Abstract: This Review describes recent trends in the development of small molecule mGlu5 positive allosteric modulators (PAMs). A large body of pharmacological, genetic, electrophysiological, and in vivo behavioral evidence has accumulated over the past decade which continues to support the hypothesis and rationale for the activation of the metabotropic glutamate receptor subtype 5 (mGlu5) as a viable and promising target for the development of novel antipsychotics. Until recently, functionally efficacious and potent mGlu5 PAMs have been somewhat structurally limited in scope and slow to emerge. This Review will discuss efforts since late 2008 which have provided novel mGlu5 PAM chemotypes, offering ligands with a diverse range of pharmacological, physicochemical, and DMPK properties that were previously unavailable. In addition, significant biological studies of importance in the past few years using the well established PAMs known as DFB, CPPHA, CDPPB, and ADX-47273 will be discussed.

Abstract: Positive and negative allosteric modulators (PAMs and NAMs, respectively) of the type 5 metabotropic glutamate (mGlu5) receptor have demonstrable therapeutic potential in an array of neurological and psychiatric disorders. Here, we have used rat cortical astrocytes to investigate how PAMs and NAMs mediate their activity and reveal marked differences between PAMs with respect to their modulation of orthosteric agonist affinity and efficacy. Affinity cooperativity factors (α) were assessed using [3H]2-methyl-6-(phenylethynyl)-pyridine (MPEP)-PAM competition binding in the absence and presence of orthosteric agonist, whereas efficacy cooperativity factors (β) were calculated from net affinity/efficacy cooperativity parameters (αβ) obtained from analyses of the abilities of PAMs to potentiate [3H]inositol phosphate accumulation in astrocytes stimulated with a submaximal (EC20) concentration of orthosteric agonist. We report that whereas 3,3′-difluorobenzaldazine (DFB) and 3-cyano-N-(1,3-diphenyl-1H-prazol-5-yl)benzamide (CDPPB) primarily exert their allosteric modulatory effects through modifying the apparent orthosteric agonist affinity at the astrocyte mGlu5 receptor, the effects of S-(4-fluoro-phenyl)-{3-[3-(4-fluoro-phenyl)-[1,2,4]oxadiazol-5-yl]-piperidinl-1-yl}-methanone (ADX47273) are mediated primarily via efficacy-driven modulation. In [3H]MPEP-NAM competition binding assays, both MPEP and 2-(2-(3-methoxyphenyl)ethynyl)-5-methylpyridine (M-5MPEP) defined similar specific binding components, with affinities that were unaltered in the presence of orthosteric agonist, indicating wholly negative efficacy-driven modulations. It is noteworthy that whereas M-5MPEP only partially inhibited orthosteric agonist-stimulated [3H]inositol phosphate accumulation in astrocytes, it could completely suppress Ca2+ oscillations stimulated by quisqualate or (S)-3,5-dihydroxyphenylglycine. In contrast, MPEP was fully inhibitory with respect to both functional responses. The finding that M-5MPEP has different functional effects depending on the endpoint measured is discussed as a possible example of permissive allosteric antagonism.

Abstract: In order to understand the interaction between the metabotropic glutamate subtype 5 (mGluR5) and N-methyl-D-aspartate (NMDA) receptors, the influence of mGluR5 positive modulators in the inhibition of NMDA receptors by the noncompetitive antagonist ketamine, the competitive antagonist D-APV and the selective NR2B inhibitor ifenprodil was investigated. This study used the multi-electrode dish (MED) system to observe field potentials in hippocampal slices of mice. Data showed that the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), as well as the positive allosteric modulators 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB) and 3,3'-difluorobenzaldazine (DFB) alone did not alter the basal field potentials, but enhanced the amplitude of field potentials induced by NMDA. The inhibitory action of ketamine on NMDA-induced response was reversed by CHPG, DFB, and CDPPB, whereas the blockade of NMDA receptor by D-APV was restored by CHPG and CDPPB, but not by DFB. Alternatively, activation of NMDA receptors prior to the application of mGluR5 modulators, CHPG was able to enhance NMDA-induced field potentials and reverse the suppressive effect of ketamine and D-APV, but not ifenprodil. In addition, chelerythrine chloride (CTC), a protein kinase C (PKC) inhibitor, blocked the regulation of mGluR5 positive modulators in enhancing NMDA receptor activation and recovering NMDA receptor inhibition. The PKC activator (PMA) mimicked the effects of mGluR5 positive modulators on enhancing NMDA receptor activation and reversing NMDA antagonist-evoked NMDA receptor suppression. Our results demonstrate that the PKC-dependent pathway may be involved in the positive modulation of mGluR5 resulting in potentiating NMDA receptor activation and reversing NMDA receptor suppression induced by NMDA antagonists.

Abstract: One of the most problematic issues in the treatment of drug-addiction is the persistence of drug craving and drug-seeking behavior long after the symptoms of acute drug withdrawal have subsided. As a result of chronic and repetitive drug use, drug-associated cues and contexts acquire an excessive degree of motivational salience. This “hypersalience” of drug-associated cues and contexts is a result of associative overlearning produced by the drug’s ability to create strong and lasting associations between its euphorigenic effects and the cues and contexts that are present when the drug is administered. As a result, these cues and contexts can often trigger memories of prior drug use and drug craving (an intense desire for the drug that is often evoked by drug-associated contexts and cues), which may result in drug-seeking behavior and ultimately relapse. In addition to associative overlearning, the ritualistic nature of chronic drug-taking behavior transitions from self-regulated to compulsive and habitual, representing a form of maladaptive instrumental overlearning. The two forms of overlearning result in behavioral inflexibility and perseverance of drug-seeking that is resistant to change during the course of treatment, and are often a cause for relapse and noncompliance with different treatment regimens (Childress et al., 1999; Cleva & Gass, 2010; Kalivas, Volkow, & Seamans, 2005). Extinction is defined as the gradual reduction of a conditioned response (CR) when the CR is no longer reinforced or the unconditioned stimulus (US) is no longer presented in the presence of conditioned stimulus (CS). Repeated pairings of a discrete CS (i.e., a light and/or tone) with a US (i.e., drug infusion) enables the CS to evoke specific behaviors since the CS is associated with and predicts the availability of the US. Extinction can be defined as a decline in the magnitude and/or frequency of the CR either within a single extinction training session as well as over successive extinction training sessions conducted on a routine basis (i.e., daily or weekly). Extinction is a process of new and active learning (Bouton, 2000, 2004), and as with other forms of learning and memory, it consists of separate phases including acquisition, consolidation, recall, and reconsolidation (Lee, 2009; Myers & Carlezon, 2010b; Taylor, Olausson, Quinn, & Torregrossa, 2009). In behavioral terms, acquisition of extinction learning occurs when the organism first begins to learn that the CS no longer results in the presentation of the US. Enhancement of the acquisition of extinction learning can be achieved via experimental manipulations performed prior to extinction training sessions. Consolidation, on the other hand, is the strengthening and storage of new CS-US expectancies and relationships that carry forward from one extinction training session to the next. Enhancement of the consolidation of extinction learning can be achieved via experimental manipulations performed following extinction training sessions. Finally, reconsolidation is a process by which prior extinction memories are recalled and then reconsolidated back into long-term memory. However, recent evidence suggests that memories that are recalled and reconsolidated are labile and amenable to modification (Lee, 2009; Taylor, Olausson, Quinn, & Torregrossa, 2009). Cellular hallmarks of learning and memory and associated synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD) of synaptic efficacy require glutamatergic transmission in order to occur (Miyamoto, 2006; Rao & Finkbeiner, 2007; Reis et al., 2009; Riedel, Platta, & Micheaub, 2003; Robbins & Murphy, 2006), suggesting that potentiation of glutamate-mediated neural plasticity could serve as an effective adjunct for facilitating extinction learning. Recent studies have shown that potentiation of glutamatergic transmission with ligands such as the N-methyl-D-aspartate (NMDA) partial agonist D-cycloserine (DCS), the α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor potentiator 2-[2,6-difluoro-4-({2-[(phenylsulfonyl)amino]ethyl}thio)phenoxy]acetamide (PEPA), or the cystine prodrug N-acetylcysteine (NAC, which stimulates the cystine-glutamate exchanger and normalizes drug-induced deficits in extracellular glutamate during drug withdrawal), reduce extinction responding following active drug self-administration, reduce re-acquisition of drug intake, or facilitate the extinction of a conditioned place preference (CPP) produced by drugs of abuse (Botreau, Paolone, & Stewart, 2006; LaLumiere, Niehoff, & Kalivas, 2010; Nic Dhonnchadha et al., 2010; Thanos, Bermeo, Wang, & Volkow, 2009; Torregrossa, Sanchez, & Taylor, 2010; Zhou & Kalivas, 2008). DCS has also been shown to facilitate the extinction of a naloxone-induced conditioned place aversion in morphine-dependent rats (Myers & Carlezon, 2010a), suggesting that this compound may facilitate the extinction of both appetitive and aversive drug-associated memories. Collectively, these findings suggest that extinction learning can be augmented through pharmacological manipulation of glutamate transmission (Cleva, Gass, Widholm, & Olive, in press; Myers & Carlezon, 2010b; Myers, Carlezon, & Davis, in press). An alternative strategy to enhancing glutamatergic transmission, and therefore possibly extinction learning, is by allosteric potentiation of the function of type 5 metabotropic glutamate receptors (mGluR5). These receptors are predominantly localized to the perisynaptic annulus of postsynaptic dendritic spines, where they are positively coupled to NMDA receptor function and mediate various forms of synaptic plasticity (Ayala et al., 2009; Gladding, Fitzjohn, & Molnar, 2009; Luscher & Huber, 2010) and learning and memory (Simonyi, Schachtman, & Christoffersen, 2010). For example, potentiation of mGluR5 function has been shown to enhance the induction of LTP and LTD in CA1 region of the hippocampus and improve performance in a spatial memory task (Ayala et al., 2009; Popkirov & Manahan-Vaughan, in press; Rosenbrock et al., 2010). Potentiation of mGluR5 function has also been shown to facilitate the extinction of a cocaine conditioned place preference (Gass & Olive, 2009), an effect that is reversed by blockade of either mGluR5 or NMDA receptors. In contrast, mice carrying a null mutation in the gene encoding the mGluR5 receptor protein show deficits in extinction learning following cue or contextual fear conditioning (Xu, Zhu, Contractor, & Heinemann, 2009). These data strongly support a role for mGluR5 receptors in extinction learning. In the present study, we sought to determine if enhancement of mGluR5 receptor function by the mGluR5 positive allosteric modulator (PAM) 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) would enhance extinction learning following intravenous cocaine self-administration, as evidenced by reductions in the number of presses on a lever that previously resulted in intravenous cocaine delivery. We also sought to determine if CDPPB-treated animals would require fewer extinction training sessions to reach pre-defined extinction criteria as compared with vehicle-treated animals. A tertiary aim of the present study was to determine if CDPPB would enhance both the acquisition and consolidation of extinction learning, as assessed by administration of the compound prior to or immediately following extinction training sessions. Finally, to rule out the possibility that any observed reductions in lever pressing during extinction produced by CDPPB might be attributable to non-specific motor effects, we also tested the effects of this compound on open field locomotor activity.

Abstract: The most common cause of inherited mental retardation, fragile X syndrome, results from a triplet repeat expansion in the FMR1 gene and loss of the mRNA binding protein, fragile X mental retardation protein (FMRP). In the absence of FMRP, signaling through group I metabotropic glutamate receptors (mGluRs) is enhanced. We previously proposed a mechanism whereby the audiogenic seizures exhibited by FMR1 null mice result from an imbalance in excitatory mGluR and inhibitory GABAB receptor (GABABR) signaling (Mol Pharmacol 76:18–24, 2009). Here, we tested the mGluR5-positive allosteric modulator 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB), the mGluR5 inverse agonist 2-methyl-6-(phenylethynyl)pyridine (MPEP), and GABAB receptor agonists, alone and in combination on receptor protein expression and audiogenic seizures in FMR1 mice. Single doses of MPEP (30 mg/kg), the GABABR orthosteric agonist R-baclofen (1 mg/kg), or the GABABR-positive allosteric modulator N,N′-dicyclopentyl-2-(methylthio)-5-nitro-4,6-pyrimidine diamine (GS-39783) (30 mg/kg), reduced the incidence of seizures. However, when administered subchronically (daily injections for 6 days), MPEP retained its anticonvulsant activity, whereas R-baclofen and GS-39783 did not. When administered at lower doses that had no effect when given alone, a single injection of MPEP plus R-baclofen also reduced seizures, but the effect was lost after subchronic administration. We were surprised to find that subchronic treatment with R-baclofen also induced tolerance to a single high dose of MPEP. These data demonstrate that tolerance develops rapidly to the antiseizure properties of R-baclofen alone and R-baclofen coadministered with MPEP, but not with MPEP alone. Our findings suggest that cross-talk between the G-protein signaling pathways of these receptors affects drug efficacy after repeated treatment.