MAO types in guinea pig liver mitochondria

MAO of guinea pig liver mitochondria actively deaminated dopamine, tyramine, serotonin and 5-methoxy-tryptamine, while tryptamine, 5-methyl-tryptamine and 7-methyl-tryptamine were moderately deaminated. Very little deamination occurred when benzylamine. noradrenaline and β-phenylethylamine were used as substrates. The in vitro inhibition patterns of MAO of guinea pig liver mitochondria by some selective inhibitors were investigated in the presence of tyramine, tryptamine and serotonin. Tryptamine oxidation showed biphasic inhibition pattern with harmaline, clorgyline and Lilly 51641, while the inhibition curves in the presence of pargyline and deprenyl were sigmoidal. The inhibition curves for tyramine oxidation were biphasic with all the inhibitors except pargyline. Serotonin-MAO inhibition curves, on the other hand, were sigmoidal with all the inhibitors except Lilly 51641. Thermal treatment of guinea pig liver mitochondria produced rapid inactivation of serotonin and tryptamine oxidizing activity, while benzylamine oxidizing activity was found to be most thermostable.     

Substrate- and inhibitor-related characteristics of human platelet monoamine oxidase

Human platelet monoamine oxidase (MAO) preferentially deaminated benzylamine and phenylethylamine, two substrates relatively specific for type B MAO, in comparison to 5-hydroxytryptamine, a substrate specific for type A MAO. In studies comparing human platelet and rat brain MAO specific activities, benzylamine and 5-hydroxytryptamine deamination by platelets was approximately 90 and 2 per cent, respectively, that of brain, while platelet deamination of dopamine, tryptamine and tyramine was 20 per cent or less than that of brain. Among sixteen drugs studied, platelet MAO activity was selectively inhibited by low concentrations of the MAO-B inhibitors, deprenyl and pargyline, and was relatively insensitive to the MAO-A inhibitors, clorgyline and Lilly 51641. These observations, in addition to the simple sigmoid inhibition curves obtained with increasing concentrations of either clorgyline or deprenyl, suggest that platelet MAO consists of essentially one distinguishable form of MAO which most closely resembles the MAO type B found in other tissues.     

Potentiation of morphine analgesia in mice after inhibition of brain type b monoamine oxidase

Morphine analgesia in mice was significantly potentiated by deprenyl and pargyline, selective inhibitors of brain type B monoamine oxidase (MAO). Clorgyline and Lilly 51641, selective inhibitors of type A MAO, did not modify the morphine effect. Morphine analgesia was also potentiated by increasing the dose of Lilly 51641 so as to block type B MAO by about 70%, or by inhibiting unspecifically both enzyme forms after either tranylcypromine or high doses of pargyline. No general correlation was found between an increased brain concentration of morphine and the enhancement of morphine analgesia induced by some of the MAO inhibitors tested. 2-Phenylethylamine, a specific substrate of type B MAO, was also found to potentiate morphine analgesia. It is suggested that endogenous substrates of type B MAO may modulate the opiate receptor binding.     

Effect of nonselective and selective inhibitors of monoamine oxidases A and B on pethidine toxicity in mice.

The LD50 of pethidine was determined in mice pretreated (4 h) either with the nonselective monoamine oxidase (MAO) inhibitor, phenelzine or with clorgyline, a selective inhibitor of MAO A or deprenyl, a selective inhibitor of MAO B. Phenelzine or combined clorgyline plus deprenyl pretreatments decreased pethidine LD50. Clorgyline or deprenyl alone did not affect pethidine toxicity. Whole brain 5-hydroxytryptamine (5-HT) concentrations were measured in the pretreated mice. 5-HT levels were approximately doubled (P less than 0.001) after phenelzine or clorgyline plus deprenyl treatment, but not after clorgyline or deprenyl given alone. These results indicate that both MAO A and MAO B need to be inhibited to increase pethidine toxicity and brain 5-HT levels. They support the involvement of 5-HT in the toxic interaction between pethidine and MAO inhibitors.     

Effect of selective monoamine oxidase inhibitor drugs on morphine tolerance and physical dependence in mice

Clorgyline and Lilly 51641, given at dose levels that inhibited selectively brain type A monoamine oxidase (MAO), significantly lowered the incidence of stereotyped jumping produced by naloxone in mice rendered dependent on morphine by subcutaneous implantation of a pellet of the drug. In contrast, selective inhibition of brain type B MAO by deprenyl or pargyline, or nonspecific inhibition of both type A and type B MAO by high doses of Lilly 51641 or pargyline did not modify the abstinence syndrome. Tolerance to the analgesic effect of morphine was unchanged regardless of the enzyme form blocked. The attenuation of withdrawal jumping by low doses of clorgyline or Lilly 51641 does not seem related to changes in brain dopamine. which was found to be deaminated by both enzyme types. The results suggest the possible implication of different and interrelated neurochemical systems in the development of morphine dependence in mice.     

Properties of monoamine oxidase in mouse neuroblastoma N1E-115 cells

Monoamine oxidase (MAO) from adrenergic mouse neuroblastoma N1E-115 cells was compared to MAO found in rat and mouse brain, rat superior cervical ganglion, and human platelet. In comparison to MAO from brain and ganglion, mouse neuroblastoma MAO deaminated 5-hydroxytryptamine (5-HT) to a proportionately greater extent than all other substrates studied, with benzylamine deamination representing only 1 per cent that of 5-HT. Neuroblastoma MAO was over 1000 times more sensitive to inhibition by clorgyline than by deprenyl. With increasing concentrations of clorgyline, inhibition of tyramine deamination was represented by a simple sigmoid curve, suggesting the presence of primarily one form of MAO. Our results are consistent with evidence for a specific form of MAO associated with sympathetic neurons and suggest that neutoblastoma N1E-115 cells are highly enriched in MAO type A.     

Inhibition of monoamine oxidation in subfractions of crude mitochondria of rat brain by clorgyline and Lilly 51641

There are at least two forms of monoamine oxidase (MAO), designated as type A and type B, which differ in their substrate specificities and sensitivities to certain inhibitors. Inhibition patterns of MAO activity of subfractions of crude mitochondria of rat brain by clorgyline and Lilly 51641 were studied using tyramine and serotonin as substrates. It was observed that type A and type B MAO are distributed in different ratios among the mitochondrial sub-fractions investigated.     

Effects of three monoamine oxidase inhibitors on ethanol preference in mice

These experiments investigated the effects of pargyline, N-[2-(o-Chlorophenoxy)-ethyl]-cyclopropylamine (Lilly 51641), and nialamide on voluntary ethanol consumption of C57BL/6J mice. Both pargyline and Lilly 51641 reduced ethanol preference; in contrast, nialamide did not affect preference, despite the fact that it inhibited MAO activity by more than 90%. A subsequent experiment determined that both pargyline and Lilly 51641 produced substantially greater elevations in acetaldehyde levels than did nialamide. It is suggested that increased acetaldehyde is the mechanism responsible for the reduction in ethanol preference observed with pargyline and Lilly 51641.     

Effect of repeated administration of antidepressants on serotonin uptake sites in limbic and neocortical structures of rat brain determined by quantitative autoradiography.

The binding of 3H-cyanoimipramine, a selective radioligand for the serotonin (5-HT) transporter, was measured by quantitative autoradiography on sections of rat brain to determine if 5-HT uptake sites are regulated by repeated administration of antidepressants. The drugs studied included selective inhibitors of the uptake of 5-HT (citalopram, sertraline) or norepinephrine (protriptyline). Also, effects of inhibitors of monoamine oxidase (MAO) that inhibit both type A and type B MAO (phenelzine), or just type B MAO (deprenyl), were investigated. In addition, the atypical antidepressant mianserin, which has antagonist properties at both alpha 2 adrenoceptors and 5-HT2 receptors, was studied. A total of 19 limbic areas and 4 regions of the parietal cortex were quantitated. The binding of 3H-cyanoimipramine was increased (14% to 31%) by phenelzine and deprenyl in a total of 3 brain areas and decreased (15% to 21%) by sertraline in 4 brain areas. Citalopram, protriptyline, and mianserin produced no statistically significant effect in any brain region examined. The results indicate that different types of antidepressants do not exert consistent or substantial regulatory effect on the density of uptake sites for 5-HT in the limbic system or parietal cortex.     

The effect of monoamine oxidase A and B inhibitors on rat serum prolactin

Serum prolactin levels were assayed in rats treated with specific inhibitors of monoamine oxidase (MAO) A or B. Monoamine oxidase A was inhibited by MD780515, clorgyline and Lilly 51641, and MAO B by pargyline and deprenyl (all at 10 mg/kg, p.o.). Serum prolactin levels were significantly reduced 1 hour after treatment with MAO A inhibitors, but were unaltered by MAO B inhibitors. The time-course of the reduction in serum prolactin did not correspond to that of MAO A inhibition. When rats were treated with clorgyline, serum prolactin returned to control values by 6 hr after treatment, whereas MAO A was inhibited by 92–94% over the whole period. At a time when MAO A inhibitors produced no change in serum prolactin (5 hr after treatment) they abolished the rise in serum prolactin induced by reserpine (5 mg/kg, i.p.), indicating a continuing absence of MAO A activity in the neurones regulating prolactin release. Monoamine oxidase B inhibitors did not alter the effect of reserpine on serum prolactin. Possible reasons for the rapid recovery of serum prolactin levels are discussed.     

N-propargylbenzylamine, a major metabolite of pargyline, is a potent inhibitor of monoamine oxidase type B in rats in vivo: a comparison with deprenyl.

In an effort to explore the contribution of the metabolites of pargyline towards the in vivo inhibition of monoamine oxidase (MAO), the effects of pargyline and its major metabolites on the production and metabolism of a number of biogenic amines were studied in rats. The administration of pargyline gave rise to three major ethyl acetate extractable metabolites: benzylamine, N-methylbenzylamine and N-propargylbenzylamine (NPB). Only NPB demonstrated in vivo monoamine oxidase inhibitory properties at an acute dose of 30 mg kg-1. The acute effects of pargyline, NPB, and deprenyl on urine and brain concentrations of a number of biogenic amines (phenylethylamine (PEA), m- and p-tyramine, noradrenaline (NA), dopamine, and 5-hydroxytryptamine (5-HT) and their metabolites were evaluated. Increased urine and brain concentrations of PEA were considered to represent in vivo inhibition of type B MAO while decreased concentrations of NA and 5-HT metabolites were regarded as indicators of an in vivo inhibition of MAO type A. NPB, like deprenyl and pargyline, significantly increased urine and brain PEA while only pargyline reduced 5-HT metabolism, suggesting that the metabolism of pargyline to NPB may contribute towards the MAO type B inhibitory effects of pargyline in vivo. Since the therapeutic benefits of MAO inhibitors in clinical practice usually require some period of chronic treatment, the chronic effects of repeated 14 daily doses of the above MAO inhibitors on central and peripheral biogenic amines were evaluated at the following times: during treatment, one day and five days after termination of treatment. The biochemical changes observed during the course of chronic NPB, pargyline and deprenyl treatments generally follow the expected in vitro characteristics of these drugs, but the detailed changes observed suggest clear differences. For example, the in vivo effect of pargyline on urine 5-hydroxyindoleacetic acid excretion was considerably weaker than its effect on the excretion of NA and dopamine metabolites. These changes are opposite to the in vitro effects of pargyline on 5-HT, dopamine and NA oxidative deamination. Inhibitions of the metabolism of all the amines studied were clearly observed during chronic MAOI treatments, but these effects were less evident five days after the end of treatment, suggesting an almost normal metabolism of biogenic amines. It is concluded that while MAO inhibitors may be the primary compound responsible for MAO inhibition, the effects of their metabolites in some cases may also play equally important roles in the regulation of monoamines both in the periphery and the brain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hypericum perforatum L (St John's wort) preferentially increases extracellular dopamine levels in the rat prefrontal cortex

The effects of hydro-alcoholic extracts of Hypericum perforatum L on extracellular serotonin (5-HT), noradrenaline (NA) and dopamine (DA) levels and the acidic metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxy-3-indoleacetic acid (5-HIAA)) were examined by in vivo microdialysis in the prefrontal cortex of awake rats. Thus, a single dose (60 mg kg(-1) i.p. or 300 mg kg(-1) p.o.) of H. perforatum increased DA concentrations to 165 and 140% of control values, respectively, and increased locomotor activity in nonhabituated rats. DOPAC and HVA levels were markedly reduced. 5-HT concentrations were elevated only moderately, while the NA levels were not affected by any treatment. The whole-tissue analysis revealed that hypericum increased, whereas the monoamine oxidase (MAO) A/B inhibitor phenelzine decreased DA and 5-HT turnover. The present data indicate that the mechanism of action of hypericum extract in vivo is more complex than the inhibition of monoamine reuptake or metabolism observed in vitro. The finding of preferential enhancement of DA transmission is in agreement with human studies measuring DA-mediated neuroendocrine responses.     

Inhibition of monoamine oxidase by N-phenacyl-cyclopropylamine

N-phenacyl-cyclopropylamine hydrobromide (54761) was evaluated in vitro and in vivo as a monoamine oxidase (MAO) inhibitor in rats. In contrast to 51641, which has an o-chlorophenoxy group in place of the phenacyl group and which is a highly selective inhibitor of type A MAO, 54761 showed a slight preference as a type B MAO inhibitor, since it inhibited phenylethylamine oxidation at slightly lower concentrations than were required to inhibit serotonin oxidation in vitro by rat liver MAO. Twelve analogs of 54761 with various substituents on the phenyl ring were also studied, but none was substantially more selective than 54761 as a type B inhibitor and most were preferential type A inhibitors. When 51641 and 54761 were injected into rats and MAO activity was assayed in tissue homogenates, the oxidation of serotonin in brain, heart and liver was inhibited more by 51641 than by 54761. In contrast, the oxidation of phenylethylamine was inhibited more by 54761 than by 51641 in brain and liver. In heart, however, 51641 was a more effective inhibitor of phenylethylamine oxidation than was 54761, supporting earlier evidence that phenylethylamine is destroyed in heart mainly by type A MAO. The oxidation of exogenous [¹⁴C]phenylethylamine was inhibited in vivo more effectively by 54761, whereas the oxidation of endogenous serotonin in brain was inhibited more by 51641. Although 54761 is not as selective an inhibitor of type B MAO as some other compounds such as deprenyl, it illustrates that a large range of selectivity in MAO inhibition can exist within the N-cyclopropylamine series. Further, selective type B inhibition could be achieved in vivo 24 hr after injection of 54761 by co-administration of harmaline. Harmaline selectively protected against the inactivation of type A MAO by 54761 but permitted the inactivation of type B MAO to occur.     

N-substituted cyclopropylamines as inhibitors of MAO-A and -B forms

Monoamine oxidase (MAO) activity in mouse neuroblastoma homogenates was preferentially inhibited by N[2-(o-chlorophenoxy)-ethyl]cyclopropylamine (L-51641) and a series of structurally related compounds. Two other congeners (L-54761 and L-54748) from this series, however, demonstrated preferential inhibition of MAO in human platelet homogenates. As neuroblastoma and human platelet cells have been identified as possessing apparently exclusive MAO-A and MAO-B characteristics, respectively, L-54761 and L-54748 appear to be preferential MAO-B inhibitors. In keeping with this conclusion, a greater proportional inhibition of phenylethylamine oxidation than serotonin oxidation by these two compounds was found in rat brain homogenates, while contrasting results were obtained with L-51641. Preferential effects of L-54761 and L-51641 on phenylethylamine oxidation and, to a lesser extent, on serotonin oxidation were also observed in vivo.     

In vivo, noradrenaline is a substrate for rat brain monoamine oxidase A and B

In vivo clorgyline (5 mg/kg) and (-)-deprenyl (5 mg/kg) selectively inhibit monoamine oxidase (MAO) type A and B activities in rat brain hypothalamus and caudate nucleus using 5-hydroxytryptamine (5-HT), noradrenaline (NA), and beta-phenylethylamine (PEA) as substrates. Clorgyline induces a significant increase in NA concentrations of hypothalamus and caudate nucleus; however (-)-deprenyl is without effect. The combination of clorgyline and (-)-deprenyl at the above doses completely inhibits both forms of MAO, resulting in an even greater increase in NA levels in both brain areas than observed with clorgyline. The non-selective inhibitor tranylcypromine (5 mg/kg) produced a similar effect. Rats pretreated with the selective or the non-selective inhibitors but given L-DOPA (50 mg/kg) have a similar pattern of brain NA, but its concentrations are higher in both brain regions. The results indicate that although in vitro NA may be an exclusive substrate for MAO type A, in vivo, when this enzyme form is selectively inhibited, NA at high concentrations can be a substrate for MAO type B.     

Substrate specificity and inhibitor sensitivity of monoamine oxidase in rat kidney mitochondria.

The deamination of the substrates 5-hydroxytryptamine (5-HT), tyramine, dopamine, β-phenylethylamine and benzylamine by rat kidney mitochondrial monoamine oxidase (MAO) was studied, and kinetic constants are reported for each substrate. By the use of the selective MAO inhibitors, clorgyline and deprenyl, 5-HT and benzylamine were found to be substrates for types A and B MAO, respectively, in this tissue, whereas the other substrates were metabolized by both forms of MAO. No evidence for any significant metabolism of 5-HT or benzylamine by other amine oxidases was obtained. However, some conditions under which the carbonyl reagents semicarbazide, isoniazid and aminoguanidine may interfere with assays for MAO, without actually affecting enzyme activity directly, are described. Preincubation of kidney mitochondria with histamine resulted in a time- and oxygen-dependent irreversible inhibition of both type A and type B MAO activity; the exact nature of the inhibitory agent and its mode of action remain to be determined.     

A study of antidepressant activity of some indole alkylamines.

The antidepressant characteristics of three indole alkylamines were investigated and compared with phenelzine and imipramine by utilising specific pharmacological tools like reserpine, amphetamine, tryptamine and tetrabenazine for determining their possible mechanism of action. Amongst the three indole compounds investigated, indole-3-(2-aminopropyl)-acetate (U-14 164E), indole-3(2-aminobutyl)-d-acetate (u-17 312E) and beta-phenethylhydrazine (phenelzine) produced complete antagonism to reserpine induced sedation, hypothermia as well as facilitation of convulsive seizures. Some of these features suggest that MAO inhibition might be a common mechanism of action of these indoles. The potentiation of CNS effects of tryptamine by these compounds is an outstanding feature of MAO inhibitors, while imipramine is ineffective. Qualitative differences between these indoles and imipramine are evident in the tetrabenazine test. The potentiation of amphetamine induced motor excitation and pentobarbitone narcosis has been explained.     

Monoamine oxidase inhibitors phenelzine and brofaromine increase plasma serotonin and decrease 5-hydroxyindoleacetic acid in patients with major depression: relationship to clinical improvement.

We have examined the effects of two monoamine oxidase (MAO) inhibitors with different mechanisms of action--phenelzine and brofaromine--on peripheral serotonergic (5-hydroxytryptamine [5-HT]) measures, sensitive to the inhibition of MAO-A (intra- and extracellular 5-HT and related metabolites in blood). Both drugs increased the concentration of 5-HT in platelet-free plasma (254%, p less than 0.001) in patients with depressive illness (DSM-III-R) after 6 weeks of daily treatment. Platelet 5-HT was also increased significantly in both drug treatment groups but more marked in the patient group treated with phenelzine. The acid/amine ratio at 6 weeks was 30% of pretreatment values (p less than 0.000) and individual variability correlated significantly with the Hamilton Rating Scale for Depression. Plasma 5-HT increased more markedly in responders than in nonresponders and a significant inverse relationship surfaced between plasma 5-HT and the Hamilton Rating Scale for Depression. The results support other reports of comparable antidepressant efficacy for brofaromine and phenelzine, both inhibitors of MAO-A in humans. The consistent relationship we found between the biochemical and clinical changes again suggests and supports a key role of 5-HT in the antidepressant effect of these MAO inhibitors.     

Binding and deamination of various substrates by types A and B monoamine oxidase in bovine brain mitochondria

The binding and deamination of four substrates by type A and type B monoamine oxidase (MAO) in bovine brain mitochondria were investigated in mixed substrate experiments. MAO activity in bovine brain mitochondria, with 5-hydroxytryptamine (5-HT) as substrate, was highly sensitive to clorgyline and less sensitive to deprenyl, while MAO activity with benzylamine or β-phenylethylamine (PEA) as substrate was highly sensitive to deprenyl and less sensitive to clorgyline. On the other hand, when tyramine plus PEA was used as substrate, the inhibition curves of clorgyline and deprenyl were both biphasic. These results indicate that 5-HT and benzylamine were preferentially deaminated by type A MAO and type B MAO, respectively, and that tyramine and PEA were deaminated by both types of MAO. Studies on the inhibition by clorgyline plus deprenyl of tyramine deamination (in the absence and presence of another substrate) showed that the deamination of tyramine by both type A and type B MAO was inhibited by PEA or benzylamine, while only type A MAO was inhibited significantly by 5-HT. The KA_i value, the dissociation constant of the type A MAO and 5-HT complex, and the KB_i values, the dissociation constants of the type B MAO and PEA or benzylamine complex, were almost equal to the K_m values of type A MAO and type B MAO respectively. The KA_i values for PEA and benzylamine were 78 and 58 μM respectively. For the type B MAO-5-HT complex, the dissociation constant KB_i was 1447 μM. These results show that type A MAO deaminates tyramine and 5-HT whereas benzylamine is not deaminated, but only binds to the substrate binding site of type A MAO with almost the same rate as that for deamination by type B MAO; with type B MAO, tyramine, PEA and benzylamine are deaminated, whereas 5-HT is not deaminated and binds to the substrate binding site of type B MAO with low affinity.     

Human brain monoamine oxidase: one molecular entity-multiple binding sites?

Monoamine oxidase (MAO) of human brain cortex was partially characterized by using different substrates and inhibitors. Two Km values were calculated for each of the three substrates tested, i.e., phenethylamine (PEA) benzylamine (BA) and 5-hydroxtryptamine (5-HT). Clorgyline and 5-HT, both known as MAO-A occupants, were able to abolish the second (high) Km deamination of PEA. 5-HT, while non-competitively inhibiting the deamination of low BA concentrations, competitively inhibited the deamination of high concentrations of this type B substrate. The kinetics of 5-HT deamination showed positive cooperation which indicates the involvement of subunits in the enzyme structure. The ability of some phospholipids to change the enzyme behaviour was considered as indication that these molecules might play a role in determining the ratio between the so-called A and B types of MAO, and in the regulation of the enzyme's activity.