Murine neuroblastoma clone N1E-115 cells possess neurotensin (NT) receptors, which are coupled to signal transduction systems resulting in polyphosphoinositide (Pl) hydrolysis and cyclic GMP synthesis. Previously, we have demonstrated that the process of down-regulation of NT receptors in N1E-115 cells involves intracellular sequestration of recyclable receptors followed by receptor degradation, causing true down-regulation. In this study, agonist-induced sequestration of NT receptors in N1E-115 cells was inhibited by an aminosteroid, 1-(6-([17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino)hexyl)-1H-pyrrole- 2,5-diane (U-73122). Pl hydrolysis elicited by NT or sodium fluoride, which stimulates GTP binding proteins, was also inhibited by U-73122, whereas Pl hydrolysis elicited by calcium ionophores, ionomycin or A23187, was not apparently affected. These data suggest that U-73122 affects a process that is distal to the cell surface receptor but not involving the sites just proximal to Pl hydrolysis or cyclic GMP synthesis. It is suggested that U-73122 may affect the coupling of GTP binding proteins and the NT receptor. We conclude that GTP binding proteins play an important role in the mechanism of agonist-induced down-regulation of NT receptors in N1E-115 cells. These results may indicate that GTP binding proteins also play a role in the mechanism of internalization of this receptor in the central nervous system in vivo.