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.
