The morphology of an identified motor neuron in the fly Drosophila melanogaster changes rhythmically every day, with smaller synaptic boutons at night when the fly is resting than during the day when the fly is active. This rhythm is largely independent of synaptic activity, is controlled from a peripheral clock and persists for at least 40 days but is no longer detectable in older flies. We are studying the ultrastructure of these synaptic boutons in wild-type flies, as well as mutant and transgenic flies in which synaptic activity was blocked during a short time, to investigate if the rhythmic change in bouton size includes a reorganization of synapses. So far our findings have shown circadian changes in the size of synaptic vesicles and suggest circadian changes in the numbers of synapses and associated organelles as presynaptic densities (“T-bars”), endosomes, multivesicular bodies and lamellar bodies, but not in mitochondria. We propose that in this motor neuron there is a circadian rhythm of synapse assembly when the fly is at rest in the dark phase, and disassembly over the light phase, when the fly is active.
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