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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.
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In the lamina L1 and L2 monopolar cells show circadian rhythms in shape and size changes, however, mechanisms of these rhythms have not been recognized yet. It has been proposed that swelling and shrinking of L1 and L2 interneurons might be due to ionic fl uxes, changes in cytoskeleton organization and in protein synthesis. This process also depends on glial cell activity. Since ion channels seem to be involved in morphological plasticity of neurons and glial cells we examined expression of the sodium pump, Na+ ,K+ -ATPase, during the day and night. The sodium pump consists of two polypeptide chains: α- and β-subunits encoded by the Atpα and Nrv2 genes, respectively. Using the real-time PCR and immunohistochemistry methods we found the α-subunit shows daily changes in mRNA and protein concentrations while in case the β-subunit such changes were not observed. Since the rhythm in the α-subunit abundance was not observed in the per0 arrhythmic mutant, this rhythm is controlled by clock genes. It seems to refl ect changes in the visual system activity.
Bruchpilot (BRP) protein is a scaffolding presynaptic protein that has been detected in the nervous system of Drosophila melanogaster. BRP assembles a presynaptic active zone and is responsible for calcium channel clustering and release of a neurotransmitter from synaptic vesicles. We have used the antibody nc82 against BRP to visualize presynaptic elements of tetrad synapses formed between the photoreceptor terminals and postsynaptic cells in the first optic neuropil (lamina) of the Drosophila optic lobe. In the lamina several circadian rhythms have been detected including rhythms in plasticity of synapses and neurons. The aim of this study was to examine whether BRP protein is involved in the circadian plasticity of tetrad synapses in the lamina. We have examined the BRP level, measured as the fluorescence intensity of immunolabeling, at different times of a light/dark (LD 12:12) regime and constant darkness (DD). We have found that BRP oscillates during the day. In LD 12:12 its level increases two times, in the morning and in the evening. However, these two peaks in BRP abundance are regulated differently. The morning peak is predominantly regulated by light since it is not present in norpA7 phototransduction mutant but it also depends on the circadian clock gene per. In turn the evening peak is regulated by the brain pacemaker. This peak is present in DD as well as in the norpA7 in LD but is absent in clock gene mutants.
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