DIFFERENTIAL BINDING PROFILE AND INTERNALIZATION PROCESS OF NEUROTENSIN VIA NEURONAL AND GLIAL RECEPTORS

Two G-protein-coupled receptors for the tridecapeptide neurotensin (NT ) have been identified and cloned in mammalian brain: a high-affinity (K-d = 0.3 nM) receptor, sensitive to the antagonist SR 48692 but inse nsitive to levocabastine, and a lower-affinity (K-d = 2-4 nM) receptor , sensitive to levocabastine but with poor affinity for SR 48692. Alth ough there is good evidence that the high-affinity site is predominant ly expressed in neurons, little is known of the cellular localization of the low-affinity receptor. In the present study, we identify by con focal microscopy selective levocabastine-sensitive, SR 48692 resistant binding of a fluorescent derivative of NT (fluo-NT) to a subpopulatio n of glial fibrillary acidic protein-immunoreactive glial cells grown in culture from the midbrain and cerebral cortex of embryonic and neon atal rats, respectively. We also demonstrate, by combining fluo-NT det ection with tyrosine hydroxylase immunofluorescence, that these glial binding sites are differentially regulated from the SR 48692-sensitive NT receptor expressed in the same cultures by mesencephalic dopamine neurons. Whereas the latter undergoes rapid ligand-induced internaliza tion followed by centripetal mobilization of ligand-receptor complexes from processes to perikarya and from perikaryal periphery to cell cen ter, the former induces the formation of cell-surface clusters that fa il to internalize. It is concluded that NT may exert its effects on bo th neurons and astrocytes in the CNS. Whereas NT neural signaling is e xerted through high-affinity receptors and may be partly effected thro ugh internalization of receptor-ligand complexes, glial signaling is e xerted through low-affinity NT receptors and appears to be transduced exclusively at the level of the plasma membrane.