Internalization and recycling of the CB1 cannabinoid receptor

Tolerance develops rapidly to cannabis, cannabinoids, and related drugs acting at the CBI cannabinoid receptor. However, little is known about what happens to the receptor as tolerance is developing. In this study, we have found that CB1 receptors are rapidly internalized following agonist binding and receptor activation. Efficacious cannabinoid agonists (WIN 55,212-2, CP 55,940, and HU 210) caused rapid internalization. Methanandamide (an analogue of an endogenous cannabinoid, anandamide) was less effective, causing internalization only at high concentration, whereas Delta(9)-tetrahydrocannabinol caused little internalization, even at 3 mu M. CB1 internalized via clathrin-coated pits as sequestration was inhibited by hypertonic sucrose. Internalization did not require activated G protein alpha(i), alpha(o), or alpha(s) subunits. A region of the extreme carboxy terminus of the receptor was necessary for internalization, as a mutant CBI receptor lacking the last 14 residues did not internalize, whereas a mutant lacking the last 10 residues did. Steps involved in the recycling of sequestered receptor were also investigated. Recovery of CB1 to the cell surface after short (20 min) but not long (90 min) agonist treatment was independent of new protein synthesis. Recycling also required endosomal acidification and dephosphorylation. These results show that CB1 receptor trafficking is dynamically regulated by cannabimimetic drugs.