Etomidate stereospecifically stimulates forebrain, but not cerebellar, 3H-diazepam binding

Distinct structural requirements for the direct and indirect actions of the anaesthetic etomidate at GABA(A) receptors

1. The intravenous anaesthetic etomidate augments GABA-gated chloride currents (indirect action) and, at higher concentrations, evokes chloride currents in the absence of GABA (direct action). 2. In order to identify amino acid residues essential for these actions, site directed mutagenesis was performed on the beta3 subunit. 3. Mutation of an asparagine to a serine residue at position 290 dramatically reduced both etomidate-induced chloride currents and its ability to enhance [3H]flunitrazepam binding in HEK293 cells expressing alpha1beta3gamma2 recombinant GABA(A) receptors. 4. In contrast, the indirect effect of etomidate was retained, though its potency was reduced. 5. These findings indicate that there are distinct requirements for these dual actions of etomidate at GABA(A) receptors.

Mechanisms of etomidate potentiation of GABAA receptor-gated currents in cultured postnatal hippocampal neurons

The effect of etomidate, an imidazole general anesthetic, on GABAA receptor function was studied in cultured hippocampal neurons. At a clinically relevant concentration of 4.1 microM, etomidate shifts the GABA dose response to the left (ED50 shift from 10.2 to 5.2 microM), with no change in the maximum current evoked by saturating concentrations of GABA. At a higher concentration of 82 microM, etomidate directly induces current in the absence of GABA. Etomidate selectively increases the amplitude and prolongs the duration of miniature inhibitory postsynaptic currents without significant effects on miniature inhibitory postsynaptic currents. The combined effects of etomidate on miniature inhibitory postsynaptic current amplitude and duration enhance the total charge transfer by 280% during a spontaneous synaptic event. Analysis of single channels opened by GABA indicates that 8.2 microM etomidate increases the probability of channels being open 13-fold and increases the effective channel open time two-fold. Given the present understanding of central inhibitory synapses, the effect of etomidate on channel kinetics is most likely to be the predominant mechanism which influences the synaptic function. In addition, etomidate, through its modulation of both channel kinetics and open probability, is likely to have a large impact on extrasynaptic GABAA receptor function.

Interaction of positive allosteric modulators with human and Drosophila recombinant GABA receptors expressed in Xenopus laevis oocytes

1. A comparative study of the actions of structurally diverse allosteric modulators on mammalian (human alpha 3 beta 2 gamma 2L) or invertebrate (Drosophila melanogaster Rdl or a splice variant of Rdl) recombinant GABA receptors has been made using the Xenopus laevis oocyte expression system and the two electrode voltage-clamp technique. 2. Oocytes preinjected with the appropriate cRNAs responded to bath applied GABA with a concentration-dependent inward current. EC50 values of 102 +/- 18 microM; 152 +/- 10 microM and 9.8 +/- 1.7 microM were determined for human alpha 3, beta 1 gamma 2L, Rdl splice variant and the Rdl receptors respectively. 3. Pentobarbitone enhanced GABA-evoked currents mediated by either the mammalian or invertebrate receptors. Utilizing the appropriate GABA EC10, the EC50 for potentiation was estimated to be 45 +/- 1 microM, 312 +/- 8 microM and 837 +/- 25 microM for human alpha 3, beta 1 gamma 2L, Rdl splice variant and Rdl receptors respectively. Maximal enhancement (expressed relative to the current induced by the EC10 concentration of GABA where this latter response = 1) at the mammalian receptor (10.2 +/- 1 fold) was greater that at either the Rdl splice variant (5.5 +/- 1.3 fold) or Rdl (7.9 +/- 0.8 fold) receptors. 4. Pentobarbitone directly activated the human alpha 3 beta 1 gamma 2L receptor with an EC50 of 1.2 +/- 0.03 mM and had a maximal effect amounting to 3.3 +/- 0.4 fold of the response evoked by the EC10 concentration of GABA. Currents evoked by pentobarbitone were blocked by 10-30 microM picrotoxin and potentiated by 0.3 microM flunitrazepam. Pentobarbitone did not directly activate the invertebrate GABA receptors. 5. 5 alpha-Pregnan-3 alpha-ol-20-one potentiated GABA-evoked currents mediated by the human alpha 3 beta 1 gamma 2L receptor with an EC50 of 87 +/- 3 nM and a maximal enhancement of 6.7 +/- 0.8 fold of that produced by the GABA EC10 concentration. By contrast, relatively high concentrations (3-10 microM) of this steroid had only a modest effect on the Rdl receptor and its splice variant. 6. A small direct effect of 5 alpha-pregnan-3 alpha-ol-20-one (0.3-10 microM) was detected for the human alpha 3 beta 1 gamma 2L receptor (maximal effect only 0.08 +/- 0.01 times that of the GABA EC10). This response was antagonized by 30 microM picrotoxin and enhanced by flunitrazepam (0.3 microM). 5 alpha-Pregnan-3 alpha-ol-20-one did not directly activate the invertebrate GABA receptors. 7. Propofol enhanced GABA-evoked currents mediated by human alpha 3 beta 1 gamma 2L and Rdl splice variant receptors with EC50 values of 3.5 +/- 0.1 microM and 8 +/- 0.3 microM respectively. The maximal enhancement was similar at the two receptor types (human 11 +/- 1.8 fold; invertebrate 8.8 +/- 1.4 fold that of the GABA EC10). 8. Propofol directly activated the human alpha 3 beta 1 gamma 2L receptor with an EC50 of 129 +/- 10 microM, and at a maximally effective concentration, evoked a current amounting to 3.5 +/- 0.5 times that elicited by a concentration of GABA producing 10% of the maximal response. The response to propofol was blocked by 10-30 microM picrotoxin and enhanced by flunitrazepam (0.3 microM). Propofol did not directly activate the invertebrate Rdl splice variant receptor. 9. GABA-evoked currents mediated by the human alpha 3 beta 1 gamma 2L receptor were potentiated by etomidate (EC50 = 7.7 +/- 0.2 microM) and maximally enhanced to 8 +/- 0.8 fold of the response to an EC10 concentration of GABA. By contrast, the Rdl, or Rdl splice variant forms of the invertebrate GABA receptor were insensitive to the positive allosteric modulating actions of etomidate. Neither the mammalian nor the invertebrate receptors, were directly activated by etomidate. 10. delta-Hexachlorocyclohexane enhanced GABA-evoked currents with EC50 values of 3.4 +/- 0.1 microM and 3.0 +/- 0.1 microM for the human alpha 3 beta 1 gamma 2L receptor and the Rdl splice variant receptor respectively. The maximal enhancement was 4.5

Etomidate potentiation of GABAA receptor gated current depends on the subunit composition

The role of the gamma 2 subunit in etomidate potentiation of GABAA receptor-gated chloride current was studied by whole cell patch clamp experiments on H293 cells expressing GABAA receptors. The GABAA receptor subunits alpha 1 beta 1 with or without the gamma 2 subunit expressed well, with an overall peak current of 157 +/- 42 pA/pF. At a clinically relevant concentration, etomidate potentiates the peak current induced by GABA equally well in receptors with or without the gamma 2 subunit. In contrast, the time course of current decay was prolonged only in receptors with the gamma 2 subunit. This gamma 2 subunit-dependent prolongation of the current time course was not blocked by the benzodiazepine receptor antagonist flumazenil. These results show that etomidate, an imidazole general anesthetic, interacts with the GABAA receptor in a gamma 2 subunit-dependent manner.

In vitro studies on the broad spectrum anticonvulsant loreclezole in the hippocampus

In hippocampal slices from guinea-pig a paired-pulse stimulation protocol was used to examine the effects of loreclezole, R-(+)-etomidate, phenobarbital and pentobarbital on orthodromic and antidromic GABAergic neuronal inhibition in the CA1 region. All four compounds increased orthodromic GABAergic inhibition, with R-(+)-etomidate and pentobarbital inducing a quantitatively larger effect than loreclezole and phenobarbital. Only R-(+)-etomidate and pentobarbital increased antidromic GABAergic inhibition. We propose that all four compounds are anticonvulsant by increasing feed-forward dendritic GABAergic inhibition, whilst only the sedative/hypnotic compounds (R-(+)-etomidate, pentobarbital) increase feedback recurrent GABAergic inhibition. Loreclezole was also shown to inhibit 'low Ca2+' and 'low Mg2+' epileptogenesis at similar concentrations to those active on inhibition. Thus loreclezole may possess other pharmacodynamic properties, beyond its ability to increase feed-forward GABAergic neuronal inhibition, which contribute to its antiepileptic action.

Field-potential assay of antiepileptic drugs in the hippocampal slice

The effects of nine clinically active antiepileptic drugs and the NMDA antagonist 2-amino-7-phosphonoheptanoic acid (2-APH) were examined in three models in the in vitro hippocampal slice. In the "low Mg2+" model, removal of Mg2+ from the perfusion fluid increased excitatory neurotransmission and led to epileptogenic field potentials. In the "low Ca2+" model, decrease of Ca2+ and increase of Mg2+ and K+ in the perfusion fluid induced spontaneous "bursts" in the absence of synaptic transmission. Paired-pulse stimulation was used to estimate the strength of recurrent inhibition in the "inhibition" model. The rank order of the potency of the compounds to antagonize the second epileptogenic population spike in the low Mg2+ model was 2-APH greater than pentobarbital greater than midazolam greater than phenytoin greater than carbamazepine greater than chlordiazepoxide greater than phenobarbital = flurazepam. Ethosuximide and valproate were inactive. In the low Ca2+ model, the rank order of the potency of the drugs to antagonize spontaneous epileptogenic bursts was phenytoin greater than carbamazepine greater than midazolam greater than pentobarbital greater than chlordiazepoxide greater than flurazepam greater than phenobarbital. 2-APH, ethosuximide, and valproate were inactive. Only pentobarbital was active in the inhibition model. These experiments demonstrate the potential of in vitro tests in the hippocampus to reveal profiles of anticonvulsant activity.

Heterogeneity of gamma-aminobutyric acid/benzodiazepine/beta-carboline receptor complex in rat spinal cord

The properties of muscimol, beta-carboline (BC), and benzodiazepine (BZD) binding to crude synaptic membranes were studied in the spinal cord and cerebellum of rats. In cerebellar membranes, the density of high-affinity [3H]muscimol and [3H]6,7-dimethoxy-4-ethyl-beta-carboline ([3H]BCCM) binding sites is almost identical to that of [3H]flunitrazepam ([3H]FLU) or [3H]flumazenil (Ro 15-1788; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a] [1-4]benzodiazepine-3-carboxylate). In contrast to the cerebellum, the number of muscimol and BC binding sites in rat spinal cord is approximately 20-25% of the number of FLU or flumazenil binding sites. Moreover, in spinal cord membranes, BC recognition site ligands displace [3H]-flumazenil bound to those sites, with low affinity and a Hill slope significantly less than 1; the potency of the different BCs in displacing [3H]flumazenil is 20-50-fold lower in the spinal cord than in the cerebellum. [3H]Flumazenil is not displaced from spinal cord membranes by the peripheral BZD ligand Ro 5-4864 (4'-chlorodiazepam), whereas it is displaced with low affinity and a Hill slope of less than 1 (nH = 0.4) by CL 218,872 (3-methyl-6-(3-trifluoromethylphenyl)-1,2,4-triazolol[4,3-b] pyridazine). These data suggest that a large number of BZD binding sites in spinal cord (approximately 80%) are of the central-type, BZD2 subclass, whereas the BZD binding sites in cerebellum are predominantly of the central-type, BZD1 subclass.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of flunitrazepam binding to intact primary cultured spinal cord neurons and its modulation by GABAergic drugs

The interaction of [3H]flunitrazepam and its modulation by various drugs was studied in intact primary cultured spinal cord neurons. In the intact cells, the [3H]-flunitrazepam binding was rapid and saturable. The benzodiazepine binding sites exhibited high affinity and saturability, with an apparent KD of 6.1 +/- 1.6 nM and Bmax of 822 +/- 194 fmol/mg protein. The association and dissociation of [3H]flunitrazepam binding exhibited monoexponential kinetics. Specifically bound [3H]flunitrazepam was displaced in a concentration-dependent manner by benzodiazepines like flunitrazepam, clonazepam, diazepam, Ro 15-1788, and beta-carbolines like methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3'-carboxylate. Specific [3H]flunitrazepam binding to intact cells was enhanced in a concentration-dependent manner by gamma-aminobutyric acid (GABA) agonists and drugs which facilitate GABAergic transmission like etazolate, (+)-etomidate, and pentobarbital. The enhancing effect of GABA agonists was antagonized by bicuculline and picrotoxinin. These results suggest that the intact cultured spinal cord neurons exhibit the properties of benzodiazepine GABA receptor-ionophore complex. Since these cells can also be studied in parallel for characterizing GABA-induced 36Cl-influx, they provide an ideal in vitro assay preparation to study GABA synaptic pharmacology.

Barbiturate and benzodiazepine modulation of GABA receptor binding and function

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) acts primarily on receptors that increase chloride permeability in postsynaptic neurons. These receptors are defined by sensitivity to the agonist muscimol and the antagonist bicuculline, and are also subject to indirect allosteric inhibition by picrotoxin-like convulsants and enhancement by the clinically important drugs, the benzodiazepines and the barbiturates. All of these drugs modulate GABA-receptor regulated chloride channels at the cellular level assayed by electrophysiological or radioactive ion tracer techniques. Specific receptor sites for GABA, benzodiazepines, picrotoxin/convulsants, and barbiturates can be assayed in vitro by radioactive ligand binding. Mutual chloride-dependent allosteric interactions between the four receptor sites indicate that they are all coupled in the same membrane macromolecular complex. Indirect effects of barbiturates on the other three binding sites define a pharmacologically specific, stereospecific receptor. All of the activities can be solubilized in the mild detergent 3-[(3-cholamidopropyl)-dimethylammonio]propane sulfonate (CHAPS) and co-purify as a single protein complex.

The gamma-aminobutyrate/benzodiazepine receptor from pig brain. Enhancement of gamma-aminobutyrate-receptor binding by the anaesthetic propanidid

The binding of [3H]muscimol, a gamma-aminobutyrate (GABA) receptor agonist, to a membrane preparation from pig cerebral cortex was enhanced by the anaesthetic propanidid in a concentration-dependent manner. At 0 degrees C, binding was stimulated to 220% of control values, with 50% stimulation at 60 microM-propanidid. At 37 degrees C, propanidid caused a more powerful stimulation of [3H]muscimol binding (340% of control values). Propanidid (1 mM) exerted little effect on the affinity of muscimol binding (KD approx. 10 nM), but increased the apparent number of high-affinity binding sites in the membrane by 2-fold. Enhancement of [3H]muscimol binding was observed only in the presence of Cl- ions, half-maximal activation being achieved at approx. 40 mM-Cl-. Picrotoxinin inhibited the stimulation of [3H]muscimol binding by propanidid with an IC50 (concentration causing 50% inhibition) value of approx. 25 microM. The enhancement of [3H]muscimol binding by propanidid was not additive with the enhancement produced by secobarbital. Phenobarbital inhibited the effect of propanidid and secobarbital. The GABA receptor was solubilized with Triton X-100 or with Chaps [3-[(3-cholamidopropyl)dimethylammonio]propanesulphonate]. Propanidid and secobarbital did not stimulate the binding of [3H]muscimol after solubilization with Triton X-100. However, the receptor could be solubilized by 5 mM-Chaps with retention of the stimulatory effects of propanidid and secobarbital. Unlike barbiturates, propanidid did not stimulate the binding of [3H]flunitrazepam to membranes. It is suggested that the ability to modulate the [3H]muscimol site of the GABA-receptor complex may be a common and perhaps functional characteristic of general anaesthetics.

Radiation inactivation of brain [35S]t-butylbicyclophosphorothionate binding sites reveals complicated molecular arrangements of the GABA/benzodiazepine receptor chloride channel complex

[35S]t-Butylbicyclophosphorothionate ([35S]TBPS), a bicyclic cage convulsant, binds to the anion gating mechanism of the GABA/benzodiazepine receptor chloride channel complex. Using a carefully calibrated radiation inactivation technique, the molecular weight of [35S]TBPS binding complexes from frozen rat cerebral cortex was estimated to be 137,000 daltons. The GABA agonist muscimol reduced [35S]TBPS binding to 0-10% of the control value, in a way which is independent of the radiation dose. This shows that the GABA receptor (Mw = 55,000 daltons) is included in the 137,000-dalton [35S]-TBPS binding complex; the [35S]TBPS binding protein alone accounts for 137,000-55,000 = 82,000 daltons. The pyrazolopyridazine etazolate (SQ 20.009) and etomidate in appropriate concentrations both reduced specific binding of [35S]TBPS. The ability of SQ 20.009 and etomidate to reduce [35S]TBPS binding was greatly reduced by exposure to low radiation doses, suggesting that SQ 20.009 and etomidate reduce [35S]TBPS binding by an allosteric mechanism requiring a molecular structure of 450,000-500,000 daltons. Benzodiazepine agonists (ethyl 4-methoxymethyl-6-benzyloxy-beta-carboline-3-carboxylate, ZK 93423) and inverse agonists (methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, DMCM) enhance and reduce [35S]TBPS binding, respectively, in repeatedly frozen and washed membrane preparations. The effects of ZK 93423 and DMCM on [35S]TBPS binding disappeared upon exposure of membranes to low radiation doses. This suggests that the benzodiazepine receptor site interacts allosterically with the [35S]TBPS binding site, requiring a molecular complex of at least c. 400,000 daltons. The [35S]TBPS site alone in these latter conditions of membrane preparation (repeatedly frozen/washed) revealed a molecular weight of 221,000 daltons (TBPS-site + GABA receptor + unknown structures). The number of binding sites for [35S]TBPS (145 pmol/g tissue) was only slightly higher than for [3H]flunitrazepam (130 pmol/g tissue) in cerebral cortex. These results are all consonant with the conclusion that the GABA/BZ receptor chloride channel complex is composed of highly integrated multimeric subunits, tentatively accounted for by a tetramic complex of molecular weight 548,000 daltons.

Specific 3H-DMCM binding to a non-benzodiazepine binding site after silver ion treatment of rat brain membranes

Specific binding of the BZ-receptor ligand 3H-DMCM to rat cortical membranes was dramatically enhanced by preincubation of the homogenate with 0.1 mM silver (Ag+) nitrate. The binding was completely inhibited by midazolam. Nevertheless, the pharmacological specificity of the Ag+-enhanced 3H-DMCM binding was different from that of BZ-receptors. Furthermore, the Bmax value, the regional distribution and the molecular target size determined by radiation inactivation analysis of the Ag+-enhanced binding site were different from those of BZ-receptors. The results indicate that Ag+-enhanced 3H-DMCM binding represent a high affinity metal complex formation between 3H-DMCM and an unknown brain specific protein of approximately 100,000 daltons molecular weight.

Barbiturates allosterically inhibit GABA antagonist and benzodiazepine inverse agonist binding

Barbiturates and the related depressant drugs, etazolate and etomidate, inhibited both the binding of [3H]bicuculline methochloride (BMC) to gamma-aminobutyric acid (GABA) receptor sites and the binding of [3H] beta-carboline-3-carboxylic acid methyl ester (beta CCM) to benzodiazepine receptor sites in mammalian brain. These concentration-dependent effects were chemically specific and stereospecific in a manner correlating with the activity of barbiturates to enhance GABA responses in neurons and to enhance GABA and benzodiazepine receptor agonist binding in vitro. The barbiturate inhibition of [3H]BMC binding involved a decrease in affinity which at high concentrations of barbiturates results in an effective complete loss of detectable binding. The maximal inhibition of [3H] beta CCM binding involved a more modest decrease in affinity (increase in KD from 1.35 to 1.85 nM). The barbiturate inhibitions of both ligands could be reversed by picrotoxin, suggesting an indirect action at previously defined picrotoxin/barbiturate modulatory sites on the GABA-benzodiazepine receptor/chloride ion channel complex.

Differential interactions of GABA agonists, depressant and convulsant drugs with [35S]-t-butylbicyclophosphorothionate binding sites in cortex and cerebellum

Effects of three GABA agonists, four GABA antagonists and convulsants (picrotoxinin, alpha-dihydropicrotoxinin [DHP], pentamethylenetetrazole [PTZ] and isopropylbicyclophosphate ester) and three depressant drugs (pentobarbital, (+)etomidate and etazolate) were investigated on [35S]t-butylbicyclophosphorothionate (TBPT) in cortex and cerebellum. All the convulsants tested were equipotent in inhibiting [35S]TBPT binding in cortex and cerebellum. Convulsants like picrotoxinin inhibited [35S]-TBPT binding competitively in both cortex and cerebellum. In contrast, gamma-aminobutyric acid (GABA) agonists (muscimol, GABA and 4,5,6,7-tetrahydroisoxazol[5,4-C]pyridine-3-ol [THIP]), and depressants like etazolate, (+)etomidate and pentobarbital were more potent inhibitors of [35S]TBPT binding in cerebellum than in cortex. GABA inhibition of [35S]TBPT binding appears to be mediated through a low-affinity site. GABA and pentobarbital inhibited [35S]TBPT binding in cortex and cerebellum noncompetitively. Depressants like pentobarbital appear to interact with the TBPT sites allosterically. These results suggest that depressant and convulsant drugs that modulate GABAergic transmission interact differently with the TBPT binding sites in cortex and cerebellum.

gamma-Aminobutyric acid activation of 36Cl- flux in rat hippocampal slices and its potentiation by barbiturates

gamma-Aminobutyric acid (GABA) increases the rate of 36Cl- efflux from preloaded rat hippocampal slices in a dose-dependent manner (EC50: 400 microM). This action has the pharmacological specificity expected of activation of GABA receptors in that it is mimicked by the agonists muscimol and 3-aminopropanesulfonic acid, and blocked by the antagonists bicuculline and picrotoxinin. GABA uptake inhibitors, nipecotic acid and 2,4-diaminobutyric acid, fail to increase 36Cl- flux. Pentobarbital produces a dose-dependent activation (EC50 = 1.5 mM) of 36Cl- efflux with maximal response greater than that of GABA. The effect of pentobarbital can be mimicked by 1,3-dimethylbutylbarbiturate, secobarbital, (+)hexobarbital but not (-)hexobarbital, and is blocked by bicuculline and picrotoxinin. Pentobarbital and the other active barbiturates also potentiate the action of GABA. Phenobarbital does not have any effect independently or in combination with GABA. It is suggested that GABA increases 36Cl- permeability by activation of a postsynaptic receptor which is in turn functionally coupled to a barbiturate receptor.

Binding characteristics and interactions of depressant drugs with [35S]t-butylbicyclophosphorothionate, a ligand that binds to the picrotoxinin site

[35S]t-Butylbicyclophosphorothionate (TBPT), a cage convulsant with picrotoxinin-like activity, binds to rat brain membranes to a single site with an apparent KD of 25.1 +/- 5.6 nM and a Bmax of 1.40 +/- 0.22 pmol/mg protein. TBPT binding to rat brain membranes was inhibited by a variety of convulsant, depressant, anxiolytic, and anticonvulsant drugs that had previously been shown to inhibit [3H]alpha-dihydropicrotoxinin binding. Depressant drugs such as pentobarbital and the nonbarbiturate (+)etomidate inhibited TBPT binding in an uncompetitive manner. Thus, pentobarbital and (+)etomidate decreased both the affinity and the number of binding sites of TBPT to whole brain membranes. The IC50 values of (+)etomidate (9 microM) and pentobarbital (90 microM) are similar to the EC50 values at which they enhance both [3H]gamma-aminobutyric acid and [3H]diazepam binding in cerebral cortex membranes. RO5-4864, which has recently been shown to be a convulsant, also inhibited TBPT binding (IC50 = 10 microM). These results suggest that TBPT binds to the picrotoxinin site and further supports the notion that the picrotoxinin site is an important modulatory site at the benzodiazepine-GABA receptor-ionophore complex.

Convulsant/depressant site of action at the allosteric benzodiazepine-GABA receptor-ionophore complex

Several classes of centrally acting convulsant, depressant, anticonvulsant and anxiolytic drugs modulate GABAergic transmission. The postsynaptic receptor with which these drugs interact is an allosteric complex with distinct binding sites for GABA, benzodiazepines, picrotoxinin and related compounds. Convulsants which inhibit GABA transmission (except bicuculline) inhibit competitively the binding of dihydropicrotoxinin (DHP) or t-butylbicyclophosphorothionate (TBPT) to the picrotoxinin site and prevent the allosteric enhancing effect of depressant drugs on GABA and benzodiazepine binding. Depressant drugs give a mixed inhibition of TBPT binding. The possible topography of the picrotoxinin site and its relationship to convulsant/depressant drug action at the benzodiazepine-GABA receptor-ionophore complex is discussed.

Comparison of the distribution of convulsant/barbiturate and benzodiazepine receptors using light microscopic autoradiography

Some convulsant drugs elicit CNS excitation by blocking neuronal activity at GABAergic synapses whereas depressant compounds may result in the enhancement of GABAergic transmission. These effects are thought to involve drug actions at a multireceptor complex involving a benzodiazepine receptor, GABA receptor, picrotoxin receptor and a chloride ionophore. A radiolabeled convulsant, [35S]t-butylbicyclophosphorothionate [( 35S]-TBT) has been developed and used to characterize the binding to the "picrotoxin" or convulsant/barbiturate site. The microscopic distribution of the convulsant/barbiturate sites are reported in this communication, as demonstrated by receptor autoradiography after labeling tissue sections with [35S]-TBT. Comparison of the distribution of these sites with those of the benzodiazepine receptors show a close regional correlation in many areas. The convulsant/barbiturate sites and the benzodiazepine receptors, however, are unevenly distributed in the rat cerebellum and exist in separate lamina.

Benzodiazepine receptor multiplicity

Receptors for gamma-aminobutyric acid (GABA) are phylogenetically old and extensive GABA receptor multiplicity had already evolved in invertebrate species. High affinity, "brain specific", benzodiazepine (BZ) receptors, present in the central nervous systems of virtually all vertebrate species, represent a heterogeneous sub-class of GABA receptors. Functional GABA-BZ-ion receptor complexes are aggregates consisting of different kinds of sub-units which are probably coded for by separate genes. These sub-units may be combined in different ways to yield different benzodiazepine receptor complexes. Different GABA-BZ-ion receptor complexes probably subserve different physiological functions and more selective drugs modifying these functions will probably be found.

Benzodiazepine-GABA receptor-ionophore complex. Current concepts

The benzodiazepine--gamma-aminobutyric acid (GABA) receptor--ionophore system is an oligomeric complex, composed of at least three interacting components. These three components have been well characterized in vitro by radioreceptor binding assays. A variety of centrally acting anxiolytic, depressant, anticonvulsant and convulsant drugs, which affect GABAergic transmission, bind to one of the sites and modulate the binding of ligands at the other sites. Thus, depressant barbiturates, nonbarbiturate hypnotics (like etomidate) and pyrazolopyridines (like etazolate), while inhibiting the binding of alpha-dihydropicrotoxinin (DHP), enhance the binding of GABA and benzodiazepines. These enhancing effects are blocked by convulsant drugs that inhibit the binding of dihydropicrotoxinin and also by bicuculline. These interactions involving barbiturates and other modulatory drugs, exhibit stereoselectivity, anion dependence and brain regional selectivity. Several classes of drugs which facilitate GABAergic transmission appear to interact with the sites for GABA and benzodiazepines allosterically via the dihydropicrotoxinin site of the oligomeric complex. The GABA system has also been implicated in a variety of pathological conditions, including anxiety, seizure activity, movement disorders, cardiovascular control, pain and in drug dependence. Since most of the GABA agonists do not pass the blood-brain barrier, future trends in the pharmacology of GABA may be the development of drugs that will activate the GABA receptor system via picrotoxinin or benzodiazepine sites.

Diazepam, pentobarbital and D-etomidate produced increases in bicuculline seizure threshold; selective antagonism by RO15-1788, picrotoxin and (+/-)-DMBB

The seizure threshold for different seizure components was measured after slow intravenous infusion of bicuculline in the rat. Clear differences were seen in the seizure threshold for tremor (TRE) and clonic-forepaw (CLOF) as compared to clonic-hindpaw (CLOH) and tonic-forepaw (TONF). Seizure threshold was measured after treatment with different doses of diazepam, pentobarbital, D-etomidate, picrotoxin, RO15-1788 and (+/-)-5-(1,3,-dimethylbutyl)-5-barbituric acid (DMBB). Direct and indirect antagonism between the agonists and antagonists was examined. The interactions between the drugs for TRE and CLOF resemble those described in in vitro receptor-binding assays using the GABA-benzodiazepine-chloride ionophore complex (GBCI). The interactions for CLOH and TONF do not show this resemblance, suggesting less involvement of GABA in these phenomena. Diazepam was selectively antagonized by RO15-1788. D-Etomidate and pentobarbital were directly antagonized by DMBB, suggesting shared activity at the barbiturate site. No evidence was found for an interaction between compounds acting at different sites within the GBCI.

Interactions of some anaesthetic, convulsant, and anticonvulsant drugs at GABA-benzodiazepine receptor-ionophore complexes in rat brain synaptosomal membranes

The effects of several anaesthetic, convulsant and anticonvulsant drugs were studied upon high affinity [3H]GABA and [3H]diazepam binding to rat brain synaptosomal membranes in chloride-containing incubation buffers at 25 degrees C, conditions under which pentobarbitone extensively enhanced binding of both ligands to GABA-benzodiazepine-receptor-ionophore complexes. Of the compounds studied, only (+)-etomidate enhanced both GABA and diazepam binding; the sedative-hypnotic glutethimide weakly enhanced GABA binding while inhibiting diazepam binding. Several drugs, including beta-butyl-beta-methyl-glutarimide, phenobarbitone, pentylenetetrazole, and ketamine reversed the enhancement of GABA binding by pentobarbitone (500 microM) while not altering basal GABA or diazepam binding. Enhancement of high affinity GABA binding does not appear to be a general property of sedative or anticonvulsant drugs.

Modulation of [3H]muscimol binding in rat cerebellar and cerebral cortical membranes by picrotoxin, pentobarbitone, and etomidate

Modulation of [3H]muscimol binding by picrotoxin, pentobarbitone, and etomidate was investigated in rat cerebellar and cerebral cortical membranes. In cerebellum, at 37 degrees C in the presence of chloride ions (150 mM), picrotoxin and picrotoxinin decreased specific [3H]muscimol binding to 43 +/- 3% of control, with an EC50 of 1.2 +/- 0.1 microM. [3H]Muscimol saturation experiments in the presence and absence of picrotoxin indicated that the picrotoxin effect was primarily due to a loss of high-affinity muscimol sites with KD approximately equal to 10 nM. Pentobarbitone enhanced specific [3H]muscimol binding to 259 +/- 3% of control, with EC50 = 292 +/- 37 microM, and etomidate increased binding to 298 +/- 18%, with EC50 = 7.1 +/- 1.0 microM. The influence of temperature and chloride ion concentration on these effects was investigated by comparing experiments at 37 and 0 degrees C in the presence or absence of chloride at constant ionic strength. The results indicate that studies at 0 degrees C underestimate the coupling between GABA receptors and barbiturate sites and that they greatly overestimate the importance of chloride ions in this phenomenon. In cerebral cortical membranes (37 degrees C, 150 mM Cl-), the effect of picrotoxin was similar to that observed in cerebellum, whereas the effects of pentobarbitone and etomidate were greater, but occurred at higher concentrations.

Enhancement of diazepam and gamma-aminobutyric acid binding by (+)etomidate and pentobarbital

(+)Etomidate and pentobarbital enhance [3H]diazepam and [3H]gamma-aminobutyric acid [( 3H]GABA) binding to cerebral cortex membranes. Both (+)etomidate and pentobarbital increase the affinity of [3H]diazepam for its binding sites. In contrast, they increase the Bmax of both the high- and low-affinity GABA receptor sites. The enhancement of [3H]diazepam and [3H]GABA by (+)etomidate and pentobarbital is blocked by GABA antagonists. These results indicate that hypnotic drugs such as (+)etomidate and pentobarbital, which are not structurally related, modulate diazepam and GABA binding sites via similar mechanisms.

Etomidate inhibits prolactin release but not through a dopaminergic mechanism

A short-acting non-barbiturate intravenous general anaesthetic, etomidate, when administered i.v. lowered serum prolactin levels in the rat. The effect was evident at rest, after surgical stress or after injection of 5-hydroxytryptophan. Haloperidol-induced hyperprolactinaemia was not modified. Administered via the intracerebroventricular route, etomidate strongly reduced serum prolactin levels in unanesthetized rats. Studies performed on superfused synaptosomes from brain areas rich in dopaminergic nerve endings did not show any influence of the etomidate, 10(-7) M-10(-6) M, on [3H]dopamine release. A peculiar GABA-like mechanism and/or possible interplay with serotonergic control of the prolactin release may be postulated in order to explain the suppressive effects of the drug on secretion of lactotrophs.

Binding of 3H-DMCM to benzodiazepine receptors; chloride dependent allosteric regulation mechanisms

DMCM is a convulsant agent with negative efficacy at benzodiazepine (BZ) receptors. 3H-DMCM binds to benzodiazepine receptors in vitro. The sensitivity of 3H-DMCM binding to agents presumed to act on chloride channels associated with the BZ/GABA-receptor-complex was investigated at 37 degrees C. Chloride ions (200 mM) enhanced the specific binding of 3H-DMCM four-fold. Similarly the specific binding of 3H-DMCM was enhanced by picrotoxinine in the absence but not in the presence of chloride ions. (+)-Etomidate and pentobarbital reduced the specific 3H-DMCM binding in a partially chloride ion dependent and picrotoxinine sensitive manner. The results obtained are consonant with the idea that 3H-DMCM binds to the BZ/GABA-receptor-chloride ionophor complex; furthermore, binding of 3H-DMCM seems to involve a chloride dependent allosteric regulation mechanism.

Modulation of benzodiazepine receptor binding: insight into pharmacological efficacy

The effects of GABA on the binding of analogues of benzodiazepines, triazolopyridazines, beta-carbolines and imidazodiazepines were examined in ligand/[3H] flunitrazepam competition experiments. GABA increased the potency of anxiolytics, like flunitrazepam, whereas the potency of benzodiazepine antagonists, like Ro15-1788, was largely insensitive to the influence of GABA. Several other agents including pyrazolopyridines, barbiturates and etomidate caused a chloride dependent enhancement of [3H] flunitrazepam binding but not an enhancement of [3H] propyl-beta-carboline-3-carboxylate binding.