Abstract
In the first optic neuropil (lamina) of the fly's visual system, two interneurons, L1 and L2 monopolar cells, and epithelial glial cells show circadian rhythms in morphological plasticity. These rhythms depend on clock gene period (per) and cryptochrome (cry) expression. In the present study, we found that rhythms in the lamina of Drosophila melanogaster may be regulated by circadian clock neurons in the brain since the lamina is invaded by one neurite extending from ventral lateral neurons; the so-called pacemaker neurons. These neurons and the projection to the lamina were visualized by green fluorescent protein (GFP). GFP reporter gene expression was driven by the cry promotor in cry-GAL4/UAS-GFP transgenic lines. We observed that the neuron projecting to the lamina forms arborizations of varicose fibers in the distal lamina. These varicose fibers do not form synaptic contacts with the lamina cells and are immunoreactive to the antisera raised against a specific region of Schistocerca gregaria ion transport peptide (ITP). ITP released in a paracrine way in the lamina cortex, may regulate the swelling and shrinking rhythms of the lamina monopolar cells and the glia by controlling the transport of ions and fluids across cell membranes at particular times of the day.
Highlights
In the visual system of flies several processes show circadian oscillations
green fluorescent protein (GFP) was not detected in the dorsal neurons DN2 and in the lateral posterior neurons (LPN)
The pattern of changes of cry-GAL4 driven GFP intensity in the 5th s-LNv resembles the pattern of the daily morphological changes of L2 dendritic trees [8]
Summary
In the visual system of flies several processes show circadian oscillations. The rhythms have been detected in the retina and in the first optic neuropil (the lamina). Circadian rhythms have been detected in changes of the number of several structures in the photoreceptor terminals [4] and of synaptic contacts [5], and in morphological plasticity of interneurons [6,7,8] and glial cells [9]. Changes in the L1 and L2 axon size are correlated with the pattern of the locomotor activity of the fly species and with the number of tetrad synapses. On the other hand, severing the housefly’s optic lobe from the rest of the brain abolishes the rhythmic swelling and shrinking of L1 and L2 This result indicates that clock neurons located in the brain, are involved in the generation of circadian rhythms in the morphological changes of monopolar and glial cells in the lamina [16,17]
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