Selective antagonism of acute ethanol-induced motor disturbances by centrally administered Ro 15-4513 in mice

GABAA receptor subtypes: the "one glass of wine" receptors

This review discusses evidence for and apparent controversy about, gamma-aminobutyric acid type A (GABAA) receptor (GABAAR) subtypes that mediate alcohol effects experienced during social drinking. GABAARs that contain the beta3 and delta subunits were shown to be enhanced by alcohol concentrations that mirror the concentration dependence of alcohol responses in humans. A mutation (alpha6R100Q) previously found in alcohol nontolerant rats in the cerebellar GABAAR alpha6 subunit is sufficient for increased alcohol-induced ataxia in rats homozygous for this mutation (alpha6-100QQ) and further increases alcohol sensitivity of tonic GABA currents (mediated by alpha6betadelta receptors) in cerebellar granule cells of alpha6-100QQ rats and in recombinant alpha6R100Qbeta3delta receptors. This provided the first direct evidence that these types of receptors mediate behavioral effects of ethanol. Furthermore, the behavioral alcohol antagonist Ro15-4513 specifically reverses ethanol enhancement on alpha4/6beta3delta receptors. Unexpectedly, native and recombinant alpha4/6beta3delta receptors bind the behavioral alcohol antagonist Ro15-4513 with high affinity and this binding is competitive with EtOH, suggesting a specific and mutually exclusive (competitive) ethanol/Ro15-4513 site, which explains the puzzling activity of Ro15-4513 as a behavioral alcohol antagonist. Our conclusion from these findings is that alcohol/Ro15-4513-sensitive GABAAR subtypes are important alcohol targets and that alcohol at relevant concentrations is more specific than previously thought. In this review, we discuss technical difficulties in expressing recombinant delta subunit-containing receptors in oocytes and mammalian cells that may have contributed to negative results and confusion. Not only because we have reproduced detailed positive results numerous times, and we and many others have built extensively on basic findings, but also because we explain and combine many previously puzzling results into a coherent and highly plausible paradigm on how alcohol exerts an important part of its action in the brain, we are confident about our findings and conclusions. However, many important open questions remain to be answered.

Role of GABAA alpha5-containing receptors in ethanol reward: the effects of targeted gene deletion, and a selective inverse agonist

GABA(A) receptors containing alpha5 subunits have been suggested to mediate the rewarding effects of ethanol. We tested this hypothesis in mice with deletion of alpha5 subunits. alpha5 knockout mice did not differ from wildtypes in operant responding for 10% ethanol/10% sucrose, but responded less for 10% sucrose. The benzodiazepine (BZ) site inverse agonist, Ro 15-4513, has higher affinity for GABA(A) receptors containing 5 subunits and dose-dependently (0-27 mg/kg, i.p.) reduced lever pressing for ethanol/sucrose in wildtype mice, but had less effect in knockout mice; lever pressing for sucrose was unaffected. These data suggest that alpha5 subunits are not essential for ethanol reward, but the reduction of operant responding for ethanol by Ro 15-4513 is mediated by alpha5-containing GABA(A) receptors. In measures of ethanol consumption, alpha5 knockout mice did not differ from wildtypes at low ethanol concentrations (2-8%), but consumed less ethanol at higher concentrations; these differences were not attributable to increased behavioural disruption of the knockout by ethanol, since no differences were seen in sensitivity to ethanol's sedative or ataxic effects. Ro 15-4513's ability to reduce ethanol consumption was unaffected, suggesting that this effect is not mediated by the alpha5 subtype. Secondly, we tested the ability of a novel alpha5-efficacy-selective benzodiazepine receptor ligand, alpha5IA-II, that possesses greater inverse agonist activity at alpha5- than at alpha1-, á2- or alpha3-containing GABA(A) receptors, to influence operant responding. alpha5IA-II (0.03-3 mg/kg) dose-dependently decreased lever pressing for 10% ethanol, the minimally effective dose of 1 mg/kg, corresponding to over 90% receptor occupancy, but did not affect lever pressing for 4% sucrose. Although inverse agonists acting at alpha5-containing receptors reduce ethanol self-administration, alpha5 subunits may not be essential to signaling ethanol reward.

Selective GABAA ??5 Benzodiazepine Inverse Agonist Antagonizes the Neurobehavioral Actions of Alcohol

BACKGROUND

Previous research has implicated the alpha5-containing GABAA receptors of the hippocampus in the reinforcing properties of alcohol. In the present study, a selective GABAA alpha5 benzodiazepine inverse agonist (e.g., RY 023) was used in a series of in vivo and in vitro studies to determine the significance of the alpha5-receptor in the neurobehavioral actions of alcohol.

METHODS

In experiment one, systemic injections of RY 023 (1 to 10 mg/kg IP) dose-dependently reduced ethanol-maintained responding by 52% to 86% of controls, whereas bilateral hippocampal infusions (0.3 to 20 microg) reduced responding by 66% to 84% of controls. Saccharin responding was reduced only with the highest intraperitoneal (e.g., 10 mg) and microinjected (e.g., 20 microg) doses. In experiment two, RY 023 (3.0 to 15 mg/kg IP) reversed the motor-impairing effects of a moderate dose of alcohol (0.75 g/kg) on an oscillating bar task in the absence of intrinsic effects. In the open field, RY 023 (3.0 to 7.5 mg/kg) produced intrinsic effects alone but attenuated the suppression of the 1.25 g/kg ethanol dose. Because the diazepam-insensitive receptors (e.g., alpha4 and alpha6) have been suggested to play a role in alcohol motor impairing and sedative actions, experiment three compared the efficacy of RY 023 with Ro 15-4513 and two prototypical benzodiazepine antagonists (e.g., flumazenil and ZK 93426) across the alpha4beta3gamma2-, alpha5beta3gamma2-, and alpha6beta3gamma2-receptor subtypes in Xenopus oocytes.

RESULTS

RY 023 produced classic inverse agonism at all receptor subtypes, whereas Ro15-4513 and the two antagonists displayed a neutral or agonistic profile at the diazepam-insensitive receptors.

CONCLUSIONS

Overall, the results extend our previous findings by demonstrating that an alpha5-subtype ligand is capable of attenuating not only the rewarding action of alcohol but also its motor impairing and sedative effects. We propose that these actions are mediated in part by the alpha5-receptors of the hippocampus. The hippocampal alpha5-receptors could represent novel targets in understanding the neuromechanisms regulating the neurobehavioral actions of alcohol in humans.

The benzodiazepine partial inverse agonist Ro15-4513 alters anticonvulsant and lethal effects of carbamazepine in amygdala-kindled rats

Ro15-4513 (ethyl-8-azido-5,6-dihydro-5methyl-6-oxo-4H-imidazo-[1,5-a]-1,4-benzodiazepine-3-carboxylate), a benzodiazepine partial inverse agonist of the GABA(A) receptor, is known to protect against alcohol toxicities. The present study was designed to determine the role of Ro15-4513 in preventing anticonvulsant, toxic, and lethal effects of carbamazepine (CBZ) in amygdala-kindled rats. Acute treatment with CBZ (25 mg/kg, i.p.) produced anticonvulsant effects in fully kindled rats characterized by a significant decrease in afterdischarge and seizure duration and stage. Repeated administration of this high dose of CBZ induced sedation and high (56%) lethality. The anticonvulsant and sedative effects of CBZ were strikingly suppressed by pretreatment with Ro15-4513 (2.5 and 5 mg/kg, i.p.), and there was no mortality in animals co-administrated with Ro15-4513 during the entire experimental period. These results indicate that Ro15-4513 protects against CBZ-induced sedation and lethality, while suppressing the anticonvulsant effects of CBZ, suggesting a role for the GABA(A) receptor in CBZ efficacy and side effects. The potential clinical implications for CBZ-induced toxicity and overdose remain to be explored.

A comparison of female and male rats' ETOH-induced ataxia and exploration following restraint or swim stress

Animal models of stress reactivity are often employed in developing treatments for humans. Many studies use shock stress, and most use male rats. These experiments compare female and male rats exposed to either restraint stress (RS) or ambient-temperature swim stress (SS), using two durations of each stressor and naive controls. The ataxic effects of a 0.6 g/kg i.p. dose of ethanol (ETOH) were measured. Females exhibited less ataxia than males following ETOH administration. There were no significant effects of stress on ETOH-induced ataxia. Exploration was also measured in an open-field test (OFT) both pre- and poststress. In the prestress OFT, females were more active than males. For the no-stress groups and the shorter-duration stress groups, exploration decreased between the first and second OFTs. However, the groups exposed to the longer-duration stress did not show this expected decrease in exploration. A key finding of this research is that while sex differences may be present at baseline, the sexes may react similarly to stress. These data extend knowledge on sex differences in stress, alcohol reactivity and exploratory behavior.

Cerebellar CB(1) receptor mediation of Delta(9)-THC-induced motor incoordination and its potentiation by ethanol and modulation by the cerebellar adenosinergic A(1) receptor in the mouse

The effect of intracerebellar microinfusion of antisense oligodeoxynucleotide to Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and other naturally occurring cannabinoid receptor (CB(1)) mRNA on Delta(9)-THC-induced motor impairment was investigated in mice. Delta(9)-THC (15-30 microgram/microliter intracerebellar) resulted in a significant motor impairment in a dose-related manner. The intracerebellar pretreatment with antisense oligodeoxynucleotide (3.0 microgram/100 nl/12 h; six administrations/mouse) virtually abolished Delta(9)-THC (15 and 25 microgram/1 microliter intracerebellar)-induced motor impairment. However, intracerebellar pretreatment with the mismatched oligodeoxynucleotide in exactly the same manner as the antisense was completely ineffective in altering the Delta(9)-THC-induced motor impairment. These results strongly suggest the involvement of CB(1) receptor in the expression of Delta(9)-THC-induced motor impairment. The intracerebellar microinfusion of adenosine A(1)-selective agonist, N(6)-cyclohexyladenosine (CHA) (4 ng/100 nl) significantly enhanced Delta(9)-THC-induced motor impairment, suggesting a cerebellar A(1) adenosinergic modulation of motor impairment. A pretreatment with the antisense and the mismatched oligodeoxynucleotide also markedly attenuated and did not alter, respectively, the cerebellar A(1) adenosinergic modulation (enhancement) of Delta(9)-THC-induced motor impairment. There was no change in the normal motor coordination due to intracerebellar pretreatment with antisense and its mismatch, in the presence as well as absence of intracerebellar CHA indicating the selectivity of interactions with Delta(9)-THC. The Delta(9)-THC-induced motor incoordination was also significantly enhanced dose-dependently by systemic (i.p.) ethanol administration suggesting behavioral synergism between the two psychoactive drugs. Pretreatment (intracerebellar) with pertussis toxin (PTX) markedly attenuated Delta(9)-THC- and Delta(9)-THC+CHA-induced motor incoordination suggesting coupling of CB(1) receptor to PTX-sensitive G-protein (G(i)/G(o)). These data suggested co-modulation by cerebellar cannabinoid and adenosine system of Delta(9)-THC-induced motor impairment. Conversely, the results in the present study also suggested co-modulation by cerebellar adenosine A(1) and CB(1) receptors of ethanol-induced motor impairment, thereby indicating a possible common signal transduction pathway in the expression of motor impairment produced by Delta(9)-THC as well as ethanol.

GABAA-benzodiazepine receptors in the striatum are involved in the sedation produced by a moderate, but not an intoxicating ethanol dose in outbred Wistar rats

The role of the dorsal striatum in mediating the sedation produced by a moderate (0.75 g/kg) and an intoxicating (1.25 g/kg) EtOH dose was investigated in the open field by determining the capacity of direct intrastriatal injections of RY 008, a partial inverse agonist of the benzodiazepine (BDZ) receptor, to antagonize EtOH's effects. SR 95531, the competitive high-affinity GABAA antagonist was used as a reference compound. Intrastriatal RY 008 (50, 500 ng) and SR 95531 (50 ng) antagonized the sedation produced by the 0.75 g/kg EtOH dose. However, RY 008 did not alter the sedation produced by the 1.25 g/kg dose. RY 008 alone was without effect. RY 008 also failed to negatively modulate GABAergic function at alpha1beta2gamma2 or alpha6beta2gamma2 receptor subtypes expressed in Xenopus oocytes. Intrastriatal modulation of the moderate EtOH dose was site specific: no antagonism by RY 008 after intraaccumbens infusions was observed. The results suggest that central GABAA-BDZ receptors in the dorsal striatum play an important role in mediating the sedation produced by a moderate EtOH dose in the open field.

Adenosinergic modulation of ethanol-induced motor incoordination in the rat motor cortex

1. On going work in our laboratory has shown that adenosine modulates ethanol-induced motor incoordination (EIMI) when given systemically as well as directly into the cerebral ventricles, cerebellum and corpus striatum of the rat and/or mouse. 2. The objective of this study was to determine what effect adenosine agonists and antagonists would have within the rat motor cortex on EIMI. 3. The participation of the motor cortex in EIMI was suggested when microinfusion of the anti-ethanol compound, Ro15-4513, an inverse agonist of the benzodiazepine binding site, directly into the motor cortex significantly attenuated EIMI. Further, the adenosine agonists N6-cyclohexyladenosine (CHA) and 2-p-(2-carboxyethyl)-phenethylamino-5'-N-carboxaminoadenosine++ + hydrochloride (CGS-21680) significantly accentuated EIMI in a dose-related manner. The adenosine A1 receptor-selective agonist, CHA, appeared most potent in this modulatory effect when compared to the A2-selective agonist, CGS-21680. 4. The extent of diffusion of the adenosine drugs within the cortical tissue after their microinfusion was also checked by measuring the dispersion of microinfused [3H]CHA. The [3H]CHA dispersion study indirectly confirmed that the results of the present investigation were based on the effect of adenosine drugs within the motor cortex only. 5. Accentuation by the A1- and A2-selective adenosine agonists was significantly attenuated by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2 receptor-selective antagonist 8-(3-chlorostyryl)caffeine (CSC) further suggesting modulation mainly by the A1-subtype. 6. Pretreatment of the motor cortex with pertussis toxin (PT) significantly reduced the capacity of both A1- and A2-selective adenosine agonists to accentuate EIMI suggesting the involvement of a PT-sensitive Gi/Go protein. 7. These data support earlier work which showed that adenosine modulates EIMI within the central nervous system (CNS), most likely via the A1 receptor, and moreover, extend that work by including the motor cortex as a brain area participating in the adenosinergic modulation of ethanol-induced motor impairment.

The striatal adenosinergic modulation of ethanol-induced motor incoordination in rats: possible role of chloride flux

Previous studies from our laboratory have provided strong evidence that brain adenosine modulates acute ethanol (i.p.)-induced motor incoordination (MI) through receptor mediated mechanism(s). Recently, we have reported the involvement of the striatum in ethanol-induced MI as well as the striatal adenosinergic modulation of the ethanol-induced motor deficit. The present study was thus designed to further characterize the modulatory effect of striatal adenosine on ethanol-induced MI and to look for its functional correlation with chloride flux within the rat striatum. Intrastriatal microinfusion of adenosine A1 receptor agonist N6-cyclohexyladenosine (CHA) and antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), significantly accentuated and attenuated, respectively, the motor incoordinating effect of ethanol while having no effect on the normal motor coordination in saline-treated control animals. These data confirmed the role of striatal adenosine in ethanol-induced MI. The selectivity of interactions between adenosine A1 agonist and antagonist and ethanol was further confirmed by the study in which neither intrastriatal CHA nor DPCPX significantly altered the MI induced by sodium pentobarbital. Previously, we have shown that intrastriatal Ro15-4513 not only significantly attenuated ethanol-induced MI but also blocked its accentuation by intrastriatal CHA. It is well known that Ro15-4513 antagonizes many, but not all, CNS effects of ethanol by blocking the ethanol potentiation of GABA-stimulated uptake of chloride. Therefore, experiments using striatal microsac preparations were carried out to investigate the possible modulation of chloride conductance by CHA and its relationship to ethanol. High concentrations of CHA (10 and 100 nM) increased the total chloride uptake by the striatal microsacs. Corresponding to the ethanol-adenosine interaction observed behaviorally, a much lower concentration (1 nM) of CHA, being ineffective itself, significantly enhanced the stimulatory action of ethanol on chloride uptake. This effect was blocked by either Ro15-4513 (100 nM) or DPCPX (10 nM). The modulatory effect of GABA and/or ethanol on chloride influx was also evaluated, and the results supported the appropriateness to use striatal microsac preparations in the present study. Overall, the data suggested a functional interaction between ethanol and striatal adenosine and further supported the hypothesis that striatal adenosine might, in part, modulate ethanol-induced MI through its effect on chloride conductance through chloride channels coupled to GABA-benzodiazepine receptor complex.

Contribution of "diazepam-insensitive" GABAA receptors to the alcohol antagonist properties of Ro 15-4513 and related imidazobenzodiazepines

Both in vivo and in vitro studies have shown that Ro 15-4513 can antagonize many of the pharmacologic actions of ethanol. In contrast to many benzodiazepine receptor (BzR) ligands, Ro 15-4513 binds with high affinity to a novel GABAA receptor subtype, referred to as "diazepam-insensitive" (DI). This study examined the contribution of DI GABAA receptors to the modulation of ethanol-induced sleep time by Ro 15-4513 and related imidazobenzodiazepines [e.g., Ro 19-4603, Ro 16-6028, and ZG-63 (t-butyl-8-chloro-5,6-dihydro-5-methyl-6-oxo-imidazo[1,5,a] [1,4]benzodiazepine-3-carboxylate)] that possess high affinities for this GABAA receptor subtype. Ro 15-4513 (0.6-5 mg/kg) significantly reduced ethanol (3.5 g/kg, i.p.) sleep time in mice (p < 0.001, analysis of variance). This effect was not blocked by BzR antagonists ZK 93426 (5 mg/kg) and Ro 14-7437 (5 mg/kg), which possess low affinities for DI but bind with high affinities to other "diazepam-sensitive" (DS) GABAA receptor isoforms. Although Ro 19-4603 (2.5 mg/kg) also reduced ethanol sleep time (p < 0.01), this effect was attenuated by coadministration of ZK 93426 (2.5 mg/kg). Ro 16-6028 (2.5 mg/kg) prolonged (p < 0.01) ethanol sleep time. However, in the presence of either Ro 19-7437 (5 mg/kg) or ZK 93426 (2.5 mg/kg) ethanol-induced sleep time was reduced to values approaching those obtained with ethanol in the presence of Ro 15-4513. A low dose (2.5 mg/kg) of ZG-63 did not significantly affect alcohol sleep time. However, in the presence of ZK 93426, ZG-63 increased sleep time (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonism by intracerebellar Ro15-4513 of acute ethanol-induced motor incoordination in mice

The possible antiethanol effect of intracerebellarly microinjected Ro15-4513 was investigated using motor incoordination as the test response. The results of this study further confirmed reports from this and other laboratories that this partially negative ligand of benzodiazepine selectively attenuated and nearly reversed the motor impairment of acute ethanol. The attenuation observed after microinjections of doses of 0.05, 0.1, and 0.5 ng was significant and dose related. There was no effect on normal coordination when the highest dose, 0.5 ng, was administered followed by saline instead of a test dose of ethanol. When 0.5 ng of Ro15-4513 alone was microinjected into the cerebellum, no significant change in the locomotor activity was observed. Even a 10-fold higher intracerebellar dose (5 ng) of Ro15-4513 administered alone produced no significant changes in locomotor activity. This suggests that attenuation of ethanol-induced motor incoordination was most likely due to the selective antiethanol effect of Ro15-4513 at the dose range used in the present investigation. The antiethanol effect of intracerebellar Ro15-4513 also reaffirmed the well-known belief that the cerebellum is an important brain region for ethanol's motor-impairing effect. The results also indirectly suggest the inhibition of GABAA-gated chloride ion channel activity as the most likely basis of Ro15-4513's antiethanol effect.