Regulation by insulin of a unique neuronal Ca2+ pool and of neuropeptide secretion

The insulin receptor is a tyrosine kinase receptor that is found in mammalian brain(1) and at high concentrations in the bag cell neurons of Aplysia(2). We show here that insulin causes an acute rise in intracellular Ca2+ concentration ([Ca2+](i)) in these neurons and triggers release of neuropeptide. The insulin-sensitive intracellular Ca2+ pool differs pharmacologically from previously described Ca2+ stores that are sensitive to inositol trisphosphate and from mitochondrial Ca2+ stores(3-7). Insulin, but not thapsigargin, stimulates Ca2+ release at the distal tips of neurites, the presumed site of neuropeptide secretion(8,9). The effects of insulin on intracellular Ca2+ release and neuropeptide secretion occur without triggering spontaneous action potentials. The insulin-sensitive rise in [Ca2+](i) moves into the distal tips of neurites after exposure to a cyclic AMP analogue, a treatment that causes a similar translocation of neuronal vesicles(10-12). Our data indicate that Ca2+ release from a distinct intracellular pool associated with secretory vesicles may contribute to secretion of neuropeptide in the absence of neuronal discharge.