Glutamate is a major excitatory neurotransmitter at neuronal synapses in the central nervous system (CNS) (1, 2). Glutamate produces excitatory effects by acting on cell-surface ionotropic glutamate or metabotropic glutamate receptors (mGluRs). The mGluRs are GTP-binding protein (G-protein)–coupled receptors that play important roles in regulating the activity of many synapses in the CNS, and many neuronal projection pathways contain mGluRs. There are eight mGluR subtypes, which are further subdivided into groups I, II, and III. The group I receptors include mGluR1 and mGluR5, and they are found predominantly in postsynaptic locations. mGluR1 is found in moderate to high density in the cerebellum, caudate, putamen, thalamus, cingulate cortex, and hippocampus, with low density in the pons. mGluR5 is usually found in moderate to high density in the frontal cortex, caudate, putamen, nucleus accumbens, olfactory tubercle, and hippocampus, with low density in the cerebellum. mGluR1 and mGluR5 are positively coupled to phospholipase C in the regulation of neuronal excitability (3). Dysfunction of mGluR1 and mGluR5 is implicated in a variety of diseases in the CNS, including anxiety, depression, schizophrenia, Parkinson’s disease, and drug addiction or withdrawal (2, 4).
Positron emission tomography (PET) and single-photon emission tomography of radioligands targeting mGluR1 can visualize and analyze mGluR1 expression in normal physiological and pathological conditions. Several radioligands have been studied for in vivo imaging of mGluR1 in the brain. 1-(2-Fluoro-3-pyridyl)-4-(2-isopropyl-1-oxo-isoindoline-5-yl)-5-methyl-1H-1,2,3-triazole (FPIT) was shown to be a selective mGluR1 with nanomolar affinity (5.4 nM) with little inhibition to mGluR5 (5). Fujinaga et al. (6) prepared and evaluated 1-(2-[18F]Fluoro-3-pyridyl)-4-(2-isopropyl-1-oxo-isoindoline-5-yl)-5-methyl-1H-1,2,3-triazole ([18F]FPIT) for use with in vivo PET imaging of mGluR1 distribution in the brain with small animals and monkeys.