Inhibitors of monoamine oxidase. Influence of methyl substitution on the inhibitory activity of beta-carbolines

Harmane Potentiates Nicotine Reinforcement Through MAO-A Inhibition at the Dose Related to Cigarette Smoking

Nicotine is the primary addictive component in cigarette smoke, and dopamine release induced by nicotine is considered a significant cause of persistent smoking and nicotine dependence. However, the effects of nicotine replacement therapy on smoking cessation were less effective than expected, suggesting that other non-nicotine constituents may potentiate the reinforcing effects of nicotine. Harmane is a potent, selective monoamine oxidase A (MAO-A) inhibitor found in cigarette smoke, but showed no effect on nicotine self-administration in previous studies, possibly due to the surprisingly high doses used. In the present study, we found that harmane potentiated nicotine self-administration on the fixed ration schedule at the dose related to human cigarette smoking by the synergistic effects in up-regulating genes in addiction-related pathways, and the effect was reduced at doses 10 times higher or lower than the smoking-related dose. The smoking-related dose of harmane also enhanced the increase of locomotor activity induced by nicotine, accompanied by increased dopamine basal level and dopamine release in the nucleus accumbens through MAO-A inhibition. Our findings provided new evidence for the important role of non-nicotine ingredients of tobacco products in smoking addiction.

Application of the Asymmetric Pictet-Spengler Reaction in the Total Synthesis of Natural Products and Relevant Biologically Active Compounds

Tetrahydroisoquinolines are the framework of numerous natural products predominantly alkaloids, an important and one of the most wide spread families of naturally occurring compounds in the plant kingdom. Tetrahydroisoquinolines are commonly constructed through an old reaction, the so-called Pictet−Spengler Reaction (PSR). In this reaction, a β-aryl ethylamine undergoes an acid mediated condensation with a suitable aldehyde or ketone, followed by ring closure. In this review, we aim to highlight the applications of the asymmetric variant of this old name reaction in the total synthesis of natural products, chiefly, alkaloids, which exhibit significant biological properties.

Design, synthesis and anticancer evaluation of tetrahydro-β-carboline-hydantoin hybrids

A series of new tetrahydro-β-carboline-hydantoin hybrids have been designed and synthesized based on the structure of the known Eg5 inhibitor HR22C16. These compounds have been evaluated for their anticancer activity against lung (A549), cervical (ME180, HeLa), prostate (PC-3) and breast (MCF-7) cancer cell lines by MTT assay. These hybrids have displayed significant in vitro cytotoxicity in comparison to etoposide against PC-3, A549, and MCF-7 cell lines. The hybrids 3a, 3b, 3c, 3e, 3f, 3g, 4b, 4c, 4e and 4f appear to be more effective against the PC-3 cell line, among which compound 4b displayed the highest cytotoxicity (6.08 ± 0.2, IC₅₀ μM). Based on these results, an attempt was made to rationalize their mechanism of action through cell cycle analysis studies. The flow-cytometric analysis of compound 4b in PC-3 cells indicated a G2/M cell cycle arrest. Molecular docking studies substantiate that these compounds indeed bind to the allosteric site of Eg5 formed from Glu116, Gly117, Glu118, Trp127, Ala133, Ile136, Pro137, Tyr211, Leu214, and Glu215 residues with the most potent compound 4b showing the most favorable interaction.

6-Hydroxy- and 6-methoxy-beta-carbolines as acetyl- and butyrylcholinesterase inhibitors

In the course of studies directed toward the discovery of novel acetyl- and butyrylcholinesterase (AChE and BChE) inhibitors for the treatment of Alzheimer's disease, we focused on beta-carbolines (BCs). 6-Oxygenated beta-carboline and beta-carbolinium derivatives based on the serotonin template were synthesized and tested in vitro for their ability to inhibit AChE and BChE, respectively. Particularly the carbolinium salts, which can be formed by intracerebral methylation out of the tertiary-BC prodrugs, show inhibitory activity levels reaching those of galantamine, physostigmine, and rivastigmine.

Catalytic N-Sulfonyliminium Ion-Mediated Cyclizations to α-Vinyl-Substituted Isoquinolines and β-Carbolines and Applications in Metathesis

[reaction: see text] Catalytic Sn(OTf)2-induced cyclization of linear, aryl-containing allylic N,O-acetals produced vinyl-substituted tetrahydroisoquinolines and tetrahydro-1H-beta-carbolines. The usefulness of the vinyl moiety in the resulting products was demonstrated via the synthesis of various key building blocks for alkaloid structures. The alpha-vinyl moiety was utilized in a [2,3] sigmatropic rearrangement, in ring-closing metathesis and a cross-metathesis-based synthesis of vincantril, an antianoxia agent, and a synthetic member of the vincamine type natural products.

A diversity oriented four-component approach to tetrahydro-β-carbolines initiated by Sonogashira coupling

A consecutive four-component synthesis of highly-substituted tetrahydro-beta-carbolines can be achieved by a coupling-aminatio-aza-annulation-Pictet-Spengler (CAAPS) sequence creating five new sigma-bonds and four new stereocenters in a one-pot fashion. The structures were unambiguously supported by X-ray structure analyses.

Photophysical properties of methyl beta-carboline-3-carboxylate mediated by hydrogen-bonded complexes--a comparative study in different solvents

The hydrogen bonding interactions of methyl beta-carboline-3-carboxylate (BCCM) in both ground and first singlet excited electronic states have been studied in solvents with different properties in the presence of acetic acid, a hydrogen-bonding donor/acceptor. The methyl ester substituent reduces the pyridinic nitrogen basicity of this beta-carboline derivative. This fact has let us study the hydrogen bonding interactions in a higher range of acetic acid concentrations than for other beta-carboline derivatives previously studied. Steady and non-steady photophysical studies have been carried out in two non-polar solvents, benzene and p-dioxane; and in two polar solvents, acetonitrile and dichloromethane. Six different fluorescence emissions have been isolated corresponding to the uncomplexed BCCM, the protonated species and four different complexes between BCCM and acetic acid whose structures we have tried to elucidate.

Elevation of blood beta-carboline alkaloids in essential tremor

BACKGROUND

beta-Carboline alkaloids are normal body constituents but are also potent tremor-producing chemicals that are naturally present in the food chain.

OBJECTIVE

To explore the hypothesis that high concentrations of beta-carboline alkaloids are associated with essential tremor (ET).

METHODS

One hundred cases and 100 controls were frequency matched on age, sex, and ethnicity. Blood concentrations of harmane and harmine were quantified by high-performance liquid chromatography, blinded to clinical information.

RESULTS

The mean log blood concentration of harmane was higher in cases than controls (0.72 +/- 0.53 vs 0.51 +/- 0.64 g(-10)/mL; p = 0.01). A nonparametric test on nontransformed data (median harmane = 5.21 g(-10)/mL in cases and 2.28 g(-10)/mL in controls) confirmed this difference (p = 0.005). The mean log blood concentration of harmine was 0.20 +/- 0.48 g(-10)/mL in cases and 0.10 +/- 0.65 g (-10)/mL in controls (p = 0.20). Log harmane concentrations were stratified based on the median value; 62% of cases vs 39% of controls had a high log harmane concentration (p = 0.001). Mean log harmane concentration was similar in the cases with (0.74 +/- 0.58 g(-10)/mL) and without (0.71 +/- 0.50 g(-10)/mL) an affected relative (p = 0.83).

CONCLUSIONS

Blood concentrations of harmane were measured in ET cases compared with controls. Concentrations were elevated in cases with and without a family history of ET.

Genotoxic potential of beta-carbolines: a review

The mutagenic and co-mutagenic properties of harman, norharman and of some of their pharmacologically important derivatives are reviewed. These compounds do not behave as true mutagens, but rather interact, directly or indirectly with DNA, leading to various consequences. This unusual behaviour is most probably related to the particular structure of the chemical nucleus common to all beta-carbolines which confers to the different derivatives the property to interact with various macromolecules and enzymatic systems. These interactions are compiled and discussed in this review. The alterations, by beta-carbolines, of some important enzymatic systems, e.g. cytochrome P-450, have been clearly demonstrated, yet many discrepancies and contradictions exist so that an interpretation of the results and the definition of some common mechanism appears premature. Since beta-carbolines are widely distributed in tissues and since they may modify and increase genotoxic and toxic consequences of other compounds, these interactions need to be clarified.

Structural studies of condensation products of biogenic amines as inhibitors of tryptophan hydroxylase

The effects of condensation products of dopamine and indoleamines on the activity of tryptophan hydroxylase (TPH) were evaluated to determine the structures associated with modulation of this enzyme activity. The compounds having a catechol structure, such as 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, markedly inhibited the activity of the enzyme prepared from the rat brain. The inhibition was non-competitive in terms of both the biopterin cofactor and the substrate L-tryptophan. Substitution on the one or two positions of catechol isoquinolines did not affect the inhibitory activity towards TPH. Among these compounds, a charged substance, 1,2[N]-dimethyl-6,7-dihydroxy-isoquinolinium ion, was an extremely potent inhibitor; the Ki values were 0.88 +/- 0.17 and 0.64 +/- 0.08 microM (mean +/- S.D.) in terms of the substrate and cofactor, respectively. By contrast, the condensation products of tryptophan and tryptamine with acetaldehyde scarcely affected TPH activities. 1-Methyl-1,2,3,4-tetrahydroisoquinoline, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ion (MPP+) were almost inactive. These results indicated that the catechol structure recognized and combined with TPH at a binding site different from that of the substrate or cofactor and the positive charge on the dopamine-derived substance enhanced the affinity to TPH. The selective inhibition of TPH by dopamine-derived catechol isoquinolines was discussed in relationship to the interactions between catecholamines, indoleamines and their metabolites in the brain under physiological and pathological conditions.

Potential bioactivated neurotoxicants, N-methylated beta-carbolinium ions, are present in human brain

Potential bioactivated neurotoxicants, 2-N-methyl-beta-carbolinium and 2,9-N,N'-dimethyl-beta-carbolinium ions, as well as N-methylation activities which form these charged species, were analyzed for the first time in the parietal association cortex and the substantia nigra of human brain using GC/MS and HPLC. The brains were taken during forensic autopsies from corpses without obvious degeneration of substantia nigra. In the cortex, 2-methyl-norharmanium ion (2-MeNH) and 2,9-dimethyl-norharmanium ion (2,9-Me2NH) were detected in almost all samples. 2-Methyl-harmanium ions (2-MeHA) and 2,9-dimethyl-harmanium ions (2,9-Me2HA) were detectable in only two samples. In substantia nigra samples pooled from 3 or 4 brains for analysis, 2-MeNH and 2,9-Me2NH levels were higher than those in the cortex, whereas 2-MeHA and 2,9-Me2HA were below detection limits. Their precursors, norharman (NH) and harman (HA), were also measured using HPLC/fluorescence detection. In both regions, NH and HA were present in almost all samples; levels of NH and HA were also significantly higher in the nigra than in the cortex. Using 9-methyl-NH and 2-MeNH as substrates, in vitro N-methylation of the 2[beta] and 9[indole] nitrogens toward beta-carbolines was measured both in the cortex and in the nigra. 2[beta]-N-Methylation activity was significantly higher than 9[indole]-N-methylation activity in both regions. Recent studies show that beta-carbolinium ions resemble the synthetic parkinsonian toxicant, MPP+, with respect to structure and neurotoxic activity. Such 'bioactivated' carbolinium ions could be endogenous causative factors in Parkinson's disease.

Gas chromatographic-mass spectrometric screening procedure for the identification of formaldehyde-derived tetrahydro-beta-carbolines in human urine

A gas chromatographic-mass spectrometric method for the identification of 1,2,3,4-tetrahydro-beta-carboline and four metabolites extracted from urine is described. In a first step the substances, formed by reaction of formaldehyde with biogenic amines, were derivatized in aqueous solution with methyl chloroformate to eliminate an artificial formation of these compounds via condensation of endogenous indole ethylamines with aldehydes or alpha-keto acids during the work-up procedure. This initial derivatization formed stable hydrophobic compounds and improved the extractability for a liquid-liquid extraction. Further clean-up was performed by solid-phase extraction on C18 sample preparation columns. The method can identify these compounds in the picogram range.

Novel S-adenosylmethionine-dependent indole-N-methylation of beta-carbolines in brain particulate fractions

Guinea pig brain S-adenosylmethionine (SAM)-dependent N-methyltransferase activity toward physiologically relevant beta-carboline (BC) substrates was examined with reverse-phase HPLC and radiochemical detection. Representative BCs, norharman and harmine, were enzymatically methylated on the 2[beta]-nitrogen by [3H]CH3-SAM in undialyzed homogenates to yield 2[beta]-methylated BCs and subsequently on the 9[indole]-nitrogen to generate 2,9-dimethylated BC products. This may be the first account of mammalian indole N-methyl transfer. There was no HPLC evidence for 9-methyl BC or (from carbon methylation) 2,6-dimethyl BC products. Capillary gas chromatography-mass spectrometry analysis confirmed the structures of the 2,9-dimethyl and 2-methyl products of norharman. The 2[beta]- and 9[indole]-N-methylation activities were mainly in the nuclear fractions and were negligible in undialyzed cytosol. This differs from the cytosolic SAM-dependent N-methylations reported with other azaheterocyclics, including 1,2,3,4-tetrahydro-BCs. The involvement of a single enzyme was suggested because the two N-methyl transfers with BC substrate had similar subcellular activity patterns, regional brain distributions, and Km and Vmax values. Sequential N-methylation of various BCs that have been observed in vivo may be a unique route to centrally retained N2,N9-dimethylated beta-carbolinium ions. Because they resemble the synthetic parkinsonian toxicant, N-methyl-4-phenylpyridinium, with respect to structure and neurotoxic activity, such "bioactivated" carbolinium ions could be endogenous causative factors in Parkinson's disease.

Mono-N-methylation of 1,2,3,4-tetrahydro-beta-carbolines in brain cytosol: absence of indole methylation

In an accompanying report we demonstrated enzyme activity in guinea pig brain cell nuclei that catalyzes S-adenosylmethionine (SAM)-dependent N-methylations of heteroaromatic beta-carbolines (BCs) on the 2[beta]-nitrogen and subsequently on the 9[indole]-nitrogen, ultimately yielding N2,N9-dimethylated BCs. Presented here are the results of a parallel study of the N-methylation of 1,2,3,4-tetrahydro-BCs (THBCs), which form endogenously via condensations of tryptophan and its derived indoles with carbonyl compounds or, like their BC oxidation products, are environmental constituents and plant alkaloids. THBCs were enzymatically methylated on the 2[beta]-nitrogen by [3H]-SAM in undialyzed homogenates of rat or guinea pig brain, but [3H]methyl transfer to the 9[indole]-nitrogen was not observed. The structure of the 2[beta]-methyl THBC product was verified with capillary gas chromatography-mass spectrometry. Furthermore, whereas BC N-methylation was largely particulate and displayed micromolar Km values for BC substrate, THBC 2[beta]-N-methylation activity was cytosolic and displayed a relatively high (millimolar) Km for THBC substrate. The N-methylation of THBCs may be due to cytosolic N-methyltransferases that others have studied using different azaheterocyclics. Our overall studies indicate that N2,N9-dimethylated BCs could be unique neurotoxic factors that are bioactivated within brain by sequential N-methylations of BCs. These results suggest the possibility of an additional route to the putative 2,9-dimethylated toxins involving, as a first step, 2[beta]-N-methylation of environmental or endogenously derived THBCs in the brain and perhaps other organs.

Isolation, structure, and properties of the beta-carboline formed from 5-hydroxytryptamine by the superoxide anion-generating system

A yellow substance was isolated by Sephadex LH-20 gel chromatography, silica gel TLC, and reversed-phase HPLC after incubation of 5-hydroxytryptamine (5-HT) with the superoxide anion (O2-)-generating system, i.e., the xanthine-xanthine oxidase system, in the presence of the Fe-EDTA complex and glycine in alkaline medium. The product gave a blue color with Ehrlich's reagent very slowly but no color with xanthydrol and Gibbs' reagent. Its reduced form, however, gave an immediate blue-violet color with all three reagents. No color was developed with ninhydrin, but the reduced form was orange-red. The chemical structure of the yellow substance was identified by 1H-nuclear magnetic resonance and field desorption-mass spectrometry as 4,9-dihydro-3H-pyrido[3,4-b]indol-6-ol (6-hydroxy-3,4-dihydro-beta-carboline, 5-hydroxy-2,3-dihydrotryptoline). The one carbon unit inserted into 5-HT came from glycine, with its 14C-2 being incorporated into C-1 of the yellow substance. The mechanism for the formation of the yellow substance from 5-HT is discussed. This compound inhibited 5-HT uptake into rat brain cortical synaptosomes with an IC50 of 1.5 X 10(-4) M and a Ki value of 1.2 X 10(-5) M.

Ethanol increases single unit activity in the inferior olivary nucleus

Single unit recording of rat inferior olivary nucleus neurons reveals significantly elevated discharge after acute intraperitoneal injection of 2 g/kg ethanol. This effect is consistent across 3 different methods of anesthesia and immobilization: local Xylocaine plus intraperitoneal D-tubocurare, intraperitoneal chloral hydrate and halothane vapor. In contrast, under urethane anesthesia acute ethanol produces significant depression of olivary discharge. Since this effect is opposite to that found under the other anesthetic conditions (including topical Xylocaine only), urethane anesthesia may compromise generalizations of electrophysiologic studies of ethanol. Neurons of the inferior olivary nucleus excite cerebellar Purkinje cells through a powerful afferent circuit; our data therefore suggest that ethanol-induced increases in cerebellar Purkinje cell complex (climbing fiber burst) spikes, obtained in our previous studies, are secondary to olivary activation.

6-Hydroxymethtryptoline is naturally occurring in mammalian urine: identification by combined chiral capillary gas chromatography and high resolution mass spectrometry

The tricyclic indole derivative 6-hydroxymethtryptoline (7-hydroxy-1-methyltetrahydro-beta-carboline) was identified as a normal constituent of human and cat urine. The identification was based on the correct retention time on a chiral capillary gas chromatographic column and on selected ion monitoring of the molecular ion with the mass spectrometer set at high resolving power (RP 10,000). The quantitation utilized a 13C-labeled analog as internal standard and demonstrated levels of 135 +/- 35 and 174 +/- 42 (pmol ml-1, mean +/- SEM) for the S(-) and R(+) enantiomers, respectively, in human urine. In cat urine, the levels were 203 +/- 55 and 108 +/- 44 (pmol ml-1, mean +/- SEM) for the S(-) and R(+) enantiomers, respectively. In human urine, the compound occurred predominantly in a conjugated form while in cat urine the free compound predominated. It was concluded that 6-hydroxymethtryptoline is naturally occurring in mammalian urine and that it most likely constitutes a metabolite of endogenous methtryptoline (1-methyltetrahydro-beta-carboline).

Combined inhibition of serotonin uptake and oxidative deamination attenuates audiogenic seizures in DBA/2J mice

We previously reported that 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THBC) attenuated audiogenic seizures (AGS) in 21-day-old DBA/2J mice and also inhibited brain monoamine oxidase-A (MAO-A) and serotonin (5-hydroxytryptamine, 5-HT) uptake leading to increased brain 5-HT concentration. In this study, the sensitivity of AGS to 5-HT manipulation was evaluated by utilizing drug combinations which paralleled the actions of 6-MeO-THBC and which also have been associated with the production of a serotonergic motor syndrome in rats. Combination of a specific 5-HT uptake inhibitor (fluoxetine or citalopram) with the MAO-A inhibitor clorgyline inhibited AGS more effectively than the individual drugs but combination with the MAO-B inhibitor deprenyl did not. Combined administration of clorgyline plus deprenyl also suppressed AGS. Inhibition of AGS by tryptophan was potentiated by combination with either of the mixed MAO inhibitors nialamide or tranylcypromine. The effects of these drugs individually and in combination on brain MAO-A and MAO-B activity and 5-HT uptake were also determined ex vivo and were consistent with expected mechanisms of action. These results suggest, first of all, that the inhibition of AGS produced by 6-MeO-THBC is a consequence of its combined MAO-A and 5-HT uptake inhibition properties. Secondly, the similarity of results of pharmacological manipulations of the 5-HT system which produce the rat motor syndrome and which inhibit AGS in the mouse suggests that AGS in 21-day-old DBA/2J mice may be a useful system for assessing functional consequences of these serotonergic manipulations.

Light microscopic autoradiography of rat retinae after intravitreal injection of [3H]6-methoxy-tetrahydro-beta-carboline

Many beta-carbolines have recently been shown to occur in the human body and animals. beta-carbolines may act as neuromodulators although their physiological role is not known. We have studied the autoradiographic retinal distribution of intravitreally injected [3H]6-methoxy-tetrahydro-beta-carboline (6-MeO-THBC) in rats. The synaptic and ganglion cell layers accumulate pronounced radioactivity, as do some intra- and extracellular locations in the inner nuclear layer and the outer segments of the photoreceptors. The pigment epithelium gradually accumulates a very dense band of radioactivity. 6-MeO-THBC (pinoline, 5-methoxytryptoline, 6-methoxy-1,2,3,4-tetrahydronorharmane) has been shown to possess many important biochemical and pharmacological properties. It has been demonstrated to occur as a normal constituent in the retina of some animals. The relationship between 6-MeO-THBC, melatonin and other 5-methoxy-indoles is discussed.

Effects of 6-methoxytetrahydro-beta-carboline on 5-hydroxytryptamine binding in rat brain

6-MeOTHBC competes for 5-HT binding sites in rat brain in-vitro and in-vivo. The beta-carboline is significantly more active at the type 1 [3H]-5-HT, than the type 2 [3H]spiperone receptors, in-vitro. Following injection, 6-MeOTHBC significantly decreases [3H]-5-HT binding in the cortex. The ineffectiveness on [3H]spiperone binding in-vivo corresponds with the low affinity in-vitro. The effect of 6-MeOTHBC on central 5-HT binding may be a significant aspect of its 5-HT-ergic activity.

Tetrahydro-beta-carbolines: effect on alcohol intake in rats

Some beta-carbolines, such as tetrahydro-beta-carboline (THBC) and 6-methoxy-THBC, occur normally in mammalian tissues, and 1-methyl-THBC has been found in human blood after alcohol intake. Continuous intraventricular (ICV) infusion of THBC and 1-methyl-THBC for 14 days was shown to increase voluntary alcohol intake in rats during the second week of infusion. In this study the experimental arrangement was slightly modified. Alcohol was offered for 7 days before the start of the 14 days of ICV infusion with Alzet minipumps and alcohol concentration (3-30% v/v) was increased every second day. The rats consumed less alcohol in the second day with the same concentration. Also, the dose of 47 nmoles/hr of 1-Me-THBC increased the voluntary alcohol intake over the controls, but only during the last 7 days. The same dose of 6-MeO-THBC, a serotonergic beta-carboline, was ineffective. Neither drug changed the total fluid intake. This study suggests that the increased voluntary alcohol intake by THBC's is not due to their serotonergic effect. A hypothesis concerning a possible involvement of opiate receptors is presented.

beta-Carbolines as selective monoamine oxidase inhibitors: in vivo implications

The inhibitory action of a range of beta-carbolines on human and rat monoamine oxidase (MAO) A and B has been studied. Concentrations of 5-hydroxytryptamine and phenylethylamine, approximately at their Km values, were used as substrates for MAO A and B respectively. A wide variation in selectivity was found, with harmaline being 10,000 times more potent an inhibitor of A than B whereas, using tetrahydro-beta-carboline and harmane, the difference was nearer to ten-fold. Of the carbolines which have been found endogenously, tetrahydro-beta-carboline, 6-methoxytetrahydro-beta-carboline and harmane are all sufficiently potent inhibitors of human MAO A, with I50 values of 5 X 10(-6), 10(-6), 5 X 10(-7) M respectively, for this property to be of possible physiological significance. Harmane, with an I50 of 5 X 10(-6) M, might also play a role as an inhibitor of MAO B.

Monamine oxidase activity in bovine retina: subcellular distribution and drug sensitivities

The activities of monoamine oxidase (MAO)-A and -B in subcellular fractions of bovine retina were determined using serotonin and beta-phenylethylamine, respectively, as substrates. The subcellular fractions were a P1 fraction containing photoreceptor cell synaptosomes and a P2 fraction enriched with small synaptosomes derived from the inner plexiform layer. MAO activities in the homogenate and P1 fraction were similar and lower than those in the P2 fraction for both MAO-A and -B. The activity of MAO-B was greater than that of MAO-A in all fractions, but the relative distribution of MAO-A and -B did not change in the different fractions. Studies using various MAO inhibitors showed effects which were generally similar to those seen in brain. These results provide further support for the localization of terminals of retinal monamine-containing neurons to the inner plexiform layer and suggest that pharmacological modification of MAO activity in the retina could play an important role in retinal function via changes in monoamine metabolism.

6-Methoxy-1,2,3,4-tetrahydro-beta-carboline: a specific monoamine oxidase-A inhibitor in CF-1 mouse brain

The activities of monoamine oxidase (MAO)-A and MAO-B (using [3H]serotonin and [14C]beta-phenylethylamine, respectively, as substrates) were determined in homogenates of whole mouse brains using a solvent extraction procedure. 6-Methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-TH beta C) had an IC50 of 1.6 microM, in vitro, for MAO-A using whole brain homogenate. A time course of apparent in vivo MAO-A inhibition using 6-MeO-TH beta C (100 mg/kg i.p.) showed a maximal inhibition by 1 h (56%), a decline to 30% at 12 h, and no inhibition at 24 h. There was a maximal inhibition of MAO-B by 4 h (16%). A dose-effect study done at 1 h after injection showed 6-MeO-TH beta C to inhibit MAO-A by 23% at 25 mg/kg, 43% at 50 mg/kg, 58% at 100 mg/kg and 73% at 150 mg/kg; for MAO-B there was no inhibition at 25 and 50 mg/kg and 16% inhibition at 100 mg/kg. The data thus indicate that, at doses less than or equal to 50 mg/kg, 6-MeO-TH beta C is a specific MAO-A inhibitor in vivo.