Inhibition of monoamine oxidase action on kynuramine by substrate amines and stereoisomeric α-methyl amines

Possible involvement of amine oxidase inhibition in the sympathetic activation induced by khat (Catha edulis) chewing in humans

The possibility that phenylalkylamines (cathinone and cathine) of khat, like amphetamines, are amine oxidase inhibitors with clinical significance is evaluated. Results show that khat chewing induces a significant increase in blood pressure, body temperature and urinary catecholamines in khat naive subjects. On the other hand, in habitual consumers a significant enhancement only of blood pressure is observed. Urinary concentrations of vanilmandelic acid show an inhibitory trend in both groups of subjects. This has been related to a decrease in catecholamine deamination. In fact, oxidative deamination of benzylamine catalyzed by beef plasma amine oxidase is non-competitively inhibited in vitro by cathinone (Ki = 0.05 mM) and cathine (Ki = 10 mM) as well as by amphetamine (Ki = 7.9 mM).

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

(+)-4-Dimethylamino-2,alpha-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(+).

Some neurochemical effects of amphetamine, methylamphetamine and p-bromomethylamphetamine in the rat

1. Low doses of D-amphetamine increased brain noradrenaline concentrations in the rat; doses greater than 5 mg/kg, however, caused a decrease. Methylamphetamine also showed this dual effect, but a reduction in brain noradrenaline concentration only occurred when doses greater than 10 mg/kg were administered. p-Bromomethylamphetamine did not significantly reduce brain noradrenaline concentrations even at a dose of 60 mg/kg. The order of potency in reducing the concentration of noradrenaline correlated with the central stimulant effects; D-amphetamine produced the greatest and p-bromomethylamphetamine the least increase in motor activity.2. D-Amphetamine and D-methylamphetamine potentiated the action of 4,alpha-dimethyl-m-tyramine (H77/77) in depleting brain noradrenaline; the greatest potentiation was produced by D-amphetamine. This suggests that the phenylethylamines may affect brain noradrenaline concentrations by acting on the reserpine resistant uptake mechanism.3. Differences were found in the effect of the three drugs on brain dopamine concentrations; D-amphetamine caused a decrease while p-bromomethylamphetamine caused an increase. Methylamphetamine had no effect on the concentration of dopamine. Only p-bromomethylamphetamine significantly reduced the depletion of brain dopamine concentrations caused by H77/77.4. Methylamphetamine and p-bromomethylamphetamine reduced the concentration of 5-hydroxytryptamine (5-HT) in the brain; administration of the same dose of D-amphetamine did not change the concentration of 5-HT.5. Changes in the blood and brain concentrations of tyrosine and tryptophan, and in the concentration of gamma-amino-n-butyric acid in the brain could not be correlated with the changes observed in the concentrations of biogenic amines in the brain.