Cellular circadian rhythms in the fly's visual system

Abstract

Circadian rhythms in the visual system have been detected in both vertebrates and invertebrates, mostly in the retinal photoreceptors of animals. They include circadian rhythms in the sensitivity of the eye measured as the electroretinogram (ERG), in the structure of insect ommatidia, in disc-shedding in vertebrate photoreceptors, and in the migration of the screening pigment in the photoreceptor and pigment cells of arthropods. The visual systems of the flies Musca domestica, Drosophila melanogaster, and Calliphora vicina show circadian rhythms in the structure of their cells and in their subcellular organization. These rhythms are a new example of structural circadian rhythms in neurons other than photoreceptor cells. They have been detected in the first optic neuropile, the lamina, in the number of synaptic contacts and the vertical migration of screening pigment in photoreceptor terminals, and in size and shape changes in two of their interneurons, L1 and L2. These are regulated by the lamina's neurotransmitters and controlled “clock Genes.” Although the changes in LI and L2's sizes are driven by inputs from the retina and a circadian clock located in the central part of brain, the epithelial glial cells in the lamina are also involved in rhythmic changes in LI and L2's lamina axon sizes. The function of circadian rhythms in LI and L2 is still unknown; these seem to correlate with cyclical variation in the fly's motor activity and with activity in the visual system. The mechanism of these rhythmic neuronal size changes is not known, but disrupting the cytoskeleton or blocking the proton pump, vacuolar ATPase, in cells of the optic lobe abolishes the daily rhythm of size changes in LI and L2.