receptor chloride channel complex takes place at different recognition sites on the complex. It further substantiates the allosteric effects of GABA and barbiturate recognition sites. Copyright 1996 S. Karger AG, Basel

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.