Brains are difficult to study because they comprise complex and densely intermingled neuronal circuits composed of vast numbers of diverse nerve cells. Transgenic approaches - highly developed in Drosophila - provide opportunities for non-invasive cell-specific manipulations and visualizations that are not limited by anatomical constraints (Borst, 2009; Venken et al., 2011). Combined with various assays, the morphology and functional roles of neurons can be determined in a highly reproducible fashion for one neuronal type at a time.
To this end, the existence of cell-specific DNA regulatory elements is exploited to generate stable transgenic fly lines in which a transcription factor derived from other organisms (usually Gal4 from yeast) is expressed specifically in a neuron of interest. For many visual interneurons, we have such lines at our disposal that can be crossed to Gal4-activatable UAS actuator lines for cell-specific visualization and manipulation. Silencing of neurons can be achieved for instance by expressing the inwardly rectifying potassium channel Kir2.1 (Baines et al., 2001), the temperature-inducible synaptic inhibitor shibirets (Kitamoto, 2001), synaptobrevin-cleaving tetanus toxin light chain (Sweeney et al., 1995), or the photo-activatable chloride channel GtACR1 (Govorunova et al., 2015; Mauss et al., 2017). As a complementary approach, neurons can be activated with heat using the TrpA1 channel (Marella et al., 2006) or with light using photo-activatable Channelrhodopsin2 (Boyden et al., 2005; Pulver et al., 2009; Haikala et al., 2013, Mauss et al., 2014).
Combined with a suitable behavioral readout (Rister et al., 2007) or whole-cell patch-clamp recordings from LPTCs (Joesch et al., 2008), detailed circuit information and function can be inferred from such manipulations (Joesch et al., 2010; Schnell et al., 2012; Bahl et al., 2013; Joesch et al., 2013; Maisak et al., 2013, Mauss et al., 2014, Mauss et al., 2015). In addition, the cell-specific expression of fluorescence reporters of neural activity like TN-XXL (Mank et al., 2008; Reiff et al., 2010), GCaMP (Akerboom et al., 2012; Maisak et al., 2013) or iGluSnFR (Marvin et al., 2013; Richter et al., 2018) can be used establish visual response properties of neurons that are not accessible to electrodes.
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