CELLULAR EXPRESSION OF MESSENGER-RNAS ENCODING MONOAMINE-OXIDASE-A AND MONOAMINE-OXIDASE-B IN THE RAT CENTRAL-NERVOUS-SYSTEM

Monoamine oxidases A and B (MAO-A and MAO-B) oxidatively deaminate neu rotransmitter and xenobiotic amines. The cellular localization of thes e isoenzymes in the central nervous system (CNS) differs markedly and only partly reflects the distribution of their presumed natural substr ates. In the present study, by using in situ hybridization with S-35-l abelled oligonucleotide probes, we examined the distribution of mRNAs encoding MAO-A and MAO-B in the rat CNS. Probes for tyrosine hydroxyla se, histidine decarboxylase, and tryptophan hydroxylase mRNAs were use d to demonstrate the catecholaminergic, histaminergic, or serotoninerg ic nature of some cell populations in adjacent sections. The radioliga nds [H-3]-Ro 41-1049 and [H-3]lazabemide (reversible and selective inh ibitors of MAO-A and MAO-B, respectively) were used to reveal the prot ein distribution by enzyme radioautography. The distribution and abund ance of transcripts for both isoenzymes in the tissues investigated di ffered markedly but, in general, correlated with the protein distribut ion. MAO-A mRNA and protein were most abundant in noradrenergic neuron s. However, moderate levels of transcript expression and protein were also detected in the serotoninergic neurons, and low but significant l evels were detected in the dopaminergic neurons. An unexpectedly remar kable degree of hybridization signal was apparent in nonaminergic cell populations, e.g., in the cerebral cortices, the hippocampal formatio n (CA1-3, dentate gyrus), the cerebellar granule cell layer, and the s pinal cord motoneurons. In contrast, MAO-B mRNA and protein were most abundant in serotoninergic and histaminergic neurons, Bergmann glial c ells, and circumventricular organs, including the ependyma. MAO-B tran scripts were also weakly expressed in nonaminergic cells, e.g., in the hippocampal formation (CA1-2). A further nonneuronal localization of MAO-B transcripts was also resolved, e.g., in the glia limitans, the o lfactory nerve layer, and the cerebellar peduncle. These findings reve al further the potential of various cell populations to synthesize the isoenzymes, and homologous (aminergic) and heterologous (nonaminergic ) patterns of expression as well as coexpression of MAO mRNAs are desc ribed. (C) 1995 Wiley-Liss, Inc.