The role of retrograde signaling between target cells and their presynaptic partners during early cell-cell interactions was examined in cell culture. Using time-lapse video microscopy and fura-2 calcium analysis, we followed contacts between presynaptic neurites of Helisoma motoneurons and muscle fibers dissociated from identified partners in chemical synaptogenesis. Cytosolic calcium rose dramatically at the region of cell-cell contact and subsequently increased throughout the entire neuron. Changes in the calcium set-point of presynaptic neurons were maintained even when connections with muscle targets were severed. Changes in presynaptic calcium concentration were not detected following contact with partners in electrical synapse formation. These data, taken together with the fact that calcium changes coincide with the time course of chemical synaptogenesis in these neuron-muscle cultures, suggest that calcium changes may be involved in early stages of synapse formation.
