The effect of the imidazodiazepine Ro 15-4513 on the anticonvulsant effects of diazepam, sodium pentobarbital and ethanol

Zolpidem generalization and antagonism in male and female cynomolgus monkeys trained to discriminate 1.0 or 2.0 g/kg ethanol

BACKGROUND

The subtypes of gamma-aminobutyric acid (GABA)(A) receptors mediating the discriminative stimulus effects of ethanol in nonhuman primates are not completely identified. The GABA(A) receptor positive modulator zolpidem has high, intermediate, and low activity at receptors containing alpha(1), alpha(2/3), and alpha(5) subunits, respectively, and partially generalizes from ethanol in several species. The partial inverse agonist Ro15-4513 has the greatest affinity for alpha(4/6)-containing receptors, higher affinity for alpha(5)- and lower, but equal, affinity for alpha(1)- and alpha(2/3)-, containing GABA(A) receptors, and antagonizes the discriminative stimulus effects of ethanol.

METHODS

This study assessed Ro15-4513 antagonism of the generalization of zolpidem from ethanol in male (n = 9) and female (n = 8) cynomolgus monkeys (Macaca fascicularis) trained to discriminate 1.0 g/kg (n = 10) or 2.0 g/kg (n = 7) ethanol (i.g.) from water with a 30-minute pretreatment interval.

RESULTS

Zolpidem (0.017 to 5.6 mg/kg, i.m.) completely generalized from ethanol (>or=80% of total session responses on the ethanol-appropriate lever) for 6/7 monkeys trained to discriminate 2.0 g/kg and 4/10 monkeys trained to discriminate 1.0 g/kg ethanol. Zolpidem partially generalized from 1.0 or 2.0 g/kg ethanol in 6/7 remaining monkeys. Ro15-4513 (0.003 to 0.30 mg/kg, i.m., 5-minute pretreatment) shifted the zolpidem dose-response curve to the right in all monkeys showing generalization. Analysis of apparent pK(B) from antagonism tests suggested that the discriminative stimulus effects of ethanol common with zolpidem are mediated by low-affinity Ro15-4513 binding sites. Main effects of sex and training dose indicated greater potency of Ro15-4513 in males and in monkeys trained to discriminate 1.0 g/kg ethanol.

CONCLUSIONS

Ethanol and zolpidem share similar discriminative stimulus effects most likely through GABA(A) receptors that contain alpha(1) subunits, however, antagonism by Ro15-4513 of zolpidem generalization from the lower training dose of ethanol (1.0 g/kg) may involve additional zolpidem-sensitive GABA(A) receptor subtypes (e.g., alpha(2/3) and alpha(5)).

Mice performance on the staircase test following acute ethanol administration

This study examined the effect of acute ethanol administration as compared to diazepam on the number of rearing events and the number of steps ascended in the mouse staircase test, an animal model sensitive to benzodiazepines. Acute ethanol administration, similar to acute diazepam administration, reduces rearing (at doses that do not reduce climbing) in the staircase test. This effect of acute ethanol administration is insensitive to the benzodiazepine antagonist flumazenil and is not consistently counteracted by the partial inverse agonist Ro15-4513. It seems that the mouse staircase test is an efficient paradigm for studying agents active at the gamma-aminobutyric acid (GABA(A)) receptor complex, including ethanol.

Discriminative stimulus effects of flumazenil in rhesus monkeys treated chronically with chlordiazepoxide

Discriminative stimulus effects of the benzodiazepine antagonist flumazenil were studied in two rhesus monkeys receiving 3.2 mg/kg/12 h of chlordiazepoxide while discriminating between vehicle and 0.056 mg/kg of flumazenil. In a drug discrimination component responding was maintained under a FR 10 schedule of stimulus-shock termination; in a non-discrimination component responding was maintained under a FR 10 schedule of food presentation. Flumazenil and Ro 15-4513 occasioned >80% flumazenil-lever responding at doses larger than 0.032 and 0.056 mg/kg, respectively. Pentylenetetrazole, ethyl-beta-carboline-3-carboxylate (betaCCE), ketamine and spiradoline failed to substitute for flumazenil although >80% drug-lever responding was observed for two of the compounds in one monkey. Flumazenil, Ro 15-4513, pentylenetetrazole, betaCCE but not ketamine or spiradoline decreased rates of responding in the food component at doses that had little effect on rates in the stimulus-shock termination component. When chlordiazepoxide injections were discontinued and saline was administered before the session, monkeys did not respond on the flumazenil lever; when flumazenil was administered under the same conditions, monkeys responded on the flumazenil lever despite not having received chlordiazepoxide for nine days. Drug stimulus control was established with flumazenil in monkeys receiving chlordiazepoxide and substitution studies suggest that this effect of flumazenil might result from antagonist actions at benzodiazepine receptors: however, lack of withdrawal-related effects after termination of chlordiazepoxide treatment precludes validation of this procedure for studying benzodiazepine dependence.

Chronic pentobarbital administration alters gamma-aminobutyric acidA receptor alpha 6-subunit mRNA levels and diazepam-insensitive [3H]Ro15-4513 binding

In order to study the chronic effects of pentobarbital, a positive GABAA receptor modulator, on the inverse agonist binding of the benzodiazepine site, binding of [3H]Ro15-4513 and levels of GABAA receptor alpha 6-subunit mRNA were investigated in the brains of pentobarbital-tolerant/dependent animals, using receptor autoradiography and in situ hybridization histochemistry in consecutive brain sections. Pentobarbital was administered to rats either 60 mg/kg, i.p., once, for acute treatment, or 300 micrograms/10 microliters/h i.c.v. continuously for 6 days via osmotic minipumps to render rats tolerant to pentobarbital. Rats assigned to the dependent group were sacrificed 24 h after discontinuance of pentobarbital infusion, while those assigned to the tolerant group were sacrificed at the end of infusion. The alpha 6 subunit mRNA was increased in the tolerant group only. Diazepam-insensitive [3H]Ro15-4513 binding was increased in the cerebellar granule layer of pentobarbital-tolerant and -dependent rats. No alterations in these parameters were observed in acutely treated animals. These data suggest that chronic pentobarbital treatment induced expression of alpha 6-subunit mRNA. This was in contrast to alpha 1- and gamma 2-subunit mRNA, which in tolerant animals are unchanged, but for which withdrawal triggers a surge in levels. Because the alpha 6-subunit is a major component of the diazepam-insensitive [3H]Ro15-4513 binding site, the increased diazepam-insensitive [3H]Ro15-4513 binding implied de novo synthesis of the receptor subunit protein.

Ro 15-4513 selectively attenuates ethanol, but not sucrose, reinforced responding in a concurrent access procedure; comparison to other drugs

The experiments described in this report used a concurrent access procedure to study ethanol reinforcement. Rats were trained to lever press for a 10% sucrose solution and a 10% ethanol/10% sucrose mixture, and both reinforcers were available on variable-interval 5-s schedules. In baseline and vehicle injection sessions, the animals distributed their responding between both solutions. When injected with the partial inverse benzodiazepine agonist Ro 15-4513 (3, 9, and 18 mg/kg), responding for the ethanol solution decreased while responding for sucrose remained intact. Ethanol injections (0.5 and 1.0 g/kg) engendered a similar profile. Chlordiazepoxide led to an increase in ethanol mix responding at 2 mg/kg and a decrease in ethanol mix responding at higher doses; no dose affected sucrose responding. Morphine (0.5-16 mg/kg) decreased responding for both the ethanol mix and sucrose solutions, more or less simultaneously. Naloxone (0.125-20 mg/kg) selectively reduced ethanol mix responding at low doses, and decreased responding for both reinforcers at high doses. In another group of animals, isocaloric alternatives were concurrently available: 10% ethanol/0.25% saccharin versus 14% sucrose. Injections of Ro 15-4513 and chloridiazepoxide produced similar results as in the first group of rats: an increase in ethanol mix responding with low dose chlordizepoxide, and a decrease in ethanol mix responding with Ro 15-4513. However, naloxone injections did not selectively affect responding for either of the reinforcers when they were isocaloric. These results are discussed in terms of ethanol's neuropharmacological actions.

Anaesthetic concentrations of alcohols potentiate GABAA receptor-mediated currents: lack of subunit specificity

Anaesthetic concentrations of ethanol (50-400 mM) and butanol (1-20 mM) were tested for their effects on GABAA receptor-mediated chloride currents in Xenopus oocytes expressing human GABAA receptor cDNAs. Significant potentiation of the currents was seen in all receptor constructs tested. Substituting the alpha 5 subunit for the alpha 1, or the beta 2 for the beta 1, did not affect the degree of ethanol potentiation. The effects of 200 mM ethanol and 20 mM butanol were also tested using a variety of GABA concentrations (0.3-1000 microM) on oocytes expressing alpha 1 beta 1 vs. alpha 1 beta 1 gamma 2S or alpha 1 beta 2 vs. alpha 1 beta 2 gamma 2S receptor constructs. The presence of the gamma 2S subunit generally did not appear to affect the degree of potentiation, except that butanol potentiation was greater in alpha 1 beta 1 than in alpha 1 beta 1 gamma 2S receptors. This phenomenon of anaesthetic concentrations of alcohols potentiating GABAA receptor responses appears to be distinct from the low (20 mM) ethanol potentiation previously reported in the literature.

Differential interactions between ethanol and Ro 15-4513 on two anxiety tests in rats

The effects of low (2 g/kg) and high (4 g/kg) doses of ethanol and their interaction with the imidazobenzodiazepine Ro 15-4513 (a partial inverse agonist of the benzodiazepine receptor) were studied in two different models of anxiety in rats: the "elevated plus-maze" test and the early acquisition of two-way (shuttlebox) avoidance. In the elevated plus-maze, ethanol (2 g/kg) increased the percentage of entries into the open arms (%EOA) and both ethanol doses increased the percentage of time spent into the open arms (%TOA), thus indicating an anxiolytic action which was reversed by Ro 15-4513 (5 mg/kg). By contrast, Ro 15-4513 did not counteract the anxiolytic effect of the low dose of ethanol in the acquisition of shuttlebox avoidance. Thus, the treatment with 2 g/kg of ethanol (plus either vehicle or Ro 15-4513) significantly increased the total number of avoidances. Conversely, animals treated with 4 g/kg of ethanol showed impaired shuttlebox avoidance acquisition, and this effect was completely reversed by Ro 15-4513. Ro 15-4513 was without effect on its own in any of the anxiety-related parameters (i.e., %EOA, %TOA, and avoidance acquisition). The results indicate a different pattern of ethanol effects and Ethanol x Ro 15-4513 interactions depending upon the task used.

Attenuation of anticonvulsant effects of diazepam after chronic treatment with bicuculline

Changes in the GABAergic system after chronic treatment with bicuculline were examined in two strains of inbred rats, Fischer 344 (F344) and Lewis (LEW). Rats received an IP injection of either bicuculline (2 mg/kg) or vehicle once a day for 12 days. After this chronic treatment, the effects of diazepam (1 mg/kg, IP) and pentobarbital (20 mg/kg, IP) on bicuculline-induced convulsions were measured. Bicuculline was acutely infused into a tail vein at 0.0415 mg/min, and the infusion was terminated when rats showed seizure. Following the chronic bicuculline treatment, the anticonvulsant effect of diazepam, but not of pentobarbital, was significantly reduced as compared to its effect following chronic vehicle treatment in both strains. Both diazepam and pentobarbital showed a significant difference in anticonvulsant effects between strains (F344 > LEW). The hypnotic effects of muscimol, barbital, pentobarbital, and ethanol following chronic bicuculline treatment were examined. There was no significant difference in sleep time induced by these drugs between bicuculline- and vehicle-treated rats. These results suggest that the attenuation of diazepam's anticonvulsant effect after chronic bicuculline treatment may result from functional changes in benzodiazepine receptors and that the anticonvulsant effects of diazepam and pentobarbital may be influenced by genetic factors. Moreover, the hypnotic effects of several drugs tested are apparently not affected by chronic bicuculline treatment.

Investigation of the involvement of opioid receptors in the action of anticonvulsants

This study investigates the possible involvement of opioid receptors in the action of a variety of anticonvulsant agents. The opioid antagonist naloxone (0.3, 1 mg/kg IP) and the selective mu-opioid antagonist cyprodime (3 mg/kg IP) significantly inhibited the increase in electroshock seizure threshold induced by phenytoin (3 mg/kg IP) in mice. The anticonvulsant effects of ethanol (1 g/kg IP) were also significantly antagonised by naloxone (1 mg/kg IP) but not by a 0.3 mg/kg IP dose or by cyprodime (3 mg/kg IP). The results with naloxone were confirmed using higher doses of phenytoin (10 mg/kg IP) and ethanol (1.5 g/kg IP). In contrast to the above findings, naloxone (0.3, 1 mg/kg IP) had no effect on the increase in seizure threshold induced by sodium valproate (200 mg/kg IP) or dizocilpine (MK801, 0.5 mg/kg IP) and paradoxically potentiated the increase in seizure threshold produced by phenobarbitone (15 mg/kg IP); carbamazepine (10 mg/kg IP) and the benzodiazepine agonist loprazolam (1 mg/kg IP), clearly differentiating these compounds from phenytoin and ethanol. These findings suggest that the anticonvulsant effects of phenytoin and ethanol (as assessed by their ability to prevent tonic hindlimb extension in the mouse electroshock model) may be mediated, at least in part, by the release of endogenous opioids and subsequent activation of opioid receptors (mu, in the case of phenytoin, but non-mu, in the case of ethanol) although direct activity at opioid receptors cannot be precluded.

Involvement of gamma-aminobutyric acid and N-methyl-D-aspartate receptors in the inhibitory effect of ethanol on pentylenetetrazole-induced c-fos expression in rat brain

The expression of c-fos mRNA in rat brain was induced by intraperitoneal administration of pentylenetetrazole (PTZ) and picrotoxin, which act on the picrotoxin binding site of the gamma-aminobutyric acid-benzodiazepine (GABA-BZ) receptor complex, by N-methyl-D-aspartate (NMDA) and kainic acid, agonists of different classes of glutamate receptors and by caffeine, an antagonist of adenosine receptors. The actions of PTZ and picrotoxin but not that of NMDA were blocked by ethanol and the NMDA-receptor antagonist, MK-801. Ro 15-4513 partially reversed the inhibitory effect of ethanol on PTZ-induced c-fos mRNA synthesis. Acute ethanol administration blocked the actions of PTZ and NMDA without affecting the response to kainic acid or caffeine. Taken together, these data suggest that ethanol blocks c-fos gene activation by noncompetitive antagonists of the GABA-BZ receptor via actions on both the NMDA and GABA receptors.

Effect of Ro 15-4513 on ethanol-induced conditioned place preference

The benzodiazepine receptor inverse agonist Ro 15-4513 reverses a number of ethanol's effects, including its reinforcing properties as measured through self-administration. The present study examined the effect of this putative ethanol antagonist in a place conditioning design that has been shown to be sensitive to ethanol's rewarding properties in mice. Using an unbiased differential conditioning procedure, DBA/2J mice received, on alternate days, pairings of a distinctive floor stimulus (CS+) with either ethanol (2 g/kg), Ro 15-4513 (3 mg/kg), or a combination of ethanol and Ro 15-4513. On alternate days, a different distinctive floor stimulus (CS-) was paired with vehicle. Under these conditions, ethanol produced a conditioned place preference that was unaffected by Ro 15-4513. Ro 15-4513 alone did not produce either a place preference or aversion. Ro 15-4513 did produce reductions in locomotor activity during conditioning, indicating it was behaviorally active. These results indicate that a dose of Ro 15-4513 that alters general activity does not affect ethanol reward.

In vivo studies on the mechanism of action of the broad spectrum anticonvulsant loreclezole

In animal models of epilepsy the anticonvulsant profile of loreclezole resembles that of barbiturates and benzodiazepines. We examined whether the increase in seizure threshold to pentylenetetrazole infusion produced by 10 mg/kg of loreclezole, pentobarbital or diazepam could be reversed by a spectrum of benzodiazepine partial inverse to full inverse agonists (FG-7142 beta-carboline carboxylate, CGS-8216, Ro-15-4513 and DMCM) or by a benzodiazepine neutral antagonist (Ro-15-1788). The doses of the benzodiazepine inverse agonists were chosen to produce a 20-40% decrease in seizure threshold. The seizure threshold increase produced by loreclezole and pentobarbital was reduced by all the benzodiazepine inverse agonists and potentiated by Ro-15-1788. Diazepam was antagonized by the benzodiazepine inverse agonists and by the neutral antagonist. The generality of this finding was examined in amygdala-kindled rats. The decrease in the duration of forepaw clonus and the reduction in behavioural stage34 produced by loreclezole, pentobarbital and diazepam was reversed by CGS-8216. Ro-15-1788, which itself showed anticonvulsant effects in this model, antagonized the effects of diazepam, but not loreclezole or pentobarbital. Thus loreclezole behaves more like a barbiturate than a benzodiazepine in these two in vivo models. This suggests a possible mechanism of action of loreclezole at a neuromodulatory site within the GABAA receptor complex, which is unlikely to be a benzodiazepine receptor.

RO15-4513 antagonizes the anxiolytic effects of ethanol in a nonshock conflict task at doses devoid of anxiogenic activity

RO15-4513 is a partial benzodiazepine inverse agonist that has been reported to antagonize some of the biochemical and neurobehavioral actions of ethanol. However, whether this antagonistic action of RO15-4513 is dependent on the drug exerting its intrinsic (inverse agonist) properties is unclear at present. The purpose of the present study was to examine whether RO15-4513 was capable of antagonizing the anxiolytic effects of ethanol in a nonshock conflict task at doses that, by themselves, do not reveal the compound's intrinsic anxiogenic properties. The consummatory conflict task employed (negative contrast) involves quantifying how animals respond to an abrupt, unexpected reduction in reward (sucrose solution), and is particularly sensitive to the effects of anxiolytic agents, including ethanol. As previously demonstrated, depressed consummatory behavior engendered by reward reduction was significantly alleviated by ethanol (0.75 g/kg). This anxiolytic effect of ethanol, however, was antagonized dose dependently by RO15-4513 (0.1875-3.0 mg/kg). Only the highest dose of RO15-4513 (3.0 mg/kg) showed evidence of further response suppression. Lower doses of RO15-4513 tested did not exert an anxiogenic effect when given alone. Thus the antagonism of EtOH's anxiolytic (contrast-reducing) effects occurred at doses of RO15-4513 (0.375-1.5 mg/kg) that did not exhibit any intrinsic anxiogenic activity. As such, these results suggest that RO15-4513 interacts with the anxiolytic effects of ethanol in a nonadditive fashion in this test situation.

Role of signal transduction in anesthetic action. Alpha 2 adrenergic agonists

The molecular mechanism for general anesthetic action is not known. The alpha 2 adrenergic agonists represent a novel class of "anesthetic-like" agent because of their selectivity for receptor binding sites and because the transmembrane signaling systems mediating their biologic responses in non-CNS systems are known. We have begun to characterize the signal transduction pathway involved in the anesthetic-like action of the alpha 2 adrenergic agonists. The alpha 2 adrenergic agonists potently decrease both central noradrenergic neurotransmission and halothane anesthetic requirements (MAC). Since MAC is only reduced by 30-40% when noradrenergic neurotransmission is totally abolished and since the reduction in MAC with the highly selective alpha 2 adrenergic agonists exceeds 90%, factors in addition to noradrenergic neurotransmission must be contributing to the anesthetic action of the alpha 2 agonists. Studies with the superselective alpha 2 agonist dexmedetomidine confirmed this, as the alpha 2 agonist could still reduce the MAC for halothane in rats depleted of their central norepinephrine stores. The profound reduction in anesthetic requirements with dexmedetomidine raised the possibility that alpha 2 adrenergic agonists may be considered an anesthetic hypnotic agent by itself. This sole anesthetic hypnotic response was established together with the confirmation that a central alpha 2 adrenoceptor mediated this action. Subsequently, data using molecular biologic techniques suggested that the alpha 2 C4 isoreceptor was the probable receptor that mediated the anesthetic response. We further explored the postreceptor effector mechanism for the signal transduction pathway for alpha 2 anesthetic action and identified the participation of two other molecular components, namely, a pertussis-toxin-sensitive G protein and a 4-aminopyridine-sensitive ion channel. Whether the signal transduction pathway for alpha 2 anesthetic action mediates the further response to other non-alpha 2 anesthetic agents needs to be defined.

Central and peripheral benzodiazepine receptors: involvement in an organism's response to physical and psychological stress

The present review discusses the current knowledge of the molecular pharmacology and neuroanatomical and subcellular localization of both the central benzodiazepine/GABA-chloride ionophore receptor complex and the peripheral benzodiazepine receptor. It then reviews all of the literature to date on how these two receptor sites are modulated by environmental stress. The possible role of these sites in learning and memory is also discussed. Finally, a theoretical model is presented which examines the differential, and perhaps complementary, alterations of these two sites in an organism's response to stress.

Transient increase in [3H]Ro 15-4513 specific binding in the superficial gray layer of the rat superior colliculus induced by visual deafferentation

The imidazodiazepine compound [3H]Ro 15-4513, a partial inverse agonist of benzodiazepine receptors of the central type, binds with high affinity (order of 10(-8) M) to a single population of benzodiazepine binding sites in the mammalian central nervous system. A quantitative autoradiographic study was carried out to determine the effects of one eye removal on [3H]Ro 15-4513 specific binding to rat brain sections in the superficial gray layer or stratum griseum superficiale (SGS) of the superior colliculus. Retinal afferent degeneration due to right eye removal, performed 3 and 7 days before sacrifice, led to a significant and symmetrical increase in the [3H]Ro 15-4513 specific binding in both right and left SGS by enhancing the binding affinity of the radioligand. This transient phenomenon disappeared when a longer survival period of 45 days was allowed to elapse. Conversely, unilateral lesion of the primary visual areas had no apparent effects on the specific binding of the radioligand. The absence of any loss of binding sites after either type of lesion suggests that the benzodiazepine receptors are probably not situated on the optic nerve axon terminals, nor on the cortical axon terminals originating from primary visual areas. In the SGS, as in other rat brain structures, benzodiazepine receptors of the central type are functionally coupled with GABAA receptors and form 'GABAA receptors/benzodiazepine receptors/chloride channel' complexes. The involvement of the local GABAergic system in the postlesion plasticity of benzodiazepine receptors was studied by testing the effects of exogenously applied GABA on [3H]Ro 15-4513 specific binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Flunitrazepam photoaffinity labeling of the GABA(A) receptor reduces inhibition of [3H]Ro15-4513 binding by GABA

The benzodiazepine drugs modulate gamma-aminobutyric acid (GABA)-mediated synaptic transmission via a high-affinity binding site that is part of the GABA(A) receptor complex, but which is distinct from the GABA binding site. Ro15-4513 is a benzodiazepine negative modulator of GABA action that displays unique anti-ethanol properties both in vivo and in vitro. Ro15-4513 has been reported to photoaffinity label nearly 100% of the benzodiazepine binding sites in rat brain homogenates. In contrast, the benzodiazepine positive modulator flunitrazepam photoaffinity labels only 25% of the sites. Here, we have examined the reversible binding of [3H]Ro15-4513, [3H]flumazenil (Ro15-1788), and [3H]flunitrazepam to embryonic chick brain membranes, and to membranes that have been photoaffinity labeled with nonradioactive flunitrazepam. Photoaffinity labeling with flunitrazepam decreased the subsequent reversible binding of [3H]flunitrazepam and [3H]flumazenil, but increased the binding of [3H]Ro15-4513. The increase in [3H]Ro15-4513 binding after flunitrazepam photoaffinity labeling was due to a decrease in the apparent Kd, with no change in Bmax. Following photoaffinity labeling, negative modulation of [3H]Ro15-4513 binding by GABA was lost, whereas positive modulation of residual [3H]flunitrazepam binding was retained. We conclude that the site photoaffinity labeled by flunitrazepam is distinct from the site responsible for reversible binding of [3H]Ro15-4513.

Differential interactions of Ro 15-4513 with benzodiazepines, ethanol and pentobarbital

The effects of the imidazobenzodiazepine Ro 15-4513 in combination with three CNS depressants (ethanol, benzodiazepine agonists and pentobarbital) were examined in three different experiments. Full antagonism of classical benzodiazepines by Ro 15-4513 was seen in all three situations. Partial antagonism of ethanol occurred in the pull up test of muscle relaxation in rats, but not in the inhibition of ultrasounds produced in rat pups by mild stress. The depressant effect of ethanol on twitching of the urethane-anaesthetised rat suprahyoid muscles was reversed. No attenuation of the effects of pentobarbital was seen in any test.

The effects of Ro 15-4513 on ethanol-induced taste aversions

Ro 15-4513 is an imidazobenzodiazepine that has been reported to block a range of behavioral effects of ethanol. In the present experiments, the effects of Ro 15-4513 were assessed on the acquisition of an ethanol-induced conditioned taste aversion. Specifically, rats were given a novel saccharin solution to drink followed by an injection of one of a range of doses of Ro 15-4513 (0.5 and 1.0 mg/kg, Experiment 1A, and 2.0 and 3.0 mg/kg, Experiment 1B) and an injection of ethanol (1.75 g/kg). Ro 15-4513 failed to block the acquisition of the ethanol-induced taste aversion. Possible reasons for this failure were discussed.

GABAergic mechanisms in the electrophysiological actions of ethanol on cerebellar neurons

We have found that the partial inverse benzodiazepine agonists Ro 15-4513 and FG 7142 antagonize the depressant electrophysiological effects of locally applied ethanol in the cerebellum. Although absolute tissue concentrations are not known, dose-response curves constructed using pressure-ejection doses as previously described we found that FG 7142 was more efficacious, but less potent than Ro 15-4513. Our observation that ethanol and inverse benzodiazepine agonists have interactions which are not competitive might suggest that these two drugs act through separate, but interactive mechanisms in order to produce the observed ethanol antagonism. If such independent interactions were mediated at different sites on a given macromolecular complex, such as the GABAa/Cl- channel, then one might expect to find allosteric interactions between those sites as well as with the functional response of the complex to GABA activation. Indeed, this hypothesis is consistent with the recent finding of Harris and collaborators that ethanol potentiates the inverse agonist actions of Ro 15-4513 and FG 7142. On the other hand, we were unable to find large ethanol-induced potentiations of GABA effects on all neurons which showed depressant responses to ethanol administration in rat cerebellum. However we did find that the GABAa antagonist, bicuculline, blocks the depressant effects of ethanol on the same neurons. We conclude that the interaction between ethanol and GABA probably does not occur directly at the GABAa receptor site, but that the GABAa mechanism does play a permissive role in the ethanol-induced depressions of cerebellar Purkinje neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Barbiturates and the GABAA receptor complex

The GABA synapse plays an important role in the pharmacologic effects of barbiturates and the mechanisms involved in barbiturate tolerance and dependence. A synopsis of the effects which have been reported to date is found in Tables 1 and 2. Although the acute changes in neurotransmitter uptake and release are nonselective, a lag in the ability of the GABA synapse to compensate for discontinuation of barbiturate exposure may be important in the symptoms of withdrawal. Barbiturates cause changes in the properties of many receptors, but manipulations of the GABAA receptor in vivo correlate with changes in the therapeutic and toxicologic responses to barbiturates, indicating that the GABAA receptor complex plays a pivotal role in the effects of barbiturates. Experiments done in several laboratories show that barbiturate tolerance and dependence cause subtle changes in the properties of the GABAA receptor complex. These observations suggest that decreased GABA-stimulated chloride channel activity and reduced ability to modulate it may be important in causing barbiturate tolerance and the symptoms observed in withdrawal. Selection of drug-resistant rodent strains suggests that there may be genetic factors involved in drug tolerance and dependence. The complexity of the responses of the GABA synapse to both acute and prolonged exposure to barbiturates indicates that it is a valuable model for understanding how the central nervous system responds to drugs and the mechanisms involved in drug addiction.

Effects of ethanol and Ro 15-4513 in an electrophysiological model of anxiolytic action

Previous research has implicated hippocampal rhythmical slow activity in the mechanisms of action of the anxiolytic drugs. In this study ethanol and a putative ethanol antagonist, Ro 15-4513, were investigated with reticular elicitation of rhythmical slow activity. Doses of ethanol between 0.6 and 3.1 g/kg were used. Ethanol reduced the frequency of reticular-elicited rhythmical slow activity in the same way as has been reported for anxiolytic barbiturates and benzodiazepines. This effect was linearly related to log dose of ethanol in the range of 1.7-3.1 g/kg. Ro 15-4513 at a dose of 2 mg/kg reduced the effect of ethanol (2.0 g/kg) but had no action itself. Ethanol also decreased the slope of the stimulation voltage-rhythmical slow activity frequency function but this effect was not reduced by Ro 15-4513. These results show that ethanol acts in a similar manner to conventional anxiolytic drugs but that only one component of this action can be reduced by Ro 15-4513.

Antagonism of methoxyflurane-induced anesthesia in rats by benzodiazepine inverse agonists

Injection of the partial benzodiazepine inverse agonist Ro15-4513 (1-32 mg/kg i.p.) or nonconvulsant i.v. doses of the full benzodiazepine inverse agonist beta-CCE immediately following cessation of exposure of rats to an anesthetic concentration of methoxyflurane significantly antagonized the duration of methoxyflurane anesthesia as measured by recovery of the righting reflex and/or pain sensitivity. This antagonism was inhibited by the benzodiazepine antagonist Ro15-1788 at doses which alone did not alter the duration of methoxyflurane anesthesia. In addition, high-dose Ro15-4513 pretreatment (32 mg/kg) antagonized the induction and duration of methoxyflurane anesthesia but was unable to prevent methoxyflurane anesthesia or affect the induction or duration of anesthesia induced by the dissociative anesthetic ketamine (100 mg/kg). These findings indicate that methoxyflurane anesthesia can be selectively antagonized by the inverse agonistic action of Ro15-4513 and beta-CCE.

Mechanistic and functional divergence between thyrotropin-releasing hormone and RO 15-4513 interactions with ethanol

Both thyrotropin-releasing hormone (TRH) and RO 15-4513 antagonize ethanol-induced depression, but this common property does not infer that both compounds share similar mechanisms of action. In the present studies, both TRH (30 mg/kg, i.p.) and RO 15-4513 (10 mg/kg, i.p.) reversed ethanol-induced depression of locomotor activity, in accord with previous reports. However, the benzodiazepine antagonist, RO 15-1788, blocked this action of RO 15-4513, while exerting no effect on the analeptic action of TRH. Using a model of seizure activity electrically elicited from the inferior colliculus, ethanol exerted a dose-related attenuation of seizure activity. This anticonvulsant action of ethanol was not altered by TRH (30 mg/kg, i.p.), but RO 15-4513 (3 mg/kg) reversed the effect of the 0.5, but not the 1.0 g/kg, dose of ethanol. In addition, pretreatment with RO 15-4513 (1 or 3 mg/kg, i.p.), but not TRH (30 mg/kg, i.p.), caused seizure generalization into the forebrain following inferior collicular stimulation, further verifying the proconvulsant properties of RO 15-4513. In conclusion, the analeptic action of TRH appears independent of benzodiazepine activity, and in contrast to RO 15-4513, TRH does not exhibit proconvulsant properties. Furthermore, because TRH did not antagonize both depressant actions of ethanol studied, it appears unlikely that TRH directly interacts with the molecular basis of ethanol action.

The effects of ethanol and Ro 15-4513 on elevated plus-maze and rotarod performance in long-sleep and short-sleep mice

The effects of ethanol and diazepam were examined in long-sleep (LS) and short-sleep (SS) mice using the elevated plus-maze. Ethanol had more pronounced effects in SS mice than in LS mice. In contrast, LS mice were more sensitive to the effects of diazepam on the elevated plus-maze. The ataxic effects of ethanol were measured by rotarod performance. SS mice were more resistant to the ataxic effects of a 2.0 g/kg dose of ethanol than LS mice. Ro 15-4513 reversed ethanol's ataxic effects when administered after ethanol in both LS mice and SS mice. Pentobarbital-induced ataxia was unaffected by treatment with Ro 15-4513. Studies of competition of Ro 15-4513 on 3H-flunitrazepam binding indicated that LS and SS mice did not differ in this measure in cortex, cerebellum or hippocampus.

Antagonism of ethanol-reinforced behavior by the benzodiazepine inverse agonists Ro15-4513 and FG 7142: relation to sucrose reinforcement

The partial inverse benzodiazepine agonist Ro15-4513 has been shown to antagonize many of ethanol's actions, including the reduction of behavior reinforced with ethanol presentation. The studies reported here compared the effects of the Ro compound on sucrose reinforcement alone and concurrently available with ethanol reinforcement. Also, a second inverse agonist, FG 7142, was tested. The result indicated that ethanol reinforcement was more sensitive to the inverse agonists compared to sucrose reinforcement. This was seen as a graded effect upon ethanol responding at doses which failed to have any effect upon sucrose-reinforced behavior. The Ro compound was approximately three times more potent than the FG compound in suppressing ethanol-reinforced responding. Possible explanations for the greater sensitivity of ethanol reinforcement compared to sucrose reinforcement was discussed in terms of ethanol's potential actions at the benzodiazepine-GABA receptor complex.

Ethanol and the benzodiazepine-GABA receptor-ionophore complex

Ethanol has a pharmacological profile similar to that of classes of drugs like benzodiazepines and barbiturates, which enhance GABAergic transmission in the mammalian CNS. Several lines of behavioral, electrophysiological and biochemical studies suggest that ethanol may bring about most of its effects by enhancing GABAergic transmission. Recently, ethanol at relevant pharmacological concentrations has been shown to enhance GABA-induced 36Cl-fluxes in cultured spinal cord neurons, synaptoneurosomes and microsacs. These enhancing effects of ethanol were blocked by GABA antagonists. Ro15-4513, an azido analogue of classical BZ antagonist Ro15-1788, reversed most of the behavioral effects of ethanol and other effects involving 36Cl-flux studies. The studies summarized below indicate that most of the pharmacological effects of ethanol can be related to its effects on GABAergic transmission.

Reduction of the intoxicating effects of ethanol by drugs acting at the benzodiazepine-GABA receptor complex

The ability of the benzodiazepine receptor partial inverse agonists Ro 15-4513, Ro 15-3505 and FG 7142, and the picrotoxin site ligands pentylenetetrazole and Ro 5-3663 to reduce ethanol-induced intoxication were investigated. Ro 15-4513 (0.3-3 mg/kg), Ro 15-3505 (3 mg/kg), pentylenetetrazole (20 and 25 mg/kg) and Ro 5-3663 (4 mg/kg) all significantly attenuated the intoxicating effects of ethanol. In contrast, FG 7142 (20 and 40 mg/kg) failed to reduce ethanol intoxication, but reversed the effect of Ro 15-4513. This pattern of results differs from that obtained using other behavioral paradigms. Since drugs which reduce the effects of GABA generally reduce the intoxicating effects of ethanol, it is suggested that the beta-carbolines may be unusual in their interaction with ethanol.

Effects of Ro 15-4513 on the motor impairment and hypnotic effects of ethanol and pentobarbital

The selectivity of Ro 15-4513 in reversing the actions of sedative-hypnotic drugs was examined. The motor impairment induced by i.p. administration of 1.8 g/kg of ethanol was partially reversed by doses of Ro 15-4513 ranging from 2-8 mg/kg. Antagonism of the similar effect induced by 20 mg/kg of pentobarbital was observed only at the 8 mg/kg dose of Ro 15-4513. Treatment with 4 mg/kg of Ro 15-4513 shifted the dose-effect curve for motor impairment by ethanol to the right but did not affect the corresponding curve for pentobarbital. Ro 15-4513 also increased the onset latency and shortened the duration of sleep time induced by ethanol but not by pentobarbital. Blood ethanol and pentobarbital levels measured at 32 min after drug administration were not affected by Ro 15-4513. The selectivity of Ro 15-4513 and the mechanism(s) underlying its proconvulsant effect in reversing the actions of ethanol are discussed.

A new alcohol antagonist: phaclofen

The ability of the GABA(B) receptor antagonist, phaclofen to alter behavioral effects of ethanol was evaluated by loss of righting reflex (sleep time), motor incoordination (bar holding), spontaneous locomotion (open field activity) and hypothermia. Pretreatment with phaclofen significantly decreased the effects of ethanol on motor incoordination, locomotor activity and hypothermia. However, phaclofen had no effect on either pentobarbitalor diazepam-induced motor incoordination. Phaclofen slightly increased the ED50 for loss of the righting reflex but did not alter either the duration of reflex loss produced by ethanol or blood ethanol levels at awakening. Our results suggest phaclofen is rapidly inactivated resulting in difficulty in observing antagonism of long duration ethanol effects. These findings suggest that the GABA(B) system may play a role in mediating several important actions of ethanol.

Attenuation of ethanol intoxication by alpha-2 adrenoceptor antagonists

The interaction of a highly potent and selective alpha-2 adrenoceptor antagonist, atipamezole with ethanol was investigated in tests assessing a number of ethanol's behavioral effects. Atipamezole antagonized ethanol's effects on directed exploration in a holeboard test, reduced observer-rated intoxication and also reduced the duration of loss of righting reflex caused by ethanol. Similar effects were produced by another alpha-2 adrenoceptor antagonist idazoxan. The magnitude of the effects was comparable to that produced in the same animal models by the imidazodiazepine Ro 15-4513, which antagonizes ethanol by an action at central benzodiazepine receptors. Whereas Ro 15-4513 possesses marked behavioral effects on its own, atipamezole is comparatively inactive in all paradigms so far tested. The data suggest that alpha-2 adrenoceptors can play an important role in modulating the intoxicating effects of ethanol.

Benzodiazepines in the treatment of alcoholism

This chapter comprises three sections that cover the main aspects of benzodiazepines and alcohol: (1) the basic pharmacology of benzodiazepines; (2) use of benzodiazepines in the treatment of withdrawal; and (3) the use of benzodiazepines in treating alcoholics. The basic studies suggest that a major site of action of alcohol may be the GABA/benzodiazepine receptor complex and that compensatory alterations in this complex may underly withdrawal. In the section on alcohol withdrawal, interactions between the GABA/benzodiazepine receptor complex, sympathetic nervous system, and hypothalamic-pituitary-adrenal axis are discussed. Use of benzodiazepines in the treatment of the alcohol withdrawal syndrome are reviewed, including the possibility that the benzodiazepines may prevent withdrawal-induced "kindling." Lastly, we review indications for, and efficacy of, benzodiazepines in long-term treatment of patients with alcoholism. Benzodiazepines are not indicated for the treatment of alcoholism. Furthermore, they have very few indications in alcoholics and their dependency-producing potency has to be appreciated when they are used in patients with alcoholism.

RO15-4513 but not FG-7142 reverses anticonvulsant effects of ethanol against bicuculline- and picrotoxin-induced convulsions in rats

The reversal of anticonvulsant effect of ethanol against chemoconvulsions by RO15-4513 was investigated in rats as this novel imidazobenzodiazepine (ethyl-8 azido-5, 6-dihydro-5-methyl-6-Oxo-4H-imidazo [1,5a] [1,4] benzodiazepine-3-carboxylate) is reported to antagonize the acute behavioral and biochemical effects of ethanol in animals. Reversal of ethanol effects on onset of myoclonic jerks, tonic extensor phase, mortality time and percent protection against mortality were compared with not only other anticonvulsant pentobarbital but also with another inverse agonist FG-7142. Pretreatment with RO15-4513 (4 mg/kg) reversed the protective effect of ethanol against bicuculline-induced tonic extensor phase and mortality (87%). This response was sensitive to reversal by RO15-1788 (10 mg/kg). However, onset of myoclonic jerks and duration of clonus were not significantly altered. It also reversed the effect against picrotoxin but the reversal against mortality was up to 50%. As compared to ethanol, RO15-4513 reversed partially the protective effect of pentobarbital against bicuculline- and picrotoxin-induced convulsions. FG-7142 failed to reverse the protective effect of ethanol and pentobarbital against bicuculline-induced tonic extensor phase although it reversed the effect against onset and mortality. It had no effect on the protective effect against picrotoxin-induced convulsions. Both RO15-4513 and FG-7142 possessed proconvulsant effects against bicuculline but not against picrotoxin. These observations suggest that RO15-4513 has a more preferential action against ethanol effects as compared to the other inverse agonist.

The benzodiazepine receptor inverse agonist RO15-4513 exacerbates, but does not precipitate, ethanol withdrawal in mice

RO15-4513, an imidazobenzodiazepine that has been reported to antagonize several behavioral and biochemical actions of ethanol, was given to C3H mice at various times during withdrawal from chronic (72 hours) continuous exposure to ethanol vapor. When administered immediately following chronic ethanol exposure, RO15-4513 (6 or 12 mg/kg) did not influence the withdrawal response. However, when given at subsequent times (3, 5, and 8 hours postethanol withdrawal), RO15-4513 significantly increased the severity of the withdrawal response in ethanol-exposed mice. Moreover, this exacerbation was completely reversed by pretreatment with the benzodiazepine receptor antagonist RO15-1788. Thus, these data indicate that the benzodiazepine inverse agonist, RO15-4513, is capable of exacerbating, but not precipitating, ethanol withdrawal.

Antagonism of ethanol effects by Ro 15-4513: an electrophysiological analysis

Ethanol (ETH) and general anesthetics have been reported to facilitate the chloride channel opening, possibly, or at least partly, through an interaction with the GABA-benzodiazepine (BZ) receptor-gated chloride ionophore "supramolecular complex". Recently Ro 15-4513, a novel BZ ligand, has been indicated as a potent and selective antagonist of various ETH-induced behavioral and biochemical effects. However, since its precise characterization is still a matter of debate, we have tested and compared the effect of Ro 15-4513, as well as its antagonism against ETH, in two objective electrophysiological parameters, i.e., the electroencephalograph (EEG) pattern in freely moving rats and single unit activity of reticulata neurons. Ro 15-4513 produced an EEG state of alertness and antagonized the behavioral impairment and the EEG deterioration by ETH. However, while its protective action was consistent against moderate doses (2 g/kg) of ETH, it was much less evident versus higher doses (4 and 8 g/kg). On reticulata cells, Ro 15-4513 potently stimulated their spontaneous firing and reversed the depression by both ETH and Na-pentobarbital. Moreover, the beta-carboline DMCM also had similar effects. The "pure" BZ antagonist Ro 15-1788 was completely inefective against ETH, yet fully cancelled the reversing actions of Ro 15-4513 and DMCM upon ETH or Na-pentobarbital effects. It is concluded that Ro 15-4513 behaves as a BZ inverse agonist, so that its opposition to ETH and Na-pentobarbital is probably the result of its "negative" coupling with the BZ recognition site that triggers the closing of chloride channels. It suggests that BZ inverse agonists might constitute, in the near future, a new class of analeptic drugs.

Antagonizing the behavioural effects of drugs: a discussion with specific reference to benzodiazepines and alcohol

It can be extremely difficult to interpret the results of experiments in which a treatment is found to antagonize the behavioural effects of a drug. This article discusses strategies that can be used to help identify the nature of the antagonism and examines what such experiments tell us about the mechanisms underlying the drug's behavioural effects. Examples are taken from research with both benzodiazepines and ethanol. It is suggested that behavioural studies using different strategies to reduce the effects of a drug will further enhance our understanding of the mechanisms mediating the drug's effects. Such studies will include comparisons between the effects of chronic drug exposure (tolerance studies), genetic manipulations, and pharmacological treatments.

Ro 15-4513: partial inverse agonism at the BZR and interaction with ethanol

The imidazobenzodiazepinone derivative Ro 15-4513 has the activity profile of a partial inverse (low efficacy) agonist at the benzodiazepine receptor (BZR). It reverses central nervous depressant effects of diazepam, and, in part, of phenobarbitone and ethanol in mice, rats and cats in behavioural, electrophysiological, and neurochemical paradigms. The interaction of Ro 15-4513 with barbiturates and ethanol is due to its inverse agonistic (negative allosteric modulatory) property at the BZR, as it was reversed by the selective BZR blocker flumazenil (Ro 15-1788). In the present experiment situations, other BZR partial inverse agonists in subconvulsant or overt convulsant doses were less effective against ethanol effects than Ro 15-4513. Possible mechanisms for this differential activity of BZR inverse agonists are discussed.

The effects of Ro 15-4513 on the behavioral actions of ethanol in an operant reaction time task and a conflict test

Ro 15-4513, an analogue of the benzodiazepine receptor antagonist Ro 15-1788, has been reported to selectively block the anxiolytic and intoxicating properties of ethanol in rats. To examine the specificity and selectivity of this ethanol antagonism, the effects of Ro 15-4513 were tested on the actions of ethanol in an operant reaction time and conflict test in rats. The operant reaction time task involved holding down a lever for 0.25-2.0 seconds to obtain food, and animals treated with 1 g/kg of ethanol showed a significant disruption in performance. This disruptive effect was reversed by Ro 15-4513 in doses of 1.5-5.0 mg/kg. Ro 15-4513 was also tested in an operant conflict paradigm sensitive to alcohol effects. Ro 15-4513 (0, 1.5, 3.0, 6.0 mg/kg) produced a significant decrease in both punished and unpunished responding in the conflict test. Ethanol (0.75 g/kg), pentobarbital (5 mg/kg) and chlordiazepoxide (5 mg/kg) all produced a significant release of punished responding that was blocked by pretreatment with 6.0 mg/kg Ro 15-4513, but again Ro 15-4513 suppressed responding on its own at this dose. These results suggest that Ro 15-4513 has inverse agonist properties that may explain its ethanol-antagonist action.

Partial reversal of ethanol-induced reductions in exploration by two benzodiazepine antagonists (flumazenil and ZK 93426)

A series of experiments was performed to examine the effects of two benzodiazepine antagonists, flumazenil and ZK 93426, on the behavioral changes caused by ethanol (2 g/kg) in a holeboard test. The interaction between ethanol and low doses of diazepam (0.2 and 0.5 mg/kg) was also investigated. Both flumazenil and ZK 93426 reversed the reduction in exploration caused by ethanol, but tended to increase exploration when administered alone. In contrast, diazepam, which also increased exploration alone, tended to enhance the reduction in exploration caused by ethanol. Diazepam reduced the motor stimulant action of ethanol. The partial reversal by the two antagonists of the reduction in exploration caused by ethanol was not due to alterations in blood ethanol concentrations but may have resulted from their intrinsic exploration-increasing effects or an antagonism of an endogenous ligand for central benzodiazepine receptors.

Antagonizing the anticonvulsant effect of ethanol using drugs acting at the benzodiazepine/GABA receptor complex

The ability of various benzodiazepine receptor ligands to antagonize the anticonvulsant action of ethanol was investigated using intravenous infusion of the GABA antagonist bicuculline. The partial inverse agonists FG 7142, RO 15-4513 and RO 15-3505 produced dose-related reductions in seizure threshold. These compounds also partially reversed the anticonvulsant action of ethanol. However, the magnitude of the effects in each case was only equivalent to the reduction in seizure threshold caused by each compound when administered alone. This is the proconvulsant effect of each compound merely subtracted from the anticonvulsant effect of ethanol. ZK 93426, a benzodiazepine receptor antagonist which alone failed to alter seizure threshold, did not affect the anticonvulsant action of ethanol. Both RO 15-4513 and RO 15-3505 also lowered the seizure threshold of barbiturate-treated mice, again in a subtractive fashion. The ability of RO 15-4513 and other inverse agonists to antagonize the anticonvulsant effect of ethanol appears to result from their intrinsic proconvulsant properties.

Interactions of ethanol with benzodiazepine receptor ligands in tests of exploration, locomotion and anxiety

The ability of benzodiazepine receptor ligands to modify the behavioral effects of ethanol in tests of exploration, locomotion and anxiety are reviewed. Drugs with inverse agonist activity appear capable of consistently antagonizing the reductions in exploration and anxiety caused by ethanol. In contrast, the locomotor stimulant action of ethanol has appeared relatively insensitive to inverse agonists, suggesting that this effect may not be mediated primarily by an action of ethanol at the benzodiazepine/GABA receptor complex.

Does RO 15-4513 reverse the anxiolytic effects of ethanol by its intrinsic properties?

In order to better understand the antagonistic effects of the partial inverse agonist of benzodiazepine receptors, RO 15-4513, against the disinhibitory action of ethanol, we examined the effects of RO 15-4513 at a dose (2.0 mg/kg) that did not alter locomotor activity, given alone or in combination with ethanol, on the behavior of mice confronted with the light/dark choice procedure and the staircase test. At this dose, RO 15-4513 given alone was found to have slight anxiogenic properties and when given in combination with ethanol, to completely reverse the disinhibitory effects of ethanol. Since we previously observed postictal depression after higher doses of RO 15-4513 given alone and antagonistic effects of these same doses on the action of ethanol, it can be suggested that the antagonistic effects of RO 15-4513 against ethanol are due to its anxiogenic or depressive properties depending on doses. However, this hypothesis can only be regarded as being in early stages of development at the present time since these results do not parallel with those of several other studies and the question whether the antagonistic action of RO 15-4513 against ethanol is additive or interactive remains open.

Interactions of three benzodiazepine receptor inverse agonists with ethanol in a plus-maze test of anxiety

The effects of RO 15-4513, RO 15-3505 and FG 7142 on the anxiolytic properties of ethanol in mice were investigated using the plus-maze test of anxiety. Before being tested on the plus-maze, the mice were tested in a holeboard apparatus. All three inverse agonists attenuated the reduction in exploration caused by ethanol in the holeboard test. In the plus-maze, only RO 15-4513 and FG 7142, which possess anxiogenic properties when administered alone, attenuated ethanol's anxiolytic effect. RO 15-3505, which alone had no effect on anxiety, failed to significantly reduce ethanol's anxiolytic effect. Neither RO 15-4513 nor FG 7142 reduced the increase in the total number of arm entries caused by ethanol. These data indicate that the interaction between ethanol and benzodiazepine receptor ligands depends both on the intrinsic properties of the ligands and the behavior under investigation.

RO 15-4513 does not protect rats against the lethal effects of ethanol

In two separate research centres the ability of RO 15-4513 to protect rats against the lethal effects of ethanol (7.5 and 15 g/kg) was investigated. In neither study did RO 15-4513 offer protection against ethanol-induced lethality or the loss of righting reflex caused by these doses of ethanol. These data fail to replicate the results of an earlier report and suggest that RO 15-4513 is unlikely to be clinically useful treating acute severe ethanol toxicity.

Molecular interactions of ethanol with GABAergic system and potential of RO15-4513 as an ethanol antagonist

The behavioral and biochemical effects of ethanol in man and animals have been investigated for a long time. A role of catecholamines in the central stimulatory action and during withdrawal has been envisaged, but more recent observations have revealed the involvement of inhibitory synaptic transmitter, GABA, in the actions of ethanol. Ethanol-induced motor incoordination, hypnosedation, antianxiety, and anticonvulsant actions are reported to be GABA-mediated. Involvement of the GABA system has been implicated in ethanol withdrawal-induced seizures in animals. More direct evidences using Cl- influx studies in synaptoneurosomes and spinal neuronal culture studies confirm such a mode of action of ethanol, probably influencing the chloride channel modulation at the GABA-benzodiazepine receptor ionophore complex. RO15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-Oxo-4H-imidazo [1,5-alpha], [1,4] benzodiazepine-3-carboxylate), a novel imidazobenzodiazepine, an analogue of the classical benzodiazepine antagonist is reported to possess alcohol antagonistic properties. RO15-4513 reverses both the behavioral and biochemical effects of ethanol, including the action of GABA-induced Cl- fluxes. But its potential clinical application may be restricted due to its inverse agonistic property. The present review focuses on the GABA-linked behavioral and biochemical actions of ethanol and discusses the potential of RO15-4513 as an alcohol antagonist.

Failure of Ro 15-4513 to antagonize ethanol in rat lines selected for differential sensitivity to ethanol and in Wistar rats

An imidazobenzodiazepine, Ro 15-4513, acting as a partial inverse agonist at the central benzodiazepine receptors has been recently reported to reverse efficiently the intoxicating effects of ethanol. In studies designed to delineate the role of benzodiazepine receptors in the ethanol-induced motor impairment difference between two rat lines selectively bred for high and low sensitivity to ethanol, however, we could not antagonize the effects of ethanol by Ro 15-4513 in the tilting plane and horizontal wire tests. Neither could we observe any consistent antagonism of ethanol actions in Han:Wistar rats, although we used a wide range of Ro 15-4513 doses, injected the drug intraperitoneally or intragastrically and before or after ethanol administration, and carried out the tests for motor impairment (rotarod, horizontal wire test and intoxication rating) at various times after the drug administration. The ex vivo assay of flunitrazepam binding in brain homogenates revealed the presence of compound(s) inhibiting the binding after administration of Ro 15-4513. Ro 15-4513 antagonized the motor impairing effects of lorazepam. In conclusion, Ro 15-4513 failed to function as a specific antagonist of moderate doses of ethanol in several tests for motor impairment in different rat lines.