In vivo, noradrenaline is a substrate for rat brain monoamine oxidase A and B.

In vivo clorgyline (5 mg/kg) and (-)-deprenyl (5 mg/kg) selectively inhibit monoamine oxidase (MAO) type A and B activities in rat brain hypothalamus and caudate nucleus using 5-hydroxytryptamine (5-HT), noradrenaline (NA), and beta-phenylethylamine (PEA) as substrates. Clorgyline induces a significant increase in NA concentrations of hypothalamus and caudate nucleus; however (-)-deprenyl is without effect. The combination of clorgyline and (-)-deprenyl at the above doses completely inhibits both forms of MAO, resulting in an even greater increase in NA levels in both brain areas than observed with clorgyline. The non-selective inhibitor tranylcypromine (5 mg/kg) produced a similar effect. Rats pretreated with the selective or the non-selective inhibitors but given L-DOPA (50 mg/kg) have a similar pattern of brain NA, but its concentrations are higher in both brain regions. The results indicate that although in vitro NA may be an exclusive substrate for MAO type A, in vivo, when this enzyme form is selectively inhibited, NA at high concentrations can be a substrate for MAO type B.