Serotonin depression of local cerebral glucose utilisation after monoamine oxidase inhibition

The major catabolic enzyme for serotonin, monoamine oxidase (MAO), is present in the endothelium of cerebral vessels. We report the effects of the intracarotid administration of serotonin on local cerebral glucose utilisation in rats following MAO inhibition with the drug clorgyline. It was found that saline, clorgyline, or serotonin alone produced no significant changes in glucose utilisation. Following the infusion of clorgyline, the administration of serotonin produced significant decreases in glucose utilisation in cortical areas of between 12 and 33% and in the caudate nucleus of 16%.     

Monoamine oxidase activity in brain microvessels determined using natural and artificial substrates: relevance to the blood-brain barrier

The possible contribution of cerebrovascular monoamine oxidase (MAO) to the blood-brain barrier to catecholamines was studied in isolated porcine and rat microvessels by determining its activity with various substrates. Michaelis-Menten kinetic constants, Km and Vmax, were determined using noradrenaline (NA) as substrate in a Tris medium. Km values were 0.25 +/- 0.05 mM in control and 0.16 +/- 0.09 mM in ultrasonically disintegrated (USD) preparations (difference not significant); Vmax in USD preparations (1.83 +/- 0.20 n.atoms O2 min-1 mg protein-1) was slightly higher (p less than 0.05) than in control preparations (1.35 +/- 0.11 n.atoms O2 min-1 mg protein-1), suggesting a certain restriction by the plasma membrane of substrate access to the enzyme. This phenomenon was confirmed in a more physiological, ionic medium; the activity was then approximately doubled for 1 mM NA, whereas that for 1 mM beta-phenylethylamine (beta-PEA), a lipid-soluble substrate, tended to decrease with USD treatment. These results show that this highly active form of MAO is unlikely to be saturated by physiological concentrations of catecholamine. It can be estimated that, for a plasma concentration of NA of 1 microM, a facilitated diffusion accelerating the entry of the catecholamine into the cells by at least 15-fold would be necessary in order to exceed the catabolic capacity of MAO. It is concluded that circulating catecholamines are not likely to cross the endothelial barrier of cerebral microvessels intact, and that the small quantities of radioactivity detected in the parenchyma in measurements of the brain uptake index essentially represent metabolites due to MAO activity.     

Interactions between relatively selective monoamine oxidase inhibitors and an inhibitor of 5-hydroxytryptamine re-uptake, clomipramine

Features of interactions with combined antidepressants in man were evoked by clomipramine in rats pretreated with both the relatively selective monoamine oxidase (MAO) inhibitors clorgyline and deprenyl, but not when clomipramine was given to rats pretreated with deprenyl or clorgyline alone, i.e. inhibition of both MAO A and B was a likely prerequisite for clomipramine to elicit the syndrome (with the larger dose of clorgyline and deprenyl, MAO A and B inhibition exceeded 95%). The features evoked were myoclonic—forelimb flexor-extensor movements, wet dog shakes and head and body twitches; hyperthermia and ECG anomalies also developed, and locomotor activity was augmented. Myoclonic phenomena were prevented when the above pretreatment also included p-chlorophenyl-alanine, but were unaffected or even intensified when pretreatment instead included -methyl-p-tyrosine; these phenomena were attenuated or abolished by pirenperone, a 5HT₂ antagonist. Relevance of these findings to safer combinations of antidepressants is discussed.     

Characterization of monoamine oxidase activity during early stages of quail embryogenesis

Catecholamines and other biogenic amines may play a role in early embryogenesis in addition to functioning as neurotransmitters after neuronal differentiation. Regulation of amine levels is mediated by several different parameters including activity levels of degradative enzymes. Since monoamine oxidase (EC 1.4.3.4) is the primary degradative enzyme for these biogenic amines, we have begun to characterize MAO activity during quail embryogenesis. Our results demonstrate that MAO activity is present at all stages of development examined (stages 2–22) and that the MAO specific activity levels are highest during the earliest stages (stages 2–6). Two types of MAO activity similar to adult avian and mammalian MAO-A and MAO-B have been demonstrated by differential clorgyline sensitivity of tryptamine deamination. In addition, SDS-PAGE of embryonic quail [³H]pargyiine-labeled MAO demonstrates that the quail MAO-A and MAO-B flavin-containing subunits have apparent molecular weights of 63,000 and 62,000 respectively.We have begun to assess the functional significance of embryonic quail MAO activity by daily injection of MAO inhibitors (clorgyline or clorgyline plus deprenyl) into fertilized eggs. Clorgyline injection selectively and completely inhibited MAO-A activity, while injection of clorgyline and deprenyl inhibited both MAO-A and MAO-B activities when embryos were assayed after either 2 or 7 days of embryonic development. This paradigm will allow a detailed examination of the effects of MAO inhibition on the developing embryo.     

Effect of acute levodopa on brain catecholamines after selective MAO and COMT inhibition in male rats

Summary Interactions between a selective catechol-O-methyltransferase (COMT) inhibitor OR-462 and a monoamine oxidase (MAO)-A inhibitor clorgyline were studied measuring concentrations of L-dopa, dopamine and their metabolites in the rat hypothalamus and striatum after administration of levodopa/carbidopa (15/30 mg/kg i.p.). Part of the experiments were performed in rats pretreated with 6-OH-dopamine (6-OHDA) intracerebroventricularly (i.c.v.) to determine whether changes in dopamine metabolism occurred inside or outside catecholaminergic neurons. OR-462 was an effective COMT inhibitor at the doses 3 and 30 mg/kg i.p. Inhibition of 3-O-methyldopa (3-OMD) formation from L-dopa was reflected in the hypothalamus (45–81% decrease) and striatum (87–88% decrease), since 3-OMD penetrates the blood-brain barrier. Homovanillic acid (HVA) was decreased only in the striatum at 30 mg/kg of OR-462. Clorgyline (8 and 32 mg/kg i.p.) decreased 3,4-dihydroxyphenylacetic acid (DOPAC) formation in the hypothalamus and striatum by 61–91%. When given together, OR-462 and clorgyline elevated hypothalamic dopamine levels 3.2–4.6-fold, but striatal dopamine only 1.3–1.9-fold. The formation of 3-OMD and DOPAC remained suppressed and even brain HVA levels were decreased by 51–97%. 6-OHDA treatment decresed striatal and hypothalamic dopamine by 50% and noradrenaline by 75%. In these animals levodopa/carbidopa increased brain L-dopa 2.4–4-fold, those of 3-OMD 1.2–1.7-fold compared to intact animals, but the synthesis and metabolism of dopamine and the effects of COMT and MAO inhibitors were not significantly changed. Levodopa/carbidopa treatment decreased significantly prolactin and thyrotropin levels in serum but none of the additional treatments changed this action.     

Type A monoamine oxidase catalyzes the intraneuronal deamination of dopamine within nigrostriatal,mesolimbic, tuberoinfundibular and tuberohypophyseal neurons in the rat

Summary Clorgyline (0.3–10mg/kg, i.p.) inhibited type A monoamine oxidase (5-hydroxytryptamine as substrate) but not type B monoamine oxidase (phenylethylamine as substrate) in homogenates of rat striatum and olfactory tubercle; deprenyl (0.3–3 mg/kg, i.p.) inhibited type B but not type A monoamine oxidase in these homogenates. The same doses of clorgyline increased concentrations of dopamine in striatum, and dopamine and norepinephrine in the olfactory tubercle, median eminence and posterior pituitary; they also reduced the concentrations of dihydroxyphenylacetic acid and the rate of synthesis of dopamine (DOPA accumulation after a decarboxylase inhibitor) in the same brain regions. On the other hand, the administration of deprenyl at doses that markedly inhibited type B monoamine oxidase did not alter the concentrations of dopamine, norepinephrine and dihydroxyphenylacetic acid or the rate of accumulation of DOPA in these brain regions. In addition, only clorgyline significantly lowered serum concentrations of prolactin. These results suggest that type A monoamine oxidase catalyzes the intraneuronal deamination of dopamine within terminals of nigrostriatal, mesolimbic, tuberoinfundibular and tuberohypophyseal dopamine neurons.     

A biochemical measure of monoamine oxidase type A and B inhibitor effects in man.

A method is described for the measurement of the effects of monoamine oxidase inhibitors (MAOI's) in human plasma. The procedure was used to estimate the levels of pargyline and clorgyline in plasma from patients with affective disorders. The results indicate that after an oral dose, pargyline is rapidly absorbed into the blood, reaches a maximum level in approximately 1 hr and has a half life of approximately 2 hr. After chronic administration, neither pargyline nor clorgyline attain steady state levels of MAO inhibiting potential which are higher than those seen 12 hr after a single dose of drug. A single 50 mg dose of pargyline causes greater than 90% inhibition of platelet MAO (in agreement with the observation that platelet MAO is MAO type B); this dose of pargyline does not significantly alter plasma amine oxidase. Clorgyline at the dose used (20 mg) does not alter platelet MAO activity.     

Regional Inhibition of Monoamine Oxidase Activity by Administration of Clorgyline in Rat Brain

Abstract: The inhibiting effect of monoamine oxidase (MAO) activities towards 5-hydroxytryptamine, dopamine and β-phenylethylamine by an acute and chronic administration of clorgyline was investigated in five locations of the rat brain. Inhibiting the percentage distributed unevenly in each hypothalamic nucleus and in the circumventricular areas, except the median eminence, the percentages were significantly low as compared with the hypothalamic lateral nucleus. A significant percentage of inhibition for type B MAO towards β-phenylethylamine was noticed in the chronically administered group. In conclusion, the MAO-inhibiting effect of clorgyline administered intraperitoneally was unevenly distributed in the discrete brain nuclei and the regional difference depended upon the dose and the duration of the clorgyline administration.     

Distribution of type A and type B monoamine oxidase activity in hypothalamic nuclei of the rat.

The regional distribution of type A, type A and B and type B activity of monoamine oxidase (MAO) in individual nuclei of the rat hypothalamus was studied according to quantitative micromethods with clorgyline as a specific inhibitor of type A MAO. As results, type A and type B MAO were distributed differently in the rat hypothalamus. It is suggested that type B MAO acts differently from type A MAO in regard to the function of the hypothalamus.     

Distribution of Type A and Type B Monoamine Oxidase Activity in Hypothalamic Nuclei of the Rat

The regional distribution of type A, type A and B and type B activity of monoamine oxidase (MAO) in individual nuclei of the rat hypothalamus was studied according to quantitative micromethods with clorgyline as a specific inhibitor of type A MAO. As results, type A and type B MAO were distributed differently in the rat hypothalamus. It is suggested that type B MAO acts differently from type A MAO in regard to the function of the hypothalamus.     

Involvement of monooxygenases and amine oxidases in hydroxyl radical generation in vivo

The levels of hydroxyl radicals in vivo were measured in rat blood plasma by determining the formation of 2,3-dihydroxybenzoate (2,3-DHB) resulting from the attack of hydroxyl radicals on injected salicylate. This approach was used to study the effects of alterations in the activities of cytochrome P-540 (CYP)-dependent hydroxylases (monooxygenases) and monoamine oxidase (MAO), two groups of enzymes that can produce reactive oxygen species in the reactions they catalyse. Pretreatment with inducers of the cytochrome P540 (CYP) hydroxylases, such as phenobarbital and dexamethasone, resulted in substantial increases in the plasma oxygen radical formation, suggesting that the action of these enzymes on endogenous substrates, or on exogenous substrates such as salicylate, may contribute to oxygen radical formation in vivo. In contrast, pretreatment with the monoamine oxidase (MAO) inhibitors clorgyline and deprenyl did not have any significant effect on the plasma 2,3-DHB levels, perhaps reflecting the different intracellular locations of MAO and the CYP-dependent hydroxylases. Injection of pentylamine, a substrate of MAO-B and semicarbazide-sensitive amine oxidase (SSAO) was also without significant effect.     

The nature of inhibition of cat brain mitochondrial monoamine oxidase by clorgyline

In the cat brain the highest monoamine oxidase (MAO) activity is observed in the hypothalamus followed by hippocampus, caudate nucleus, pons and median cortex. Tyramine was the most actively deaminated substrate tested followed by dopamine. Clorgyline was more selective in its inhibitory action and could distinguish between tyramine and dopamine MAO deaminating system. The latter being more resistant to inhibition. The multiple forms of solubilized MAO as separated by polyacrylamide gel electrophoresis have varying phospholipid phosphorus content and sensitivity to inhibition by clorgyline in vitro and in vivo.     

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

Summary 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 with³H-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 following³H-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 using³H-pargyline, specific labelling and axonal transport of type A MAO in striatal neurons projecting to the substantia nigra was demonstrated.     

Effect of ICV infusion of CRF on blood pressure and adrenal steroids in rabbits

The effect of prolonged, 22 h long, intracerebroventricular (i.c.v.) infusion of corticotropin-releasing hormone (CRF) on plasma cortisol, corticosterone and electrolyte concentrations, mean arterial blood pressure (MAP) and heart rate (HR) were investigated in conscious rabbits. During i.c.v. infusion of CRF, 1 and 3 μ/h, at a rate of 17 μl/h, plasma cortisol and corticosterone concentrations rose to the level noted after ACTH stimulation in rabbits. Plasma [Na] did not change, but plasma [K] was reduced and plasma osmolality increased during the infusion of CRF, 3 μ/h. MAP and HR, recorded continuously during i.c.v. infusion of CRF, changed only with the higher dose of CRF: MAP was elevated during the first 5 h of infusion, and then returned to the control level. HR was lower than control at the end of the first hour of infusion and again between 9 and 15 h of infusion. The prolonged rise of CRF concentration in the brain induced a sustained rise in circulating adrenal steroid hormones. MAP did not increase to the level noted after bolus i.c.v. injection of CRF and the rise in MAP was not sustained.     

Monoamine oxidase A rather than monoamine oxidase B inhibition increases nicotine reinforcement in rats

Although nicotine is considered to be responsible for the addictive properties of tobacco, growing evidence underlines the importance of non-nicotine components in smoking reinforcement. It has been shown that tobacco smoke contains monoamine oxidase (MAO) A and B inhibitors and decreases MAO-A and MAO-B activity in smokers. Here, we investigated the effects of clorgyline hydrochloride (irreversible MAO-A inhibitor; 2 mg/kg/day), selegiline (irreversible MAO-B inhibitor; 4 mg/kg) and the beta-carboline norharmane hydrochloride (reversible MAO-B inhibitor; 5 mg/kg/day) treatments on nicotine self-administration (30 microg/kg/infusion, free base) in rats. Independent of the responsiveness to novelty and locomotor activity stimulation, only clorgyline hydrochloride treatment increased the intake of nicotine in a fixed-ratio schedule (FR5) of reinforcement. When a progressive-ratio schedule was implemented, both clorgyline hydrochloride and norharmane hydrochloride treatments potentiated the reinforcing effects of nicotine, whereas selegiline had no effect. Taken together, these results indicate that MAO-A inhibition interacts with nicotine to enhance its rewarding effects in rats and suggest that other compounds present in tobacco, such as beta-carboline, may also play an important role in sustaining smoking behavior in humans.     

The inhibition of tyramine oxidation and the tyramine hypertensive response (“cheese effect”) may be independent phenomena

Summary Although the selective monoamine oxidase (MAO) inhibitor, (–)-deprenyl, substantially inhibits tyramine-oxidizing ability in the pig, intravenous tyramine challenge after pretreatment with this drug failed to produce the characteristic pressor response (cheese effect) associated with other irreversible MAO inhibitors. Conversely, pretreatment with the tyramine oxidation-sparing selective MAO inhibitor, clorgyline, followed by intravenous tyramine, paradoxically resulted in a profound pressor response. We suggest that the action of standard MAO-inhibiting drugs may be compounded of two separate actions, usually associated but, in fact, unrelated.     

Chemical sympathectomy and clorgyline-induced stimulation of rat pineal melatonin synthesis

Summary The response to administration of the specific monoamine oxidase A (MAO-A) blocker clorgyline was investigated in adult male Sprague-Dawley rats which were sympathectomized by injection of the false neurotransmitter 6-hydroxydopamine as newborns. In intact animals which served as controls, the contents of pineal indoles melatonin, serotonin, 5-hydroxytryptophan were augmented, and the content of 5-hydroxyindoleacetic acid decreased 90 min following clorgyline injections when compared to saline receiving rats. Sympathectomized animals exhibited similar responses but these were less pronounced. It is suggested that blocking of the oxidation of both MAO-A substrates, noradrenaline and serotonin, upon clorgyline administration results in the observed increase in melatonin synthesis which is thought to contribute to the antidepressant effects of MAO inhibition.     

Regional cerebral blood flow in conscious stroke-prone spontaneously hypertensive rats

Regional cerebral blood flow (rCBF) was measured autoradiographically with [14C]iodoantipyrine as a diffusible tracer in two strains of conscious normotensive rats (Wistar Kyoto and local Wistar) and in two groups of spontaneously hypertensive stroke-prone rats (SHRSP) with a mean arterial pressure (MAP) below or above 200 mm Hg. In spite of the large differences in arterial pressure, rCBF did not differ significantly between the hypertensive and the normotensive groups in any of the 14 specified brain structures measured. However, rCBF increased asymmetrically within part of the caudate-putamen in two of nine SHRSP with a MAP above 200 mm Hg, indicating a regional drop in the elevated cerebrovascular resistance.     

Evidence that brain MAO A activity does not correspond to MAO A genotype in healthy male subjects.

BACKGROUND: A functional polymorphism in the promoter region of the monoamine oxidase A (MAO A) gene has two common alleles that are referred to as the high and low MAO A genotypes. We report the first in vivo human study to determine whether there is an association between MAO A genotype and brain MAO A activity in healthy male subjects. METHODS: Brain MAO A activity was measured with positron emission tomography and [(11)C]clorgyline in 38 healthy adult male nonsmokers genotyped for MAO A polymorphism. RESULTS: There was no significant difference in brain MAO A activity between the high (n = 26) and low (n = 12) MAO A genotypes. CONCLUSIONS: The lack of an association between the high and low MAO A genotype and brain MAO A activity suggests that this polymorphism by itself does not contribute to differences in brain MAO A activity in healthy adult male subjects.     

Down-regulation of tryptamine receptors following chronic administration of clorgyline

Chronic treatment with clorgyline, a type A monoamine oxidase (MAO) inhibitor (1 mg/kg/day for 11 days), reduced the number (Bmax) but not the affinity (Kd) of [3H]tryptamine binding sites in rat frontal/parietal cortical membranes. Binding was reduced for at least 36 days following the last injection. The reduction in [3H]tryptamine binding was dose-related and appeared to be maximal following a daily dose of 3 mg/kg. Chronic treatment with deprenyl, a type B MAO inhibitor (1 mg/kg/day for 11 days), did not affect [3H]tryptamine binding. Acute clorgyline administration (11 mg/kg) also had no effect. These data suggest that [3H]tryptamine binds to neurotransmitter receptors for tryptamine since mere chemical recognition sites would not be expected to be modulated by chronic drug treatment. Also, since [3H]tryptamine binding was down-regulated by a type A, but not a type B, MAO inhibitor, tryptamine may be selectively metabolized by type A MAO in vivo.