m-OCTOPAMINE, p-OCTOPAMINE AND PHENYLETHANOLAMINE IN RAT BRAIN: A SENSITIVE, SPECIFIC ASSAY AND THE EFFECTS OF SOME DRUGS

Abstract— An enzyme radiochemical assay for p -octopamine, m -octopamine (norphenylephrine) and phenylethanolamine based on the N -methylation of these amines by the enzyme phenylethanolamine N -methyl transferase ( S -adenosyl- l -methionine: phenylethanolamine N -methyl transferase (EC 2.1.1.28) has been developed. ³H]Methyl- S -adenosyl- l -methionine was used as methyl donor. The reaction products are converted to their dansyl derivatives and separated by TLC in three different solvent systems prior to liquid scintillation counting. The method exhibits a sensitivity of less than 10 pg for each amine and is suitable for the measurement of endogenous p -octopamine levels in mammalian brain. The highest levels of p -octopamine were found in the hypothalamus (3.4 ng/g) but despite the sensitivity of the assay, neither phenylethanolamine nor m -octopamine could be detected. After MAO inhibition, however, both of these amines were found to be present. p -Octopamine was increased substantially in all brain regions following the administration of an MAO inhibitor, whereas pretreatment with reserpine produced a significant decrease in the hypothalamus.