INTRODUCTION: Circadian rhythms are ~24-hour cycles that regulate physiology and behaviour. They are generated by cell-autonomous molecular clocks and synchronized by environmental cues, like light–dark cycles. In Drosophila melanogaster, the circadian system consists of central pacemaker neurons and peripheral clocks. The visual system (namely the lamina, the first optic lobe neuropil) shows circadian plasticity in synaptic number and morphology of neurons and glia. R1–R6 photoreceptors function as peripheral clock neurons and form tetrad synapses with interneurons, glial cells, and amacrine cells in the lamina. These synapses undergo daily structural changes that relate with locomotor activity rhythms.

AIM(S): This project examines how external cues (light and temperature) and internal factors (oxidative stress) regulate circadian synaptic plasticity of tetrad synapses in the lamina. It also seeks to identify molecular pathways underlying these cyclic structural changes.

METHOD(S): BRUCHPILOT (BRP), a presynaptic active zone protein, served as a marker of tetrad presynaptic structure. Its levels were quantified using immunohistochemistry and confocal microscopy in various light conditions: standard (LD 12:12), short (LD 8:16), long (LD 16:8) photoperiods, constant darkness and light. To complement anatomical data, bulk transcriptomic profiling will be conducted at two circadian time points (morning activity peak and midnight sleep), followed by differential gene expression analysis.

RESULTS: Synapse number and size peak in the morning and evening and decline at midday and midnight. BRP levels follow the same pattern: the morning peak is light-dependent, while the evening peak persists in constant darkness, indicating endogenous clock control.

CONCLUSIONS: Photoreceptors integrate environmental and physiological signals with circadian timing to regulate synaptic plasticity. Ongoing molecular analyses will further clarify the genetic pathways linking circadian rhythms to structural synaptic remodeling in the visual system.

FINANCIAL SUPPORT: INCITE (Z/heu/00049)