Null mutation in shaking‐B eliminates electrical, but not chemical, synapses in the Drosophila giant fiber system: A structural study

Mutations in the Drosophila shaking‐B gene perturb synaptic transmission and dye coupling in the giant fiber escape system. The GAL4 upstream activation sequence system was used to express a neuronal‐synaptobrevin‐green fluorescent protein (nsyb‐GFP) construct in the giant fibers (GFs); nsyb‐GFP was localized where the GFs contact the peripherally synapsing interneurons (PSIs) and the tergotrochanteral motorneurons (TTMns). Antibody to Shaking‐B protein stained plaquelike structures in the same regions of the GFs, although not all plaques colocalized with nsyb‐GFP. Electron microscopy showed that the GF‐TTMn and GF‐PSI contacts contained many chemical synaptic release sites. These sites were interposed with extensive regions of close membrane apposition (3.25 nm ± 0.12 separation), with faint cross striations and a single‐layered array of 41‐nm vesicles on the GF side of the apposition. These contacts appeared similar to rectifying electrical synapses in the crayfish and were eliminated in shaking‐B2 mutants. At mutant GF‐TTMn and GF‐PSI contacts, chemical synapses and small regions of close membrane apposition, more similar to vertebrate gap junctions, were not affected. Gap junctions with more vertebratelike separation of membranes (1.41 nm ± 0.08) were abundant between peripheral perineurial glial processes; these were unaffected in the mutants. J. Comp. Neurol. 404:449–458, 1999. © 1999 Wiley‐Liss, Inc.