Prenatal effects of acute harmaline exposure on fetal brain biogenic amine metabolism

Harmaline, a known type A monoamine oxidase (MAO) inhibitor in adult brain of various species was found to elevate whole brain levels of dopamine and serotonin (5-HT) in rat fetuses of mothers injected 2-4 h before Caesarean delivery. Similar stimulatory effects were observed for the norepinephrine metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG), however, no significant effect was obtained for norepinephrine. The dopamine metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC) and the 5-HT metabolite 5-hydroxyindole acetic acid (5-HIAA) were decreased with the same treatment. These results imply that harmaline or one of its metabolites may cross the placental barrier to affect the fetal brain system not merely as a type A MAO inhibitor (i.e., relatively 5-HT-specific), but possibly also as a stimulatory agent for aldehyde reductase or catechol-O-methyltransferase (COMT) or alternately as an agent inhibiting the conjugation, efflux, or turnover of biogenic amine metabolites such as MHPG.