Mice lacking metabotropic glutamate receptor 4 do not show the motor stimulatory effect of ethanol

A Role for GABAA Receptor β3 Subunits in Mediating Harmaline Tremor Suppression by Alcohol: Implications for Essential Tremor Therapy

Background

Essential tremor patients may find that low alcohol amounts suppress tremor. A candidate mechanism is modulation of α6β3δ extra-synaptic GABAA receptors, that in vitro respond to non-intoxicating alcohol levels. We previously found that low-dose alcohol reduces harmaline tremor in wild-type mice, but not in littermates lacking δ or α6 subunits. Here we addressed whether low-dose alcohol requires the β3 subunit for tremor suppression.

Methods

We tested whether low-dose alcohol suppresses tremor in cre-negative mice with intact β3 exon 3 flanked by loxP, and in littermates in which this region was excised by cre expressed under the α6 subunit promotor. Tremor in the harmaline model was measured as a percentage of motion power in the tremor bandwidth divided by overall motion power.

Results

Alcohol, 0.500 and 0.575 g/kg, reduced harmaline tremor compared to vehicle-treated controls in floxed β3 cre- mice, but had no effect on tremor in floxed β3 cre+ littermates that have β3 knocked out. This was not due to potential interference of α6 expression by the insertion of the cre gene into the α6 gene since non-floxed β3 cre+ and cre- littermates exhibited similar tremor suppression by alcohol.

Discussion

As α6β3δ GABAA receptors are sensitive to low-dose alcohol, and cerebellar granule cells express β3 and are the predominant brain site for α6 and δ expression together, our overall findings suggest alcohol acts to suppress tremor by modulating α6β3δ GABAA receptors on these cells. Novel drugs that target this receptor may potentially be effective and well-tolerated for essential tremor.

Highlights

We previously found with the harmaline essential tremor model that GABAA receptors containing α6 and δ subunits mediate tremor suppression by alcohol. We now show that β3 subunits in α6-expressing cells, likely cerebellar granule cells, are also required, indicating that alcohol suppresses tremor by modulating α6β3δ extra-synaptic GABAA receptors.

Regulation and functional consequences of mGlu4 RNA editing

Metabotropic glutamate receptor 4 (mGlu₄) is one of eight mGlu receptors within the Class C G protein-coupled receptor superfamily. mGlu₄ is primarily localized to the presynaptic membrane of neurons where it functions as an auto and heteroreceptor controlling synaptic release of neurotransmitter. mGlu₄ is implicated in numerous disorders and is a promising drug target; however, more remains to be understood about its regulation and pharmacology. Using high-throughput sequencing, we have validated and quantified an adenosine-to-inosine (A-to-I) RNA editing event that converts glutamine 124 to arginine in mGlu₄; additionally, we have identified a rare but novel K129R site. Using an in vitro editing assay, we then validated the pre-mRNA duplex that allows for editing by ADAR enzymes and predicted its conservation across the mammalian species. Structural modeling of the mGlu₄ protein predicts the Q124R substitution to occur in the B helix of the receptor that is critical for receptor dimerization and activation. Interestingly, editing of a receptor homodimer does not disrupt G protein activation in response to the endogenous agonist, glutamate. Using an assay designed to specifically measure heterodimer populations at the surface, however, we found that Q124R substitution decreased the propensity of mGlu₄ to heterodimerize with mGlu₂ and mGlu₇ Our study is the first to extensively describe the extent and regulatory factors of RNA editing of mGlu₄ mRNA transcripts. In addition, we have proposed a novel functional consequence of this editing event that provides insights regarding its effects in vivo and expands the regulatory capacity for mGlu receptors.

The role of mGlu4 receptors within the nucleus accumbens in acquisition and expression of morphine-induced conditioned place preference in male rats

Background

Several studies have shown that glutamate neurotransmission in the nucleus accumbens (NAc) is required for the development of morphine-induced conditional place preference (CPP). In addition, metabotropic glutamate receptors (mGluRs) in NAc play important roles in the reward pathways. However, the precise role of mGluR4 in different steps of the morphine-induced CPP is less well known. In the present study the effect of bilateral intra-accumbal infusion of VU0155041, as a specific mGluR4 agonist on the acquisition and expression of morphine induced CPP in male Wistar rats was investigated. The animals were bilaterally implanted with guide cannulae above the NAc. In the first step of the study, the VU0155041 was administered at doses of 10, 30 and 50 μg/0.5 μL saline per side into the NAc during the 3 days of morphine (5 mg/kg) conditioning (acquisition) phase of morphine-induced CPP. In the second step of the study, the rats bilaterally received VU0155041 at the dose of 50 μg/0.5 μL, 5 min before the post-conditioning test in order to check the effect of VU0155041 on the expression of morphine-induced CPP.

Results

The results showed that the intra-accumbal injection of VU0155041 inhibits the acquisition of morphine-induced CPP in a dose dependent manner, but had no effect on expression.

Conclusions

The data indicated that intra-NAc administration of VU0155041 dose dependently blocks the establishment of morphine-induced CPP and reduces the rewarding properties of morphine. These effects may be related to changes in glutamate activity in the NAC and/or learning dependent mechanism of glutamate neurotransmission in reward pathway(s).

The effect of the mGlu8 receptor agonist, (S)-3,4-DCPG on acquisition and expression of morphine-induced conditioned place preference in male rats

Background

The nucleus accumbens (NAc) plays a principal role in drug reward. It has been reported that metabotropic glutamate receptors (mGlu receptors) play a key role in the rewarding pathway(s). Previous studies have shown the vast allocation of the different types of mGlu receptors, including mGlu8 receptors, in regions that are associated with opioid rewards, such as the NAc. The aim of the present study was to evaluate the role of mGlu8 receptors within the NAc in the acquisition and expression phases of morphine induced conditioned place preference (CPP). Adult male Wistar rats were bilaterally implanted by two cannulas' in the NAc and were evaluated in a CPP paradigm. Selective mGlu8 receptor allosteric agonist (S-3,4-DCPG) was administered at doses of 0.03, 0.3, and 3 μg/0.5 μL saline per side into the NAc on both sides during the 3 days of morphine (5 mg/kg) conditioning (acquisition) phase, or before place preference test, or post-conditioning (expression) phase of morphine-induced CPP.

Results

The results revealed that intra-accumbal administration of S-3,4-DCPG (0.3 and 3 μg) markedly decreased the acquisition in a dose-dependent manner but had no effect on expression of morphine-induced CPP.

Conclusions

The findings suggest that activation of mGlu8 receptors in the NAc dose-dependently blocks the establishment of morphine-induced CPP and reduces the rewarding properties of morphine which may be related to the glutamate activity into the NAc and in reward pathway(s). These data suggest that mGlu8 receptor may be involved in conditioned morphine reward.

LSP2-9166, an orthosteric mGlu4 and mGlu7 receptor agonist, reduces cocaine self-administration under a progressive ratio schedule in rats

Cocaine addiction is a serious health issue in Western countries. Despite the regular increase in cocaine consumption across the population, there is no specific treatment for cocaine addiction. Critical roles for glutamate neurotransmission in the rewarding effects of psychostimulants as well as relapse have been suggested and accumulating evidence indicates that targeting mGlu group III receptors could represent a promising strategy to develop therapeutic compounds to treat addiction. In this context, the aim of our study was to examine the effect of LSP2-9166, a mGlu4/mGlu7 receptor orthosteric agonist, on the motivation for cocaine intake. We used an intravenous self-administration paradigm in male Wistar rats as a reliable model of voluntary drug intake. We first evaluated the direct impact of cocaine on Grm4 and Grm7 gene expression. Voluntary cocaine intake under a fixed ratio schedule of injections induced an increase of both mGlu4 and mGlu7 receptor transcripts in nucleus accumbens and hippocampus. We then evaluated the ability of LSP2-9166 to affect cocaine self-administration under a progressive ratio schedule of reinforcement. We found that this compound inhibits the motivation to obtain the drug, although it induced a hypolocomotor effect which could biais motivation index. Our findings demonstrate that mGlu group III receptors represent new targets for decreasing motivation to self-administer cocaine.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

The Modulation of Pain by Metabotropic Glutamate Receptors 7 and 8 in the Dorsal Striatum

The dorsal striatum, apart from controlling voluntary movement, displays a recently demonstrated pain inhibition. It is connected to the descending pain modulatory system and in particular to the rostral ventromedial medulla through the medullary dorsal reticular nucleus. Diseases of the basal ganglia, such as Parkinson's disease, in addition to being characterized by motor disorders, are associated with pain and hyperactivation of the excitatory transmission. A way to counteract glutamatergic hyperactivation is through the activation of group III metabotropic glutamate receptors (mGluRs), which are located on presynaptic terminals inhibiting neurotransmitter release. So far the mGluRs of group III have been the least investigated, owing to a lack of selective tools. More recently, selective ligands for each mGluR of group III, in particular positive and negative allosteric modulators, have been developed and the role of each subtype is starting to emerge. The neuroprotective potential of group III mGluRs in pathological conditions, such as those characterized by elevate glutamate, has been recently shown. In the dorsal striatum, mGluR7 and mGluR8 are located at glutamatergic corticostriatal terminals and their stimulation inhibits pain in pathological conditions such as neuropathic pain. The two receptors in the dorsal striatum have instead a different role in pain control in normal conditions. This review will discuss recent results focusing on the contribution of mGluR7 and mGluR8 in the dorsal striatal control of pain. The role of mGluR4, whose antiparkinsonian activity is widely reported, will also be addressed.

Allosteric modulation of metabotropic glutamate receptors in alcohol use disorder: Insights from preclinical investigations

Metabotropic glutamate (mGlu) receptors are family C G protein-coupled receptors (GPCRs) that modulate neuronal excitability and synaptic transmission throughout the nervous system. Owing to recent advances in development of subtype-selective allosteric modulators of mGlu receptors, individual members of the mGlu receptor family have been proposed as targets for treating a variety of neurological and psychiatric disorders, including substance use disorders. In this chapter, we highlight preclinical evidence that allosteric modulators of mGlu receptors could be useful for reducing alcohol consumption and preventing relapse in alcohol use disorder (AUD). We begin with an overview of the preclinical models that are used to study mGlu receptor involvement in alcohol-related behaviors. Alcohol exposure causes adaptations in both expression and function of various mGlu receptor subtypes, and pharmacotherapies aimed at reversing these adaptations have the potential to reduce alcohol consumption and seeking. Positive allosteric modulators (PAMs) of mGlu₂ and negative allosteric modulators of mGlu₅ show particular promise for reducing alcohol intake and/or preventing relapse. Finally, this chapter discusses important considerations for translating preclinical findings toward the development of clinically useful drugs, including the potential for PAMs to avoid tolerance issues that are frequently observed with repeated administration of GPCR agonists.

GPCR and Alcohol-Related Behaviors in Genetically Modified Mice

G protein-coupled receptors (GPCRs) constitute the largest class of cell surface signaling receptors and regulate major neurobiological processes. Accordingly, GPCRs represent primary targets for the treatment of brain disorders. Several human genetic polymorphisms affecting GPCRs have been associated to different components of alcohol use disorder (AUD). Moreover, GPCRs have been reported to contribute to several features of alcohol-related behaviors in animal models. Besides traditional pharmacological tools, genetic-based approaches mostly aimed at deleting GPCR genes provided substantial information on how key GPCRs drive alcohol-related behaviors. In this review, we summarize the alcohol phenotypes that ensue from genetic manipulation, in particular gene deletion, of key GPCRs in rodents. We focused on GPCRs that belong to fundamental neuronal systems that have been shown as potential targets for the development of AUD treatment. Data are reviewed with particular emphasis on alcohol reward, seeking, and consumption which are behaviors that capture essential aspects of AUD. Literature survey indicates that in most cases, there is still a gap in defining the intracellular transducers and the functional crosstalk of GPCRs as well as the neuronal populations in which their signaling regulates alcohol actions. Further, the implication of only a few orphan GPCRs has been so far investigated in animal models. Combining advanced pharmacological technologies with more specific genetically modified animals and behavioral preclinical models is likely necessary to deepen our understanding in how GPCR signaling contributes to AUD and for drug discovery.

The dopamine transporter VNTR polymorphism moderates the relationship between acute response to alcohol and future alcohol use disorder symptoms

Alcohol use disorder (AUD) is a genetically influenced disease with peak onset in young adulthood. Identification of factors that predict whether AUD symptoms will diminish or persist after young adulthood is a critical public health need. King and colleagues previously reported that acute response to alcohol predicted future AUD symptom trajectory. Genes associated with brain dopamine signaling, which underlies alcohol's rewarding effects, might influence this finding. This study analyzed whether variation at a variable number tandem repeat polymorphism in DAT1/SLC6A3, the gene encoding the dopamine transporter, moderated the predictive relationships between acute response to alcohol and future AUD symptoms among participants enrolled in the Chicago Social Drinking Project (first two cohorts). Heavy-drinking young adults (N = 197) completed an alcohol challenge, in which acute response (liking, wanting, stimulation, and sedation) was measured. Alcohol use disorder symptoms were assessed over the following 6 years. DAT1 genotype significantly moderated the interactions between follow-up time and alcohol liking (P = 0.006) and wanting (P = 0.006) in predicting future AUD symptoms. These predictive effects were strongest among participants who carried the DAT1 9-repeat allele, previously associated with enhanced striatal dopamine tone relative to the 10-repeat allele. Exploratory analyses indicated that DAT1 effects on the relationship between alcohol liking and AUD symptoms appeared stronger for females (n = 79) than males (n = 118) (P = 0.0496). These data suggest that heavy-drinking DAT1 9-repeat allele carriers who display high alcohol-induced reward in young adulthood may be predisposed to persistent AUD symptoms and support combining genotypic and phenotypic information to predict future AUD risk.

Metabotropic Glutamate Receptors in Alcohol Use Disorder: Physiology, Plasticity, and Promising Pharmacotherapies

Developing efficacious treatments for alcohol use disorder (AUD) has proven difficult. The insidious nature of the disease necessitates a deep understanding of its underlying biology as well as innovative approaches to ameliorate ethanol-related pathophysiology. Excessive ethanol seeking and relapse are generated by long-term changes to membrane properties, synaptic physiology, and plasticity throughout the limbic system and associated brain structures. Each of these factors can be modulated by metabotropic glutamate (mGlu) receptors, a diverse set of G protein-coupled receptors highly expressed throughout the central nervous system. Here, we discuss how different components of the mGlu receptor family modulate neurotransmission in the limbic system and other brain regions involved in AUD etiology. We then describe how these processes are dysregulated following ethanol exposure and speculate about how mGlu receptor modulation might restore such pathophysiological changes. To that end, we detail the current understanding of the behavioral pharmacology of mGlu receptor-directed drug-like molecules in animal models of AUD. Together, this review highlights the prominent position of the mGlu receptor system in the pathophysiology of AUD and provides encouragement that several classes of mGlu receptor modulators may be translated as viable treatment options.

Impact of a novel environment on alcohol-induced locomotor activity in Wistar rats

Clinical studies have shown a positive correlation between novelty-seeking behavior and the susceptibility to consume drugs of abuse. Although several animal studies have demonstrated this correlation with psychostimulants or morphine, studies with alcohol have shown conflicting results. The aim of this work was to investigate alcohol-induced motor effects in Wistar rats with different responses to novelty. Animals were classified as Low- (LR) or High-Responders (HR) to novelty, depending on their horizontal activity in an automated open field. Motor activity was recorded in naïve, saline, and alcohol-administered rats at different doses (0.1, 0.25, 0.5, 1.0, or 2.5 g/kg). Horizontal movements, rearings, and stereotyped behaviors were evaluated. After the behavioral test, animals were sacrificed and blood alcohol concentrations (BACs) were measured. Low (0.1 and 0.25 g/kg) and high (2.5 g/kg) alcohol doses decreased horizontal movements in LR animals, whereas 1.0 g/kg increased this parameter in HR rats. Rearings were increased by alcohol 1.0 g/kg in LR animals. In HR rats, alcohol doses of 0.5 and 1.0 g/kg also increased this parameter. Stereotyped behaviors were decreased by an alcohol dose of 2.5 g/kg in LR animals, but were increased by an intermediate dose (1.0 g/kg) in HR rats. Differences in horizontal movements and rearings were found between LR and HR animals at certain ethanol doses. Horizontal movements (0.25 g/kg) and rearings (0.5 g/kg) were lower in LR than HR rats; however, rearings were lower in HR than LR rats at 1.0 g/kg. BACs were similar between LR and HR rats at all ethanol doses. These findings suggest that HR rats are more responsive to the stimulant effects of intermediate alcohol doses, whereas LR animals are sensitive to low/high doses of the drug. Sensitivity to alcohol motor effects may substantially depend on the initial animal's response to a novel environment. The stimulant effects of alcohol may constitute important behavioral traits significantly associated with the rewarding properties of the drug.

Role of mGlu7 receptor in morphine rewarding effects is uncovered by a novel orthosteric agonist

Opiate dependence is a major health issue and despite the existence of opioid substitution treatment, relapse frequently occurs. Group III metabotropic glutamate (mGlu) receptors has received much attention as a putative target in ethanol and cocaine addiction, but no data on opiate addiction exist. So we investigated the role of group III mGlu receptors in morphine rewarding effects through the expression and the reinstatement of conditioned place preference (CPP) using a newly synthesized mGlu4/mGlu7 receptor orthosteric agonist, LSP2-9166. We found that LSP2-9166 blocked morphine CPP expression and reinstatement after extinction. Blockade of CPP expression with LSP2-9166 was abolished when using XAP044, a mGlu7 antagonist. We also found that LSP2-9166 at the dose active for blocking morphine reward was devoid of any effect on locomotion, hedonic state, spatial memory, anxiety or depression. Altogether our data demonstrated that group III mGlu receptors, and more specifically mGlu7, might be a valuable target in opiate addiction.

Metabotropic and ionotropic glutamate receptors as potential targets for the treatment of alcohol use disorder

Emerging evidence indicates that dysfunctional glutamate neurotransmission is critical in the initiation and development of alcohol and drug dependence. Alcohol consumption induced downregulation of glutamate transporter 1 (GLT-1) as reported in previous studies from our laboratory. Glutamate is the major excitatory neurotransmitter in the brain, which acts via interactions with several glutamate receptors. Alcohol consumption interferes with the glutamatergic signal transmission by altering the functions of these receptors. Among the glutamate receptors involved in alcohol-drinking behavior are the metabotropic receptors such as mGluR1/5, mGluR2/3, and mGluR7, as well as the ionotropic receptors, NMDA and AMPA. Preclinical studies using agonists and antagonists implicate these glutamatergic receptors in the development of alcohol use disorder (AUD). Therefore, the purpose of this review is to discuss the neurocircuitry involving glutamate transmission in animals exposed to alcohol and further outline the role of metabotropic and ionotropic receptors in the regulation of alcohol-drinking behavior. This review provides ample information about the potential therapeutic role of glutamatergic receptors for the treatment of AUD.

Genes and Alcohol Consumption: Studies with Mutant Mice

In this chapter, we review the effects of global null mutant and overexpressing transgenic mouse lines on voluntary self-administration of alcohol. We examine approximately 200 publications pertaining to the effects of 155 mouse genes on alcohol consumption in different drinking models. The targeted genes vary in function and include neurotransmitter, ion channel, neuroimmune, and neuropeptide signaling systems. The alcohol self-administration models include operant conditioning, two- and four-bottle choice continuous and intermittent access, drinking in the dark limited access, chronic intermittent ethanol, and scheduled high alcohol consumption tests. Comparisons of different drinking models using the same mutant mice are potentially the most informative, and we will highlight those examples. More mutants have been tested for continuous two-bottle choice consumption than any other test; of the 137 mouse genes examined using this model, 97 (72%) altered drinking in at least one sex. Overall, the effects of genetic manipulations on alcohol drinking often depend on the sex of the mice, alcohol concentration and time of access, genetic background, as well as the drinking test.

Glutamatergic transmission in drug reward: implications for drug addiction

Individuals addicted to drugs of abuse such as alcohol, nicotine, cocaine, and heroin are a significant burden on healthcare systems all over the world. The positive reinforcing (rewarding) effects of the above mentioned drugs play a major role in the initiation and maintenance of the drug-taking habit. Thus, understanding the neurochemical mechanisms underlying the reinforcing effects of drugs of abuse is critical to reducing the burden of drug addiction in society. Over the last two decades, there has been an increasing focus on the role of the excitatory neurotransmitter glutamate in drug addiction. In this review, pharmacological and genetic evidence supporting the role of glutamate in mediating the rewarding effects of the above described drugs of abuse will be discussed. Further, the review will discuss the role of glutamate transmission in two complex heterogeneous brain regions, namely the nucleus accumbens (NAcc) and the ventral tegmental area (VTA), which mediate the rewarding effects of drugs of abuse. In addition, several medications approved by the Food and Drug Administration that act by blocking glutamate transmission will be discussed in the context of drug reward. Finally, this review will discuss future studies needed to address currently unanswered gaps in knowledge, which will further elucidate the role of glutamate in the rewarding effects of drugs of abuse.

Group III metabotropic glutamate receptors and drug addiction

Neuroadaptations of glutamatergic transmission in the limbic reward circuitry are linked to persistent drug addiction. Accumulating data have demonstrated roles of ionotropic glutamate receptors and group I and II metabotropic glutamate receptors (mGluRs) in this event. Emerging evidence also identifies Gαi/o-coupled group III mGluRs (mGluR4/7/8 subtypes enriched in the limbic system) as direct substrates of drugs of abuse and active regulators of drug action. Auto- and heteroreceptors of mGluR4/7/8 reside predominantly on nerve terminals of glutamatergic corticostriatal and GABAergic striatopallidal pathways, respectively. These presynaptic receptors regulate basal and/or phasic release of respective transmitters to maintain basal ganglia homeostasis. In response to operant administration of common addictive drugs, such as psychostimulants (cocaine and amphetamine), alcohol and opiates, limbic group III mGluRs undergo drastic adaptations to contribute to the enduring remodeling of excitatory synapses and to usually suppress drug seeking behavior. As a result, a loss-of-function mutation (knockout) of individual group III receptor subtypes often promotes drug seeking. This review summarizes the data from recent studies on three group III receptor subtypes (mGluR4/7/8) expressed in the basal ganglia and analyzes their roles in the regulation of dopamine and glutamate signaling in the striatum and their participation in the addictive properties of three major classes of drugs (psychostimulants, alcohol, and opiates).

Behavioral actions of alcohol: phenotypic relations from multivariate analysis of mutant mouse data

Behavioral studies on genetically diverse mice have proven powerful for determining relationships between phenotypes and have been widely used in alcohol research. Most of these studies rely on naturally occurring genetic polymorphisms among inbred strains and selected lines. Another approach is to introduce variation by engineering single-gene mutations in mice. We have tested 37 different mutant mice and their wild-type controls for a variety (31) of behaviors and have mined this data set by K-means clustering and analysis of correlations. We found a correlation between a stress-related response (activity in a novel environment) and alcohol consumption and preference for saccharin. We confirmed several relationships detected in earlier genetic studies, including positive correlation of alcohol consumption with saccharin consumption and negative correlations with conditioned taste aversion and alcohol withdrawal severity. Introduction of single-gene mutations either eliminated or greatly diminished these correlations. The three tests of alcohol consumption used (continuous two-bottle choice and two limited access tests: drinking in the dark and sustained high alcohol consumption) share a relationship with saccharin consumption, but differ from each other in their correlation networks. We suggest that alcohol consumption is controlled by multiple physiological systems where single-gene mutations can disrupt the networks of such systems.

The selective metabotropic glutamate receptor 7 allosteric agonist AMN082 prevents reinstatement of extinguished ethanol-induced conditioned place preference in mice

Alcohol dependence is considered a major public health problem in modern societies. The role for glutamatergic neurotransmission in the reinforcing effects of ethanol is becoming increasingly evident. Our previous findings have shown that in rats, the mGluR7 positive allosteric agonist AMN082, but not its allosteric antagonist MMPIP, prevented ethanol consumption and preference in the two-bottle choice paradigm. This study was conducted to determine the effects of AMN082 and MMPIP on the extinction and reinstatement of ethanol-elicited place preference (CPP) in C57BL/6 mice. AMN082 and MMPIP were administered during extinction of ethanol CPP to determine whether mGluR7 signaling is required. Furthermore, the effects of AMN082 and MMPIP on reinstatement of CPP were also evaluated. Finally, spontaneous locomotor activity and ethanol pharmacokinetics were assessed following systemic administration of AMN082 and MMPIP. Our results indicate that mGluR7 pharmacological modulation had no effect on ethanol-elicited CPP extinction. In contrast, mGluR7 activation using AMN082 reduced ethanol-induced CPP reinstatement, an effect reversed by co-administration of MMPIP. Collectively, these results indicate, for the first time, that activation of the mGluR7 receptor is effective in reducing the reinstatement of conditioned rewarding effects of ethanol. Taken together, the efficacy of AMN082 on the various phases of alcohol-CPP could represent an interesting pharmacological approach and could open a new line of research for the development of therapies to reduce ethanol intake in patients.

Metabotropic glutamate receptors for new treatments in schizophrenia

Metabotropic glutamate receptors (mGluRs) represent exciting targets for the development of novel therapeutic agents for schizophrenia. Recent studies indicate that selective activation of specific mGluR subtypes may provide potential benefits for not only the positive symptoms, but also the negative symptoms and cognitive impairments observed in individuals with schizophrenia. Although optimization of traditional orthosteric agonists may still offer a feasible approach for the activation of mGluRs, important progress has been made in the discovery of novel subtype-selective allosteric ligands, including positive allosteric modulators (PAMs) of mGluR2 and mGluR5. These allosteric mGluR ligands have improved properties for clinical development and have served as key preclinical tools for a more in-depth understanding of the potential roles of these different mGluR subtypes for the treatment of schizophrenia.

Neuroimmune regulation of alcohol consumption: behavioral validation of genes obtained from genomic studies

Analysis of mouse brain gene expression, using strains that differ in alcohol consumption, provided a number of novel candidate genes that potentially regulate alcohol consumption. We selected six genes [beta-2-microglobulin (B2m), cathepsin S (Ctss), cathepsin F (Ctsf), interleukin 1 receptor antagonist (Il1rn), CD14 molecule (Cd14) and interleukin 6 (Il6)] for behavioral validation using null mutant mice. These genes are known to be important for immune responses but were not specifically linked to alcohol consumption by previous research. Null mutant mice were tested for ethanol intake in three tests: 24-hour two-bottle choice, limited access two-bottle choice and limited access to one bottle of ethanol. Ethanol consumption and preference were reduced in all the null mutant mice in the 24-hour two-bottle choice test, the test that was the basis for selection of these genes. No major differences were observed in consumption of saccharin or quinine in the null mutant mice. Deletion of B2m, Ctss, Il1rn, Cd14 and Il6 also reduced ethanol consumption in the limited access two bottle choice test for ethanol intake; with the Il1rn and Ctss null mutants showing reduced intake in all three tests (with some variation between males and females). These results provide the most compelling evidence to date that global gene expression analysis can identify novel genetic determinants of complex behavioral traits. Specifically, they suggest a novel role for neuroimmune signaling in regulation of alcohol consumption.

The role of metabotropic glutamate receptors in addiction: evidence from preclinical models

Addiction is a chronic disorder characterised by repeated bouts of drug taking, abstinence and relapse. The addicted state may be in part due to drug-induced neuroadaptations in the mesocorticolimbic and corticostriatal pathways. Recently focus has been on the role of aberrant glutamate transmission and its contribution to the hierarchical control over these systems. This review will expand our current knowledge of the most recent advances that have been made in preclinical animal models that provide evidence that implicate metabotropic glutamate receptors (mGluRs) in contributing to the neuroadaptations pertinent to addiction, as well as the role of Homer proteins in regulating these responses. The recent discovery of receptor mosaics will be discussed which add an additional dimension to the complexity of understanding the mechanism of glutamate mediated behaviours. Finally this review introduces a new area related to glutamatergic responses, namely microRNAs, that may become pivotal in directing our future understanding of how to best target intervention strategies to prevent addictive behaviours.

Allosteric modulation of metabotropic glutamate receptors

The development of receptor subtype-selective ligands by targeting allosteric sites of G protein-coupled receptors (GPCRs) has proven highly successful in recent years. One GPCR family that has greatly benefited from this approach is the metabotropic glutamate receptors (mGlus). These family C GPCRs participate in the neuromodulatory actions of glutamate throughout the CNS, where they play a number of key roles in regulating synaptic transmission and neuronal excitability. A large number of mGlu subtype-selective allosteric modulators have been identified, the majority of which are thought to bind within the transmembrane regions of the receptor. These modulators can either enhance or inhibit mGlu functional responses and, together with mGlu knockout mice, have furthered the establishment of the physiologic roles of many mGlu subtypes. Numerous pharmacological and receptor mutagenesis studies have been aimed at providing a greater mechanistic understanding of the interaction of mGlu allosteric modulators with the receptor, which have revealed evidence for common allosteric binding sites across multiple mGlu subtypes and the presence for multiple allosteric sites within a single mGlu subtype. Recent data have also revealed that mGlu allosteric modulators can display functional selectivity toward particular signal transduction cascades downstream of an individual mGlu subtype. Studies continue to validate the therapeutic utility of mGlu allosteric modulators as a potential therapeutic approach for a number of disorders including anxiety, schizophrenia, Parkinson's disease, and Fragile X syndrome.

Behavioral differences between C57BL/6J x FVB/NJ and C57BL/6J x NZB/B1NJ F1 hybrid mice: relation to control of ethanol intake

C57BL/6J x FVB/NJ F1 (B6 x FVB) mice consume more alcohol than C57BL/6J x NZB/B1NJ F1 (B6 x NZB) mice and this high alcohol consumption is stable after abstinence whereas B6 x NZB show reduced consumption, thus providing models of Sustained Alcohol Preference (SAP) and Reduced Alcohol Preference (RAP). In female hybrids, we assessed several behavioral responses to define behaviors which might predict SAP and RAP. B6 x FVB exhibited less severe ethanol-induced conditioned taste aversion and were less sensitive to ethanol-induced loss of righting reflex than B6 x NZB. Both hybrids demonstrated ethanol-induced place preference and a low ethanol withdrawal severity. We found that these hybrids differ in their sensitivity to the aversive and sedative, but not rewarding, effects of ethanol. Results of elevated plus maze, mirror chamber, and locomotor tests reveal B6 x FVB mice are less anxious and more active than B6 x NZB mice. Results obtained offer insights about factors that determine SAP and RAP in these new genetic models of alcohol consumption.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the central nervous system. The mGluRs bind glutamate within a large extracellular domain and transmit signals through the receptor protein to intracellular signaling partners. A great deal of progress has been made in determining the mechanisms by which mGluRs are activated, proteins with which they interact, and orthosteric and allosteric ligands that can modulate receptor activity. The widespread expression of mGluRs makes these receptors particularly attractive drug targets, and recent studies continue to validate the therapeutic utility of mGluR ligands in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, anxiety, depression, and schizophrenia.

Role of glutamate in schizophrenia: integrating excitatory avenues of research

Schizophrenia is a debilitating lifelong disorder affecting up to 1% of the population worldwide, producing significant financial and emotional hardship for patients and their families. As yet, the causes of schizophrenia and the mechanism of action of antipsychotic drugs are unknown, and many patients do not respond well to currently available medications. Attempts to find risk factors for the disorder using epidemiological methods have shown that schizophrenia is highly heritable, and path analyses predict that the disorder is caused by several genes in combination with nongenetic factors. Therefore, intensive research efforts have been made to identify genes creating vulnerability to schizophrenia and also genes predicting response to treatment. Interactions of the glutamatergic system with dopaminergic and serotonergic circuitry are crucial for normal brain function, and their disruption may be a mechanism by which the pathophysiology of schizophrenia is manifest. Genes within the glutamatergic system are therefore strong candidates for investigation, and these include the glutamate receptor genes in addition to genes encoding neuregulin, dysbindin, D-amino acid oxidase and G72/G30. These genetic studies could eventually reveal new targets for antipsychotic drug treatment, which currently focuses on inhibition of the dopaminergic system. However, a recent breakthrough indicates clinical efficacy of a drug stimulating the metabotropic glutamate receptor II, LY2140023, which has improved efficacy for negative and cognitive symptoms of schizophrenia. Studies of larger patient samples are required to consolidate these data. Further investigation of glutamatergic targets is likely to reinvigorate antipsychotic drug development.

Metabotropic glutamate receptor 5 (mGluR5) regulation of ethanol sedation, dependence and consumption: relationship to acamprosate actions

Recent studies have demonstrated that metabotropic glutamate receptor 5 (mGluR5) antagonists decrease alcohol self-administration and suggest that the anti-craving medication, acamprosate, may also act to decrease mGluR5 function. To address the role of mGluR5 in behavioural actions of ethanol and acamprosate, we compared mutant mice with deletion of the mGluR5 gene and mice treated with a mGluR5 antagonist (MPEP) or acamprosate. Lack of mGluR5 or administration of MPEP reduced the severity of alcohol-induced withdrawal (AW), increased the sedative effect of alcohol (duration of loss of righting reflex; LORR), and increased basal motor activity. The motor stimulation produced by ethanol was blocked by deletion of mGluR5, but not by injection of MPEP. Both acamprosate and MPEP increased ethanol-induced LORR and reduced AW. Importantly, the protective effects of both MPEP and acamprosate on AW were found when the drugs were injected before, but not after, injection of ethanol. This indicates that the drugs prevented development of dependence rather than merely producing an anticonvulsant action. No effects of acamprosate or MPEP on ethanol-induced LORR and AW were found in mGluR5 knockout mice, demonstrating that mGluR5 is required for these actions. mGluR5 null mutant mice showed decreased alcohol consumption in some, but not all, tests. These data show the importance of mGluR5 for several actions of alcohol and support the hypothesis that some effects of acamprosate require mGluR5 signalling.

Comparison of ethanol locomotor sensitization in adolescent and adult DBA/2J mice

RATIONALE

The mammalian adolescent period is characterized by enhanced vulnerability to drug-induced neuroadaptations. Epidemiological evidence indicates that individuals who start drinking alcohol during adolescence are four times more likely to develop alcohol dependence in adulthood, but little is known about the adaptive mechanism(s) that may underlie this observation. Behavioral sensitization in rodents is a model of neurobehavioral plasticity that occurs following repeated drug exposure and may underlie components of addiction.

OBJECTIVES

The goal of this study was to determine if adolescent mice are differentially sensitive to ethanol-induced locomotor sensitization as compared to adults.

MATERIALS AND METHODS

Adolescent and adult DBA/2J mice were treated with saline or ethanol (1.0, 1.5, 2.0, 2.5 g/kg) for 7, 11, or 15 days and tested for acute and sensitized locomotor activity. Blood ethanol clearance (BEC) was also assessed 10, 60, and 180 min following treatment with ethanol 2 g/kg.

RESULTS

Adolescent mice were more sensitive than adult mice to the acute locomotor activating effects of ethanol. However, adolescent mice were less sensitive than adult mice to locomotor sensitization, as only the highest dose of ethanol (2.5 g/kg) induced sensitization in the adolescent mice, while lower doses of ethanol elicited sensitization in the adult mice. The differential response to ethanol sensitization was not related to duration of treatment or differential BEC.

CONCLUSIONS

These results indicate that adolescent mice are less sensitive to ethanol sensitization, and this blunted behavioral response in adolescents might reflect differential ethanol-induced neurobehavioral adaptations.

Neuropharmacology of alcohol addiction

Despite the generally held view that alcohol is an unspecific pharmacological agent, recent molecular pharmacology studies demonstrated that alcohol has only a few known primary targets. These are the NMDA, GABA(A), glycine, 5-hydroxytryptamine 3 (serotonin) and nicotinic ACh receptors as well as L-type Ca(2+) channels and G-protein-activated inwardly rectifying K(+) channels. Following this first hit of alcohol on specific targets in the brain, a second wave of indirect effects on a variety of neurotransmitter/neuropeptide systems is initiated that leads subsequently to the typical acute behavioural effects of alcohol, ranging from disinhibition to sedation and even hypnosis, with increasing concentrations of alcohol. Besides these acute pharmacodynamic aspects of alcohol, we discuss the neurochemical substrates that are involved in the initiation and maintenance phase of an alcohol drinking behaviour. Finally, addictive behaviour towards alcohol as measured by alcohol-seeking and relapse behaviour is reviewed in the context of specific neurotransmitter/neuropeptide systems and their signalling pathways. The activity of the mesolimbic dopaminergic system plays a crucial role during the initiation phase of alcohol consumption. Following long-term, chronic alcohol consumption virtually all brain neurotransmission seems to be affected, making it difficult to define which of the systems contributes the most to the transition from controlled to compulsive alcohol use. However, compulsive alcohol drinking is characterized by a decrease in the function of the reward neurocircuitry and a recruitment of antireward/stress mechanisms comes into place, with a hypertrophic corticotropin-releasing factor system and a hyperfunctional glutamatergic system being the most important ones.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Alcohol-related genes: contributions from studies with genetically engineered mice

Since 1996, nearly 100 genes have been studied for their effects related to ethanol in mice using genetic modifications including gene deletion, gene overexpression, gene knock-in, and occasionally by studying existing mutants. Nearly all such studies have concentrated on genes expressed in brain, and the targeted genes range widely in their function, including most of the principal neurotransmitter systems, several neurohormones, and a number of signaling molecules. We review 141 published reports of effects (or lack thereof) of 93 genes on responses to ethanol. While most studies have focused on ethanol self-administration and reward, and/or sedative effects, other responses studied include locomotor stimulation, anxiolytic effects, and neuroadaptation (tolerance, sensitization, withdrawal). About 1/4 of the engineered mutations increase self-administration, 1/3 decrease it, and about 40% have no significant effect. In many cases, the effects on self-administration are rather modest and/or depend on the specific experimental procedures. In some cases, genes in the background strains on which the mutant is placed are important for results. Not surprisingly, review of the systems affected further supports roles for serotonin, gamma-aminobutyric acid, opioids and dopamine, all of which have long been foci of alcohol research. Novel modulatory effects of protein kinase C and G protein-activated inwardly rectifying K+ (GIRK) channels are also suggested. Some newer research with cannabinoid systems is promising, and has led to ongoing clinical trials.