Internalization of metabotropic glutamate receptor in C6 cells through clathrin-coated vesicles.

C6 glioma cells were treated for hours with 100 microM L-glutamate, quisqualate or trans-ACPD. In all cases, phospholipase C-coupled metabotropic glutamate receptors (mGluRs) present in these cells are down-regulated after this agonist treatment. Cell surface metabotropic glutamate receptor density was minimum at 6 h of agonist treatment and reached near control values after 30 h of treatment. This recovery was associated with a progressive increase in mGluR1 and mGluR1a mRNA level between 6 and 24 h and was not due to agonist removal. Specific L-[3H]glutamate or 3Htrans-ACPD binding decrease detected in C6 cells after 6 h of 100 microM L-glutamate treatment was associated with a remarkable increase of specific L-[3H]glutamate binding detected in clathrin-coated vesicles isolated from these treated cells. Moreover, this decrease was blocked in the presence of 0.5 M sucrose or 1 microM phenylarsine oxide, suggesting that desensitization and down-regulation of mGluR can be due to an endocytosis process through clathrin-coated pits and vesicles.