• Giovanna Guerrero, Dierk F Reiff, Dierk F Rieff, Gautam Agarwal, Robin W Ball, Alexander Borst, Corey S Goodman, and Ehud Y Isacoff.
• Department of Molecular and Cell Biology, 279 Life Sciences Addition, University of California Berkeley, Berkeley, California 94720, USA.
• Nat. Neurosci. 2005 Sep 1; 8 (9): 1188-96.

AbstractAt the Drosophila melanogaster larval neuromuscular junction (NMJ), a motor neuron releases glutamate from 30-100 boutons onto the muscle it innervates. How transmission strength is distributed among the boutons of the NMJ is unknown. To address this, we created synapcam, a version of the Ca2+ reporter Cameleon. Synapcam localizes to the postsynaptic terminal and selectively reports Ca2+ influx through glutamate receptors (GluRs) with single-impulse and single-bouton resolution. GluR-based Ca2+ signals were uniform within a given connection (that is, a given bouton/postsynaptic terminal pair) but differed considerably among connections of an NMJ. A steep gradient of transmission strength was observed along axonal branches, from weak proximal connections to strong distal ones. Presynaptic imaging showed a matching axonal gradient, with higher Ca2+ influx and exocytosis at distal boutons. The results suggest that transmission strength is mainly determined presynaptically at the level of individual boutons, possibly by one or more factors existing in a gradient.

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