H. Parvez

Monoamine oxidase expression during development and aging.

Monoamine oxidase (MAO) isoenzymes play a major role in regulating the concentration of several bioactive amines, including serotonin and catecholamines. Both in the nervous system and in peripheral organs, MAOs can potentially modulate all the processes involving these bioactive amines. In the present article, we review some of the most significant articles published so far on changes in MAOs during development and aging. The data available on development refer mainly to the mammal brain at fetal and post-fetal stages. Very little work has been done on studying MAO ontogenesis during early development, that is, at stages prior to organogenesis, and what has been done refers to non-mammal vertebrates such as fish, amphibians and birds. MAO A and MAO B changes have been measured as values of enzymatic activity, as amount of protein or, more rarely, as amount of mRNAs. A knowledge of MAO developmental changes not only provides a basis for the investigation of factors regulating MAO expression, but can also contribute to a better understanding of the possible trophic and/or morphogenetic role of monoaminergic neurotransmitters in the developing brain. Transgenic mice lacking MAO A and rodents treated with MAO inhibitors during gestation have been very useful in this second case. The investigations of changes in MAO A and MAO B during aging in the literature refer mostly to humans, mice and rats. Interest in studies on aging is stimulated, among other things, by the observation that age-related diseases leading to neurodegenerative phenomena could be accompanied by changes in MAO activity.

Alcohol and central neurotransmission

The study of the relationships between alcohol consumption and central neurotransmission is difficult: they are different from one individual to another, from one neurotransmission system to another and from one cerebral area to another. Moreover, there is no fully satisfactory animal model of alcoholism and the human studies have to cope with a lot of methodological problems. In spite of these difficulties a bidirectional relationship between alcohol and central neurotransmission is well established. Neuronal dysfunctions are the neurobiological basis for the alcohol behaviour, and ethanol craving seems specifically related to hypofunction of the noradrenergic, GABAergic and serotoninergic systems, and maintained by a positive reinforcement mediated by the dopaminergic and opioid systems. Ethanol alters almost all membrane functions, but it behaves essentially like a barbiturate-type GABAergic agonist. In the short-term, it also stimulates central monoaminergic neurotransmissions. With chronic intoxication, membrane tolerance develops, which is the substratum for tolerance and dependence. Concurrently there are adaptative processes and a depletion of the capacities for synthesis of neurotransmitters, therefore a hypofunctioning of all neurotransmission systems. This hypofunctioning is an additive mechanism for tolerance and dependence, pushing the individual into drinking always more alcohol to palliate it; it is sharply revealed during withdrawal, particularly the GABAergic deficiency.

Oxidation of N-Methyl-1,2,3,4-Tetrahydroisoquinoline into the N-Methyl-Isoquinolinium Ion by Monoamine Oxidase

Abstract: N‐Methyl‐1,2,3,4‐tetrahydroisoquinoline (NMTIQ) was found to be oxidized by monoamine oxidase (MAO) into N‐methylisoquinolinium ion, which was proved to inhibit enzymes related to the metabolism of catecholamines, such as tyrosine hydroxylase, aromatic‐L‐amino acid decarboxylase, and MAO. NMTIQ was oxidized by both types A and B MAO in human brain synaptosomal mitochondria. Oxidation was dependent on the amount of MAO sample and the reaction time. Enzyme activity with respect to NMTIQ reached optimum at a pH of ∼7.25, as was the case with other substrates. Type A MAO had higher activity for this substrate than type B. The Km and Vmax values of the oxidation by types A and B MAO were 571 ± 25 μM and 0.29 ± 0.06 pmol/min/mg protein, and 463 ± 43 μM and 0.16 ± 0.03 pmol/min/mg protein, respectively. The Vmax values of types A and B MAO for NMTIQ were much smaller than those for other substrates such as kynuramine. NMTIQ was the first tetrahydroisoquinoline shown to be oxidized into the isoquinolinium ion by MAO in the brain.

Simple purification of aromatic l-amino acid decarboxylase from human pheochromocytoma using high-performance liquid chromatography☆

We purified aromatic L-amino acid decarboxylase (AADC) homogeneously and rapidly from human pheochromocytoma using high-performance liquid chromatography. HPLC with gel permeation and hydrophobic columns was highly effective, and the entire purification could be finished within 3 days. Purified AADC showed a single band with an Mr of 50,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and decarboxylated L-3,4-dihydroxyphenylalanine, L-5-hydroxytryptophan, and L-threo-3,4-dihydroxyphenylserine (a synthetic precursor of natural norepinephrine). Amino acid analysis of purified AADC was performed.

ANIMAL MODELS OF VASCULAR DEMENTIA WITH EMPHASIS ON STROKE‐PRONE SPONTANEOUSLY HYPERTENSIVE RATS

1. Two experimental models designed to reflect different aspects of vascular dementia (rats with cerebrovascular occlusion and rats with cerebral embolization) and stroke‐prone spontaneously hypertensive rats (SHRSP) have been evaluated. The focus was on SHRSP as a model for vascular dementia.

Apoptosis induced by an endogenous neurotoxin, N-methyl (R) salsolinol, in dopamine neurons

A dopamine-derived neurotoxin, 1(R),2(N)-dimethyl-6,7-dihydroxy-1,2, 3,4-tetrahydroisoquinoline [N-methyl(R)salsolinol] was found to cause parkinsonian in rats and to deplete selectively dopamine neurons in the substantia nigra after infusion in the striatum. This isoquinoline occurs enantio-specifically in the nigra-striatum of human brains. The biosynthesis from dopamine is catalyzed by two enzymes, (R)salsolinol synthase and (R)salsolinol N-methyltransferase. The isoquinoline increases in the cerebrospinal fluid from parkinsonian patients, and the increase is ascribed to high activity of its synthesizing neutral (R)salsolinol N-methyltransferase, as shown by analyses in lymphocytes. The cell death caused by this neurotoxin in dopaminergic human neuroblastoma SH-SY5Y cells proved to be apoptotic. Apoptosis by this neurotoxin is mediated by intracellular sequential process, loss of mitochondrial membrane potential, activation of caspases and DNA fragmentation. These results are discussed in relation to the role of apoptosis in neurodegenerative diseases and the involvement of the endogenous toxin in the pathogenesis of Parkinsons disease.

N-methylisoquinolinium ion as an inhibitor of tyrosine hydroxylase, aromatic l-amino acid decarboxylase and monoamine oxidase

The effect of the N-methylisoquinolinium ion (NMIQ(+)) on the activity of enzymes related to metabolism of dopamine was studied using a rat clonal pheochromocytoma PC12h cell line. The activities of tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase (AADC) and monoamine oxidase (MAO) were inhibited by NMIQ(+), but the mechanism of inhibition of these enzymes differed from each other. TH activity in the cells was inhibited by NMIQ(+) with an IC(50) of about 75 ?M. Aromatic l-amino acid decarboxylase (AADC) was also inhibited by NMIQ(+) but in competition with a co-factor, pyridoxal-5-phosphate, and the K(i) value was 90 ?M. MAO was inhibited by NMIQ(+) in competition with a substrate, kynuramine, and the K(i) value was 20 ?M. In vivo effects of NMIQ(+) on these enzymes in PC12h cells were examined by culture of the cells in the presence of 100 nM-1 mM NMIQ(+) for 6 days. After 6 days culture, TH activity was reduced in cells cultured with NMIQ(+) at concentrations higher than 10 ?M, but the activities of AADC and MAO were reduced only in cells cultured with 1 mM NMIQ(+). In addition, NMIQ(+) was transported into the cells by a transport system specific for dopamine. These data suggest that NMIQ(+) may perturb the catecholamine metabolism of a dopaminergic system in the brain, as a naturally-occurring compound.

Effect of sex steroids on rat endometrial epithelium and stroma cultured separately

Abstract Rat endometrium was separated into its epithelial and stromal components. Primary cell culture was established and some effect of sex steroids studied on these two cell populations. Progesterone added to the culture medium, reduced the rate of epithelial cell proliferation but not that of stromal cells. The hormone increased the incorporation of [ 3 H]-uridine and [ 3 H]-leucine into TCA precipitates of epithelial cells. Estradiol was found to be less effective. Epithelial and stromal cell content of glycogen was determined. A high amount was found in both types of cells cultured under progesterone. Enzyme monoamine oxidase activity evaluated in epithelial cells was also markedly increased by this hormone. Results suggest that endometrial epithelium and stroma in cell culture respond to progesterone by metabolic changes similar to those observed in vivo .

The effect of diabetes mellitus on the morphology and physiology of monoamine oxidase in the pancreas.

Monoamine oxidase (MAO) is an ubiquitous, non-soluble, membrane-bound enzyme, located in the outer membrane of mitochondria. MAO consists of two subtypes, MAO-A and MAO-B, depending on their substrates and sensitivity to inhibitors. MAO consists of two units joined together by a disulphide bond. The two units of MAO and flavin adenine dinucleotide (FAD) form a polymer in the outer membrane of mitochondria. The function of MAO-A is highly dependent on the lipid constituent of mitochondrial membrane, whereas the function of MAO-B does not depend on the lipid status of mitochondrial membrane. Hydrogen peroxide and ammonia are generated during MAO-induced metabolism of its substrates. MAO and its substrates are present in both the exocrine as well as the endocrine parts of the pancreas. In the islet of Langerhans, MAO-A is observed in about 50% of the cells, whereas MAO-B is less abundant and located mainly in the periphery of pancreatic islets. MAO-B is also demonstrated in centroacinar cells and in pancreatic ducts. Electron microscopy studies suggest that MAO is co-localised with insulin in secretory granules of pancreatic beta cells. Pharmacologically, beta-2-adrenoreceptors agonists such as terbutaline can stimulate MAO activity. In contrast, cholinergic muscarinic stimulation does not affect islet MAO activity. MAO activity in pancreatic tissue is significantly reduced in diabetes. This decrease in MAO activity is associated with an increase in pancreatic tissue levels of adrenaline (ADR) and noradrenaline (NA). Studies on the level of 5-hydroxyindoleacetic acid of pancreatic tissues suggest that serotonin level is also increased in diabetics. Many studies show that MAO inhibits insulin secretion. However, some of its substrates including, serotonin, adrenaline and noradrenaline have been shown to stimulate insulin secretion. In conclusion, the activity and subcellular localisation of MAO suggests that MAO may play an important role in pancreatic beta cell function and hence in the pathogenesis of diabetes mellitus.

Accumulation of phosphorylated tyrosine hydroxylase into insoluble protein aggregates by inhibition of an ubiquitin–proteasome system in PC12D cells

Tyrosine hydroxylase (TH) is a rate-limiting enzyme for the biosynthesis of catecholamines including dopamine. The relationship between proteasomal dysfunction and the etiology of Parkinson’s disease has been suggested, but it is unknown if TH protein is affected by proteasomal dysfunctions. Here, we examined the effect of inhibition of ubiquitin–proteasomal pathway on biochemical characteristics of TH protein in the neuronal cells. Inhibition of 20S or 26S proteasome by proteasome inhibitor I, or MG-132 in NGF-differentiated PC12D cells induced dot-like immunoreactivities with the anti-40Ser-phosphorylated TH (p40-TH) antibody. These dots were tightly co-localized with ubiquitin and positive to Thioflavine-S staining. These dot-like immunoreactivities were not obvious when immunostaining was performed against total-TH or choline acetyltransferase. Western blotting analysis showed time-dependent increase of p40-TH in the Triton-insoluble fractions. We also examined the effect of okadaic acid, an inhibitor of protein phosphatase 2A, which is a phosphatase acting on p40-TH. Okadaic acid increased the amount of insoluble p40-TH. These data suggest that p40-TH is prone to be insolubilized and aggregated by dysfunction of an ubiquitin–proteasome system in PC12D cells.