MAOI supplier

[trimethoprim]

I need to measure the amount of Serotonin (5-HT) in rat brain cells.
In order to prevent degradation of 5-HT, I want to add MAOI into the cell lysate. Is there any supplier for MAOI?
What is the suggested amount?
Other than MAOI, I also add protease inhibitor cocktail. Is there anymore chemicals to be added to prevent degradation of serotonin?
Please help. Thank you very much.

[JackBean]

I don't think that protease inhibitors are added to prevent serotonin degradation, as is it low-weigth molecule, not peptide