Radiolabeled Ligands Specific for the G Protein‐Coupled State of Neurotensin Receptors

: Radiolabeled analogues of neuromedin N have been prepared by acylation of the α, ε1, and ε2 amino groups of [Lys2]neuromedin N (Lys‐Lys‐Pro‐Tyr‐Ile‐Leu) either with the 125I‐labeled Bolton‐Hunter reagent or with N‐succinimidyl[2,3‐3H]propionate. The binding properties of the purified analogues toward newborn mouse brain homogenate or toward membranes of cells transitorily (COS) or permanently (AA1) transfected with the cloned rat brain neurotensin receptor cDNA were evaluated and compared with those of radiolabeled neurotensin. The α‐modified analogue of [Lys2]neuromedin N behaves exactly like neurotensin in these binding experiments, whereas the ε1‐ and ε2‐modified analogues selectively recognize the fraction of neurotensin binding sites that is sensitive to GTPγS. The proportion of neurotensin receptors coupled to GTP binding proteins is ∼50% in membranes of newborn mouse brain or of AA1 cells that respond to neurotensin by an increase of the intracellular inositol trisphosphate concentration. By contrast, membranes of transitorily transfected COS cells that do not respond to neurotensin exhibit very low levels of GTP‐sensitive receptors labeled with the ε1‐ or ε2‐modified analogues. These radiolabeled peptides offer new tools to selectively detect active neurotensin receptors.