Fig. 11. Possible involvement of serotonin in the coupling mechanism between the optic lobe pacemakers. The circadian pacemakers in the optic lobe synchronize not only to the environmental light/dark cycle through photic information from the compound eye (1) but also to their contralateral partner. The medulla bilateral neurons (MBNs) are the major component of the coupling system and receive photic information from the photoreceptor (2) and circadian information from the pacemaker (3) on their own side. Serotonin (5-HT) is released from the serotonergic neurons under regulation by the circadian pacemaker (4) and the contralateral MBNs (5). The released serotonin shifts the phase of the pacemaker (6) in a phase-dependent manner, and at the same time, it reduces the coupling signals (7) by suppressing the activity of MBNs.