Serotonin metabolism and the nature of monoamine oxidase in squid central nervous system

Monoamine oxidase activity, using type A and type B substrates and inhibitors, was investigated in the central brain and synaptosomes prepared from the optic lobe of squid. Although substantial MAO activity towards serotonin (5-HT), phenylethylamine and tyramine was present, the enzyme could not be differentiated into the two subtypes previously described in the vertebrate central nervous system. However, this enzyme is functional since5-HT levels, as identified by immunocytochemical procedures, are low in the squid nervous system and show significant increases in pargyline treated squids.