Biochemical characterization of an isolated and functionally reconstituted gamma-aminobutyric acid/benzodiazepine receptor

Recent developments on ultrasound-assisted one-pot multicomponent synthesis of biologically relevant heterocycles

Heterocycles are the backbone of organic compounds. Specially, N- &O-containing heterocycles represent privileged structural subunits well distributed in naturally occurring compounds with immense biological activities. Multicomponent reactions (MCRs) are becoming valuable tool for synthesizing structurally diverse molecular entities. On the other hand, the last decade has seen a tremendous outburst in modifying chemical processes to make them sustainable for the betterment of our environment. The application of ultrasound in organic synthesis is fulfilling some of the goals of 'green and sustainable chemistry' as it has some advantages over the traditional thermal methods in terms of reaction rates, yields, purity of the products, product selectivity, etc. Therefore the synthesis of biologically relevant heterocycles using one-pot multi-component technique coupled with the application of ultrasound is one of the thrusting areas in the 21st Century among the organic chemists. The present review deals with the "up to date" developments on ultrasound assisted one-pot multi-component synthesis of biologically relevant heterocycles reported so far.

Recent Developments on Ultrasound-Assisted Synthesis of Bioactive N-Heterocycles at Ambient Temperature

N-Heterocycles represent privileged structural subunits well distributed in naturally occurring compounds with immense biological activities. The last decade has seen a tremendous practice to carry out reactions at ambient temperature avoiding harsh reaction conditions. By applying ultrasonic radiation in organic synthesis we can make synthetic protocols more sustainable and can carry out reactions at room temperature avoiding the traditional thermal harsh reaction conditions. Therefore the synthesis of biologically relevant N-heterocycles at room temperature under the influence of ultrasonic irradiation is one of the advancing areas in the 21st century among organic chemists. The present review summarises the latest developments on ultrasound-assisted synthesis of biologically relevant N-heterocycles at ambient temperature.

Comparison of responses of spontaneously active cells in the cerebellar Purkinje layer to parallel fibre stimulation in slice preparations and urethane-anaesthetised rats: effects of benzodiazepine receptor ligands

1. GABA-mediated inhibitory responses were induced in spontaneously active Purkinje cells by parallel fibre stimulation in cerebellar slices or in urethane-anaesthetised rats. Effects of agonist and inverse agonist benzodiazepine (BDZ) receptor ligands were compared in the preparations. 2. Purkinje cells fired simple spikes at higher rates in slice preparations while complex spikes were seldom (in vivo) or never observed (slice). Cells fired more regularly in vivo resulting in the occurrence of rhythmic postinhibitory responses in the PSTH analysis in some preparations. 3. Single pulse stimulation of parallel fibres at just suprathreshold intensity induced inhibition of Purkinje cell activity in both preparations. At lower firing rates there was a marked increase in the duration of this response, which was more evident in vivo where more slowly firing cells were encountered. 4. BDZ receptor ligands modified inhibitory responses in slice preparations with only weak effects on the firing rates of the cells. These compounds predominately induced changes in firing rate in the anaesthetised rat with little evidence of direct modification of GABA-mediated synaptic transmission. 5. In a few experiments, following injection of the partial inverse agonists beta-CCE and beta-CCM, block of the inhibitory response was observed independent of changes in firing rate. Bidirectional efficacy of BDZ receptor ligand (agonists decrease firing and increase inhibitory response, inverse agonists increase firing and decrease inhibitory response) was demonstrated for modulation of inhibitory responses in slices and for changes in firing rate in vivo. The increased firing rate response in vivo was biphasic the magnitude of the later phase being correlated with efficacy of inverse agonists. 6. It is concluded that cerebellar slice preparations are more appropriate for studying direct effects of BDZ receptor ligands on GABA-mediated synaptic inhibition than in vivo preparations.

Receptor binding sites and uptake activities mediating GABA neurotransmission in chronic alcoholics with Wernicke encephalopathy

Superior frontal cortex (SFC) and primary motor cortex tissue was obtained at autopsy from thirteen severe chronic alcoholics with neuropathologically confirmed Wernicke Encephalopathy (WE) and 22 controls. Cases with both WE and cirrhosis showed markedly fewer neurones in SFC than did WE cases without cirrhosis. The extent of the apparent neuronal loss corresponded to an increase in post-synaptic GABAA receptor sites, as assessed by the binding of [3H]muscimol to synaptic membranes. Increased [3H]muscimol binding was not accompanied by an increase in 'central-type' benzodiazepine binding sites: as assessed by [3H]flunitrazepam binding, these sites were apparently unaltered, while as assessed by [3H]diazepam binding, they were decreased. The affinities of the two benzodiazepine ligands varied differently with disease. These discrepancies between [3H]flunitrazepam and [3H]diazepam binding could not be accounted for, either by the presence of a second, diazepam-preferring, 'central-type' benzodiazepine binding site, or by loss of 'peripheral-type' sites. The changes in the post-synaptic GABAA-benzodiazepine receptor sites did not reflect any regional, disease-related deficit of afferent GABAergic terminals, as assessed by synaptosomal high-affinity [3H]GABA uptake. On a number of indices, it appears most likely that the data reflect both a loss of receptor sites, and a change in the population of receptor sub-types.

Characterization of the influence of unsaturated free fatty acids on brain GABA/benzodiazepine receptor binding in vitro

We have investigated the effect of unsaturated free fatty acids (FFAs) on the brain GABA/benzodiazepine receptor chloride channel complex from mammalian, avian, amphibian, and fish species in vitro. Unsaturated FFAs with a carbon chain length between 16 and 22 carbon atoms enhanced [3H]diazepam binding in rat brain membrane preparations, whereas the saturated analogues had no effect. The enhancement of [3H]diazepam binding by oleic acid was independent of the incubation temperature (0-30 degrees C) of the binding assay and not additive to the enhancement by high concentrations of Cl-. In rat brain preparations, the stimulation of [3H]diazepam binding by oleic acid (10(-4) M) was independent of the ontogenetic development. Phylogenetically, large differences were found in the effect of unsaturated FFAs on [3H]diazepam and [3H]muscimol binding: In mammals and amphibians, unsaturated FFAs enhanced both [3H]-muscimol and [3H]diazepam binding to 150-250% of control binding. In 17 fish species studied, oleic acid (10(-4) M) stimulation of [3H]diazepam binding was weak (11 species), absent (four species), or reversed to inhibition (two species), whereas stimulation of [3H]muscimol binding was of the same magnitude as in mammals and amphibians. In 10 bird species studied, only weak enhancement of [3H]muscimol binding (110-130% of control) by oleic acid (10(-4) M) was found, whereas [3H]diazepam binding enhancement was similar to values in mammal species. Radiation inactivation of the receptor complex in situ from frozen rat cortex showed that the functional target size for oleic acid to stimulate [3H]flunitrazepam binding has a molecular mass of approximately 200,000 daltons.(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstitution of purified GABAA receptors: ligand binding and chloride transporting properties

GABAA receptors have been solubilized from bovine brain membranes and, following purification by benzodiazepine affinity chromatography, have been reconstituted into phospholipid vesicles. Reconstituted vesicles were about 120 nm in diameter, and, on average, each vesicle contained fewer than one GABAA receptor which was reconstituted in an outside-out orientation. These preparations have been used in parallel studies of radiolabeled ligand binding and chloride flux, the latter being measured by following the fluorescence changes of a chloride-sensitive probe which was trapped within the vesicles at the time of reconstitution. The benzodiazepine [3H]flunitrazepam binds to an apparently homogeneous population of sites in these preparations (Kd of 5 nM) whereas the GABA analogue [3H]muscimol binds to both high- and low-affinity sites (KdS of 10 nM and 0.27 microM). Muscimol stimulated chloride flux with an EC50 of 0.2 microM and, at similar concentrations (EC50 = 0.16 microM), potentiated [3H]flunitrazepam binding, suggesting that occupancy of the low-affinity sites may be important for these effects. Diazepam shifted the dose-response curve for muscimol-stimulated flux to about 4-fold lower concentrations without affecting the maximum response. Diazepam did not, however, alter the equilibrium binding of [3H]muscimol. The purified receptor showed densensitization since flux responses were abolished by prior exposure to muscimol. The competitive antagonist bicuculline and the channel blocker picrotoxin completely inhibited ion flux mediated by 3 microM muscimol with EC50 values of 5.3 and 2.5 microM, respectively. These results are discussed in terms of possible mechanisms for activation, inhibition, and modulation of GABAA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of the antidepressant phenelzine and its N-acetyl analogue: effects on GABAergic function

The MAO inhibitor phenelzine (2-phenylethylhydrazine; PLZ) is used widely in psychiatry for the treatment of depression and panic disorder. Its N-acetyl metabolite, N2-acetylphenelzine (N2AcPLZ) is a reasonably potent nonselective inhibitor of monoamine oxidase (MAO) that causes elevation in brain levels of the biogenic amines. In the studies reported here, PLZ (0.05 mmol/kg/day), N2AcPLZ (0.10 mmol/kg/day) or vehicle were administered to male rats for 28 days s.c. with Alzet minipumps, and their effects on GABAergic function were examined. Whole brain concentrations of gamma-aminobutyric acid (GABA) were significantly elevated in the PLZ but not in the N2AcPLZ-treated group. PLZ was found to inhibit the anabolic enzyme glutamic acid decarboxylase (GAD) and, to a greater extent, the catabolic enzyme GABA transaminase (GABA-T). The results of these investigations suggest that the free hydrazine moiety in PLZ is crucial to producing the elevated levels of GABA, probably through inhibition of GABA-T. Despite the considerable increase in whole brain GABA levels in the PLZ-treated rats, there were no significant differences in GABAA or benzodiazepine receptor binding parameters (KD or Bmax) between the groups as measured using 3H-muscimol and 3H-flunitrazepam in radioligand binding assays.

Characterization of two cerebellar binding sites of [3H]Ro 15-4513

Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H- imidazo[1,5-a][1,4]benzodiazepine-3-carboxylate), a partial inverse agonist of central benzodiazepine receptors, binds to two distinct sites in the cerebellum. The binding to diazepam-sensitive (DZ-S) sites is displaced by different benzodiazepine receptor ligands, whereas the other site is insensitive to benzodiazepine agonists [diazepam-insensitive (DZ-IS)]. The binding of [3H]Ro 15-4513 was studied in pig cerebellar membranes and in receptors solubilized and purified from these. Micromolar concentrations of gamma-aminobutyric acid (GABA) decreased DZ-S binding at both 0 and 37 degrees C, whereas it had no effect on DZ-IS binding at 0 degrees C and was stimulatory at 37 degrees C. The pH profiles of [3H]Ro 15-4513 binding were quite similar in both binding sites in the pH range of 5.5-10.5 but differed at acidic pH values from those reported for flunitrazepam and Ro 15-1788 (flumazenil; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H- imidazol[1,5-a][1,4]benzodiazepine-3-carboxylate) binding in DZ-S sites, suggesting that [3H]Ro 15-4513 does not interact with a histidine residue apparently present in the binding site. Zn2+, Cu2+, Co2+, and Ni2+ enhanced the binding to DZ-S sites, and the first three mentioned also enhanced the binding to DZ-IS sites. [3H]Ro 15-4513 binding activity was solubilized by various detergents. All detergents tested were more efficient in solubilizing DZ-S binding activity. High ionic strength improved especially the solubility of DZ-IS binding activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitive procedures for measuring chloride fluxes mediated by the purified glycine receptor incorporated into phospholipid vesicles

Two novel methods have been developed to directly measure chloride influx into purified glycine receptor-containing phospholipid vesicles. Using a method based on the fluorescence quenching of a chloride-sensitive probe entrapped into the vesicles, a chloride influx was detected which could be enhanced by glycine and completely abolished by the antagonist strychnine. In addition, by tracing the 36Cl- influx into the proteoliposomes, a stimulatory effect of both glycine and beta-alanine could be seen, which can be inhibited by strychnine and other glycine antagonists. These data, together with a previous report demonstrating ligand-mediated iodide fluxes in the same preparation (Biochemistry, 28 (1989) 6405-6409), clearly demonstrate the utility of the reconstituted receptor preparation to investigate some ion channel and pharmacological properties of the glycine receptor.

Functional reconstitution of a GABAA receptor purified from bovine brain

The GABAA/benzodiazepine receptor has been solubilized from bovine brain membranes and purified by benzodiazepine affinity chromatography. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed two major protein species of 53 and 56 kDa. The purified protein has been reconstituted, in a functionally active form, into phospholipid vesicles. Chloride flux responses of the reconstituted preparations were investigated in stopped-flow experiments by monitoring fluorescence changes of a chloride-sensitive dye trapped within the vesicles. Flux was rapidly stimulated by muscimol and this response was potentiated by diazepam and blocked by desensitization of the receptor and by preincubation with the channel blocker, picrotoxin.