Extracellular long-term recordings of the isolated accessory medulla, the circadian pacemaker center of the cockroach Leucophaea maderae, reveal ultradian and hint circadian rhythms

Abstract

In the cockroach Leucophaea maderae transplantation studies located the circadian pacemaker center, which controls locomotor activity rhythms, to the accessory medulla (AMe), ventromedially to the medulla of the brain’s optic lobes. The AMe is densely innervated via GABA- and manyfold peptide-immunoreactive neurons. They express ultradian action potential oscillations in the gamma frequency range and form phase-locked assemblies of synchronously spiking cells. Peptide application resulted in transient rises of extracellularly recorded activity. It remained unknown whether transient rises in spontaneous electrical activity as a possible indication of peptide release occur in the isolated circadian clock in a rhythmic manner. In extracellular glass electrode recordings of the isolated AMe in constant darkness, which lasted at least 12 h, the distribution of daytime-dependent changes in activity independently of the absolute action potential frequency was examined. Rapid, transient changes in activity preferentially occurred at the mid-subjective night, with a minimum at the middle of the subjective day, hinting the presence of circadian rhythms in the isolated circadian clock. Additionally, ultradian rhythms in activity change that are multiples of a fundamental 2 h period were observed. We hypothesize that circadian rhythms might originate from coupled ultradian oscillations, possibly already at the single cell level.