In the visual system of D. melanogaster numerous circadian rhythms have been described. The most pronounced rhythmic changes have been detected in L2 monopolar cells located in the first neuropil of the optic lobe (the lamina). The L2’s dendritic tree perimeter oscillates in a circadian manner and is the largest at the beginning of the day. The aim of our study was to understand molecular mechanisms controlling cyclic L2 dendritic tree changes. We have examined a possible involvement of TOR and PI3K class 1 proteins in this process. First the expression of tor and pi3k class 1 genes was investigated in the fly’s brain at different times of the day in 12 h of light and 12 h of darkness (LD12:12) or constant darkness (DD) by means of Real-Time PCR. Next the analysis of circadian changes in the perimeter of GFP labeled L2 dendritic trees was examined in control and in flies with down-regulated expression of tor or pi3k class 1 genes. The obtained results showed that the expression of tor gene changes during the 24 h cycle and is the highest when the L2 cell dendritic trees are largest. The silencing of tor gene expression resulted in the disruption of circadian changes in L2 interneurons. It indicates that TOR signaling pathway is involved in the mechanism of circadian neuroplasticity in the visual system of D. melanogaster.
Neurons and glial cells in the fly’s visual system exhibit circadian rhythms through changes in shape and size. Moreover, the number of synaptic contacts between these cells changes during the day and night and in the case of one type of synapses, feedback synapses, is maintained under constant conditions indicating an endogenous origin of this rhythm. The structural changes described above, involving the oscillations in the number of synapses and the size of interneurons and glial cells, are examples of plasticity in the central nervous system driven by internal inputs from a circadian clock and by external stimuli such as light. They are also modulated by visual and other sensory stimuli and by motor activity.