Preferential localization of monoamine oxidase type A activity in neurons of the locus coeruleus and type B activity in neurons of the dorsal raphe nucleus of the rat: A detailed enzyme histochemical study

Using enzyme histochemistry for monoamine oxidase (MAO) activity, we have examined whether MAO type A or type B or both are localized in neurons of the locus coeruleus (LC) and dorsal raphe nucleus (DR) of the rat. After pretreatment with various concentrations of the MAO type A inhibitor clorgyline or the type B inhibitor deprenyl, non-fixed frozen sections of the brain were histochemically stained for MAO activity with tyramine as a common substrate for the two types. MAO activity of the stained neuron was determined by measuring optical density of the staining. Percentage inhibition of the control MAO activity was plotted against increasing concentrations of the inhibitors. MAO activity of LC neurons was inhibited by low concentrations of clorgyline with a monophasic dose-response curve but not with a biphasic curve. Higher concentrations of deprenyl were needed to inhibit the MAO activity of LC neurons. MAO activity of DR neurons was inhibited by low concentrations of deprenyl with a monophasic dose-response curve. Clorgyline inhibited the MAO activity of DR neurons at only much higher concentrations. When the sections without inhibitor pretreatment were incubated with the type A preferential substrate serotonin, the MAO activity was strongly stained in LC neurons but very weakly in DR neurons. With the type B preferential substrate beta-phenylethylamine, the staining was intense in DR neurons while very faint in LC neurons. These findings suggest that (i) almost all of the MAO activity in LC neurons is of type A, and (ii) the MAO activity in DR neurons is predominantly of type B.