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Abstract

In spite of decades of work by neuroscientists, understanding how our brain functions is still a milestone question. A range of techniques, such as electrical measurements with single or multiple electrodes, pharmacological testing, non-invasive computational methods and BOLD fMRI is being used to address this fundamental issue. None of these techniques provides complete understanding. To achieve a better appreciation of brain function and dysfunction, a multimodal approach is required that can combine the advantages of each technique. We propose the development of a new measurement technique. Our objective is to develop glutamate (Glu) responsive contrast agents (RCAs) to image changes in different parts of the brain upon neural activation. Glutamate is abundantly present in the mammalian central nervous system, and plays a critical role in mediating excitatory signals through both G-proteincoupled metabotropic receptors and ligand-gated ionotropic receptors present on postsynaptic neuronal cells. The metabotropic Glu receptor subtype 5 (mGluR5) is known to be actively involved in transducing excitatory signals via G-protein coupled secondary messengers between neurons through Glu. In this work, we have developed two series of Glu-RCAs containing various specific mGluR5 antagonists (based on aromatic alkyne and dipyridyl/heterobiaryl amide)[1] to exploit their potential application as responsive MR imaging probes. 1) The first set of molecules is derived from GdDOTA where antagonists were integrated into these structures in a modular fashion. 2) The second set of molecules is derived from trans 1,3-N,N-DO2A where 2nd-N of macrocycle was appended with a lumophore to allow visualization using luminescence methods and 4th-N was incorporated with antagonists to target the receptors. The biocompatibility of these Glu-RCAs was evaluated on mGluR5 expressing primary astrocytes. In vitro MR relaxivity measurements showed a significant (~40%) increase in relaxation rate in the presence of high abundance of mGluR5 in cortical primary astrocytes cell suspensions[2]. The reversibility of probe binding to the receptor sites, induced by added Glu, was demonstrated using optical emission, and the antagonistic activity of complexes was defined by calcium binding assays. The lumophore coupled RCAs are being evaluated using in cellulo luminescence measurements assays under appropriate physiological conditions.