Circadian Rhythm and Neuropeptides

Circadian rhythms are driven by the circadian clock in which a self-sustaining mechanism produces ca. 24-h rhythmicity. Genetic ablation or rescue experiments have shown that many kinds of neuropeptides are necessary for circadian rhythms. However, relatively few neuropeptides have been well characterized physiologically in relation to circadian rhythms. In Rhyparobia maderae, ten neuropeptides have been immunohistochemically identified in accessory medulla neurons, which constitute the circadian clock system. In Drosophila melanogaster, 12 neuropeptides have been reported of their presence in brain clock neurons. Pigment-dispersing factor (PDF) is the most well-characterized peptide regarding the activity rhythm. PDF is important to set an activity phase in D. melanogaster, R. maderae, and Gryllus bimaculatus, possibly by coordinating the clock neuron network. Diuretic hormone 31 is known as an output factor regulating fly sleep and temperature preference under the control of the circadian clock. Short neuropeptide F works for coupling of the central and peripheral clocks by inhibiting prothoracicotropic hormone-producing neurons, thereby controlling the circadian eclosion rhythm. Transcriptome and connectome studies on clock neurons in D. melanogaster suggest a variety of neuropeptides play important roles in organizing circadian rhythms.