We investigated the regulatory pathways responsible for agonist-induced internalization and down-regulation of G(q) protein-coupled histamine H(1)-receptors in Chinese hamster ovary cells. Histamine-induced internalization and down-regulation of H(1)-receptors were detected as the loss of [(3)H]mepyramine binding sites on intact cells accessible to hydrophilic and hydrophobic H(1)-receptor antagonists, pirdonium and mepyramine, respectively. Pretreatment of cells with 0.1 mM histamine for 30 min at 37 degrees C induced internalization as well as down-regulation of H(1)-receptors, both of which were inhibited either in the presence of an inhibitor against G protein-coupled receptor kinases (ZnCl(2)) or under hypertonic conditions where clathrin-dependent endocytosis is known to be inhibited, but were not affected by inhibitors against caveolae/raft-dependent endocytosis (filipin and nystatin). Down-regulation of H(1)-receptors, but not their internalization, was inhibited by protein kinase C inhibitors (chelerythrin or GF109203X), a ubiquitin E1 inhibitor (UBEI-41) and proteasome inhibitors (lactacystin and MG-132). Neither a Ca(2+)/calmodulin-dependent protein kinase II inhibitor (KN-62) nor lysosomal protease inhibitors (E-64, leupeptin, chloroquine and NH(4)Cl) affected the internalization and down-regulation of H(1)-receptors. These results suggest that H(1)-receptors internalize upon agonist stimulation via G protein-coupled receptor kinase/clathrin-dependent but caveolae/raft-independent mechanisms and are delivered to proteasomes, preferentially to lysosomes, for their prompt down-regulation.