Immunologic differentiation of two high-affinity neurotensin receptor isoforms in the developing rat brain

Earlier studies have demonstrated overexpression of NT1 neurotensin recepto rs in rat brain during the first 2 weeks of Life. To gain insight into this phenomenon, we investigated the identity and distribution of NT1 receptor proteins in the brain of 10-day-old rats by using two different NT1 antibod ies: one (Abi3) directed against the third intracellular loop and the other (Abi4) against the C-terminus of the receptor. Immunoblot experiments that used Abi3 revealed the presence of two differentially glycosylated forms o f the NT1 receptor in developing rat brain: one migrating at 54 and the oth er at 52 kDa. Whereas the 54-kDa form was expressed from birth to adulthood , the 52-kDa form was detected only at 10 and 15 days postnatal. Only the 5 2-kDa isoform was recognized by Abi4. By immunohistochemistry, both forms o f the receptor were found to be predominantly expressed in cerebral cortex and dorsal hippocampus, in keeping with earlier radioligand binding and in situ hybridization data. However, whereas Abi4 immunoreactivity was mainly concentrated within nerve cell bodies and extensively colocalized with the Golgi marker a-mannosidase II, Abi3 immunoreactivity was predominantly loca ted along neuronal processes. These results suggest that the transitorily e xpressed 52-kDa protein corresponds to an immature, incompletely glycosylat ed and largely intracellular form of the NT1 receptor and that the 54-kDa p rotein corresponds to a mature, fully glycosylated, and largely membrane-as sociated form. They also indicate that antibodies directed against differen t sequences of G-protein-coupled receptors may yield isoform-specific immun ohistochemical labeling patterns in mammalian brain. Finally, the selective expression of the short form of the NT1 receptor early in development sugg ests that it may play a specific role in the establishment of neuronal circ uitry. J. Comp. Neurol. 425: 45-57, 2000. (C) 2000 Wiley-Liss, Inc.