The C-Terminal Domain of the mGluR1 Metabotropic Glutamate Receptor Affects Sensitivity to Agonists

The metabotropic glutamate receptor (mGluR) subtype 1 exists as at least three variants (-1a, -1b, and -1c) generated by alternative splicing at the C-terminal domain. Fluorometric Ca²⁺ measurements were used to compare the concentration dependency of agonist-induced rises in intracellular free Ca²⁺ concentration ([Ca²⁺](i)) in human embryonic HEK 293 cells transiently expressing rat mGluR1a, mGluR1b, or mGluR1c. The rank order of agonist potencies was quisqualate >> (2S, 1'S,2'S)-2- (carboxycyclopropyl)glycine (L-CCG-I) > (1S,3R)-1-aminocyclopentane-1,3- dicarboxylic acid [(1S,3R)-ACPD] and did not differ among the splice variants. However, agonists were consistently more potent at mGluR1a than at mGluR1c and mGluR1b. In the same system, we characterized the agonist pharmacology of two chimeric rat mGluR3/1 receptors where the first and/or the second intracellular loop(s) and the C-terminal domain were exchanged with the corresponding mGluR1a or mGluR1c sequences and that were previously shown to mediate elevations in [Ca²⁺](i) in response to agonists. The potency of agonists was higher at the chimera having the C-terminus of mGluR1a as compared with those having the mGluR1c C-terminus. Both chimeric mGluR3/1 receptors had the same rank order of agonist potencies: L-CCG-I >> (1S,3R)-ACPD ~ quisqualate. These data support the hypothesis that the C- terminal domain of mGluRs plays a role in determining the potency of agonists for inducing mGluR-mediated functional responses.