Peptidergic circadian clock circuits in the Madeira cockroach

•The insect circadian coupling factor PDF is the functional orthologue of VIP in the mammalian circadian clock.•Neuropeptidergic pacemaker neurons express ultradian and circadian action potential rhythms.•PDF forms ensembles of phase-locked circadian pacemakers to gate clock outputs.•Circadian pacemakers with neuropeptide-autoreceptors form self-organized 'labeled lines' to clock outputs.•External stimuli can reconfigure clock networks neuropeptide-dependently.

Circadian clocks control physiology and behavior of organisms in synchrony with external light dark cycles in changing photoperiods. The Madeira cockroach Rhyparobia maderae was the first model organism in which an endogenous circadian clock in the brain was identified. About 240 neurons constitute the cockroach circadian pacemaker network in the accessory medulla. The expression of high concentrations of neuropeptides, among them the most prominent circadian coupling factor pigment-dispersing factor, as well as their ability to generate endogenous ultradian and circadian rhythms in electrical activity and clock gene expression distinguish these pacemaker neurons. We assume that entrainment to light-dark cycles and the control of 24 h rest-activity rhythms is achieved via peptidergic circuits forming autoreceptive labeled lines.