Benzodiazepine inhibition of adenosine uptake is not prevented by benzodiazepine antagonists

Opposite effects of midazolam and beta-carboline-3-carboxylate ethyl ester on the release of dopamine from rat nucleus accumbens measured by in vivo microdialysis

This report describes the effects of midazolam and beta-carboline-3-carboxylate ethyl ester (beta-CCE) on extracellular concentrations of dopamine in the nucleus accumbens of freely moving rats measured by in vivo microdialysis. The two compounds had opposite effects, midazolam (0.075 and 0.15 mg/kg i.v.) dose dependently decreasing, and beta-CCE (3 and 10 mg/kg i.p.) dose dependently increasing, dialysate concentrations of dopamine. Flumazenil (6 micrograms/kg i.v.) did not affect the efflux of dopamine but it prevented the effects of both midazolam and beta-CCE on dopamine efflux. N6-Cyclohexyladenosine (0.1, and 1 mg/kg i.p.), a selective adenosine A1 agonist, dose dependently increased the efflux of dopamine. This effect was blocked by 8-cyclopentyl-1,3-dipropylxanthine (25 mg/kg i.p.), a selective adenosine A1 receptor antagonist, a dose which given alone did not affect dopamine efflux; responses to midazolam were not affected. 3,7-Dimethyl-1-propargylxanthine (1 and 3 mg/kg i.p.), a selective adenosine A2 receptor antagonist, did not mimic the effects of beta-CCE. The results suggest that midazolam and beta-CCE modulate dopamine release in the nucleus accumbens by an action at the benzodiazepine binding site associated with the GABAA receptor complex.

Evidence for a role for dopamine in the diazepam locomotor stimulating effect

It is well known that benzodiazepines produce dependence in humans and locomotor stimulation in experimental animals. In this study the possible involvement of catecholamines in the diazepam-induced locomotor stimulation in mice were investigated. Diazepam was found to have a biphasic effect; increasing locomotor activity at a low dose (0.25 mg/kg), while decreasing it at higher doses (greater than 0.5 mg/kg). The locomotor stimulating effect of diazepam was effectively blocked by pretreatment with the benzodiazepine receptor antagonist flumazenil, as well as with the catecholamine synthesis inhibitor alpha-methyltryrosine and the dopamine receptor antagonists haloperidol, spiperone and SCH 23390. Taken together, these data indicate that the locomotor stimulating effect observed after low doses of diazepam is due to activation of brain dopaminergic systems involved in locomotor activity. The observations are discussed in relation to the hypothesis that dependence-producing drugs activate specific brain reward systems.

Chronic benzodiazepine treatment and cortical responses to adenosine and GABA

The effects of chronic treatment of mice with clonazepam have been examined on the responses of neocortical slices to adenosine, 5-hydroxytryptamine (5-HT) and gamma-aminobutyric acid (GABA). Responses to these agonists were measured as changes in the depolarisation induced by N-methyl-D-aspartate (NMDA). Added to the superfusion medium diazepam blocked responses to adenosine but not 5-HT; this effect was not observed with 2-chloroadenosine or in the presence of 2-hydroxynitrobenzylthioguanosine. GABA was inactive in control slices but chronic treatment with clonazepam induced responses to GABA and enhanced responses to adenosine but not 5-HT. It is suggested that the induction of GABA responses may reflect the up-regulation of GABA receptors, but the increase of adenosine responses by clonazepam implies that there is no simple relationship between adenosine receptor binding and functional responses.

Chronic administration of diazepam downregulates adenosine receptors in the rat brain

Following chronic administration (10 or 20 days) of diazepam (5 mg/kg/day, subcutaneous pellets) or RO 15-1788 (5 mg/kg/day, intraperitoneally), adenosine and benzodiazepine receptors in different rat brain areas were assessed by radioligand binding studies using [3H]R-PIA for A1 receptors, [3H]NECA and [3H]R-PIA for A2 receptors and [3H]FNZ for benzodiazepine receptors. Chronic administration of diazepam for 10, but not for 20 days, decreased A2 receptors in the striatum by 46% (p less than 0.05) and A1 receptors in the hippocampus by 13% (p less than 0.05). Administration of diazepam for 10 days and 20 days failed to alter [3H]FNZ binding in all brain areas studied. However, 20 days of diazepam administration decreased the magnitude of GABA enhancement of [3H]FNZ binding in the cortex by 25% (p less than 0.05). In contrast, chronic administration of RO 15-1788 failed to alter [3H]R-PIA, [3H]NECA and [3H]FNZ binding in all brain areas. These results suggest that adenosine receptors may play a role in the CNS actions of benzodiazepines.

Blockade of non-opioid analgesia in intruder mice by selective neuronal and non-neuronal benzodiazepine recognition site ligands

In male mice, the biologically significant experience of social defeat is associated with an acute non-opioid form of analgesia. Recent studies have shown that this reaction is sensitive to certain benzodiazepine receptor ligands but is unaffected by others. The present experiments were designed to assess the possibility that activity at "non-neuronal" benzodiazepine binding sites might account for this unusual pharmacological profile. Our results show that defeat analgesia was blocked by clonazepam (0.06-3 mg/kg), Ro05-4864 (2.5-20 mg/kg), Ro05-5115 (20 mg/kg), PK11195 (5-20 mg/kg) and PK14067 (10-20 mg/kg). Furthermore, when given in combination, subthreshold doses of PK11195 (2.5 mg/kg) and clonazepam (0.03 mg/kg) totally prevented defeat analgesia. All of these effects were observed in the absence of intrinsic activity on basal nociception. Together with earlier findings, current data imply that inhibition of defeat analgesia by ligands for neuronal and/or non-neuronal benzodiazepine recognition sites is most probably unrelated to their activity at these sites. Alternative explanations for the overall patterns of results are considered.

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)

The effect of diazepam and Ro 15-1788 on extracellular ascorbic acid, DOPAC and 5-HIAA in the striatum of anaesthetized and conscious freely moving rats, as measured by differential pulse voltammetry

The effects of diazepam (10 mg/kg i.p.) and the central benzodiazepine receptor antagonist, Ro 15-1788 (30 mg/kg i.p.), on extracellular ascorbate, 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) were examined using differential pulse voltammetry in anaesthetized and freely moving rats. In anaesthetized animals, diazepam did not significantly alter the heights of peak 1 (ascorbate) or peak 3 (5-HIAA), but significantly reduced that of peak 2 (DOPAC). In freely moving rats, diazepam greatly reduced the heights of all 3 peaks. Ro 15-1788, injected 2 h after diazepam, reversed the effect of diazepam on peak 3, but not on peaks 1 and 2.

Are the analgesic effects of social defeat mediated by benzodiazepine receptors?

Social conflict in mice is associated with at least two forms of analgesia. A long-lasting opioid reaction is evident in intruder mice exposed to prolonged attack, whilst an acute non-opioid analgesia is seen in response to either defeat experience per se or the territorial scent-marking of an aggressive conspecific. Recent work from this laboratory has suggested that the non-opioid analgesic reaction to defeat experience may be mediated via benzodiazepine receptor mechanisms. The present studies were designed to further test this tentative hypothesis. Results confirmed that defeat analgesia is dose-dependently blocked by Ro15-1788 (20-40 mg/kg) and diazepam (2-4 mg/kg), and also indicated partial antagonism of the reaction by CGS8216 (2.5 mg/kg). The partial agonists CGS9896 (2.5-20 mg/kg) and ZK91296 (2.5-20 mg/kg) were ineffective in blocking the reaction, a finding also obtained with the full agonist ZK93423 (0.05-10 mg/kg). However, the antagonist/weak inverse agonist ZK93426 was found to possess significant intrinsic analgesic activity (10 mg/kg) and to enhance defeat analgesia (5-10 mg/kg). Although several interpretative frameworks for the current pharmacological profile are considered, it is concluded that full clarification of the substrates of defeat analgesia must await further investigations.

Dissimilarities between benzodiazepine-binding sites and adenosine uptake sites in astrocytes and neurons in primary cultures

The question whether the benzodiazepine receptor site in astrocytes or in neurons might be identical to the adenosine uptake site was studied by determining pharmacological profiles, inhibition types, and the effects of benzodiazepine antagonists in primary cultures of either astrocytes or neurons. Fourteen different benzodiazepines and five different adenosine uptake inhibitors displaced [3H] diazepam and inhibited adenosine uptake in both astrocytes and neurons. However, the rank orders (determined as IC50 values) with which these two parameters were affected were profoundly different, indicating dissimilarities between these two sites. For several of the compounds a difference in inhibition type (competitive vs. noncompetitive) was observed between the benzodiazepine-binding site and the adenosine uptake site in astrocytes and/or neurons, which further corroborated the conclusion of a difference between the benzodiazepine-binding site and the adenosine uptake site. Finally, the neuronal benzodiazepine antagonists RO 15-1788 and CGS-8216 and the astrocytic benzodiazepine antagonist PK 11195, which reverse the action of benzodiazepines, were not able to reverse inhibition of adenosine uptake by diazepam but exerted an inhibitory effect of their own.

The suppression of hippocampal potentials by the benzodiazepine antagonist Ro 15-1788 may be mediated by purines

The benzodiazepine antagonist Ro 15-1788 has been reported to suppress paired pulse inhibition in the hippocampal slice. It is now shown that the depression of orthodromic synaptic transmission by Ro 15-1788 can be prevented by the adenosine antagonist 8-phenyl-theophylline, or by adenosine deaminase. Since Ro 15-1788 has previously been shown to inhibit adenosine-uptake into rat brain tissue, it is suggested that this property, leading to an accumulation of extracellular adenosine, may underlie its effects on synaptic transmission.

Intrinsic actions of the benzodiazepine receptor antagonist Ro 15-1788

The imidazodiazepine Ro 15-1788 is a benzodiazepine receptor antagonist that was initially reported to be lacking in intrinsic activity in a variety of test situations in which benzodiazepine-like effects can be identified. However, many recent studies have shown that this compound does indeed have intrinsic activity in a variety of behavioural, neurological, electrophysiological and biochemical preparations in both animals and man. The purpose of the present review is firstly to describe these intrinsic actions, and secondly to consider to what extent these intrinsic actions of Ro 15-1788 have implications for current concepts of the functioning of the benzodiazepine receptor.

Basic and clinical aspects of adenosinergic neuromodulation

Adenosine and the methylxanthines have marked and opposite effects on behavior both of which are now thought to be mediated by cell surface adenosine receptors present in brain. These receptor sites have now been characterized using simple radioreceptor ligand binding techniques. Pharmacologic, autoradiographic and behavioral studies involving adenosine and the methylxanthines strongly suggest a neuromodulatory role for adenosine and indicate that adenosinergic neurons constitute an important central nervous system depressant system. A key component of the adenosinergic system is the adenosine uptake site which represents the inactivation mechanism for receptor mediated adenosine action. The adenosine uptake site can be identified as distinct from the adenosine receptor using a specific ligand. The two key components of the adenosine system, i.e., the receptor and uptake site, can therefore be studied using simple binding techniques. This should facilitate the development of new drugs specific for each system. Adenosine agonists can be expected to have sedative, anticonvulsant and anxiolytic actions whereas adenosine antagonists such as caffeine have stimulant and anxiogenic properties. Adenosine uptake blockers should have pharmacologic actions similar to adenosine agonists. The adenosinergic system, therefore, offers unique opportunities for developing new and potentially useful clinical agents.

Antagonism of caffeine-induced seizures in mice by Ro15-1788

The effect of the benzodiazepine antagonist Ro15-1788 has been tested against caffeine (200-300 mg/kg i.p.) induced convulsions in mice. It offered protection in lower doses (10 mg/kg i.p.) than had been effective previously against other convulsants such as bicuculline and leptazol. In contrast, diazepam was much less active against caffeine than against the other convulsants and offered no significant protection in doses up to 1 mg/kg i.p. If Ro15-1788 only acts on benzodiazepine receptors then one explanation for the increased activity of Ro15-1788 compared with diazepam against caffeine is that caffeine exerts its convulsant action through a direct effect on benzodiazepine receptors but is more potent at displacing diazepam than Ro15-1788 or alternatively it acts on a sub-class of benzodiazepine receptors that bind Ro15-1788 more effectively than diazepam.

Inhibition of adenosine accumulation by a CNS benzodiazepine antagonist (Ro 15-1788) and a peripheral benzodiazepine receptor ligand (Ro 05-4864)

Although benzodiazepines can inhibit adenosine uptake into central neurones, this effect is not antagonized by behaviourally effective 'benzodiazepine antagonists' such as Ro 15-1788. We now report that Ro 15-1788 and the 'peripheral' benzodiazepine ligand Ro 05-4864 themselves inhibit adenosine accumulation by rat brain synaptosomes. The inhibition of adenosine accumulation may thus underlie those behavioural effects of benzodiazepines which are mimicked but not antagonized by Ro 15-1788.