Ro 15-1788 both antagonizes and potentiates adenosine-evoked depression of cerebral cortical neurons

The amygdala regulates the antianxiety sensitization effect of flumazenil during repeated chronic ethanol or repeated stress

BACKGROUND

The benzodiazepine receptor antagonist flumazenil reduces anxiety-like behavior and sensitization of anxiety-like behavior in various models of ethanol withdrawal in rodents. The mechanism and brain region(s) that account for this action of flumazenil remain unknown. This investigation explored the potential role of several brain regions (amygdala, raphe, inferior colliculus, nucleus accumbens, and paraventricular hypothalamus) for these actions of flumazenil.

METHODS

Rats were surgically implanted with guide cannulae directed over the brain region of interest and then treated with an ethanol diet for three 7-day dietary cycles (5 days on ethanol diet followed by 2 days on control diet). At approximately 4 hours, flumazenil was administered intracranially into each of the first 2 withdrawals. Examinations of anxiety-like behavior followed 1 week later during a third withdrawal. In other animals, restraint stress sessions or intra-amygdala DMCM (methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) injections, preceded by intraperitoneal flumazenil injections, were substituted for the first 2 ethanol treatment cycles to assess the potential anxiety-sensitizing action of stress or a benzodiazepine receptor inverse agonist, respectively.

RESULTS

Flumazenil treatment of the amygdala during the first 2 withdrawals blocked the development of sensitized anxiety seen during a third withdrawal. Similar actions of flumazenil were found when stress sessions substituted for the first 2 cycles of ethanol exposure and withdrawal. Amygdala treatment with DMCM magnified the anxiety response to the single subthreshold chronic ethanol treatment, and prophylactic flumazenil blocked this effect.

CONCLUSIONS

Intra-amygdala flumazenil inhibits the development of anxiety sensitized by repeated ethanol withdrawal, stress/ethanol withdrawal, or DMCM/ethanol withdrawal. These actions suggest that site-specific and persistent effects of flumazenil on gamma-aminobutyric acid-modulatory processes in this brain region are relevant to sensitized behavioral effects seen in alcoholism.

Chronic Ro 15-1788 treatment increases REM sleep in rats

Administration of Ro 15-1788, a benzodiazepine antagonist (3.6 mg/kg/day in drinking water for 14 days), increased total sleep and rapid eye movement (REM) sleep in rats. Standard six-hour EEG recording periods were obtained on day 0, 1, 3, 7, 10, 14, as well as 24 and 72 hours following withdrawal. Enhanced REM sleep reached significance on day 7 of continuous drug treatment and remained significantly increased on day 10 and 14, as well as at 24 and 72 hours following drug withdrawal. The present data show that chronic administration of Ro 15-1788 increases total sleep time due to increases in REM sleep. The actions of Ro 15-1788 presumably occur through either adenosinergic or cholinergic mechanisms.

Interaction of caffeine with the GABAA receptor complex: alterations in receptor function but not ligand binding

Behavioral and neurochemical evidence indicates interactions between caffeine and other adenosine receptor ligands and the gamma-aminobutyric acid (GABA)-benzodiazepine system. To assess the effects of caffeine on binding and function at the GABAA receptor, we studied the effects of behaviorally-active doses of caffeine on benzodiazepine and Cl- channel binding and on overall function of the GABAA receptor as measured by Cl- uptake. There was no effect of caffeine on benzodiazepine receptor binding in cortical synaptosomal membranes at concentrations of 1-100 microM. No effects on benzodiazepine binding were found ex vivo in mice treated with caffeine, 20 and 40 mg/kg. At the putative Cl- channel site labeled by t-butylbicyclophosphorothionate (TBPS), binding was unchanged in vitro after caffeine treatment (1 and 10 microM) in washed and unwashed membranes. However, in ex vivo studies caffeine (20 and 40 mg/kg) increased numbers of TBPS sites in unwashed but not washed membranes. Muscimol-stimulated Cl- uptake into cortical synaptoneurosomes was decreased in mice treated with caffeine, 20 and 40 mg/kg. Similar results were observed in in vitro preparations treated with 50 microM but not 100 microM caffeine. These results indicate that caffeine administration significantly alters the Cl- transport function of the GABAA receptor complex.

Effects of estradiol on cerebral cortical neurons and their responses to adenosine

The effects of iontophoretically applied 17 alpha- and 17 beta-estradiol on the spontaneous firing of, and actions of purines (adenosine, adenosine 5'-N-ethylcarboxamide) on, neurons in the rat cerebral cortex have been determined. Both steroids, applied as acetate, sulphate or hemisuccinate esters, depressed the spontaneous firing of approximately half of the neurons tested. 17 beta-Estradiol potentiated the inhibitory actions of adenosine on some neurons (26%) and antagonized the effects of adenosine on others (42%). 17 beta-Estradiol antagonized the inhibitory actions of adenosine 5'-N-ethylcarboxamide on 76% of the neurons tested, but did not enhance the actions of this uptake resistant adenosine analog. 17 alpha-Estradiol potentiated the actions of adenosine and adenosine 5'-N-ethylcarboxamide but failed to antagonize either purine. It is suggested that the potentiating effects of both stereoisomers on adenosine are a result of their ability to block adenosine uptake and that 17 beta-estradiol is also able to antagonize the actions of these purines. Antagonism of the effects of endogenously released adenosine may account for the excitant actions of 17 beta-estradiol on the central nervous system.

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.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

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 role of adenosine in the central actions of the benzodiazepines

1. Evidence is presented which indicates that the central actions of the benzodiazepines cannot be fully accounted for by assuming an action only at the GABAA-Cl- channel supramolecular complex. 2. The hypothesis is presented, together with supporting evidence, that inhibition of adenosine uptake can account for many of the actions of the benzodiazepines. 3. New findings showing that Ro 15-1788 and Ro 5-4864 have both potentiative and antagonistic interactions with adenosine are discussed. 4. The proconvulsant beta-carbolines are shown to be adenosine antagonists. 5. The concept that benzodiazepine action may involve several mechanisms is presented.