Molecular Imaging Probes for N-Methyl-D-Aspartate Receptors

Abstract

The non-invasive detection of neuronal transmission is of prime importance in order to understand brain function better. This will aid cognitive neuroscience, as well as medical science, in the early detection of diseased states. Herein, approaches to molecular imaging of the NMDA receptor, a receptor subtype of the excitatory neurotransmitter glutamate, through the use of targeted contrast agents, is described.
Initially, a series of NMDA receptor-targeted MRI contrast agents was developed based upon a known competitive NMDA receptor antagonist, appended to an N-linked ‘Gd-DOTA’ core that possesses a fast-exchanging water molecule. Their use as responsive MR imaging probes was evaluated in vitro using a neuronal cell line model, and three contrast agents showed large enhancements in cellular relaxation rates.
In order to confirm NMDA receptor localisation, derivatives of the lead compounds were also prepared. The derivatives contained a biotin moiety, which allowed direct visualisation of the cell-surface receptors, after addition of an AvidinAlexaFluor®-488 conjugate. Using these derivatives, the specificity and reversibility (in the presence of glutamate) of binding at the NMDA receptor was demonstrated in living cells using laser scanning confocal microscopy.
In an attempt to generate a single-component NMDA receptor-targeted optical imaging agent, a very bright europium complex conjugated to an NMDA receptor-binding moiety was synthesised. Unfortunately, upon incubation with a neuronal cell line model, complex localisation appeared to be dictated by the ligand structure and not by the receptor-binding moiety.
One emerging imaging technique with potential applications in neuronal imaging is photoacoustic imaging. Two NMDA receptor-targeted photoacoustic imaging agents were synthesised and their ability to label NMDA receptors assessed in vitro. Finally, preliminary in vivo evaluation of the most promising photoacoustic imaging agent is described.