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

A neurogenic signature involving monoamine Oxidase-A controls human thermogenic adipose tissue development

Mechanisms that control 'beige/brite' thermogenic adipose tissue development may be harnessed to improve human metabolic health. To define these mechanisms, we developed a species-hybrid model in which human mesenchymal progenitor cells were used to develop white or thermogenic/beige adipose tissue in mice. The hybrid adipose tissue developed distinctive features of human adipose tissue, such as larger adipocyte size, despite its neurovascular architecture being entirely of murine origin. Thermogenic adipose tissue recruited a denser, qualitatively distinct vascular network, differing in genes mapping to circadian rhythm pathways, and denser sympathetic innervation. The enhanced thermogenic neurovascular network was associated with human adipocyte expression of THBS4, TNC, NTRK3, and SPARCL1, which enhance neurogenesis, and decreased expression of MAOA and ACHE, which control neurotransmitter tone. Systemic inhibition of MAOA, which is present in human but absent in mouse adipocytes, induced browning of human but not mouse adipose tissue, revealing the physiological relevance of this pathway. Our results reveal species-specific cell type dependencies controlling the development of thermogenic adipose tissue and point to human adipocyte MAOA as a potential target for metabolic disease therapy.

Synthesis of 2′-(1,2,3-triazoyl)-acetophenones: molecular docking and inhibition of in vitro monoamine oxidase activity

The synthesis of 2′-(1,2,3-triazoyl)-acetophenones by a CuAAC using thiourea as a ligand, molecular docking and MAO activity analyses were performed.

Monoamine oxidase-B inhibitors as potential neurotherapeutic agents: An overview and update

Monoamine oxidase (MAO) inhibitors have made significant contributions and remain an indispensable approach of molecular and mechanistic diversity for the discovery of antineurodegenerative drugs. However, their usage has been hampered by nonselective and/or irreversible action which resulted in drawbacks like liver toxicity, cheese effect, and so forth. Hence, the search for selective MAO inhibitors (MAOIs) has become a substantial focus in current drug discovery. This review summarizes our current understanding on MAO-A/MAO-B including their structure, catalytic mechanism, and biological functions with emphases on the role of MAO-B as a potential therapeutic target for the development of medications treating neurodegenerative disorders. It also highlights the recent developments in the discovery of potential MAO-B inhibitors (MAO-BIs) belonging to diverse chemical scaffolds, arising from intensive chemical-mechanistic and computational studies documented during past 3 years (2015-2018), with emphases on their potency and selectivity. Importantly, readers will gain knowledge of various newly established MAO-BI scaffolds and their development potentials. The comprehensive information provided herein will hopefully accelerate ideas for designing novel selective MAO-BIs with superior activity profiles and critical discussions will inflict more caution in the decision-making process in the MAOIs discovery.

Evidence that formulations of the selective MAO-B inhibitor, selegiline, which bypass first-pass metabolism, also inhibit MAO-A in the human brain

Selegiline (L-deprenyl) is a selective, irreversible inhibitor of monoamine oxidase B (MAO-B) at the conventional dose (10 mg/day oral) that is used in the treatment of Parkinson's disease. However, controlled studies have demonstrated antidepressant activity for high doses of oral selegiline and for transdermal selegiline suggesting that when plasma levels of selegiline are elevated, brain MAO-A might also be inhibited. Zydis selegiline (Zelapar) is an orally disintegrating formulation of selegiline, which is absorbed through the buccal mucosa producing higher plasma levels of selegiline and reduced amphetamine metabolites compared with equal doses of conventional selegiline. Although there is indirect evidence that Zydis selegiline at high doses loses its selectivity for MAO-B, there is no direct evidence that it also inhibits brain MAO-A in humans. We measured brain MAO-A in 18 healthy men after a 28-day treatment with Zydis selegiline (2.5, 5.0, or 10 mg/day) and in 3 subjects receiving the selegiline transdermal system (Emsam patch, 6 mg/day) using positron emission tomography and the MAO-A radiotracer [(11)C]clorgyline. We also measured dopamine transporter (DAT) availability in three subjects from the 10 mg group. The 10 mg Zydis selegiline dose significantly inhibited MAO-A (36.9±19.7%, range 11-70%, p<0.007)) but not DAT; and while Emsam also inhibited MAO-A (33.2±28.9 (range 9-68%) the difference did not reach significance (p=0.10)) presumably because of the small sample size. Our results provide the first direct evidence of brain MAO-A inhibition in humans by formulations of selegiline, which are currently postulated but not verified to target brain MAO-A in addition to MAO-B.

Xanthoangelol and 4-Hydroxyderricin Are the Major Active Principles of the Inhibitory Activities against Monoamine Oxidases on Angelica keiskei K

Monoamine oxidase inhibitors (MAOI) have been widely used as antidepressants. Recently, there has been renewed interest in MAO inhibitors. The activity-guided fractionation of extracts from Angelica keiskei Koidzumi (A. keiskei K.) led to the isolation of two prenylated chalcones, xanthoangelol and 4-hydroxyderricin and a flavonoid, cynaroside. These three isolated compounds are the major active ingredients of A. keiskei K. to inhibit the MAOs and DBH activities. Xanthoangelol is a nonselective MAO inhibitor, and a potent dopamine β-hydroxylase (DBH) inhibitor. IC₅₀ values of xanthoangelol to MAO-A and MAO-B were calculated to be 43.4 μM, and 43.9 μM. These values were very similar to iproniazid, which is a nonselective MAO inhibitor used as a drug against depression. The IC₅₀ values of iproniazid were 37 μM, and 42.5 μM in our parallel examination. Moreover, IC₅₀ value of xanthoangelol to DBH was calculated 0.52 μM. 4-Hydroxyderricin is a potent selective MAO-B inhibitor and also mildly inhibits DBH activity. The IC₅₀ value of 4-hydroxyderricin to MAO-B was calculated to be 3.43 μM and this value was higher than that of deprenyl (0.046 μM) used as a positive control for selective MAO-B inhibitor in our test. Cynaroside is a most potent DBH inhibitor. The IC₅₀ value of cynaroside to DBH was calculated at 0.0410 μM. Results of this study suggest that the two prenylated chalcones, xanthoangelol and 4-hydroxyderricin isolated from A. keiskei K., are expected for potent candidates for development of combined antidepressant drug. A. keiskei K. will be an excellent new bio-functional food material that has the combined antidepressant effect.

Effects of combined treatment with clorgyline and selegiline on extracellular noradrenaline and serotonin levels

Kitaichi Y, Inoue T, Nakagawa S, Boku S, Koyama T. Effects of combined treatment with clorgyline and selegiline on extracellular noradrenaline and serotonin levels.

Objective Combined treatment with clorgyline, an irreversible monoamine oxidase (MAO)-A inhibitor, and selegiline, an irreversible MAO-B inhibitor, reportedly increases extracellular serotonin levels in the raphe nuclei more than clorgyline does alone. However, the effects of combination of these MAO inhibitors on extracellular noradrenaline have not been reported.

Methods Using in vivo microdialysis, we measured extracellular noradrenaline and serotonin levels after administration of clorgyline and/or selegiline in the medial prefrontal cortex of rats.

Results Administration of clorgyline (10 mg/kg) significantly increased both extracellular serotonin and noradrenaline levels. Combined treatment using clorgyline (10 mg/kg) and selegiline (3 mg/kg) increased extracellular serotonin and noradrenaline levels more than each drug alone did.

Conclusions These findings of this study suggest the augmented antidepressant action of the combination of MAO-A inhibition and MAO-B inhibition. The addition of a MAO-A inhibitor to selegiline or increasing dose of selegiline to achieve full MAO-A inhibition might be the promising strategy for the antidepressant treatment in partial responders or non-responders to selegiline.

Combined treatment with MAO-A inhibitor and MAO-B inhibitor increases extracellular noradrenaline levels more than MAO-A inhibitor alone through increases in beta-phenylethylamine

Monoamine oxidase inhibitors (MAO inhibitors) have been widely used as antidepressants. However, it remains unclear whether a difference exists between non-selective MAO inhibitors and selective MAO-A inhibitors in terms of their antidepressant effects. Using in vivo microdialysis methods, we measured extracellular noradrenaline and serotonin levels following administration of Ro 41-1049, a reversible MAO-A inhibitor and/or lazabemide, a reversible MAO-B inhibitor in the medial prefrontal cortex (mPFC) of rats. We examined the effect of local infusion of beta-phenylethylamine to the mPFC of rats on extracellular noradrenaline and serotonin levels. Furthermore, the concentrations of beta-phenylethylamine in the tissue of the mPFC after combined treatment with Ro 41-1049 and lazabemide were measured. The Ro 41-1049 alone and the combined treatment significantly increased extracellular noradrenaline levels compared with vehicle and lazabemide alone. Furthermore, the combined treatment increased noradrenaline levels significantly more than Ro 41-1049 alone did. The Ro 41-1049 alone and the combined treatment significantly increased extracellular serotonin levels compared with vehicle and lazabemide alone, but no difference in serotonin levels was found between the combined treatment group and the Ro 41-1049 group. Local infusion of low-dose beta-phenylethylamine increased extracellular noradrenaline levels, but not that of serotonin. Only the combined treatment significantly increased beta-phenylethylamine levels in tissues of the mPFC. Our results suggest that the combined treatment with a MAO-A inhibitor and a MAO-B inhibitor strengthens antidepressant effects because the combined treatment increases extracellular noradrenaline levels more than a MAO-A inhibitor alone through increases in beta-phenylethylamine.

Long-term administration of monoamine oxidase inhibitors alters the firing rate and pattern of dopamine neurons in the ventral tegmental area

Monoamine oxidase inhibitors (MAOIs) exert their antidepressant action by increasing the function of the serotonin (5-HT), norepinephrine and dopamine (DA) systems. There is, however, limited electrophysiological data on the effects of MAOIs on DA neurons. The effects of 2-d and 21-d administration of three MAOIs were investigated (clorgyline, selective MAOI-A; deprenyl, selective MAOI-B; phenelzine, non-selective MAOI) on the firing activity of DA neurons in the ventral tegmental area using in-vivo electrophysiology in rats. Short-term clorgyline (1 mg/kg) and phenelzine (2.5 mg/kg) was devoid of effect on DA neurons, whereas prolonged administration significantly decreased their firing rate (by 30% and 20%, respectively), number of bursts (by 80% and 45%, respectively), and percentage of spikes occurring in bursts only in clorgyline-treated rats (70%). Deprenyl (0.25 mg/kg) was without effects. DA firing was restored in clorgyline-treated rats by inhibiting 5-HT synthesis using para-chlorophenylalanine (p-CPA; 300 mg/kg. d for three consecutive days). The 5-HT3 antagonist ondansetron (0.5 mg/kg) was devoid of effect in control rats, but completely reversed the alterations of DA neuronal activity in clorgyline-treated rats. An attenuation of DA neuronal activity was thus produced by prolonged blockade of MAOA activity. The absence of effect of MAOA inhibition after subacute administration suggested an indirect mechanism. This was confirmed by the observation that p-CPA antagonized the effects of clorgyline. Since ondansetron completely reversed the effects of clorgyline on DA neuronal activity, the effects of MAOA inhibition appeared to be mediated by 5-HT3 receptors.

How antidepressants work: cautionary conclusions based on clinical and laboratory studies of the longer-term consequences of antidepressant drug treatment

Time-dependent alterations in the functional activity of adrenergic and serotonergic neurotransmitter systems and, in particular, a frequently observed down-regulation of brain beta-adrenoceptors have been implicated in antidepressant drug effects. Current studies of catecholamine and serotonin neurotransmitter systems suggest that the net physiological output changes in neuroendocrine responses, blood pressure, sleep and motor activity which follow various antidepressant treatments in psychiatric patients, normal controls and different experimental animals are not indicative of a common response pattern to all therapeutically effective agents. Rather, antidepressant treatment effects differ according to many variables, including the pre-existing state of the organism (e.g. depressed, stressed or normal), the species, the duration of treatment and the particular brain or peripheral circuits investigated. Examples are cited from our studies of the effects of monoamine oxidase inhibitors and other antidepressants on noradrenergic-serotonergic interactions that affect melatonin release and other neuroendocrine responses, on some additional functional end-points, and on depressive mood and other symptoms in patients with depression or other tricyclic-responsive disorders. These examples illustrate the complexity found in attempts to identify a unitary mechanism of antidepressant drug action.

Effects of a tyramine-enriched meal on blood pressure response in healthy male volunteers treated with selegiline transdermal system 6 mg/24 hour

BACKGROUND

Monoamine oxidase inhibitors are well recognized as effective antidepressant agents but are rarely used due, in part, to the risk of hypertensive crisis following the ingestion of foods high in tyramine ("cheese reaction"). A selegiline transdermal system (STS) was developed to provide antidepressant concentrations of selegiline in the brain, while preserving the gastrointestinal monoamine oxidase A (MAO-A) barrier. The present study was conducted to determine the effect of the STS 6 mg/24 hour on cardiovascular safety following the ingestion of approximately 400 mg of tyramine consumed as a component of aged cheeses.

METHODS

In this open-label, single-center phase I study, cardiovascular vital signs were recorded following tyramine challenges during placebo and STS 6 mg/24 hr treatment. Subjects were observed for clinical signs and symptoms of a pressor response and/or potential hypertensive crisis during and following the challenges.

RESULTS

Ingestion of tyramine-enriched meals following 13 consecutive days of treatment with the STS 6 mg/24 hr (pharmacokinetic steady-state) produced no clinically significant changes in cardiovascular vital signs in 12 healthy adult male subjects. No evidence of a tyramine pressor effect on systolic blood pressure or evidence of hypertensive crisis occurred during the STS treatment.

CONCLUSION

These results suggest that STS 6 mg/24 hr may be administered without concern for dietary tyramine consumption.

Transdermal selegiline: the new generation of monoamine oxidase inhibitors

The clinical use of monoamine oxidase inhibitors (MAOIs) has declined due to concerns about food and drug interactions and waning physician experience. Evidence indicates that MAOIs are effective in depressive disorders, in particular depression with atypical features. Efforts to address safety issues have led to the development of more selective and reversible MAOIs, such as moclobemide. Selegiline, a selective monoamine oxidase B inhibitor, has been approved for the adjunctive treatment of Parkinson's disease at low doses. At higher doses, oral selegiline is also effective in major depressive disorder (MDD) but loses its selectivity and has the potential for tyramine interactions. To overcome these problems, a transdermal formulation of selegiline, the selegiline transdermal system (STS), was developed with novel pharmacokinetic and pharmacodynamic properties. Compared with oral administration, transdermal selegiline leads to sustained plasma concentrations of the parent compound, increasing the amount of drug delivered to the brain and decreasing metabolite production. In addition, STS allows targeted inhibition of central nervous system monoamine A (MAO-A) and monoamine B isoenzymes with minimal effects on MAO-A in the gastrointestinal and hepatic systems, thereby reducing the risk of interactions with tyramine-rich foods (the "cheese-reaction"). Clinical trials have found 6 mg/24 hours of STS to be effective in MDD without the need for dietary restrictions. The efficacy and safety profile of STS supports its use in MDD. It is possible that STS may demonstrate benefit in MDD with atypical features or MDD resistant to other antidepressants. However, more research is needed. Clinicians should familiarize themselves with the properties and indications for the new generation of MAOIs.

Positron emission tomography quantification of [11C]-harmine binding to monoamine oxidase-A in the human brain

This article describes the kinetic modeling of [(11)C]-harmine binding to monoamine oxidase A (MAO-A) binding sites in the human brain using positron emission tomography (PET). Positron emission tomography studies were performed in healthy volunteers at placebo conditions and after treatment with clinical doses of moclobemide. In either condition, a two-tissue compartment model (2CM) provided better fits to the data than a one-tissue model. Estimates of k(3)/k(4) values from an unconstrained 2CM were highly variable. In contrast, estimates of the specifically bound radioligand distribution volume (DV(B)) from an unconstrained 2CM were exceptionally stable, correlated well with the known distribution of MAO-A in the brain (cerebellum <frontal cortex approximately putamen <temporal cortex approximately cingulate <thalamus) and thus provided reliable indices of MAO-A density. Total distribution volume (DV) values were also highly stable and not different from those estimated with the Logan approach. Fixing the DV of free and nonspecifically bound radiotracer (DV(F + NS)) or coupling DV(F + NS) between brain regions enabled more stable estimates of k(3)/k(4) as compared with an unconstrained 2CM. Moclobemide treatment leads to a 64% to 79% MAO-A blockade across brain regions, a result that supports the specificity of [(11)C]-harmine binding to MAO-A. The stability and reliability of DV(B) values obtained from an unconstrained 2CM, together with the computational simplicity associated with this method, support the use of DV(B) as an appropriate outcome measure for [(11)C]-harmine. These results indicate the suitability of using [(11)C]-harmine for quantitative evaluation of MAO-A densities using PET and should enable further studies of potential MAO-A dysregulation in several psychiatric and neurologic illnesses.

Daily transdermal administration of selegiline to guinea-pigs preferentially inhibits monoamine oxidase activity in brain when compared with intestinal and hepatic tissues

Selegiline has been formulated in an acrylic polymer adhesive mixture to be employed as a constant release topical patch for daily transdermal administration. Application of this selegiline transdermal system (STS) to guinea-pigs resulted in an average delivery of 1.185 mg selegiline/cm(2) patch/24 h. STS dose-response curves were generated by altering patch size (cm(2)). A transdermal dose range was identified which inhibited guinea-pig brain monoamine oxidase-B (MAO-B) by greater than 95% yet provided for a dose-dependent inhibition of monoamine oxidase-A (MAO-A) activity. The ID50 for inhibition of MAO-A activity in response to a 21-day daily regimen with transdermal selegiline was approximately 7.5-fold lower for cortical and striatal brain regions compared with that obtained for duodenum; hepatic MAO-A was unaffected following the same dosing regimen. By contrast, orally administered selegiline inhibited brain and duodenal MAO-A to the same extent, and generated a shallower dose-inhibition curve for brain MAO-A inhibition. In addition, transdermal delivery was approximately 6-8-times more potent than oral selegiline for the inhibition of brain MAO-A activity. It is concluded that daily transdermal selegiline administration may provide therapeutic advantages over oral treatment, based on its preferential, dose-dependent inhibition of brain vs peripheral MAO-A activity.

Moclobemide: evolution, pharmacodynamic, and pharmacokinetic properties

The benzamide moclobemide is a reversible inhibitor of monoamine-oxidase-A (RIMA). It has been extensively evaluated in the treatment of a wide spectrum of depressive disorders and less extensively in anxiety disorders. While clinical aspects will be presented in a subsequent review, this article focuses primarily on moclobemide's evolution, pharmacodynamic and pharmacokinetic properties. In particular, the effects on neurotransmission and intracellular signal transduction, the neuroendocrine system, the tyramine pressure response and animal models of depression are surveyed. In addition, other CNS effects are reviewed with special respect to experimental serotonergic syndrome, anxiolytic and antinociceptive activity, sleep, cognition and driving performance, neuroprotection and seizures.

Comparative effects of dehydropirlindole and other compounds on rat brain monoamine oxidase type A

Dehydropirlindole (DHP) is the dehydroderivative of pirlindole, a short-acting inhibitor of monoamine oxidase type A (MAO-A). DHP would be formed in vivo from oxidation of pirlindole by MAO-A. The aim of this work is to compare the inhibitory potency of DHP with three reference compounds: harmaline, befloxatone and clorgyline; the two former are reversible inhibitors and the later is an irreversible inhibitor of MAO-A. Both in vitro and ex vivo assays were performed on rat brain homogenates, and IC50 and ID50 were calculated by a fluorometric method with octopamine as selective MAO-A substrate. In vitro clorgyline and befloxatone were more potent inhibitors than DHP and harmaline with IC50 values of 1.6 and 7.7 nM vs. 40 and 55 nM; ex vivo ID50 values were 1.5 and 32 micromol/kg vs. 41 and 49 micromol/kg. Befloxatone had an ID50/IC50 ratio four to five times higher than DHP and harmaline. Preincubation time experiments did not distinguish befloxatone from DHP and harmaline. In conclusion, this study shows that DHP behaves as a reversible MAO-A inhibitor whose potency is situated between that of befloxatone and harmaline.

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.

Regional action of brofaromine on rat brain MAO-A and MAO-B

1. Brofaromine (CGP 11,305 A) belongs to a new generation of monoamine oxidase (MAO) inhibitors. These compounds induce short, reversible and selective inhibition of brain MAO of type A. 2. The aim of this work is to study monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) activities of several rat brain regions after increasing doses of brofaromine. 3. Brofaromine inhibits MAO-A activities in a dose dependent manner in all brain regions examined. 4. The largest reduction was found in hippocampal formation, striatum and prefrontal cortex respectively. ID50 is 2 times lower in hippocampus than in remaining brain. 5. Brofaromine does not inhibit MAO-B activities in the different regions examined. 6. Brofaromine is a very selective inhibitor of rat brain MAO-A with a preferential action on telencephalic monoaminergic nerve terminals.

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.

Disruption of the activity-rest cycle by MAOI treatment: dependence on light and a secondary visual pathway to the circadian pacemaker

The disruptive effects on the activity-rest cycle of the monoamine oxidase inhibitor (MAOI) clorgyline and of continuous light were examined in Syrian hamsters. When administered in dim and moderate light intensities, clorgyline delayed the daily onset of wheel-running. When administered in bright light, it dissociated the circadian rhythm of wheel-running. This dissociation was prevented by lesions of the intergeniculate leaflet of the ventral lateral geniculate nucleus. Constant darkness restored the circadian rhythm of wheel-running in hamsters with disrupted circadian rhythms. The phase of the restored rhythm of wheel-running was shifted 6-12 h later than the phase of wheel-running prior to dissociation. Our results suggest that MAOI treatment weakens the coupling between oscillators that comprise the circadian pacemaker, and augments the disruptive effects of continuous light acting via the intergeniculate leaflet region of the ventral lateral geniculate nucleus. These effects on the circadian pacemaker may be responsible for disruptions of the sleep-wake cycle that occur as side effects when MAOIs are used clinically to treat depression and might play a role in the induction of mania and rapid cycling by antidepressants.

Evidence of a coupled mechanism between monoamine oxidase and peroxidase in the metabolism of tyramine by rat intestinal mitochondria

The relationship between monoamine oxidase (EC 1.4.3.4; MAO) and peroxidase (EC 1.11.1.7; POD) in the metabolism of tyramine was investigated using the crude mitochondrial fraction of rat intestine. When tyramine was incubated with mitochondria, the formation of the peroxidase-catalysed oxidation product, 2,2'-dihydroxy-5,5'-bis(ethylamino)diphenyl (dityramine), identified by mass spectrometric analysis, was monitored spectrophotometrically. After an initial lag time, the formation rate of dityramine was linear up to 2 hr, amounting to 17 nmol x hr(-1) x mg protein(-1). A similar value was found for the oxidative deamination of tyramine catalysed by intestinal MAO. Either 10(-3) M clorgyline or 10(-3) M NaCN suppressed this reaction by completely inhibiting MAO or POD, respectively. In the former case, however, addition of H2O2 to the incubation mixture promptly started the reaction. Selective inhibition of MAO-A and MAO-B was achieved with 3 x 10(-7) M clorgyline and 3 x 10(-7) M deprenyl, respectively, and the formation rate of dityramine decreased in a corresponding manner. Preincubation with histamine or spermidine reduced the lag time without affecting the steady-state reaction rate. Higher levels of dityramine were also detected in vivo in rat intestine after oral administration of tyramine. These results indicate that the peroxidase-dependent metabolism of tyramine in the gut may be driven by H2O2 produced by MAO activities and that MAO-A is mainly responsible for this process, as well as for the oxidative deamination of tyramine.

Monoamine oxidase inhibitors. An update on drug interactions

After initial enthusiasm, the use of monoamine oxidase inhibitors (MAOIs) has been limited by the wide range of MAOI-drug and MAOI-food interactions that are possible, particularly with sympathomimetic medications or tyramine-containing foods, resulting in hypertensive reactions. Despite their clinical benefits, this has led to a reduction in use of such medications. Discovery of the 2 main subgroups of monoamine oxidase, types A and B, led to the synthesis of MAOIs selective for one or other of these isoenzymes. Consequently, selegiline (deprenyl), a selective MAO-B inhibitor, was developed for the treatment of idiopathic Parkinson's disease. This drug is useful in the treatment of the early stages of the disease and later on as an adjunct to other drug therapies. Although the selective MAO-A inhibitor, clorgiline (clorgyline), was found to be effective in the treatment of depression, it still retained the potential to cause hypertensive reactions. Recently, agents that are not only selective, but reversible in their inhibition of MAO-A (RIMAs) have been synthesised (e.g. moclobemide and toloxatone), and have proven antidepressant efficacy. Whilst they are less likely to induce hypertensive reactions with the concomitant administration of sympathomimetic drugs or with tyramine-rich foodstuffs, it still seems wise to advocate care in co-prescribing potentially interacting medications and to advise a degree of caution with regard to the dietary intake of foodstuffs likely to contain a high tyramine content. Although these newer drugs represent an advance in safety, their use has, as yet, only been established in the treatment of depression. RIMAs also retain a potential for adverse interaction with other drugs. Concomitant prescription of serotonin-enhancing drugs should only be undertaken with caution for patients on moclobemide, toloxatone or selegiline. Coprescription of sympathomimetic drugs should also be avoided with these newer MAOIs and patients should be advised against purchasing over-the-counter preparations that may contain sympathomimetic drugs.

Circadian rhythms and the pharmacology of affective illness

The chronic effects of antidepressant drugs (ADs) on circadian rhythms of behavior, physiology and endocrinology are reviewed. The timekeeping properties of several classes of ADs, including tricyclic antidepressants, selective serotonin reuptake inhibitors, monoamine oxidase inhibitors, serotonin agonists and antagonists, benzodiazepines, and melatonin are reviewed. Pharmacological effects on the circadian amplitude and phase, as well as effects on day-night measurements of motor activity, sleep-wake, body temperature (Tb), 3-methoxy-4-hydroxyphenylglycol, cortisol, thyroid hormone, prolactin, growth hormone and melatonin are examined. ADs often lower nocturnal Tb and affect the homeostatic regulation of sleep. ADs often advance the timing and decrease the amount of slow wave sleep, reduce rapid eye movement sleep and increase or decrease arousal. Together, AD effects on nocturnal Tb and sleep may be related to their therapeutic properties. ADs sometimes delay nocturnal cortisol timing and increase nocturnal melatonin, thyroid hormone and prolactin levels; these effects often vary with diagnosis, and clinical state. The effects of ADs on the coupling of the central circadian pacemaker to photic and nonphotic zeitgebers are discussed.

Autonomic and behavioral thermoregulation in the golden hamster during subchronic administration of clorgyline

Chronic administration of clorgyline, a type-A monoamine oxidase inhibitor, leads to a decrease in peritoneal (i.e., core) temperature of golden hamsters. To better understand the mechanisms of clorgyline's thermoregulatory effects, autonomic and behavioral thermoregulatory effectors were measured in Syrian hamsters following chronic infusion of clorgyline via a minipump (2 mg/kg/day). Metabolic rate, evaporative water loss, motor activity, and core temperature were measured after 60 min of exposure to ambient temperatures (Ta) of 5, 20, 30, and 35 degrees C. Behavioral thermoregulatory responses were assessed by measuring selected Ta and motor activity of the same animals in a temperature gradient over the course of 23 h. Metabolic rate and motor activity were significantly elevated in clorgyline-treated hamsters exposed to a Ta of 5 degrees C. There were no effects of clorgyline on evaporative water loss. In the temperature gradient the mean selected Ta of clorgyline-treated hamsters was nearly equal to that of the saline-treated hamsters, 30.7 and 31.2 degrees C, respectively. On the other hand, the mode of selected Ta in the clorgyline group was 2.8 degrees C higher than that of the saline group. Motor activity in the gradient was significantly elevated and food consumption was depressed by clorgyline treatment. Overall, these findings indicate that chronic clorgyline treatment in the golden hamster results in novel autonomic and behavioral modification; it stimulates metabolic thermogenesis during cold exposure, but appears to increase the behavioral zone of thermoneutrality. This latter effect may mean an improvement in heat tolerance, suggesting that this drug might assist in the adaptation to warm temperatures.

Effect of a selective MAO-A inhibitor (Ro 41-1049) on striatal L-dopa and dopamine metabolism: an in vivo study

We administered Ro 41-1049, an inhibitor of the enzyme monoamine oxidase type A (MAO-A) to rats and monitored extracellular catecholamine levels in the corpus striatum before and after the intraperitoneal (IP) administration of a bolus of L-dopa. Acute administration of Ro 41-1049 (1-50 mg/kg IP) produced a dose-dependent decrease in basal levels of the dopamine metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and an increase in basal levels of dopamine. In rats treated with Ro 41-1049 (20 mg/kg IP), L-dopa administration (100 mg/kg IP) produced a greater increase in striatal levels of dopamine than it did in controls, while DOPAC and HVA formation was attenuated. We conclude that inhibition of central MAO-A activity promotes synaptic accumulation of dopamine following administration of pharmacological doses of L-dopa.

Clorgyline effect on pineal melatonin biosynthesis in adrenalectomized rats pretreated with 6-hydroxydopamine

The response to administration of the specific monoamine oxidase A (MAO-A) blocker clorgyline was investigated in adult male Sprague-Dawley rats which were adrenalectomized four days prior to treatment or were additionally sympathectomized as newborns by injection of 6-hydroxydopamine. In both groups, the contents of pineal indoles melatonin and N-acetylserotonin were augmented, and the contents of 5-hydroxyindoleacetic acid and 5-hydroxyindoletryptophol decreased 90 min following clorgyline injections when compared to rats receiving saline. The observed responses were less pronounced in rats both adrenalectomized and sympathectomized. The results are in line with the hypothesis that preservation from oxidation of both MAO-A substrates, noradrenaline and serotonin, upon clorgyline administration contributes to the observed increase in melatonin biosynthesis thought to be associated with the anti-depressant effects of MAO inhibition.

The trace amines and their acidic metabolites in depression--an overview

1. Investigations of the role of the trace amines (phenylethylamine, tryptamine, m- and p-tyramine) and their acidic metabolites (phenylacetic, indoleacetic, m- and p-hydroxyphenylacetic acids) in depression are reviewed. 2. The evidence for the phenylethylamine hypothesis of depression is mixed. 3. Reduced phenylacetic acid levels in urine, plasma and CSF and changes in those levels during treatment with antidepressants show potential as state markers for depression. 4. Impaired p-tyramine conjugation following a tyramine challenge may be a good trait marker for depression.

Clinical overview on moclobemide

1. Moclobemide is a novel benzamide reversible inhibitor of monoamine oxidase A and has clinical efficacy in a wide spectrum of depressive illness including endogenous and non-endogenous depression, in younger adults and in the elderly. 2. Comparisons have shown similar efficacy to all main classes of antidepressants and much greater tolerability and safety in overdose than tricyclic antidepressants. Clinically, it is neither sedative nor alerting. 3. There is no need for dietary restrictions for patients on moclobemide on a normal diet, and drug interactions are few and usually mild. Specific cautions are noted with pethidine and with selective serotonin re-uptake inhibitor antidepressants. 4. Moclobemide is a useful addition to the range of antidepressants in clinical practice.

The new generation of monoamine oxidase inhibitors

Irreversible and unspecific inhibitors of MAO were the first modern antidepressants, but after an initial success they fell into discredit due to adverse side effects. In the past two decades interest in MAO inhibitors has been renewed because of progress in basic research, a milestone being the finding that there are two subtypes of MAO, MAO-A and MAO-B. These are distinct proteins with high amino acid homology, coded by separate genes both located on the short arm of the human chromosome X. The enzyme subforms show different substrate specificities in vitro and different distributions within the central nervous system and in peripheral organs. In the central nervous system of man MAO-A seems to be mainly involved in the metabolism of 5 HT and noradrenaline, whereas 2-phenylethylamine and probably dopamine are predominantly deaminated by MAO-B. In the intestinal tract tyramine is mainly metabolized by MAO-A. These characteristics indicate distinct physiological functions of the two MAO-subforms. Several irreversible and reversible non-hydrazine inhibitors with relative selectivities for one of the MAO-subforms have been developed. They belong to various chemical classes with different modes of enzyme inhibition. These range from covalent mechanism based interaction (e.g. by propargyl- and allylamine derivatives) to pseudosubstrate inhibition (e.g. by 2-aminoethyl-carboxamides) and non-covalent interaction (e.g. by brofaromine, toloxatone and possibly moclobemide). The most important pharmacological effects of the new types of MAO inhibitors are those observed in neuropsychiatric disorders. The inhibitors of MAO-A show a favorable action in various forms of mental depression. The drugs seem to have about the same activity as other types of antidepressants, including tricyclic and related compounds as well as classical MAO inhibitors. The onset of action of the MAO-A inhibitors is claimed to be relatively fast. Other possible indications of these drugs include disorders with cognitive impairment, e.g. dementia of the Alzheimer type. In subjects with Parkinson's disease the MAO-B inhibitor L-deprenyl exerts a L-dopa-sparing effect, prolongs L-dopa action and seems to have a favorable influence regarding on-off disabilities. The action is in general transitory (months to several years). In addition L-deprenyl has been shown to delay the necessity for L-dopa treatment in patients with early parkinsonism. Whether the drug influence the progression of the disease is still a matter of debate. L-deprenyl also appears to have some antidepressant effect (especially in higher doses) and to exert a beneficial influence in other disorders, e.g. dementia of the Alzheimer type.(ABSTRACT TRUNCATED AT 400 WORDS)

Minipump clorgyline administration and CSF amine metabolites in unrestrained monkeys

The irreversible MAO-A inhibitor clorgyline was administered in doses of 0.5 mg/kg (N = 1), 1 mg/kg (N = 3), and 2 mg/kg (N = 1) to 5 young (age 5.5 to 23.9 months) pigtail (M. nemestrina) monkeys using a 28-day (Alza 2ML4) osmotic minipump. CSF MHPG, 5-HIAA, HVA, and plasma MHPG were measured before and at approximately weekly intervals after pump implantation. Implants were well tolerated. CSF MHPG decreased about 75%, 5-HIAA 30%, and HVA from 30-50% with a tendency to plateau by the second week. Plasma MHPG decreased to undetectable levels. The findings demonstrate that long-term inhibition of MAO-A can be produced in unrestrained monkeys by minipump administered clorgyline. There is an apparently greater effect on the norepinephrine system relative to the serotonin and dopamine systems.

[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.

New directions in monoamine oxidase A and B selective inhibitors and substrates

Identification, cellular localization, and cDNA cloning of MAO subtypes A and B have increased the insight into the pharmacology of these enzymes, whose primary functions are intra- and extraneuronal inactivation of neurotransmitter (dopamine, noradrenaline and serotonin) and other biogenic amines. In addition, MAO oxidizes the inert uncharacteristic tertiary amine, MPTP, to the parkinson inducing dopaminergic neurotoxin, MPP+, and the novel secondary amine anticonvulsant milacemide to the inhibitory amino acid neurotransmitter, glycine. These recent developments have provided new therapeutic perspectives for the management of Parkinson's disease and seizure disorders via the use of selective inhibitors and amino acid amine prodrug substrates of MAO-B.

Modulation of vocal and nonvocal behavior in adult squirrel monkeys by selective MAO-A and MAO-B inhibition

The acute effects of monoamine oxidase inhibitors L-deprenyl (0.5-5.0 mg/kg), clorgyline (1.0-10.0 mg/kg), and milacemide (100-400 mg/kg) on the behavior of adult male squirrel monkeys were examined during brief social separations beginning 60 min after subcutaneous drug administration. All three drugs selectively reduced the rate of calling during social separation at doses which did not affect time spent in locomotion, nor the frequency of vigilance-checking. Deprenyl and milacemide, but not clorgyline, produced concurrent decreases in locomotion at the higher doses tested. At threshold doses, clorgyline, but not deprenyl or milacemide, increased call duration and decreased call peak frequency compared to vehicle control values. Plasma levels of MHPG were decreased by an optimal dose of clorgyline but not by deprenyl or milacemide, indicating that substrate specificity was maintained at the drug doses employed. We conclude that different MAO substrates mediate different aspects of vocal and nonvocal behavior in adult male squirrel monkeys.

Current issues in monoamine oxidase inhibitor treatment

Interest in the monoamine oxidase (MAO) inhibitors has been revived over the past fifteen years since the publication of studies and reviews which argue their relative safety and efficacy in certain patient subgroups. The authors conclude that there is a continuing role for the MAO inhibitors in treating atypical depressive illness and panic disorders. Careful selection of patients to be commenced on MAO inhibitor treatment is advised. When prescribed alone or in combination with lithium there is evidence to support a usefulness for MAO inhibitors in depression resistant to the tricyclic antidepressants. Their efficacy in the depressed phase of bipolar affective disorder or in combination with the tricyclic antidepressants remains unproven. The risk of anaesthesia while a patient is receiving MAO inhibitor treatment may be less than heretofore believed. There is realistic hope of finding among the short-acting selective MAO inhibitors an effective antidepressant which is also free from tyramine related effects. With the increasing difficulties of evaluating new drugs on account of ethical and governmental constraints, it may however be some time before the preliminary hopeful findings can be substantiated in larger groups of patients.

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

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.

Comparative studies of the effects of RS-8359 and safrazine on monoamine oxidase in-vitro and in-vivo in mouse brain

The effect of RS-8359, pyrimidine on monoamine oxidase (MAO) has been compared with a hydrazinic MAO inhibitor, safrazine (beta-piperonylisopropylhydrazine hydrochloride,) which is a MAO inhibitor used clinically. In-vitro radiochemical determination of MAO activity showed that the IC50 of RS-8359 was 0.52 microM for the deamination of 5-hydroxytryptamine (5-HT) in the mouse brain mitochondrial preparation, while beta-phenylethylamine (PEA) deamination was inhibited by only 20% at 100 microM of the drug. 5-HT deamination in the brain homogenate prepared from mice killed 60 min after administration of RS-8359 was inhibited significantly by 14 and 48%, at 30 and 100 mg kg-1 (p.o.), respectively, while deamination of PEA was little affected at the same doses. On the other hand, safrazine strongly inhibited both 5-HT and PEA deaminations, but showed no selectivity toward the substrate used. The extent of MAO inhibition by RS-8359, measured fluorometrically with kynuramine as a substrate in the brain homogenate, was independent of preincubation up to 80 min. In contrast, the inhibitory potency of safrazine was strengthened by preincubation in a time-dependent manner. Oral administration of RS-8359 (3-30 mg kg-1) caused a dose-dependent increase in endogenous monoamines in mouse brain, which disappeared a few hours after its administration. Increase in monoamine content caused by safrazine lasted for at least 24 h. These results indicate that RS-8359 is a reversible and specific inhibitor of MAO-A, while safrazine is an irreversible and non-specific MAO inhibitor, in-vivo and in-vitro in mouse brain.

Role of serotonergic input in the down-regulation of beta-adrenoceptors following long-term clorgyline treatment

Administration of the selective monoamine oxidase (MAO) type A-inhibiting antidepressant clorgyline (1 mg/kg per day) to rats for 21 days caused a significant decrease in cortical [3H]dihydroalprenolol binding. Selective lesioning of central serotonergic axons by 5,7-dihydroxytryptamine (5,7-DHT; confirmed by the presence of the serotonin syndrome in response to a 40 mg/kg dose of 5-hydroxytryptophan (5-HTP) or inhibition of 5-HT synthesis by parachlorophenylalanine (PCPA) caused significant 5-HT and 5-HIAA depletions in the cortex without much effect on NE and DA concentrations, but did not have any significant effect on beta-adrenoceptor density, and furthermore failed to attenuate clorgyline-induced decreases in beta-adrenoceptor density. Clorgyline treatment partially antagonized 5-HT depletion by the 5,7-DHT lesion or PCPA treatment. These findings suggest that due to their ability to raise 5-HT concentrations, MAO-inhibiting antidepressants may be a better alternative than the tricyclics in treating depressed patients with reduced 5-HT if down-regulation of beta-adrenoceptors is critical for antidepressant efficacy.

Is monoamine oxidase inhibitor induced myoclonus serotoninergically mediated?

In the present study a single case observation of myoclonus during sleep-wave transition was monitored in a depressed patient treated with the monoamine oxidase inhibitor, phenelzine. The myoclonus had a rhythm of 1 c/second and lasted for two years, the duration of phenelzine treatment. Myoclonus appeared neither during wakefulness nor during sleep, but at wake-sleep-wake transitions. This "switch" myoclonus was associated with phasic muscle hyperactivity during REM sleep. Methysergide a 5-HT suppressor, decreased the switch myoclonus frequency and the REM muscle hyperactivity, indicating serotoninergic involvement in the mechanism of phenelzine induced myoclonus.

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.