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Differential Involvement of Intracellular Domains of the Rat NTS1 Neurotensin Receptor in Coupling to G Proteins: A Molecular Basis for Agonist-Directed Trafficking of Receptor Stimulus

Institut National de la Santé et de la Recherche Médicale, Unité 339, Hôpital Saint-Antoine, Paris, France (D.S., A.-M.L., A.L., P.F., M.Y., W.R., D.P.); and Laboratoire de Pharmacologie Expérimentale, Université Catholique de Louvain 54.10, Brussels, Belgium (E.H.)

In this work, we evidenced characteristic features of agonist-induced trafficking of receptor stimulus for the rat neurotensin receptor 1 (NTS1). Thus, reverse potency orders between two agonists, EISAI-1 and neuromedin N, were observed in inositol 1,4,5-trisphosphate and cAMP assays in Chinese hamster ovary cells transfected with this receptor. Indeed, compared with other agonists, EISAI-1 presented lower relative potency toward inositol 1,4,5-trisphosphate production than toward cAMP accumulation, guanosine 5'-O -(3-[³⁵ S]thio)triphosphate binding, and [³H]arachidonic acid production. These results indicated pathway-dependent differences in EISAI-1 intrinsic efficacies, favoring activations of G_s- and G_i/o-related pathways over the G_q/11-related pathway. Moreover, although coupling to G_q/11 and G_i/o involved the third intracellular loop and the C-terminal domain of the NTS1 receptor, respectively, we demonstrated that deletion of the latter domain suppressed agonist-induced cAMP accumulation, suggesting that this domain also mediated coupling to G_s. Together, these results indicated that, unlike other agonists, EISAI-1 discriminated between the pathways involving the receptor C-terminal domain and that involving the third intracellular loop. These properties of EISAI-1 were also observed in cortical neurons endogenously expressing the NTS1 receptor. They were further attributed to the functionalization of its COOH end by an ethyl group, because the unesterified analog EISAI-2 presented normal behavior on inositol 1,4,5-trisphosphate production.

These findings support the hypothesis of agonist-selective receptor states with distinct conformations or accessibilities of intracellular domains. They also suggest that the differential involvement of these domains in coupling to G proteins might represent a molecular basis for agonist-selective responses through G protein-coupled receptors.

Address correspondence to: Dr. D. Pelaprat, INSERM U339, Hôpital Saint-Antoine, 184 rue du faubourg Saint-Antoine, 75571 Paris Cedex 12, France. E-mail: pelaprat{at}st-antoine.inserm.fr

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