Presence of monoamine oxidase type B protein but absence of associated enzyme activity in neurons within the inferior olive nucleus of the rat.

A previous study demonstrated that monoamine oxidase type B (MAOB) mRNA is located in the inferior olive complex (IO). The purpose of the present study was to examine whether neuronal cell bodies within the IO also express MAOB protein and whether they exhibit associated MAOB enzyme activity. Using immunohistochemistry and enzyme histochemistry, we demonstrated that IO neuronal cell bodies were positive for MAOB immunohistochemistry but negative for MAOB enzyme histochemistry. These findings indicate that IO neuronal cell bodies express MAOB mRNA and produce MAOB protein but curiously do not exhibit MAOB enzyme activity, as might be expected. The mechanism responsible for the failure of MAOB protein to result in enzymatic activity in IO neuronal cell bodies is clearly of significance in terms of functionality but remains to be elucidated.