Neuropeptide release by efficient recruitment of diffusing cytoplasmic secretory vesicles

Neuropeptides are slowly released from a limited pool of secretory vesicles . Despite decades of research, the composition of this pool has remained un known. Endocrine cell studies support the hypothesis that a population of d ocked vesicles supports the first minutes of hormone release. However, it h as been proposed that mobile cytoplasmic vesicles dominate the releasable n europeptide pool. Here, to determine the cellular basis of the releasable p ool, single green fluorescent protein-labeled secretory vesicles were visua lized in neuronal growth cones with the use of an inducible construct or to tal internal reflection fluorescence microscopy. We report that vesicle mov ement follows the diffusion equation. Furthermore, rapidly moving secretory vesicles are used more efficiently than stationary vesicles near the plasm a membrane to support stimulated release. Thus, randomly moving cytoplasmic vesicles participate in the first minutes of neuropeptide release. Importa ntly, the preferential recruitment of diffusing cytoplasmic secretory vesic les contributes to the characteristic slow kinetics and limited extent of s ustained neuropeptide release.