Antidepressant-induced Switch of β₁-Adrenoceptor Trafficking as a Mechanism for Drug Action*

Abstract

Reduction in surface β₁-adrenoceptor (β1AR) density is thought to play a critical role in mediating the therapeutic long term effects of antidepressants. Since antidepressants are neither agonists nor antagonists for G protein-coupled receptors, receptor density must be regulated through processes independent of direct receptor activation. Endocytosis and recycling of the β1AR fused to green fluorescent protein at its carboxyl-terminus (β1AR-GFP) were analyzed by confocal fluorescence microscopy of live cells and complementary ligand binding studies. In stably transfected C6 glioblastoma cells, β1AR-GFP displayed identical ligand-binding isotherms and adenylyl cyclase activation as native β1AR. Upon exposure to isoproterenol, a fraction of β1AR-GFP (10–15%) internalized rapidly and colocalized with endocytosed transferrin receptors in an early endosomal compartment in the perinuclear region. Chronic treatment with the tricyclic antidepressant desipramine (DMI) did not affect internalization characteristics of β1AR-GFP when challenged with isoproterenol. However, internalized receptors were not able to recycle back to the cell surface in DMI-treated cells, whereas recycling of transferrin receptors was not affected. Endocytosed receptors were absent from structures that stained with fluorescently labeled dextran, and inhibition of lysosomal protease activity did not restore receptor recycling, indicating that β1AR-GFP did not immediately enter the lysosomal compartment. The data suggest a new mode of drug action causing a “switch” of receptor fate from a fast recycling pathway to a slowly exchanging perinuclear compartment. Antidepressant-induced reduction of receptor surface expression may thus be caused by modulation of receptor trafficking routes.