Barbiturate receptor sites are coupled to benzodiazepine receptors

New epilepsy therapies in development

Epilepsy is a common brain disorder, characterized by spontaneous recurrent seizures, with associated neuropsychiatric and cognitive comorbidities and increased mortality. Although people at risk can often be identified, interventions to prevent the development of the disorder are not available. Moreover, in at least 30% of patients, epilepsy cannot be controlled by current antiseizure medications (ASMs). As a result of considerable progress in epilepsy genetics and the development of novel disease models, drug screening technologies and innovative therapeutic modalities over the past 10 years, more than 200 novel epilepsy therapies are currently in the preclinical or clinical pipeline, including many treatments that act by new mechanisms. Assisted by diagnostic and predictive biomarkers, the treatment of epilepsy is undergoing paradigm shifts from symptom-only ASMs to disease prevention, and from broad trial-and-error treatments for seizures in general to mechanism-based treatments for specific epilepsy syndromes. In this Review, we assess recent progress in ASM development and outline future directions for the development of new therapies for the treatment and prevention of epilepsy.

Binding site location on GABAA receptors determines whether mixtures of intravenous general anaesthetics interact synergistically or additively in vivo

BACKGROUND AND PURPOSE

General anaesthetics can act on synaptic GABA_A receptors by binding to one of three classes of general anaesthetic sites. Canonical drugs that bind selectively to only one class of site are etomidate, alphaxalone, and the mephobarbital derivative, R-mTFD-MPAB. We tested the hypothesis that the general anaesthetic potencies of mixtures of such site-selective agents binding to the same or to different sites would combine additively or synergistically respectively.

EXPERIMENTAL APPROACH

The potency of general anaesthetics individually or in combinations to cause loss of righting reflexes in tadpoles was determined, and the results were analysed using isobolographic methods.

KEY RESULTS

The potencies of combinations of two or three site-selective anaesthetics that all acted on a single class of site were strictly additive, regardless of which single site was involved. Combinations of two or three site-selective anaesthetics that all bound selectively to different sites always interacted synergistically. The strength of the synergy increased with the number of separate sites involved such that the percentage of each agent's EC₅₀ required to cause anaesthesia was just 35% and 14% for two or three sites respectively. Propofol, which binds non-selectively to the etomidate and R-mTFD-MPAB sites, interacted synergistically with each of these agents.

CONCLUSIONS AND IMPLICATIONS

The established pharmacology of the three anaesthetic binding sites on synaptic GABA_A receptors was sufficient to predict whether a mixture of anaesthetics interacted additively or synergistically to cause loss of righting reflexes in vivo. The principles established here have implications for clinical practice.

GABAA receptor: Positive and negative allosteric modulators

gamma-Aminobutyric acid (GABA)-mediated inhibitory neurotransmission and the gene products involved were discovered during the mid-twentieth century. Historically, myriad existing nervous system drugs act as positive and negative allosteric modulators of these proteins, making GABA a major component of modern neuropharmacology, and suggesting that many potential drugs will be found that share these targets. Although some of these drugs act on proteins involved in synthesis, degradation, and membrane transport of GABA, the GABA receptors Type A (GABAAR) and Type B (GABABR) are the targets of the great majority of GABAergic drugs. This discovery is due in no small part to Professor Norman Bowery. Whereas the topic of GABABR is appropriately emphasized in this special issue, Norman Bowery also made many insights into GABAAR pharmacology, the topic of this article. GABAAR are members of the ligand-gated ion channel receptor superfamily, a chloride channel family of a dozen or more heteropentameric subtypes containing 19 possible different subunits. These subtypes show different brain regional and subcellular localization, age-dependent expression, and potential for plastic changes with experience including drug exposure. Not only are GABAAR the targets of agonist depressants and antagonist convulsants, but most GABAAR drugs act at other (allosteric) binding sites on the GABAAR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABAAR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABAAR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABAAR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABAAR subtype-dependent extracellular domain sites. Thus GABAAR subtypes possess pharmacologically specific receptor binding sites for a large group of different chemical classes of clinically important neuropharmacological agents. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

GABAA circuit mechanisms are associated with ether anesthesia-induced unconsciousness

OBJECTIVE

An emerging paradigm for understanding how anesthetics induce altered arousal is relating receptor targeting in specific neural circuits to electroencephalogram (EEG) activity. Enhanced gamma amino-butyric acid A (GABAA) inhibitory post-synaptic currents (IPSCs) manifest with large-amplitude slow (0.1-1Hz) and frontally coherent alpha (8-12Hz) EEG oscillations during general anesthesia. Therefore, we investigated the EEG signatures of modern day derivatives of ether (MDDE) anesthesia to assess the extent to which we could obtain insights into MDDE anesthetic mechanisms.

METHODS

We retrospectively studied cases from our database in which patients received isoflurane anesthesia vs. isoflurane/ketamine anesthesia (n=10 each) or desflurane anesthesia vs. desflurane/ketamine anesthesia (n=9 each). We analyzed the EEG recordings with spectral power and coherence methods.

RESULTS

Similar to known GABAA circuit level mechanisms, we found that MDDE anesthesia induced large amplitude slow and frontally coherent alpha oscillations. Additionally, MDDE anesthesia also induced frontally coherent theta (4-8Hz) oscillations. Reduction of GABAergic IPSCs with ketamine resulted in beta/gamma (13-40Hz) oscillations, and significantly reduced MDDE anesthesia-induced slow, theta and alpha oscillation power.

CONCLUSIONS

Large amplitude slow oscillations and coherent alpha and theta oscillations are moderated by ketamine during MDDE anesthesia.

SIGNIFICANCE

These observations are consistent with the notion that GABAA circuit-level mechanisms are associated with MDDE anesthesia-induced unconsciousness.

Structural comparisons of ligand-gated ion channels in open, closed, and desensitized states identify a novel propofol-binding site on mammalian γ-aminobutyric acid type A receptors

BACKGROUND

Most anesthetics, particularly intravenous agents such as propofol and etomidate, enhance the actions of the neurotransmitter γ-aminobutyric acid (GABA) at the GABA type A receptor. However, there is no agreement as where anesthetics bind to the receptor. A novel approach would be to identify regions on the receptor that are state-dependent, which would account for the ability of anesthetics to affect channel opening by binding differentially to the open and closed states.

METHODS

The open and closed structures of the GABA type A receptor homologues Gloeobacter ligand-gated ion channel and glutamate-gated chloride channel were compared, and regions in the channels that move on channel opening and closing were identified. Docking calculations were performed to investigate possible binding of propofol to the GABA type A β3 homomer in this region.

RESULTS

A comparison between the open and closed states of the Gloeobacter ligand-gated ion channel and glutamate-gated chloride channel channels identified a region at the top of transmembrane domains 2 and 3 that shows maximum movement when the channels transition between the open and closed states. Docking of propofol into the GABA type A β3 homomer identified two putative binding cavities in this same region, one with a high affinity and one with a lower affinity. Both cavities were adjacent to a histidine residue that has been photolabeled by a propofol analog, and both sites would be disrupted on channel closing.

CONCLUSIONS

These calculations support the conclusion of a recent photolabeling study that propofol acts at a site at the interface between the extracellular and transmembrane domains, close to the top of transmembrane domain 2.

Allosteric ligands and their binding sites define γ-aminobutyric acid (GABA) type A receptor subtypes

GABAA receptors (GABA(A)Rs) mediate rapid inhibitory transmission in the brain. GABA(A)Rs are ligand-gated chloride ion channel proteins and exist in about a dozen or more heteropentameric subtypes exhibiting variable age and brain regional localization and thus participation in differing brain functions and diseases. GABA(A)Rs are also subject to modulation by several chemotypes of allosteric ligands that help define structure and function, including subtype definition. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABA(A)Rs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Two classes of pharmacologically important allosteric modulatory ligand binding sites reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site and the high-affinity, relevant to intoxication, ethanol site. The benzodiazepine site is specific for certain GABA(A)R subtypes, mainly synaptic, while the ethanol site is found at a modified benzodiazepine site on different, extrasynaptic, subtypes. In the transmembrane domain are allosteric modulatory ligand sites for diverse chemotypes of general anesthetics: the volatile and intravenous agents, barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are endogenous positive allosteric modulators. X-ray crystal structures of prokaryotic and invertebrate pentameric ligand-gated ion channels, and the mammalian GABA(A)R protein, allow homology modeling of GABA(A)R subtypes with the various ligand sites located to suggest the structure and function of these proteins and their pharmacological modulation.

Analysis of γ-aminobutyric acid (GABA) type A receptor subtypes using isosteric and allosteric ligands

The GABAA receptors (GABAARs) play an important role in inhibitory transmission in the brain. The GABAARs could be identified using a medicinal chemistry approach to characterize with a series of chemical structural analogues, some identified in nature, some synthesized, to control the structural conformational rigidity/flexibility so as to define the 'receptor-specific' GABA agonist ligand structure. In addition to the isosteric site ligands, these ligand-gated chloride ion channel proteins exhibited modulation by several chemotypes of allosteric ligands, that help define structure and function. The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABAARs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Also in the trans-membrane domain are allosteric modulatory ligand sites, mostly positive, for diverse chemotypes with general anesthetic efficacy, namely, the volatile and intravenous agents: barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids. The last are apparent endogenous positive allosteric modulators of GABAARs. These binding sites depend on the GABAAR heteropentameric subunit composition, i.e., subtypes. Two classes of pharmacologically very important allosteric modulatory ligand binding site reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site, and the low-dose ethanol site. The benzodiazepine site is specific for certain subunit combination subtypes, mainly synaptically localized. In contrast, the low-dose (high affinity) ethanol site(s) is found at a modified benzodiazepine site on different, extrasynaptic, subtypes.

Anxiolytic-like and antinociceptive effects of 2(S)-neoponcirin in mice

Study aims: 2(S)-neopincirin (NEO) is a constituent from of Clinopodium mexicanum, which is used in traditional Mexican herbal medicine for its tranquilizing and analgesic properties. This study investigated the anxiolytic-like, sedative and antinociceptive effects of NEO in several mice models. Material and methods: The anxiolytic-like effect was evaluated in the hole-board (HBT) and Open Field Tests (OFT); sedative effect was evaluated in sleeping time induced by sodium pentobarbital, and its antinociceptive actions were measured in the hot plate test. To evaluate if the GABA receptor could be involved in the anxiolytic-like effect produced by NEO, in independent experiments, the effects produced by co-administration of NEO plus muscimol (MUS) and NEO plus Pitrotoxin (PTX) were evaluated in the HBT. Results: NEO was isolated from Clinopodium mexicanum leaves. The NMR, MS and optic rotation data helped establish its identity as (2S)-5-hydroxy-4′-methoxyflavanone-7-O-{β-glucopyranosyl-(1→6)-β-rhamnoside}. NEO showed an anxiolytic-like effect and was able to counter the nociception induced by a thermal stimulus in a dose-dependent manner. PTX blocked the anxiolytic-like effect of NEO, while MUS was able to enhance it. Conclusions: The findings of present work demonstrated that NEO possesses anxiolytic-like and antinociceptive effects in mice. Such effects are not associated with changes in the locomotor activity. These results supported the notion that anxiolytic-like effect of NEO involves the participation of GABAergic system.

How theories evolved concerning the mechanism of action of barbiturates

The barbiturate phenobarbital has been in use in the treatment of epilepsy for 100 years. It has long been recognized that barbiturates act by prolonging and potentiating the action of γ-aminobutyric acid (GABA) on GABA(A) receptors and at higher concentrations directly activating the receptors. A large body of data supports the concept that GABA(A) receptors are the primary central nervous system target for barbiturates, including the finding that transgenic mice with a point mutation in the β3 GABA(A) -receptor subunit exhibit diminished sensitivity to the sedative and immobilizing actions of the anesthetic barbiturate pentobarbital. Although phenobarbital is only modestly less potent as a GABA(A) -receptor modulator than pentobarbital, phenobarbital is minimally sedating at effective anticonvulsant doses. Possible explanations for the reduced sedative effect of phenobarbital include more regionally restricted action; partial agonist activity; reduced propensity to directly activate GABA(A) receptors (possibly including extrasynaptic receptors containing δ subunits); and reduced activity at other ion channel targets, including voltage-gated calcium channels. In recent years, substantial progress has been made in defining the structural features of GABA(A) receptors responsible for gating and allosteric modulation by drugs. Although the precise sites of action of barbiturates have not yet been defined, the second and third transmembrane domains of the β subunit appear to be critical; binding may involve a pocket formed by β-subunit methionine 286 as well as α-subunit methionine 236. In addition to effects on GABA(A) receptors, barbiturates block AMPA/kainate receptors, and they inhibit glutamate release through an effect on P/Q-type high-voltage activated calcium channels. The combination of these various actions likely accounts for their diverse clinical activities. Despite the remarkable progress of the last century, there is still much to learn about the actions of barbiturates that can be applied to the discovery of new, more therapeutically useful agents.

The actions of chloride channel blockers, barbiturates and a benzodiazepine on Caenorhabditis elegans glutamate- and ivermectin-gated chloride channel subunits expressed in Xenopus oocytes

The pharmacology of Caenorhabditis elegans glutamate-gated chloride (GluCl) channels was determined by making intracellular voltage-clamp recordings from Xenopus oocytes expressing GluCl subunits. As previously reported (Cully et al. 1994), GluClalpha1beta responded to glutamate (in a picrotoxin sensitive manner) and ivermectin, while GluClbeta responded only to glutamate and GluClalpha1 only to ivermectin. This assay was used to further investigate the action of chloride channel compounds. The arylaminobenzoate, NPPB, reduced the action of glutamate on the heteromeric GluClalpha1beta channel (IC(50) 6.03 +/- 0.81 microM). The disulphonate stilbene, DNDS, blocked the effect of both glutamate and ivermectin on GluClalpha1beta channels, the action of glutamate on GluClbeta subunits, and the effect of ivermectin on GluClalpha1 subunits (IC(50)s 1.58-3.83 microM). Surprisingly, amobarbital and pentobarbital, otherwise known as positive allosteric modulators of ligand-gated chloride channels, acted as antagonists. Both compounds reduced the action of glutamate on the GluClalpha1beta heteromer (IC(50)s of 2.04 +/- 0.5 and 17.56 +/- 2.16 microM, respectively). Pentobarbital reduced the action of glutamate on the GluClbeta homomeric subunit with an IC(50) of 0.59 +/- 0.09 microM, while reducing the responses to ivermectin on both GluClalpha1beta and GluClalpha1 with IC(50)s of 8.7 +/- 0.5 and 12.9 +/- 2.5 microM, respectively. For all the antagonists, the mechanism is apparently non-competitive. The benzodiazepine, flurazepam had no apparent effect on these glutamate- and ivermectin-gated chloride channel subunits. Thus, arylaminobenzoates, disulphonate stilbenes, and barbiturates are non-competitive antagonists of C. elegans GluCl channels.

Expression levels of the alpha4 subunit of the GABA(A) receptor in differentiated neuroblastoma cells are correlated with GABA-gated current

The alpha4 subunit of the GABA(A) receptor (GABAR) is capable of rapid plasticity, increased by chronic exposure to positive GABA modulators, such as the neurosteroid 3alpha-OH-5alpha[beta]-pregnan-20-one (THP). Here, we show that 48 h exposure of differentiated neuroblastoma cells (IMR-32) to 100 nM THP increases alpha4 expression, without changing the current density or the concentration-response curve. Increased expression of alpha4-containing GABAR was verified by a relative insensitivity of GABA (EC(20))-gated current to modulation by the benzodiazepine (BZ) lorazepam (0.01-100 microM), and potentiation of current by flumazenil and RO15-4513, characteristic of alpha4betagamma2 pharmacology. In contrast to THP, compounds which decrease GABA-gated current, such as the BZ inverse agonist DMCM, the GABAR antagonist gabazine and the open channel blocker penicillin, decreased alpha4 expression after a 48 h exposure, without changing BZ responsiveness. However, pentobarbital, another positive GABA modulator, increased alpha4 expression, while the BZ antagonist flumazenil had no effect. In order to test whether changes in current were responsible for increased alpha4 expression, decreases in the Cl(-) driving force were produced by chronic exposure to the NKCC1 blocker bumetanide (10 microM). When applied under these conditions of reduced GABA-gated current, THP failed to increase alpha4 expression. The results of this study suggest that alpha4 expression is correlated with changes in GABA-gated current, rather than simply through ligand-receptor interactions. These findings have relevance for GABAR subunit plasticity produced by fluctuations in endogenous steroids across the menstrual cycle, when altered BZ sensitivity is reported.

Individually monitoring ligand-induced changes in the structure of the GABAA receptor at benzodiazepine binding site and non-binding-site interfaces

The mechanisms by which the GABA and benzodiazepine (BZD) binding sites of the GABA-A receptor are allosterically coupled remain elusive. In this study, we separately monitored ligand-induced structural changes in the BZD binding site (alpha/gamma interface) and at aligned positions in the alpha/beta interface. alpha(1)His101 and surrounding residues were individually mutated to cysteine and expressed with wild-type beta2 and gamma2 subunits in Xenopus laevis oocytes. The accessibilities of introduced cysteines to modification by methanethiosulfonate ethylammonium (MTSEA)-Biotin were measured in the presence and absence of GABA-site agonists, antagonists, BZDs, and pentobarbital. The presence of flurazepam or the BZD-site antagonist flumazenil (Ro15-1788) decreased the rate of modification of alpha(1)H101C at the BZD binding site. GABA and muscimol each increased MTSEA-Biotin modification of alpha(1)H101C located at the BZD-site, gabazine (SR-95531, a GABA binding site antagonist) decreased the rate, whereas pentobarbital had no effect. Modification of alpha(1)H101C at the alpha/beta interface was significantly slower than modification of alpha(1)H101C at the BZD site, and the presence of GABA or flurazepam had no effect on its accessibility, indicating the physicochemical environments of the alpha/gamma and alpha/beta interfaces are different. The data are consistent with the idea that GABA-binding site occupation by agonists causes a GABA binding cavity closure that is directly coupled to BZD binding cavity opening, and GABA-site antagonist binding causes a movement linked to BZD binding cavity closure. Pentobarbital binding/gating resulted in no observable movements in the BZD binding site near alpha(1)H101C, indicating that structural mechanisms underlying allosteric coupling between the GABA and BZD binding sites are distinct.

General anaesthesia: from molecular targets to neuronal pathways of sleep and arousal

The mechanisms through which general anaesthetics, an extremely diverse group of drugs, cause reversible loss of consciousness have been a long-standing mystery. Gradually, a relatively small number of important molecular targets have emerged, and how these drugs act at the molecular level is becoming clearer. Finding the link between these molecular studies and anaesthetic-induced loss of consciousness presents an enormous challenge, but comparisons with the features of natural sleep are helping us to understand how these drugs work and the neuronal pathways that they affect. Recent work suggests that the thalamus and the neuronal networks that regulate its activity are the key to understanding how anaesthetics cause loss of consciousness.

Molecular targets underlying general anaesthesia

The discovery of general anaesthesia, over 150 years ago, revolutionised medicine. The ability to render a patient unconscious and insensible to pain made modern surgery possible and general anaesthetics have become both indispensible as well as one of the most widely used class of drugs. Their extraordinary chemical diversity, ranging from simple chemically inert gases to complex barbiturates, has baffled pharmacologists, and ideas about how they might work have been equally diverse. Until relatively recently, thinking was dominated by the notion that anaesthetics acted 'nonspecifically' by dissolving in the lipid bilayer portions of nerve membranes. While this simple idea could account for the chemical diversity of general anaesthetics, it has proven to be false and it is now generally accepted that anaesthetics act by binding directly to sensitive target proteins. For certain intravenous anaesthetics, such as propofol and etomidate, the target has been identified as the GABA(A) receptor, with particular subunits playing a crucial role. For the less potent inhalational agents, the picture is less clear, although a relatively small number of targets have been identified as being the most likely candidates. In this review, I will describe the work that led up to the identification of the GABA(A) receptor as the key target for etomidate and propofol and contrast this with current progress that has been made in identifying the relevant targets for other anaesthetics, particularly the inhalational agents.

Benzodiazepine modulation of partial agonist efficacy and spontaneously active GABA(A) receptors supports an allosteric model of modulation

Benzodiazepines (BZDs) have been used extensively for more than 40 years because of their high therapeutic index and low toxicity. Although BZDs are understood to act primarily as allosteric modulators of GABA(A) receptors, the mechanism of modulation is not well understood. The applicability of an allosteric model with two binding sites for gamma-aminobutyric acid (GABA) and one for a BZD-like modulator was investigated. This model predicts that BZDs should enhance the efficacy of partial agonists. Consistent with this prediction, diazepam increased the efficacy of the GABA(A) receptor partial agonist kojic amine in chick spinal cord neurons. To further test the validity of the model, the effects of diazepam, flurazepam, and zolpidem were examined using wild-type and spontaneously active mutant alpha1(L263S)beta3gamma2 GABA(A) receptors expressed in HEK-293 cells. In agreement with the predictions of the allosteric model, all three modulators acted as direct agonists for the spontaneously active receptors. The results indicate that BZD-like modulators enhance the amplitude of the GABA response by stabilizing the open channel active state relative to the inactive state by less than 1 kcal, which is similar to the energy of stabilization conferred by a single hydrogen bond.

Comparative tolerability of sedative agents in head-injured adults

Sedative agents are widely used in the management of patients with head injury. These drugs can facilitate assisted ventilation and may provide useful reductions in cerebral oxygen demand. However, they may compromise cerebral oxygen delivery via their cardiovascular effects. In addition, individual sedative agents have specific and sometimes serious adverse effects. This review focuses on the different classes of sedative agents used in head injury, with a discussion of their role in the context of clinical pathophysiology. While there is no sedative that has all the desirable characteristics for an agent in this clinical setting, careful titration of dose, combination of agents, and a clear understanding of the pathophysiology and pharmacology of these agents will allow safe sedative administration in head injury.

Direct and indirect enhancement of GABAergic neurotransmission by ammonia: implications for the pathogenesis of hyperammonemic syndromes

Experimental evidence indicates that ammonia causes neuroexcitation and seizures. This contrasts with the lethargy, confusion and other manifestations of global CNS depression commonly considered to be major components of hyperammonemic encephalopathies. Substantial data now indicates that ammonia can modulate GABAergic neurotransmission through direct and indirect mechanisms. This modulation consists of an enhancement of GABAergic neurotransmission at concentrations commonly encountered in hyperammonemic states and precedes the suppression of inhibitory neuronal function observed at higher (>1mM) ammonia concentrations. Not only is this increase in GABAergic neurotransmission consistent with the clinical picture of lethargy, ataxia and cognitive deficits associated with liver failure and congenital hyperammonemia, but it also provides a mechanism for testing new therapeutic modalities for the treatment of hyperammonemic encephalopathy.

Sedatives used in pediatric imaging: comparison of IV pentobarbital with IV pentobarbital with midazolam added

OBJECTIVE

This study was designed to evaluate safety, efficacy, and success of adding IV midazolam to an established IV pentobarbital protocol for pediatric sedation for radiologic imaging. Outcomes included sedation and discharge times as well as adverse events

SUBJECTS AND METHODS

This prospective study compared two different sedation protocols developed by the radiology sedation committee and approved by the hospital sedation committee at our institution. Patients in the pentobarbital group received IV pentobarbital alone, and patients in the pentobarbital--midazolam group received a combination of IV pentobarbital and midazolam. A total of 1070 infants and children were enrolled, and sedation data were entered into a computer database and reviewed at bimonthly radiology sedation committee meetings for safety, efficacy, efficiency, failed sedations, and adverse outcomes.

RESULTS

Mean age distribution, sex, American Society of Anesthesiologists physical status classification, fasting status, weight, and types of examinations were similarly distributed between the two study groups. Analysis of variance indicated longer times were required to sedate and to discharge patients who had received pentobarbital--midazolam (p < 0.001 for both times), even after adjusting for differences in the patients' ages and weights. The pentobarbital--midazolam group required more time to be successfully sedated and more time to discharge from the recovery room. The rates of adverse events and failed sedations were similar for both groups.

CONCLUSION

Midazolam does not have a beneficial effect on pentobarbital sedation and has no effect on the rate of adverse events. The prolonged time needed both to sedate and to discharge (timed from the initial dose of sedation) pediatric patients who have received midazolam should discourage physicians from combining it with pentobarbital for pediatric sedation.

Profile of acute tolerance to three sedative anxiolytics

Acute tolerance, defined as a decreasing drug effect relative to drug-plasma levels (DPL) over a period of minutes to a few hours, is pronounced following single doses of diazepam or pentobarbital. Both of these lipid-soluble drugs produce an early peak behavioral impairment and subsequent rapid recovery component that is followed by a much slower blood-drug rise time. These pronounced early peak effects were not shared by alcohol, and contribute significantly to the lack of correlation between impairment and DPL.

A role for chloride in the hyperpolarizing effect of acetylcholine in isolated frog vestibular hair cells

Acetylcholine (ACh) is the dominant transmitter released from inner ear efferent neurons. In frog vestibular organs, these efferent neurons synapse exclusively with type II hair cells. Hair cells isolated from the frog saccule hyperpolarize following the application of 50 microM ACh, thereby demonstrating the presence of an ACh receptor. A role for Cl(-) in the response of hair cell-bearing organs to efferent nerve activation or ACh application was suggested some years ago. Perfusion with solutions in which most of the Cl(-) was replaced by large impermeant anions decreased the cholinergic inhibition of afferent firing in the cat and turtle cochleas, and frog semicircular canal. Our previous work in the intact organ demonstrated that substitution of large impermeant anions for Cl(-) or use of Cl(-) channel blockers reduced the effect of ACh on saccular afferent firing. Using the perforated-patch clamping technique, replacement of Cl(-) by methanesulfonate, iodide, nitrate, or thiocyanate attenuated the hyperpolarizing response to ACh in hair cells isolated from the frog saccule. The chloride channel blockers picrotoxin and 4,4'-dinitrostilbene-2,2'-disulfonic acid were also tested and found to inhibit the ACh response. Thus, the present work demonstrates that the effects of Cl(-) substitutions or Cl(-) channel blockers on the ACh response in the intact saccule can be explained completely by effects on the hair cell. Evidence is also presented for the presence of the messenger RNA for a calcium-dependent chloride channel in all hair cells but especially saccular hair cells. This channel may be involved in the response to ACh. The precise role for chloride in this response, whether as a distinct ion current, as a transported ion, or as a permissive ion for other components, is discussed.

Autoradiography of [3H]glutamate binding during pentobarbital tolerance and withdrawal in the rat

The influence of centrally administered pentobarbital on [3H]glutamate receptor binding in the rat brain was examined. Animals were rendered tolerant by intracerebroventricular (i.c.v.) infusion through osmotic minipumps with pentobarbital (300 microg/10 microl/h, for 6 days), and dependent, by 24 h after withdrawal from pentobarbital. Pentobarbital tolerant rats have significant increases in [3H]glutamate binding in the cortex and hippocampus area. Pentobarbital withdrawal produced increases in glutamate binding in many regions, e.g., the cortex, hippocampus area, thalamus, and cerebellum. These results show that chronic i.c.v. infusion with pentobarbital increases N-methyl-D-aspartate (NMDA) displaceable [3H]glutamate binding, suggesting that an increase in NMDA binding sites may play an important role in the development of tolerance to and withdrawal from pentobarbital.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Chronic pentobarbital administration alters gamma-aminobutyric acidA receptor alpha 6-subunit mRNA levels and diazepam-insensitive [3H]Ro15-4513 binding

In order to study the chronic effects of pentobarbital, a positive GABAA receptor modulator, on the inverse agonist binding of the benzodiazepine site, binding of [3H]Ro15-4513 and levels of GABAA receptor alpha 6-subunit mRNA were investigated in the brains of pentobarbital-tolerant/dependent animals, using receptor autoradiography and in situ hybridization histochemistry in consecutive brain sections. Pentobarbital was administered to rats either 60 mg/kg, i.p., once, for acute treatment, or 300 micrograms/10 microliters/h i.c.v. continuously for 6 days via osmotic minipumps to render rats tolerant to pentobarbital. Rats assigned to the dependent group were sacrificed 24 h after discontinuance of pentobarbital infusion, while those assigned to the tolerant group were sacrificed at the end of infusion. The alpha 6 subunit mRNA was increased in the tolerant group only. Diazepam-insensitive [3H]Ro15-4513 binding was increased in the cerebellar granule layer of pentobarbital-tolerant and -dependent rats. No alterations in these parameters were observed in acutely treated animals. These data suggest that chronic pentobarbital treatment induced expression of alpha 6-subunit mRNA. This was in contrast to alpha 1- and gamma 2-subunit mRNA, which in tolerant animals are unchanged, but for which withdrawal triggers a surge in levels. Because the alpha 6-subunit is a major component of the diazepam-insensitive [3H]Ro15-4513 binding site, the increased diazepam-insensitive [3H]Ro15-4513 binding implied de novo synthesis of the receptor subunit protein.

A behavioural and neurochemical study in rats of the pharmacology of loreclezole, a novel allosteric modulator of the GABAA receptor

Loreclezole is an anticonvulsant and anxiolytic compound which has been reported to potentiate GABA via a novel allosteric site on the beta-subunit of the receptor. We have now studied in rats both the in vivo and in vitro pharmacology of the compound. The dose of loreclezole required to increase by 50% the dose of intravenous pentylenetetrazol eliciting a seizure was comparable to that of barbiturates and chlormethiazole (in mg/kg): diazepam, 1.3; pentobarbitone, 16; chlormethiazole, 22; loreclezole, 25; pentobarbitone, 36. Loreclezole dose-dependently decreased locomotion (dose to decrease locomotion by 50% (in mg/kg): chlormethiazole, 9; pentobarbitone, 16; loreclezole, 25). Loreclezole, chlormethiazole and pentobarbitone all failed to displace [3H]muscimol and [3H]flunitrazepam binding from a rat cortical membrane preparation. All three compounds fully displaced [35S]TBPS binding (IC50 values: loreclezole, 4.34 +/- 0.68 microM; pentobarbitone, 37.39 +/- 3.24 microM; chlormethiazole, 82.10 +/- 8.52 microM). Addition of bicuculline (10 microM) produced a major rightward shift in the loreclezole and pentobarbitone displacement curves, increasing IC50 values for [35S]TBPS binding by 25 times (loreclezole), 6 times (pentobarbitone) and 2.7 times (chlormethiazole), suggesting a greater involvement of GABA in the interaction of loreclezole with the chloride channel than in the case of chlormethiazole. Anticonvulsant activity of the compounds did not appear to relate to [35S]TBPS binding activity. Other binding data suggested that although the evidence of others indicates that loreclezole interacts with a specific allosteric site on the beta-subunit, it nevertheless also alters the binding characteristics of other modulatory sites.

L-lysine is a barbiturate-like anticonvulsant and modulator of the benzodiazepine receptor

Our earlier observations showed that L-lysine enhanced the activity of diazepam against seizures induced by pentylenetetrazol (PTZ), and increased the affinity of benzodiazepine receptor binding in a manner additive to that caused by gamma-aminobutyric acid (GABA). The present paper provides additional evidence to show that L-lysine has central nervous system depressant-like characteristics. L-lysine enhanced [3H]flunitrazepam (FTZ) binding in brain membranes was dose-dependent and stimulated by chloride, bromide and iodide, but not fluoride. Enhancement of [3H]FTZ binding by L-lysine at a fixed concentration was increased by GABA but inhibited by pentobarbital between 10(-7) to 10(-3)M. While GABA enhancement of [3H]FTZ binding was inhibited by the GABA mimetics imidazole acetic acid and tetrahydroisoxazol pyridinol, the enhancement by pentobarbital and L-lysine of [3H]FTZ binding was dose-dependently increased by these two GABA mimetics. The above results suggest that L-lysine and pentobarbital acted at the same site of the GABA/benzodiazepine receptor complex which was different from the GABA binding site. The benzodiazepine receptor antagonist imidazodiazepine Ro15-1788 blocked the antiseizure activity of diazepam against PTZ. Similar to pentobarbital, the anti-PTZ effect of L-lysine was not blocked by Ro15-1788. Picrotoxinin and the GABA, receptor antagonist bicuculline partially inhibited L-lysine's enhancement of [3H]FTZ binding with the IC50s of 2 microM and 0.1 microM, respectively. The convulsant benzodiazepine Ro5-3663 dose-dependently inhibited the enhancement of [3H]FTZ binding by L-lysine. This article shows the basic amino acid L-lysine to have a central nervous system depressant characteristics with an anti-PTZ seizure activity and an enhancement of [3H]FTZ binding similar to that of barbiturates but different from GABA.

The effect of midazolam premedication on implicit memory activation during alfentanil-nitrous oxide anaesthesia

Eighty-three patients were given midazolam 0.1 mg.kg-1 by intramuscular injection as premedication before general anaesthesia with alfentanil-nitrous oxide. During anaesthesia patients were presented (through headphones) with either statements about common facts of some years ago (group A) (n = 43) or new verbal associations, e.g. names of fictitious, nonfamous people (group B) (n = 40). In a previous study with the same anaesthetic technique, but without premedication there was significant activation of implicit memory (p < 0.001). In this study we found no explicit or implicit memory for the auditory information presented during anaesthesia. Midazolam premedication can prevent implicit memory activation during alfentanil-nitrous oxide anaesthesia.

Synergistic inhibition by benzodiazepine and barbiturate drugs of the clock control of ovulation in hamsters

A surge of pituitary luteinizing hormone (LH) into the bloodstream occurs in hamsters every 4 days between 1:30 p.m. and 3 p.m. in response to a signal from a biological clock. This surge initiates behavioral estrus approximately 2 h later and ovulation approximately 12 h later. Phenobarbital at a dose > or = 100 mg/kg consistently blocks LH release. Barbiturate and benzodiazepine drugs have separate binding sites in the GABAA receptor/chloride channel complex. Binding of either drug increases GABA-mediated chloride conductance, which suppresses the postsynaptic neuron. Barbiturate binding also increases benzodiazepine binding. This suggested that these drugs might synergize to inhibit LH release. A combination of triazolam and phenobarbital at doses of 10 mg/kg injected s.c. at 1:30 p.m. inhibited ovulation and extended the 4-day vaginal cycle in all treated hamsters. Either drug dose injected alone at 1:30 p.m., or the combination at 3 p.m., was completely ineffective. Bicuculline prevented inhibition by the combination at 1:30 p.m. The clock signal for LH release may act by antagonizing GABA transmission, which may be chronically inhibiting LH release. The combination delimited a 75-min period (1:30-2:45 p.m.) within which the clock signal for LH release occurred in all individuals (ET50 = 2:08 p.m.). This period appears to arise from individuals with different but constant clock settings rather than from a 75-min variation in the clock setting of the individual.

Binding characteristics of [3H]flunitrazepam in pentobarbital-withdrawal rats

The effects of chronic pentobarbital (PB) treatment on the binding characteristics of [3H]flunitrazepam (FLU) in rat brain were examined. Saline or sodium PB (500 micrograms/10 microliters/hr) was infused into the lateral cerebral ventricles of rats for 6 days using osmotic pumps. Immediately before withdrawal, there were no significant differences in [3H]FLU binding constants (KD and Bmax) between saline and PB groups. However, 24 hr withdrawal caused an increase in Bmax with no changes in KD. The enhancement of [3H]FLU binding by in vitro addition of chloride ions and PB was not affected after the PB infusion. The PB enhancement of [3H]FLU binding was inhibited by the convulsant, picrotoxinin. PB withdrawal did not cause significant differences in the binding constants of [3H]Ro 15-1788, a benzodiazepine (BZ) antagonist, between the saline and PB groups. Pretreatment of membranes with 0.02 mM of 3-[(3-cholamidopropyl)-dimethylammonio]-1- propanesulfonate (CHAPS), a zwitterionic detergent, caused decreases in both KD and Bmax in FLU binding in PB-withdrawal membrane, but not in the saline-treated membrane. The enhancement of [3H]FLU binding by chloride ions and PB was not affected by the CHAPS treatment. These results suggest that the change in BZ receptors induced by PB withdrawal is functionally linked to the GABA-BZ-barbiturate receptor complex and that PB withdrawal induces some conformational changes in BZ receptors.

GABA receptor molecules of insects

Receptors for 4-aminobutyric acid (GABA) have been identified in both central and peripheral nervous systems of several invertebrate phyla. To date, much of the information derived from physiological and biochemical studies on insect GABA receptors relates to GABA-gated chloride channels that show some similarities with vertebrate GABAA receptors. Like their vertebrate central nervous system (CNS) counterparts, agonist activation of such insect GABA receptors leads to a rapid, picrotoxin-sensitive increase in chloride ion conductance across the cell membrane. In insects, responses to GABA can be modulated by certain benzodiazepines and barbiturates. However, recent studies have detected a number of striking pharmacological differences between GABA-gated chloride channels of insects and vertebrates. Receptor binding, electrophysiological and 36Cl- flux assays have indicated that many insect receptors of this type are insensitive to the vertebrate GABAA antagonists bicuculline and pitrazepin. Benzodiazepine binding sites coupled to insect GABA receptors display a pharmacological profile distinct from that of corresponding sites in vertebrate CNS. Receptor binding studies have also demonstrated differences between convulsant binding sites of insect and vertebrate receptors. Insect GABA receptor molecules are important target sites for several chemically-distinct classes of insecticidally-active molecules. By characterizing these pharmacological properties in detail, it may prove possible to exploit differences between vertebrate and insect GABA receptors in the rational design of novel, more selective pest control agents. The recent application of the powerful techniques of molecular biology has revealed a diversity of vertebrate GABAA receptor subunits and their respective isoforms that can assemble in vivo to form a multiplicity of receptor subtypes. Molecular cloning of insect GABA receptor subunits will not only enhance our understanding of invertebrate neurotransmitter receptor diversity but will also permit the precise identification of the sites of action of pest control agents.

Effects of pentobarbital tolerance to and dependence on GABAB receptor-binding

Effects of pentobarbital pellet implantation on [3H]baclofen binding in the frontal cortex of cerebellum of rat brains were examined. In the frontal cortex, pentobarbital tolerance caused an increase in the number of binding sites (Bmax) without changing their affinity (KD). Twenty-four hours after withdrawal of the pentobarbital pellets, there was a significant increase in the KD and Bmax values. Cerebellar binding, in contrast, was not significantly changed in any of the treatment groups. Addition of 1 mM of pentobarbital directly to binding assays using cortical membrane produced as increase in KD without a change in Bmax. In vitro, pentobarbital affected neither the KD nor the Bmax in the cerebellar [3H]baclofen binding. These results suggest that like the GABAA receptor, [3H]baclofen binding to the GABAB receptor in rat frontal cortex was affected by pentobarbital tolerance and dependence, and that there are regional differences in the properties of the GABAB receptor.

Halothane enhances the binding of diazepam to synaptic membranes from rat cerebral cortex

The effects of halothane on diazepam binding to the rat cerebral synaptic membranes were investigated. Halothane at the concentrations of 0.63, 1.3 and 4.1 mmol/l increased the binding by 12, 23 and 33%, respectively, compared to the control in the absence of halothane. The increments were dependent on the presence of Cl-. The findings indicate that halothane and diazepam have an interaction at receptor level.

The GABAA receptor family in the mammalian brain

The GABAA-benzodiazepine receptor protein from bovine brain was purified by affinity chromatography and the subunit composition examined by gel electrophoresis in sodium dodecyl sulfate. Protein staining revealed a doublet at 51-53 kDa, a band at 55 kDa, and a broad band at 57-59 kDa. The 51 and 53 kDa bands co-migrated with the alpha 1 and alpha 2 gene products identified by Western blotting with subtype-specific antibodies. These two bands were also photoaffinity labeled by [3H]flunitrazepam, as was a breakdown product at 44 kDa. Partial sequencing of proteolytic fragments of these polypeptides yielded sequences found in all alpha clones, and identified the benzodiazepine binding site within residues 8-297 and probably between 106-297 of alpha 1; the 44 kDa and 31 kDa bands yielded fragments containing alpha 3 sequence. The native alpha 3 polypeptide was identified with subtype-specific antibody at 57 kDa overlapping with the two major bands photolabeled with [3H]muscimol at 55 and 58 kDa. Antisera to a beta-selective peptide recognized four bands at 60, 58, 57 and 55 kDa. Thus, one can identify 6-8 distinct polypeptides with the possibility of another 4-6 in purified GABAA receptor proteins, depending on brain region, consistent with the family of gene products suggested by molecular cloning.

t-[35S]butylbicyclophosphorothionate binding under equilibrium and nonequilibrium conditions: differential effects of barbiturates and gamma-aminobutyric acid in the long-sleep and short-sleep selected mouse lines

Significant differences were demonstrated between the long-sleep (LS) and short-sleep (SS) selected mouse lines in the abilities of barbiturates and gamma-aminobutyric acid (GABA) to inhibit t-[35S]butylbicyclophosphorothionate [( 35S]TBPS) binding to well-washed cerebral cortical membranes. Thus, using phenobarbital to initiate the dissociation of [35S]TBPS, the extent of inhibition was significantly greater in LS mice (but not SS mice) than would be predicted using equilibrium conditions. Pentobarbital had the opposite effect, causing [35S]TBPS to dissociate to a greater extent in SS than LS membranes. [35S]TBPS binding was dissociated from LS and SS membranes by GABA to a greater and lesser extent, respectively, than would be predicted from equilibrium studies. Because no line differences in the potencies of these drugs to inhibit [35S]TBPS binding were found using equilibrium conditions, these results indicate that the association rates of barbiturates and GABA may be different between these lines. These findings are consistent with neurochemical studies indicating differences in the benzodiazepine/GABA receptor-chloride channel complex in these selected lines and may explain their differential sensitivities to certain agents acting through this supramolecular complex.

Pharmacokinetic-pharmacodynamic modeling of the central nervous system effects of heptabarbital using aperiodic EEG analysis

The concentration EEG effect relationship of heptabarbital was modeled using effect parameters derived from aperiodic EEG analysis. Male Wistar rats (n = 10) received an intravenous infusion of heptabarbital at a rate of 6-9 mg/kg per min until burst suppression with isoelectric periods of 5 sec or longer. Arterial blood samples were obtained and EEG was measured continuously until recovery of baseline EEG and subjected to aperiodic analysis for quantification. Two EEG parameters, the amplitudes per second (AMP) and the total number of waves per second (TNW), in five discrete frequency ranges and for two EEG leads were used as descriptors of the drug effect on the brain. The EEG parameters responded both qualitatively and quantitatively different to increasing concentrations of heptabarbital. Monophasic concentration effect curves (decrease) were found for the frequency ranges greater than 2.5 Hz and successfully quantified with a sigmoidal Emax model after collapsing the hysteresis by a nonparametric modeling approach. For the parameter TNW in the 2.5-30 Hz frequency range the value of the pharmacodynamic parameters EC50, Emax, and n (means +/- SD) were 78 +/- 7 mg/L, 11.4 +/- 1.7 waves/sec and 5.0 +/- 1.5, respectively. For other discrete frequency ranges, differences in EC50 were observed, indicating differences in sensitivity to the effect of heptabarbital. In the 0.5 +/- 2.5 Hz frequency range biphasic concentration effect relationships (increase followed by decrease) were observed. To fully account for the hysteresis in these concentration effect relationships, postulation of two effect compartments was necessary. To characterize these biphasic effect curves two different pharmacodynamic models were evaluated. Model 1 characterized the biphasic concentration effect relationship as the summation of two sigmoidal Emax models, whereas Model 2 assumed the biphasic effect to be the result of only one inhibitory mechanism of action. With Model 1 however realistic parameter estimation was difficult because the maximal increase could not be measured, resulting in high correlations between parameter estimates. This seriously limits the value of Model 1. Model 2 involves besides estimation of the classical pharmacodynamic parameters Emax, EC50, and n also estimation of the maximal disinhibition Amax. This model is a new approach to characterize biphasic drug effects and allows, in principle, reliable estimation of all relevant pharmacodynamic parameters.

Mechanisms of toxic action and structure-activity relationships for organochlorine and synthetic pyrethroid insecticides

The mechanisms and sites of action of organochlorine (DDT-types and chlorinated alicyclics) and synthetic pyrethroid insecticides are presented with discussion of symptoms, physiological effects, and selectivity. The structural requirements for toxicity are assessed, and structure-activity relationships are considered for each subclass. Lipophilicity is important for all the groups because it facilitates delivery of these neurotoxicants to the site of action in the nerve. Steric factors including molecular volume, shape, and isomeric configuration greatly influence toxicity. Electronic parameters also have been demonstrated to affect biological activity in some of the groups of insecticides, e.g., Hammett's sigma and Taft's sigma * as indicators of electronegativity. New synthetic pyrethroids continue to be developed, with varied structures and different physicochemical and biological properties.

Cerebral radioprotection by pentobarbital: dose-response characteristics and association with GABA agonist activity

Pentobarbital reduces cerebral radiation toxicity; however, the mechanism of this phenomenon remains unknown. As an anesthetic and depressant of cerebral metabolism, pentobarbital induces its effects on the central nervous system by stimulating the binding of gamma-aminobutyric acid (GABA) to its receptor and by inhibiting postsynaptic excitatory amino acid activity. The purpose of this study is to investigate the role of these actions as well as other aspects of the radioprotective activity of pentobarbital. Fischer 344 rats were separated into multiple groups and underwent two dose-response evaluations. In one set of experiments to examine the relationship of radioprotection to pentobarbital dose, a range of pentobarbital doses (0 to 75 mg/kg) were given intraperitoneally prior to a constant-level radiation dose (70 Gy). In a second series of experiments to determine the dose-response relationship of radiation protection to radiation dose, a range of radiation doses (10 to 90 Gy) were given with a single pentobarbital dose (60 mg/kg intraperitoneally). Further groups of animals were used to evaluate the importance of the timing of pentobarbital administration, the function of the (+) and (-) isomers of pentobarbital, and the role of an alternative GABA agonist (diazepam). In addition, the potential protective effects of alternative methods of anesthesia (ketamine) and induction of cerebral hypometabolism (hypothermia) were examined. Enhancement of survival time from acute radiation injury due to high-dose single-fraction whole-brain irradiation was maximal with 60 mg/kg of pentobarbital, and occurred over the range of all doses examined between 30 to 90 Gy. Protection was seen only in animals that received the pentobarbital before irradiation. Administration of other compounds that enhance GABA binding (Saffan and diazepam) also significantly enhanced survival time. Ketamine and hypothermia were without protective effect. Protection from acute radiation-induced mortality by pentobarbital in the rat model is a reproducible phenomenon and is associated with the GABA agonistic activity of the compound. This property of GABA agonists offers the potential for a novel approach to enhancement of the efficacy of radiation therapy in the treatment of brain tumors.

Barbiturates and the GABAA receptor complex

The GABA synapse plays an important role in the pharmacologic effects of barbiturates and the mechanisms involved in barbiturate tolerance and dependence. A synopsis of the effects which have been reported to date is found in Tables 1 and 2. Although the acute changes in neurotransmitter uptake and release are nonselective, a lag in the ability of the GABA synapse to compensate for discontinuation of barbiturate exposure may be important in the symptoms of withdrawal. Barbiturates cause changes in the properties of many receptors, but manipulations of the GABAA receptor in vivo correlate with changes in the therapeutic and toxicologic responses to barbiturates, indicating that the GABAA receptor complex plays a pivotal role in the effects of barbiturates. Experiments done in several laboratories show that barbiturate tolerance and dependence cause subtle changes in the properties of the GABAA receptor complex. These observations suggest that decreased GABA-stimulated chloride channel activity and reduced ability to modulate it may be important in causing barbiturate tolerance and the symptoms observed in withdrawal. Selection of drug-resistant rodent strains suggests that there may be genetic factors involved in drug tolerance and dependence. The complexity of the responses of the GABA synapse to both acute and prolonged exposure to barbiturates indicates that it is a valuable model for understanding how the central nervous system responds to drugs and the mechanisms involved in drug addiction.

Steroid and barbiturate modulation of the GABAa receptor. Possible mechanisms

This review describes the modulation of the GABAa receptor by steroid hormones and barbiturates and proposes guidelines for further research. Having examined the complex organization of the GABAa receptor complex and the multiple allosteric interactions between its drug and transmitter/modulator binding sites, the possibility that conformational changes of the receptor molecule may explain most of its characteristics is explored. On the basis of considerable evidence, we propose that the GABAa receptor may adopt as many as five different conformations. However, the heterogeneity of central GABAa receptor binding cannot only be explained by different configurations of a single protein. It also has been shown that different GABAa receptor subtypes exist within different brain regions. These receptor subtypes may differ from each other in their subunit composition. By describing the GABAa receptor as a macromolecular complex that may adopt different conformations and whose subunit composition may vary, it becomes possible to understand the molecular mechanisms by which steroid hormones modulate the receptor. This has led to two models of hormone actions. A first model addresses the direct effects that steroids exert on the GABAa receptor and predicts that steroid hormones may cause the conformation of the receptor complex to change between active and inactive states. A second model, which addresses the observed heterogeneity of GABAa receptor binding within the brain, suggests that steroid hormones may change the expression of the different subunits of the receptor complex by acting at the genomic level. This review complements other recent reviews describing the modulation of the GABAa receptor (Olsen and Venter, 1986; Gee, 1988).

The modulation by chlormethiazole of the GABAA-receptor complex in rat brain

1. The interactions of chlormethiazole with gamma-aminobutyric acid (GABA) synthesis and release, and with ligand binding to sites associated with the GABAA-receptor complex and the GABAB-receptor have been studied in the rat. The GABAA-receptor was studied using [3H]-muscimol, [3H]-flunitrazepam was used to label the benzodiazepine modulatory site, and [35S]-butyl-bicyclophosphorothionate ([35S]-TBPS) to label the chloride channel. 2. Chlormethiazole had no effect on GABA synthesis in the cortex, hippocampus and striatum or on GABA release from cortical slices in vitro. Chlormethiazole did not displace [3H]-baclofen binding to the GABAB-receptor. 3. Chlormethiazole (IC50 = 140 microM) and pentobarbitone (IC50 = 95 microM) both inhibited [35S]-TBPS binding by increasing the rate of [35S]-TBPS dissociation. In addition, chlormethiazole caused an apparent decrease in the affinity of [35S]-TBPS binding. 4. Chlormethiazole enhanced the binding of [3H]-muscimol but had no effect on [3H]-flunitrazepam binding. In contrast, the sedative barbiturate pentobarbitone enhanced both [3H]-muscimol and [3H]-flunitrazepam binding. 5. It is concluded that the sedative and anticonvulsant effects of chlormethiazole are probably mediated through an action at the GABAA-receptor. However, chlormethiazole does not interact with the GABAA-receptor complex in an identical manner to the sedative barbiturate pentobarbitone.

Fluorescence measurements of anion transport by the GABAA receptor in reconstituted membrane preparations

A fluorescence assay for measuring the functional properties of the GABAA receptor in reconstituted membrane vesicles is described. This assay is based on a method previously described to measure monovalent cation transport mediated by the nicotinic acetylcholine receptor in membranes from Torpedo electric organ [Moore, H.-P.H., & Raftery, M. A. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 4509-4513]. The GABAA receptor has been solubilized from bovine brain membranes and reconstituted into phospholipid vesicles. Influx of chloride or iodide into the vesicles has been measured in stopped-flow experiments by monitoring the fluorescence quench of an anion-sensitive fluorophore trapped within the vesicles. Muscimol, a GABAA receptor agonist, stimulated a rapid uptake of either chloride or iodide. Stimulation of chloride influx was dependent on the concentration of muscimol, and the midpoint of the dose-response curve occurred at approximately 0.3 microM. Agonist-stimulated uptake was enhanced by diazepam and blocked by desensitization and by the antagonists bicuculline and picrotoxin. These receptor-mediated effects are shown to be qualitatively similar to measurements of 36Cl- and 125I- efflux using synaptoneurosomes prepared from rat cerebral cortex. The advantages of the fluorescence method in terms of its improved time resolution, sensitivity, and suitability for quantitating GABAA receptor function are discussed.

Steroid modulation of the GABA/benzodiazepine receptor-linked chloride ionophore

Recent findings suggest that steroids with sedative-hypnotic properties interact specifically with the gamma-aminobutyric acidA/benzodiazepine receptor-chloride ionophore complex (GBRC). They show positive heterotropic cooperativity by allosterically enhancing the binding of GABA agonists and the clinically useful benzodiazepines (BZs) to their respective recognition sites. These steroids have stringent structural requirements for activity at the GBRC, with the essential requirements for high potency being a 3 alpha-hydroxyl group and a 5 alpha-reduced A-ring. Some of these steroids are naturally occurring metabolites of progesterone and deoxycorticosterone and have nanomolar potencies as potentiators of chloride channel conductance. These 3 alpha-hydroxylated, 5 alpha-reduced steroids do not act through any known sites on the GBRC. Thus, the exact site and mechanism of action remain to be determined. Together with the observation that physiological levels of these metabolites are sufficient to influence the function of the GBRC, the evidence clearly suggests a role for these steroids in the normal regulation of brain excitability by potentiating the postsynaptic effects of gamma-aminobutyric acid (GABA). Pharmacological studies of the GBRC-active steroids show that they possess anxiolytic and anticonvulsant activities. The potential therapeutic application of these steroids in the treatment of mood disorders and catamenial exacerbation of seizures associated with the menstrual cycle is discussed. Collectively, the evidence from the studies of these steroids imply that another mechanism by which the endocrine system influences brain function has been identified. Its characterization will provide important insight into how steroids modulate brain excitability under normal and pathophysiological states.

Anion regulation of agonist and inverse agonist binding to benzodiazepine receptors

Binding of the benzodiazepine inverse agonist [3H]methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate [( 3H]DMCM) and the agonist [3H]flunitrazepam [( 3H]FNZ) was compared in rat cortical membranes. Halide ions enhanced [3H]DMCM binding three- to fourfold, increasing both the apparent affinity and the number of binding sites for this radioligand. The effect was present at both 0 and 37 degrees C. In contrast, the magnitude of halide stimulation of [3H]FNZ binding was much smaller, resulting solely from an increase in the apparent affinity for this radioligand, and was not observed at 37 degrees C. The potencies but not the efficacies of a series of anions to stimulate both [3H]DMCM and [3H]FNZ binding to benzodiazepine receptors were highly correlated with their relative permeabilities through gamma-aminobutyric acid (GABA)-gated chloride channels. Two stress paradigms (10 min of immobilization or ambient-temperature swim stress), previously shown to increase significantly the magnitude of halide-stimulated [3H]FNZ binding, did not significantly affect [3H]DMCM binding. Phospholipase A2 treatment of cortical membrane preparations was equipotent in preventing the stimulatory effect of chloride on both [3H]DMCM and [3H]FNZ binding. These data strongly suggest that anions modify the binding of [3H]DMCM and [3H]FNZ by acting at a common anion binding site that is an integral component of the GABA/benzodiazepine receptor chloride channel complex.

Modulation of the GABAA receptor by depressant barbiturates and pregnane steroids

1. The modulation of the gamma-aminobutyric acidA (GABAA) receptor by reduced metabolites of progesterone and deoxycorticosterone has been compared with that produced by depressant barbiturates in: (a) voltage-clamp recordings from bovine enzymatically isolated chromaffin cells in cell culture, and (b) an assay of the specific binding of [3H]-muscimol to a preparation of porcine brain membranes. 2. The progesterone metabolites 5 alpha- and 5 beta-pregnan-3 alpha-ol-20-one (greater than or equal to 30 nM) reversibly and dose-dependently enhanced the amplitude of membrane currents elicited by locally applied GABA (100 microM), and over the concentration range 30 nM-100 microM stimulated the binding of [3H]-muscimol. In contrast, 5 alpha- and 5 beta-pregnan-3 beta-ol-20-one (30 nM-100 microM) had little effect in either assay, indicating a marked stereoselectivity of steroid action. 3. Scatchard analysis of the ligand binding data suggested an apparent increase in the number, rather than the affinity, of detectable [3H]-muscimol binding sites as the principle action of the active steroid isomers. 4. GABA-evoked currents were also potentiated by androsterone (1 microM) and the deoxycorticosterone metabolite 5 alpha-pregnane-3 alpha,21-diol-20-one (100 nM). 5. Secobarbitone (10-100 microM), pentobarbitone (10-300 microM) and phenobarbitone (100-500 microM) reversibly and dose-dependently potentiated the amplitude of GABA-evoked currents in the absence of any change in their reversal potential. 6. At relatively high concentrations (greater than or equal to 30 microM) secobarbitone and pentobarbitone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor-channel activation since they share a common reversal potential with GABA-evoked responses (approximately 0 mV), are reversibly antagonized by bicuculline (3 microM), and potentiated by either diazepam (1 microM) or 5 beta-pregnan-3 alpha-ol-20-one (500 nM). 7. Secobarbitone (1 microM-1 mM) dose-dependently enhanced the binding of [3H]-muscimol. In common with the active steroids, an increase in the apparent number of binding sites was responsible for this effect. 8. A saturating concentration (1 mM) of secobarbitone in the ligand binding assay did not suppress the degree of enhancement of control binding produced by 5 beta-pregnan-3 alpha-ol-20-one (30 nM-100 microM). Similarly the steroid, at a concentration of 100 microM, did not influence the enhancement of [3H]-muscimol binding by secobarbitone (1 microM-1 mM). In all combinations of concentrations tested, the effects of secobarbitone and 5#-pregnan-3a-ol-20-one on [3H]-muscimol binding were additive. 9. In conjunction with previously published observations, the present data indicate close similarities in the GABA-mimetic and potentiating actions of barbiturates and steroids. However, the results obtained with combinations of steroids and barbiturates in the ligand binding assay appear inconsistent with the two classes of compound interacting with a common site to modulate the GABAA receptor activity.