Distinct calcium signaling within neuronal growth cones and filopodia

Previous findings indicate that spatial restriction of intracellular calcium levels within growth cones can regulate growth cone behavior at many levels, ranging from filopodial disposition to neurite extension. By combining techniques for focal stimulation of growth cones with those for measurement of filopodia and for capturing low intensity calcium signals, we demonstrate that filopodia on individual growth cones can respond to imposed stimuli independently from one another. Moreover, filopodia and their parent growth cones appear to represent functionally and morphologically distinct domains of calcium regulation, possessing distinct calcium sources and sinks. Both are sensitive to calcium influx; however, application of the calcium ionophore A23187 to cells in calcium-free medium demonstrated the presence of potential intracellular calcium pools in the growth cone proper, but not in isolated filopodia. Thapsigargin significantly reduced the rise in growth cone calcium levels associated with excitatory neurotransmitters, further implicating release from calcium pools as one component of growth cone calcium regulation. The relative contributions of these pools were examined in response to excitatory neurotransmitters by quantitative calcium measurements made in both growth cones and isolated filopodia. Striking differences were observed; filopodia were sensitive to a low concentration of dopamine and serotonin, while growth cones displayed an amplified rise at a higher concentration. The spatial distribution of organelles that could serve as morphological correlates to such calcium amplification was examined using confocal microscopy. While the majority of organelles were located in the central core of the growth cone proper, peripheral organelles were detected at the base of a subset of filopodia. The distinctive distribution of calcium regulation within motile growth cones suggests one mechanism by which growth cones may regulate their complex behavior.