The effect of tryptolines (1, 2, 3, 4-tetrahydro-β-carbolines) on monoamine metabolism and the platelet aggregation response in human platelets

Tryptolines (1, 2, 3, 4-tetrahydro-β-carbolines) are structurally related to 5-hydroxytryptamine. Using the human platelet preparation as a model of central serotoninergic neurons the action of various tryptolines on 5-hydroxytryptamine and epinephrine metabolism have been examined. All these compounds are competitive selective inhibitors of monoamine oxidase ‘type A’, 5-hydroxytryptamine being ‘type A’ substrate. 5-hydroxytryptoline and 5-methoxytryptoline are the most active monoamine oxidase inhibitors with IC₅₀ 0.5 μm and 1.5 μm respectively using 5-hydroxytryptamine as substrate. Tetrahydro-β-carbolines are also potent uptake inhibitors of 5-hydroxytryptamine and epinephrine; a significantly greater uptake inhibitory selectivity being shown for 5-hydroxytryptamine than epinephrine. Platelet aggregation response induced by 5-hydroxytryptamine, epinephrine and adenosine 5'-diphosphate are mediated by specific receptors on the platelet. Tetrahydro-β-carbolines by themselves do not induce platelet aggregation but are potent inhibitors of 5-hydroxytryptamine and epinephrineinduced aggregation. However, they are ineffective on aggregation induced by adenosine 5'-diphosphate. The comparison of the inhibition kinetics of tetrahydro-β-carbolines for 5-hydroxytryptamine and epinephrine uptake with that of the platelet aggregation response to these amines have shown that 5-hydroxymethtryptoline, methtryptoline, and tryptolin are poor inhibitors of uptake. However, 5-hydroxymethtryptoline and tryptoline are potent inhibitors of the aggregation response. It is suggested that tryptoline derivatives can distinguish between receptors for uptake and the platelet aggregation response to 5-hydroxytryptamine. In all respects 5-hydroxytryptoline and 5-methoxytryptoline showed greater pharmacological activity than the tryptoline and methtryptoline.Although the in vivo formation of tryptolines has been a matter of controversy, they have profound pharmacological activity. This is reflected in their actions described above. The results confirm what has been observed using rat brain preparations and would suggest that human platelets can be used as a model to study the action of these compounds on central serotoinergic neurons.     

Participation of brain monoamine oxidase B form in the neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine: relationship between the enzyme inhibition and the neurotoxicity

A neurotoxin for nigrostriatal dopaminergic neurons, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its oxidized metabolite, 1-methyl-4-phenylpyridine (MPP+), both dose-dependently inhibited rat striatal and forebrain monoamine oxidase (MAO) activity with monoamine oxidase A (MAO-A) selectively reversible (competitive, Ki = 4.5 and 2.0 microM) inhibition. A comparison of the Ki values indicated the affinity of MPP+ for MAO-A to be greater than that of MPTP. MPTP inhibited monoamine oxidase B (MAO-B) with both a reversible (competitive, Ki = 116 microM) and an irreversible time-dependent component, but inhibition by MPP+ was reversible and competitive (Ki = 180 microM). These results, together with previous findings on metabolism of MPTP to MPP+ by brain MAO-B, suggest that MPP+ is a simple inhibitor of MAO-A and MAO-B, but MPTP might be a 'suicide substrate' inhibitor for MAO-B.     

Histochemistry of MPTP oxidation in the rat brain: sites of synthesis of the parkinsonism-inducing toxin MPP+

Systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is toxic to the nigrostriatal dopaminergic neurons and produces a syndrome similar to Parkinson's disease. Conversion of MPTP to 1-methyl-4-phenylpyridine (MPP+) by monoamine oxidase-B (MAO-B) appears necessary for this neurotoxicity. When MPTP was used as the substrate for the histochemical localization of monoamine oxidase activity on sections of the rat brain, only a few specific sites were found in which MPTP oxidation to MPP+ occurs. These include the noradrenergic and serotoninergic neurons of the brainstem and the histamine neurons of the caudal hypothalamus. Dopamine neurons themselves do not display the capacity to oxidize MPTP. It is proposed that the conversion of MPTP to MPP+ occurs via MAO-B within serotonin and histamine neurons which may innervate the substantia nigra where the toxin MPP+ could be released and then taken up into the dopamine neurons.     

The effect of epileptiform discharges evoked by intrahippocampal injection of kainic acid on cholinergic and catecholaminergic hippocampal afferents

The influence of epileptiform seizures evoked by intrahippocampal injection of kainic acid on morphological changes of hippocampus and related brain regions was analyzed in rabbits using catecholamine histofluorescence, monoamine oxidase, acetylcholinesterase and Nissl staining methods. It was found that kainic acid induced generalized electroencephalographic seizures and a disappearance of hippocampal neurons. These effects did not affect the volume of neurons in septum and locus coeruleus. In the injected hippocampus, kainic acid destroyed hippocampal pyramidal cells and induced some sprouting of catecholamine, acetylcholinesterase-positive and monoamine oxidase-positive nerve fibers near the injection site.These results indicate that intrahippocampal kainic acid injection does not provoke a retrograde, transsynaptic degeneration in the medial septum and locus coeruleus, the brain regions which innervate the hippocampus.     

Platelet monoamine oxidase activity in Down's syndrome

The activity of platelet monoamine oxidase in Down's syndrome cases was significantly lower than that of controls. This difference was found for both males and females, and with tyramine, tryptamine and β-phenethylamine as substrate. The K_m values of the monoamine oxidase towards tryptamine as substrate from controls and Down's syndrome patients were similar.     

Immunization against monoamine oxidase slows extinction of a conditioned active escape reflex in rats

Immunization of white rats against bovine plasma monoamine oxidase increased the level of retention of a developed active escape reflex. This phenomenon can serve as a basis for determining the physiological role of bovine plasma monoamine oxidase. Department of Human and Animal Physiology, M. V. Lomonosov Moscow State University, Vorob’ev Heights, Moscow 119899, Russia. Translated from Rossiiski Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 83, No. 7, pp. 19–22, July, 1997.     

Preclinical profile of befloxatone, a new reversible MAO-A inhibitor.

Befloxatone, a novel oxazolidinone derivative, is a potent, selective and reversible monoamine oxidase A (MAO-A) inhibitor in vitro (K1A = 1.9-3.6 nM) and ex vivo (ED50 MAO-A = 0.02 mg/kg, p.o.). It does not interact with a large number of receptors, monoamine transporters or other amine oxidases. Binding studies with [3H]-befloxatone in rat brain sections show that it labels with high affinity (Kd = 1.3 nM) a single population of sites with the pharmacological characteristics and regional distribution of MAO-A. In the rat brain, befloxatone (0.75 mg/kg, i.p.) increases tissue levels of monoamines and decreases levels of their deaminated metabolites. Acute administration of befloxatone (0.75 mg/kg, i.p.) induces an increase in extracellular striatal dopamine and cortical norepinephrine but not cortical serotonin levels in the rat. Befloxatone (1 mg/kg, i.p.) potently inhibits the firing rate of serotonergic neurons, partially decreases the firing of noradrenergic neurons and has no effect on the firing of dopaminergic neurons (a mirror image of its effects on monoamine release in terminal regions), suggesting that the relative effects of befloxatone on monoamine release may be governed by autoreceptor-mediated control of monoaminergic neurons at the cell body level. Befloxatone (0.03-0.3 mg/kg, p.o.) exhibits potent activity in behavioural models predictive of antidepressant activity. Befloxatone (up to 1.5 mg/kg, p.o.) does not potentiate the pressor effects of orally administered tyramine at centrally active doses and duodenal [3H]-befloxatone binding is displaced by increasing doses of orally administered tyramine (0.1-40 mg/kg, i.p.). These results suggest that befloxatone is a potent reversible MAO-A inhibitor with antidepressant potential and a wide safety margin with regard to the potentiation of the pressor effect of tyramine.     

Demonstration of monoamine oxidase-A and -B in the human brainstem by a histochemical technique

The distribution of both monoamine oxidase subtypes, monoamine oxidase-A and -B, is demonstrated in brainstems from 16 humans by use of a histochemical technique. The results presented here, focus primarily upon the aminergic areas of the substantia nigra, the locus coeruleus and the raphe nuclei. While dopaminergic neurons of the substantia nigra revealed no staining for monoamine oxidase, noradrenergic neurons of the locus coeruleus stained positively with the monoamine oxidase-A substrate serotonin, and serotonergic neurons of the raphe nuclei were stained by the monoamine oxidase-B substrate beta-phenylethylamine. In addition, data are presented showing that glial cells stain predominantly for monoamine oxidase-B.     

Monoamine and diamine oxidase activities in atopic eczema

Increased plasma histamine levels were associated with significantly lowered diamine and type B monoamine oxidase activities in platelet rich plasma of atopic eczema patients.     

Monoamine oxidase activity, lipid peroxidation, and morphological changes in human hypothalamus during aging

Progressive gliosis of the hypothalamus during aging in humans is accompanied by activation of monoamine oxidase B activity, intensification of lipid peroxidation, and inhibition of SDH. Density of capillaries decreases. The role of monoamine oxidase B in the induction of lipid peroxidation is discussed.     

Cofactor levels of mono- and diamine oxidase in atopic eczema

We have recently shown that increased plasma histamine levels were associated with significantly lowered diamine and type B monoamine oxidase activities in platelet rich plasma of atopic eczema patients. We now report almost normal cofactor (pyridoxal phosphate and Cu2+) levels for diamine oxidase but lowered cofactor (flavin adenine dinucleotide and Fe2+) levels for type B monoamine oxidase.     

Comparative Analysis of Age-Related Changes in Activities of Monoamine Oxidase-B and Antioxidant Defense Enzymes in Various Structures of Human Brain

Activities of monoamine oxidase B, Cu-Zn-dependent superoxide dismutase (SOD), and catalase, and concentration of enzyme-active ceruloplasmin were measured in brain preparations from 43 humans died at the age of 21-92 years. Activity of monoamine oxidase B in the neocortex, cerebellum, basal ganglia, and stem structures increased with age, while SOD activity decreased and catalase and ceruloplasmin concentrations slightly increased. A contribution of age-related increase of cerebral monoamine oxidase activity into ontogenetic impairment of the antioxidant defense in human brain is discussed.     

Comparative study of age-related activity of monoamine oxidase-B,antioxidant defense enzymes,and tolerance to oxidative stress in various segments of human spinal cord

Activities of monoamine oxidase B, Cu-Zn-dependent superoxide dismutase (SOD), and catalase, the concentration of enzyme-active ceruloplasmin, and resistance of the nerve tissue to oxidative stress were examined in spinal cord preparations from humans (n=43) died at the age of 21-92 years. Age-related activation of monoamine oxidase B was found only in thoracic segments, while age-related decrease in SOD activity was demonstrated in thoracic segments and cervical intumescence of the spinal cord. Age-related accumulation of enzyme-active ceruloplasmin was observed in all segments of the spinal cord. Activation of catalase and increased sensitivity to oxidative stress were observed in the cervical and lumbosacral intumescences. Heterotopic changes in the examined indices suggest that activity of monoamine oxidase B cannot serve as a universal factor of age-related changes in antioxidant defense in the spinal cord and its sensitivity to oxidative stress.     

Altered Expression of Tyrosine Hydroxylase in the Locus Coeruleus Noradrenergic System in Citalopram Neonatally Exposed Rats and Monoamine Oxidase A Knock Out Mice

In rodents, noradrenergic (NE) locus coeruleus (LC) neurons are well known to express tyrosine hydroxylase (TH) immunoreactivity. However, due to its very low enzyme activity, NE cortical fibers do not typically express TH immunoreactivity, thus dopamine‐β‐hydroxylase (DBH) immunoreactivity is commonly utilized as a marker for NE cortical fibers. In this study, we performed double and/or triple immunofluorescent staining using antibodies against TH, DBH, and/or norepinephrine transporter (NET) to investigate the altered NE TH expression of cortical fibers in citalopram (CTM)‐exposed rats and monoamine oxidase (MAO) A knock out (KO) mice. We have noted the following novel findings: (1) neonatal exposure to the selective serotonin reuptake inhibitor (SSRI) CTM enhanced NE TH immunoreactive fibers throughout the entire neocortex, and a few of them appeared to be hypertrophic; (2) slightly enhanced NE cortical TH immunoreactive fibers were also noted in MAO A KO mice, and many of them revealed varicosities compared with the rather smooth NE cortical TH immunoreactive fibers in wild‐type (WT) mice; (3) LC dendrites of MAO A KO mice exhibited beaded morphology compared with the smooth LC dendrites in WT mice. Our findings suggest that both genetic and environmental factors during early development may play a critical role in the regulation and proper function of NE TH expression in the neocortex. Anat Rec, 2011. © 2011 Wiley‐Liss, Inc.     

Effect of monoamine oxidase A knockout on resistance to long-term exposure to ethanol

It was shown that transgenic Tg8 mice with monoamine oxidase A (MAO A) gene knockout demonstrate higher resistance to acute ethanol exposure compared to wild type C3H mice. This difference was observed at the early age (28-30 days). Long-term ethanol treatment changed the resistance to hypnotic, but not hypothermic action of this agent. Seven-day exposure increased the resistance to ethanol-induced narcotic sleep in Tg8 and C3H mice. After 30-day ethanol treatment the duration of narcotic sleep sharply decreased in C3H mice and increased in Tg8 mice, which attested to their decreased tolerance to ethanol.     

Developmental Role of Hyaluronate

Recent evidence has revealed that lysyl oxidase, plasma amine oxidase and diamine oxidase each contain copper and pyrroloquinoline quinone at their active sites as cofactors essential to their catalytic functions. It thus seems likely that these enzymes will share similar mechanisms of action. Since mechanism-based inhibitors of lysyl oxidase have important chemotherapeutic potential for the control of fibrotic disease, the relative inhibitory potential of such agents toward catalytically similar amine oxidases was assessed in the present study using purified preparations of lysyl oxidase, diamine oxidase, plasma amine oxidase and the flavin-dependent mitochondrial monoamine oxidase A and B. The results indicate that there is sufficient difference between the sensitivities of lysyl oxidase and the other amine oxidases to β-aminopropionitrile to warrant its consideration as an antifibrotic agent in vivo, while also revealing that aminoguanidine, clorgyline and deprenyl are sufficiently selective for diamine oxidase, monoamine oxidase A and monoamide oxidase B, respectively, to differentiate between lysyl oxidase and these enzymes at appropriate concentrations.     

Changes in Serotonin Metabolism in the Rat Brain on Presentation of a Habituated Stimulus

The levels of serotonin and the serotonin metabolite 5-hydroxyindoleacetic acid, monoamine oxidase activity, and the kinetic parameters of the oxidative deamination of serotonin were studied in various brain structures in rats after repeated presentation of a contextual stimulus. These experiments showed that presentation of the habituated stimulus was accompanied by increases in serotonin metabolism and the active transport of its metabolite in the amygdaloid complex, striatum, and midbrain, with no significant changes in the hippocampus or prefrontal cortex. Increases in monoamine oxidase activity in various brain areas resulted from different catalytic mechanisms: an increase in the rate of formation of the enzyme-substrate complex (a decrease in the Michaelis constant) in the amygdaloid complex, and faster conversion of the enzyme-substrate complex (increases in the maximum reaction rate) in the striatum and midbrain. It is concluded that activation of the presynaptic mechanism of serotonin transmission in the amygdaloid complex and striatum may be involved in the process of suppressing the biological significance of and attention to a repeatedly presented stimulus.Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 90, No.1, pp. 11–19, January, 2004.     

Pharmacological inactivation of the vesicular monoamine transporter can enhance 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurodegeneration of midbrain dopaminergic neurons, but not locus coeruleus noradrenergic neurons

The vesicular monoamine transporter in the brain can sequester the neurotoxin 1-methyl-4-phenylpyridinium into synaptic vesicles and protect catecholamine-containing neurons from degeneration. Mouse nigrostriatal dopaminergic neurons, and to a lesser extent locus coeruleus noradrenergic neurons, are vulnerable to toxicity produced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The present study sought to determine whether pharmacological inactivation of the vesicular monoamine transporter in the brain would enhance the degeneration of substantia nigra dopaminergic neurons and locus coeruleus noradrenergic neurons in 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine-treated animals. Mice were treated subacutely with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine alone, or in combination with vesicular monoamine transporter inhibitors (tetrabenazine or Ro4-1284), and 10-24 days later striatal dopamine and cortical norepinephrine levels were measured using chromatographic methods. In the same animals, substantia nigra and locus coeruleus catecholaminergic neurons were counted using tyrosine hydroxylase immunohistochemical staining with computer imaging techniques. Mice in which pharmacological blockage of the vesicular monoamine transporter enhanced the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity in the depletion of striatal dopamine concentrations also exhibited enhanced degeneration of substantia nigra dopaminergic neurons. In the same animals, however, vesicular monoamine transporter blockade did not enhance the effects of 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine in the locus coeruleus noradrenergic system. These data are consistent with the hypothesis that the vesicular monoamine transporter can protect catecholamine-containing neurons from 1-methyl-4-phenylpyridinium-induced degeneration by sequestration of the toxin within brain vesicular monoamine transporter-containing synaptic vesicles. Since the amount of vesicular monoamine transporter in locus coeruleus neurons is more than in substantia nigra neurons, and because 1-methyl-4-phenylpyridinium is sequestered within locus coeruleus neurons to a far greater extent than within substantia nigra neurons, it may be that a greater amount of vesicular monoamine transporter inhibition is required for 1-methyl-4-phenylpyridinium to be toxic to locus coeruleus neurons than to substantia nigra dopaminergic neurons.     

Noradrenaline-degrading activity of monoamine oxidase is localized in noradrenergic neurons of the locus coeruleus of the rat.

We found intense monoamine oxidase (MAO) activity in rat locus coeruleus (LC) neurons by means of a histochemical method using noradrenaline as a substrate. This MAO activity was abolished by clorgyline, a specific inhibitor of MAO type A. Fluorescence immunohistochemistry for tyrosine hydroxylase (TH) combined with MAO histochemistry revealed intense MAO activity in virtually all TH-immunoreactive LC neurons (i.e. noradrenergic neurons). The results indicate that noradrenaline produced in LC neurons might be degraded by MAO type A activity.     

Effects of beta-amyloid protein on serotoninergic, noradrenergic, and cholinergic markers in neurons of the pontomesencephalic tegmentum in the rat

The effects on serotoninergic, noradrenergic and cholinergic markers on neurons of the pontomesencephalic tegmentum nuclei were studied in rats following local administration of fibrillar beta-amyloid peptide (Abeta1-40) into the left retrosplenial cortex. Focal deposition of Abeta in the retrosplenial cortex resulted in a loss of serotoninergic neurons in the dorsal and median raphe nuclei. The dorsal raphe nucleus showed a statistically significant reduction of 31.7% in the number of serotoninergic neurons and a decrease (up to 17.38%) in neuronal density in comparison with the same parameters in uninjected controls. A statistically significant reduction of 50.3%, together with a significant decrease of 53.94% in the density of serotoninergic neurons, was also observed in the median raphe nucleus as compared with control animals. Furthermore, a significant reduction of 35.07% in the number of noradrenergic neurons as well as a statistically significant decrease of 56.55% in the density of dopamine-beta-hydroxylase-immunoreactive neurons were also found in the locus coeruleus as compared with the corresponding hemisphere in uninjected controls. By contrast, a reduction of 24.37% in the number of choline acetyltransferase-positive neurons and a slight decrease (up to 22.28%) in the density of cholinergic neurons, which were not statistically significant, was observed in the laterodorsal tegmental nucleus in comparison with the same parameters in control animals. These results show that three different neurochemically defined populations of neurons in the pontomesencephalic tegmentum are affected by the neurotoxicity of Abeta in vivo and that Abeta might indirectly affect serotoninergic, noradrenergic and cholinergic innervation in the retrosplenial cortex.