mGlu receptors and drug addiction

Abstract

Historically, brain catecholamine systems have been the primary focus of studies examining the neural substrates of drug addiction. In the past two decades, however, a wealth of evidence has accumulated indicating a pivotal role for glutamatergic neurotransmission in mediating addictive behaviors as well as long-term neuroplasticity associated with chronic drug use. As a result, there has been increased interest in developing glutamate-based therapies for the treatment of addictive disorders. Metabotropic glutamate (mGlu) receptors are classified into subcategories designated as Group I (mGlu1 and mGlu5), Group II (mGlu2 and mGlu3), and Group III (mGlu4, mGlu6, mGlu7, and mGlu8), and have received a great deal of attention due to their mediation of slower modulatory excitatory neurotransmission. Pharmacological ligands targeting these receptors have demonstrated reduced incidences of excitotoxicity or severe adverse side effects as compared to those targeting ionotropic glutamate (iGlu) receptors. Behavioral genetic and pharmacological studies have explored the role of individual mGlu receptor subtypes in regulating various addiction-related behaviours and several mGlu receptor ligands have been the subject of clinical testing for other medical conditions.

L-Glutamate is the most abundant excitatory neurotransmitter in the central nervous system and governs many processes in the brain including fast and slow excitatory neurotransmission, control of basal neuronal activity, and synaptic plasticity. Historically, studies examining the neural basis of drug addiction have focused primarily on brain catecholamineergic neurotransmission. However, a tremendous amount of preclinical and clinical evidence has recently accumulated suggesting that glutamate plays a pivotal role in drug self-administration, reward-related processes, drug-induced conditioned associations, and relapse^1-3.

Glutamatergic signalling in the brain is primarily mediated by both ionotropic (iGlu) and metabotropic (mGlu) glutamate receptors. While pharmacological manipulations of iGlu receptors have been shown to decrease numerous addiction-related behaviors, iGlu receptor ligands produce many adverse side effects, including excitotoxicity, psychosis-like symptoms and dissociative states, and alterations in general CNS excitability. mGlu receptors, on the other hand, exert more subtle modulatory influences on excitatory neurotransmission. Many animal and human studies conducted to date have shown that mGlu receptor ligands have potential for the treatment of numerous disorders of the CNS including chronic pain, schizophrenia, depression, anxiety, epilepsy, Parkinson’s disease and other neurodegenerative diseases^4-7, and, as will be described in this review, drug addiction.

To date, eight different mGlu receptor subtypes have been cloned and characterized, and each appears to have a diverse neuroanatomical distribution as well as unique pharmacological and intracellular signalling properties^8,9. mGlu receptors are typically subcategorized into Group I receptors (mGlu1 and mGlu5), which are coupled to various classes of G-proteins such as G_q/11. Upon activation, these receptors stimulate the release of calcium from intracellular stores. In contrast, Group II (mGlu2 and mGlu3) and Group III (mGlu4, mGlu6, mGlu7, and mGlu8) receptors are coupled to G_i/o protein signalling mechanisms, and upon activation, reduce the formation of intracellular cyclic adenosine monophosphate (cAMP) via inhibition of adenylyl cyclase activity.

The purpose of the present review is to summarize studies conducted to date that have provided strong evidence that individual mGlu receptor subtypes regulate various addiction-related behaviors. We will also review the safety, tolerability, efficacy, and side effects profile of several mGlu receptor ligands that are currently in clinical trials for other medical conditions. These clinical studies are critical for establishing the feasibility of utilizing such compounds as pharmacological aids in the treatment of drug addiction.

GROUP I mGlu RECEPTORS AND ADDICTION

mGlu1 and mGlu5 receptors have an almost complementary pattern of anatomical distribution in the brain¹⁰. High levels of mGlu5 receptors are found in forebrain regions such as the cerebral cortex, dorsal and ventral striatum (including the nucleus accumbens, NAc), olfactory bulb and tubercle, lateral septum, and hippocampus^11,12. In contrast, expression levels of mGlu1 receptors in these regions are relatively weak, except for the hippocampus, where this receptor is highly expressed primarily in the CA3 region and dentate gyrus. Regions containing high levels of expression of mGlu1 receptors that have low levels of mGlu5 expression include the cerebellum, thalamus, hypothalamus, and pallidum¹³. Electron microscopy studies have shown that Group I mGlu receptors are primarily localized on postsynaptic elements and are rarely found on presynaptic terminals¹².

mGlu5 receptors

Although the mGlu5 receptor was first cloned and characterized in the early 1990s¹⁴, its role in drug addiction did not become evident until 2001 in a highly influential study published by Chiamulera and colleagues¹⁵. These investigators demonstrated that mice carrying a null mutation for the gene encoding the mGlu5 receptor failed to demonstrate hyperlocomotion in response to acute administration of various doses of cocaine (10, 20 or 40 mg/kg i.p.), and also failed to acquire intravenous self-administration of cocaine at various doses, while wildtype mice demonstrated typical patterns of self-administration. Chiamulera and colleagues also showed that pharmacological antagonism of mGlu5 receptors with 2-methyl-6-(phenylethynyl)-pyridine (MPEP) dose-dependently reduced intravenous cocaine self-administration. This pivotal study provided the first evidence that either genetic or pharmacological inhibition of mGlu5 receptors could reduce the locomotor stimulant and reinforcing effects of cocaine.

Although the findings of Chiamulera and colleagues were highly influential in instigating the study of mGlu5 receptors in addiction research, it should be noted that several subsequent studies have failed to completely replicate these results. For example, Bird and colleagues¹⁶ demonstrated that acute administration of cocaine (20 mg/kg i.p.) increased locomotor activity in both wildtype and mGlu5-deficient mice, although the effects in mGlu5-deficient mice were somewhat delayed incomparison to those observed in wildtype mice. In addition, using cell type-specific RNA interference to selectively reduce mGlu5 expression on dopamine D1 receptor-expressing neurons in the dorsal striatum, Novak and colleagues¹⁷ showed that patterns of cocaine self-administration were similar in wildtype and mGlu5 knockdown mice,. These findings suggest that the dorsal striatum does not mediate the effects mGlu5 receptor inhibition on cocaine reinforcement. As will be discussed below, there is evidence to suggest that mGlu5 receptors in regions of the ventral striatum, particularly the NAc, are in fact implicated in mediating drug reinforcement and relapse-like behaviors.

Despite these two studies that seemingly contradict the original findings of Chiamulera and colleagues, a host of pharmacological studies have subsequently shown that blockade of mGlu5 receptors attenuates the acquisition and/or expression of the conditioned rewarding effects of cocaine¹⁸, nicotine¹⁹, amphetamine²⁰, morphine^21-23, and alcohol²⁴, as measured by the conditioned place preference (CPP) paradigm. On the other hand, however, recent studies have reported contradictory findings that MPEP actually potentiates the acquisition of CPP for heroin^25,26, ketamine^25,26, cocaine²⁶, nicotine²⁶, and morphine²⁷ and reinstates for heroin and ketamine-induced CPP²⁵. In addition, these same investigators demonstrated that intravenous MPEP induces CPP on its own²⁸, although relatively high doses (3 and 10 mg/kg i.v.) were needed to observe such effects, and MPEP is known to exert non-selective effects on other glutamate receptors at high concentrations²⁹. While these phenomena have not yet been demonstrated for other more selective mGlu5 antagonists, these findings suggest the possibility that the ability of such compounds to reduce intravenous drug self-administration (see below) may be due to a pharmacologically induced increase in the rewarding effects of these abused substances.

In animals actively self-administering various drugs of abuse (intravenously) or alcohol (orally), pharmacological blockade or genetic inactivation of mGlu5 receptors reduces self-administration of cocaine^15,30-37, nicotine^33,38-41, heroin⁴², ketamine⁴², methamphetamine⁴³, and alcohol^24,44-53, and can also reduce breakpoints for reinforcement for various drugs of abuse in a progressive ratio paradigm^43,54,55. mGlu5 receptor antagonism can also interfere with the interoceptive effects of some drugs of abuse such as alcohol⁵⁶, nicotine⁵⁷, and cocaine³⁴, which may contribute to the ability of mGlu5 receptor antagonists to reduce self-administration of these drugs. In studies of animals self-administering natural reinforcers such as food, mGlu5 receptors appear to be required for the acquisition of the incentive properties of reinforcer-associated conditioned stimuli⁵⁸ as well as operant sensation-seeking⁵⁹, which may also contribute to the ability of mGlu5 antagonists to attenuate drug self-administration. Interestingly, it has been demonstrated that in rats self-administering heroin, intravenous MPEP will substitute for heroin and maintain lever-pressing²⁸, albeit to a limited degree. Additional studies examining this phenomenon with more selective mGlu5 antagonists are needed to confirm these observations.

One of the most problematic obstacles to the successful treatment of drug addiction is the phenomenon of relapse. Pharmacological blockade of mGlu5 receptors has been shown to attenuate the reinstatement of drug-seeking behavior induced by drug priming, stress, or drug-associated cues^{33,34,36,43,44,52,60-66}, which are established animal models of relapse. In addition, mGlu5 receptor antagonism also reduces the surge in alcohol intake when it is reintroduced following a period of forced abstinence^44,49. Together, these studies suggest that mGlu5 antagonists not only reduce on-going drug use, but may aid in the prevention of relapse.

While all of the aforementioned studies have utilized systemic administration of mGlu5 receptor antagonists or embryonic genetic deletion of the mGlu5 receptor gene, only a handful of studies to date have examined the precise neuroanatomical loci where drug reward, reinforcement, or reinstatement of drug-seeking behavior is mediated by these receptors. Blockade of mGlu5 receptors in the nucleus accumbens (NAc) has been shown to attenuate both intravenous and oral alcohol reinforcement^67-69, as well as the interoceptive effects of alcohol⁷⁰. Similarly, blockade of mGlu5 receptors in the NAc has also been shown to attenuate cocaine- or cue-primed reinstatement of cocaine-seeking behavior^60,71. Finally, Novak and colleagues showed that knockdown of mGlu5 expression on dopamine D1 receptor-expressing neurons in the dorsal striatum did not patterns of cocaine self-administration, but attenuated cue-induced reinstatement of cocaine-seeking¹⁷. Thus, it appears that mGlu5 receptors in dopaminoceptive regions of the forebrain differentially mediate cocaine reinforcement vs. the incentive salience of drug-associated cues.

As with all studies that document a pharmacological suppression of drug reward, reinforcement, or drug-seeking behavior, it is important to consider the possibility that any observed effects may be due to nonspecific inhibitory effects of the test compounds on locomotor activity. This is especially important in light of the fact that fenobam, an mGlu5 antagonist currently in clinical trials for the treatment of Fragile X syndrome, has been reported to produce mild sedation⁷². While many of the aforementioned studies have demonstrated that mGlu5 antagonists such as MPEP and MTEP do not alter spontaneous locomotor activity, some have reported that these ligands, particularly a higher doses, produce moderate impairments of locomotor activity^33,54,73. This underscores the importance of incorporating appropriate locomotor control experiments into studies on the ability of mGlu5 antagonists (or any mGlu receptor ligand) on addiction-related behaviors to monitor for the possibility of confounding motor effects.

mGlu1 receptors

Compared with the extensive literature reviewed above on the role of mGlu5 receptors in mediating drug reward, reinforcement, and relapse, relatively few studies have been published on the specific role of mGlu1 receptors. Several investigators have found a lack of effect of mGlu1 receptor antagonism on voluntary alcohol self-administration in rodents^48,49. However, Lominac and colleagues found an inhibitory effect of an mGlu1 receptor antagonist on alcohol self-administration in mice²⁴. A more recent study showed that the potent mGlu1 receptor antagonist JNJ16259685 reduces alcohol self-administration as well as breakpoints for alcohol reinforcement in a progressive ratio paradigm, but nonspecific impairments in motor activity were also induced by this compound^54,84. Thus, to date the role of mGlu1 receptors in regulating alcohol consumption are still not clearly defined. Further studies are needed to examine the effects of mGlu1 receptor antagonists that are devoid of motor side effects on self-administration of alcohol and other drugs of abuse.

With regards to relapse-like behaviors, it has been demonstrated that the selective mGlu1 receptor antagonist (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone (EMQMCM) attenuates the reinstatement of nicotine-seeking behavior elicited by nicotine priming and nicotine-associated cues⁸⁵. Antagonism of mGlu1 receptors in the dorsal hippocampus has been reported to attenuate the reinstatement of cocaine-seeking behavior evoked by cocaine-associated contextual cues⁸⁶, consistent with a well-defined role of the hippocampus in contextual memory. Thus, mGlu1 receptors may play a role in the processing of drug-associated contextual cues. Studies examining the effects of pharmacological manipulation of mGlu1 receptors in other brain regions known to be involved in addiction (i.e., amygdala, ventral tegmental area, prefrontal cortex, etc.) on drug self-administration and relapse are needed to provide a more comprehensive understanding of the neural circuitry in which these receptors regulate various aspects of drug addiction.

GROUP II mGlu RECEPTORS AND ADDICTION

mGlu2 and mGlu3 receptors

mGlu2 and mGlu3 receptors are predominantly expressed as heterodimers on presynaptic membranes of glutamatergic terminals and negatively regulate the release of glutamate, although there is also evidence that these receptors are also located in peripheral regions of the synapse^87-89. mGlu2 and mGlu3 receptors are found in moderate to high abundance in the olfactory bulb, cerebral cortex, septal region, hippocampus, dorsal and ventral striatum, amygdala, and cerebellum^89-92. Since these receptors are predominantly found as heterodimers, the receptor complexes are frequently referred to as mGlu2/3 receptors, although homodimers of each receptor subtype have been identified.

One of the first demonstrations that mGlu2/3 agonists might possess therapeutic efficacy in the treatment of drug addiction was published in 1997, when it was shown that the mGlu2/3 agonist LY354740 suppressed somatic signs of nicotine withdrawal in rats⁹³. Since this study, there have been numerous reports that mGlu2/3 agonists reduce active drug self-administration and drug-seeking behavior. For example, mGlu2/3 agonists such as LY379268 and LY404039 have been shown to suppress self-administration of cocaine^37,94-96, nicotine⁴⁰, and ethanol^52,97,98, as well as cue-, stress- and drug priming-induced reinstatement of drug-seeking^{52,94,96,98,99}. mGlu2/3 agonists have also been shown to attenuate the phenomenon of “incubation” of cocaine craving¹⁰⁰, which is characterized by a progressive increase in the magnitude of cue-induced reinstatement over time following extinction^101,102. Some of the aforementioned effects of mGlu2/3 agonists must be interpreted with caution, however, since higher doses (3-5 mg/kg i.p.) of mGlu2/3 agonists such as LY379268 have been shown to decrease spontaneous locomotor activity⁹⁸ as well as self-administration of natural reinforcers such as food⁹⁵ and sweetened solutions⁹⁷. In light of these nonspecific effects of mGlu2/3 agonists, recent studies have shown that such effects are lacking following administration of mGlu2 PAMs, such as 3’-(((2-cyclopentyl-6,7-dimethyl-1-oxo-2,3-dihydro-1H-inden-5-yl)oxy)methyl)biphenyl-4-carboxylic acid (BINA)¹⁰³ and 3-chloro-3’-((2-cyclopentyl-3-oxo-2,3-dihydrobenzo[d]isothiozal-6-yloxy)methyl)biphenyl-4-carboxylic acid¹⁰⁴, while maintaining their ability to suppress drug self-administration. Positive allosteric modulation of mGlu2 (and possibly mGlu3) receptors may therefore be novel pharmacological approach by which to reduce drug self-administration without motoric side effects. It should also be mentioned that tolerance to the motor-impairing of classical mGlu2/3 agonists such as LY379268 has been reported¹⁰⁵, suggesting that unwanted motoric side effects of such ligands may dissipate following repeated administration.

Studies employing intracerebral microinjection techniques have revealed some of the neural circuitries involved in the ability of mGlu2/3 agonists to reduce drug reinforcement or reinstatement. For example, infusion of LY379268 into the ventral tegmental area attenuates nicotine self-administration¹⁰⁶ as well as contextual cue-induced reinstatement of heroin-seeking¹⁰⁷. Infusions of LY379268 into the NAc shell attenuate nicotine but not food reinforcement¹⁰⁶, while infusions of LY379268 into the NAc attenuate both cocaine and food-primed reinstatement of cocaine- and food-seeking, respectively⁹⁵. Infusion of this ligand into the NAc core or shell also reduces context-induced reinstatement of heroin-seeking without affecting sucrose-seeking¹⁰⁸. Finally, it was recently demonstrated that infusions of LY379268 into the central nucleus of the amygdala attenuates the “incubation” of cocaine craving¹⁰⁰. Thus, structures of the mesolimbic reward system and extended amygdala appear to be regions where mGlu2/3 receptors mediate addiction-related behaviors.

GROUP III mGlu RECEPTORS AND ADDICTION

The existing literature on the role of Group III mGlu receptors in mediating drug addiction is somewhat minute, largely because of the general lack of availability of pharmacological ligands that are selective for individual subtypes of Group III mGlu receptors. The expression patterns of Group III receptors in the brain is also more limited than those of Group I and II mGlu receptors. mGlu6 receptor expression is confined to the retina¹⁰⁹, thus making it an unattractive target for potential anti-addiction pharmacotherapies. mGlu4 receptors have a limited distribution in the central nervous system, being expressed predominantly on presynaptic terminals at high levels in the cerebellum, basal ganglia, thalamus, and hippocampus¹¹⁰. In contrast, mGlu7 receptors are expressed much more widely throughout the central nervous system including the cerebral cortex, striatum, hippocampus, hypothalamus, and cerebellum, where they are predominantly localized to presynaptic terminals¹¹¹. Finally, mGlu8 receptors have a restricted distribution in the brain, primarily being confined to olfactory bulb, piriform cortex, hippocampus, thalamus, and pons¹¹².

mGlu4 receptors

To our knowledge, only one study has been conducted that specifically examined the role of mGlu4 receptors in addiction-related behaviors. Blednov and colleagues¹¹³ demonstrated that mice lacking mGlu4 receptors showed normal levels of ethanol consumption, but lacked a locomotor stimulant effect of low doses of alcohol. These findings indicate that behavioral responses to acute administration of alcohol do not necessarily predict alcohol consumption patterns in genetically altered mice.

mGlu7 receptors

Similar to findings from studies on mGlu2/3 receptors, stimulation of presynaptic mGlu7 receptors with the selective mGlu7 agonist AMN082 results in suppression of cocaine self-administration, and similar effects are observed when this ligand is infused into the nucleus accumbens or ventral pallidum¹¹⁴. AMN082 has also been reported to attenuate cocaine-primed reinstatement of cocaine-seeking behavior¹¹⁵. Doses of AMN082 that are similar to those used in these studies have also been found to suppress alcohol self-administration¹¹⁶, but nonselective inhibitory effects of this compound on sucrose self-administration and locomotor activity were also found. Thus, the development of mGlu7-selective ligands devoid of these nonselective effects is necessary in order to fully establish a role for these receptors in drug reward and reinforcement. Despite these limitations, gene mapping techniques in mice have revealed that the gene encoding the mGlu7 receptor is a cis-regulated gene that predisposes the organism towards higher levels of alcohol consumption¹¹⁷, suggesting that the mGlu7 receptor may indeed be a potential target for suppressing high levels of alcohol intake.

mGlu8 receptors

Backstrom and Hyytia demonstrated that administration of the mGlu8 agonist (S)-3,4-dichlorophenylglycine ((S)-3,4-DCPG) suppressed alcohol self-administration as well as cue-induced reinstatement of alcohol-seeking behavior ⁹⁸. However, doses of (S)-3,4-DCPG that were effective at reducing alcohol consumption and reinstatement were also accompanied by reductions in locomotor activity. Thus, as with mGlu2/3 and mGlu7 receptors, there is need for the development of improved mGlu8 ligands that are devoid of non-specific motoric effects.

EFFECTS OF ADDICTIVE DRUGS ON mGlu RECEPTOR EXPRESSION AND FUNCTION

The preceding sections have summarized findings on the effects of mGlu receptor ligands in rodent models of addictive behaviors, such as conditioned drug reward, drug self-administration, and reinstatement. However, since addictive drugs exert long-lasting effects on the brain such as changes in synaptic transmission, gene expression, protein localization and function, neuronal morphology, and the integrity of neural circuits, it important to consider effects of long-term exposure to drugs of abuse on the expression and/or function of mGlu receptors, particularly if these receptors are to be targeted by novel pharmacotherapeutics for the treatment of addictive disorders.

Studies using quantitative in situ hybridization techniques have shown that repeated exposure to amphetamine produces lasting reductions in expression of mGlu5 mRNA in the dorsal striatum and NAc, while expression of mGlu8 mRNA is increased in these regions¹¹⁸ and mGlu1 mRNA levels are only transiently affected¹¹⁹. In contrast, increases in the expression of mGlu5 mRNA in these regions have been observed following repeated exposure to cocaine¹²⁰. Repeated cocaine exposure also disrupts mGlu1- and mGlu2/3-mediated signalling in the NAc^121,122. Chronic alcohol consumption has been shown to reduce mGlu1, mGlu3, mGlu5, and mGlu7 mRNA levels in various subregions of the hippocampus while leaving the expression of mGlu2, mGlu4, and mGlu8 unchanged¹²³.

While examinations of changes in mRNA levels are general indirect measures of changes in receptor levels, a small number of recent studies have examined drug-induced changes in overall protein levels as well as cellular distribution and trafficking of mGlu receptors. Chronic ethanol consumption in alcohol-preferring P rats has been shown to increase the expression of mGlu1 and mGlu5 levels in the NAc core and central nucleus of the amygdala¹²⁴, while repeated cocaine exposure results in a decrease in the expression of mGlu2/3 receptors in the NAc core¹²⁵. Using electron microscopy methods, Mitrano and colleagues¹²⁶ found minimal effects of repeated cocaine exposure on the percentage of membrane-bound mGlu1 or mGlu5 receptors in the NAc core and shell subregions. However, investigators utilizing subcellular fractionation techniques have shown that repeated cocaine increases the levels of mGlu2/3 and mGlu5 receptors in synaptosomal membrane fractions from the NAc core¹²⁵.

Together, these drug-induced alterations in mGlu mRNA and protein levels as well as the subcellular redistribution of mGlu receptors may underlie some of the functional changes in glutamatergic transmission and synaptic plasticity that occur as a result of chronic drug use^127,128. It therefore follows that such changes may influence the ability of mGlu ligands to alter addiction-related behaviors. This point is exemplified in a study by Sidhpura and colleagues⁵², who demonstrated that the mGlu2/3 agonist LY379268 suppressed foot-shock stress-induced reinstatement of alcohol-seeking behavior in rats both before and after induction of physical dependence, whereas the mGlu5 antagonist MTEP appeared to be less effective in reducing footshock-induced reinstatement following the induction of dependence.

CURRENT STATUS OF mGlu RECEPTOR LIGANDS IN HUMAN CLINICAL TRIALS

Several mGlu5 (but not mGlu1) receptor antagonists (or negative allosteric modulators (NAMs), which dampen the activity of the receptor without rendering it completely inactive) have now entered Phase I and II clinical trials for the treatment of other medical conditions, including Fragile X syndrome, gastroesophageal reflux disease (GERD), anxiety, depression, and L-dopa induced dyskinesia in Parkinson’s disease^5-7,129,130. Since the number of published pilot clinical studies is relatively small, the data thus far on the clinical efficacy, safety, tolerability, and side effect profiles of these compounds are relatively limited in detail.

Approximately a decade prior to the isolation and characterization of the mGlu5 receptor in 1992, several clinical studies were conducted with a compound called fenobam (N-(3-chlorophenyl-N’-(4,5)-dihydro-1-methyl-4-oxo-1H-imidazol-2-yl)urea) for the treatment of anxiety^131-134. At the time of these studies, fenobam had an unknown mechanism of action and was referred to as a “non-benzodiazepine anxiolytic”. In 2005, fenobam was re-characterized and found to be an mGlu5 receptor antagonist with greater selectivity for the receptor than MPEP^135,136.

The results of these early clinical studies demonstrated significant anxiolytic effects of fenobam that were equivalent to those produced by diazepam, but adverse side effects such as nausea, dizziness, insomnia, blurred vision, depersonalization, and perceptual alterations were reported in approximately 25-40% of patients. Many patients who experienced these side effects discontinued the study. These adverse side effects occurred primarily in patients taking higher doses of fenobam (200-600 mg/day). Unfortunately, despite its apparent efficacy as an anxiolytic drug, high dropout rates and the prevalence of adverse side effects associated with fenobam resulted in a discontinuation of clinical trials of this compound.

More recently, however, re-examinations of the therapeutic potential of fenobam have been initiated for the treatment of Fragile X syndrome, a common form of mental retardation that involves abnormalities in mGlu5-mediated synaptic plasticity and neuronal development^5-7,105,106. In a recent preliminary report on the safety and tolerability of fenobam in subjects with Fragile X syndrome, fenobam (50-150 mg/day) produced clinical improvements in cognitive function in 9 out of 12 patients, including increased eye contact and social interaction, and reduced anxiety and hyperactivity⁷². The only side effect observed in 3 of the 12 patients was mild sedation, and no subjects discontinued the trial. Thus, fenobam appeared to be well tolerated in this study.

The results of a two clinical trials of the mGlu5 receptor NAM ADX10059 for the treatment of GERD have also recently been published¹³⁷. Few details are known about the precise pharmacological properties of ADX10059, yet the mechanistic rationale for the treatment of GERD with mGlu5 NAMs is based on the expression of mGlu5 receptors on brainstem vagal afferents that control the contractility of the lower esophageal sphincter, thereby regulating reflux of stomach acid into the esophagus^138-140. In a small within-subjects placebo-controlled study by Keywood and colleagues¹³⁷, 24 subjects with GERD received placebo prior to a high fat meal on the first day of reflux assessment, followed by ADX10059 at a dose of either 50 or 250 mg/day (n=12 per group) prior to a high fat meal on the second day of reflux assessment. ADX10059 significantly reduced acid reflux, and no subjects taking either dose of ADX10059 discontinued the trial. In subjects receiving the 50 mg dose, 1 out of 12 subjects reported mild sedation, cough, and rhinorrhea. In subjects receiving the 250 mg dose, side effects such as dizziness and nausea occurred in 9 out of 12 participants, but these side effects resolved following the first or second dose of ADX10059.

In a larger randomized double-blind placebo-controlled clinical trial by the same investigators¹⁴¹, 103 GERD patients received either placebo (n=53) or ADX10059 (n=50, 120 mg) twice daily for 2 weeks. The results of this study indicated that patients receiving ADX10059 reported a significant reduction in GERD symptoms and reflux, as well as a significantly reduced need to take additional “rescue” antacid medications, as compared to those receiving placebo. Three patients (6%) that received ADX10059 discontinued the trial to due adverse side effects. The most common adverse side effects reported by patients taking ADX10059 were dizziness (16% vs. 4% in patients receiving placebo) and vertigo (12% vs. 2% in patients receiving placebo), and all adverse side effects were reported to be mild to moderate in severity.

Thus, like fenobam, ADX10059 appears to be fairly well tolerated with few adverse side effects that necessitate discontinuation of the medication. The initiation of clinical trials examining the effects of these medications on drug craving, use, and relapse in human drug addicts is eagerly awaited.

A Group II mGlu receptor ligand has also recently advanced to clinical trials with promising initial results. In a recent clinical proof-of-concept study by Patil and colleagues¹⁴², it was reported that LY2140023, a prodrug of the orthosteric mGlu2/3 receptor agonist LY404039, was superior to placebo in alleviating both positive and negative symptoms of schizophrenia and equivalent to effects observed with the atypical antipsychotic olanzapine. Placebo-treated patients demonstrated the highest dropout rate, primarily due to a lack of improvement in symptoms. The onset of symptom alleviation was relatively rapid (i.e., within 1 week). Adverse side effects in patients receiving LY2140023 were mild to moderate in severity and included insomnia, affect lability, nausea, headache, somnolence, and elevated serum creatine phosphokinase levels. However, dyskinesia, akathesia, and parkinsonian-like symptoms, which are problematic side effects often associated with typical and atypical antipsychotic medications, were not observed in LY2140023-treated patients. These positive findings of the mGlu2/3 receptor agonist prodrug LY2140023 in alleviating the symptoms of schizophrenia without severe side effects are very encouraging. Given the wealth of preclinical data reviewed above showing potential anti-addiction effects of mGlu2/3 receptor agonists and PAMs (the latter of which may produce even fewer side effects and more selectively target mGlu2 or mGlu3 receptors^4,143), clinical trials on the efficacy of LY2140023 or similar compounds in the treatment of drug addiction need to be initiated.

Conclusion

In summary, the preclinical data published to date provide strong evidence that antagonists or NAMs of Group I (particularly mGlu5) receptors, as well as agonists or PAMs of Group II mGlu receptors, hold the most promise for potential utility as pharmacological aids in the treatment of drug addiction. Given that mGlu5 receptor antagonists and NAMs as well as agonists for mGlu2/3 receptors are currently in clinical trials for various other medical conditions, investigations into the clinical efficacy of these compounds in reducing drug craving, use, and relapse in human drug addicts should be promptly undertaken. However, recent positive preclinical findings with mGlu1 antagonists and mGlu7 agonists should not preclude further investigation into developing ligands for these receptors for potential use as addiction pharmacotherapies.

Table 1.

Summary of primary findings on effects of mGlu ligands in rodent models of addiction

Receptor	Pharmacological action	Behavioral effect	Drugs
mGlu1	antagonism	reduced reinstatement of drug-seeking	cocaine, nicotine
mGlu5	antagonism or negative allosteric modulation	reduced conditioned reward (CPP)	alcohol, amphetamine, cocaine, morphine, nicotine
		reduced self-administration	alcohol, cocaine, ketamine, methamphetamine, nicotine
		reduced reinstatement of drug-seeking	alcohol, cocaine, methamphetamine, nicotine
	positive allosteric modulation	reduced extinction responding	cocaine
		reversal of drug-induced cognitive deficit	methamphetamine
mGlu2/3	agonism or positive allosteric modulation	reduced self-administration	alcohol, cocaine, nicotine
		reduced reinstatement of drug-seeking	alcohol, cocaine, heroin, nicotine
		reduced “incubation” of drug craving	cocaine
mGlu7	agonism	reduced drug self-administration	alcohol, cocaine
		reduced reinstatement of drug-seeking	cocaine
mGlu8	agonism	reduced drug self-administration	alcohol
		reduced reinstatement of drug-seeking	alcohol

Positive allosteric modulation of mGlu5 receptors – a novel mechanism to enhance extinction learning and cognitive deficits associated with drug addiction?

While mGlu5 receptor blockade attenuates the rewarding and reinforcing effects of multiple drugs of abuse, there is also evidence that mGlu5 receptor blockade also impairs certain forms of learning and memory^74,75. This is likely due to the fact that postsynaptic mGlu5 receptor function is positively coupled to that of postsynaptic N-methyl-D-aspartate (NMDA) receptors, which are critical for the induction of synaptic plasticity associated with learning and memory. Recently, increased attention has been given to the development of pharmacological compounds that enhance, rather than inhibit, mGlu5 receptor function. These compounds are collectively known as mGlu5 positive allosteric modulators (PAMs), as they bind to an allosteric site that is distinct from the orthosteric glutamate binding site of the receptor. mGlu5 receptor PAMs are devoid of any agonistic activity on their own, but potentiate receptor response to endogenously released glutamate. Commonly used systemically active mGlu5 receptor PAMs include 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB)⁷⁶ and [S-(4-fluoro-phenyl)-[3-[3-(4-fluoro-phenyl)-[1,2,4]-oxadiazol-5-yl]-piperidin-1-yl]-methanone] (ADX47273)^77,78.

Drug-associated memories evoked by exposure to contexts or distinct environmental stimuli that have been repeatedly paired with drug use often result in drug craving and relapse. Therefore, extinction of the motivational salience of drug-associated cues and contexts may lead to reduced incidences of relapse⁷⁹, and recent evidence suggests that mGlu5 receptors play an important role in extinction learning processes. For example, mGlu5 receptor-deficient mice fail to extinguish conditioned fear behaviors⁸⁰. In contrast, the mGlu5 PAM CDPPB facilitates the extinction of the conditioned rewarding effects of cocaine⁸¹ as well as cocaine-seeking behavior following intravenous self-administration⁸². In addition, mGlu5 PAMs appear to have some cognition-enhancing properties, as CDPPB can reverse cognitive deficits produced by extended access to methamphetamine⁸³. Since cognitive deficits are frequently observed in chronic drug users, mGlu5 PAMs may represent a novel approach to facilitating the extinction of the motivational salience of drug-associated stimuli, as well as reversing some of the cognitive deficits caused by chronic drug use.