Chapter 3. Benzodiazepine Receptors

Acidic hydrolysis of bromazepam studied by high performance liquid chromatography. Isolation and identification of its degradation products

A kinetic study on the acidic hydrolysis of bromazepam was carried out in 0.01 M hydrochloric acid solution at 25 and 95 degrees C. A reversed-phase HPLC method was developed and validated for the determination of bromazepam and its degradation products. Bromazepam degraded by a consecutive reaction with a reversible first step. Two degradation products were isolated and identified by infrared, 1H and 13C nuclear magnetic resonance and mass spectroscopy. Spectroscopic data indicated that N-(4-bromo-2-(2-pyridylcarbonyl)phenyl)-2-aminoacetamide was the intermediate degradation product of this acid hydrolysis, whereas 2-amino-5-bromophenyl-2-pyridylmethanone was the final one. Therefore, the mechanism of this acid-catalysed hydrolysis involved initial cleavage of the 4,5-azomethine bond, followed by slow breakage of the 1,2-amide bond. Statistical evaluation of the HPLC method revealed its good linearity and reproducibility. Detection limits were 3.8 x 10(-7) M for bromazepam, 6.25 x 10(-7) M for the intermediate and 8.16 x 10(-7) M for the benzophenone derivative.

S-adenosyl-L-homocysteine and 5'-methylthioadenosine inhibit binding of [3H]flunitrazepam to rat brain membranes

The effect of inhibitors of transmethylation reactions, S-adenosylhomocysteine and 5'-methylthioadenosine, on [3H]flunitrazepam-specific binding to the rat brain membranes has been investigated. Both S-adenosylhomocysteine and 5'-methylthioadenosine are able to inhibit binding with Ki values of 7.9 microM and 15.8 microM respectively. These compounds therefore may be candidate endogenous benzodiazepine-receptor ligands. In light of these and other data possible correlations between phospholipid methylation and gamma-aminobutyric acid (GABA) receptor function are discussed.

Tonic inhibition of striatal dopamine transmission: effects of benzodiazepine and GABAA receptor antagonists on extracellular dopamine levels

At present, it is unclear whether ligands which bind at the benzodiazepine/GABA receptor complex play a tonic modulatory role with regard to striatal dopamine (DA) transmission. The present study was designed to examine the effects of Ro15-1788, a benzodiazepine (BZ) receptor antagonist, and SR 95531, a GABAA receptor antagonist, on striatal extracellular DA (DA[e]) concentrations in anesthetized and awake rats using the technique of in vivo microdialysis. Local administration of Ro15-1788 resulted in a dose-dependent increase in DA[e] in both anesthetized and awake animals. The Ro15-1788-induced increase in DA[e] was blocked by coadministration of the BZ agonist diazepam, as well as GABA. Local administration of SR 95531 also resulted in a dose-dependent alteration in striatal DA levels in both anesthetized and awake animals. The SR 95531-induced increase in DA was blocked by coadministration of GABA. The results suggest that GABA may play a tonic inhibitory role with regard to striatal DA transmission.

Regionally specific alterations in the low-affinity GABAA receptor following perinatal exposure to diazepam

Alterations in a low affinity form of the GABAA receptor were examined with [3H]bicuculline methylchloride in the adult rat following perinatal exposure to diazepam. Perinatal exposure resulted in a significant reduction in [3H]bicuculline binding in the cingulate cortex. A significant decrease in the ability of GABA to displace bound [3H]bicuculline was observed only in the hypothalamus. The results suggest that the effects of perinatal exposure to diazepam are regionally specific and that benzodiazepine receptors and low affinity GABAA receptors are functionally linked during the perinatal period.

GABAergic mechanisms in the electrophysiological actions of ethanol on cerebellar neurons

We have found that the partial inverse benzodiazepine agonists Ro 15-4513 and FG 7142 antagonize the depressant electrophysiological effects of locally applied ethanol in the cerebellum. Although absolute tissue concentrations are not known, dose-response curves constructed using pressure-ejection doses as previously described we found that FG 7142 was more efficacious, but less potent than Ro 15-4513. Our observation that ethanol and inverse benzodiazepine agonists have interactions which are not competitive might suggest that these two drugs act through separate, but interactive mechanisms in order to produce the observed ethanol antagonism. If such independent interactions were mediated at different sites on a given macromolecular complex, such as the GABAa/Cl- channel, then one might expect to find allosteric interactions between those sites as well as with the functional response of the complex to GABA activation. Indeed, this hypothesis is consistent with the recent finding of Harris and collaborators that ethanol potentiates the inverse agonist actions of Ro 15-4513 and FG 7142. On the other hand, we were unable to find large ethanol-induced potentiations of GABA effects on all neurons which showed depressant responses to ethanol administration in rat cerebellum. However we did find that the GABAa antagonist, bicuculline, blocks the depressant effects of ethanol on the same neurons. We conclude that the interaction between ethanol and GABA probably does not occur directly at the GABAa receptor site, but that the GABAa mechanism does play a permissive role in the ethanol-induced depressions of cerebellar Purkinje neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sensitivity of "central" and "peripheral" type benzodiazepine receptors to phospholipase A2

The effects of preincubating cerebral cortical membranes with phospholipase A2 (PLA2) were examined on radioligand binding to benzodiazepine receptors of the "central" and "peripheral" types. PLA2 (0.005-0.1 U/ml) increased [3H]flunitrazepam and [3H]3-carboethoxy-beta-carboline binding by increasing the apparent affinities of these ligands with no concomitant change in the maximum number of binding sites. In contrast, neither gamma-aminobutyric acid (GABA)-enhanced [3H]flunitrazepam binding nor [3H]Ro 15-1788 binding was altered by preincubation with PLA2 at concentrations as high as 2 U/ml. Both pyrazolopyridine (SQ 65,396)- and barbiturate (pentobarbital)-enhanced [3H]flunitrazepam binding and [35S]t-butylbicyclophosphorothionate (TBPS) binding were markedly reduced by as little as 0.0025-0.005 U/ml of PLA2. These findings suggest that PLA2 inactivates the TBPS binding site on the benzodiazepine-GABA receptor chloride ionophore complex, which results in a selective loss of allosteric "regulation" between the components of this complex. PLA2 also reduced the apparent affinity of [3H]Ro 5-4864 to peripheral-type benzodiazepine receptors in cerebral cortical, heart, and kidney membranes, but increased the number of [3H]PK 11195 binding sites with no change in apparent affinity. These data demonstrate that PLA2 can differentially affect the lipid microenvironment of "central" and "peripheral" types of benzodiazepine receptors.

Are the toxicities of pentobarbital and ethanol mediated by the GABA-benzodiazepine receptor-chloride ionophore complex?

Both barbiturates and ethanol have been reported to interact with the GABA-benzodiazepine receptor-chloride ionophore 'supramolecular complex'. These observations raise the possibility that some of the pharmacologic actions of barbiturates and ethanol may be mediated through this complex. In this study we have administered a series of drugs which bind to various components of the complex in an attempt to antagonize the lethality of sodium pentobarbital, and ethanol-induced loss of righting reflex in mice. It was found that isopropylbicyclophosphate (IPPO), a cage convulsant which binds at or near the chloride ionophore, greatly reduces the overall mortality (and increases latency to death) of animals pretreated with a lethal dose of pentobarbital. Picrotoxin also decreases pentobarbital lethality, but only at doses which were usually lethal when given alone. Picrotoxin shortened, rather than increased, latency to death. Strychnine did not prevent pentobarbital lethality, suggesting that the IPPO effect is not shared by convulsants in general. IPPO did not prevent ketamine-induced deaths, which supports the notion that the protective actions of IPPO are specific for depressant drugs which act at the chloride ionophore. IPPO also significantly reduced the duration of loss of righting reflex induced by ethanol. These observations suggest that the use of compounds which have a high affinity for the chloride ionophore in vitro might be fruitful in developing a clinical treatment for barbiturate or ethanol toxicity.

Preclinical pharmacology of buspirone hydrochloride

Buspirone is an azaspirodecanedione that has anxiolytic actions in man and anticonflict actions in experimental animals. In contrast to many commonly used anxiolytic agents, buspirone does not appear to have significant sedative, muscle relaxant or anticonvulsant actions. Consequently, it has been termed an "anxioselective" agent. The effects of buspirone and related compounds have been examined in a number of neurotransmitter systems that have been linked to the actions of other anxiolytic agents. Buspirone has been shown to affect dopaminergic, serotonergic and noradrenergic pathways, as well as the GABA-benzodiazepine receptor chloride ionophore complex. Nonetheless, the doses (in vivo) and/or concentrations (in vitro) of buspirone (and related compounds) needed to affect these systems are not consistent with a causal relationship between the anxiolytic actions of buspirone and any of these systems. The neurochemical mechanism by which buspirone and related compounds exert both anticonflict and anxiolytic actions remains unknown.

Structure-activity relationships in kynurenine, diazepam and some putative endogenous ligands of the benzodiazepine receptors

Kynurenine, an endogenous cerebral and peripheral neuroactive metabolite of 1-tryptophan, exerts stimulant and convulsant effects in mice, rats and frogs. In mice it (intracerebroventricularly, ICV) antagonized the anticaffeine effect of diazepam and in smaller doses potentiated its sedative action. In rats 1-kynurenine (ICV) potentiated the convulsant action of caffeine. The effect of pentylenetetrazol was not altered in either species. The convulsant effect of 1-kynurenine is the most resistant among various convulsants towards the protective action of diazepam. The structure of 1-kynurenine is similar to benzophenones, metabolites of diazepam, and has four structural fragments common with diazepam. Putative endogenous and non-endogenous ligands of the benzodiazepine receptors have from one to three of these common fragments. Among the antagonists of diazepam exhibiting stimulant and convulsant action ethyl-beta-carboline-3-carboxylate has the same four fragments, Ro 5-3663 and Ro 15-1788 have three and caffeine two. The most striking dissimilarity is a diazo-moiety (N-C-C-N or N-C=C-C=N) absent in the structure of 1-kynurenine. This moiety seems to be the most important for the binding to the benzodiazepine receptors. A role of each fragment and their combinations as well as the stereoconfiguration for the pharmacological activity is considered. It is suggested that 1-kynurenine is a putative endogenous modulator or, less probably, ligand of the benzodiazepine receptor of either type (most probably that which mediates anxiolytic action of benzodiazepines) or a part of this receptor. The benzodiazepine receptor might be a phylogenetically transformed kynurenine receptor. Highly selective antagonism of purines to 1-kynurenine suggests that it can modulate the function of the benzodiazepine receptors via purinergic mechanisms. Stimulant and convulsant action of 1-kynurenine can be related to a moiety of succinic acid (O=C-C-C-C=O) which is typical of quinolinic acid, the strongest endogenous convulsant among kynurenines, and aspartic acid, an excitatory amino acid. 1-Kynurenine is suggested to be an anxiogenic and convulsigenic endogenous factor.

The inhibitory effect of diazepam on conditioned defensive burying is reversed by picrotoxin

The ability of picrotoxin to reverse the effect of diazepam was studied using the conditioned defensive burying paradigm. Although picrotoxin alone had no detectable effect on the conditioned defensive burying response of rats, picrotoxin was able to reverse the usual inhibitory effect of diazepam on defensive burying. These results suggest that the anxiolytic effect of diazepam may depend upon the integrity of GABAergic neural systems.