Optimization of novel mGluR3-selective allosteric modulators
The hippocampus is a limbic cortical structure that plays an important role in a number of normal physiological processes and is a primary site of pathology in certain neurological disorders, such as Alzheimer's disease and temporal lobe epilepsy (see (2) for review). Because of its important role in both normal and pathological processes, a great deal of effort has been focused on developing a more ..
The hippocampus is a limbic cortical structure that plays an important role in a number of normal physiological processes and is a primary site of pathology in certain neurological disorders, such as Alzheimer's disease and temporal lobe epilepsy (see (2) for review). Because of its important role in both normal and pathological processes, a great deal of effort has been focused on developing a detailed understanding of the cellular mechanisms involved in regulation of transmission through the hippocampal circuit. Evidence suggests that the metabotropic glutamate receptors (mGlus) play a variety of roles in modulating transmission and cell excitability at each of the major excitatory synapses in the hippocampus (1,3). In particular, the Gi/o-coupled group II mGlus (mGlu2 and mGlu3) can have significant physiological effects in the hippocampus. Activation of group II mGlus reduces transmission at perforant path-dentate gyrus synapses (9), the mossy fiber synapse (11), and at synapses onto certain interneuron populations (5). However, although presynaptic group II mGlus do not directly reduce synaptic transmission through actions on glutamatergic terminals at the SC-CA1 synapse (7,14,6), we and others previously reported extensive studies demonstrating that group II mGlus in this region are involved in a novel form of glial-neuronal communication. In particular, activation of group II mGlus induces a marked potentiation of cAMP responses elicited by activation of beta-adrenergic receptors (betaARs) in astrocytes, leading to release of adenosine from the astrocytes and inducing a profound depression of transmission at the SC-CA1 synapse via activation of presynaptic A1 adenosine receptors (14,10). This unique response provides a mechanism by which activation of group II mGlus only reduces transmission at the SC-CA1 synapses under conditions where betaARs and group II mGlus are coincidentally activated. This could provide a protective mechanism to reduce risk of excitotoxicity during periods in which there is excessive excitatory drive when noradrenergic inputs are highly active, such as during periods of intense or prolonged stress.
Unfortunately, despite concerted efforts by multiple groups, selective reagents that differentiate between mGlu2 and mGlu3 have not been available to allow determination of the specific mGlu subtype involved in modulating betaAR-mediated cAMP responses and effects on synaptic transmission. However, within our mGlu5 program, we have discovered compounds with mGlu3 NAM activity, such as VU0092273 (13), that have excellent aqueous solubility and achieve high CNS exposure. However, this compound also acts as an mGlu5 PAM. While the mGlu5 PAM activity prevents use of VU0092273 as a selective mGluR3 NAM probe, discovery of this compound still represents a major breakthrough in that it is the first compound that selectively blocks mGlu3 relative to mGlu2 and establishes the feasibility of optimizing novel molecules that act at mGlu3 without effects on the closely related mGlu2 subtype. Thus, it will now be critical to further optimize compounds based on VU0092273 that are selective for mGlu3 relative to all other mGlu subtypes. Establishing novel, highly selective mGlu3 probes represents a critical need that could directly impact our understanding of mGlu3 function and could have a major impact on the direction of current drug discovery efforts focused on discovery and development of agents that activate or inhibit group II mGlus.
Compounds will be initially screened by performing concentration-response curves (CRCs, 10 points, ranging from approximately 30 microM-1 nM at 0.3% final DMSO concentration) to determine the potency and efficacy of novel compounds for mGlu3 in a thallium flux assay measuring coupling of mGlu3 to G Protein-coupled Inwardly Rectifying Potassium (GIRK) Channels (12,8,4). Compounds with IC50 values less than 1 microM will next be evaluated for potency versus mGlu2 in a thallium flux CRC assay. Compounds demonstrating at least 20-fold selectivity for mGlu3 versus mGlu2 will then be evaluated for the mechanism of mGlu3 antagonism through Schild functional thallium flux assays at mGlu3 to determine if the compounds are antagonizing mGlu3 in a competitive or noncompetitive manner. For noncompetetive compounds passing the above steps, we will pursue follow-up studies to determine their selectivity for mGlu3 relative to other mGlu subtypes. Our goal for this project would be to generate an mGlu3-selective compound, with an IC50 under 1 microM, and with reasonable solubility in a solvent generally useful for in vitro experimentation (i.e., DMSO). The ultimate goal of this project from the PI's perspective is to generate compounds that would be useful proof-of-concept molecules to determine the mGlu subtype involved in glial-neuronal communication in the hippocampus.
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