Abecarnil, a metabolically stable, anxioselective beta-carboline acting at benzodiazepine receptors.

Abecarnil (isopropyl 6-benzyloxy-4-methoxymethyl-beta-carboline-3-carboxylate) is a novel ligand for central benzodiazepine (BZ) receptors, possessing anxiolytic and anticonvulsant properties, but with considerably reduced muscle relaxant effects in comparison to diazepam (DZP). In vitro, abecarnil inhibited the binding of the BZ [3H]lormetazepam to rat cerebral cortex membranes with an IC50 value of 0.82 nM in comparison to 56 nM for DZP. The ability of abecarnil to displace [3H]lormetazepam was enhanced 1.24-fold in the presence of 30 microM gamma-aminobutyric acid; the corresponding value for DZP was 2.8-fold. DZP and abecarnil were equally effective in enhancing the binding of t-[35S]butylbicyclophosphorothionate to rat cortical membranes. In vivo, abecarnil exhibited a 3- to 6-fold higher affinity to forebrain BZ receptors than DZP. Abecarnil was from 2 to 10 times more potent than DZP in most rodent tests of anxiolytic activity, and in reducing locomotor activity in mice and rats thoroughly habituated to the test chamber. However, in rats newly exposed to a novel cage, abecarnil was less potent than DZP in reducing locomotor activity. In tests of motor coordination, abecarnil, in contrast to DZP, showed no or only weak activity, and in potentiating the effects of ethanol and hexobarbital on motor performance abecarnil was 4 to 25 times less potent than DZP. Abecarnil antagonized the effects of BZs in the chimney and loss of righting reflex tests, but not in the rotarod test.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant action of the beta-carboline abecarnil: studies in rodents and baboon, Papio papio.

Abecarnil (ZK 112119; isopropyl-6-benzyloxy-4-methoxymethyl-beta-carboxylate) is a metabolically stable beta-carboline derivative with potent anxiolytic and few sedative and ataxic effects in rodents. The anticonvulsant and muscle relaxant actions of abecarnil have been evaluated in mice, rats, gerbils and baboons. Abecarnil raised the threshold for tonic electroconvulsions in mice after corneal but not after auricular application, had no effect on maximal electroshock-induced tonic convulsions triggered by either method, protected mice against the tonic hindlimb extension in PTZ-, picrotoxin- and 3-mercaptopropionate-induced seizures and blocked clonus after PTZ, DMCM (methyl-4-ethyl-6,7-dimethoxy-9H-pyrido-(3,4-b)-indol-3-carboxylate) and 3-mercaptopropionate. Abecarnil had no effect on convulsions induced by bicuculline and strychnine. Furthermore, abecarnil blocked kindled seizures after chronic administration of PTZ and FG 7142 (beta-carboline-3-carboxylic acid methylamide) and protected mice and rats against limbic convulsions induced by pilocarpine. Severity and afterdischarge duration of amygdala-kindled seizures were reduced in rats treated with abecarnil. Abecarnil also antagonized selectively convulsions induced by i.c.v. administration of kainate, but not those triggered by N-methyl-D-aspartate or quisqualate. In genetic models of reflex epilepsy, abecarnil was effective against sound-induced convulsions in DBA/2 mice, against air blast-induced generalized seizures in gerbils and against myoclonus in baboons Papio papio. The anticonvulsant effect of abecornil in a PTZ seizure model in mice was potentiated by ethosuximide, whereas no significant potentiation was found with diazepam, clonazepam, diphenylhydantoin, carbamazepine and phenobarbital. Electromyographic monitoring in a etorphine model of muscle rigidity in rats showed no or little muscle relaxant effect of abecarnil.(ABSTRACT TRUNCATED AT 250 WORDS)

Close correlation between behavioural response and binding in vivo for inhibitors of the rolipram-sensitive phosphodiesterase.

The antidepressant rolipram interacts in vitro with a binding site in brain tissue labeled by 3H-rolipram. A 3H-rolipram binding assay was employed in vivo to compare the affinity of rolipram-related compounds and reference phosphodiesterase (PDE) inhibitors with their potency in behavioural measures for potential antidepressant property. In two species, mice and rats, the potency of a number of compounds to antagonise reserpine-induced hypothermia (mice) and to induce head twitches (rats) was determined, as well as their potency to displace 3H-rolipram from forebrain binding sites in vivo. The treatment schedules for the two series of experiments were identical. Significant correlations between pharmacological effects and displacement of 3H-rolipram binding in vivo were observed in both species. Since the reference PDE inhibitors closely fit into the binding-pharmacological activity relationship, the PDE inhibitory properties of the substances involved are discussed.

Stereospecific binding of the antidepressant rolipram to brain protein structures.

The characteristics for the binding of the selective cAMP phosphodiesterase inhibitor and antidepressant agent rolipram to brain and peripheral organs were investigated. (+/-)-[3H]Rolipram equilibrium binding and Scatchard analysis revealed saturable, reversible, stereospecific, Mg2+-dependent and heat-sensitive binding with an apparent Hill number of 1. Binding was detected both to membrane-bound and soluble sites, with dissociation constants Kd of 1.2 and 2.4 nM, respectively, and binding site concentrations (Bmax) of 19.3 and 23.6 pmol/g rat forebrain. The (-)-enantiomer of rolipram was ca. 20 times more effective than the (+)-enantiomer in displacing (+/-)-[3H]rolipram from membranes. Rolipram bound to brain tissue of all mammalian species tested including man, while tissue from bird and fish showed less binding. Organs other than brain exhibited only negligible binding. Only specific cAMP phosphodiesterase inhibitors (ICI 63.197, Ro 20-1724) were potent competitors, while rolipram itself was inactive in a variety of receptor binding assays of neuroactive ligands. The kinetics of (-)-[3H]rolipram binding to the particulate fraction revealed a complex association and dissociation behaviour. The nature of the rolipram binding protein(s) is not clear, but the low affinity binding site evident from binding kinetics may represent a rolipram-sensitive phosphodiesterase isoenzyme also common to some peripheral organs, while the high affinity binding site(s) may be related to PDE isoenzymes more confined to the central nervous system.

Beta-carbolines with agonistic and inverse agonistic properties at benzodiazepine receptors of the rat.

The anxiogenic or anxiolytic properties of five beta-carbolines were assessed in rats trained in two-lever operant chambers to operate one lever to obtain food reward when treated with a training drug, and the other when treated with saline. Two such drug-discrimination tests were used, employing either chlordiazepoxide (CDP) or pentylenetetrazol (PTZ) as training drugs. The benzodiazepine (BZ) agonist, ZK 93 423, substituted for the CDP cue and antagonized the PTZ cue. The inverse agonists DMCM and FG 7142 showed the opposite effects. An antagonist, ZK 93 426, antagonized the CDP cue but did not substitute for PTZ, while a partial agonist was identified as CDP-like but only partially antagonized the PTZ cue. An anxiolytic profile (substitution for CDP and antagonism of PTZ) was associated with GABA ratios of about 2, and an anxiogenic profile with GABA ratios less than 1. The antagonist and partial agonist displayed intermediate values. These observations are generally consistent with the beta-carbolines modulating anxiety through their activity at BZ receptors influencing GABAergic neurotransmission.

ZK 91296, a partial agonist at benzodiazepine receptors.

5-benzyloxy-4-methoxymethyl-beta-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 sedative 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.

Evaluation of the beta-carboline ZK 93 426 as a benzodiazepine receptor antagonist.

We describe here biochemical and pharmacological effects of the beta-carboline ZK 93426 was compared with Ro 15-1788 and CGS 8216, two compounds previously described as BZ receptor antagonists. Certain effects of ZK 93426, Ro 15-1788 and CGS 8216 were quite similar (e.g., 3H-FNM displacement, "GABA ratio", "photo-shift"). In most pharmacological tests ZK 93426 and Ro 15-1788 lacked overt effects; Ro 15-1788 was a weak agonist in some paradigms, while ZK 93426 exhibited a potent proconflict effect but also a weak anticonvulsant effect. This interesting finding with ZK 93426 suggests that BZ receptor ligands may possess differential efficacy at BZ receptor subtypes. In contrast, CGS 8216 exhibited potent proconvulsant effects in several paradigms in addition to proconflict and pentylenetetrazol generalizing effects. ZK 93426, Ro 15-1788 and CGS 8216 were almost equally potent as antagonists of the effects of BZ receptor agonists, such as diazepam and lorazepam. However, ZK 93426 was the most potent inhibitor of the convulsions produced by the BZ receptor inverse agonist DMCM.

Interaction of convulsive ligands with benzodiazepine receptors.

The gamma-aminobutyric acid (GABA)-benzodiazepine receptor complex, which is composed of distinct proteins embedded in the neuronal plasma membrane, is important for several effects of benzodiazepines, including protection afforded against convulsions. During structural modification of ethyl beta-carboline-3-carboxylate an agent was discovered which has high affinity for brain benzodiazepine receptors but which is a potent convulsant. Also in contrast to benzodiazepines, this type of benzodiazepine receptor ligand favors benzodiazepine receptors in the non-GABA-stimulated conformation, which may explain the convulsive properties.