Localization of cellular storage compartments and sites of constitutive and activity-dependent release of nerve growth factor (NGF) in primary cultures of hippocampal neurons

The characteristics of the constitutive and activity-mediated secretion of NGF from native hippocampal slices are the same as those from hippocampal cultures transfected with an NGF-overexpressing plasmid (Blochl and Thoenen, 1995). In these cultures, the distribution of intracellular NGF immunoreactivity-including the co-localization with endoplasmic reticulum (ER) and Golgi markers-in soma, dendrites, and axons, as visualized by confocal microscopy, is compatible with a localization of NGF in an ER-like compartment. Since the positively charged NGF molecule is bound, at the sites of its release, to the negatively charged neuronal surface, at low salt buffer concentrations, it was possible to attribute the different release mechanisms to specific neuronal surface sites. Constitutive secretion of NGF is confined to the neuronal soma and the very proximal parts of dendrites. In contrast, the activity-dependent secretion, initiated by high potassium or glutamate also occurs all along the neuronal processes, in particular dendrites. This release is independent of extracellular calcium, but depends on calcium released from intracellular calcium stores and is mediated by sodium influx via voltage-gated sodium channels and non-NMDA glutamate receptors. The confocal intensity analysis of the NGF surface staining permitted quantitative assessment of the different release mechanisms to different neuronal domains.