In Vivo Preclinical Molecular Imaging of Repeated Exposure to an N-methyl-d-aspartate Antagonist and a Glutaminase Inhibitor as Potential Glutamatergic Modulators.

Abstract

Glutamate is the principal excitatory neurotransmitter in the brain and is at the base of a wide variety of neuropathologies, including epilepsy, autism, Fragile X, and obsessive compulsive disorder. Glutamate has also become the target for novel drugs in treatment and in fundamental research settings. However, much remains unknown on the working mechanisms of these drugs and the effects of chronic administration on the glutamatergic system. This study investigated the chronic effects of two glutamate-modulating drugs with imaging techniques to further clarify their working mechanisms for future research opportunities. Animals were exposed to saline (1 ml/kg), (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) (0.3 mg/kg), or ebselen (10 mg/kg) for 7 consecutive days. At the sixth injection, animals underwent a positron emission tomography (PET)/computed tomography (CT) with (3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-O-11C-methyl-oxime) (ABP-688) to visualize the metabotropic G protein-coupled glutamate receptor 5 (mGluR5). After the seventh injection, animals underwent a magnetic resonance spectroscopy (MRS) scan to visualize glutamate and glutamine content. Afterward, results were verified by mGluR5 immunohistochemistry (IHC). PET/CT analysis revealed that animals receiving chronic MK-801 or ebselen had a significant (P < 0.05) higher binding potential (2.90 ± 0.47 and 2.87 ± 0.46, respectively) when compared with saline (1.97 ± 0.39) in the caudate putamen. This was confirmed by mGluR5 IHC, with 60.83% ± 6.30% of the area being highlighted for ebselen and 57.14% ± 9.23% for MK-801 versus 50.21% ± 5.71% for the saline group. MRS displayed significant changes on the glutamine level when comparing chronic ebselen (2.20 ± 0.40 µmol/g) to control (2.72 ± 0.34 µmol/g). Therefore, although no direct effects on glutamate were visualized, the changes in glutamine suggest changes in the total glutamate-glutamine pool. This highlights the potential of both drugs to modulate glutamatergic pathologies.