Distribution of neurokinin A-like and serotonin immunoreactivities within the vertical lobe complex in Sepia officinalis

Immunohistochemistry, using antibodies raised against mammalian neurokinin A (NKA) and serotonin (5-HT), was applied in double-staining experiments to map these molecules within the vertical lobe complex (inferior frontal, superior frontal, post-frontal, vertical, subvertical and precommissural lobes). NKA-like and 5-HT immunoreactivities were detected in all the lobes of the vertical lobe complex but were never colocalized in cell bodies or fibres. Except for the cell layers of the superior frontal lobe, both types of labelled cell bodies were observed in all the lobes. Both types of immunoreactive fibres were detected in all the neuropils and interestingly revealed clear subdivisions within some lobes, e.g., 5-HT-IR fibres were more abundant in the peripheral part of the vertical lobe whereas NKA-IR ones were widely observed in both the peripheral and central parts. In cephalopods, the vertical lobe complex is involved in learning and memory; thus, our results strongly suggest that one or more NKA-like and 5-HT molecules may function as neurochemical messengers in these cognitive processes.