In vivo Proton NMR spectroscopy of genetic mouse models BALB/cJ and C57BL/6By: variation in hippocampal glutamate level and the metabotropic glutamate receptor, subtype 7 (Grm7) gene.

Glutamatergic neurotransmission in the brain is modulated by metabotropic glutamate receptors (mGluR). In recent studies, we identified a cis-regulated variant of a gene (Grm7) which codes for mGluR subtype 7 (mGluR7), a presynaptic inhibitory receptor. The genetic variant derived from the BALB/cJ mouse strain (Grm7 (BALB/cJ)) codes for higher abundance of mGluR7 mRNA in the hippocampus than the C57BL/6By strain-derived variant (Grm7 (C57BL/6By)). Here, we used localized in vivo (1)H NMR spectroscopy to test the hypothesis that Grm7 (BALB/cJ) is also associated with lower glutamate concentration in the same brain region. All data were obtained on a 7.0 T Agilent (Santa Clara, CA, USA) 40-cm bore system using experimentally naive adult male inbred C57BL/6By, BALB/cJ, and congenic mice (B6By.C.6.132.54) constructed in our laboratory carrying Grm7 (BALB/cJ) on C57BL/6By genetic background. The voxel of interest size was 6 μL (1 × 2 × 3 mm(3)) placed in the hippocampal CA1 region. The results showed that the hippocampal level of glutamate in the congenic mouse strain was significantly lower than that in the background C57BL/6By strain which carried the Grm7 (C57BL/6By) allele. Because the two inbred strains are genetically highly similar except at the region of the Grm7 gene, the results raise the possibility that allelic variation at the Grm7 locus contributes to the strain differences in both hippocampal mRNA abundance and glutamate level which may modulate complex behavioral traits, such as learning and memory, addiction, epilepsy, and mood disorders.