Antagonism of the anxiolytic action of diazepam and chlordiazepoxide by the novel imidazopyridines, EMD 39593 and EMD 41717

Substituted imidazo[1,2-b]pyridazines

A large range of substituted imidazo[1,2-b]pyridazines have been synthesized, and a number of potent ligands at central benzodiazepine (Bz) receptors on rat brain membranes have been identified in initial binding screens using [3H]diazepam. For those tested more extensively, binding studies conducted in the presence and absence of gamma-aminobutyric acid suggest that they were full receptor agonists. Some preliminary evidence was found suggesting some species selectivity, i.e. several of the compounds were more active in in vivo tests in rats than in mice. The agonist activity of these 2-phenyl (and substituted phenyl) imidazo[1,2-b]pyridazines is consistent with the model of Bz receptor ligands as proposed by Fryer [Raven Press, 1983, pp. 7-20]. Several compounds were identified which had more selective activity at peripheral-type (mitochondrial) Bz binding sites. Thus, substituted imidazo[1,2-b]pyridazines represent yet another class of low molecular mass compounds which have activity at Bz receptor sites.

Myorelaxant effect of baclofen injected to the nucleus accumbens septi

The GABAergic modulation in the nucleus accumbens septi (NAS) of muscle tone was investigated in rats using behavioral tests. The GABAB receptor agonist baclofen dose-dependently decreased muscle tone in the wire-mesh and bar holding tests both after local injection into the NAS (1.0 and 2.5 micrograms), and after intraperitoneal administration in a dose of 20 mg/kg. In the Wirth's test haloperidol (5 mg/kg i.p.), produced catalepsy, whereas baclofen (20 mg/kg, i.p.) significantly deteriorated rats' performance. Intraaccumbens microinjections of muscimol, midazolam, nicardipine, as well as peripheral injections of haloperidol and midazolam failed to modify muscle tone in the wire-mesh test. These findings argue against the involvement of GABAA receptors, benzodiazepine receptors, as well as dopaminergic- and calcium channel-related mechanisms in the effect of baclofen. Hence, the muscle relaxant effect of baclofen seems to be also mediated through GABAB receptor sites within the NAS.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

CGS 8216, Ro 15-1788 and methyl-beta-carboline-3-carboxylate, but not EMD 41717 antagonize the muscle relaxant effect of diazepam in genetically spastic rats

Diazepam (0.4-4 mg/kg i.p.) reduced the spontaneous tonic activity in the electromyogram (EMG) recorded from the gastrocnemius-soleus muscle of spastic mutant Han-Wistar rats in a dose-dependent manner. The muscle relaxant effect of diazepam was antagonized by the benzodiazepine antagonists Ro 15-1788 (5 mg/kg i.p.), beta-CCM (2 mg/kg i.p.) and CGS 8216 (5 mg/kg i.p.), but not by EMD 41717 (50 mg/kg i.p.). These results add further support to the hypothesis that Ro 15-1788, CGS 8216 and beta-CCM do antagonize all pharmacological effects of benzodiazepines while EMD 41717 displays more selectivity in antagonizing the different actions of benzodiazepines.

Molecular aspects of the action of benzodiazepine and non-benzodiazepine anxiolytics: a hypothetical allosteric model of the benzodiazepine receptor complex

The availability of radiolabeled benzodiazepines has resulted in the identification of high affinity receptors in the central nervous system for this class of psychotherapeutic agent which are linked to recognition sites for the inhibitory neurotransmitter, GABA. Evaluation of new, synthetic compounds in the benzodiazepine radioligand binding assay has resulted in the identification of nine classes of non-benzodiazepine putative anxiolytic agents, some of which may be more anxioselective than the benzodiazepines. At least three and possibly five subclasses of benzodiazepine receptor have been identified in mammalian tissues using radioligand binding assays. The possibility exists that one of these receptor subclasses may mediate the anxiolytic effects of the benzodiazepines while the remainder may be involved in the mediation of the sedative, ataxic and anticonvulsant properties associated with benzodiazepine-like agents. Several endogenous ligands for the benzodiazepine receptor(s) have been postulated. These include various proteins and peptides, purines and the beta-carbolines. This latter group, which competitively antagonizes the pharmacological and biochemical effects of the benzodiazepines, has the highest affinity for the benzodiazepine receptor of all compounds thus far examined; however, none of these compounds has been conclusively identified as the endogenous ligand akin to the enkephalins and endorphins at the opiate receptor. The majority of available evidence would indicate that the endogenous ligand for the benzodiazepine receptor(s) is an antagonist of the benzodiazepines and other putative anxiolytic agents.

In vitro characterization of agonist, antagonist, inverse agonist and agonist/antagonist benzodiazepines

The efficacy of coupling between the benzodiazepine receptor and chloride channel as well as the coupling to the GABA receptor is differentially affected by different benzodiazepine ligands. In general, the order of efficacy with regard to allosteric effects of benzodiazepine ligands on the chloride channel ([35S]TBPS) and GABA receptor ([3H]muscimol), is: agonist greater than agonist/antagonist greater than partial agonist greater than antagonist; with inverse agonists acting in a manner opposite to the classical benzodiazepine agonists. The chloride ionophore is allosterically modulated both by benzodiazepine and GABA receptors.