Discovering novel subtype-selective glutamate receptor antagonists for the study of hippocampal synaptic plasticity

The main aim of this work is to design and synthesize new chemical substances to be used as tools to investigate the fundamental mechanism of learning and memory. This research may lead to new treatments for disorders that involve cognitive dysfunction, such as schizophrenia, Alzheimer’s and Parkinson’s disease.
Communication between one nerve cell (neuron) and others in the brain is effected at the junctions (synapses). One of the main mechanisms by which one cell in a neuronal chain communicates with the next cell is by releasing an amino acid, glutamate from its multiple synaptic endings. We have helped establish that glutamate can interact with a family of structurally related proteins known as glutamate receptor subtypes each performing different functions in the central nervous system (CNS).
Whereas glutamate activates all the subtypes of glutamate receptor, we aim to produce tools that block the activation of only one subtype of receptor. By observing what effect specific activation or blockade of a particular glutamate receptor subtype has on the functioning of the CNS, one can deduce the particular role of that receptor subtype in the integrated pattern of nervous activity that underlies learning and memory.

NMDA receptors (NMDARs) are ligand gated ion channels that belong to the glutamate receptor (GluR) family and are expressed throughout the central nervous system (CNS). NMDARS are tetrameric assemblies of NR1, NR2A-D and NR3A and NR3B subunits. NMDARs have been identified as key players in synaptic plasticity and CNS disorders. However, the roles the different NMDAR subunits play in synaptic plasticity in the hippocampus are poorly understood. A related family of GluRs, the metabotropic glutamate receptors (mGluRs), play a modulatory role in CNS function. Of particular interest are subtypes of group III mGluRs (mGluR7 and 8) as these are known to be expressed in the hippocampus and play a role in synaptic plasticity in this brain region. However, the lack of pharmacological tools that are capable of distinguishing between the various types of NMDAR subunits or the subtypes of mGluRs is hampering our understanding of the functions of NMDARs and mGluRs in synaptic plasticity in the hippocampus. We plan to develop antagonists that are selective for mGluR7, mGluR8, NR2A, NR2B and NR3A. Two interrelated approaches will be adopted to develop isoform specific antagonists, 1) rational drug design based on known structure-activity data combined with the use of X-ray crystal structures and homology models to target unique amino acid residues in the binding site of the protein of interest and 2) validation of hypotheses generated in modelling studies by testing compounds on appropriate point mutations of the protein target. Once mGluR7 and mGluR8 selective antagonists have been identified, we will use them, in conjunction with knockout mice, to elucidate the roles of mGluR7 and mGluR8 in hippocampal synaptic plasticity. The roles of NR2A and NR2B in long-term potentiation (LTP) and long-term depression (LTD) are controversial, largely due to the lack of suitably selective pharmacological tools. The NR2A and NR2B selective antagonists developed here will be used to probe the roles of these subunits in hippocampal synaptic function. Very little is known about the functions of NR3 subunits in LTP and LTD. We will therefore use selective antagonists to investigate plasticity at CA3-CA1 synapses where the NR3 subunit is expressed in young animals. We also plan to develop biotin-conjugated photoaffinity ligands and fluorescently labelled compounds based on the isoform specific antagonists identified during this project. These will be used to investigate the molecular organisation, developmental regulation, regional, cellular and subcellular distribution and trafficking of the native NMDARs and mGluRs.