Serotonin immunoreactivity in the optic lobes of the sphinx moth Manduca sexta and colocalization with FMRFamide and SCPB immunoreactivity.

In the optic lobes (OLs) of the sphinx moth Manduca sexta, 300-350 neurons per hemisphere are immunoreactive with an antiserotonin antiserum. Two groups of weakly serotonin-immunoreactive cells (OL1) appear to be amacrine cells of the medulla, whereas more intensely immunoreactive cells (OL2) are probably centrifugal neurons that innervate the lobula, medulla, and lamina, as well as the superior protocerebrum. At least one other OL2 cell is a local optic-lobe interneuron with arborizations in the dorsal medulla and lobula. The serotonin-immunoreactive cells are also immunoreactive with an antiserum against Drosophila melanogaster DOPA decarboxylase. All OL2 cells, but not the OL1 cells, are furthermore immunoreactive with an anti-FMRFamide antiserum and an anti-SCPB antiserum. This suggests that neuropeptides related or identical to FMRFamide and SCPB are localized and may serve as cotransmitters with serotonin in OL2 optic-lobe interneurons.