(+)-4-dimethylamino-2, α-dimethylphenethylamine (FLA 336(+)), a selective inhibitor of the a form of monoamine oxidase in the rat brain

(+)-4-Dimethylamino-2, α-dimethylphenethylamine (FLA 336(+)) and its N-demethylated secondary amino derivative FLA 788(+) were examined for their monoamine oxidase (MAO) inhibitory effects in the rat brain. They were found to be reversible and very selective inhibitors of the A form of monoamine oxidase in vitro and in vivo after oral administration. FLA 788(+) was 2–6 times more active than FLA 336(+) in vitro depending on the assay technique employed but the two compounds had similar potency after oral administration. Both compounds inhibited competitively the deamination of 5-hydroxytryptamine by hypothalamic mitochondria. Although the irreversible inhibitor clorgyline was 60 times more potent than FLA 336(+) in vitro, it was equipotent with FLA 336(+) and FLA 788(+) in the rat brain after oral administration. There was a high correlation between the log plasma concentration of FLA 788(+) and the MAO inhibition in hypothalamic slices. The plasma concentration of the metabolite FLA 788(+) exceeded that of FLA 336(+) after oral administration of the latter compound. Thus, the MAO inhibition produced by FLA 336(+) in vivo, appears in part to be due to the metabolite FLA 788(+).     

Effects of acute and repeated administration of amiflamine on monoamine oxidase inhibition in the rat

The inhibitory effect on monoamine oxidase (MAO) of the reversible MAO-A inhibitor (+)-4-dimethylamino-2,alpha-dimethylphenethylamine [amiflamine, FLA 336(+)] was evaluated in the rat after acute and repeated (twice daily for two weeks) oral treatment. MAO activity was measured ex vivo in slices from the hypothalamus and the duodenum for both MAO-A and MAO-B. Amiflamine selectively inhibited the A form of MAO after repeated as well as after acute treatment (ED50 approximately 7 mumoles/kg both acute and repeated). In the brain slices this inhibition corresponded to a decrease in the concentration of 5-hydroxyindoleacetic acid (5-HIAA) and to an increase in the concentration of 5-HT in the hypothalamus, the hippocampus and the striatum. The concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) was decreased in the striatum to the same extent as the decrease in the 5-HIAA concentrations. The effect on the homovanillic acid (HVA) concentration was somewhat weaker as was the increase in the concentration of dopamine. No essential difference was found after acute and chronic treatment on the amine and metabolite levels. The MAO activity returned to normal 24 hours after final dosing. A large difference between the neuronal and the extraneuronal protection against the phenelzine-induced irreversible MAO inhibition in the hypothalamus was found after both acute and repeated treatment. The ED50 of the protection within the serotonergic neurons was 1.3 mumoles/kg p.o. (acute) and 0.75 mumoles/kg p.o. (repeated). Amiflamine was 3 times less potent within noradrenergic neurons than within serotonergic neurons. A brain to plasma ratio of about 20:1 was found for amiflamine and its metabolites. The plasma and the brain concentrations of the N-demethylated metabolite [FLA 788(+)] exceeded that of amiflamine after a single dose, whereas the N,N-demethylated [FLA 668(+)] was found in low concentrations. The effect on MAO-A correlated significantly with the plasma and the brain concentration of FLA 788(+).     

Effects of amiflamine and related compounds on the accumulation of biogenic monoamines in rat brain slices in vitro and ex vivo in relation to their behavioural effects

The inhibition of the accumulation of serotonin, noradrenaline and dopamine in rat brain (occipital cortex and striatum) slices by amiflamine and its two major metabolites FLA 788(+) and FLA 668(+) was examined and compared with that of amine releasing compounds e.g. p-chloroamphetamine (PCA) and alpha-ethyltryptamine and uptake inhibitors, e.g. alaproclate, citalopram, desipramine, fluoxetine and norzimeldine. It was found that amiflamine and FLA 788(+) inhibited the accumulation of serotonin and noradrenaline but only slightly that of dopamine in vitro and ex vivo with a mechanism similar to that of PCA and alpha-ethyltryptamine, i.e. with higher potency in reserpinized rats than in normal animals suggesting amine releasing mechanisms. Similar to alpha-ethyltryptamine and PCA, amiflamine and FLA 788(+) caused behavioural changes (serotonin syndrome) which were particularly pronounced in reserpinized rats but also observed after a second administration of amiflamine in normal rats. Upon repeated administration of amiflamine the behavioural changes disappeared which indicates a functional down-regulation of the serotonin receptors responsible for this syndrome. FLA 668(+) inhibited the accumulation of noradrenaline in vitro and ex vivo but had less effect on the accumulation of dopamine and particularly that of serotonin. It is concluded that amiflamine and FLA 788(+) inhibit the accumulation of serotonin and noradrenaline by releasing mechanisms and that the released serotonin triggers the behavioural changes observed.     

Monoamine oxidase-A selective inhibition in human hypothalamus and liver in-vitro by amiflamine and its metabolites

Amiflamine (FLA 336(+] and its two metabolites, FLA 788(+) and FLA 668(+) were found to be competitive inhibitors of the activity of monoamine oxidase-A in homogenates of human hypothalamus and liver obtained at autopsy. Ki values, determined at pH 7.2, were 1.3, 0.3 and 22 microM (liver) and 0.8, 0.2 and 14 microM (hypothalamus) for amiflamine, FLA 788(+) and FLA 668(+), respectively. Monoamine oxidase-B activity was only weakly inhibited by the compounds.     

Selective inhibition of monoamine oxidase in monoaminergic neurons in the rat brain

The prevention by six reversible and selective monoamine oxidase-A (MAO-A) inhibitors (alpha-ethyl-tryptamine, harmaline, 4-methoxyamphetamine, amiflamine [FLA 336(+)], N-desmethylamiflamine [FLA 788(+)] and N,N-desmethylamiflamine [FLA 668(+)] of the phenelzine-induced irreversible MAO inhibition in the rat brain was examined. By using crude synaptosome preparations of hypothalamus and striatum incubated with low substrate concentrations of 14C-serotonin (1 X 10(-7) M), 14C-noradrenaline (2.5 X 10(-7) M) and 14C-dopamine (2.5 X 10(-7) M) in the absence and presence of selective amine uptake inhibitors (alaproclate, maprotiline and amfonelic acid, respectively), it was possible to determine the deaminating activities inside and outside the specific aminergic synaptosomes. Thus, with this technique the protection of MAO by the reversible inhibitors administered orally 1 h prior to the subcutaneous injection of phenelzine against the phenelzine effect could be determined inside and outside the specific aminergic neurons. It was found that alpha-ethyltryptamine, 4-methoxyamphetamine and particularly amiflamine and FLA 788(+) were more potent inside than outside the serotonergic neurons. FLA 668(+) was a selective inhibitor of noradrenergic MAO, to which also 4-methoxyamphetamine, amiflamine and FLA 788(+), but not alpha-ethyltryptamine had some preference. Harmaline had no certain preference for MAO in any of the aminergic neurons. At high doses of FLA 668(+) a preference for dopaminergic MAO was observed. Since pretreatment of the rats with norzimeldine or desipramine antagonized the preferences for serotonergic or noradrenergic MAO, it is plausible to conclude that the compounds showing these preferences are accumulated in the neurons by the membranal uptake systems.     

Stereoselective inhibition of monoamine oxidase and semicarbazide-sensitive amine oxidase by 4-dimethylamino-2,α-dimethylphenethylamine (FLA 336)

Summary The in vitro inhibition by amiflamine [FLA 336(+)] and related compounds of the activity of rat monoamine oxidase (MAO)-A and-B, rat semicarbazide sensitive amine oxidase (SSAO) and human platelet poor plasma benzylamine oxidase was studied. Amiflamine was an MAO-A selective inhibitor, but also inhibits SSAO with both a reversible (competitive, K _i=200 mol/l) and a small time-dependent component which was irreversible in nature. The optical isomer FLA 336(–) was ten times less potent towards MAO-A. However, this compound was much more potent an inhibitor of SSAO (competitive, K _i=4.6 mol/l). The amiflamine metabolites FLA 788(+) and FLA 668(+) inhibited SSAO, but only at concentrations considerably higher than required for MAO-A inhibition. Ex vivo experiments indicated that there was no significant irreversible inhibition of rat heart and lung SSAO after both single and repeated administration of amiflamine at doses up to 20 times higher than required for inhibition of MAO-A within central serotoninergic neurones.     

Selective inhibition of the a form of monoamine oxidase by 4-dimethylamino-α-methylphenylalkylamine derivatives in the rat

The inhibitory effects on monoamine oxidase (MAO) of some dimethylamino-α-phenylalkylamine derivatives were examined in a rat brain mitochondrial preparation in vitro and in rat brain slices following oral administration. In the in vitro assay the compounds were shown to be selective inhibitors of the A form of MAO, being 100–600 limes more potent in inhibiting the deamination of [¹⁴C]5-hydroxytryptamine than that of [¹⁴C]phenetylamine. Using an ex vivo brain slice technique it was found that the new compounds were reversible and very selective inhibitors of type A MAO in the rat brain and the most potent compounds (FLA 405, 314, 336 and 558) were equipotent with clorgyline. The compounds increased the monoamine concentrations in whole rat brain, particularly that of 5-hydroxytryptamine, in the same dose range which produced MAO inhibition. Some of the new compounds, e.g. FLA 336 and FLA 717, caused only weak potentiation of the vaso-pressor effect of orally administered tyramine.     

On the reversibility of reversible MAO inhibitors

The aim of this study was to assess a) the validity of an ex vivo approach for the estimation of the in vivo MAO A inhibitory properties of the new short-acting MAO A inhibitors, amiflamine, brofaremine, cimoxatone and moclobemide, by studying the effect of dilution of brain and liver homogenates from pretreated rats on the degree of enzyme inhibition; b) the displaceability of the inhibitors from the enzyme by substrate in brain and liver homogenates from pretreated rats; c) idem, in the in vivo situation in the brain, by increasing the availability of the substrate by releasing it from its endogenous stores by tetrabenazine. The following results were obtained: The ex vivo approach was found to be valid for moclobemide in brain and liver and for cimoxatone in brain tissue; a slight underestimation of the MAO A inhibitory effect of the latter in the liver is likely. Definite underestimation occurred with amiflamine in both tissues. Kinetic investigations using homogenates from pretreated rats showed amiflamine to be a competitive inhibitor; cimoxatone was competitive in the liver but showed a more complex pattern in the brain. Moclobemide was noncompetitive in both tissues, as has been shown previously for brofaremine. Moclobemide prevented the deamination of dopamine and serotonin released from their striatal stores by tetrabenazine nearly as efficiently as clorgyline at an otherwise equieffective dose; cimoxatone was somewhat less effective relative to the reference compound, as was brofaremine, which was however given at a more effective dose. Amiflamine was much less effective than clorgyline at protecting dopamine, but equieffective with respect to serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of monoamine oxidase A-form and semicarbazide-sensitive amine oxidase by selective and reversible monoamine oxidase-A inhibitors, amiflamine and FLA 788(+)

In vitro studies demonstrated that two selective monoamine oxidase (MAO)-A inhibitors, amiflamine and FLA 788(+), have been shown to inhibit semicarbazide-sensitive amine oxidase (SSAO) in rat testis and lung homogenates in a concentration-dependent way. The inhibition was not greatly influenced by pretreatment of the preparations with either clorgyline (10(-3) mol/l), l-deprenyl (10(-3) mol/l) or SKF 525A (10(-4) mol/l). The two compounds showed a time-dependent inhibition of SSAO, and for the initial phase of the inhibition, amiflamine is a competitive inhibitor with a Kislope of 135 mumol/l, but FLA 788(+) is a noncompetitive inhibitor with a Ki value of 180 mumol/l. After preincubation for 60 min at 37 degrees C, however, inhibition by amiflamine was found to be essentially irreversible whereas that produced by FLA 788(+) was still noncompetitive and reversible. These two compounds also reversibly and competitively inhibited rat testis MAO-A with FLA 788(+) being much more selective towards this MAO (Kislope = 0.26 mumol/l for FLA 788(+) and 7 mumol/l for amiflamine, respectively). The present results indicate that both MAO-A-selective inhibitors also inhibit SSAO in vitro, but their properties as SSAO inhibitors differ from those as MAO-A inhibitors.     

Cumulative effects of irreversible MAO inhibitors in vivo

The effects of repeated treatment with clorgyline, pargyline, deprenyl and tranylcypromine on MAO activity in rat brain and liver were investigated. MAO was measured with the substrates serotonin (5HT), phenethylamine (PEA) and, in some cases, t brain tissue after single and repeated administrations of 10 mg/kg s.c. clorgyline or deprenyl were also compared. Single doses of clorgyline (1 and 10 mg/kg s.c.) completely blocked the deamination of 5-HT. PEA deamination gradually decreased during the 14-day treatment. Pargyline in a dose of 0.3 mg/kg s.c. reduced both 5-HT and PEA deamination progressively over the same period. In the course of repeated treatment the effects of clorgyline and deprenyl on 5-HT and PEA deamination increased in intensity, by a factor of about 10 in the brain and about 3 in the liver. The potentiation of the effect of tranylcypromine was less marked (brain: × 4; liver: × 2). The rates of recovery of MAO activity were not greater after repeated than after single administrations of high doses of clorgyline and deprenyl, suggesting that the withdrawal of the drugs is not followed by a rebound phenomenon. Our results indicate that repeated treatment with suitable doses of clorgyline or deprenyl leads to specific reduction of either MAO A or B activity in brain, without producing any appreciable effect in the liver.     

Enzymic and molecular characteristics of a new form of monoamine oxidase, distinct from form-A and form-B

The present study was undertaken to clarify the enzymic and molecular properties of monoamine oxidase (MAO) in carp brain. In particular, its sensitivities to selective MAO inhibitors, kinetic properties and molecular weight were compared with those of the enzyme in carp liver. The selective and potent MAO-A and MAO-B inhibitors FLA 788(+), FLA 336(+), MD 780236 and benzylcyanide caused dose-dependent inhibitions of MAO activity in both carp brain and liver; the inhibition curves were all single-sigmoidal, and the degrees of inhibition of the activities towards 5-hydroxytryptamine (5-HT, selective MAO-A substrate), tyramine (substrate for both forms of MAO) and beta-phenylethylamine (PEA, selective MAO-B substrate) were similar. This was also the case for inhibition of activity in carp brain by the irreversible and selective MAO-A and MAO-B inhibitors clorgyline and I-deprenyl, indicating the presence in both preparations of a single MAO which differs from either form of MAO. Studies on the substrate specificities and Km values for these three substrates and the inhibitory effects of some compounds suggested that the enzymic characters of MAO in carp preparations were similar and that these enzymes might be FAD-containing enzymes, like MAO in various mammals. By labelling the preparations with radioactive pargyline and then subjecting them to sodium dodecyl sulfate electrophoresis, the apparent molecular weights of carp brain and liver MAO were estimated as 60,000 daltons. The same value was also obtained for rat brain and liver mitochondrial MAO-B. These results indicate that by the present definitions of MAO-A and MAO-B, MAO in carp brain and liver is similar to, but distinct from, both these forms of MAO.     

Studies of monoamine oxidases: Inhibition of bovine brain MAO in intact mitochondria by selective inhibitors

The inhibition of mitochondrial monoamine oxidase (MAO) from beef brain cortex by the selective inhibitors, clorgyline, harmaline, Deprenyl and pargyline, was compared using five substrates: serotonin (5-HT), β-phenylethylamine (PEA), tyramine, tryptamine and dopamine. Dose-response studies, consistent with the classification of MAO, types A and B, indicated that serotonin deamination was more sensitive to clorgyline and harmaline inhibition than was phenylethylamine. However, the curves for all substrates were double-sigmoidal, rather than being a single sigmoid curve for 5-HT and PEA. Deprenyl and pargyline did not exhibit any marked selectivity for inhibiting PEA deamination without prior preincubation of enzyme and inhibitor. The rate of inhibition was variable and was dependent upon the substrate, the nature of the inhibitor and the inhibitor concentration. Dual inhibitor studies, using the “type A” inhibitor, clorgyline, and the “type B” inhibitor, Deprenyl, together, resulted in almost complete MAO inhibition, regardless of substrate. Combining the two type A inhibitors, clorgyline and harmaline, or the two type B inhibitors, deprenyl and pargyline, resulted in inhibitions that were equal to or only slightly greater than the inhibition produced by a single inhibitor. These results suggested that there are at least two distinct sites in beef brain MAO from cortical mitochondria which may be interacting. The deamination of all substrates occurs at both sites.     

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.     

A comparison of the cardiovascular and sedative actions of the alpha-adrenoceptor agonists, FLA-136 and clonidine, in the rat

1 The degree of selective monoamine oxidase (MAO) inhibition produced by (-)-deprenyl, clorgyline, LY51641 and tranylcypromine was examined in relation to modification of tyramine and noradrenaline contractile responses of the rat isolated vas deferens. 2 All inhibitors possessed reversible alpha-adrenoceptor blocking activity, determined against noradrenaline on the denervated vas deferens. For LY51641 and tranylcypromine, antagonism was competitive, with pA2 values of 6.17 and 5.16. 3 Clorgyline, LY51641 and (-)-deprenyl (10(-5) M) inhibited the tyramine response while present in the organ bath: LY51641, which was the most potent as an alpha-adrenoceptor blocker, produced this effect at 10(-6) M. Responses to tyramine and noradrenaline were potentiated on wishing out the inhibitors, but noradrenaline potentiation was seen only when tyramine had been present in the system. 4 Tranylcypromine (10(-6) M) potentiated responses to noradrenaline and tyramine while present in the organ bath. 5 Potentiation of tyramine responses by clorgyline and LY51641 occurred at 91% and 64% inhibition of MAO type A respectively, although full potentiation of the tyramine response was elicited only when substantial inhibition of both enzyme types occurred. Selective inhibition of MAO type B by 67% (with deprenyl) was not associated with tyramine potentiation.     

Characterization of rat skeletal muscle monoamine oxidase.

Optimal conditions for deamination of 5-hydroxytryptamine in rat skeletal muscle were determined. The presence of monoamine oxidase (MAO) A and MAO B isozymes was demonstrated by the use of tyramine (a substrate of both forms), specific substrates (serotonin and benzylamine), and specific inhibitors (clorgyline and deprenyl) of MAO A and B respectively. A 6.5:3.5 ratio of MAO A to B was found using a whole muscle homogenate, while a 7.5:2.5 ratio was found with isolated mitochondria. Thermal inactivation studies demonstrated that skeletal muscle MAO A is more susceptible to heat inactivation than MAO B. The approximate proportion of muscle homogenate MAO which is present in sympathetic nerves was found to be 18 per cent, as determined by treating rats with 6-hydroxydopamine and quantifying the decrease in activity. Significant inhibition of MAO activity was observed after administration in vivo of the MAO inhibitors pargyline, tranylcypromine and harmaline.     

The effects of amiflamine, a reversible MAO-A inhibitor, on the first pass metabolism of tyramine in dog intestine

The effects of amiflamine on tyramine deamination were studied using isolated loops of intestine in anesthetized dogs. In the pretreatment experiment, dogs were dosed with amiflamine (3.5 mg/kg/day) once daily for 3 days, with the study being carried out 3 hr after the final dose. [14C] Tyramine (50 mg and 50 microCi) in 10 ml of normal saline was introduced into the isolated loops of gut, and tyramine and p-hydroxyphenylacetic acid in the venous blood were separated by HPLC and measured by scintillation spectrometry. In the untreated dogs, approximately 15% of the tyramine passed through the gut wall unchanged. When tyramine and amiflamine (0.06 to 3.5 mg/kg) were administered simultaneously to the gut loop, about 27 to 65% of the tyramine passed through the gut wall unchanged. On the contrary, after pretreatment with amiflamine for 3 days, percentage of tyramine passing through the gut wall was not increased in comparison with the control. These results suggest that pretreatment with amiflamine does not produce drug concentrations in the lining cells of the gut sufficient to effectively inhibit the deamination of oral tyramine, which is administered at least 3 hr after the final dose of amiflamine.     

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.     

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.     

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.     

In vivo evidence for the reversible action of the monoamine oxidase inhibitor brofaromine on 5-hydroxytryptamine release in rat brain

We have used intracerebral microdialysis to examine the reversibility of the action of brofaromine, a selective inhibitor of monoamine oxidase-A (MAO, E.C. 1.4.3.4.), on 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) output in rat frontal cortex. Brofaromine significantly increased the 5-HT output to about 200% of basal values 4 h after the s.c. administration of 10 and 30 mg/kg (but not 3 mg/kg) and reduced the concentration of 5-HIAA in the dialysate dose-dependently (61%, 53% and 41% of basal value with doses of 3, 10 and 30 mg/kg, respectively). At this time, cortical 5-HT concentration was increased and cortical 5-HIAA concentration was decreased in a dose-dependent manner.Treatment of rats with 10 mg/kg brofaromine plus 2.5 mg/kg of the irreversible MAO-B inhibitor l-deprenyl increased the concentration of 5-HT in the dialysate more than did brofaromine alone (503% vs 206% of the basal value, 4 h after administration). Similarly, clorgyline (5 mg/kg) plus l-deprenyl (2.5 mg/kg) increased the concentration of 5-HT in the dialysate to 461 % of the control value. This indicates that the concurrent inhibition of both types of MAO increases 5-HT output more than the selective blockade of either enzyme subtype. We have used this characteristic to examine, in vivo, the reversibility of the interaction of brofaromine with MAO-A. The output of 5-HT and 5-HIAA was examined 19–21 h after treatment with l-deprenyl plus clorgyline or l-deprenyl plus brofaromine. At this time, the concentration of 5-HT in the dialysate was increased six-fold in the clorgyline plus l-deprenyl group, as compared to pre-treatment values, but did not differ significantly from these in the brofaromine plus L-deprenyl group. Also, 5-HIAA concentration in the dialyste was still reduced (48% of basal value) in the clorgyline plus l-deprenyl group but not in the brofaromine plus l-deprenyl group. Cortical 5-HT and 5-HIAA concentrations in these animals, measured at the end of the microdialysis experiments (21 h after treatment), displayed changes that paralleled those in the extracellular compartment.These results indicate that: (a) as with clorgyline, the inhibition of MAO-A with brofaromine has a more pronounced effect on tissue 5-HT concentrations than on extracellular 5-HT concentrations; (b) the simultaneous inhibition of both forms of MAO with l-deprenyl and either brofaromine or clorgyline increased the concentration of 5-HT in the dialysate shortly (1–4 h) after administration more than did treatment with either of the MAO-A inhibitors alone; (c) dialysate and tissue 5-HT concentrations were greatly increased one day after the irreversible inhibition of MAO by clorgyline plus l-deprenyl, whereas they had returned to pretreatment values in those animals treated with brofaromine plus l-deprenyl. These transient effects on 5-HT and 5-HIAA in vivo provide further support for the conclusion that the interaction of brofaromine with brain MAO-A is reversible.