Ralph Schmiechen

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.

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

We describe here biochemical and pharmacological effects of the β-carboline ZK 93426, a new and potent benzodiazepine (BZ) receptor antagonist. 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.

ZK 91296, a partial agonist at benzodiazepine receptors

ZK 91296 (ethyl 5-benzyloxy-4-methoxymethyl-β-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.

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 by3H-rolipram. A3H-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 displace3H-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 of3H-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.

β-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.

Sensitivity of a cAMP PDE to Rolipram in Different Organs

Manipulation of cAMP-dependent mechanisms in aminergic transmission by selective phosphodiesterase (PDE) inhibitors may provide a novel therapetic approach for the management of depression (Wachtel, 1983). The PDE inhibitor rolipram has been shown in recent clinical trials to possess antidepressant activity (Horowski and Sastre, 1985). Antidepressant drug therapy needs several days before relief of symptoms is observed. Time-dependent neuronal adaptation processes may be based on an altered function of regulatory proteins, as e.g. resulting from protein phosphorylation. The therapeutic effect following rolipram treatment could therefore be explained by a mechanism involving a cAMP dependent protein kinase.

Dopaminanaloge 1,2,3,4-Tetrahydro-β-carboline

Die Beziehungen zwischen Struktur und neuropsychotroper Wirkung bei 6 neuen 1,2,3,4‐Tetrahydro‐β‐carbolinen werden untersucht. Die aktivste Verbindung 4b hat ein pharmakologisches Profil, das dem Chlorpromazin ähnelt. Keine der Substanzen hat dopaminerge Eigenschaften.

β-Carboline-3-Carboxylic Acid Ethyl Ester: a Lead for New Psychotropic Drugs

Beta-carboline-3-carboxylic-acid ethyl ester (β-CCE) was isolated by Claus Braestrup while he was searching for the endogenous ligand of the benzodiazepine receptor (BZ receptor). Although this compound was not the endogenous ligand but an artifact of the isolation procedure, it was considered a promising lead for the development of psychotropic drugs acting via the BZ receptor because of the following features: High affinity (better than diazepam) Partial selectivity for specific brain areas (in contrast to BZs) Pharmacological effects mediated by the BZ receptor but differing from BZs (antagonistic or even partial inverse agonistic) A rather simple chemical structure permitting a vast number of modifications