NMDA Receptor Activity in Neuropsychiatric Disorders

Mood disorders are common, chronic, recurrent mental illnesses that affect the lives of millions of individuals worldwide. To date, the monoaminergic systems (serotonergic, noradrenergic and dopaminergic) in the brain have received the greatest attention in neurobiological studies of mood disorders, and most therapeutics target these systems. However, there is growing evidence that the glutamatergic system is central to the neurobiology and treatment of these disorders. Here, we review data supporting the involvement of the glutamatergic system in mood-disorder pathophysiology as well as the efficacy of glutamatergic agents in mood disorders. We also discuss exciting new prospects for the development of improved therapeutics for these devastating disorders.

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels widely expressed in the central nervous system that play key roles in excitatory synaptic transmission. Because of their involvement in numerous neurological disorders, NMDARs are also targets of therapeutic interest. NMDARs occur as multiple subtypes which differ in their subunit composition and in their biophysical and pharmacological properties. In particular, NMDARs contain a diversity of sites at which endogenous ligands or pharmacological agents can act to modulate receptor activity in a subunit-selective manner, and recent structural and functional data have started to reveal the molecular determinants for this subunit selectivity. These include the binding sites for glutamate, the ion-channel pore and the recently identified allosteric sites on the N-terminal domain. Other potential sites yet unexplored by medicinal chemistry programs are also considered, in particular at the interface between subunits. Given the growing body of evidence that diverse brain disorders implicate different NMDAR subtypes, such as NR2B in pain or NR3A in white matter injury, there is a growing interest in exploiting the pharmacological heterogeneity of NMDARs for the development of novel NMDAR subtype-selective compounds.

Amyloid-β oligomers may cause cognitive deficits in Alzheimer's disease by impairing neuronal NMDA-type glutamate receptors, whose function is regulated by the receptor tyrosine kinase EphB2. Here we show that amyloid-β oligomers bind to the fibronectin repeats domain of EphB2 and trigger EphB2 degradation in the proteasome. To determine the pathogenic importance of EphB2 depletions in Alzheimer's disease and related models, we used lentiviral constructs to reduce or increase neuronal expression of EphB2 in memory centres of the mouse brain. In nontransgenic mice, knockdown of EphB2 mediated by short hairpin RNA reduced NMDA receptor currents and impaired long-term potentiation in the dentate gyrus, which are important for memory formation. Increasing EphB2 expression in the dentate gyrus of human amyloid precursor protein transgenic mice reversed deficits in NMDA receptor-dependent long-term potentiation and memory impairments. Thus, depletion of EphB2 is critical in amyloid-β-induced neuronal dysfunction. Increasing EphB2 levels or function could be beneficial in Alzheimer's disease.