Determinants of metabotropic glutamate receptor-5-mediated Ca ²⁺ and inositol 1,4,5-trisphosphate oscillation frequency: Receptor density versus agonist concentration

Diverse patterns of Ca ²⁺ _i release differentially regulate Ca ²⁺-sensitive enzymes and gene transcription, and generally the extent of agonist activation of phospholipase C-linked G protein-coupled receptors determines the type of Ca ²⁺ signal. We have studied global Ca ²⁺ oscillations arising through activation of the metabotropic glutamate receptor mGluR5a expressed in Chinese hamster ovary cells and find that these oscillations are largely insensitive to agonist concentration. Using an inducible receptor expression system and a non-competitive antagonist, in conjunction with the translocation of eGFP-PH _PLCδ to monitor inositol 1,4,5-trisphosphate (InsP ₃) oscillations in single cells, we show that mGluR5a density determines the frequency of these oscillations. The predominant underlying mechanism resulted from a negative feedback loop whereby protein kinase C (PKC) inhibited InsP ₃ generation. Down-regulation of PKC by prolonged exposure to phorbol ester revealed a second form of Ca ²⁺ _i oscillation at low agonist concentrations. These Ca ²⁺ _i signals showed features typical of classic repetitive Ca ²⁺ _i-induced Ca ²⁺ release and were sensitive to agonist concentration. Therefore, a single receptor can stimulate two types of InsP ₃-mediated Ca ²⁺ signal dependent upon feedback inhibition, producing two distinct means of controlling the final pattern of Ca ²⁺ _i release. Our results have physiological implications for Ca ²⁺ signaling in general and emphasize the importance of mGluR5 surface expression for modulating synaptic plasticity.