ZK 91296, a partial agonist at benzodiazepine receptors

ZK 91296 (ethyl 5-benzyloxy-4-methoxymethyl--carboline-3-carboxylate) is a potent and selective ligand for benzodiazepine (BZ) receptors. Biochemical investigations indicate that ZK 91296 may be a partial agonist at BZ receptors. Such partial agonism may explain to some extent why ZK 91296 needs higher BZ receptor occupancy than diazepam for the same effect against chemical convulsants and for behavioural effects. The lack of sedatiye effects, and the very potent inhibition of reflex epilepsy, spontaneous epilepsy and DMCM-induced seizures suggest, furthermore, that ZK 91296 may possess pharmacological selectivity for a particular type of BZ receptor interaction, perhaps including topographic as well as receptor subtype differentiation.     

Searching for endogenous benzodiazepine receptor ligands

The discovery of benzodiazepine receptors in 1977 prompted an intensive search for endogenous ligands with benzodiazepine-like activity or the opposite. Derivatives of β-carboline 3-carboxyclic acid are very potent and seem interesting. It is also interesting that benzodiazepine receptors and GABA receptors are tightly coupled at the molecular level.     

Escitalopram versus citalopram: the surprising role of the <Emphasis Type="Italic">R</Emphasis>-enantiomer

Rationale Citalopram is a racemate consisting of a 1:1 mixture of the R(–)- and S(+)-enantiomers. Non-clinical studies show that the serotonin reuptake inhibitory activity of citalopram is attributable to the S-enantiomer, escitalopram. A series of recent non-clinical and clinical studies comparing escitalopram and citalopram to placebo found that equivalent doses of these two drugs, i.e. containing the same amount of the S-enantiomer, showed better effect for escitalopram. These results suggested that the R-citalopram in citalopram inhibits the effect of the S-enantiomer.Objective To review the pharmacological and non-clinical literature that describes the inhibition of escitalopram by R-citalopram, as well as the implications of this inhibition for the clinical efficacy of escitalopram compared to citalopram.Methods The information in this review was gathered from published articles and abstracts.Results In appropriate neurochemical, functional, and behavioural non-clinical experiments, escitalopram shows greater efficacy and faster onset of action than comparable doses of citalopram. The lower efficacy of citalopram in these studies is apparently due to the inhibition of the effect of the S-enantiomer by the R-enantiomer, possibly via an allosteric interaction with the serotonin transporter. Data from randomised clinical trials consistently show better efficacy with escitalopram than with citalopram, including higher rates of response and remission, and faster time to symptom relief.Conclusion The R-enantiomer present in citalopram counteracts the activity of the S-enantiomer, thereby providing a possible basis for the pharmacological and clinical differences observed between citalopram and escitalopram.     

Modulation of GABA binding to rat brain membranes by alkyl β-carboline-3-carboxylate esters

The effects of the methyl, ethyl and propyl esters of β-carboline-3-carboxylic acid were assessed on low affinity binding of GABA to rat brain membranes, and the enhancement of such binding by diazepam. The propyl ester acted as a benzodiazepine agonist in enhancing low affinity GABA binding, while the methyl and ethyl esters acted as benzodiazepine antagonists in reversing the stimulation of GABA bindingby diazepam. These effects on low affinity GABA binding in vitro are consistent with pharmacological and behavioural actions of these esters in vivo and support the hypothesis that such actions are mediated via a GABA-benzodiazepine receptor complex.     

Reduction of 3H-spiroperidol binding in rat striatum and frontal cortex by chronic amphetamine: Dose response,time course and role of sustained dopamine release

After 5 days of continuous treatment with d-amphetamine base in doses greater than 0.5 mg/kg/h maintained by subcutaneously implanted osmotic minipumps, specific binding of 3H-spiroperidol was reduced in rat striatum and frontal cortex as previously reported. These effects were dose-dependent at lower doses of amphetamine, whereas with higher doses an apparent ceiling for the reduction in binding was reached at approximately 70% of control values. Similarly, increasing the exposure time to amphetamine for up to 14 days only slightly augmented the reduction in 3H-spiroperidol binding already present after 5 days of treatment. In rats treated for 5 days with amphetamine, concomitant treatment with the dopamine (DA) synthesis inhibitor alpha-methyl-p-tyrosine prevented the decrease in 3H-binding in corpus striatum, and attenuated the decrease in frontal cortex. Furthermore, in rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal DA tract, 5 days of chronic amphetamine had no significant effect on 3H-spiroperidol binding in the denervated striatal tissue. Since a major effect of amphetamine is to release DA from nerve terminals, these results indicate that the reduction of DA receptors by chronic amphetamine in the striatum is mediated by sustained release of DA.     

Benzodiazepine receptor ligands with positive and negative efficacy

Recent studies have shown that benzodiazepine receptors can be affected not only by benzodiazepine agonists and antagonists but also by a new class of ligands which produce effects opposite to those of benzodiazepines, that is they produce convulsions and anxiety. These ligands can be described as having a negative efficacy at the receptor; tentatively they are named “inverse agonists”. Pharmacological experiments indicate that agonists, antagonists and inverse agonists comprise a whole continuum of agents with a graduated variety of efficacy at the receptor. Biochemical studies, supported by published electrophysiological data, indicate that the benzodiazepine receptor allosterically up- or down-regulates the gain in the GABAergic system depending on the nature of the ligand.     

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.