Neurotensin (NT) receptors NTS1 and NTS2 are known to display considerable distributional overlap in mammalian central nervous system (CNS). Using co-immunoprecipitation approaches, we demonstrated here that NTS1 forms constitutive heterodimers with NTS2 in transfected COS-7 cells. We also showed that co-expression of NTS2 with NTS1 markedly decreases the cell surface density of NTS1 without affecting ERK1/2 MAPK activity or NT-induced NTS1 internalization. However, radioligand-binding studies indicated that upon prolonged NT stimulation, cell surface NTS1 receptors are more resistant to down-regulation in cells co-expressing NTS1 and NTS2 than in cells expressing NTS1 alone. Taken together, these data suggest that NTS1/NTS2 heterodimerization affects the intracellular distribution and trafficking of NTS1 by making it more similar to that of NTS2 as witnessed in cells expressing NTS2 alone. NTS1/NTS2 heterodimerization might therefore represent an additional mechanism in the regulation of NT-triggered responses mediated by NTS1 and NTS2 receptors. (c) 2006 Elsevier Inc. All rights reserved.