Subcellular localization of types A and B monoamine oxidase in rat brain

Study on the Neuroprotective, Radical-Scavenging and MAO-B Inhibiting Properties of New Benzimidazole Arylhydrazones as Potential Multi-Target Drugs for the Treatment of Parkinson's Disease

Oxidative stress is a key contributing factor in the complex degenerating cascade in Parkinson's disease. The inhibition of MAO-B affords higher dopamine bioavailability and stops ROS formation. The incorporation of hydroxy and methoxy groups in the arylhydrazone moiety of a new series of 1,3-disubstituted benzimidazole-2-thiones could increase the neuroprotective activity. In vitro safety evaluation on SH-SY5Y cells and rat brain synaptosomes showed a strong safety profile. Antioxidant and neuroprotective effects were evaluated in H2O2-induced oxidative stress on SH-SY5Y cells and in a model of 6-OHDA-induced neurotoxicity in rat brain synaptosomes, where the dihydroxy compounds 3h and 3i demonstrated the most robust neuroprotective and antioxidant activity, more pronounced than the reference melatonin and rasagiline. Statistically significant MAO-B inhibitory effects were exerted by some of the compounds where again the catecholic compound 3h was the most potent inhibitor similar to selegiline and rasagiline. The most potent antioxidant effect in the ferrous iron induced lipid peroxidation assay was observed for the three catechols-3h and 3j, 3q. The catecholic compound 3h showed scavenging capability against superoxide radicals and antioxidant effect in the iron/deoxyribose system. The study outlines a perspective multifunctional compound with the best safety profile, neuroprotective, antioxidant and MAO-B inhibiting properties.

Teaching an old drug new tricks: repositioning strategies for spinal muscular atrophy

Spinal muscular atrophy (SMA) is a childhood disorder caused by loss of the SMN gene. Pathological hallmarks are spinal cord motor neuron death, neuromuscular junction dysfunction and muscle atrophy. The first SMN genetic therapy was recently approved and other SMN-dependent treatments are not far behind. However, not all SMA patients will reap their maximal benefit due to limited accessibility, high costs and differential effects depending on timing of administration and disease severity. The repurposing of commercially available drugs is an interesting strategy to ensure more rapid and less expensive access to new treatments. In this mini-review, we will discuss the potential and relevance of repositioning drugs currently used for neurodegenerative, neuromuscular and muscle disorders for SMA.

Neurovascular-neuroenergetic coupling axis in the brain: master regulation by nitric oxide and consequences in aging and neurodegeneration

The strict energetic demands of the brain require that nutrient supply and usage be fine-tuned in accordance with the specific temporal and spatial patterns of ever-changing levels of neuronal activity. This is achieved by adjusting local cerebral blood flow (CBF) as a function of activity level - neurovascular coupling - and by changing how energy substrates are metabolized and shuttled amongst astrocytes and neurons - neuroenergetic coupling. Both activity-dependent increase of CBF and O₂ and glucose utilization by active neural cells are inextricably linked, establishing a functional metabolic axis in the brain, the neurovascular-neuroenergetic coupling axis. This axis incorporates and links previously independent processes that need to be coordinated in the normal brain. We here review evidence supporting the role of neuronal-derived nitric oxide (^•NO) as the master regulator of this axis. Nitric oxide is produced in tight association with glutamatergic activation and, diffusing several cell diameters, may interact with different molecular targets within each cell type. Hemeproteins such as soluble guanylate cyclase, cytochrome c oxidase and hemoglobin, with which ^•NO reacts at relatively fast rates, are but a few of the key in determinants of the regulatory role of ^•NO in the neurovascular-neuroenergetic coupling axis. Accordingly, critical literature supporting this concept is discussed. Moreover, in view of the controversy regarding the regulation of catabolism of different neural cells, we further discuss key aspects of the pathways through which ^•NO specifically up-regulates glycolysis in astrocytes, supporting lactate shuttling to neurons for oxidative breakdown. From a biomedical viewpoint, derailment of neurovascular-neuroenergetic axis is precociously linked to aberrant brain aging, cognitive impairment and neurodegeneration. Thus, we summarize current knowledge of how both neurovascular and neuroenergetic coupling are compromised in aging, traumatic brain injury, epilepsy and age-associated neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, suggesting that a shift in cellular redox balance may contribute to divert ^•NO bioactivity from regulation to dysfunction.

Expression of A and B types of monoamine oxidase in neuroblastoma hybrid cells

Monoamine oxidase (MAO) activity towards kynuramine as substrate was measured in 6 hybrid cells derived by fusion of neuroblastoma and glioma, liver or brain cells, and was compared with that of parental or non-parental clones. Activities varied from the lowest level of less than 0.15 pmol/min/mg protein in a neuroblastoma clone NB2A to the highest level of 127 pmol/min/mg protein in NCB20 mouse neuroblastoma x Chinese hamster embryo brain hybrid cells. The relative proportions of A and B types of MAO activities were determined in homogenates of each cell line by inhibition curves with clorgyline and deprenyl. Although the A type activity was found in all cell lines measured, MAO A was predominant in 9 clones, except for NCB20 hybrid cells, N4G-B-a neuroblastoma x glioma hybrid cells, and G8-1 myoblast. The ratio of type A/type B activity in NCB20, N4G-B-a and G8-1 cells was 20/80, 75/25 and 95/5, respectively. The results suggest that NCB20 cells are highly enriched in MAO type B, and that the NCB20 cell is an excellent model for studying the type B activity found in the brain in vivo.

Neurotoxic catecholamine metabolite in nociceptors contributes to painful peripheral neuropathy

The neurotoxic effects of catecholamine metabolites have been implicated in neurodegenerative diseases. As some sensory neurons express tyrosine hydroxylase and monoamine oxidase (MAO), we investigated the potential contribution of catecholamine metabolites to neuropathic pain in a model of alcoholic neuropathy. The presence of catecholamines in sensory neurons is supported by capsaicin-stimulated epinephrine release, an effect enhanced in ethanol-fed rats. mRNA for enzymes in dorsal root ganglia involved in catecholamine uptake and metabolism, dopamine beta-hydroxylase and MAO-A, were decreased by neonatal administration of capsaicin. Ethanol-induced hyperalgesia was attenuated by systemic and local peripheral administration of inhibitors of MAO-A, reduction of norepinephrine transporter (NET) in sensory neurons and a NET inhibitor. Finally, intradermal injection of 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL), a neurotoxic MAO-A catecholamine metabolite, produced robust mechanical hyperalgesia. These observations suggest that catecholamines in nociceptors are metabolized to neurotoxic products by MAO-A, which can cause neuronal dysfunction underlying neuropathic pain.

Striatal damage and oxidative stress induced by the mitochondrial toxin malonate are reduced in clorgyline-treated rats and MAO-A deficient mice

Intrastriatal administration of the succinate dehydrogenase (SDH) inhibitor malonate produces neuronal injury by a "secondary excitotoxic" mechanism involving the generation of reactive oxygen species (ROS). Recent evidence indicates dopamine may contribute to malonate-induced striatal neurodegeneration; infusion of malonate causes a pronounced increase in extracellular dopamine and dopamine deafferentation attenuates malonate toxicity. Inhibition of the catabolic enzyme monoamine oxidase (MAO) also attenuates striatal lesions induced by malonate. In addition to forming 3,4-dihydroxyphenylacetic acid, metabolism of dopamine by MAO generates H2O2, suggesting that dopamine metabolism may be a source of ROS in malonate toxicity. There are two isoforms of MAO, MAO-A and MAO-B. In this study, we have investigated the role of each isozyme in malonate-induced striatal injury using both pharmacological and genetic approaches. In rats treated with either of the specific MAO-A or -B inhibitors, clorgyline or deprenyl, respectively, malonate lesion volumes were reduced by 30% compared to controls. In knock-out mice lacking the MAO-A isoform, malonate-induced lesions were reduced by 50% and protein carbonyls, an index ROS formation, were reduced by 11%, compared to wild-type animals. In contrast, mice deficient in MAO-B showed highly variable susceptibility to malonate toxicity precluding us from determining the precise role of MAO-B in this form of brain damage. These findings indicate that normal levels of MAO-A participate in expression of malonate toxicity by a mechanism involving oxidative stress.

Differential subcellular location of mitochondria in rat serotonergic neurons depends on the presence and the absence of monoamine oxidase type B

Monoamine oxidase type A and type B are major neurotransmitter-degrading enzymes in the CNS. The type A is present on mitochondrial outer membranes in the whole extent of noradrenergic and dopaminergic neurons, including their axon terminals. The type B is present in serotonergic neurons, but its subcellular localization has not been elucidated. In the present study, we used both a double-labeling immunofluorescence method and electron microscopic immunohistochemistry to examine the subcellular localization of monoamine oxidase type B in serotonergic neurons projecting from the dorsal raphe nucleus to the suprachiasmatic nucleus in the rat brain. In the dorsal raphe nucleus, serotonin-positive neuronal cell bodies were clustered, and virtually all of these cell bodies were also positive for monoamine oxidase type B. By contrast, serotonin-negative neuronal cell bodies were mostly free of this enzyme. Within the neuronal cell bodies and dendrites that were positive for monoamine oxidase type B, most mitochondria contained this enzyme on their outer membranes, but a substantial proportion of mitochondria lacked this enzyme. In the suprachiasmatic nucleus, serotonin-positive varicosities were concentrated, but none of these varicosities exhibited monoamine oxidase type B. In this nucleus, mitochondria were found in almost all serotonin-positive axon terminals, but monoamine oxidase type B was not observed in any axon terminal that contained mitochondria. Our results show that there are two kinds of mitochondria in serotonergic neuronal cell bodies and dendrites: one containing monoamine oxidase type B on their outer membranes, and the other lacking this enzyme. In addition, mitochondria in serotonergic axon terminals do not possess monoamine oxidase type B. It is suggested in serotonergic neurons that only mitochondria lacking monoamine oxidase type B are transported by axonal flow up to axon terminals. It is also probable that mitochondria containing monoamine oxidase type B are transported along the axons, but that this enzyme undergoes a change, for example, conformational change, decomposition or removal from the membranes.

Region-specific alterations in the concentrations of catecholamines and indoleamines in the brains of young and old F344 rats after L-deprenyl treatment

The effects of L-deprenyl, a monoamine oxidase-B (MAO-B) inhibitor, on the concentrations of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), serotonin (5-hydroxytryptamine), and 5-hydroxyindoleacetic acid (5-HIAA) in medial basal hypothalamus (MBH), substantia nigra (SN), striatum (Str), and nucleus accumbens (NAc) of young (3 month) and old (21 month) male F344 rats were examined after a 7-day wash-out period following 1, 15, or 30 days of deprenyl treatment in young rats and a 9-day wash-out period after a 10-week deprenyl treatment in old rats. The brain areas were microdissected and the concentrations of neurotransmitters were measured by High Performance liquid chromatography with electrochemical detection (HPLC-EC). Deprenyl administration following the drug wash-out period increased the concentrations of DOPAC in the SN, Str, and in the NAc of young rats but it was decreased in the NAc of old rats. The concentration of HVA was lower in the Str of young deprenyl-treated rats, and in the Str and NAc of old deprenyl-treated rats, but it was higher in the SN of young deprenyl-treated rats. The concentration of 5-HIAA was increased in the MBH, SN, and in the NAc of young deprenyl-treated rats, but it was decreased in the Str and NAc of old deprenyl-treated rats. The concentration of NE was increased in the MBH, SN, Str, and in the NAc of young rats treated with deprenyl and in the MBH of old deprenyl-treated rats. The concentration of 5-HT was increased in the SN of young deprenyl-treated rats. The concentration of DA increased in the Str of both young and old deprenyl-treated rats. We concluded that a drug wash-out period after deprenyl treatment differentially affects the metabolism of catecholamines and indoleamine depending on the region of the brain and that this effect may be due to variation in the kinetics of MAO inhibition.

A simple enzyme histochemical method for the simultaneous demonstration of acetylcholinesterase and monoamine oxidase in fixed-frozen sections

We describe an enzyme histochemical technique for the simultaneous demonstration of acetylcholinesterase (AChE) and monoamine oxidase (MAO) (Types A, B, or A+B) in fixed-frozen sections. Several regions in the mesencephalon and brainstem were examined for both somatic and neuropil labeling. The results obtained are equivalent or superior to those obtained using previous methods for the individual localization of these enzymes. The simultaneous visualization of AChE and MAO in the same section allows the relationship of the two enzymes to be easily assessed with brightfield microscopy.

Monoamine oxidase isoenzymes in rat brain: differential changes during postnatal development but not aging

Differential development of monoamine oxidase (MAO) isoenzymes in rat whole brain is described in postnatally developing Sprague-Dawley rats. Total MAO and isoenzyme activity was measured using nonspecific and specific substrates. Total MAO activity measured using tyramine, increased postnatally up to 24 weeks of age and attained a plateau afterward. The increase in total MAO activity was significant at all age groups (18 days to 36 months) investigated as compared to new born rats. MAO-A and MAO-B activities were measured using octopamine and benzylamine respectively. We also observed a marginal increase of MAO-A activity and a significant increase of MAO-B activity upon development. Furthermore, at 12 weeks of age, MAO-B activity increased by 10-fold as compared to new born and was consistent up to 36 months of age. The qualitative localization of the enzyme activity on non SDS-PAGE by nitroblue tetrazolium staining confirmed the increase of MAO-B during the development. It is suggested that the maturational increase of total MAO activity in brain is predominantly due to the increase of MAO-B isoenzyme.

Chronic L-deprenyl or L-amphetamine: equal cognitive enhancement, unequal MAO inhibition

The effect of chronic (4 month), subcutaneous injections of saline, L-deprenyl (0.25 mg/kg), or L-amphetamine (0.25 mg/kg) on the acquisition of a learned spatial habit in a modified Morris Water Maze was investigated in middle aged rats. Injections, given three times weekly starting at 6 months of age, were continued during behavioral testing, which occurred at 10 months of age. The cognitive performance of the middle aged rats was compared to that of 2-month-old control rats. Twenty-four hours after the last behavioral test, the rats were sacrificed and their brains were removed, dissected, and frozen in liquid nitrogen. The activities of MAO-A and MAO-B in the lateral cortex were determined. Results indicate that rats in the L-deprenyl group, the L-amphetamine group, and the young control group all learned the water maze task equally rapidly and significantly faster than rats in the saline group. MAO-A did not differ among the saline, amphetamine, and young control rats, but MAO-B was significantly higher in the middle aged saline and L-amphetamine rats than in the young controls. Both MAO-A and MAO-B activities were significantly lower in the L-deprenyl group than in the other three groups. This indicates that low-dose L-deprenyl can also inhibit MAO-A following chronic SC administration. Moreover, the improved cognitive performance produced by L-deprenyl may not be due to its ability to inhibit MAO-B, but rather to some other effect such as the activation of growth factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular distribution of monoamine oxidase A in selected regions of rat and monkey brain and spinal cord

Monoamine oxidase A and B (MAO A and B; EC 1.4.3.4) are integral proteins of the outer mitochondrial membrane that degrade monoamines including the neurotransmitters norepinephrine, dopamine, and serotonin. In this study, monoclonal antibodies that recognize rat or monkey MAO A were used in immunocytochemical studies to visualize the subcellular localization of this enzyme within neurons in the central nervous system of these species. The regions examined included the locus coeruleus, substantia nigra, spinal cord, and pallidostriatum, which are known to contain MAO A-positive structures. Ultrastructural studies revealed that most MAO A staining was associated with the outer membrane of mitochondria, within the cell bodies, dendrites, axons and terminals. However, some immunoreactive staining for MAO A was also observed in the rough endoplasmic reticulum in the cell bodies. Staining for mitochondrial MAO A in dendrites was observed in terminal fields of the monoamine system, including the spinal cord and the pallidostriatum. The intensity of staining also increased in the subsynaptic density. MAO A was also found associated with mitochondria in ependymal cells lining the fourth ventricle adjacent to the locus coeruleus and in the endothelial cells lining the blood vessels. Localization of MAO A in noradrenergic neurons, ependymal cells, and subsynaptic regions of dendrites in monoamine terminal fields supports the concept that this neurotransmitter-degrading enzyme may play a protective role in the central nervous system.

Is ammonia a pathogenetic factor in Alzheimer's disease?

An attempt was made to review experimental evidence in favor of the idea that ammonia plays a role in dementia of the Alzheimer type (DAT). Hyperammonemia causes biochemical and cellular dysfunctions in the brain, which can be found in brains of DAT patients. The most conspicuous among these findings are astrocytosis, impairment of glucose utilization, and a decreased rate of energy metabolism, and the impairment of neurotransmission, with a net increase in excitability and glutamate release. The derangement of lysosomal processing of proteins is another potential site of ammonia action. This aspect is especially important in view of the growing evidence for the role of the endosomal-lysosomal system in the formation of amyloidogenic fragments from beta-amyloid precursor protein. Ammonia is not considered a primary factor of the disease. However, since hyperammonemia and release of ammonia from the brains of DAT patients is well supported by published observations, ammonia should be taken into account as a factor that contributes to manifestations and the progression of DAT. If elevated ammonia concentrations turn out to be indeed as important in DAT, as is suggested in this review, rational therapeutic avenues can be envisaged that lead to the amelioration of symptoms and progression of the disease.

Effect of repeated electroconvulsive shock on striatal L-dopa and dopamine metabolism: an in vivo study

A course of treatments with electroconvulsive shock (ECS) has been reported to reestablish L-dopa efficacy in patients with advanced Parkinson's disease. We wished to determine if ECS could modify L-dopa and dopamine metabolism in an animal model of Parkinson's disease. Therefore, we administered repeated ECS (8 ECS at 48 hr intervals) to rats with partial destruction of the nigrostriatal dopamine pathway and used the cerebral microdialysis technique to monitor extracellular concentrations of dopamine and dopamine metabolites (DOPAC and HVA) in the corpus striatum. The control group of animals received sham-ECS treatments. Basal dopamine levels were decreased by 20% in animals receiving repeated-ECS versus sham-ECS. DOPAC levels, on the other hand, were increased by 84% in animals receiving repeated-ECS. HVA levels were equal in the two groups. Following L-dopa administration, dopamine and HVA levels increased equally in control animals and animals which had previously received repeated-ECS. DOPAC concentrations were uniformly greater in rats receiving repeated-ECS. When ECS was administered acutely, dopamine levels increased 390% and returned to baseline values in 75 minutes, DOPAC and HVA were unchanged, and 5HIAA levels decreased 30%. We conclude that 1) acute ECS administration produces a transient, marked release of striatal dopamine and 2) repeated ECS can reset the level of basal dopamine release, a finding compatible with ECS-induced dopamine receptor supersensitivity, and 3) neither single nor repeated administration of ECS has a major effect on the formation of dopamine or HVA from exogenously administered L-dopa although there was a strong tendency for increased DOPAC formation. ECS may exert its putative antiparkinsonian effect by enhancing dopamine receptor sensitivity.

Monoamine oxidase, dopamine and Parkinson's disease

Four aspects about monoamine oxidase (MAO; E.C. 1.4.3.4) are of obvious interest in relation to Parkinson's disease and its treatment with the irreversible and selective MAO-B inhibitor L-deprenyl and are discussed in this review: 1) To what extent the two forms of MAO are of importance for the deamination of dopamine and to what degree MAO localised inside and outside of dopaminergic nerve terminals contributes 2) The kinetics of the MAO-protein, i.e. the rate of recovery of MAO after irreversible inhibition. 3) To what extent MAO may be changed as a consequence of the pathophysiological processes. 4) To what extent MAO may be involved as a force in the pathophysiological processes.

Improvement of cognitive function by MAO-B inhibitor L-deprenyl in aged rats

This study evaluated the ability of the selective MAO-B inhibitor, L-deprenyl, to reverse cognitive impairments appearing in aged rats, using the reference memory, Morris Water Maze paradigm. L-Deprenyl significantly improved learning and memory deficits associated with old age in doses of 1.25 and 5 mg/kg PO (escape latency measure) and doses of 1.25, 2.5 and 5 mg/kg PO (path length measure). L-Deprenyl also improved reversal learning impairments in doses of 1.25, 2.5 and 5 mg/kg PO, as expressed by the escape latency measure. The data suggest that L-deprenyl possesses potential cognitive enhancement abilities probably due to an increase in dopaminergic activity.

[3H]harman binding experiments. II: Regional and subcellular distribution of specific [3H]harman binding and monoamine oxidase subtypes A and B activity in marmoset and rat

[3H]Harman (1-[3H]methyl-beta-carboline) was used in a novel radioligand binding assay to label selectively and with high affinity monoamine oxidase (MAO) type A. The concentration of the enzyme was determined in six CNS regions of the primate species marmoset (Callithrix jacchus) and of the rat: hypothalamus, hippocampus, cerebellum, cerebral cortex, striatum, and spinal cord. The specific [3H]harman binding in the CNS of the marmoset reveals the same pharmacological profile and other characteristics (affinity, saturability, and reversibility) as in the CNS of the rat. The regional distribution of the [3H]harman binding density (Bmax) in the CNS exhibits a distinct pattern in the marmoset and the rat and a 35 (hypothalamus) to 75% (hippocampus) lower Bmax in the marmoset than in the rat. The Bmax values of [3H]harman binding in the CNS of the marmoset and the rat combined as well as those from visceral organs of the rat (liver, heart, lung, thymus, spleen, and kidney) correlated positively and highly significantly with the respective Vmax values of specific MAO activity of the A type but not of the B type, determined with kynuramine as the substrate. In subcellular fractionation experiments with rat cerebral cortex, the highest [3H]harman binding density (Bmax) and MAO-A activity (Vmax) were detected in mitochondrial fractions and severalfold lower values in the synaptosomal membrane fraction. In conclusion, we suggest that [3H]harman binding is a biochemical tool as a selective marker to quantify MAO-A in the CNS of different mammalian species as well as in extraneuronal tissues.

Monoamine oxidase B in brains from patients with Alzheimer's disease: a biochemical and autoradiographical study

In vitro quantitative autoradiography using [3H]L-deprenyl, an irreversible and preferential inhibitor of monoamine oxidase B, was performed to investigate the localization of the enzyme in brains from senile dementia of Alzheimer type and control cases. Brains from three male patients with the clinical diagnosis of senile dementia of Alzheimer type and from three male control patients, without any known clinical history of neurological disorder, were obtained at autopsy. Cryosections of 100 microns thickness were mounted on gelatinized glass plates and dried over desiccant for one week at -20 degrees C. The sections were incubated with 10 nM [3H]L-deprenyl for 1 h and then exposed to film for four weeks. The autoradiographs were analysed by computer-assisted densitometry. Monoamine oxidase-B activities were also estimated in 1% homogenates from 10 different regions, using 10 microM beta-[ethyl-14C]phenylethylamine, in order to study the consonance between the autoradiographical and biochemical techniques. Both [3H]L-deprenyl binding and monoamine oxidase-B activities in senile dementia of Alzheimer type were higher than in the controls in all brain regions studied. The increase was highest in the white matter (about 70%) and in the order of 20-50% in the various gray matter regions. A high correlation coefficient (r approximately 0.9) was obtained between [3H]L-deprenyl binding and monoamine oxidase-B activity, both in the senile dementia of Alzheimer type and in the control brains.

Enhancement of rat brain cytosolic monoamine oxidase activity by clorgyline. Comparison with (-)-deprenyl and MDL 72145

The presence of unsedimentable forms of monoamine oxidase (EC 1.4.3.4) in liver and brain homogenates has prompted fresh studies on the effects of inhibitors on this cytosolic monoamine oxidase. Clorglycine is a specific monoamine oxidase A inhibitor and (-)-deprenyl and MDL 72145 are specific monoamine oxidase B inhibitors. We investigated the effects of (-)-deprenyl, MDL 72145 and clorgyline on the purified enzyme from mitochondria and cytosolic monoamine oxidase along with high speed cytosol and 1% Triton X-100 treated mitochondrial preparations. Clorgyline enhanced the activity of the purified enzyme several-fold. (-)-Deprenyl and MDL 72145 also enhanced and inhibited the activity of cytosolic monoamine oxidase in a concentration-dependent manner.

Ontogenesis of monoamine oxidase-A and -B in the human brain frontal cortex

The effect of several antemortem and postmortem factors (patients' age, sex, postmortem delay, storage time, laterality and brain weight) on both monoamine oxidase-A (MAO-A) and -B (MAO-B) activity was investigated in the frontal cortex of human brains. The MAO-A activity decreases rapidly during the first two years of life and remains constant thereafter. In contrast, the MAO-B activity keeps unchanged during early childhood and raises during advanced age. These findings seem to be consistent with a genetic regulation and a variation in cell type assembling during brain development and aging.

The cholinergic neurotoxin ethylcholine mustard aziridinium (AF64A) induces an increase in MAO-B activity in the rat brain

Recently it was reported that there is an increase in monoamine oxidase B (MAO-B) activity in post-mortem brains of patients with Alzheimer's disease. It was postulated that this increase in MAO-B activity was due to gliosis associated with neuronal degeneration. The aim of the present investigation was to evaluate the effect on MAO of neuronal degeneration primarily affecting the cholinergic system. The specific cholinergic toxin AF64A (3 and 4.5 nmol) was injected bilaterally into the cerebral ventricles of rats. We then estimated MAO-A, MAO-B, dopamine (DA) uptake rates and choline acetyltransferase (ChAT) activities in hippocampus, striatum and cortex, 1, 2.5 and 4.5 weeks after the injection. Marked long-lasting reduction in ChAT activities appeared only in hippocampus, consistent with previous reports. The MAO-A activity was unchanged as were DA uptake rates. Neither was there any change in MAO-B activity found 1 week after the injection. However, a significant increase in MAO-B activity appeared after 2.5 weeks and persisted after 4.5 weeks in all 3 brain regions investigated. This result is likely to reflect progressive gliosis after cholinergic neuronal degeneration. Previous results have shown an increased MAO-B activity with age and a further accelerated increase in Alzheimer's disease. Experimentally, hemitransection and injection of kainic acid have been shown to cause a similar increase. The present results show that changes in MAO-B activity also reflect degenerative processes in brain mainly affecting the cholinergic system.

Topographic immunocytochemical mapping of monoamine oxidase-A, monoamine oxidase-B and tyrosine hydroxylase in human post mortem brain stem

Immunocytochemical demonstration of monoamine oxidase-A, monoamine oxidase-B and tyrosine hydroxylase was performed in the human brain stem using monoclonal antibodies to monoamine oxidase-A and monoamine oxidase-B and polyclonal antibodies to tyrosine hydroxylase. In most of the brain areas examined, except the serotonergic dorsal nucleus of raphe, the noradrenergic locus coeruleus and the dorsal efferent nucleus of vagus, tyrosine hydroxylase-positive neurons were in greater number than monoamine oxidase-A-stained or monoamine oxidase-B-stained neurons. The dorsal nucleus of raphe showed no tyrosine hydroxylase immunoreactivity, but reacted positively to serotonin- and monoamine oxidase-B antibodies, while monoamine oxidase-A staining was moderate. In none of the investigated brain areas did neurons exclusively react with monoamine oxidase-B antibodies without expressing monoamine oxidase-A in a few neurons, while in some areas neurons expressed both monoamine oxidase-A and tyrosine hydroxylase (locus coeruleus; dorsal efferent nucleus of vagus). The oculomotor nucleus stained only with monoamine oxidase-A antibodies, substantia nigra neurons reacted only with tyrosine hydroxylase antibodies. Glial staining in most of the brain areas examined seemed, with slight differences, to have the same intensity with monoamine oxidase-A and monoamine oxidase-B antibodies used. No glial staining was obtained with tyrosine hydroxylase antibodies.

Monoamine oxidase activity in single nerve cell bodies from substantia nigra of rat and man

In single nerve cell bodies isolated from the substantia nigra of man and rat the active forms of MAO A and MAO B were found by the use of the microdiver technique and specific inhibitors.

Demonstration of a distinct class of high-affinity binding sites for [3H]norharman [( 3H]beta-carboline) in the rat brain

Specific binding sites were demonstrated for some beta-carbolines in the rat brain with [3H]norharman as a ligand. The ligand displayed a high affinity for synaptosomal membranes which had been fractionated by a sucrose gradient. The calculated apparent KD value was 1.55 nmol/l and the maximum number of binding sites 148 fmol/mg protein. Displacement studies showed an exclusive specificity for a small group of beta-carbolines but not for the previously described inverse agonists at the benzodiazepine receptor nor for tryptamine and other indoles, as well as pargyline, a monoamine oxidase inhibitor. Further analysis revealed other binding sites for [3H]norharman, with an apparent KD value of 36 nmol/l that are presumably located on mitochondrial membranes. Binding to these sites was also not displaced by pargyline. Pargyline displaced [3H]norharman from a third population of binding sites on mitochondrial membranes with the apparent KD value of 46 nmol/l. These findings could explain the pharmacological effects of norharman and other beta-carbolines in vivo.

Immunohistochemical localization of monoamine oxidase-B in the cat brain: clues to understanding N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity

The immunohistochemical localization of monoamine oxidase-B in normal cat brain was examined. The enzyme was localized in both neural and nonneural elements of the cat brain. Neurons in the hypothalamus (lateral, dorsal, ventromedial, dorsomedial, and supraoptic nuclei), raphe system, dorsal tegmental nucleus, locus ceruleus, Kölliker-Fuse nucleus, dorsal parabrachial region, and central tegmental field were positive. No substantia nigra pars compacta, retrorubral, or ventral tegmental neurons stained positively. Glial cells (astrocytes) stained positively for monoamine oxidase-B in many regions of the central nervous system, however, there was a significantly greater number of monoamine oxidase-B-positive glial cells in the substantia nigra pars compacta than in other adjacent dopaminergic regions. Because nigra compacta neurons are specifically damaged by the neurotoxin MPTP and because the toxicity of the drug is expressed only in the presence of monoamine oxidase-B, it is possible that the preferential loss of substantia nigra pars compacta neurons in the cat brain may be related to the regional and cellular localization of monoamine oxidase-B.

Effects of 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine and its metabolite 1-methyl-4-phenylpyridine on acetylcholine synthesis in synaptosomes from rat forebrain

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+), have been shown to cause a number of lesions in dopaminergic pathways of the nigro-striatal region of the brain. However, data on the effects of these neurotoxins on other aspects of brain metabolism are scarce. The data presented here show that MPTP and MPP+ inhibit glucose oxidation via the tricarboxylic acid cycle, and acetylcholine synthesis in synaptosomal preparations from rat forebrain. Monoamine oxidase B inhibitors (e.g., pargyline, MDL 72145) relieve the inhibition caused by MPTP but not MPP+. The inhibitory effects of MPP+ on glucose oxidation and acetylcholine synthesis are a consequence of the decreased glucose metabolism in synaptosomes and are consistent with its role as an inhibitor of the Complex I (NADH-CoQ reductase) of the mitochondrial respiratory chain.

Amine oxidases and their endogenous substrates (with special reference to monoamine oxidase and the brain)

The roles of MAO, BzO, DAO and PAO in the metabolism of endogenous substrates and the functional implications of their action and inhibition is reviewed, the emphasis being on MAO on one hand and on brain on the other. The major issues are the following: There is no discrete subdivision into substrates selective for MAO-A, MAO-B, or mixed ones, but rather a continuum. Tissue differences in substrate specificity are not likely to be due to molecular variability of MAO. For the deamination of DA, 5-HT and PEA at least, the relative participation of either MAO form in a given tissue is primarily determined by the relative abundance of the two forms; only at 10(-5) M and above, substrate concentration begins to matter also. In vivo, compartmentation is of paramount importance: since there seems to be more MAO-A than B inside monoaminergic neurons, DA, 5-HT and NA are predominantly metabolized by MAO-A if metabolism occurs mainly intraneuronally. Conversely, since MAO-B is more abundant extraneuronally, e.g. in glia cells, the relative participation of this form increases if a significant portion of these amines is deaminated outside monoaminergic neurons. In vivo, monoamine deamination is reduced concomitantly with the degree of MAO inhibition, whereas signs of increased transmitter function are only observed if enzyme inhibition is at least 80%. This is likely to be the result of the action of compensatory mechanisms such as feedback inhibition of transmitter release and synthesis. BzO is particularly abundant in vascular tissue, lung and bone. Low levels are found in brain. Endogenous substrates and physiological function are not known. DAO also occurs only in minimal amount in brain, if at all. Its principal substrates are histamine and the polyamines, and the disposal of these amines is probably its main function. Of the PAO's, the type of enzyme found in the rat liver attacks the secondary amino groups and may have a more prominent role in the metabolism of polyamines in the brain than in the periphery. Bovine plasma PAO, which attacks primary amino groups, is only found in the serum of ruminants, but not other species. Its function in the metabolism of polyamines is not known.

Time course of the metabolite patterns of intraventricularly injected [3H]noradrenaline in rat brain regions

In the hypothalamus, septum, pons with medulla, and hippocampus regions of rat brain, the level of radioactivity of [3H]noradrenaline and of five of its metabolites were determined up to 6 h after intraventricular injection of the tritiated amine. The following main results were found: In anterior hypothalamus and septum, the [3H]noradrenaline level declined in two phases. Similar turnover curves were obtained for the primary deaminated metabolites, with almost the same final half-lives as for [3H]noradrenaline. The level of the initial methylation product, normetanephrine, also showed a biphasic decline, which did not correspond to that of [3H]noradrenaline but rather was faster throughout the experiment. The final metabolites (i.e., the glycol sulfates) reached maximal levels in hypothalamus and septum earlier than in other regions. Thereafter, their levels declined with almost similar rates in all areas tested, but always faster than the [3H]noradrenaline level. The following conclusions were drawn: In areas rich in catecholaminergic nerve terminals, there seems to be a site, in addition to the vesicular storage pool, that accumulates exogenous noradrenaline and then releases it with relatively short half-lives. The contents of primary deaminated metabolites followed the turnover of [3H]noradrenaline at both sites. Exogenous [3H]noradrenaline seems to be methylated at two extraneuronal sites, which are distinguished by the rates of subsequent deamination. The size of the pool of slowly deaminated [3H]normetanephrine that is formed immediately after [3H]noradrenaline injection determined the apparent turnover of this product throughout the experiment and, thus, like the final metabolites, reflects for several hours the initial degradation of the unstored [3H]noradrenaline, rather than the metabolism of the stored amine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of maternal methadone administration on the development of multiple forms of monoamine oxidase in rat brain and liver

The development of total monoamine oxidase (MAO) and MAO-A and MAO-B in the forebrain, brainstem, cerebellum and liver of methadone-treated and normal rats were monitored with tryptamine, serotonin and benzylamine, respectively. Daily administration of methadone (10 mg/kg, s.c.) to pregnant and nursing rats substantially retarded the development of total and the A-form of MAO in the brain regions of pups but had no effect on that of MAO-B. The effect of methadone on the development of total MAO and MAO-A in the liver was only transient and less significant. This finding indicates that the perinatal opiate syndrome associated with maternal exposure to methadone is caused by the inhibition in MAO-A development in monoaminergic neurons in the brain.

Brain dialysis: in vivo metabolism of dopamine and serotonin by monoamine oxidase A but not B in the striatum of unrestrained rats

A dialysis cannula was implanted into rat striatum while the animals were anesthetized, and the area was perfused with Ringer solution while the animals were unanesthetized after at least 3 days following surgery. Concentrations of the metabolites of 3,4-dihydroxyphenylethylamine (DA) and 5-hydroxytryptamine (5-HT) in the perfusate were determined by HPLC with electrochemical detection. Levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the perfusate significantly decreased after pargyline administration (50 mg/kg i.p.), which may inhibit not only monoamine oxidase (MAO)-B but also MAO-A in these high doses. The level of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) also decreased after pargyline treatment, although change in the relative level of 5-HIAA was less than that of DOPAC or HVA. To clarify the mechanisms for the metabolism of monoamines in rat striatum, highly specific MAO-A and -B inhibitors were used in the following experiments. Treatment with l-deprenyl (10 mg/kg), a specific inhibitor for MAO-B, did not cause any statistically significant change in DOPAC, HVA, and 5-HIAA levels. No significant change was found in rat striatal homogenates at 2 h after the same treatment with l-deprenyl. In contrast, low-dose treatment (1 mg/kg) with clorgyline, a specific inhibitor for MAO-A, caused a significant decrease in levels of these three metabolites in both the perfusates and tissue homogenates. In addition to the above three metabolites, the level of 3-methoxytyramine, which is an indicator of the amount of DA released, greatly increased after treatment with a low dose (1 mg/kg) of clorgyline.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo labelling and axonal transport of monoamine oxidase in the rat basal ganglia using radioactive pargyline

The enzyme monoamine oxidase was labelled in the rat striatum or substantia nigra with locally injected radioactive pargyline. The binding was prevented by a pretreatment with non-radioactive pargyline, or with a combination of clorgyline and deprenyl. Most of the MAO labelled with 3H-pargyline was of the B-type, but also some MAO-A was labelled, as shown in rats pretreated with clorgyline or deprenyl separately. Seven days after the injection of (3H)-pargyline into the striatum a significant labelling was observed in the substantia nigra. This labelling was clorgyline sensitive, indicating type A MAO, and was not present when striatal neurons were destroyed with kainic acid. Labelling of the striatum following 3H-pargyline injection into the substantia nigra was also less in kainate intoxicated striata. Damage of nigral dopamine neurons with 6-hydroxydopamine did not influence the distribution of the label. Thus by using 3H-pargyline, specific labelling and axonal transport of type A MAO in striatal neurons projecting to the substantia nigra was demonstrated.

Selective inhibition of monoamine oxidase by p-aminosubstituted phenylalkylamines in catecholaminergic neurones

The in vivo inhibition of monoamine oxidase (MAO) inside and outside noradrenergic and dopaminergic nerve terminals in the hypothalamus and striatum, respectively, was examined in the rat after oral administration of a series of substituted p-aminophenethylamines and some related compounds. This was achieved by measuring their ability to protect MAO from irreversible inhibition by phenelzine, determined by the deaminating activity of synaptosomal preparations in the absence and presence of maprotiline, a selective inhibitor of the uptake of noradrenaline, or of amfonelic acid, a potent inhibitor of the uptake of dopamine, with small (0.25 microM) concentrations of [14C]noradrenaline or [14C]dopamine as substrate. It was found that several of these compounds were much more potent in protecting MAO within the noradrenergic neurones than MAO in other cells. Since the inhibitors of the uptake of noradrenaline, desipramine and CPP 199 antagonized this preference for noradrenergic MAO it is concluded that these MAO inhibitors are accumulated in the noradrenergic neurones by the membranal uptake carrier. Hence the selectivity for MAO within noradrenergic neurones seems to reflect the ability of the compounds to be transported by this carrier. The structure-activity relationship obtained showed the greatest selectivity for the unsubstituted p-dimethylamino-(FLA 289), p-methylamino-(FLA 727) and p-amino-(FLA 334)-amphetamines, whereas the 2-fluoro compound (FLA 558) had the greatest potency. N,N-didesmethylamiflamine [FLA 668(+)] had an almost specific effect in the noradrenergic nerve terminals. The primary p-amino derivatives, FLA 334 and FLA 668, produced a marked selective protection of MAO in dopaminergic nerve terminals, whereas the tertiary and secondary derivatives had much less preference for dopaminergic MAO.(ABSTRACT TRUNCATED AT 250 WORDS)

Intra- and extraneuronal monoamineoxidase-A and -B activities after central axotomy (hemisection) on rats

Hemitransection of the left side of rat brain results in a selective increase (40%) in the activity of MAO-B in the left side striatum, as compared to the right, unoperated side. This increase is shown to be the result of an increase in the activity of extraneuronal MAO-B using a "low substrate concentration method" with dopamine as substrate. This result is compatible with the hypothesis, that in certain degenerative processes such as aging, Alzheimer's disease, Huntington's disease and axotomy there is a stimulated growth of extraneuronal cells, which are relatively rich in MAO-B activity.

Kinetics of the mechanism of action of monoamine oxidase in the regulation of Na+,K+-ATPase activity in rat brain

Kinetic studies on the action of monoamine oxidase (MAO) in the regulation of Na+,K+-ATPase were performed using 3-methoxy-4-hydroxybenzaldehyde (MHB), which is an analogue of 3-methoxy-4-hydroxy-phenylacetylaldehyde (product of MAO-catalysed reaction with dopamine as substrate). It was observed that at 2.6 microM MHB, the activation of Na+,K+-ATPase may be the result of the removal of the inhibitory Ca2+, thereby increasing the Vmax. Double-reciprocal plots of Pi versus MHB showed that Ca2+ counteracted the effect of the aldehyde not by changing the Km, but be decreasing the Vmax of the Na+,K+-ATPase stimulation. The removal of 3',5'-cyclic AMP-dependent protein kinase from the microsomes by sodium dodecyl sulphate treatment abolished the activation and/or inhibition of the Na+,K+-ATPase by aldehyde; it can therefore be inferred that 3',5'-cyclic AMP-dependent protein kinase is involved in the regulation of Na+,K+-ATPase.

Comparison of the properties of semipurified mitochondrial and cytosolic monoamine oxidases from rat brain

Mitochondrial and cytosolic monoamine oxidases were purified 220- and 129-fold, respectively, from rat brain. The purification procedure involved extraction (without the use of detergents for mitochondrial monoamine oxidase), ammonium sulfate precipitation, and chromatography on Sephadex G-25 and a DEAE-cellulose column. The properties of both enzymes with kynuramine as substrate, including Km values and pH optima at different kynuramine concentrations; the Rf values on polyacrylamide gel electrophoresis; and the thermal inactivation patterns were different. 2-Mercaptoethanol, together with heat treatment, released the flavin and decreased the enzyme activity differentially for the two enzymes. The absorption spectrum showed a "Red shift" in the absorption maxima when the spectra of the non-Triton-treated purified preparations were compared with those of the Triton-treated ones, thus possibly revealing that the mitochondrial and the cytosolic monoamine oxidases may be two different enzyme entities.

Age-related changes in aldehyde dehydrogenase activity of rat brain, liver, and heart

Aldehyde dehydrogenase (ALDH) activity was measured in brains, livers, and hearts of 23-26-month-old and 3-month-old rats. A significant increase of ALDH activity was found in whole brain of old rats with both acetaldehyde (39%) and propionylaldehyde (15%) used as substrates. In different brain areas of old rats, with acetaldehyde used as substrate, a significant increase of ALDH activity was found in striatum (30-50%) and cerebral cortex (37%). However, no significant difference in ALDH activity was found in livers and hearts of young and old rats. Preliminary experiments showed a significant increase of aldehyde reductase activity (52%) with p-nitrobenzaldehyde used as substrate in whole brain of old rats compared with young rats. The present work indicates that an increase of ALDH activity in brain of old rats may be an adaptive phenomenon.

Localization of rat striatal monoamine oxidase activities towards dopamine, serotonin and kynuramine by gradient centrifugation and nigro-striatal lesions

Subfractionation of the crude synaptosomal-mitochondrial fraction of rat striatum in a continuous sucrose gradient in a zonal rotor led to the following results. The distribution pattern of monoamine oxidase (MAO) activity towards dopamine (DA) was very similar to the pattern of MAO activity towards serotonin (5HT), but differed from the pattern of MAO activity towards kynuramine (KYN). As 5HT is specifically deaminated by MAO-A while KYN is a common MAO substrate, this supports earlier suggestions that in rat striatal preparations DA is deaminated preferentially by MAO-A. The patterns of the MAO activities towards DA and 5HT were clearly dissimilar, despite considerable overlap, to the patterns of tyrosine hydroxylase (TH) and DOPA decarboxylase (DD) activity, both marking the presence of striatal dopaminergic synaptosomes. The peak activities were separated and all patterns were symmetrical without showing a shoulder. This indicates that rat striatal MAO activity towards DA and 5HT is not specifically or for the greater part localized in dopaminergic terminals. We also investigated the effects of electrolytic and 6-hydroxydopamine lesions of the substantia nigra, both causing extensive degeneration of striatal dopaminergic terminals as appeared from the large decrease of striatal TH and DD activity. However, neither type of lesion induced a reduction of the MAO activity towards any of the substrates used. It is concluded that the amount of MAO activity towards DA and 5HT (probably MAO-A activity) present in dopaminergic terminals is very low compared with the total activity of this enzyme in rat striatal tissue.

The deamination of noradrenaline and 5-hydroxytryptamine by rat brain and heart monoamine oxidase and their inhibition by cimoxatone, toloxatone and MD 770222

In both rat brain and heart, noradrenaline and 5-hydroxytryptamine are metabolised predominantly by monoamine oxidase-A. The Km values for 14C-noradrenaline in the rat brain and heart are 290 microM and 300 microM, respectively, whereas for 14C-5-hydroxytryptamine the values are 180 microM and 140 microM, respectively. In the rat brain, mixed substrate experiments suggested that 14C-noradrenaline and 14C-5-hydroxytryptamine are metabolised at the same active centre. Both substrates are inhibited with similar Ki values by the reversible inhibitors cimoxatone, toloxatone and MD 770222.

Neurochemical insights into monoamine oxidase inhibitors, with special reference to deprenyl (selegiline)

Monoamine oxidase (MAO) is distributed in neurons and non-neuronal tissue in the human central nervous system. It occurs there as MAO type A and MAO type B. It is not, however, established where both types are located intra- and/or extra-neuronally. Recently, the use of selective MAO-B blockers has shown beneficial effects in the treatment of Parkinson's disease (PD). Knowledge about the locus of action of MAO inhibitors is therefore of great importance. Our findings indicate that MAO-B inhibitors like deprenyl act by blocking neuronal and extra-neuronal MAO-B. This demonstrates that in the early stages of PD the action of deprenyl improves dopamine neurotransmission and hormonal action, whereas in the advanced stages of the disease, when there is progressive loss of dopaminergic neurons accompanied by gliosis, the drug seems to exert beneficial effects via the hormonal route.

The effect of deprenyl (selegiline) on intra- and extraneuronal dopamine oxidation

It was found that in the rat striatum DA was oxidized extrasynaptosomally to 11% by MAO-A and to 3% by MAO-B. The corresponding intrasynaptosomal oxidations were 84% and 2%, respectively. Those figures were virtually unchanged even if the rat brain MAO-B was selectively inhibited to 87% by deprenyl. In the human brain extrasynaptosomal oxidation was 16% and 66%, respectively, by MAO-A and -B. Intrasynaptosomally the corresponding figures were 12% and 6%, respectively. Selective inhibition of human caudate MAO-B was calculated to give a total reduction of DA oxidation of 63%. The differences between man and rat are due to the proportionately greater oxidation of DA by MAO-B in man, which is a consequence of a higher ratio of concentration of MAO-A/-B in the rat.

The metabolism of dopamine by both forms of monoamine oxidase in the rat brain and its inhibition by cimoxatone

In the rat brain, dopamine is metabolised by both A and B forms of monoamine oxidase (MAO), although the A form of the enzyme is the major component. The Km of MAO-A toward dopamine (120 microM) is lower than the Km of MAO-B toward this substrate (340 microM). The activity of MAO-A was lower in old rats than in young rats, and the same degree of decrease was found for 5-hydroxytryptamine as for dopamine as substrates for this enzyme form. The activity of MAO-B was higher in the old rats, the degree of increase being the same for dopamine as for beta-phenethylamine as substrates for this enzyme form. The Ki values of the inhibition of MAO-A by cimoxatone and MD770222 (the principal plasma metabolite of cimoxatone) were independent of the substrate used to assay for activity, but were lower than the Ki values for the inhibition of MAO-B by these compounds.

Mechanism of the involvement of monoamine oxidase in the regulation of (Na+ + K+)-ATPase in rat brain

Increasing concentrations of dopamine fail to give a biphasic response to (Na+ + K+)-ATPase activity in various subcellular fractions of rat brain preincubated with monoamine oxidase inhibitors, viz. 1 X 10(-4) M clorgyline and 1 X 10(-4) M deprenyl. The product of the monoamine-oxidase-catalysed reaction with dopamine as substrate is 3-methoxy-4-hydroxyphenylacetaldehyde. An analogue of this product is 3-methoxy-4-hydroxybenzaldehyde. This analogue, when incubated with the subcellular fractions which had been preincubated with monoamine oxidase inhibitors and dopamine, gave a more pronounced biphasic response to (Na+ + K+)-ATPase activity than that observed in the fractions incubated with dopamine alone.