Unequal coupling between locomotor pacemakers of the German cockroach, Blattella germanica (L.)

Abstract

Male adult German cockroaches, Blattella germanica (L.), expressed robust locomotor circadian rhythmicity under 28°C and constant darkness (DD) conditions. By surgically severing the connections between the optic lobes and midbrain and their subsequent regeneration, we demonstrated that the locomotor circadian pacemaker was located in the optic lobes and that it controlled the locomotor circadian rhythm through neural pathways. From the results that unilaterally optic tract severed males still showed locomotor circadian rhythmicity (93.1%, n=29) without significantly changing the circadian period (τ) or level of motor activity, we concluded that the right and left optic lobes each contain a circadian pacemaker competent to drive the locomotor circadian rhythm. These two pacemakers were strongly coupled since only one rhythm was expressed when the pacemakers were independently exposed to opposite lighting conditions (DD or LL) at the same time. However, an unequal contribution of each pacemaker in determining the overt circadian period was found under constant dim light (10 lux) conditions, revealing a major–minor coupling relationship between these two pacemakers, so that the unilaterally blinded male expressed either a LL-rhythm with a circadian period of 24.27±0.21 h (41.7%, n=24) or a DD-rhythm with a circadian period of 23.43±0.19 h (58.3%, n=24). However, higher intensity of photic information (200–300 lux) could overpower this relationship and cause the minor pacemaker to lead the rhythm.