Activity dependent active zone remodeling in the Drosophila visual system. Atsushi Sugie^1,2, Takashi Suzuki², Gaia Tavosanis¹. 1) DZNE, Bonn, Germany; 2) Titech, Yokohama, Japan.

   Neural activity contributes to the regulation of the precise localization and the number of synapses formed in a sensory system, allowing for adjustment to a changing environment. It is a fundamental question how synaptic molecular components are regulated to achieve synaptic plasticity. In this study, we visualized presynaptic active zones in photoreceptors of adult flies using UAS-Brp^{short mCherry} expressed in photoreceptor 8 (R8) with Rh6-Gal4. Brp^{short mCherry} accumulates in discrete puncta, presumably representing individual active zones as their number and distribution corresponds to previous EM data. Surprisingly, the discrete puncta of UAS-Brp^{short mCherry} observed in adult flies maintained in a 12h light/12h dark (LD) cycle were largely lost and the distribution of this marker became diffuse if the flies were kept in continuous light (LL) over a period of a day. This phenotype was reversible. We developed software-based detection of puncta distribution for quantitative analysis of UAS-Brp^{short mCherry} localization. The redistribution of UAS-Brp^{short mCherry} depended on activity as this phenotype was suppressed in norpA mutant, which abolishes the light-evoked photoreceptor potential. Conversely, the expression of UAS-TrpA1 that leads to sustained activation of the photoreceptors caused diffused distribution of UAS-Brp^{short mCherry} even in flies maintained in continuous darkness (DD). The activity requirement, though, is not cell-autonomous. Indeed, in hisCl1¹³⁴,ort¹ mutant flies or blocking photoreceptor transmission with UAS-Shibire^ts the diffuse phenotype was suppressed even in LL. Thus, postsynaptic neurons regulate the activity-dependent synaptic modification in photoreceptors. These data demonstrate that activity can modulate the molecular composition of active zones and suggest a model of feed-back regulation within the circuit.