Action of pyrazolopyridines as modulators of [3H]flunitrazepam binding to the gaba/benzodiazepine receptor complex of the cerebellum

One-pot multicomponent synthesis of novel pyridine derivatives for antidiabetic and antiproliferative activities

Background: Due to the close relationship of diabetes with hypertension reported in various research, a set of pyridine derivatives with US FDA-approved drug cores were designed and integrated by artificial intelligence. Methods: Novel pyridines were designed and synthesized. Compounds MNS-1-MNS-4 were evaluated for their structure and were screened for their in vitro antidiabetic (α-amylase) activity and anticancer (HepG2) activity by methyl thiazolyl tetrazolium assay. Comparative 3D quantitative structure-activity relationship analysis and pharmacophore generation were carried out. Results: The study revealed MNS-1 and MNS-4 as good alternatives to acarbose as antidiabetic agents, and MNS-2 as a more viable, better alternative to doxorubicin in the methyl thiazolyl tetrazolium assay. Conclusion: This combination of studies identifies new and more active analogs of existing FDA-approved drugs for the treatment of diabetes.

Synthesis and discovery of pyrazolo-pyridine analogs as inflammation medications through pro- and anti-inflammatory cytokine and COX-2 inhibition assessments

This article briefs about the efforts taken to synthesis, characterize and develop (E)-5-methyl-2-phenyl-3-(thiophen-2-yl)-7-(thiophen-2-ylmethylene)-3,3a,4,5,6,7-hexahydro-2H-pyrazolo[4,3-c]pyridine and their analogs. In the two-step reaction, the first step is the synthesis of (3Z,5E)-1-methyl-3,5-bis(thiophen-2-ylmethylene)piperidin-4-one derivatives (3a-l) by stirring the mixture of 1-methylpiperidin-4-one and substituted thiophene-carbaldehydes in presence of methanol. In the second and final step, compounds 3a-l were refluxed with phenyl-hydrazine to achieve the target compounds (E)-5-methyl-2-phenyl-3-(thiophen-2-yl)-7-(thiophen-2-ylmethylene)-3,3a,4,5,6,7-hexahydro-2H-pyrazolo[4,3-c]pyridine and their analogs (5a-l) in good yield. These compounds were used to assess their inflammation regulation properties in macrophages by executing quantitative pro-inflammatory and anti-inflammatory proteins such as TNF-α, IL-1β, IL6, and IL-10 respectively. In silico and in vitro COX-2 inhibition studies helped to understand the molecular interaction or plausible mechanism during the inflammation regulation that showed by the compounds. In the results, among the 12-member family of pyrazolo-pyridines (5a-l), 5a, 5b, 5g, and 5j were showed excellent in silico binding affinity (1-10 nM), least binding energy (-12.45 to -14.27 kcal/mol) and in vitro COX-2 inhibition (relative percentage activity maximum 96.42%). Thus, these compounds perhaps to be future anti-inflammatory drugs.

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".

Allosteric modulation of [3H]flunitrazepam binding to recombinant GABAA receptors

The allosteric modulation of [3H]flunitrazepam binding by gamma-aminobutyric acid (GABA), pentobarbital, (+)-etomidate, etazolate, alphaxalone, propofol and chlormethiazole was investigated in cerebellar membranes and membranes from human embryonic kidney (HEK) 193 cells transfected with alpha 1 beta 3 gamma 2 or alpha 1 gamma 2 subunits. Results obtained indicate that [3H]flunitrazepam binding to recombinant GABAA receptors consisting of alpha 1 beta 3 gamma 2 subunits could be modulated by these compounds in a way and with a potency similar to that observed in cerebellar membranes. In addition, it was demonstrated that not only receptors consisting of alpha 1 beta 3 gamma 3, but also those consisting of alpha 1 gamma 2 subunits exhibited [3H]flunitrazepam binding which could be stimulated by GABA. In contrast to alpha 1 beta 3 gamma 2 receptors, however, [3H]flunitrazepam binding to recombinant alpha 1 gamma 2 receptors was inhibited by pentobarbital, (+)-etomidate, etazolate, alphaxalone, propofol and chlormethiazole. This seems to indicate that binding sites for these compounds are present on alpha 1 gamma 2 receptors, but that their allosteric interaction with [3H]flunitrazepam binding sites is different from that of alpha 1 beta 3 gamma 2 receptors.

Beta-alanine potentiation of [3H]flunitrazepam binding to rat spinal cord homogenates

The effect of beta-alanine, gamma-aminobutyric acid (GABA), and other functionally related amino acids on [3H]flunitrazepam binding to rat spinal cord homogenates was studied. beta-Alanine potentiated [3H]flunitrazepam binding by 40% and GABA by 88%. Taurine increased the binding by 19%. Hypotaurine produced an 11% increase. No significant effect was seen in glycine, alanine, serine, valine or the dipeptide carnosine. The beta-alanine increase in [3H]flunitrazepam binding was completely inhibited by 10 microM strychnine, whereas the GABA increase required 0.1 mM strychnine to be fully suppressed. Results suggest that beta-alanine specifically potentiates binding of [3H]flunitrazepam in rat spinal cord homogenates.

N-0437: a selective D-2 dopamine receptor agonist in in vitro and in vivo models

The selectivity of the potent dopamine D-2 agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) was examined in a series of in vivo and in vitro pharmacological models. In radioligand binding assays, N-0437 showed high potency (Ki = 0.69 nM) and selectivity for D-2 receptors as compared to its potency and selectivity at various other neuronal receptors (Ki in nM): D-1 (678) dopamine, alpha 1-(534) and alpha 2-(195) adrenoceptor, S1-(6940) and S2-(5900) serotonin and muscarine (2660). Very low activity (Ki greater than 10(-5) M) was seen at the beta-adrenoceptor, A1-adenosine, GABAA and benzodiazepine receptors. Furthermore, N-0437 inhibited the calcium-dependent release of [3H]dopamine (IC50: 4 nM) and [3H]acetylcholine (IC50: 6.3 nM) from rabbit striatal slices in the nanomolar range. These effects of N-0437 were mediated through activation of D-2 dopamine autoreceptors and D-2 dopamine heteroreceptors, respectively. Presynaptic dopaminergic activity in vivo was measurable as an inhibition of the locomotor activity of mice, and in this model N-0437 was more effective than apomorphine. Moreover, the effect of N-0437 could be antagonized by sulpiride but not by yohimbine. N-0437 was equipotent with apomorphine in inducing circling behaviour in 6-OHDA-lesioned rats. N-0437 had almost no serotonergic activity in vivo. The results show that N-0437 is a selective dopamine D-2 agonist, and thus, that it is a new ligand of choice for studies on the D-2 receptor.

Convulsant/barbiturate activity on the soluble gamma-aminobutyric acid-benzodiazepine receptor complex

Cage convulsant t-butyl bicyclophosphoro[35S]thionate binding activity in rat brain membrane homogenates was solubilized with the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio]propane sulfonate (Chaps) and shown to co-purify with the benzodiazepine--gamma-aminobutyric acid (GABA) receptor complex on gel filtration and affinity chromatography. Whereas convulsant binding activity, but not GABA and benzodiazepine receptor binding, was eliminated by solubilization in other detergents like sodium deoxycholate or Triton X-100, or by addition of Triton X-100 to the extracts solubilized in the zwitterionic detergent, convulsant activity was not irreversibly lost or selectively unstable, but could be restored by exchanging the protein back into the detergent Chaps. The GABA-benzodiazepine receptor activity solubilized in Chaps alone, containing convulsant activity, and a sample in Chaps supplemented with Triton X-100 and lacking convulsant activity, did not differ in size as measured by gel filtration column chromatography or by radiation inactivation target size analysis. This suggests that convulsant binding activity does not require any additional protein subunits or other macromolecules nor any unique aggregation state relative to GABA and benzodiazepine receptor binding, and that all three activities reside on the same protein complex. As in intact brain, the target size for convulsant binding activity was 3-5 times that of benzodiazepine binding activity, suggesting that an oligomeric protein structure of the receptor complex with intact strong subunit interactions present in the native membrane environment is needed for convulsant activity, and that this and other properties are more preserved in Chaps than in other detergents.

Solubilisation of the gamma-aminobutyric acid/benzodiazepine receptor from rat cerebellum: optimal preservation of the modulatory responses by natural brain lipids

We have solubilised the gamma-aminobutyric acid/benzodiazepine (GABA/BDZ) receptor from rat cerebellum using 3-[(3-cholamidopropyl)dimethylammonio] 1-propane sulphonate (CHAPS) in the presence of a natural brain lipid extract and cholesteryl hemisuccinate. The soluble material shows a homogeneous [3H]flunitrazepam ([3H]FNZ) binding population with an equilibrium dissociation constant (KD) of 4.4 +/- 0.2 nM compared to a KD of 2.3 +/- 0.2 nM in cerebellar synaptosomal membranes. The receptor complex in solution retains the characteristic facilitation of [3H]flunitrazepam binding induced by GABA, the pyrazolopyridine cartazolate, and the depressant barbiturate pentobarbital to the same extent as that observed in synaptosomal membranes. Furthermore, these responses are retained both quantitatively and qualitatively when this preparation is stored for 48 h at 4 degrees C. This is contrary to the results obtained with a CHAPS-soluble preparation including asolectin in which these responses are anomalous and extremely labile on storage.

Polyclonal antibodies to agonist benzodiazepines

Benzodiazepine-binding, immunoglobulin G class antibodies have been raised in three rabbits immunised with a conjugate of kenazepine coupled to keyhole limpet haemocyanin. The antibodies were assayed by [3H]flunitrazepam binding, followed by adsorption onto Staphylococcus aureus cells. Measurement of the rates of association and dissociation of [3H]flunitrazepam binding, together with saturation analysis of equilibrium binding, revealed varying degrees of heterogeneity in the affinity constants of the three rabbit antisera (equilibrium KD values 0.18 to 4.13 nM at 20-22 degrees). Specificity of the antibodies was investigated by testing a wide variety of compounds (at concentrations of up to 10-100 microM) for their ability to inhibit [3H]flunitrazepam binding. Only benzodiazepines known to act as agonists at their receptor sites in the central nervous system (CNS) caused an inhibition of binding. The rank orders of the IC50 values of these drugs for inhibition of [3H]flunitrazepam binding to IgG from two out of the three rabbits correlated significantly with that previously published for displacement of CNS receptor binding. The agonist beta-carboline derivative ZK 93423, the anxiolytic cyclopyrrolones suriclone and zopiclone and the purines inosine and hypoxanthine all failed to inhibit antibody binding, supporting previous suggestions that these drugs may bind at non-benzodiazepine recognition sites on the CNS receptor. The antibodies described are expected to provide useful reagents for raising anti-idiotypic antibodies directed against the CNS receptor and for the identification and purification of possible endogenous benzodiazepine receptor agonists in the CNS.

Modulation of [3H]flunitrazepam binding to rat cerebellar benzodiazepine receptors by phosphatidylserine

The influence of phosphatidylserine on ligand binding to the benzodiazepine/GABA receptor complex was assessed in rat cerebellar synaptic membranes and in a detergent-solubilized membrane preparation. Intact synaptic membranes or membranes solubilized with the zwitterionic detergent CHAPS (3-[(3-cholamidopropyl)-dimethylammonio]propanesulfonate) were incubated with a range of concentrations of phosphatidylserine for 2 h at 4 degrees C, prior to use in radioligand binding assays. Phosphatidylserine, an endogenous membrane phospholipid, facilitated the site-specific binding of [3H]flunitrazepam to synaptic membranes and CHAPS-solubilized preparations. In addition, phosphatidylserine inhibited the facilitation of [3H]flunitrazepam binding induced by either cartazolate or gamma-aminobutyric acid (GABA). Although the maximum effect (38% facilitation of [3H]flunitrazepam binding; greater than 90% inhibition of the cartazolate action) was produced using 130 microM phosphatidylserine, a significant enhancement of [3H]flunitrazepam binding could be observed upon preincubation of synaptic membranes with concentrations of phosphatidylserine as low as 5 microM. These results suggest that endogenous phosphatidylserine may play a role in the regulation of benzodiazepine/GABA receptor function, possibly through modulation of the mechanisms which functionally link the various components of this complex receptor system.

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.

Solubilization of the benzodiazepine/gamma-aminobutyric acid receptor complex: comparison of the detergents octylglucopyranoside and 3-[(3-cholamidopropyl)-dimethylammonio]1-propanesulfonate (CHAPS)

Octylglucopyranoside (OCTG) was three times more efficient than 3-[(3-cholamidopropyl)-dimethylammonio]1-propanesulfonate (CHAPS) in solubilizing the benzodiazepine (BDZ)/gamma-aminobutyric acid (GABA) receptor complex from rat cerebellar synaptic membranes. OCTG-solubilized receptor preparations had ligand binding characteristics that were significantly different from those of the CHAPS-solubilized receptors. The inclusion of phospholipids in the solubilization media improved the binding characteristics of both soluble receptor preparations and appeared absolutely necessary for the maintenance of chloride facilitation of flunitrazepam (FNZ) binding to OCTG-solubilized receptors. FNZ and ethyl-beta-carboline-3-carboxylate bound to OCTG-solubilized preparations with equilibrium dissociation constants of 2.2 nM and 1.6 nM, respectively, and chloride (150 mM) and GABA (100 microM) + chloride facilitated the binding of FNZ by 15% and 55%, respectively; these ligand binding characteristics are similar to those of membrane-located BDZ receptors. Cartazolate, a pyrazolopyridine that facilitated the binding of FNZ to membrane-located and CHAPS-solubilized receptors, did not facilitate FNZ binding to OCTG-solubilized receptors. These results are discussed in terms of an interaction between the membrane lipid phosphatidylserine (PS) and cartazolate; PS appears to have the capacity to inhibit the effects of cartazolate on FNZ binding. Storage of the soluble receptor preparations for 24 h at 4 degrees C resulted in the loss of several characteristic BDZ receptor binding properties. Incorporation of the OCTG-solubilized receptor complex into liposomes prevented these losses but this procedure did not protect the CHAPS-solubilized receptors. We conclude that OCTG may have some advantages over CHAPS as the detergent of choice for the solubilization and reconstitution in liposomes of a functional BDZ/GABA receptor-chloride ionophore complex.

DIDS, an anion transport blocker, modulates ivermectin-induced enhancement of benzodiazepine receptor binding in rat brain

Ivermectin enhancement of [3H]diazepam binding was abolished by pretreatment of membranes with 0.05% Triton X-100, whereas GABA-induced enhancement was not changed after this treatment. Ivermectin enhancement was neither affected by picrotoxinin nor dependent on the chloride ion. 4,4'-Diisothiocyano-2,2'-disulfonic acid stilbene (DIDS) dose-dependently reduced [3H]diazepam binding enhanced by 10(-6) M ivermectin, without affecting the basal specific binding. The effects of DIDS were derived from reduction of increased binding affinity of benzodiazepine receptors by ivermectin, but were not dependent on chloride ion in the assay medium. DIDS inhibited GABA- and pentobarbital- but not chloride ion-induced enhancement of [3H]diazepam binding.

Reduction of inhibition by a benzodiazepine antagonist, Ro15-1788, in the rat hippocampal slice

The effects of extracellular applications of benzodiazepine agonists and the benzodiazepine antagonist, Ro15-1788, were investigated on pyramidal neurons in the CA1 region of rat hippocampal slices. The benzodiazepine agonists, chlordiazepoxide and diazepam, enhanced gamma-aminobutyrate synaptic inhibition, as tested by extracellular recordings during a paired-pulse inhibition paradigm. In contrast, Ro15-1788 (0.1-1 microM) depressed paired-pulse inhibition in a dose-dependent manner that suggested agonist activity at higher (10-100 microM) concentrations. Intracellular recordings from CA1 neurons showed that Ro15-1788 reduced both orthodromically and antidromically evoked inhibitory postsynaptic potentials. The reduction of the inhibitory postsynaptic potential probably resulted from a postsynaptic effect on the conductance mechanism of the inhibitory postsynaptic potential, since there were no changes in resting input resistance, the inhibitory postsynaptic reversal potential or the frequency of spontaneous inhibitory postsynaptic potentials. These data suggest that in the hippocampal slice preparation either (1) an endogenous benzodiazepine agonist exists that can be displaced by Ro15-1788 or (2) Ro15-1788 has inverse agonist activity.

Avermectin B1a modulation of gamma-aminobutyric acid/benzodiazepine receptor binding in mammalian brain

The anthelminthic natural product avermectin B1a (AVM) modulates the binding of gamma-aminobutyric acid (GABA) and benzodiazepine (BZ) receptor ligands to membrane homogenates of mammalian brain. The potent (EC50 = 40 nM) enhancement by AVM of [3H]diazepam binding to rat or bovine brain membranes resembled that of barbiturates and pyrazolopyridines in being inhibited (partially) by the convulsants picrotoxin, bicuculline, and strychnine, and by the anticonvulsants phenobarbital and chlormethiazole. The maximal effect of AVM was not increased by pentobarbital or etazolate. However, AVM affected BZ receptor subpopulations or conformational states in a manner different from pentobarbital. Further, unlike pentobarbital and etazolate, AVM did not inhibit allosterically the binding of the BZ receptor inverse agonist [3H]beta-carboline-3-carboxylate methyl ester, nor did it inhibit, but rather enhanced, the binding of the cage convulsant [35S]t-butyl bicyclophosphorothionate to picrotoxin receptor sites. AVM at submicromolar concentrations had the opposite effect of pentobarbital and etazolate on GABA receptor binding, decreasing by half the high-affinity binding of [3H]GABA and related agonist ligands, and increasing by over twofold the binding of the antagonist [3H]bicuculline methochloride, an effect that was potentiated by picrotoxin. AVM also reversed the enhancement of GABA agonists and inhibition of GABA antagonist binding by barbiturates and pyrazolopyridines. These overall effects of AVM are unique and require the presence of another separate drug receptor site on the GABA/BZ receptor complex.

Enhancement of diazepam activities induced by denzimol in mice

Denzimol, a new anticonvulsant drug, enhances the depressant and antimetrazol activities of diazepam in mice, in a dose and time-dependent fashion. The depressant and anticonvulsant activities of phenobarbital were not affected by Denzimol. It is suggested that Denzimol induces an increase in the number of the benzodiazepine receptors, and that such increase might be responsible for the enhancement of the diazepam's activities.

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.

Ivermectin interactions with benzodiazepine receptors in rat cortex and cerebellum in vitro

The anthelmintic macrolide, ivermectin, enhances the binding of benzodiazepine agonist ( [3H]-diazepam) and antagonist ( [3H] beta-carboline ethyl ester) ligands to rat cortical and cerebellar membrane preparations. Enhancement of benzodiazepine agonist binding is partially additive with that of gamma-aminobutyric acid (GABA) and is inhibited by etazolate, bicuculline, and the steroid GABA antagonist R5135. Ivermectin-stimulated benzodiazepine antagonist binding is enhanced by bicuculline and inhibited by GABA and etazolate. The modulatory effects of bicuculline are chloride-dependent. The stimulatory effects of ivermectin, while quantitatively different in cortex and cerebellum, are qualitatively similar in both brain regions and are reduced in the presence of chloride. Ivermectin effects on benzodiazepine ligand binding to the benzodiazepine receptor complex and the differences in the effects of GABA, bicuculline, and R5135 on ivermectin-stimulated agonist and antagonist binding may provide evidence for distinct differences in the recognition sites for the two classes of benzodiazepine receptor ligand and their interactions with other components of the receptor complex.

Saturable binding of 35S-t-butylbicyclophosphorothionate to the sites linked to the GABA receptor and the interaction with GABAergic agents

35S-t-Butylbicyclophosphorothionate (35S-TBPS) binds in a concentration-saturable manner to specific sites on membranes from rat cerebral cortex. Using a filtration assay at 25 degrees C, in 250 mM NaCl, specific binding of 35S-TBPS constitutes about 84 to 94 percent of total binding, depending on radioligand concentrations. 35S-TBPS binding is optimal in the presence of NaCl or NaBr and substantially less in the presence of NaI or NaF. It is sensitive to the treatment with 0.05 percent Triton X-100 but not to repeated freezing and thawing, procedures which increase 3H-GABA binding. Pharmacological studies show that 35S-TBPS binding is strongly inhibited by GABA-A receptor agonists (e.g., GABA and muscimol) and by the noncompetitive antagonist, picrotoxin, but not the competitive antagonist, bicuculline. Compounds which enhance binding of radioactive GABA and benzodiazepines, such as the pyrazolopyridines, cartazolate and tracazolate, and a diaryltriazine, LY81067, are also potent inhibitors of 35S-TBPS binding, with LY81067 being the most effective. The effects of GABA, picrotoxin and LY81067 on the saturable binding of 35S-TBPS in cortical membranes are compared. The present findings are consistent with the interpretation that 35S-TBPS binds at or near the picrotoxin-sensitive anion recognition sites of the GABA/benzodiazepine/picrotoxin receptor complex.

Role of the gamma-aminobutyric acid receptor-ionophore complex in seizure disorders

The possibility of a role for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in seizure disorders has been strengthened by biochemical studies showing that various nervous system depressant drugs can modulate GABA receptor binding in vitro. In particular, two classes of anticonvulsant agents, the benzodiazepines and the barbiturates, have modulatory receptor sites on the GABA receptor-ionophore protein complex of the postsynaptic membrane. Furthermore, it is well established that direct block of GABA function causes seizures and that augmentation of GABA function can protect against seizure activity. Direct evidence for altered GABA synaptic markers has been obtained in some animal models of epilepsy, as well as in human focal epilepsy. We present preliminary evidence for a deficit in benzodiazepine receptor binding in the midbrain of seizure-susceptible Mongolian gerbils. These data would be consistent with an impairment of GABA-mediated inhibitory synaptic transmission that contributes to susceptibility to the genesis or spread of seizures in some kinds of epilepsy.

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.

Benzodiazepine receptor mediated discriminative cues: effects of GABA-ergic drugs and inverse agonists

Rats were exposed to a two-lever drug discrimination procedure using the benzodiazepine (BZ) receptor inverse agonists N'-methyl-beta-carboline-3-carboxamide (FG 7142) or methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). FG 7142 (30 mg/kg) failed to acquire discriminative stimulus control, although it did suppress responding. The same group of animals was trained successfully to discriminate diazepam (DZP, 2.5 mg/kg) from vehicle. The DZP cue was potentiated by the GABA agonist 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol (THIP, 1,3 mg/kg); THIP alone produced vehicle-appropriate responding. In addition, clonazepam (0.2 mg/kg) and chlordiazepoxide (5 mg/kg) substituted for DZP (with potencies of 7.5 and 0.25 times that of DZP, respectively). In antagonism tests, FG 7142 (5-17.5 mg/kg), methyl-beta-carboline-3-carboxylate (beta-CCM, 2.5 mg/kg) did not effect, bicuculline (2 mg/kg) and DMCM (1 mg/kg) partially blocked, and the BZ receptor antagonist Ro 15-1788 (40 mg/kg) completely blocked the discriminative stimulus effects of DZP. In animals trained to discriminate DMCM (0.2 mg/kg) from vehicle, 95.2% substitution occurred with bicuculline (2 mg/kg); DZP (1-5 mg/kg) completely antagonized DMCM. These results indicate that the DZP cue is mediated by GABA-coupled BZ receptors and that GABA may modulate the efficacy of a BZ at its receptor site. However, since inverse BZ receptor agonists (FG 7142, DMCM and beta-CCM) were, at best, only marginally effective in antagonizing DZP, the DZP cue may be mediated by a distinct subclass of BZ receptors.

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.

The effect of ethanol on GABAergic transmission

One of the many actions of ethanol involves the GABAergic system. The interaction of ethanol with GABAergic neurons is a complex one involving both presynaptic and postsynaptic sites. Through a presumed fluidization of membranes after a single dose of ethanol, the available in vitro evidence suggests that ethanol disrupts the normal functioning of the GABA-benzodiazepine-chloride ionophore complex in a complicated manner involving a sequential activation of different active sites leading to the facilitation of GABA transmission. This finding has been supported in vivo using electrophysiological techniques. Presynaptic GABAergic neurons may experience a reduced activity, especially at low doses of ethanol. After chronic ethanol treatment, GABAergic transmission may be reduced, especially during an ethanol withdrawal syndrome. Also, other changes in the GABA-benzodiazepine-chloride ionophore complex suggest GABA transmission is suppressed postsynaptically. Drugs which enhance the actions of GABA may be suitable inhibitors of the ethanol withdrawal syndrome. In particular a new class of drugs, the triazolopyridazines, may be promising compounds for treatment of withdrawal with a more specific mode of action and fewer side effects.

The interactions of ethanol with the benzodiazepine-GABA receptor-ionophore complex

Ethanol, barbiturates and benzodiazepines have similar pharmacological effects. All of these drugs facilitate the inhibitory transmission mediated by GABA. Drugs that facilitate GABAergic transmission are effective in alleviating ethanol withdrawal symptoms. Acute ethanol administration increases the number of low affinity GABA receptor sites, while, during ethanol withdrawal, the affinity of this site is decreased. This decreased affinity during withdrawal correlated with the audiogenic seizure activity. These results indicate that in vivo ethanol interacts with GABA receptors, and this interaction may be responsible for some of the effects of ethanol and for some symptoms of withdrawal. Ethanol, like pentobarbital, also enhances [3H]diazepam binding to the Lubrol-solubilized membrane fraction in vitro. This effect was dose-related and was blocked by picrotoxinin and other GABA antagonists. Enhancement of [3H]diazepam binding by various alcohols did not correlate with lipid:water partition coefficients. Ethanol also partially inhibited the binding of [3H]-alpha-dihydropicrotoxinin in the Lubrol-solubilized fraction. These results indicate that ethanol, like pentobarbital, may modulate the benzodiazepine binding component of the benzodiazepine-GABA receptor-ionophore complex via the picrotoxinin site. The possible interpretation of these results, in relation to the GABAergic transmission, is discussed.

Solubilization by CHAPS detergent of barbiturate-enhanced benzodiazepine-GABA receptor complex

Barbiturates enhance the binding of [3H]flunitrazepam to benzodiazepine receptors solubilized with the detergent 3-[(3-cholamidopropyl)-dimethylammonio]propanesulfonate (CHAPS) from bovine cortex. The enhancement by the barbiturates is seen as a decrease in the dissociation constant, KD, for specific benzodiazepine binding, with no effect on the number of binding sites. The effect of the barbiturates is facilitated by chloride ions, is concentration-dependent, and has a specificity that correlates well with the anesthetic potency of barbiturates. [3H]Flunitrazepam binding activity is stable with storage at 4 degrees C, but barbiturate enhancement of soluble benzodiazepine binding activity decayed rapidly (t 1/2 = 48 h). [3H]Muscimol binding (GABA receptor) activity was also enhanced by barbiturates. Agarose gel filtration column chromatography of the CHAPS-solubilized receptor proteins showed the same elution profile as receptors solubilized with sodium deoxycholate, and enhancement by barbiturates was observed for both the benzodiazepine and GABA binding activities.

Attenuation of induced-anxiety in rats by chlordiazepoxide: role of raphe dorsalis benzodiazepine binding sites and serotoninergic neurons

In chronically implanted awake rats, microinjections of chlordiazepoxide (5 x 10(-7) M) into the dorsal raphé significantly attenuated the inhibition of lever-pressing for food elicited by a signal of punishment. This effect is abolished by prior application of 5,7-dihydroxytryptamine into the dorsal raphé (3 weeks after the infusion of the neurotoxin, dorsal raphé tryptophan hydroxylase activity was reduced to 25% of control values). Furthermore, the disinhibitory effect of intra raphé chlordiazepoxide can be mimicked or potentiated by intra raphé dorsalis application of serotonin (10(-7) or 10(-8) M, respectively). Further evidence for a crucial interaction between benzodiazepines and serotoninergic processes are provided by in vitro experiments showing that chlordiazepoxide or diazepam (10(-5) M) are able to facilitate the K+-evoked [3H]serotonin release from rat midbrain slices. Finally, a high density of [3H]flunitrazepam binding sites was found in the dorsal (and the median) raphé nucleus, the Kd and Bmax values being not altered by prior infusion of 5,7-dihydroxytryptamine. These in vitro data suggest possible means by which intra raphé (and perhaps peripherally administered) benzodiazepines may affect the activity of serotoninergic neurons and thereby produce their effects on experimental anxiety.

Molecular interactions of etazolate with benzodiazepine and picrotoxinin binding sites

Pyrazolopyridines, such as etazolate (SQ 20009), enhance [3H]diazepam binding to a Lubrol-solubilized fraction that has specific binding sites for 3H benzodiazepines, [alpha-3H]dihydropicrotoxinin (DHP) and [3H]muscimol. Etazolate enhancement of [3H]diazepam binding was inhibited by picrotoxinin. Furthermore, etazolate inhibited the [3H]DHP binding in a Lubrol-solubilized fraction with an IC50 value of 6-8 microM. These results provide evidence that etazolate, like pentobarbital, modulates benzodiazepine binding via the DHP-sensitive site of the benzodiazepine-GABA receptor-ionophore complex.