Effects of benzodiazepines and non-benzodiazepine compounds on the GABA-induced response in frog isolated sensory neurones

Etizolam and Its Major Metabolites: A Short Review

Etizolam is a benzodiazepine (BZD). Etizolam is structurally different from BZDs as a thiophene replaces the benzene ring and a triazole ring is fused to the diazepine ring, but etizolam's pharmacological profile is similar. Etizolam has been used to treat anxiety and panic disorders, to reduce depressive and somatization symptoms and to induce muscle relaxation. Etizolam is used recreationally due to its reinforcing and sedative effects. Etizolam is available in tablet or powder form or administered on blotter paper that can be placed on the tongue for oral absorption. Etizolam metabolizes into two major metabolites: α-hydroxyetizolam and 8-hydroxyetizolam, and all three compounds can be detected in different biological specimens using various common analytical techniques such as immunoassay, chromatography and mass spectrometry. Etizolam is a controlled drug in many countries around the globe but is approved for medical use in some countries, such as Japan, South Korea and Italy. This work is a collation and review of available literature on etizolam to help improve the fundamental understanding of its toxicology, outline best analytical practice, and aid interpretation of toxicology results.

Structural and dynamic mechanisms of GABAA receptor modulators with opposing activities

γ-Aminobutyric acid type A (GABA_A) receptors are pentameric ligand-gated ion channels abundant in the central nervous system and are prolific drug targets for treating anxiety, sleep disorders and epilepsy. Diverse small molecules exert a spectrum of effects on γ-aminobutyric acid type A (GABA_A) receptors by acting at the classical benzodiazepine site. They can potentiate the response to GABA, attenuate channel activity, or counteract modulation by other ligands. Structural mechanisms underlying the actions of these drugs are not fully understood. Here we present two high-resolution structures of GABA_A receptors in complex with zolpidem, a positive allosteric modulator and heavily prescribed hypnotic, and DMCM, a negative allosteric modulator with convulsant and anxiogenic properties. These two drugs share the extracellular benzodiazepine site at the α/γ subunit interface and two transmembrane sites at β/α interfaces. Structural analyses reveal a basis for the subtype selectivity of zolpidem that underlies its clinical success. Molecular dynamics simulations provide insight into how DMCM switches from a negative to a positive modulator as a function of binding site occupancy. Together, these findings expand our understanding of how GABA_A receptor allosteric modulators acting through a common site can have diverging activities.

GABA_A receptors are important targets for anxiety, sedation and anesthesia. Here, the authors present structures bound by zolpidem (Ambien), the most prescribed hypnotic in the US, and DMCM, a negative modulator, providing insights into receptor modulation.

Self-administration of bretazenil under progressive-ratio schedules: behavioral economic analysis of the role intrinsic efficacy plays in the reinforcing effects of benzodiazepines

Previous research suggests that intrinsic efficacy of benzodiazepines is an important determinant of their behavioral effects. We evaluated the reinforcing effects of the benzodiazepine partial agonist bretazenil using behavioral economic models referred to as "consumer demand" and "labor supply". Four rhesus monkeys were trained under a progressive-ratio (PR) schedule of i.v. midazolam injection. A range of doses of bretazenil (0.001-0.03 mg/kg/injection and vehicle) was evaluated for self-administration with an initial response requirement of 40 that doubled to 640; significant self-administration was maintained at doses of 0.003-0.03 mg/kg/injection. Next, a dose of bretazenil that maintained peak injections/session was made available with initial response requirements doubling from 10 to 320 (maximum possible response requirements of 160 and 5120, respectively), and increasing response requirements decreased self-administration (mean number of injections/session) of a peak dose (0.01 mg/kg/injection). Analyses based on consumer demand revealed that a measure of reinforcing strength termed "essential value", for bretazenil was similar to that previously obtained with midazolam (non-selective full agonist), but less than that observed for zolpidem (full agonist, selective for α1 subunit-containing GABA(A) receptors). According to labor supply analysis, the reinforcing effects of bretazenil were influenced by the economic concept referred to as a "price effect", similar to our previous findings with midazolam but not zolpidem. In general, behavioral economic indicators of reinforcing effectiveness did not differentiate bretazenil from a non-selective full agonist. These findings raise the possibility that degree of intrinsic efficacy of a benzodiazepine agonist may not be predictive of relative reinforcing effectiveness.

CGP-37157 inhibits the sarcoplasmic reticulum Ca²+ ATPase and activates ryanodine receptor channels in striated muscle

7-Chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one [CGP-37157 (CGP)], a benzothiazepine derivative of clonazepam, is commonly used as a blocker of the mitochondrial Na+/Ca²+ exchanger. However, evidence suggests that CGP could also affect other targets, such as L-type Ca²+ channels and plasmalemma Na+/Ca²+ exchanger. Here, we tested the possibility of a direct modulation of ryanodine receptor channels (RyRs) and/or sarco/endoplasmic reticulum Ca²+-stimulated ATPase (SERCA) by CGP. In the presence of ruthenium red (inhibitor of RyRs), CGP decreased SERCA-mediated Ca²+ uptake of cardiac and skeletal sarcoplasmic reticulum (SR) microsomes (IC₅₀ values of 6.6 and 9.9 μM, respectively). The CGP effects on SERCA activity correlated with a decreased V(max) of ATPase activity of SERCA-enriched skeletal SR fractions. CGP (≥ 5 μM) also increased RyR-mediated Ca²+ leak from skeletal SR microsomes. Planar bilayer studies confirmed that both cardiac and skeletal RyRs are directly activated by CGP (EC(50) values of 9.4 and 12.0 μM, respectively). In summary, we found that CGP inhibits SERCA and activates RyR channels. Hence, the action of CGP on cellular Ca²+ homeostasis reported in the literature of cardiac, skeletal muscle, and other nonmuscle systems requires further analysis to take into account the contribution of all CGP-sensitive Ca²+ transporters.

Bretazenil, a benzodiazepine receptor partial agonist, as an adjunct in the prophylactic treatment of OP poisoning

Benzodiazepines, mainly diazepam, are commonly used as anticonvulsants in the treatment of organophosphate casualties. Although very effective, diazepam usually is not used in prophylactic treatments because of its adverse effects on task performance and its abuse liability. Benzodiazepine (BZ) partial agonists are unique in that they are able to occupy all the population of a given receptor without eliciting the maximal physiological response. The BZ receptor agonistic occupancy was found to differ among the various physiological responses in the following order: antipanic > anticonvulsion > sedation > muscle relaxation. Thus, partial agonists, by the use of which controlled levels of agonistic activity can be achieved, might serve as effective anticonvulsants, with fewer side-effects. Bretazenil, a partial agonist, was found to counteract metrazol-induced convulsions in rats. At the anticonvulsive doses (125-250 microg x kg(-1), i.p.) bretazenil, in combination with pyridostigmine (100 microg x kg(-1), i.m.) and aprophen (4 mg x kg(-1), i.m.), conferred prophylactic protection against sarin and soman poisoning (protective ratios 2.6 and 2.1, respectively). Relevant doses of bretazenil (50-400 microg x kg(-1), i.p.) also were tested for general behavioural effects in the open field and for its anti-anxiety properties in the plus maze. The incapacitation was much lower compared with diazepam. Bretazenil should be considered as a candidate for incorporating into a prophylactic mixture as a central nervous system protectant, with significant advantages concerning incapacitation.

Benzodiazepines induce a conformational change in the region of the gamma-aminobutyric acid type A receptor alpha(1)-subunit M3 membrane-spanning segment

Benzodiazepine binding to gamma-aminobutyric acid type A (GABA(A)) receptors allosterically modulates GABA binding and increases the currents induced by submaximal GABA concentrations. Benzodiazepines induce conformational changes in the GABA-binding site in the extracellular domain, but it is uncertain whether these conformational changes extend into the membrane-spanning domain where the channel gate is located. Alone, benzodiazepines do not open the channel. We used the substituted-cysteine-accessibility method to investigate diazepam-induced conformational changes in the region of the alpha(1)-subunit M3 membrane-spanning segment. In the absence of diazepam or GABA, pCMBS(-) did not react at a measurable rate with cysteine-substitution mutants between alpha(1)Phe296 and alpha(1)Glu303. In the presence of 100 nM diazepam, pCMBS(-) reacted with alpha(1)F296C, alpha(1)F298C, and alpha(1)L301C but not with the other cysteine mutants between alpha(1)Phe296 and alpha(1)Glu303. These three mutants are a subset of the five residues that we previously showed reacted with pCMBS(-) applied in the presence of GABA. The pCMBS(-) reaction rates with these three cysteine mutants were similar in the presence of diazepam and GABA. Thus, diazepam, which binds to the extracellular domain, induces a conformational change in the membrane-spanning domain that is similar to a portion of the change induced by GABA. Because diazepam does not open the channel, these results provide structural evidence that the diazepam-bound state represents an intermediate conformation distinct from the open and resting/closed states of the receptor. The diazepam-induced conformational change in the M3 segment vicinity may be related to the mechanism of allosteric potentiation.

Enhancement of GABA-activated current by muscarine in rat dorsal root ganglion neurons

The modulation of GABA-gated ion channel responses to GABA, pentobarbital and diazepam by muscarine was studied in freshly isolated rat dorsal root ganglion neurons using a whole-cell patch-clamp technique. Muscarine enhanced current activated by 5 microM GABA dose-dependently with an EC50 of 40 +/- 2 microM. This potentiation was not blocked by pirenzepine, gallamine and atropine, the specific and non-specific muscarinic receptor antagonists. Muscarine shifted the GABA dose-response curve to the left, with the GABA EC50 decreased from 45 +/- 2 to 13 +/- 2 microM. The maximal response to GABA was suppressed to 89.3 +/- 4.6% as compared with the control (100%) by 80 microM muscarine. Muscarine potentiated GABA (1-100 microM)-activated current in a voltage-independent manner. Muscarine shifted the dose-response curve for pentobarbital enhancement of GABA-activated current to the left, and the enhancement of GABA-activated current by muscarine was additive to that of pentobarbital over all pentobarbital concentrations. Muscarine shifted the dose-response curve for diazepam (1-100 nM) enhancement of GABA-activated current to the left. However, muscarine attenuated the facilitatory effect of saturating concentrations of diazepam (> 100 nM). The potentiating effect of muscarine was blocked by 1 nM ethyl-beta-carboline-3-carboxylate, the inverse agonist of benzodiazepine receptors. These results suggest that GABA-gated ion channel responses to GABA and pentobarbital were potentiated by muscarine and the binding site(s) for muscarine might be related to those for diazepam.

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.

From GABAA receptor diversity emerges a unified vision of GABAergic inhibition

Transmitter receptor diversity often indicates differences in transmitter receptor transduction mechanisms. This is not the case for gamma-aminobutyric acid subtype A (GABAA) receptor subtypes despite the presence of 16 genes to encode the 5 families of native GABAA receptor subtypes. Similar considerations apply to GABAC receptors and GABAB receptors. Both GABAA and GABAB receptors cause hyperpolarization of neuronal membranes and inhibition of neuronal excitability, but their mechanisms differ. GABAB receptors involve an efflux of K+ rather than an influx of Cl-, as in the case of GABAA and GABAC receptors. The stimulation of GABAA receptors can sometimes cause depolarization by Cl- efflux; this efflux is not the result of a transduction mechanism modification, but of Cl(-)-concentration gradient modification. Presumably, GABAA receptor diversity is directly linked to the inhibitory activity of basket cells and other interneuron axons, each innervating several postsynaptic neurons (cortical and hippocampal pyramidal cells for instance). Since the role of this inhibition is to entrain hippocampal and cortical pyramidal neurons into columnary activity, the GABAA receptor diversification may be a mechanism expressed by these postsynaptic neuron populations that uses different GABA potencies to synchronize pyramidal neurons into columnary activity. Thus, GABA potency variability, which emerges from GABAA receptor diversity, plays a unifying role in the intrinsic functional mechanism of laminated structures. GABAA receptor structural differences also play a role in diazepam tolerance, which is a mechanism operative in neuronal circuit adaptation to the extreme amplification of GABA-gated Cl- current intensities. Partial agonists (such as imidazenil), which modestly amplify GABA action at many GABAA receptor subtypes, fail to cause tolerance, dependence, ataxia, or ethanol and barbiturate potentiation. Partial agonists might become a new class of anxiolytic and anticonvulsant drugs that are virtually devoid of the side effects that cause serious concerns in the clinical use of full allosteric positive modulators of GABA action, such as diazepam, alprazolam, triazolam, and others. None of the above can be used as anticonvulsants because of an extremely high tolerance liability. When there is tolerance to diazepam, signs of sensitization to proconvulsive action are exhibited simultaneously. After tolerance, associated changes in GABAA recepter subtype expression are virtually reversed in 72 h. Also, 96 h after termination of long-term diazepam treatment, rats exhibit anxiety and are more sensitive to kainic acid-elicited convulsions. At the same time, these rats have an increase in brain expression of GLuR1, R2, and R3. It is believed that the supersensitivity to kainic acid, convulsions and anxiety, and the increased expression of GLuR1, R2, and R3 may be parts of the mechanism of diazepam dependence.

Sarmazenil-precipitated withdrawal: a reliable method for assessing dependence liability of benzodiazepine receptor ligands

The benzodiazepine receptor partial inverse agonist sarmazenil exhibits in vivo proconvulsive, but not convulsant, effects in different paradigms in rodents. Intravenous sarmazenil challenge given at several fixed intervals following the termination of repeated treatment with a markedly sedative dose of diazepam in squirrel monkeys was effective in precipitating withdrawal signs, but had no comparable effects in vehicle-treated controls. The precipitated withdrawal reaction was not only robust, but it was consistently observed in all of the diazepam-treated monkeys. Thus, the use of sarmazenil challenge in the precipitated withdrawal paradigm provides a reliable method for assessing the development of physical dependence during repeated treatment with benzodiazepine receptor agonists.

Potentiation by sevoflurane of the gamma-aminobutyric acid-induced chloride current in acutely dissociated CA1 pyramidal neurones from rat hippocampus

1. The effects of a new kind of volatile anaesthetic, sevoflurane (Sev), on gamma-aminobutyric acid (GABA)-gated chloride current (Icl) in single neurones dissociated from the rat hippocampal CA1 area were examined using the nystatin perforated patch recording configuration under the voltage-clamp condition. All drugs were applied with a rapid perfusion system, termed the "Y-tube' method. 2. When the concentrations were higher than 3 x 10(-4) M, Sev, itself, induced an inward current (ISev) at a holding potential (VH) of -40 mV. The concentration-response curve of ISev was bell-shaped, with a suppressed peak and plateau currents at high concentrations (above 2 x 10(-3) M). The reversal potential of ISev (ESev) was close to the theoretical Cl- equilibrium potential, indicating that ISev was carried mainly by Cl-. 3. ISev was reversibly blocked by bicuculline (Bic), an antagonist of the GABAA receptor, in a concentration-dependent manner with a half-inhibitory concentration (IC50) of 7.2 x 10(-7) M. But ISev was insensitive to strychnine (Str), an antagonist of the glycine receptor. 4. At low concentrations (between 3 x 10(-4) and 10(-3) M), Sev markedly enhanced the 10(-6) M GABA induced current (IGABA) but reduced the IGABA with accelerating desensitization accompanied by a "hump' current after washout at high concentrations (higher than 2 x 10(-3) M). 5. Sev, 10(-3) M potentiated the current induced by low concentrations of GABA (between 10(-7) and 3 x 10(-6) M) but reduced the current induced by high concentrations (higher than 10(-5) M) of GABA with a clear acceleration of IGABA desensitization. 6. Sev, like pentobarbitone (PB), pregnanolone (PGN) or diazepam (DZP), potentiated the 10(-6) M GABA-induced response without shifting the reversal potential of IGABA. 7. ISev was augmented by PB, PGN, or DZP at concentrations that maximally potentiated IGABA, suggesting that Sev enhanced IGABA at a binding site distinct from that for PB, PGN, or DZP. 8. It is concluded that Sev acts on the GABAA receptor complex mimicking the GABA-induced chloride current at high concentrations. At low concentrations, Sev enhances GABA-gated chloride current at a binding site independent of the allosteric modulator sites of barbiturates, benzodiazepines or neurosteroids. The reversible potentiation of the inhibitory GABAA receptor-mediated Cl- current may result in the depressing of postsynaptic excitability and may, at least in part, underlie the anaesthetic action of Sev.

Benzodiazepine antagonists reduce epileptiform discharges in rat hippocampal slices

PURPOSE

The antiepileptic effects of benzodiazepine-receptor (BZR) agonists have been well documented. Surprisingly, an antiepileptic effect for the BZR antagonist, flumazenil, has also been described, the mechanism of which is unknown. We investigated the effects of nanomolar concentrations of flumazenil and a structurally dissimilar BZR antagonist, propyl-beta-carboline-3-carboxylate (beta-CCP), on normal synaptic responses and epileptiform discharges induced by a variety of methods in the CA1 region of rat hippocampal slices.

METHODS

Extracellular field potentials were recorded from stratum pyramidale of the CA1 region. Orthodromic stimulation was delivered by a bipolar electrode placed in the stratum radiatum at the border of the CA2/CA3 regions. Drugs were bath applied, and epileptiform discharges were quantified by using the Coastline Bursting Index, which calculates the total length of the discharge waveform of evoked multiple population spikes. For statistical comparisons, we calculated the Coastline Bursting Index for the average of five traces at the end of the control period (20 min), drug application (20 min), and washout (20-40 min).

RESULTS

Flumazenil was without effect on normal synaptic responses; however, flumazenil reduced epileptiform discharges evoked in the presence of high [K+]o, leu-enkephalin, the BZR inverse agonist, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), or after a cold-shock procedure. beta-CCP exhibited an action similar to that observed for flumazenil, suggesting that the antiepileptic effect is due to properties common to BZR antagonists.

CONCLUSIONS

We suggest that the antiepileptic effect we observed for flumazenil and beta-CCP is mediated at the BZR and might be due to competition with endogenous BZR inverse agonists released preferentially during epileptiform activity.

Effects of clobazam and its active metabolite on GABA-activated currents in rat cerebral neurons in culture

PURPOSE

The antiepileptic effects of clobazam, a 1,5-benzodiazepine, have been well documented in animal experiments and clinical trials. However, the drug's mechanisms of antiepileptic actions are still undetermined. The purpose of this study was to learn how clobazam and its active metabolite modulate gamma-aminobutyric acid (GABA)-activated currents in rat cerebral neurons in culture.

METHODS

Whole-cell voltage-clamp recordings were performed on cultured cerebral neurons of the rat. Clobazam or its metabolite N-desmethylclobazam was dissolved in the extracellular solution and applied for 2 s by pressure ejection from a micropipette. To maintain GABA-activated currents, 2 mM Mg adenosine triphosphate (ATP) was added to the intracellular solution.

RESULTS

GABA elicited outward currents that were mediated by GABAA receptor-coupled Cl- channels. Applying clobazam with 10 microM GABA elicited enhanced outward currents. Flumazenil, an antagonist of the benzodiazepine receptor, inhibited the enhancing effect of clobazam. The enhancement ratio increased as much as 2.28-fold in a dose-dependent manner at a concentration of 3 microM clobazam. However, it started to decrease at a concentration of 10 microM clobazam. The metabolite N-desmethylclobazam was tested in the same manner, and exhibited an identical dose-dependent enhancement of GABA-activated currents.

CONCLUSIONS

The antiepileptic effects of the 1,5-benzodiazepines are attributed to the enhancement of GABAergic inhibitory neurotransmission. The antiepileptic effects of clobazam are thought to depend mainly on its active metabolite N-desmethylclobazam, which is present in high concentrations in patients who receive long-term clobazam. Clobazam's enhancement of GABA-activated currents was most marked on weaker GABA currents. We therefore infer that clobazam acts more efficiently on tissues in which the release of GABA is diminished.

Allosteric modulation of GABAA receptors in acutely dissociated neurons of the suprachiasmatic nucleus

The gamma-aminobutyric acid (GABA)-induced response was investigated in acutely dissociated suprachiasmatic nucleus (SCN) neurons of 11- to 14-day-old rats, under the voltage-clamp condition of nystatin-perforated patch recording. At a holding potential of -40 mV, application of GABA induced inward currents in a concentration-dependent manner. Pentobarbital and 5 beta-pregnan-3 alpha-ol-20-one (pregnanolone) similarly induced inward currents. GABA-induced inward currents were suppressed in a concentration-dependent manner by pretreating neurons with a GABAA receptor antagonist, bicuculline. Bicuculline (3 x 10(-6) M) shifted the concentration-response curve of GABA to the left in a competitive manner. Reversal potential of the GABA response (EGABA) was -3.4 +/- 0.7 mV, close to the theoretical Cl- equilibrium potential of -4.1 mV. Pretreating SCN neurons with diazepam, pentobarbital, and pregnanolone enhanced the 3 x 10(-6) M GABA response. Diazepam (3 x 10(-8) M), pentobarbital (3 x 10(-5) M), and pregnanolone (10(-7) M) shifted the concentration-response curve of GABA to the left without changing the maximal amplitude of GABA responses. EGABA in the presence of diazepam, pentobarbital, or pregnanolone was the same as that in their absence. These results show that the GABA response in acutely dissociated SCN neurons is mediated by the GABAA receptor. Because the GABAA receptor of SCN neurons is allosterically augmented by diazepam, pentobarbital, and pregnanolone, similarly as in other regions of the central nervous system, the present study opens up ways to functionally modulate the GABAA receptors in SCN.

Contribution of "diazepam-insensitive" GABAA receptors to the alcohol antagonist properties of Ro 15-4513 and related imidazobenzodiazepines

Both in vivo and in vitro studies have shown that Ro 15-4513 can antagonize many of the pharmacologic actions of ethanol. In contrast to many benzodiazepine receptor (BzR) ligands, Ro 15-4513 binds with high affinity to a novel GABAA receptor subtype, referred to as "diazepam-insensitive" (DI). This study examined the contribution of DI GABAA receptors to the modulation of ethanol-induced sleep time by Ro 15-4513 and related imidazobenzodiazepines [e.g., Ro 19-4603, Ro 16-6028, and ZG-63 (t-butyl-8-chloro-5,6-dihydro-5-methyl-6-oxo-imidazo[1,5,a] [1,4]benzodiazepine-3-carboxylate)] that possess high affinities for this GABAA receptor subtype. Ro 15-4513 (0.6-5 mg/kg) significantly reduced ethanol (3.5 g/kg, i.p.) sleep time in mice (p < 0.001, analysis of variance). This effect was not blocked by BzR antagonists ZK 93426 (5 mg/kg) and Ro 14-7437 (5 mg/kg), which possess low affinities for DI but bind with high affinities to other "diazepam-sensitive" (DS) GABAA receptor isoforms. Although Ro 19-4603 (2.5 mg/kg) also reduced ethanol sleep time (p < 0.01), this effect was attenuated by coadministration of ZK 93426 (2.5 mg/kg). Ro 16-6028 (2.5 mg/kg) prolonged (p < 0.01) ethanol sleep time. However, in the presence of either Ro 19-7437 (5 mg/kg) or ZK 93426 (2.5 mg/kg) ethanol-induced sleep time was reduced to values approaching those obtained with ethanol in the presence of Ro 15-4513. A low dose (2.5 mg/kg) of ZG-63 did not significantly affect alcohol sleep time. However, in the presence of ZK 93426, ZG-63 increased sleep time (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Tolerance to the effects of diazepam, clonazepam and bretazenil on GABA-stimulated Cl- influx in flurazepam tolerant rats

The effect of chronic flurazepam treatment on the GABA (gamma-aminobutyric acid) receptor/chloride channel complex was studied using GABA-stimulated 36Cl- influx into brain microsacs, and its potentiation by diazepam, clonazepam and bretazenil. Rats were given flurazepam for 1 week, then microsacs were prepared from cerebral cortices of rats that were still receiving flurazepam, and from those that had stopped treatment 48 h earlier. Diazepam and clonazepam produced concentration-dependent increases in GABA-stimulated 36Cl- influx while bretazenil produced a much smaller effect, which did not reach statistical significance in the tissue from control rats. There was no significant change in the basal or 10 microM GABA-stimulated 36Cl- influx between control and treated groups. Tolerance was shown by a significantly reduced effect of diazepam and clonazepam to enhance GABA-stimulated 36Cl- influx in the tissue prepared from non-withdrawn rats. However, for both diazepam and clonazepam, there was no tolerance 48 h after chronic treatment. The results suggest that changes in the GABA receptor/Cl- channel complex on cerebral cortical neurons contribute to cross-tolerance from flurazepam to other benzodiazepines.

A comparison of Ro 16-6028 with benzodiazepine receptor 'full agonists' on GABAA receptor function

Ro 16-6028 (bretazenil) has a pharmacological profile characteristic of a partial agonist at the gamma-aminobutyric acidA (GABAA) receptor-linked benzodiazepine site. The present study utilized modulation of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding and enhancement of GABA-stimulated 36Cl- uptake to further assess Ro 16-6028's partial agonist profile in vitro. Ro 16-6028 was the most potent benzodiazepine examined, exhibiting an IC50 (concentration at which half-maximal inhibition of specific [35S]TBPS binding occurs) of 6.1 nM, compared to clonazepam (7.9 nM), flunitrazepam (13.6 nM) and diazepam (91.1 nM). The rank order of potency for inhibition of [35S]TBPS binding was identical to that for inhibition of [3H]flunitrazepam binding. However, Ro 16-6028 was less efficacious in that it produced 27% inhibition of specific [35S]TBPS binding, compared to clonazepam (34%), flunitrazepam (41%) or diazepam (49%). Ro 16-6028 antagonized the inhibition of [35S]TBPS binding produced by 10 microM diazepam. Ro 16-6028 was also more potent and less efficacious than diazepam in potentiating GABA-stimulated 36Cl- uptake. These results provide further evidence that Ro 16-6028 is acting as a partial agonist at the benzodiazepine receptor in modulating function of the GABAA receptor complex.

Differential properties of type I and type II benzodiazepine receptors in mammalian CNS neurones

1. The effects of benzodiazepine receptor (BZR) partial agonists, Y-23684 and CL218,872, were compared with its full agonist, diazepam, on gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) in acutely dissociated rat cerebral cortex (CTX), cerebellar Purkinje (CPJ) and spinal ventral horn (SVH) neurones, by the whole-cell mode patch-clamp technique. 2. The GABA-induced responses were essentially the same in both SVH and CPJ neurones, but the KD value of the GABA response in CTX neurone was lower than those in the other two brain regions. 3. Enhancement of the GABA response by the two partial agonists was about one-third of that by diazepam in the SVH neurones (where type II subtype of BZR, BZ2, is predominant), whereas these partial agonists potentiated the GABA response as much as diazepam in CPJ neurones (where the type I subtype of BZR, BZ1, is predominant). In CTX neurones where both type I and II variants are expressed, the augmentation ratio of the GABA response by diazepam was between the values in CPJ and SVH neurones. 4. In concentration-response relationships of BZR partial agonists, the threshold concentrations, KD values and maximal augmentation ratio of the GABA response were similar in all CTX, CPJ and SVH neurones. Also, in all preparations, the threshold concentration and KD values of diazepam action were 10 fold less than those induced by partial agonists. 5. All BZR agonists shifted the concentration-response relationship for GABA to the left without changing the maximum current amplitude, indicating that activation of both BZ1 and BZ2 increase the affinity of the GABAA receptor for GABA. 6. The results are important in clarifying the mechanism of anxiety and might explain the anxioselectivity of BZR partial agonists.

Ammonia potentiates GABAA response in dissociated rat cortical neurons

The ammonium ion (NH4+) evoked no response itself but facilitated the GABA-induced Cl- current (ICl) in dissociated rat cortical neurons. The GABA concentration-response curve shifted parallely to the left without changing the maximum response. Reversal potential of GABA response did not change in the presence of NH4+ at the concentration range between 0.1 and 1 mM. Ro 15-1788, a selective benzodiazepine receptor antagonist, did not affect the facilitatory action of NH4+ on GABAA receptor. The results suggest that NH4+ modifies the affinity of GABAA receptor for GABA, without the involvement of benzodiazepine receptors.

Pharmacological and Electrophysiological Properties of Recombinant GABAA Receptors Comprising the alpha3, beta1 and gamma2 Subunits

To assess the role of subunits for channel function and drug modulation in recombinant GABAA receptors, the alpha3beta1gamma2 subunits and the dual combinations alpha3beta1, beta1gamma2 and alpha3gamma2 were expressed by transfection of human embryonic kidney cells and by RNA injection in Xenopus oocytes (alpha3beta1gamma2 combination). GABA-induced chloride currents were recorded using the whole-cell configuration of the patch-clamp technique (transfected cells) or the voltage-clamp technique (oocytes). The currents recorded from the alpha3beta1gamma2 subunit combination in transfected cells were reduced by bicuculline and picrotoxin, enhanced by flunitrazepam in a flumazenil-sensitive manner and reduced by beta-carboline-3-carboxylic acid methyl ester (beta-CCM). The GABA-induced current was reduced by beta-CCM in all combinations containing the gamma2 subunit, but potentiation by flunitrazepam was only obtained when the gamma2 subunit was coexpressed in the presence of the alpha3 subunit (alpha3beta1gamma2 or alpha3gamma2). The GABA sensitivities of the receptors were similar when the alpha3beta1gamma2 combination was expressed in oocytes (half-maximum effective concentration=240 microM) or in the kidney cell line (270 microM). However, the currents were less potentiated by flunitrazepam in oocytes (129% of controls) than in transfected cells (189%). These results suggest that the alpha3beta1gamma2 subunit combination, which is coexpressed in various brain regions as shown by in situ hybridization histochemistry, may represent a building block of functional GABAA receptors in situ.

Electroencephalogram effect measures and relationships between pharmacokinetics and pharmacodynamics of centrally acting drugs

Electroencephalogram (EEG) effect parameters may be useful in pharmacokinetic-pharmacodynamic modelling studies of drug effects on the central nervous system (CNS). Effect parameters derived from a quantitative analysis of the EEG appear to be perfectly suited to characterise the relationships between pharmacokinetics and pharmacodynamics of benzodiazepines and intravenous anaesthetics. EEG parameters represent many of the characteristics of ideal pharmacodynamic measures, being continuous, objective, sensitive and reproducible. These features provide the opportunity to derive concentration-effect relationships for these drugs in individuals, which yield important quantitative information on the potency and intrinsic efficacy of these drugs. The EEG techniques presented can be used to study the influences of factors such as age, disease, chronic drug use and drug interactions on the concentration-effect relationships of psychotropic drugs. An important issue is the choice of the EEG parameter to characterise the CNS effects of the compounds. More attention must be paid to evaluating the relevance of EEG parameters to the pharmacological effects of the drugs. Knowledge of the relationship between EEG effect parameters and clinical effects of drugs under different physiological and pathophysiological conditions is crucial to determining the value of EEG parameters in drug effect monitoring. Pharmacodynamic parameters derived from the concentration-EEG effect relationship may be correlated to pharmacodynamic parameters obtained from other in vitro and in vivo effect measurements. These comparisons revealed that changes in the amplitudes in the beta frequency band of EEG signals is a relevant measure of pharmacological effect intensity of benzodiazepines, which reflects their affinity and intrinsic efficacy at the central gamma-aminobutyric acid (GABA) benzodiazepine receptor complex. The exact EEG correlates of the anxiolytic, anticonvulsant, sedative and hypnotic actions of benzodiazepines have not yet clearly been elucidated. For intravenous anaesthetics, close correlations between the potency determined with EEG measurements and clinical measures of anaesthetic depth have been established, suggesting that, in principle, EEG parameters can adequately reflect depth of anaesthesia. However, more study is required to further substantiate these findings.

Differences in intrinsic efficacy of benzodiazepines are reflected in their concentration-EEG effect relationship

1. The relevance of EEG effect parameters as a measure of the central nervous system effects of benzodiazepines was evaluated. The concentration-EEG effect relationships of the benzodiazepine agonist midazolam, partial agonist bretazenil, antagonist flumazenil and inverse agonist Ro 19-4603 were quantified and compared with the intrinsic efficacy and affinity of these compounds at the gamma-aminobutyric acid (GABA)-benzodiazepine receptor complex. 2. The pharmacokinetics and pharmacodynamics of the compounds were determined after a single intravenous bolus administration of 5 mg kg-1 midazolam, 2.5 mg kg-1 bretazenil, 10 mg kg-1 flumazenil or 2.5 mg kg-1 Ro 19-4603 to male Wistar derived rats. In a separate experiment the distribution between blood, cerebrospinal fluid and brain concentrations of these compounds was determined. A sensitive assay was developed to measure bretazenil and Ro 19-4603 concentrations in small samples of biological fluids. 3. The benzodiazepine-induced changes in amplitudes in the 11.5-30 Hz frequency band, as determined by aperiodic analysis, was used as EEG effect measure. Concentration-EEG effect relationships were derived by a pharmacokinetic-pharmacodynamic modelling procedure and in the case of midazolam, bretazenil and Ro 19-4603 successfully quantified by the sigmoidal Emax model. Large differences in maximal effect of midazolam (Emax = 73 +/- 2 microVs-1), bretazenil (Emax = 19 +/- 1 microVs-1) and Ro 19-4603 (Emax = -6.5 +/- 0.4 microVs-1) were observed, reflecting their differences in intrinsic efficacy. A close correlation was found between the EC50 values based on free drug concentration and receptor affinity as determined by displacement of [3H]-flumazenil in a washed brain homogenate at 37 degrees C. In the concentration range of receptor saturation flumazenil did not produce any changes in the EEG effect measure.4. The study demonstrated that the change in amplitudes in the 11.5-30 Hz frequency band of the EEG is a relevant measure of the pharmacological effect intensity of benzodiazepines, because it seems to reflect their affinity and intrinsic efficacy at the central GABA-benzodiazepine receptor complex.

Molecular mechanisms of the partial allosteric modulatory effects of bretazenil at gamma-aminobutyric acid type A receptor

In central nervous system gamma-aminobutyric acid (GABA) inhibits neuronal activity by acting on GABA type A (GABAA) receptors. These heterooligomeric integral membrane proteins include a GABA-gated Cl- channel and various allosteric modulatory sites where endogenous modulators and anxiolytic drugs act to regulate GABA action. In vivo, various anxiolytic drugs exhibit a wide range of variability in their modulatory efficacy and potency of GABA action. For instance, bretazenil modulatory efficacy is much lower than that of diazepam. Such low efficacy could be due either to a preferential modulation of specific GABAA receptor subtypes or to a low modulatory efficacy at every GABAA receptor subtype. To address these questions we studied drug-induced modifications of GABA-activated Cl- currents in native GABAA receptors of cortical neurons in primary cultures and in recombinant GABAA receptors transiently expressed in transformed human embryonic kidney cells (293) after transfection with cDNAs encoding different molecular forms of alpha, beta, and gamma subunits of GABAA receptors. In cortical neurons the efficacy of bretazenil was lower than that of diazepam, whereas the potency of the two drugs was similar. In cells transfected with gamma 2 subunits and various molecular forms of alpha and beta subunits bretazenil efficacy was always lower than that of diazepam. However, in cells transfected with gamma 1 or gamma 3 subunits and various forms of alpha and beta subunits the efficacy of both diazepam and bretazenil was lower and always of similar magnitude. When bretazenil and diazepam were applied together to GABAA receptors including a gamma 2 subunit, the action of diazepam was curtailed in a manner related to the dose of bretazenil.

Non-competitive inhibition of GABAA responses by a new class of quinolones and non-steroidal anti-inflammatories in dissociated frog sensory neurones

1. The interaction of a new class of quinolone antimicrobials (new quinolones) and non-steroidal anti-inflammatory agents (NSAIDs) with the GABAA receptor-Cl- channel complex was investigated in frog sensory neurones by use of the internal perfusion and 'concentration clamp' techniques. 2. The new quinolones and the NSAIDs (both 10(-6)-10(-5) M) had little effect on the GABA-induced chloride current (ICI) when applied separately. At a concentration of 10(-4) M the new quinolones, and to a lesser degree the NSAIDs, produced some suppression of the GABA response. 3. The co-administration of new quinolones and some NSAIDs (10(-6)-10(-14) M) resulted in a marked suppression of the GABA response. The size of this inhibition was dependent on the concentration of either the new quinolone or the NSAID tested. The inhibitory potency of new quinolones in combination with 4-biphenylacetic acid (BPAA) was in rank order norfloxacin (NFLX) much greater than enoxacin (ENX) greater than ciprofloxancin (CPFX) much greater than ofloxacin (OFLX), and that of NSAIDs in combination with ENX was BPAA much greater than indomethacin = ketoprofen greater than naproxen greater than ibuprofen greater than pranoprofen. Diclofenac, piroxicam and acetaminophen did not affect GABA responses in the presence of ENX. 4. In the presence of ENX or BPAA, there was a small shift to the right of the concentration-response curve for GABA without any effect on the maximum response. However, the co-administration of these drugs suppressed the maximum of the GABA concentration-response curve, indicating a non-competitive inhibition, for which no voltage-dependency was observed.5. Simultaneous administration of ENX and BPAA also suppressed pentobarbitone (PB)-gated Icl. On the other hand, both PB and phenobarbitone reversed the inhibition of GABA-induced Ic, by coadministration of ENX and BPAA.6. The effect on GABAA responses of co-administration of new quinolones and NSAIDs was not via an interaction with benzodiazepine receptors coupled to the GABAA receptor, since this effect was not reversed by Rol5-1788 or diazepam.7. It is concluded that the co-administration of new quinolones and some of the NSAIDs inhibit GABAergic transmission, and could result in convulsions.

Action of diazepam on the voltage-dependent Na+ current. Comparison with the effects of phenytoin, carbamazepine, lidocaine and flumazenil

The influence of diazepam, an agonist, and flumazenil (Ro 15-1788), an antagonist of the benzodiazepine receptor, on repetitive firing of action potentials in cultured spinal neurons and on voltage-dependent Na+ currents in cultured N2A neuroblastoma cells was examined. The effects were compared to those of the antiepileptics phenytoin and carbamazepine and the local anesthetic lidocaine. The whole-cell configuration of the patch-clamp technique was used for potential and current recording. Diazepam (10 microM) or phenytoin (10 microM) reduced the duration of repetitive action potential discharges in 50 or 67% of the spinal neurons, respectively. At a concentration of 100 microM repetitive firing was completely blocked. Flumazenil (100 microM) had no effect. In N2A neuroblastoma cells diazepam, phenytoin, carbamazepine and lidocaine, but not flumazenil, at a concentration of 100 microM reduced the Na+ current to 60-67% of control. At 10 microM no or only a weak depression was seen with any drug. In the presence of diazepam (100 microM) the Na+ channel inactivation curve was shifted in the hyperpolarizing direction by -4.8 +/- 0.5 mV. Phenytoin, carbamazepine and lidocaine (all 100 microM) caused stronger shifts of -17.4 +/- 2.1, -10.6 +/- 0.9 and -17.0 +/- 2.1 mV, respectively. Inhibition of the Na+ current by diazepam increased use-dependently over 9 depolarizing pulses repeated at high frequency (200 Hz), whereas use-dependent effects of the other compounds developed less rapidly. At a low stimulation rate (7 Hz) use-dependent block was pronounced with lidocaine, but weak or absent with diazepam and carbamazepine.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel anxiolytics that act as partial agonists at benzodiazepine receptors

Benzodiazepines in clinical use have a range of pharmacological activities. Some, e.g. sedation, tolerance and addiction, are not welcome. Undesirable side-effects of drugs are often controlled by developing compounds that bind more selectively to one particular receptor subtype. An alternative approach, discussed here by Willy Haefely and colleagues, is the development of partial agonists which exploit regional differences in receptor reserve to tease apart biological responses. Partial agonists for the benzodiazepine modulatory site on the GABAA complex have been developed and their pharmacological profiles can be interpreted to suggest that neurons mediating anticonvulsant and anti-anxiety effects do indeed have a higher receptor reserve than neurons mediating other unwanted effects. This suggests that benzodiazepine receptor partial agonists may have important therapeutic potential.