Development and maintenance of a simple reflex circuit in small‐patch mosaics of Drosophila: Effects of altered neuronal function and developmental arrest

Abstract

A combined genetic, anatomical, and behavioral approach has been undertaken to study the developmental and functional plasticity of identified bristle mechanosensory neurons in Drosophila. A stereotyped grooming reflex in decapitated flies enabled simple but reliable assessments of the functional output of individual bristle sensory cells to correlate with their axonal projections and terminal arbors revealed by the cobalt backfill technique. Construction of small-patch mosaics that contain only a single mutant bristle allowed functional perturbation of individual neurons within an otherwise normal environment. Mutations that affect nerve excitability and membrane recycling have been used to examine their effects on neuronal pathfinding, arborization, and the initiation and maintenance of functional connections. Previous studies (Burg and Wu, 1986, J. Neurosci. 6:2968-2976; 1989, Dev. Biol. 131:505-514) have demonstrated that para(ts)nap(ts) double-mutant sensory neurons, in which action potentials are unconditionally blocked by defects in sodium currents, and eag Sh double-mutant sensory cells, in which membrane excitability is increased by alterations in potassium currents, can establish and maintain central projections that are indistinguishable from their functionally normal counterparts. Mutations of the shi(ts) gene cause a temperature-sensitive, reversible block of the membrane recycling process, resulting in arrest of neuronal growth in culture (Kim and Wu, 1987, J. Neurosci. 7:3245-3255) and depletion of synaptic vesicles that leads to transmission blockade at established synapses (Ikeda, Ozawa, and Hagiwara, 1976, Nature 259:489-491; Koenig and Ikeda, 1983, J. Neurobiol. 14:411-419; 1989, J. Neurosci. 9:3844-3860). Prolonged heat treatments (up to 16% of total development time) of small-patch shi(ts) mosaics at different pupal stages did not prevent the establishment of central projections characteristic of the various sensory cell types. However, none of the shi(ts) sensory neurons heat-pulsed during the initial or the final 16% of pupal development were able to initiate the reflex behavior, although a proportion of those treated in other periods apparently established functional contacts with appropriate targets to support the characteristic cleaning reflex. The possibility exists that the membrane recycling process blocked in shi(ts) cells provides a crucial mechanism for cell-cell interactions taking place during initial differentiation and final synaptic stabilization, and possibly competition, in the developing sensory neuron. Heat treatments of adult shi(ts) mosaics blocked the reflex initiated by the mutant (but not the surrounding normal) bristles, as expected from the effect of synaptic block.(ABSTRACT TRUNCATED AT 400 WORDS)

Altered mechanoreceptor response in Drosophila bang-sensitive mutants

“…An aspect of sensory neuron physiology that has not been examined here is synaptic transmission at axonal terminals. The CNS projections of the APA sensory neuron have been characterized (Ghysen 1980; Burg et al 1993), and they are not accessible with the current experimental approach. It is still possible that hyperexcitation of the nerve terminals may result in "sensory overload", which may be suppressed by nap Is.…”

Social interaction-mediated lifespan extension of Drosophila Cu/Zn superoxide dismutase mutants

“…3B). The most extreme case of reduced locomotive activity was achieved by using decapitated helpers (with heads glued back with petroleum jelly to avoid dehydration), which are known to perform several reflex behaviors, including cleaning and grooming (25). The decapitated helpers stood motionlessly but were able to passively respond to Sod flies' approach with leg-kicking and wing-flipping (data not shown).…”

“…Unless stated otherwise, 5 helpers were added to each cohousing vial. All lifespan assays were carried out in the incubators set at the indicated temperature (17,23,25, and 29°C) under 12/12 L/D cycles, except for the constant dark (DD) experiments.…”

Brain plasticity in Diptera and Hymenoptera

“…Small patches of sensilla can be made mutant for a temperature-sensitive allele of shibire , which encodes dynamin that acts during clathrin mediated endocytosis at nerve terminals (27). In such flies, sensory axons from the mosaic patches of shibire mutant sensilla project into their target neuropile and form terminals that look normal (28), but brief exposure to a non-permissive temperature temporarily blocks their reflex responses to stimulation, irreversibly so if the heat pulse is 8 hours long (28). Despite the irreversible reflex blockade induced by heat pulses of long duration, terminals of the corresponding non-transmitting mechanoreceptor axons fail to change their shape, indicating that the structure of a terminal arborisation is maintained even when the terminal is non-functional.…”