Ethanol-induced locomotor stimulation in C57BL/6 mice following RO15-4513 administration

Role of Sigma Receptors in Alcohol Addiction

Pharmacological treatments for alcohol use disorder (AUD) are few in number and often ineffective, despite the significant research carried out so far to better comprehend the neurochemical underpinnings of the disease. Hence, research has been directed towards the discovery of novel therapeutic targets for the treatment of AUD. In the last decade, the sigma receptor system has been proposed as a potential mediator of alcohol reward and reinforcement. Preclinical studies have shown that the motivational effects of alcohol and excessive ethanol consumption involve the recruitment of the sigma receptor system. Furthermore, sigma receptor antagonism has been shown to be sufficient to inhibit many behaviors related to AUDs. This paper will review the most current evidence in support of this receptor system as a potential target for the development of pharmacological agents for the treatment of alcohol addiction.

Sigma Receptors and Alcohol Use Disorders

Although extensive research has focused on understanding the neurobiological mechanisms underlying alcohol addiction, pharmacological treatments for alcohol use disorders are very limited and not always effective. This constraint has encouraged the search for novel pharmacological targets for alcoholism therapy. Sigma receptors were shown to mediate some of the properties of cocaine and amphetamine, which was attributed to the direct binding of psychostimulants to these receptors. More recently, the role of sigma receptors in the rewarding and reinforcing effects of alcohol was also proposed, and it was suggested that their hyperactivity may result in excessive alcohol drinking. This chapter reviews current knowledge on the topic, and suggests that the sigma receptor system may represent a new therapeutic target for the treatment of alcohol use disorders.

Ethanol-related behaviors in mice lacking the sigma-1 receptor

RATIONALE

The Sigma-1 receptor (Sig-1R) is a chaperone protein that has been implicated in drug abuse and addiction. Multiple studies have characterized the role the Sig-1R plays in psychostimulant addiction; however, fewer studies have specifically investigated its role in alcohol addiction. We have previously shown that antagonism of the Sig-1R reduces excessive drinking and motivation to drink, whereas agonism induces binge-like drinking in rodents.

OBJECTIVES

The objectives of these studies were to investigate the impact of Sig-1R gene deletion in C57Bl/6J mice on ethanol drinking and other ethanol-related behaviors.

METHODS

We used an extensive panel of behavioral tests to examine ethanol actions in male, adult mice lacking Oprs1, the gene encoding the Sig-1R. To compare ethanol drinking behavior, Sig-1 knockout (KO) and wild type (WT) mice were subject to a two-bottle choice, continuous access paradigm with different concentrations of ethanol (3-20% v/v) vs. water. Consumption of sweet and bitter solutions was also assessed in Sig-1R KO and WT mice. Finally, motor stimulant sensitivity, taste aversion and ataxic effects of ethanol were assessed.

RESULTS

Sig-1R KO mice displayed higher ethanol intake compared to WT mice; the two genotypes did not differ in their sweet or bitter taste perception. Sig-1R KO mice showed lower sensitivity to ethanol stimulant effects, but greater sensitivity to its taste aversive effects. Ethanol-induced sedation was instead unaltered in the mutants.

CONCLUSIONS

Our results prove that the deletion of the Sig-1R increases ethanol consumption, likely by decreasing its rewarding effects, and therefore indicating that the Sig-1R is involved in modulation of the reinforcing effects of alcohol.

Ro 15-4513 Antagonizes Alcohol-Induced Sedation in Mice Through αβγ2-type GABA(A) Receptors

Ethyl alcohol (ethanol) has many molecular targets in the nervous system, its potency at these sites being low compared to those of sedative drugs. This has made it difficult to discover ethanol's binding site(s). There are two putative binding sites at γ-aminobutyric acid (GABA) type A receptor subtypes for the proposed ethanol antagonist Ro 15-4513, the established γ2 subunit-dependent benzodiazepine site and the recently reported δ subunit-dependent Ro 15-4513/ethanol binding site. Here, we aimed at clarifying the in vivo role of Ro 15-4513 at these two sites. We found that the antagonism of ethanol actions by Ro 15-4513 in wildtype mice was dependent on the test: an open field test showed that light sedation induced by 1.5–1.8 g/kg ethanol was sensitive to Ro 15-4513, whereas several tests for ethanol-induced anxiolytic effects showed that the ethanol-induced effects were insensitive to Ro 15-4513. Antagonism of ethanol-induced sedation by Ro 15-4513 was unaffected in GABA_A receptor δ subunit knockout mice. By contrast, when testing the GABA_A receptor γ2 subunit F77I knock-in mouse line (γ2I77 mice) with its strongly reduced affinity of the benzodiazepine sites for Ro 15-4513, we found that the ethanol-induced sedation was no longer antagonized by Ro 15-4513. Indeed, γ2I77 mice had only a small proportion of high-affinity binding of [³H]Ro 15-4513 left as compared to wildtype mice, especially in the caudate–putamen and septal areas, but these residual sites are apparently not involved in ethanol antagonism. In conclusion, we found that Ro 15-4513 abolished the sedative effect of ethanol by an action on γ2 subunit-dependent benzodiazepine sites.

Brain stress systems in the amygdala and addiction

Dysregulation of the brain emotional systems that mediate arousal and stress is a key component of the pathophysiology of drug addiction. Drug addiction is a chronically relapsing disorder characterized by a compulsion to seek and take drugs and the development of dependence and manifestation of a negative emotional state when the drug is removed. Activation of brain stress systems is hypothesized to be a key element of the negative emotional state produced by dependence that drives drug-seeking through negative reinforcement mechanisms. The focus of the present review is on the role of two key brain arousal/stress systems in the development of dependence. Emphasis is placed on the neuropharmacological actions of corticotropin-releasing factor (CRF) and norepinephrine in extrahypothalamic systems in the extended amygdala, including the central nucleus of the amygdala, bed nucleus of the stria terminalis, and a transition area in the shell of the nucleus accumbens. Compelling evidence argues that these brain stress systems, a heretofore largely neglected component of dependence and addiction, play a key role in engaging the transition to dependence and maintaining dependence once it is initiated. Understanding the role of the brain stress and anti-stress systems in addiction not only provides insight into the neurobiology of the "dark side" of addiction but also provides insight into the organization and function of basic brain emotional circuitry that guides motivated behavior.

Tonic for what ails us? high-affinity GABAA receptors and alcohol

Ethanol interactions with gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the brain, play key roles in acute intoxication. However, the exact mechanisms of these ethanol interactions have been the subject of considerable confusion and controversy. Many studies suggest that ethanol potentiates the function of the type A GABA receptor (GABAA-R). However, these findings have not been consistently replicated in experiments that directly examined the effects of ethanol on GABAA-R-mediated ion current. Differences in ethanol sensitivity of different GABAA-R subtypes have been invoked as a potential explanation for the inconsistent findings, and recent work suggests that GABAA-Rs that contain the delta subunit and/or mediate tonic extrasynaptic GABA responses may be especially ethanol sensitive. However, considerable disagreement has arisen over these findings. This special issue of Alcohol contains articles from eight research groups that are examining this issue. The authors present their work, their views on the present state of this area of alcohol research, and their ideas about how to proceed with future studies that may help to address the present confusion and controversy. This editorial provides an introduction to this line of research and the current findings and controversies.

FG 7142- and restraint-induced alterations in the ataxic effects of alcohol and midazolam in rats are time dependent

The purpose of this study was to examine whether acute stress exposure would alter the ataxic properties of midazolam or ethanol in rats. Rats were administered either vehicle or FG 7142 (10 mg/kg) and placed back in their home cages, or placed in restraining tubes for 90 min. Three and one-half or 24 h following injection all subjects were then administered an ataxic dose of either ethanol or midazolam and after 10 min, motoric impairment was assessed by rotarod performance. Neither FG 7142 administration nor restraint had an impact on rotarod performance 3-1/2 h later for ethanol nor 24 h later in response to midazolam. However, midazolam-induced ataxia was significantly modified 3-1/2 h following both restraint and FG 7142 exposure. Similarly, at the 24-h time point, both manipulations had a significant effect on ethanol-induced motor incoordination. Importantly, prior exposure to FG 7142 and restraint was without effect on rotarod performance in saline-treated subjects. Functional alterations in behavioral reactivity to low doses of two classes of CNS depressants by the acute stress of restraint and/or FG 7142 administration suggest the anxiogenic nature of these stressors may be the critical factor.

Pharmacological treatment of alcoholism

1. Pharmacological treatments are effective as part of a treatment plan that includes substantial education, psychological therapy and social support. This paper reviews recent literature on animal models of and treatment for alcohol abuse under seven categories: agents to block craving or reduce alcohol intake, agents to induce aversion to alcohol, agents to treat acute alcohol withdrawal, agents to treat protracted alcohol withdrawal, agents to diminish drinking by treating associated psychiatric pathology, agents to decrease drinking by treating associated drug abuse, and agents to induce sobriety in intoxicated individuals. 2. The benzodiazepines provide safe and effective treatment for detoxification, although current research focuses on finding drugs with a smaller likelihood of dependence. As yet, there are no drugs that effectively reverse the intoxicating effects of alcohol. 3. Currently, only two major groups of drugs that are relatively safe have shown any effect at reducing alcohol consumption: aversives such as disulfiram, and opioid antagonists such as naltrexone. 4. Finally, it is important to customize therapy for each patient rather than putting everyone through a standard treatment plan, especially in regards to the use of antidepressant or antipsychotic medications. Tailoring the program to the patient's needs dramatically improves the outcome of therapy and reduces the risk of adverse effects.

Ro 15-4513 alteration of pentobarbital dependence

This study investigated the ability of the benzodiazepine inverse agonist, Ro 15-4513, to alter the expression of physical dependence on pentobarbital. Male Sprague-Dawley rats were made physically dependent on pentobarbital by continuous. IP, infusion of escalating doses of pentobarbital for 12 days. In Experiment 1, pentobarbital dependent rats received either vehicle or Ro 15-4513, in doses of 5, 10, or 15 mg/kg, IP, periodically during the pentobarbital abstinence period. As expected, Ro 15-4513 produced a significant, dose-dependent, exacerbation of withdrawal signs in the pentobarbital dependent rats. In Experiment 2, either vehicle or Ro 15-4513, at a dose of 15 mg/ kg, was administered, IP, once daily during the 12 days of continuous pentobarbital infusion. During the subsequent pentobarbital abstinence period it was noted that the withdrawal signs were significantly reduced in the rats receiving the daily administration of Ro 15-4513. It is hypothesized that the benzodiazepine inverse agonist, Ro 15-4513, may inhibit the development of physical dependence on pentobarbital through an opposing action on the GABA-A receptor.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Low-level hyperbaric exposure antagonizes locomotor effects of ethanol and n-propanol but not morphine in C57BL mice

Low-level hyperbaric exposure antagonizes a broad range of behavioral effects of ethanol in a direct, reversible, and competitive manner. This study investigates the selectivity of the antagonism across other drugs. C57BL/6 mice were injected with saline, ethanol, n-propanol, or morphine sulfate, and then were exposed to 1 atmosphere absolute (ATA) air, 1 ATA helium-oxygen gas mixture (heliox), or 12 ATA heliox. Locomotor activity was measured from 10 to 40 min following injection. N-propanol produced a dose-dependent depression of locomotor activity from 1.0 g/kg. Morphine produced a dose-dependent stimulation of locomotor activity at doses of 3.75-12.0 mg/kg. Exposure to 12 ATA heliox significantly antagonized the locomotor depressant effects of 1.0 g/kg n-propanol and 2.5 g/kg ethanol, without significantly affecting blood concentrations of these drugs measured at 40 min postinjection. Exposure to 12 ATA heliox did not significantly antagonize the locomotor-stimulating effects of the two morphine doses tested (3.75 and 7.5 mg/kg). These findings suggest that exposure to 12 ATA heliox antagonizes the behavioral effects of intoxicant-anesthetic drugs like ethanol and n-propanol, which are believed to act via perturbation or allosteric modulation of functional proteins, but does not antagonize the effects of drugs like morphine, which act via more direct mechanisms. This demonstration of selective antagonism adds important support for the hypothesis that low-level hyperbaric exposure is a direct mechanistic ethanol antagonist, with characteristics similar to a competitive pharmacological antagonist.

Interactions of Ro15-4513, Ro15-1788 (flumazenil) and ethanol on measures of exploration and locomotion in rats

The present study investigated the role of the GABAA-benzodiazepine (BDZ) receptor complex in mediating ethanol (ETOH)-induced increases in exploration (head-dipping) and locomotion of rats in a holeboard test. Male Sprague-Dawley rats were selected based on low basal exploratory rates to increase the likelihood that ETOH would increase these behaviors. The effects of the BDZ partial inverse agonist, Ro15-4513 (2.5 mg/kg), and the BDZ receptor antagonist, Ro15-1788 (flumazenil) (8.0 mg/kg), alone, and in combination with ETOH (0.25, 0.50 and 0.75 g/kg, IP) were investigated. The 0.25 and 0.50 g/kg doses of ETOH markedly increased both exploration and locomotion in low exploratory rats. The ETOH-induced increases were prevented by Ro15-4513 on both measures at a dose that produced no observable intrinsic action; however, this apparent lack of intrinsic activity on exploration may have been related to the low basal rates of responding in the subjects. The BDZ antagonist, flumazenil, completely reversed the antagonistic action produced by Ro15-4513 of the ETOH-induced stimulant effects on locomotion, however, flumazenil exerted only a marginal statistically significant effect on Ro15-4513's actions on head-dipping. When flumazenil was given alone, it increased head-dipping, but was without effect on locomotion. Flumazenil did not affect ETOH-induced increases in locomotion; however, ETOH and flumazenil appeared to show agonistic effects on exploration. The different effects exerted by flumazenil alone, and in combination with ETOH on head-dipping and locomotion suggest that the actions of flumazenil on these behaviors are mediated through separate mechanisms. The research further suggests that both the anxiolytic and locomotor activational effects of ETOH are mediated through the GABAA-BDZ receptor complex.

Effects of ethanol, MK-801, and chlordiazepoxide on locomotor activity in different rat lines: dissociation of locomotor stimulation from ethanol preference

Several lines of research have suggested a link between the reward value of a drug and its ability to stimulate locomotion. One goal of the present study was to determine whether ethanol preferentially stimulates locomotor activity in lines of rat that show a preference for ethanol. A secondary goal was to determine the extent to which the benzodiazepine-like and NMDA antagonistic action of ethanol accounted for its effect on locomotor activity. To meet these goals, the effects of varying doses of ethanol (0.125-1.0 g/kg), MK-801 (0.1-0.3 mg/kg), and chlordiazepoxide (0.3-3 mg/kg) on locomotor activity were studied in several lines of rats that had been habituated to the testing procedure. The effect of low doses of ethanol on motor activity in the Alcohol-Preferring (P) and Fawn-Hooded rats, which show a strong ethanol preference, were similar to those of the alcohol-nonpreferring (NP), Flinders Sensitive Line, and Flinders Resistant Line rats. Only the Flinder Resistant Line rats showed a small, but significant increase in locomotor activity after the administration of ethanol. The highest dose of ethanol (1.0 g/kg) produced locomotor depression in all lines except the P and NP lines, which were not tested at this dose. These findings do not support a link between locomotor stimulation by ethanol and ethanol preference. In contrast, all lines exhibited locomotor stimulation after moderate (0.1-0.3 mg/kg) doses of MK-801, but did not exhibit increases in activity following any dose of chlordiazepoxide. These data indicate that the profiles of activity after MK-801 and chlordiazepoxide were distinct from that of ethanol in the various rat lines.(ABSTRACT TRUNCATED AT 250 WORDS)

Low-level hyperbaric antagonism of ethanol-induced locomotor depression in C57BL/6J mice: dose response

This study characterized the antagonistic effects of hyperbaric exposure on the dose-response curve for ethanol-induced depression of locomotor activity. Drug-naive, male C57BL/6 mice were injected intraperitoneally with saline, 1.5, 2.0, 2.5, or 3.0 g/kg ethanol, and were exposed to 1 atmosphere absolute (ATA) air or 12 ATA helium-oxygen gas mixtures (heliox) at temperatures that offset the hypothermic effects of ethanol and helium. Locomotor activity was measured 10-30 min after injection. In addition, the effects of exposure to 12 ATA heliox on blood ethanol concentrations were tested in a separate group of mice injected with 2.5 g/kg ethanol. Ethanol produced a dose-dependent depression of locomotor activity beginning at 2.0 g/kg. Exposure to 12 ATA heliox completely antagonized the locomotor depressant effects of 2.0 and 2.5 g/kg ethanol and partially blocked the effects of 3.0 g/kg. Activity in mice given 1.5 g/kg ethanol was not significantly affected at 1 ATA air, but was significantly increased at 12 ATA heliox. Low-level hyperbaric exposure shifted the ethanol dose-response curve to the right with a resultant increase in the ED50 of ethanol for locomotor depression from 2.6 to 3.3 g/kg. Exposure to 12 ATA heliox did not alter blood ethanol concentrations in mice injected with 2.5 g/kg ethanol. These findings with 12 ATA heliox present key new evidence for the hypothesis that low-level hyperbaric exposure acts directly, with a pattern analogous to a competitive, mechanistic antagonist of ethanol.

The role of GABAB receptors in mediating the stimulatory effects of ethanol in mice

Recently, the GABAB receptor antagonist phaclofen has been shown to attenuate the stimulation of locomotor activity induced by ethanol (Allan and Harris 1989). In the present study, the effects of a range of recently developed GABAB receptor antagonists (phaclofen, 2-hydroxysaclofen, beta-phenyl-beta-alanine, CGP 35348) and the GABAB receptor agonist baclofen, were studied for their ability to block the locomotor stimulation induced by a low dose of ethanol administered IP to mice (1.75 g/kg). Results showed that phaclofen, 2-hydroxysaclofen, BPBA and baclofen all dose-dependently decreased ethanol-induced locomotor activity, and, of these, baclofen and BPBA did so at doses which did not attenuate locomotor activity when administered alone. CGP 35348 had no effect on the activity produced by ethanol. The action of baclofen on ethanol-induced activity appeared to be GABAB receptor mediated, as the effects were stereospecific and were reversed by the antagonist, CGP 35348. However phaclofen, 2-hydroxysaclofen and BPBA failed to reverse the effects of baclofen. These results suggest that the GABAB receptor may modulate locomotor stimulation induced by low doses of ethanol, and furthermore, that agonist, rather than antagonist activity at the GABAB receptor is responsible for this reduction. The GABAB receptor subtype responsible for modulating the effects of ethanol may have properties different from those GABAB receptors characterised to date.

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.

Indomethacin does not antagonize the anxiolytic action of ethanol in the elevated plus-maze

The present study was designed to examine whether the prostaglandin (PG) synthesis inhibitor indomethacin (INDO) could antagonize the anxiolytic effects of ethanol (EtOH) in the elevated plus-maze test of anxiety. EtOH (1.6 g/kg) significantly increased the percentage of open arm entries and time spent on the open arms in both inbred C57BL/6J and outbred CD-1 mouse strains. However, this anxiolytic effect of EtOH was not significantly antagonized by pretreatment with INDO (5 and 10 mg/kg) in either strain. EtOH also significantly increased total arm entries in CD-1 mice, but not in the C57BL/6J strain. These data from C57BL/6J mice indicate that the low-dose stimulant properties of EtOH can be dissociated from the anxiolytic action of the drug in the plus-maze task. Finally, although INDO did not antagonize the stimulant effect of EtOH in the plus-maze task (in CD-1 mice), it did attenuate EtOH-induced stimulation of locomotor activity in an open-field arena. Taken together, these results suggest some specificity with regard to the role of PGs in mediating (or modulating) the neurobehavioral actions of EtOH, and further support the notion that the anxiolytic and stimulant effects of EtOH may be mediated by different mechanisms.