Selectivity of clorgyline and pargyline as inhibitors of monoamine oxidases A and B in vivo in man

Quinoline-Malononitrile-Based Aggregation-Induced Emission Probe for Monoamine Oxidase Detection in Living Cells

A quinoline-malononitrile (QM)-based aggregation-induced emission probe was developed to detect MAOs in cells through an enzymatic reaction followed by β-elimination. After being incubated at 37 °C, QM-NH2 responded to the MAO enzymes with great specificity and within just 5 min. This 5 min responsive mechanism was fast, with the limit of detection (LOD) at 5.49 and 4.76 µg mL−1 for MAO-A and MAO-B, respectively. Moreover, QM-NH2 displayed high enzyme specificity even in the presence of high concentrations of biological interferences, such as oxidizing and reducing agents, biothiols, amino acids, and glucose. Furthermore, QM-NH2 demonstrated biocompatibility as the cells retained more than 70% viability when exposed to QM-NH2 at concentrations of up to 20 µM. As a result, QM-NH2 was used to detect MAO-A and MAO-B in SH-SY5Y and HepG2 cells, respectively. After 1h incubation with QM-NH2, the cells exhibited enhanced fluorescence by about 20-fold. Moreover, the signal from cells was reduced when MAO inhibitors were applied prior to incubating with QM-NH2. Therefore, our research recommends using a QM probe as a generic method for producing recognition moieties for fluorogenic enzyme probes.

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, "general chemicals," natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10-15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Three -(pyridin-2-yl)-2-(pyridin-2-ylimino)thiazolidin-4-one as a novel inhibitor of cerebral MAO-B activity with antioxidant properties and low toxicity potential

Some brain diseases are associated with oxidative stress and altered monoamine oxidase (MAO) activity. The objective of this study was to evaluate the antioxidant and neuroprotective actions through MAO inhibition of 3-(pyridin-2-yl)-2-(pyridine-2-ylimino) thiazolidin-4-one (PPIT, a synthetic molecule containing a thiazolidinone nucleus), as well as its effects on toxicity parameters in Swiss female mice. Five in vitro assays were carried out to verify the PPIT antioxidant capacity: protein carbonylation (PC), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), 1,1-diphenyl-2-picryl-hydrazil (DPPH), ferric ion (Fe³⁺ ) reducing antioxidant power (FRAP), and superoxide dismutase (SOD)-like activity. The results showed that PPIT reduced the level of PC in the homogenate of the brain. This compound did not demonstrate SOD mimetic activity, but it acted as a free radical scavenger (ABTS and DPPH) and exhibited reducing activity in the FRAP assay. In addition, the effects of PPIT on cerebral MAO activity (MAO-A and B isoforms) were investigated in vitro. Our data revealed inhibition of the MAO-B activity by PPIT with no effects on MAO-A. Lastly, an acute oral toxicity test was conducted in mice. No changes in food intake, body weight, and biochemical markers of kidney and liver damage were detected in mice treated with a high dose of PPIT (300 mg/kg). In conclusion, the present study demonstrated that PPIT exhibits antioxidant activity and selectively inhibits the MAO-B isoform without causing apparent toxicity. These findings suggest PPIT as a potential therapeutic candidate to be tested in preclinical models of brain diseases involving perturbations of MAO-B activity and redox status.

Monoamine oxidase A inhibitor-near-infrared dye conjugate reduces prostate tumor growth

Development of anti-cancer agents with high tumor-targeting specificity and efficacy is critical for modern multidisciplinary cancer research. Monoamine oxidase A (MAOA), a mitochondria-bound enzyme, degrades monoamine neurotransmitters and dietary monoamines. Recent evidence suggests a correlation between increased MAOA expression and prostate cancer (PCa) progression with poor outcomes for patients. MAOA induces epithelial-mesenchymal transition (EMT) and augments hypoxic effects by producing excess reactive oxygen species. Thus, development of MAOA inhibitors which selectively target tumors becomes an important goal in cancer pharmacology. Here we describe the design, synthesis, and in vitro and in vivo evaluation of NMI, a conjugate that combines a near-infrared dye for tumor targeting with the moiety derived from the MAOA inhibitor clorgyline. NMI inhibits MAOA with low micromolar IC50, suppresses PCa cell proliferation and colony formation, and reduces migration and invasion. In mouse PCa xenografts, NMI targets tumors with no detectable accumulation in normal tissues, providing effective reduction of the tumor burden. Analysis of tumor specimens shows reduction in Ki-67(+) and CD31(+) cells, suggesting a decrease of cell proliferation and angiogenesis and an increase in M30(+) cells, indicating increased apoptosis. Gene expression profiles of tumors treated with NMI demonstrate reduced expression of oncogenes FOS, JUN, NFKB, and MYC and cell cycle regulators CCND1, CCNE1, and CDK4/6, along with increases in the levels of tumor suppressor gene TP53, cell cycle inhibitors CDKN1A and CDKN2A, and MAOA-downstream genes that promote EMT, tumor hypoxia, cancer cell migration, and invasion. These data suggest that NMI exerts its effect through tumor-targeted delivery of a MAOA-inactivating group, making NMI a valuable anti-tumor agent.

Effects of the neurotoxin MPTP and pargyline protection on extracellular energy metabolites and dopamine levels in the striatum of freely moving rats

The neurotoxin MPTP is known to induce dopamine release and depletion of ATP in the striatum of rats. Therefore, we studied the changes induced by MPTP and pargyline protection both on striatal dopamine release and on extracellular energy metabolites in freely moving rats, using dual asymmetric-flow microdialysis. A dual microdialysis probe was inserted in the right striatum of rats. MPTP (25mg/kg, 15mg/kg, 10mg/kg) was intraperitoneally administered for three consecutive days. MAO-B inhibitor pargyline (15mg/kg) was systemically administered before neurotoxin administration. The first MPTP dose induced an increase in dialysate dopamine and a decrease of DOPAC levels in striatal dialysate. After the first neurotoxin administration, increases in striatal glucose, lactate, pyruvate, lactate/pyruvate (L/P) and lactate/glucose (L/G) ratios were observed. Subsequent MPTP administrations showed a progressive reduction of dopamine, glucose and pyruvate levels with a concomitant further increase in lactate levels and L/P and L/G ratios. At day 1, pargyline pre-treatment attenuated the MPTP-induced changes in all studied analytes. Starting from day 2, pargyline prevented the depletion of dopamine, glucose and pyruvate while reduced the increase of lactate, L/P ratio and L/G ratio. These in vivo results suggest a pargyline neuroprotection role against the MPTP-induced energetic impairment consequent to mitochondrial damage. This neuroprotective effect was confirmed by TH immunostaining of the substantia nigra.

Serotonin and blood pressure regulation

5-Hydroxytryptamine (5-HT; serotonin) was discovered more than 60 years ago as a substance isolated from blood. The neural effects of 5-HT have been well investigated and understood, thanks in part to the pharmacological tools available to dissect the serotonergic system and the development of the frequently prescribed selective serotonin-reuptake inhibitors. By contrast, our understanding of the role of 5-HT in the control and modification of blood pressure pales in comparison. Here we focus on the role of 5-HT in systemic blood pressure control. This review provides an in-depth study of the function and pharmacology of 5-HT in those tissues that can modify blood pressure (blood, vasculature, heart, adrenal gland, kidney, brain), with a focus on the autonomic nervous system that includes mechanisms of action and pharmacology of 5-HT within each system. We compare the change in blood pressure produced in different species by short- and long-term administration of 5-HT or selective serotonin receptor agonists. To further our understanding of the mechanisms through which 5-HT modifies blood pressure, we also describe the blood pressure effects of commonly used drugs that modify the actions of 5-HT. The pharmacology and physiological actions of 5-HT in modifying blood pressure are important, given its involvement in circulatory shock, orthostatic hypotension, serotonin syndrome and hypertension.

The monoamine oxidase inhibitor phenelzine enhances the discriminative stimulus effect of nicotine in rats

In addition to delivering nicotine, tobacco smoke also inhibits monoamine oxidase (MAO). Although MAO inhibitors (MAOIs) can increase nicotine self-administration in rodents, the effects of MAOIs on the discriminative stimulus effect of nicotine are not known. This study examined the effects of three MAOIs (phenelzine, clorgyline and pargyline) with varying selectivity for MAOA and MAOB in the nicotine drug discrimination procedure in rats. Adult male Sprague-Dawley rats were trained to discriminate nicotine (0.3 mg/kg, subcutaneously) from saline in a standard, two-lever food-reinforced operant task. Once the discrimination was acquired, the ability of each MAOI to substitute for or alter the discriminative stimulus effect of nicotine was determined. In substitution tests, nicotine (0.03-0.3 mg/kg) produced full, dose-dependent substitution. Although the selective MAOA inhibitor clorgyline (3-56 mg/kg) and the selective MAOB inhibitor pargyline (3-56 mg/kg) did not elicit any nicotine-appropriate responding, partial substitution was obtained with the nonselective MAO inhibitor phenelzine (1-17 mg/kg). Phenelzine (10 mg/kg) also enhanced the discriminative stimulus effect of a low dose of nicotine (0.056 mg/kg) and prolonged the time course effect of the nicotine-training dose. These findings indicate that concomitant inhibition of MAOA and MAOB can enhance the discriminative stimulus effect of nicotine in rats.

Are MAO-A deficiency states in the general population and in putative high-risk populations highly uncommon?

Lack of monoamine oxidase A (MAO-A) due to either Xp chromosomal deletions or alterations in the coding sequence of the gene for this enzyme are associated with marked changes in monoamine metabolism and appear to be associated with variable cognitive deficits and behavioral changes in humans and in transgenic mice. In mice, some of the most marked behavioral changes are ameliorated by pharmacologically-induced reductions in serotonin synthesis during early development, raising the question of possible therapeutic interventions in humans with MAO deficiency states. At the present time, only one multi-generational family and a few other individuals with marked MAO-A deficiency states have been identified and studied in detail. Although MAO deficiency states associated with Xp chromosomal deletions were identified by distinct symptoms (including blindness in infancy) produced by the contiguous Norrie disease gene, the primarily behavioral phenotype of individuals with the MAO mutation is less obvious. This paper reports a sequential research design and preliminary results from screening several hundred volunteers in the general population and from putative high-risk groups for possible MAO deficiency states. These preliminary results suggest that marked MAO deficiency states are very rare.

Differential trace amine alterations in individuals receiving acetylenic inhibitors of MAO-A (clorgyline) or MAO-B (selegiline and pargyline)

Marked, dose-dependent elevations in the urinary excretion of phenylethylamine, para-tyramine, and meta-tyramine were observed in depressed patients treated for three or more weeks with 10, 30, or 60 mg/day of the partially-selective inhibitor of MAO-B, selegiline (l-deprenyl). In comparative studies with other, structurally similar acetylenic inhibitors of MAO, pargyline, an MAO-B > MAO-A inhibitor used in doses of 90 mg/day for three or more weeks, produced elevations in these trace amines which were similar to those found with the highest dose of selegiline studied. Clorgyline, a selective inhibitor of MAO-A used in doses of 30 mg/day for three or more weeks (a dose/time regimen previously reported to reduce urinary, plasma, and cerebrospinal fluid 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) > 80%, indicating a marked inhibitory effect on MAO-A in humans in vivo) produced negligible changes in trace amine excretion. In comparison to recent studies of individuals lacking the genes for MAO-A, MAO-B, or both MAO-A and MAO-B, the lack of change in trace amine excretion in individuals with a mutation affecting only MAO-A is in agreement with the observed lack of effect of clorgyline in the present study. Selegiline produced larger changes in trace amines--at least at the higher doses studied--than found in individuals lacking the gene for MAO-B, in agreement with other data suggesting a lesser selectivity for MAO-B inhibition when selegiline was given in doses higher than 10 mg/day. Overall, trace amine elevations in individuals receiving the highest dose of deprenyl or receiving pargyline were approximately three to five-fold lower than the elevations observed in individuals lacking the genes for both MAO-A and MAO-B, suggesting that these drug doses yield incomplete inhibition of MAO-A and MAO-B.

Can an adjuvant treatment or a pharmacologic prevention be common to several diseases? The case of those associated to neuromediator defects

The author has tried to extrapolate on several neurological diseases of the ageing, Knoll's proposition to treat Parkinson's disease chronically for relapse prevention, by MAO-B. At this occasion, the author makes a critical review of the clinical trial results concerning the new different series of depression treatments, as most authors propose today to use some new ones not only in depression, but in Parkinson's and Alzheimer's diseases and in ageing.

Selective inhibition of MAO-A, not MAO-B, results in antidepressant-like effects on DRL 72-s behavior

The effects of monoamine oxidase inhibitors (MAOIs) that selectively inhibit the MAO-A or MAO-B forms of MAO were studied in rats performing under a differential-reinforcement-of-low-rate 72-s (DRL 72-s) schedule of reinforcement. Clorgyline and CGP11'305A, irreversible and reversible MAO-A inhibitors, respectively, increased the reinforcement rate, decreased the response rate, and enhanced temporal discrimination. The irreversible MAO-B inhibitor (-)-deprenyl did not produce similar effects. Pargyline did not increase the reinforcement rate at low doses that selectively inhibit MAO-B, but did increase the reinforcement rate at doses that inhibit MAO-A by more than 90%. The present results are in accord with clinical data demonstrating that MAO-A inhibitors are effective therapeutic agents in treating depression while MAO-B inhibitors are of questionable antidepressant efficacy. The present findings provide further evidence that the DRL 72-s schedule may be useful both as a screen for identifying new antidepressants and for investigating the neurochemical effects of antidepressant drugs that are responsible for their therapeutic effects.

Effect of brofaromine and pargyline on human plasma melatonin concentrations

1. Plasma melatonin was used to determine the influence of two monoamine oxidase inhibitor drugs in 11 normal subjects. 2. Acute oral administration of the selective reversible MAO-A inhibitor brofaromine but not of the - in low doses - selective MAO-B inhibitor pargyline increased daytime melatonin with large variations in onset, degree and duration. 3. Further investigation of this selective action on melatonin might help to better understand the action of the therapeutically effective antidepressive therapy with selective MAO-A inhibitors.

Effect of selective monoamine oxidase inhibitors on rat pineal melatonin synthesis in vitro

Freshly cultured pineal glands respond to the monoamine oxidase A (MAO A) inhibitor clorgyline and to high concentrations of the MAO B inhibitor, deprenyl, with an increase in serotonin N-acetyltransferase activity and N-acetylated indoles and a fall in 5-hydroxylated serotonin degradation products. Glands cultured for 48 hours before challenge respond less. Response is absent in glands cultured for 72 hours and in glands from ganglionectomized animals cultured for 48 hours before challenge. These data are consistent with the hypothesis that these MAO inhibitors stimulate melatonin synthesis by protecting norepinephrine from degradation.

The effects of amiflamine on cerebrospinal fluid amine metabolites in the rhesus monkey

Amiflamine, a drug reported to be a reversible inhibitor of monoamine oxidase type A (MAO-A) selective for serotonergic neurons in rodents, was administered to rhesus monkeys over a 12-fold dosage range (0.5-6 mg/kg). Amiflamine produced small, essentially equivalent reductions in cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA, 1-28%), 3-methoxy-4-hydroxyphenylglycol (MHPG, 4-26%), and homovanillic acid (HVA, 7-29%), suggesting that the effects of amiflamine are approximately equal on serotonin, norepinephrine and dopamine metabolism in nonhuman primates. Concentrations of amiflamine were very low in CSF 3-6 h after drug administration (less than 7 nmol/l), while those of its two major, biologically active metabolites were higher (22-150 nmol/l) and varied in relative proportions among the monkeys. Further investigation is required of some preliminary observations of a possible association between drug metabolite variations and the substantial individual differences in the amine metabolite changes following amiflamine treatment. MAO-B in platelets was not inhibited by 6 mg/kg amiflamine, indicating that MAO-A selectivity was maintained. At low amiflamine doses, early and transient increases in CSF 5-HIAA and HVA concentrations were observed, suggesting an amine-releasing effect of the drug within brain serotonergic and dopaminergic neurons.

Rhesus monkey cerebrospinal fluid amine metabolite changes following treatment with the reversible monoamine oxidase type-A inhibitor cimoxatone

The effects of cimoxatone, a reversible inhibitor of monoamine oxidase type A (MAO-A), on the deaminated metabolites of norepinephrine, dopamine, and serotonin were examined in continuously collected rhesus monkey cerebrospinal fluid (CSF). Cimoxatone, 0.5-8 mg/kg given PO, produced dose-proportionate reductions of 24-h mean CSF 3-methoxy, 4-hydroxy phenylglycol (MHPG) concentrations of 21%-52%. Homovanillic acid (HVA) concentrations also decreased 27%-55%, while CSF 5-hydroxyindoleacetic acid (5-HIAA) decreases were somewhat smaller (7%-32% from baseline). All three metabolite concentrations reached a nadir approximately 6-10 h after drug administration, and required over 40 h to gradually return towards baseline following drug discontinuation. HVA concentration reductions in particular persisted during the entire 24-h period following treatment and were the slowest to return to baseline values. CSF concentrations of cimoxatone and its MAO-inhibiting O-demethyl metabolite showed a parallel time course, peaking 6-10 h after treatment and persisting for up to 24 h in the case of cimoxatone and over 48 h for its metabolite. Single simultaneous time point determinations revealed 10-to 20-fold lower concentrations of cimoxatone and its metabolite in CSF compared to plasma 2 h after treatment. MAO-B activity in platelet-rich plasma was not inhibited by 8 mg/kg cimoxatone, indicating that this drug maintains MAO-A selectivity in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of clorgyline on catecholamine and indoleamine metabolites in the cerebrospinal fluid of rhesus monkeys

The effects of acute and chronic administration of clorgyline, an irreversible inhibitor of monoamine oxidase type A (MAO-A), on the deaminated metabolites of norepinephrine, dopamine and serotonin were examined in rhesus monkey cerebrospinal fluid (CSF). Acute clorgyline treatment resulted in highly significant, dose-dependent reductions in 3-methoxy-4-hydroxyphenylglycol (MHPG) of 50% (1 mg/kg) and 68% (2 mg/kg) compared to pretreatment values. Chronic clorgyline administration (0.25 to 0.5 mg/kg X 24 days) resulted in a 67% reduction in CSF MHPG. In contrast, the concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) were less affected by acute clorgyline administration, being reduced significantly only after the 2 mg/kg dose, which lowered 5-HIAA 27% and HVA 48%. Chronic clorgyline treatment had no significant effect on the CSF concentrations of HVA and 5-HIAA. These data, which suggest that MAO-A inhibition by clorgyline in vivo is more closely associated with changes in the noradrenergic than the serotonergic or dopaminergic systems in nonhuman primates, are in general agreement with the effects of clorgyline on CSF and urinary biogenic amine metabolites in man. They differ from several in vitro studies which indicate a primary role of MAO-A in the metabolism of serotonin and of MAO-B in norepinephrine degradation in primate brain. The discrepancies may reflect modulating effects of synaptic feedback mechanisms on the actions of clorgyline in vivo or perhaps a failure of CSF metabolites to adequately reflect brain amine metabolism changes. The lack of change in platelet MAO-B activity during clorgyline treatment together with the minimal changes in HVA concentrations indicate that the selective inhibitory effects of clorgyline on MAO-A were maintained during chronic administration of low drug doses.

Effect of selective monoamine oxidase inhibition by clorgyline and deprenyl on the norepinephrine receptor-coupled adenylate cyclase system in rat cortex

The consequences of selective monoamine oxidase (MAO) inhibition on the norepinephrine(NE)-sensitive adenylate cyclase system were determined in slices of rat cerebral cortex. The chronic administration of clorgyline, which selectively inhibited the activity of MAO-A, caused a significant decrease in the responsiveness of the noradrenergic cyclic AMP-generating system. The noradrenergic subsensitivity was accompanied by a significant decrease in the density of beta-adrenoceptors, as measured by 3H-dihydroalprenolol (DHA) binding, without altering the Kd value. However, selective inhibition of MAO-B by deprenyl did not alter the sensitivity of the cyclic AMP-generating system to NE or the specific DHA binding. The basal levels of cyclic AMP in the cortex were unaltered by the drugs. Since inhibition of MAO-A, but not MAO-B, increases the availability of NE, the results support the hypothesis that a persistent NE-receptor interaction is one of the prerequisites for the in vivo densitization of the NE-sensitive adenylate cyclase and the concomitant down-regulation of the number of beta-adrenoceptors in brain.

REM sleep suppression induced by selective monoamine oxidase inhibitors

The effects of 4 weeks of treatment with the selective monoamine oxidase (MAO) inhibiting antidepressant clorgyline and pargyline on the sleep of affectively disordered patients were studied. Both inhibitors resulted in near total suppression of REM sleep, a decrease in total sleep time, and an increase in the percent of stage 2 sleep. Clorgyline also increased awake time and decreased total recording period and sleep latency. In general, changes were greater for clorgyline than for pargyline and were about 50% slower to return to baseline after clorgyline compared to pargyline discontinuation. The results were consistent with the hypothesis that selective inhibition of the MAO type A, as produced by clorgyline, is sufficient to induce marked sleep changes. MAO inhibitor-induced receptor changes are proposed to account for the time course of the REM suppression and the REM rebound observed upon withdrawal.

Monoaminergic mechanisms and experimental cataplexy

The effects of pharmacological alteration of the monoamine systems were investigated in a canine model of narcolepsy. Cataplexy was quantified in eight severely affected dogs by means of the food-elicited cataplexy test. The specific norepinephrine (NE) uptake blocker nisoxetine, and (to a much lesser extent) the specific serotonin (5-HT) uptake blocker fluoxetine, significantly suppressed cataplexy, as did the tricyclic antidepressants protriptyline, amitriptyline, and chlorimipramine. Thus, experimental cataplexy is suppressed more by inhibition of the uptake of NE than of 5-HT. Methylphenidate, the alpha-adrenoreceptor blocker clonidine, and the dopamine receptor blocker pimozide also suppressed cataplexy in dogs. The beta-adrenergic blocker propranolol, the fatty acid gamma-hydroxybutyrate, and the monoamine oxidase inhibitors clorgyline and pargyline had little or no effect. With one exception (pimozide), all the drugs that suppressed cataplexy are known to be potent suppressors of REM sleep. The suppression of cataplexy induced by nisoxetine or protriptyline was reversed by the anticholinesterase physostigmine, further supporting a postulated aminergic-cholinergic interaction in the mechanisms for cataplexy.

Tyramine infusions and selective monoamine oxidase inhibitor treatment. I. Changes in pressor sensitivity

The enhanced sensitivity to the pressor effects of tyramine, an indirect-aging sympathomimetic found abundantly in the diet, is a well-known potentially dangerous side effect occurring during treatment with commonly used non-selective monoamine oxidase (MAO) inhibitors. The effects of treatment with the selective MAO-A inhibitor clorgyline and the partially selective MAO-B inhibitors pargyline and deprenyl on tyramine's pressor effects were studied in depressed patients using an IV steady-state tyramine infusion technique. After 4 weeks of treatment, clorgyline produced a significantly greater increase in tyramine sensitivity in comparison to a medication-free baseline (29-fold) than did pargyline (12-fold) or deprenyl (1.7-fold). The pressor effects of tyramine were significantly prolonged after cessation of infusion during both clorgyline and pargyline, but not deprenyl treatment. These data from IV tyramine administrations suggest that intestinal MAO inhibition is not the major determinant of the enhanced tyramine pressor sensitivity produced by clorgyline and pargyline.

Tyramine infusions and selective monoamine oxidase inhibitor treatment. II. Interrelationships among pressor sensitivity changes, platelet MAO inhibition, and plasma MHPG reduction

The relationship between changes in IV tyramine pressor sensitivity accompanying selective monoamine oxidase (MAO) inhibitor treatment and estimates of MAO-A and MAO-B inhibition in vivo were studied. Reductions in platelet MAO activity provided an index of MAO-B inhibition, while changes in plasma 3-methoxy-4-hydroxyphenethylene glycol (MHPG) were used as an hypothesized reflection of MAO-A inhibition. Chronic treatment with the MAO-A inhibitor clorgyline and the MAO-B inhibitor pargyline showed significant inhibition of the alternate MAO enzyme as well, although this crossover effect was greater for pargyline than clorgyline. The MAO-B inhibitor deprenyl appeared to maintain the greatest degree of MAO inhibition selectivity in vivo. Tyramine pressor sensitivity changes accompanying administration of the MAO inhibitors were highly correlated with decreases in plasma MHPG (r = 0.92), supporting our previous data indicating the rank order of clorgyline greater than pargyline greater than deprenyl for enhancement of tyramine pressor sensitivity and, thus, suggesting that tyramine potentiation is primarily a function of MAO-A rather than MAO-B inhibition. Changes in plasma MHPG are suggested to provide a potentially useful clinical index of in vivo MAO-A inhibition.

Dopamine oxidation and its inhibition by (-)-deprenyl in man

Dopamine is predominantly oxidized by a (-)-deprenyl sensitive form of MAO in the human striatum, and (-)-deprenyl, acting at some suitably low selective inhibitory concentration may, therefore, be of benefit in Parkinson's disease. 10(-6)M was the most effective (-)-deprenyl concentration in vitro for discriminating between the inhibition of MAO A and B. The correlation between the A/B ratio present in different human brain regions and the sensitivity of dopamine oxidation to 10(-6)M deprenyl was 0.84 (p<0.001). This suggests that all dopamine oxidation can be accounted for by the joint contribution of MAO A and B and that it is unnecessary to postulate a special form of the enzyme which metabolizes dopamine. In the brain, the striatum has the highest proportion of MAO B, and in several cortical regions, relatively more dopamine is oxidized by MAO A. In other human tissues also the deprenyl sensitivity of dopamine oxidation correlated with the known A/B ratio, the placenta, lung and jejeunum having the lowest sensitivity and being the richest in MAO A. Km values for dopamine for MAO A and B are similar, 130 and 140 uM respectively, so that the proportion oxidized by the two forms should not vary with substrate concentration.

Comparative behavioral effects of clorgyline and pargyline in man: a preliminary evaluation

The antidepressant and other behavioral effects of clorgyline, a preferential inhibitor of monoamine oxidase (MAO) type A, were compared with those of pargyline, a preferential inhibitor of MAO type B, in 16 depressed patients. In a subgroup of more severely depressed patients, clorgyline treatment for 4 weeks resulted in significant improvement on both observer-rated and self-rated scales, while minimal changes occurred during pargyline treatment. Similarly, in a crossover study that included 8 patients examined with multiple scales, clorgyline had generally greater antidepressant and antianxiety effects than did pargyline, although pargyline had some activating effects and also tended to produce more side effects. MAO type A inhibition may be more important than MAO type B inhibition for antidepressant efficacy.

The central noradrenergic system and affective response to MAO inhibitors

1. In humans, norepinephrine (NE) has been postulated to be involved in the regulation of mood and behavior and to be altered in patients with manic-depressive illness. 2. Recent methodological advances have made possible a more direct assessment of central noradrenergic activity by the accurate measurement of the small amounts of NE and of the enzyme responsible for the conversion of dopamine to NE, dopamine-beta-hydroxylase (DBH), found in cerebrospinal fluid (CSF). 3. Cerebrospinal fluid samples were obtained from depressed patients both before and after treatment with two monoamine oxidase-inhibiting antidepressant drugs, clorgyline and pargyline. 4. Patients were rated twice daily by nursing staff on a modified 15-point scale for severity of global depression and anxiety. Patients were also rated using the Hamilton depression rating scale. 5. High negative correlations were observed between the drug-related changes in CSF NE and the changes in depression ratings on both the global ratings (r = -.95, p less than .001) and the Hamilton rating scale (r = -.81, p less than .01). Changes in NE were also highly correlated with changes in global anxiety ratings (r = -.85, p less than .01) calculated on the basis of changes from baseline for each measurement. Drug-related changes in CSF DBH similarly showed negative correlations with clinical response (r = -.79, r = -.38, r = -.68 respectively). In contrast, no significant correlations were found when drug-related changes in CSF MHPG were compared to changes in clinical state.