Partial chemical characterization of cyclopyrrolones ([3H] suriclone) and benzodiazepines ([3H]flunitrazepam) binding site: differences

The mechanism of action of zopiclone

The mechanism of action of the cyclopyrrolone hypnotic drug zopiclone involves allosteric modulation of the GABAAreceptor. Zopiclone displaces the binding of [3H]-flunitrazepam with an affinity of 28 nM, and enhances the binding of the channel blocker [35S]-TBPS. The binding of zopiclone, unlike that of hypnotic benzodiazepines, is not facilitated by GABA. Zopiclone does not distinguish between GABAA receptors containing different α-subunits (BZ1and BZ2phenotype). Studies with protein-modifying agents (egdiethylpyrocarbonate) and photoaffinity labelling suggest that cyclopyrrolones bind to a domain on the GABAA receptor different from the benzodiazepine binding domain. The consequence of this interaction with the GABAAreceptor is to potentiate responses to GABA, as can be demonstrated by electrophysiological methods. Subchronic treatment of mice with high doses of zopiclone does not produce the changes in sensitivity of the GABAAreceptor that are observed with hypnotic benzodiazepines.

Benzodiazepine binding to GABAA receptors: differential effects of sulfhydryl modification

The effects of sulfhydryl modification on benzodiazepine binding to gamma-aminobutyric acid type A (GABAA) receptors have been studied in membrane preparations from bovine cerebral cortex. After reduction of membranes with increasing concentrations of dithiothreitol, the binding of the partial inverse agonist [3H]Ro15-4513 (2.5 nM) was inhibited with an IC50 of 4.6 mM, while the binding of the classical agonist [3H]flunitrazepam was affected only at much higher concentrations (> 30 mM). Prior desensitization of the GABAA receptor by incubation with 10 microM muscimol had no effect on dithiothreitol inhibition of [3H]Ro15-4513 binding. In equilibrium assays, 10 mM dithiothreitol increased the KD for [3H]Ro15-4513 binding by more than three-fold and reduced the density of binding sites by more than one-third. The binding sites for [3H]Ro15-4513 were protected from the effects of dithiothreitol by carrying out the reduction step in the presence of 10 microM Ro15-4513 or 10 microM flunitrazepam. Alkylation of brain membranes by N-ethylmaleimide inhibited the binding of both [3H]Ro15-4513 and [3H]flunitrazepam with a similar concentration dependence. Equilibrium binding assays in the presence of 10 mM N-ethylmaleimide showed that both the affinity and number of binding sites for [3H]Ro15-4513 were reduced, whereas only the KD for [3H]flunitrazepam binding was affected. These results demonstrate that sulfhydryl modification of GABAA receptors has different effects on the binding of a classical benzodiazepine agonist and a partial inverse agonist, suggesting differences in the modes of binding of the two ligands.

Modification of benzodiazepine receptors supports the distinctive role of histidine residues

The effect of selective protein modifying reagents was examined on benzodiazepine (BZ) receptors in synaptosomal membrane preparations of rat whole brain and cerebellum. The potency of diethyl pyrocarbonate, a histidine modifying reagent, to inactivate BZ receptor binding, correlated with the rank order of agonist-inverse agonist efficacies of BZ ligands, the binding of the partial inverse agonist [3H]Ro 15-4513 was inactivated least. Diethyl pyrocarbonate slightly enhanced the displacing potency of Ro 15-4513 and enhanced its binding in low concentrations (1-2 mM). Diazepam-sensitive and -insensitive components of [3H]Ro 15-4513 binding were separated in cerebellum. Diethyl pyrocarbonate inactivated the diazepam-sensitive component with a potency (IC50 = 1.8 mM) similar to that on the binding of other benzodiazepines, while the diazepam-insensitive component was resistant to diethyl pyrocarbonate. Tetranitromethane and 2,3-butanedione (diacetyl), reagents specific for tyrosine and arginine residues respectively, exerted concentration-dependent partial inactivation of [3H]Ro 15-4513 binding. The diazepam-insensitive component of cerebellar Ro 15-4513 binding was more sensitive to inactivation by diacetyl but less sensitive to inactivation by tetranitromethane. These findings are consistent with a distinctive role of histidine-101 in alpha 1, alpha 2, alpha 3 and alpha 5 subunits of the gamma-aminobutyric acidA receptor complex and the His is replaced by an arginine residue in the alpha 6 subunit of the diazepam-insensitive cerebellar benzodiazepine receptors. The only other point of the protein sequence where histidine residues conserved in alpha 1, alpha 2, alpha 3 and alpha 5 subunits are replaced in alpha 6 is tyrosine-214 but this residue does not appear to contribute to benzodiazepine binding.

The effects of acute and repeated doses of suriclone on subjective sleep, psychomotor performance and cognitive function in young and elderly volunteers

Suriclone is a new anxiolytic drug belonging to the family of cyclopyrrolones. The effects of acute and repeated doses of suriclone on subjective sleep, psychomotor performance and cognitive function were compared to those of placebo in young and elderly volunteers. Young volunteers randomly received suriclone 0.2 mg, 0.3 mg, 0.4 mg or placebo tid, and the elderly received suriclone 0.1 mg, 0.2 mg or placebo tid. After the first single dose and after a three-day treatment, subjects completed at 1, 2, 4, 12 and 24 h after drug administration the following battery of psychomotor and cognitive tests: critical flicker fusion threshold, choice reaction time, simulated car tracking test, the stroop test and the Sternberg memory scanning task. Visual analogue scales and the Leeds sleep evaluation questionnaire were also administered during the study. No significant effects of suriclone compared to placebo were seen on the psychomotor tests both in young and elderly volunteers. The only significant result was an improvement of the ease of getting to sleep in the young with 0.4 mg suriclone tid. In conclusion, there is little evidence to suggest that suriclone produces any measurable behavioural toxicity, so often seen with many of the benzodiazepines, in either young or elderly subjects.

Controlled comparison of the efficacy and safety of four doses of suriclone, diazepam, and placebo in generalized anxiety disorder

The anxiolytic activity and tolerance of four doses of suriclone (0.1, 0.2, 0.3 and 0.4 mg tid), diazepam (5 mg tid), and placebo were compared in six parallel groups of 54-59 outpatients with generalized anxiety disorder (DSM III-R). After a 1-week placebo run-in period, the patients were treated for 4 weeks, with assessments at baseline and after 1, 2, and 4 weeks by the Hamilton anxiety scale and the Clinical Global Impressions. Results showed better improvement with active drugs as compared to placebo, without significant differences among the four different doses of suriclone and diazepam. The number of adverse events, particularly drowsiness, was significantly higher with diazepam than with suriclone, particularly 0.1 and 0.2 mg tid which did not differ from placebo. These results demonstrate that suriclone at daily doses ranging from 0.1 to 0.4 mg tid is an effective anxiolytic, better tolerated than diazepam.

Sleep laboratory studies on single dose effects of suriclone

1. In a double-blind, placebo-controlled sleep laboratory study single doses of suriclone, a new non-benzodiazepine anxiolytic binding to benzodiazepine receptors, were investigated with respect to sleep and awakening. 2. Sixteen healthy young volunteers spent 10 nights in the sleep laboratory: 1 adaptation night, 1 baseline night and 4 drug nights (placebo; 0.2 mg, 0.4 mg suriclone; 2 mg lorazepam as reference drug) and 4 subsequent wash-out nights (drug-interval: 1 week). Somnopolygraphic investigations (22.30 h to 06.00 h) were commenced 0.5 h after drug-intake. A self-rating scale for sleep and awakening quality as well as psychometric tests were completed in the morning. 3. Hypnotic effects were most pronounced after lorazepam in regard to total sleep time and sleep efficiency. After lorazepam as well as after 0.4 mg suriclone nocturnal awakenings decreased significantly as compared with placebo, which was reflected in an improved subjective sleep quality after both dosages. Suriclone 0.2 mg did not induce any alterations in all night sleep. 4. In the morning, well-being, drowsiness and reaction time performance deteriorated after lorazepam as compared with placebo but not after suriclone. The latter was significantly superior to lorazepam with respect to subjective awakening quality, well-being, emotional rapport, drowsiness and attention. 5. Blood pressure and pulse remained unchanged after all of the drugs. Critical flicker frequency and muscle strength decreased only after lorazepam as compared with placebo.

Cyclopyrrolones, unlike some benzodiazepines, do not induce physical dependence in mice

In a model of physical dependence in mice, treatment with cyclopyrrolones such as zopiclone and suriclone (from 4 to 400 mg/kg/day), did not modify the sensitivity of the gamma-aminobutyric acid (GABA) receptor complex to the partial inverse agonist FG 7142 following their withdrawal, whereas sensitivity changes were observed after treatment and withdrawal from some benzodiazepines (e.g. lorazepam, diazepam, flunitrazepam and triazolam). These data suggest that, in contrast to some benzodiazepines, zopiclone and suriclone may not produce physical dependence.

Preferential decrease in dopamine utilization in prefrontal cortex by zopiclone, diazepam and zolpidem in unstressed rats

This study has compared the effects of a cyclopyrrolone, zopiclone, a benzodiazepine, diazepam, and an imidazopyridine, zolpidem, on dopamine (DA) and DOPAC levels, and DA utilization (DOPAC/DA ratio) in rat striatum and prefrontal cortex. The endogenous levels of DA were significantly increased by both zopiclone (2.5, 10 and 40 mg kg-1 p.o.) and diazepam (10 and 40 mg kg-1 p.o.) in the prefrontal cortex, whereas striatal DA content was significantly increased only with the highest dose of diazepam (40 mg kg-1 p.o.). Diazepam (10 and 40 mg kg-1 p.o.) decreased cortical level of DOPAC more markedly than striatal levels, whereas zopiclone (40 mg kg-1 p.o.) only slightly decreased striatal DOPAC levels. Zopiclone and diazepam dose-dependently decreased DA utilization, an effect which was more marked in prefrontal cortex than in striatum. This result was confirmed with zolpidem, another benzodiazepine ligand. Zopiclone was most potent at decreasing DA utilization at the cortical level. The diazepam-induced decreases in DA metabolism and utilization were antagonized by Ro 15-1788, suggesting that the effects seen were mediated by specific benzodiazepine receptors. Thus, our results clearly show that ligands acting on the benzodiazepine receptor GABA receptor chloride ionophore complex can decrease the utilization of dopamine in unstressed rats. The preferential decrease in cortical DA utilization induced by benzodiazepine ligands may be compared to the well-known activation by stress of the mesocortical DAergic system.

In vivo determination of the profile of benzodiazepine ligands by comparing the inhibition of 3H-Ro 15-1788 binding to the modulation of cGMP levels in mouse cerebellum

The in vivo effects of various benzodiazepine (BZD) ligands belonging to different chemical families were studied comparatively in mouse cerebellum using displacement of 3H-Ro 15-1788 binding and cGMP content as biochemical tools. It was possible to differentiate four classes of compounds with regard to these biochemical parameters. The first class of compounds such as diazepam and suriclone induced a net effect on in vivo 3H-Ro 15-1788 binding and a dose-dependent decrease of cGMP levels. A second class of drugs such as ZK 91296 and CGS 9896 showed in vivo activities in displacement studies but relatively small or moderate activities on cGMP levels. A third class was represented by Ro 15-1788 itself which prevented dose-dependently the in vivo 3H-Ro 15-1788 binding but was devoid of effect on cGMP levels. Finally, a fourth class of compounds (CGS 8216, FG 7142, beta-CCM and DMCM) showed in vivo displacement of 3H-Ro 15-1788 with concomitant increase of cGMP levels. The first class of compounds represents full agonists, the second class, partial agonists, the third class, the antagonist Ro-15-1788 itself, and the fourth class corresponds to inverse agonists. Thus it is proposed to use 3H-Ro 15-1788 binding and cGMP levels to differentiate in vivo BZD ligands acting on the BZD receptor/GABA receptor/chloride ionophore complex.

Benzodiazepine receptors in human brain: characterization, subcellular localization and solubilization

1. Benzodiazepine receptors have been characterized in human brain mainly using [3H]-Ro 15-1788 and [3H]-flunitrazepam. Both ligands present a very high affinity for the receptor sites (Kd values of 0.56 and 1.53 nM respectively). 2. GABA enhanced the affinity of [3H]-flunitrazepam and [3H]-diazepam, but not that of [3H]-Ro 15-1788 and [3H]-methyl-beta-carboline 3-carboxylate for their specific binding sites as well in cerebral as in cerebellar human cortex. 3. Subcellular distribution of the benzodiazepine receptors revealed a main synaptosomal localization in human cerebral cortex, cerebellum and striatum. 4. Solubilized benzodiazepine receptors were obtained using 0.5% sodium deoxycholate and were characterized with [3H]-Ro 15-1788. The solubilized receptors are still coupled to GABA receptors since the [3H]-flunitrazepam specific binding was enhanced in the presence of micromolar concentrations of GABA.

Suriclone and diazepam in the treatment of neurotic anxiety. A double-blind cross-over trial

Suriclone is a new anxiolytic drug belonging to the family of cyclopyrrolones. Although chemically entirely different from the benzodiazepines, it acts as a benzodiazepine agonist with very high affinity for the benzodiazepine receptors. In the present cross-over study, 33 out-patients with a diagnosis of neurotic anxiety were treated with suriclone (mean dose 2 mg/day) and diazepam (25 mg/day) in two 6-week periods. Both drugs had a significant anxiolytic effect, but diazepam appeared to have a better effect within the first 2 weeks of treatment, while no significant difference was seen after treatment, while no significant difference was seen after treatment for 6 weeks. Suriclone and diazepam had a different side effect profile: suriclone produced mainly dizziness, while diazepam caused sedation. This may reflect the fact that suriclone and benzodiazepines bind to distinct sites or different allosteric conformations of the benzodiazepine receptors.