Catabolism of dopamine and 5-hydroxytryptamine by monoamine oxidase in the ixodid tick, amblyomma hebraeum

Abstract

The purpose of this study was to assess biogenic amine catabolism in Amblyomma hebraeum Koch (an African cattle tick). We assayed haemolymph and saliva for a variety of biogenic amines (usually following injection of substrate into the haemolymph) in partially fed females using HPLC coupled to electrochemical detection. Dopamine (DA) and 5-hydroxytryptamine (5-HT) were rapidly converted to dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole-acetic acid (5-HIAA) respectively, indicating that monoamine oxidase (MAO) constitutes a major catabolic pathway for biogenic amines in this species. We could not detect N-acetylated or γ-glutamyl conjugated metabolites of these important neurotransmitters. [In a few samples we looked for but could not detect homovanillic acid or 3-methoxytyramine ( O-methylated metabolites of DOPAC and DA respectively).] Deprenyl was about 44–72 times more potent an inhibitor of MAO than clorgyline when either DA or 5-HT was offered as substrate, suggesting that this MAO is of the MAO B type. Conversion of DA to DOPAC was also detected in several tissues incubated with DA in vitro; in descending order of MAO activity (pmol mg −1h −1 at about 18°C) tissues tested were: skeletal muscle (∼100), Malpighian tubule (∼50), ovary (∼45), salivary gland (∼30), midgut (∼20), and haemolymph (∼4–5). This study suggests that ticks differ considerably from insects in utilizing MAO as an important metabolic pathway for biogenic amines.