Immunoelectron microscopic evidence of nerve growth factor receptor metabolism and internalization in rat nucleus basalis neurons

Nerve growth factor receptor (NGFr) immunoreactive neurons of the adult rat nucleus basalis magnocellularis were examined by electron microscopy. Prominent NGFr immunoreactivity (IR) was consistently present along the perikaryal cell membrane and frequently in intracellular sites of protein synthesis and modification such as the rough endoplasmic reticulum and Golgi apparatus, respectively. Immunoperoxidase reaction product was also seen along the nuclear membrane. Membrane-bound aggregates of immunoreactive vesicles were scattered throughout the perikaryon, being more concentrated in the perinuclear region and in the proximal neurites. These may represent either aggregates of receptor-containing vesicles on their way to/from the cell membrane or secondary lysosomes where NGFr reaction product is degraded. Immunostained cytoplasmic vesicles which possessed an electron-dense coat and were adjacent to or contiguous with the plasmalemma probably represented internalized receptor. This ultrastructural study of the subcellular distribution of NGFr-IR in basal forebrain neurons therefore demonstrates sites of receptor metabolism and potential receptor-ligand interaction.