The mushroom bodies (MBs) are known to be the most prominent olfactory memory center in Drosophila. The MB Kenyon cells receive olfactory (via projection neurons, PNs) and gustatory (via dopaminergic neurons, DANs) input. The coincident detection of these stimuli elicits then the formation of associative memory. However, little is known whether and how this high order brain center affects naïve behaviors such as olfaction and gustation. We use fruit fly larvae to test the innate salt avoidance in a simple choice assay. Drosophila larvae are tested for 3 minutes for their preference for 1.5M NaCl. We show that salt avoidance is enhanced upon optogenetic activation of the Kenyon cells. This effect was also observed after ablation of pPAM DANs with simultaneous optogenetic activation of the KCs. We were able to phenocopy this effect by optogenetic activation of TH-Gal4 positive dopaminergic neurons, however not after artificial activation of single DANs of the DL1 cluster. In contrast, larvae showed no salt avoidance after optogenetic activation of sNPF-Gal4 positive neurons. Neuropeptidergic and dopaminergic signaling are tightly involved in modulation of the MBs. However, it is not clear whether our observations depend on coherent neuronal pathways. Therefore, we use simultaneous manipulations of different components of the MB, dopaminergic and sNPF circuitries to unravel the neural mechanisms of modulation of naïve gustatory behaviors in the Drosophila larvae.

MBON02 trans-tango image from Kaun Lab. Left image shows trans-tango signal in red, right image shows overlay of all channels (green for MBON02, red for trans-tango, blue for nc82):

My MBON02 trans-tango images stained with anti-HA instead of anti-RFP. Flies generated by crossing trans-tango mkII line (BDSC: 95317) with MB399B (Brembslab stock number: 319). Left shows trans-tango signal in green, right shows overlay of al channels (green for trans-tango, blue for MBON02, red for nc82):

Janelia Flylight image of MB399B with 20XUAS-CsChrimson-mVenus trafficked in attP18 reporter:

5 days old larvae (3rd instar) were tested for their salt (1.5 M) avoidance behavior. Dopaminergic neurons from the TH-D’ cluster were activated upon ChR2-XXL expression and blue light exposure, respectively.

Staining done by anti-GFP (MBON02), anti-RFP (post-synaptic cells), nc82 (for background stainingm not shown here). MBON02 cells are clearly visible on green channel but there is no clear cell signal on red channel (MBON02 in green channel, postsynaptic cells in red channel).

Here is the trans-tango image of MBON02 (postsynaptic cells of MBON02) from Kaun Lab:

Retro-Tango of b3 mns look worse, nearly no cell signal both in green (b3 mn) and red channels (b3 mn presynaptic neurons)