The discovery of allosteric modulators of G protein-coupled receptors (GPCRs) provides a promising new strategy with potential for developing novel treatments for a variety of central nervous system (CNS) disorders. Traditional drug discovery efforts targeting GPCRs have focused on developing ligands for orthosteric sites which bind endogenous ligands. Allosteric modulators target a site separate from the orthosteric site to modulate receptor function. These allosteric agents can either potentiate (positive allosteric modulator, PAM) or inhibit (negative allosteric modulator, NAM) the receptor response and often provide much greater subtype selectivity than do orthosteric ligands for the same receptors. Experimental evidence has revealed more nuanced pharmacological modes of action of allosteric modulators, with some PAMs showing allosteric agonism in combination with positive allosteric modulation in response to endogenous ligand (ago-potentiators) as well as “bitopic” ligands that interact with both the allosteric and orthosteric sites. Drugs targeting the allosteric site allow for increased drug selectivity and potentially decreased adverse side effects. Promising evidence has demonstrated potential utility of a number of allosteric modulators of GPCRs in multiple CNS disorders, including neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease, as well as psychiatric or neurobehavioral diseases such as anxiety, schizophrenia, and addiction.
Allosteric modulator; CNS; Drug discovery; GPCR; Metabotropic glutamate receptor; Muscarinic acetylcholine receptor
Metabotropic glutamate receptor subtype 5 (mGlu5) activators have emerged as a novel approach to the treatment of schizophrenia. Positive allosteric modulators (PAMs) of mGlu5 have generated tremendous excitement and fueled major drug discovery efforts. Although mGlu5 PAMs have robust efficacy in preclinical models of schizophrenia, preliminary reports suggest that these compounds may induce seizure activity. Prototypical mGlu5 PAMs do not activate mGlu5 directly but selectively potentiate activation of mGlu5 by glutamate. This mechanism may be critical to maintaining normal activity-dependence of mGlu5 activation and achieving optimal in vivo effects.
Using specially engineered mGlu5 cell lines incorporating point mutations within the allosteric and orthosteric binding sites, as well as brain slice electrophysiology and in vivo electroencephalography and behavioral pharmacology, we found that some mGlu5 PAMs have intrinsic allosteric agonist activity in the absence of glutamate.
Both in vitro mutagenesis and in vivo pharmacology studies demonstrate that VU0422465 is an agonist PAM that induces epileptiform activity and behavioral convulsions in rodents. In contrast, VU0361747, an mGlu5 PAMs optimized to eliminate allosteric agonist activity, has robust in vivo efficacy and does not induce adverse effects at doses that yield high brain concentrations.
Loss of the absolute dependence of mGlu5 PAMs on glutamate release for their activity can lead to severe adverse effects. The finding that closely related mGlu5 PAMs can differ in their intrinsic agonist activity provides critical new insights that is essential for advancing these molecules through clinical development for treatment of schizophrenia.
Glutamate; allosteric modulators; agonist; schizophrenia; seizure; convulsions; mGlu5
Currently available therapeutic agents for treatment of schizophrenia target signaling by monoaminergic neurotransmitters; however, these treatments do not adequately treat the range of symptoms observed in patients. While these therapies treat the positive symptoms, they do not have efficacy in treating the negative symptoms and cognitive deficits that are associated with the disease. Evidence suggests that molecules that modulate signaling by the neurotransmitter acetylcholine (ACh) could provide a more comprehensive treatment of schizophrenia than currently prescribed antipsychotics. Molecules that broadly increase ACh-signaling have been demonstrated to have efficacy in treating numerous symptom clusters in schizophrenia patients. Unfortunately, these compounds induce adverse effects via activation of peripheral receptors that limit their clinical utility. One proposed strategy for retaining the efficacy of cholinergic treatments, without the adverse effects, is to target specific cholinergic receptor subtypes in the brain. Several cholinergic receptors are able to modulate brain circuits that are dysregulated in schizophrenia patients including receptors belonging to both the muscarinic family (i.e., M1, M4, and M5), and the nicotinic family (i.e., α7, α4β2). Recently, great strides have been made in developing small molecules with high specificity for these receptors, and several of these novel molecules have robust efficacy in several preclinical models predictive of both anti-psychotic and pro-cognitive effectiveness. Promising studies suggest that targeting M1 and α7 may be beneficial for pro-cognitive effects, while molecules that target M4 may be ideally suited to address the positive symptoms. Since these receptor subtypes are distinct from those responsible for the adverse effects observed with non-selective cholinergic treatments, there is hope that molecules targeting these receptors could provide novel therapeutics. Further research is needed to examine the utility of such compounds as therapeutics that could be used either alone, or in combination with existing medications, to better treat schizophrenia.
Recent evidence suggests that the functions of presynaptic metabotropic glutamate receptors (mGluRs) are tightly regulated by protein kinases. We previously reported that cAMP-dependent protein kinase (PKA) directly phosphorylates mGluR2 at a single serine residue (Ser843) on the C-terminal tail region of the receptor, and that phosphorylation of this site inhibits coupling of mGluR2 to GTP-binding proteins. This may be the mechanism by which the adenylyl cyclase activator forskolin inhibits presynaptic mGluR2 function at the medial perforant path-dentate gyrus synapse. We now report that PKA also directly phosphorylates several group III mGluRs (mGluR4a, mGluR7a, and mGluR8a), as well as mGluR3 at single conserved serine residues on their C-terminal tails. Furthermore, activation of PKA by forskolin inhibits group III mGluR-mediated responses at glutamatergic synapses in the hippocampus. Interestingly, β-adrenergic receptor activation was found to mimic the inhibitory effect of forskolin on both group II and III mGluRs. These data suggest that a common PKA-dependent mechanism may be involved in regulating the function of multiple presynaptic group II and group III mGluRs. Such regulation is not limited to the pharmacological activation of adenylyl cyclase but can also be elicited by the stimulation of endogenous Gs-coupled receptors, such as β-adrenergic receptors.
β-adrenergic; cAMP-dependent protein kinase; mGluR4; mGluR7; mGluR8; phosphorylation
Treatment options for schizophrenia that address all symptom categories (positive, negative, and cognitive) are lacking. Novel compounds that regulate signaling by the major excitatory neurotransmitter in the brain, glutamate, are emerging as a novel approach for the treatment of this disorder. Currently available medications ameliorate positive symptoms but do not have efficacy in reducing negative symptoms or cognitive disturbances. It is possible that agents that target glutamatergic signaling in the CNS could have efficacy in reducing all major symptom clusters, providing a more comprehensive treatment strategy, and also avoiding some of the adverse effects that are seen with currently available treatments. Three major approaches for targeting glutamate signaling are now advancing in preclinical and clinical development. First are inhibitors for a transporter for glycine termed GlyT1. Glycine is a co-agonist with glutamate for a specific subtype of glutamate receptor, termed the NMDA receptor, which is thought to be critically involved in brain circuits that are disrupted in schizophrenia patients. Inhibiting GlyT1 increases glycine levels and can selectively increase NMDA receptor signaling. Another promising approach is to increase activity of another family of glutamate receptors, termed metabotropic glutamate receptors (mGlus), which play important modulatory roles in brain circuits that are thought to be disrupted in schizophrenia patients. Activation of the group I (mGlu5) and the group II (mGlu2 and mGlu3) mGlus is hypothesized to normalize the disruption of aberrant signaling in these circuits. Novel drug-like molecules that increase activity of these receptors have robust efficacy in animal models that predict efficacy in treatment of schizophrenia. Early clinical studies provide some support for potential utility of these targets in reducing symptoms in schizophrenia patients. Clinical studies that are underway will provide further insights into the potential utility of these compounds in the treatment of multiple symptom domains in schizophrenia patients.
Treatment options for schizophrenia that address all symptom categories (positive, negative, and cognitive) are lacking in current therapies for this disorder. Compounds targeting the metabotropic glutamate (mGlu) receptors hold promise as a more comprehensive therapeutic alternative to typical and atypical antipsychotics and may avoid the occurrence of extrapyramidal side effects that accompany these treatments. Activation of the group II mGlu receptors (mGlu2 and mGlu3) and the group I mGlu5 are hypothesized to normalize the disruption of thalamocortical glutamatergic circuitry that results in abnormal glutamaterigic signaling in the prefrontal cortex (PFC). Agonists of mGlu2 and mGlu3 have demonstrated efficacy for the positive symptom group in both animal models and clinical trials with mGlu2 being the subtype most likely responsible for the therapeutic effect. Limitations in the chemical space tolerated by the orthosteric site of the mGlu receptors has led to the pursuit of compounds that potentiate the receptor’s response to glutamate by acting at less highly conserved allosteric sites. Several series of selective positive allosteric modulators (PAMs) for mGlu2 and mGlu5 have demonstrated efficacy in animal models used for the evaluation of antipsychotic agents. In addition, evidence from animal studies indicates that mGlu5 PAMs hold promise for the treatment of cognitive deficits that occur in schizophrenia. Hopefully, further optimization of allosteric modulators of mGlu receptors will yield clinical candidates that will allow full evaluation of the potential efficacy of these compounds in the treatment of multiple symptom domains in schizophrenia patients in the near future.
metabotropic; glutamate; schizophrenia; NMDA; allosteric
This letter describes the development and SAR of a novel series of GlyT1 inhibitors derived from a scaffold hopping approach, in lieu of an HTS campaign, which provided intellectual property position. Members within this new [3.3.0]-based series displayed excellent GlyT1 potency, selectivity, free fraction, and modest CNS penetration. Moreover, enantioselective GlyT1 inhibition was observed, within this novel series and a number of other piperidine bioisosteric cores.
GlyT1; Scaffold hopping; transporter; schizophrenia
This letter describes the development and SAR of a novel series of GlyT1 inhibitors derived from a scaffold hopping approach that provided a robust intellectual property position, in lieu of a traditional, expensive HTS campaign. Members within this new [3.1.0]-based series displayed excellent GlyT1 potency, selectivity, free fraction, CNS penetration and efficacy in a preclinical model of schizophrenia (prepulse inhibition).
GlyT1; Scaffold hopping; transporter; schizophrenia
Finding new therapies for Parkinson’s disease (PD) is a slow process. We assembled an international committee of experts to examine drugs potentially suitable for repurposing to modify PD progression. This committee evaluated multiple drugs currently used, or being developed, in other therapeutic areas, as well as considering several natural, non-pharmaceutical compounds. The committee prioritized which of these putative treatments were most suited to move immediately into pilot clinical trials. Aspects considered included known modes of action, safety, blood-brain-barrier penetration, preclinical data in animal models of PD and the possibility to monitor target engagement in the brain. Of the 26 potential interventions, 10 were considered worth moving forward into small, parallel ‘learning’ clinical trials in PD patients. These trials could be funded in a multitude of ways through support from industry, research grants and directed philanthropic donations. The committee-based approach to select the candidate compounds might help rapidly identify new potential PD treatment strategies for use in clinical trials.
Drug repositioning; disease modification; neuroprotection
A functional high throughput screen and subsequent multi-dimensional, iterative parallel synthesis effort identified the first muscarinic acetylcholine receptor (mAChR) negative allosteric modulator (NAM) selective for the M5 subtype. ML375 is a highly selective M5 NAM with sub-micromolar potency (human M5 IC50 = 300 nM, rat M5 IC50 = 790 nM, M1–4 IC50 >30 μM), excellent multi-species PK, high CNS penetration, and enantiospecific inhibition.
Muscarinic acetylcholine receptor; M5; negative allosteric modulator (NAM); ML375; MLPCN probe
Allosteric modulation of G protein-coupled receptors (GPCRs) represents a novel approach to the development of probes and therapeutics that is expected to enable subtype-specific regulation of central nervous system target receptors. The metabotropic glutamate receptors (mGlus) are class C GPCRs that play important neuromodulatory roles throughout the brain, as such they are attractive targets for therapeutic intervention for a number of psychiatric and neurological disorders including anxiety, depression, Fragile X Syndrome, Parkinson’s disease and schizophrenia. Over the last fifteen years, selective allosteric modulators have been identified for many members of the mGlu family. The vast majority of these allosteric modulators are thought to bind within the transmembrane-spanning domains of the receptors to enhance or inhibit functional responses. A combination of mutagenesis-based studies and pharmacological approaches are beginning to provide a better understanding of mGlu allosteric sites. Collectively, when mapped onto a homology model of the different mGlu subtypes based on the β2-adrenergic receptor, the previous mutagenesis studies suggest commonalities in the location of allosteric sites across different members of the mGlu family. In addition, there is evidence for multiple allosteric binding pockets within the transmembrane region that can interact to modulate one another. In the absence of a class C GPCR crystal structure, this approach has shown promise with respect to the interpretation of mutagenesis data and understanding structure-activity relationships of allosteric modulator pharmacophores.
allosteric modulator; chimeric receptors; G protein-coupled receptors; neuromodulation; site-directed mutagenesis; homology model
Development of SAR in an aryl ether series of mGlu5 NAMs leading to the identification of tool compound VU0409106 is described in this Letter. VU0409106 is a potent and selective negative allosteric modulator of mGlu5 that binds at the known allosteric binding site and demonstrates good CNS exposure following intraperitoneal dosing in mice. VU0409106 also proved efficacious in a mouse marble burying model of anxiety, an assay known to be sensitive to mGlu5 antagonists as well as clinically efficacious anxiolytics.
Positive allosteric modulators (PAMs) of metabotropic glutamate receptor 5 (mGlu5) represent a promising therapeutic strategy for the treatment of schizophrenia. Both allosteric agonism and high glutamate fold-shift have been implicated in the neurotoxic profile of some mGlu5 PAMs; however, these hypotheses remain to be adequately addressed. To develop tool compounds to probe these hypotheses, the structure-activity relationship of allosteric agonism was examined within an acetylenic series of mGlu5 PAMs exhibiting allosteric agonism in addition to positive allosteric modulation (ago-PAMs). PAM 38t, a low glutamate fold-shift allosteric ligand (maximum fold-shift ~3.0), was selected as a potent PAM with no agonism in the in vitro system used for compound characterization and in two native electrophysiological systems using rat hippocampal slices. PAM 38t (ML254) will be useful to probe the relative contribution of cooperativity and allosteric agonism to the adverse effect liability and neurotoxicity associated with this class of mGlu5 PAMs.
Starting from a singleton chromanone high throughput screening (HTS) hit, we describe a focused medicinal chemistry optimization effort leading to the identification of a novel series of phenoxymethyl-dihydrothiazolopyridone derivatives as selective positive allosteric modulators (PAMs) of the metabotropic glutamate 5 (mGlu5) receptor. These dihydrothiazolopyridones potentiate receptor responses in recombinant systems. In vitro and in vivo drug metabolism and pharmacokinetic (DMPK) evaluation allowed us to select compound 16a for its assessment in a preclinical animal screen of possible antipsychotic activity. 16a was able to reverse amphetamine-induced hyperlocomotion in rats in a dose-dependent manner without showing any significant motor impairment or overt neurological side effects at comparable doses. Evolution of our medicinal chemistry program, structure activity, and properties relationships (SAR and SPR) analysis as well as a detailed profile for optimized mGlu5 receptor PAM 16a are described.
Development of SAR in an octahydropyrrolo[3,4-c]pyrrole series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. The octahydropyrrolo[3,4-c]pyrrole scaffold was chosen as an isosteric replacement for the piperazine ring found in the initial hit compound. Characterization of selected compounds in protein binding assays was used to identify the most promising analogs, which were then profiled in P450 inhibition assays in order to further assess the potential for drug-likeness within this series of compounds.
Glutamate; GPCR; mGlu1; Allosteric modulator; CNS; Octahydropyrrolo[3,4-c]pyrrole
Of the five muscarinic receptor subtypes, the M5 receptor is the only one detectable in midbrain dopaminergic neurons, making it an attractive potential therapeutic target for treating disorders in which dopaminergic signaling is disrupted. However, developing an understanding of the role of M5 in regulating midbrain dopamine neuron function has been hampered by a lack of subtype-selective compounds. Here, we extensively characterize the novel compound VU0238429 and demonstrate that it acts as a positive allosteric modulator with unprecedented selectivity for the M5 receptor. We then used VU0238429, along with M5 knock-out mice, to elucidate the role of this receptor in regulating substantia nigra pars compacta (SNc) neuron physiology in both mice and rats. In sagittal brain slices that isolate the SNc soma from their striatal terminals, activation of muscarinic receptors induced Ca2+ mobilization and inward currents in SNc dopamine neurons, both of which were potentiated by VU0238429 and absent in M5 knock-out mice. Activation of M5 also increased the spontaneous firing rate of SNc neurons, suggesting that activation of somatodendritic M5 increases the intrinsic excitability of SNc neurons. However, in coronal slices of the striatum, potentiation of M5 with VU0238429 resulted in an inhibition in dopamine release as monitored with fast scan cyclic voltammetry. Accordingly, activation of M5 can lead to opposing physiological outcomes depending on the location of the receptor. Although activation of somatodendritic M5 receptors on SNc neurons leads to increased neuronal firing, activation of M5 receptors in the striatum induces an inhibition in dopamine release.
acetylcholine; allosteric; dopamine; M5; mAChR; muscarinic
is a powerful and highly addictive stimulant that disrupts the normal
reward circuitry in the central nervous system (CNS), producing euphoric
effects. Cocaine use can lead to acute and life threatening emergencies,
and abuse is associated with increased risk for contracting infectious
diseases. Though certain types of behavioral therapy have proven effective
for treatment of cocaine addiction, relapse remains high, and there
are currently no approved medications for the treatment of cocaine
abuse. Evidence has continued to accumulate that indicates a critical
role for the metabotropic glutamate receptor subtype 5 (mGlu5) in the modulation of neural circuitry associated with the addictive
properties of cocaine. While the small molecule mGlu5 negative
allosteric modulator (NAM) field is relatively advanced, investigation
into the potential of small molecule mGlu5 NAMs for the
treatment of cocaine addiction remains an area of high interest. Herein
we describe the discovery and characterization of a potent and selective
compound 29 (VU0463841) with good CNS exposure in rats.
The utility of 29 (VU0463841) was demonstrated by its
ability to attenuate drug seeking behaviors in relevant rat models
of cocaine addiction.
mGlu5; negative allosteric modulator; noncompetitive antagonist; CNS; cocaine; addiction
Highly selective positive allosteric modulators (PAMs) of metabotropic glutamate receptor subtype 5 (mGluR5) have emerged as a potential approach to treat positive symptoms associated with schizophrenia. mGluR5 plays an important role in both long term potentiation (LTP) and long term depression (LTD), suggesting that mGluR5 PAMs may also have utility in improving impaired cognitive function. However, if mGluR5 PAMs shift the balance of LTP and LTD or induce a state in which afferent activity induces lasting changes in synaptic function that are not appropriate for a given pattern of activity, this could disrupt rather than enhance cognitive function. We determined the effect of selective mGluR5 PAMs on induction of LTP and LTD at the Schaffer collateral – CA1 synapse in the hippocampus. mGluR5-selective PAMs significantly enhanced threshold theta burst stimulation (TBS)-induced LTP. In addition, mGluR5 PAMs enhanced both DHPG-induced LTD and LTD induced by delivery of paired-pulse low frequency stimulation. Selective potentiation of mGluR5 had no effect on LTP induced by suprathreshold TBS or saturated LTP. The finding that potentiation of mGluR5-mediated responses to stimulation of glutamatergic afferents enhances both LTP and LTD supports the hypothesis that activation of mGluR5 by endogenous glutamate contributes to both forms of plasticity. Furthermore, two systemically active mGluR5 PAMs enhanced performance in the Morris water maze, a measure of hippocampus-dependent spatial learning. Discovery of small molecules that enhance both LTP and LTD in an activity-appropriate manner demonstrates a unique action on synaptic plasticity that may provide a novel approach for treatment of impaired cognitive function.
mGluR5; group I mGluR; synaptic plasticity; LTP; LTD; allosteric potentiator
In the previous work, we reported a method that utilized imaging data collected from 60 to 120 min following [18F]fallypride administration to estimate the distribution volume ratio DVR′ (DVR′ ∝ DVR; DVR = 1 + BPND, where BPND is a measure of receptor density, DA D2 in this case). In this work, we use this method to assess the effects of isoflurane anesthesia on [18F]fallypride DVR′.
Rats were injected with [18F]fallypride either unconsciously under ~1.5% isoflurane via the tail vein (Group 1) or consciously via a catheter inserted either in the jugular vein (Group 2) or the tail vein (Group 3). After about 1 h of free access to food and water the rats were anesthetized with 1.5% isoflurane and imaged in a microPET for 60 min. The rats that were injected consciously (Groups 2 and 3) were placed in a rat restrainer during [18F]fallypride injection. They were habituated in that restrainer for 3 days prior to the experiment day to minimize restraint-related stress. For comparison, a control group of rats was imaged for 120 min simultaneously with the administration of [18F]fallypride i.v. while under 1.5% isoflurane. The DVR′ estimates from the 60 min acquisitions were compared with the DVR′ from the last 60 min of the 120 min acquisitions (after neglecting the first 60 min). In addition, the striatal time–activity curves were fit with a 2-tissue + plasma compartment model using an arbitrary simulated plasma input function to obtain k3/k4 (≈ BPND) for the 60 and 120 min acquisitions.
Isoflurane anesthesia caused a significant reduction, up to 22%, in the DVR′ estimates, which were 15.7 ± 0.3 (mean ± SE) for the controls, 17.7 ± 0.3 for Group 1, 19.2 ± 0.4 for Group 2, and 18.8 ± 0.7 for Group 3. The compartmental model fit produced similar results, ~30% reduction in k3/k4 for the 120-min acquisitions compared with the 60-min acquisitions (initial conscious uptake of the radiotracer).
The results of this study demonstrate that isoflurane anesthesia significantly decreases striatal [18F]fallypride BPND in rats. Of similar importance, this work demonstrates the effectiveness of delayed scans following radiotracer injection and the implication that different types of studies can be conducted simultaneously with this method, including studies of behavioral and environmental impact on brain receptors.
[18F]fallypride; dopamine (DA) receptors; isoflurane; graphical analysis; microPET
Metabotropic glutamate (mGlu) receptors play important roles in regulating CNS function and are known to function as obligatory dimers. Although recent studies have suggested heterodimeric assembly of mGlu receptors in vitro, the demonstration that distinct mGlu receptor proteins can form heterodimers or hetero-complexes with other mGlu subunits in native tissues, such as neurons, has not been shown. Using biochemical and pharmacological approaches, we demonstrate here that mGlu2 and mGlu4 form a hetero-complex in native rat and mouse tissues which exhibits a distinct pharmacological profile. These data greatly extend our current understanding of mGlu receptor interaction and function and provide compelling evidence that mGlu receptors can function as heteromers in intact brain circuits.
Development of SAR in an N-acyl-N′-arylpiperazine series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. Characterization of selected compounds in protein binding assays was used to aid in selecting VU0469650 for further profiling in ancillary pharmacology assays and pharmacokinetic studies. VU0469650 demonstrated an excellent selectivity profile and good exposure in both plasma and brain samples following intraperitoneal dosing in rats.
Glutamate; GPCR; mGlu1; Allosteric modulator; CNS; Piperazine
The newly formed Academic Drug Discovery Consortium (ADDC) aims to support the growing numbers of university centres engaged in drug discovery that have emerged in response to recent changes in the drug discovery ecosystem.
A multi-dimensional, iterative parallel synthesis effort identified a series of highly selective mGlu3 NAMs with sub-micromolar potency and good CNS penetration. Of these, ML337 resulted (mGlu3 IC50 = 593 nM, mGlu2 IC50 >30 μM) with B:P ratios of 0.92 (mouse) to 0.3 (rat). DMPK profiling and shallow SAR led to the incorporation of deuterium atoms to address a metabolic soft spot, which subsequently lowered both in vitro and in vivo clearance by >50%.
Metabotropic glutamate receptor; mGlu3; negative allosteric modulator (NAM); ML337; MLPCN probe
Recent studies suggest that subtype specific activators of metabotropic glutamate receptors (mGluRs) have exciting potential for the development of novel treatment strategies for numerous psychiatric and neurological disorders. A number of positive allosteric modulators (PAMs) have been identified that are highly selective for mGluR1, including the compounds Ro 01-6128, Ro 67-4853, and Ro 67-7476. These PAMs have been previously found to interact with a site distinct from that of negative allosteric modulators (NAMs), typified by R214127. These mGluR1 PAMs do not have an effect on baseline calcium levels but induce leftward shifts in the concentration response of mGluR1 to agonists. However, their effects on a variety of signaling pathways and their mechanism of action have not been fully explored and are of critical importance for further development of mGluR1 allosteric modulators as novel drugs. In baby hamster kidney (BHK) cells, mGluR1 activates calcium mobilization, cAMP production, and extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation; signaling cascades which are distinct and differentially regulated. In contrast to their effects on calcium mobilization, these compounds were found to activate ERK1/2 phosphorylation in the absence of exogenously added agonist, an effect that was fully blocked by both orthosteric (LY341495) and allosteric (R214127) mGluR1 antagonists. The mGluR1 PAMs were also found to activate cAMP production in the absence of agonist. Thus, these mGluR1 PAMs have qualitatively different effects on a variety of mGluR1-mediated signal transduction cascades. Together, these data provide further evidence that allosteric compounds can differentially modulate the coupling of a single receptor to independent signaling pathways or act in a system-dependent manner.
mGluR1; allosteric potentiator; glutamate; ligand-induced differential signaling
Group III metabotropic glutamate receptors (mGluRs) reduce synaptic transmission at the Schaffer collateral-CA1 (SC-CA1) synapse in rats by a presynaptic mechanism. Previous studies show that low concentrations of the group III-selective agonist, L-AP4, reduce synaptic transmission in slices from neonatal but not adult rats, whereas high micromolar concentrations reduce transmission in both age groups. L-AP4 activates mGluRs 4 and 8 at much lower concentrations than those required to activate mGluR7, suggesting that the group III mGluR subtype modulating transmission is a high affinity receptor in neonates and a low affinity receptor in adults. The previous lack of subtype selective ligands has made it difficult to test this hypothesis. We have measured fEPSPs in the presence of novel subtype selective agents to address this question. We show that the effects of L-AP4 can be blocked by LY341495 in both neonates and adults, verifying that these effects are mediated by mGluRs. In addition, the selective mGluR8 agonist, DCPG, has a significant effect in slices from neonatal rats but does not reduce synaptic transmission in adult slices. The mGluR4 selective allosteric potentiator, PHCCC, is unable to potentiate the L-AP4-induced effects at either age. Taken together, our data suggest that group III mGluRs regulate transmission at the SC-CA1 synapse throughout development but there is a developmental regulation of the subtypes involved so that that both mGluR8 serves this role in neonates but not adults whereas mGluR7 is involved in regulating transmission at this synapse in throughout postnatal development.
metabotropic glutamate receptor; group III; hippocampus; DCPG; L-AP4