Quantitative enzyme radioautography with 3H-Ro 41-1049 and 3H-Ro 19- 6327 in vitro: localization and abundance of MAO-A and MAO-B in rat CNS, peripheral organs, and human brain

Abstract

Monoamine oxidases A and B (MAO-A and MAO-B) oxidatively deaminate neurotransmitter and xenobiotic amines. Since the cellular localization of the isoenzymes in the CNS and peripheral organs determines to a large extent which substrate has access to which isoenzyme, knowledge of their tissue distribution and cellular localization is essential. Here we describe how reversible and selective inhibitors of MAO-A and MAO-B [Ro 41–1049 and Ro 19–6327 (lazabemide), respectively] can be used, as tritiated radioligands, to map the distribution and abundance of the enzymes in microscopic regions of the rat CNS and peripheral organs, and human brain by quantitative enzyme radioautography. The in vitro binding characteristics of both radiolabeled inhibitors revealed them to be selective, high-affinity ligands for the respective enzymes. KD and Bmax values for 3H-Ro 41–1049 in rat cerebral cortex were 10.7 nM and 7.38 pmol/mg protein, respectively, and for 3H-Ro 19–6327 were 18.4 nM and 3.45 pmol/mg protein, respectively. In accordance with their potencies as enzyme inhibitors, binding to MAO-A and MAO-B was competitively inhibited by clorgyline (IC50 = 1.4 nM) and L-deprenyl (selegiline; IC50 = 8.0 nM), respectively. The capacities of various rat and human tissues to bind the radioligands correlated extremely well with their corresponding enzyme activities. As revealed by the respective binding assays, the distribution and abundance of MAO-A and MAO-B in the tissues investigated differed markedly. MAO-A was most abundant in the locus coeruleus, paraventricular thalamus, bed nucleus of the stria terminalis, median habenular nucleus, ventromedial hypothalamus, raphe nuclei, solitary tract nucleus, inferior olives, interpeduncular nucleus, claustrum, and numerous peripheral tissues, including liver, vas deferens, heart, superior cervical ganglion, and exocrine and endocrine pancreas. In contrast, MAO-B was most abundant in the ependyma, circumventricular organs, olfactory nerve layer, periventricular hypothalamus, cingulum, hippocampal formation, raphe nuclei, paraventricular thalamus, mammillary nuclei, cerebellar Bergmann glia cells, liver, posterior pituitary, renal tubules, and endocrine pancreas. The cellular localization of the isoenzymes in both rat and human brain differs markedly and does not reflect the distribution of the presumed natural substrates, for example, absence of MAO-A in serotoninergic neurons. Indeed, the present evidence suggests that, whereas MAO-A is found in noradrenergic and adrenergic neurons, MAO-B occurs in astrocytes, serotoninergic neurons, as well as ventricular cells, including most circumventricular organs. The physiological roles of the enzymes are discussed in the light of these findings, some of which were unexpected.(ABSTRACT TRUNCATED AT 400 WORDS)

Elevated Monoamine Oxidase A Binding During Major Depressive Episodes Is Associated with Greater Severity and Reversed Neurovegetative Symptoms

“…In brain tissue, MAO-A level is highly correlated with MAO-A activity and MAO-A V T is primarily an index of MAO-A level (Saura et al, 1992). Elevated MAO-A activity is related to several important downstream effects, as MAO-A participates in the metabolism of serotonin, dopamine, and norepinephrine, produces the pro-oxidant hydrogen peroxide and influences the predisposition toward apoptosis (Fitzgerald et al, 2007;Ou et al, 2006b;Youdim et al, 2006).…”

“…MAO-A level is highly correlated with MAO-A activity in brain (Nelson et al, 1979;Saura et al, 1992) and indices of MAO-A level in the prefrontal (PFC) and anterior cingulate cortex (ACC) were elevated by 25-40% across the three studies that investigated the most common, early onset MDD (Johnson et al, 2011;Meyer et al, 2006;Meyer et al, 2009). Elevated MAO-A level in these regions in MDE is consistent with other specific findings: MDE is a highstress state, and glucocorticoid administration increases transcription rate and catalytic activity of MAO-A in human neuroblastoma and glioblastoma cell lines (Ou et al, 2006a).…”

Monoamine Oxidase Inhibitors: Benzylidene-prop-2-ynyl-amines Analogues

“…Clorgyline and pargyline are commercial MAO inhibitors and their IC 50 were included as references (24, 25 in Table 1). 12,13) As expected, nearly all compounds show good inhibitory activity toward MAO-A/B. As can be seen from Table 1, benzhydrylidene-prop-2-ynyl-amine (2, IC 50 ϭ 32 nM) and (3,4-dimethoxy-benzylidene)-prop-2-ynyl-amine (10, IC 50 ϭ14 nM) are the best inhibitors toward monoamine oxidase A and B respectively, while compound 1 (SIϭ58.96) is the most selective inhibitor toward MAO-A and compound 2 (SIϭ0.34) is the most selective inhibitor toward MAO-B.…”

Greater Monoamine Oxidase A Binding in Alcohol Dependence

“…For example, reduced mitochondrial respiration, an index of reduced mitochondrial functioning, occurs when MAO-A activity is increased (54), and AD is associated with reduced N-acetylaspartate levels, an index of reduced mitochondrial function (55,56). Also, in contrast to many other brain proteins, MAO-A is present homogenously throughout the cortex, including glial cells (6), and when MAO-A activity is greater, glial cell lines are predisposed toward apoptosis (24). MAO-A activity itself creates oxidative stress (4), and markers of oxidative damage to mitochondrial DNA have been reported in AD (51,57).…”

“…MAO-A metabolizes monoamines such as serotonin, norepinephrine, and dopamine, and levels of MAO-A in brain tissue show a strong, positive correlation with MAO-A activity (5,6). Previous investigations of MAO-A activity in postmortem brain of AD were primarily negative, with no change reported in prefrontal cortex, and a decrease reported in the hypothalamus and caudate (7,8).…”