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.

Motivational states are important determinants of behavior. In fruit flies appetitive memory expression is constrained by satiety and promoted by hunger. Here we identify a neural mechanism that integrates the motivational state of hunger and memory. We show that stimulation of neurons that express Neuropeptide F (dNPF), an ortholog of mammalian NPY, mimicks food-deprivation and promotes memory performance in satiated flies. Robust appetitive memory performance requires the dNPF receptor in six dopaminergic neurons that innervate a distinct region of the mushroom bodies. Blocking these dopaminergic neurons releases memory performance in satiated flies whereas stimulation suppresses memory performance in hungry flies. Therefore dNPF and dopamine provide a motivational switch in the mushroom body that controls the output of appetitive memory.

Drosophila vision is mediated by inputs from three types of photoreceptor neurons: R1–R6 mediate achromatic motion detection while R7 and R8 constitute two chromatic channels. Neural circuits for processing chromatic information are not known. Here we identified the first-order interneurons downstream of the chromatic channels. Serial-EM revealed that small-field projection neurons Tm5 and Tm9 receive direct synaptic input from R7 and R8, respectively, and indirect input from R1–R6, qualifying them to function as color-opponent neurons. Wide-field Dm8 amacrine neurons receive input from 13–16 UV-sensing R7s and provide output to projection neurons. Using a combinatorial expression system to manipulate activity in different neuron subtypes, we determined that Dm8 neurons are both necessary and sufficient for phototaxis to ultraviolet in preference to green light. We propose that Dm8 sacrifices spatial resolution for sensitivity by relaying signals from multiple R7s to projection neurons, which then provide output to higher visual centers.