Despite the importance of the insect nervous system for functional and developmental neuroscience, descriptions of insect brains have suffered from a lack of uniform nomenclature. Ambiguous definitions of brain regions and fiber bundles have contributed to the variation of names used to describe the same structure. The lack of clearly determined neuropil boundaries has made it difficult to document precise locations of neuronal projections for connectomics study. To address such issues, a consortium of neurobiologists studying arthropod brains, the Insect Brain Name Working Group, has established the present hierarchical nomenclature system, using the brain of Drosophila melanogaster as the reference framework, while taking the brains of other taxa into careful consideration for maximum consistency and expandability. The following summarizes the consortium's nomenclature system and highlights examples of existing ambiguities and remedies for them. This nomenclature is intended to serve as a standard of reference for the study of the brain of Drosophila and other insects.

In Drosophila, the most widely used system for generating spatially restricted transgene expression is based on the yeast GAL4 protein and its target upstream activating sequence (UAS). To permit temporal as well as spatial control over UAS-transgene expression, we have explored the use of a conditional RU486-dependent GAL4 protein (GeneSwitch) in Drosophila. By using cloned promoter fragments of the embryonic lethal abnormal vision gene or the myosin heavy chain gene, we have expressed GeneSwitch specifically in neurons or muscles and show that its transcriptional activity within the target tissues depends on the presence of the activator RU486 (mifepristone). We used available UAS-reporter lines to demonstrate RU486-dependent tissuespecific transgene expression in larvae. Reporter protein expression could be detected 5 h after systemic application of RU486 by either feeding or ''larval bathing.'' Transgene expression levels were dose-dependent on RU486 concentration in larval food, with low background expression in the absence of RU486. By using genetically altered ion channels as reporters, we were able to change the physiological properties of larval bodywall muscles in an RU486-dependent fashion. We demonstrate here the applicability of GeneSwitch for conditional tissue-specific expression in Drosophila, and we provide tools to control pre-and postsynaptic expression of transgenes at the larval neuromuscular junction during postembryonic life.

The distribution and morphology of glutamatergic synapses on Drosophila bodywall muscle fibers were examined at the single-synapse level using immunocytochemistry and electrophysiology. We find that glutamate-immunoreactive motor endings innervate the entire larval bodywall musculature, with each muscle fiber receiving at least one glutamatergic ending. The innervation is initiated at stereotyped locations on each muscle fiber from where moderately branched varicose nerve processes project over the internally facing muscle surface. Individual muscle fibers have distinct stereotypic patterns of nerve endings that occupy characteristic regions on the cell surface. The muscle-specific branching pattern of motor endings is reiterated by segmentally homologous fibers. Two morphological types of innervating nerve processes can be distinguished by their bouton size distributions: (1) Type I processes, which have localized branching and a broad size distribution of relatively large varicosities ranging up to 8 microns (mean diameter, 3.1 +/- 1.6 microns; +/- SD, n = 521), and (2) thinner Type II processes, which have a narrower distribution of small varicosities with a mean diameter of only 1.4 +/- 0.6 microns (+/- SD, n = 214). Immunoelectron microscopy with peroxidase-labeled second antibody demonstrates that the varicosities are surrounded by a subsynaptic reticulum, that they contain immunoreactive vesicles of about 30-50 nm, and thus probably represent synaptic release sites. By iontophoretic application of glutamate we mapped the responsive sites on the muscle surface and found an excellent correspondence between transmitter sensitivity and the patterns of endings as described by immunocytochemistry. In contrast to our finding of numerous glutamate iontophoresis-sensitive sites, we did not detect any aspartate-responsive muscles. These data provide strong new evidence for glutamate being an endogenous transmitter at the Drosophila larval neuromuscular junction.

The Drosophila neuromuscular junction has attracted widespread attention as an excellent model system for studying the cellular and molecular mechanisms of synaptic development and neurotransmission. In Drosophila the advantages of invertebrate small systems, where individual cells can be examined with single-cell resolution, are combined with the powerful techniques of patch-clamp analysis and molecular genetics. In this review we examine myogenesis and motoneuron development, the problems of axon outgrowth and target selection, the differentiation of the synapse, and the mechanisms of both synaptic function and plasticity in this model genetic system.

Fray is a serine/threonine kinase expressed by the peripheral glia of Drosophila, whose function is required for normal axonal ensheathment. Null fray mutants die early in larval development and have nerves with severe swelling and axonal defasciculation. The phenotype is associated with a failure of the ensheathing glia to correctly wrap peripheral axons. When the fray cDNA is expressed in the ensheathing glia of fray mutants, normal nerve morphology is restored. Fray belongs to a novel family of Ser/Thr kinases, the PF kinases, whose closest relatives are the PAK kinases. Rescue of the Drosophila mutant phenotype with PASK, the rat homolog of Fray, demonstrates a functional homology among these proteins and suggests that the Fray signaling pathway is widely conserved.