Imaging of Calcium in Drosophila Larval Motor Nerve Terminals

Karunanithi, Shanker, John Georgiou, Milton P. Charlton, and Harold L. Atwood. Imaging of calcium in Drosophila larval motor nerve terminals. J. Neurophysiol. 78: 3465–3467, 1997. Calcium measurements in the presynaptic terminal are essential in the investigation of mechanisms underlying neurotransmitter release. To enhance the genetic analysis of secretory mechanisms, we have developed Ca 2+ imaging techniques for Drosophila larval motor nerve terminals. We studied Ca 2+ signals in “big” (type Ib) and “small” (type Is) boutons that innervate ventral longitudinal muscles 6 and 7 in each abdominal segment of Canton-S (CS)-strain 3rd instar larvae. The indicator fluo-3 in conjunction with confocal microscopy was used to detect stimulus-dependent changes in [Ca 2+ ] i . The Ca 2+ signals were reliable and reproducible, and the resting fluorescence remained constant throughout the experiments. The Ca 2+ signals increased with stimulus frequency from 5 to 20 Hz for both bouton types. No significant differences in the Ca 2+ signals were seen between the two bouton types at 5 and 20 Hz, but there was a difference at 10 Hz. The decay of the Ca 2+ signal was more prolonged after 20-Hz stimulation than after 5 and 10 Hz. At the single-synapse level, the secretory efficacy of Is synapses is greater than that of Ib synapses, but our data show that factors other than differences in Ca 2+ entry may govern the strength of synaptic transmission.