Heterogeneity in synaptic transmission along a Drosophila larval motor axon

Abstract

At 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. *Note: In the version of this article initially published online, the second author’s name was misspelled. The correct spelling should be Dierk F Reiff. The error has been corrected in the HTML version of the article. This correction has been appended to the PDF and print versions.