Susan K. Hemrick

Effects of fenfluramine and norfenfluramine on brain serotonin metabolism in rats.

Summary Fenfluramine and its N-demethyl metabolite, norfenfluramine (3-tri-fluoromethyl-amphetamine), lowered brain serotonin, 5-hydroxyindoleacetic acid, and tryptophan hydroxylase in rats acutely and the effect persisted for more than 1 week after a single ip dose of the drugs. Serotonin depletion by fenfluramine and norfenfluramine was blocked by pretreatment with fluoxetine, an inhibitor of uptake into serotonin neurons. Serotonin turnover, as measured by the rate of 5-hydroxyindoleacetic acid accumulation after probenecid administration, was decreased significantly still at 1 week after fenfluramine administration. The effects of fenfluramine and norfenfluramine on brain serotonin neurons in rats resemble those of p-chloroamphetamine and probably occur via similar mechanisms.

Enhanced selectivity of pargyline as an inhibitor of type B monoamine oxidase in harmaline-treated rats.

Abstract Pargyline, a slightly selective inhibitor of type B monoamine oxidase (MAO), inhibited phenylethylamine oxidation by 88 ± 1% and 81 ± 1% in rat brain and liver, respectively, at 24 hrs after injection of a 30 mg/kg i.p. dose. Serotonin oxidation was inhibited to a lesser extent, 68 ± 4% and 68 ± 2%, respectively, in brain and liver. In rats treated with harmaline, a short-lasting reversible MAO inhibitor selective for type A MAO, the inhibition of phenylethylamine oxidation after pargyline injection still occurred but the inhibition of serotonin oxidation was prevented. These results illustrate that a selective MAO inhibitor can be used to enhance the selectivity of an irreversible inhibitor, presumably by occupying active sites on a certain form of MAO temporarily and thereby preventing its inactivation. In heart, inhibition of both phenylethylamine and serotonin oxidation by pargyline was prevented by harmaline; this finding supports other evidence that phenylethylamine is metabolized by type A MAO in rat heart.

Studies of noradrenergic influence on cardiac ornithine decarboxylase in rats

Abstract Ornithine decarboxylase activity in rat heart was increased significantly by the injection of l -norepinephrine or nisoxetine, an inhibitor of norepinephrine uptake. The enzyme was also increased after 3 h of forced exercise. Propranolol, a β-noradrenergic blocking agent, prevented the enzyme increase after norepinephrine, nisoxetine and exercise. Chemical denervation with 6-hydroxydopamine prevented the effect of nisoxetine but not the effect of l -norepinephrine or exercise. Neither propranolol nor 6-hydroxydopamine significantly affected enzyme activity by themselves, suggesting that tonic noradrenergic input is not necessary for maintenance of basal ornithine decarboxylase activity in heart. These results extend previous literature indicating that cardiac ornithine decarboxylase can be elevated by increased noradrenergic input acting via β-receptors.

Inhibition of monoamine oxidase by N-phenacyl-cyclopropylamine

Abstract 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 [ 14 C]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.

PHARMACOLOGIC MODIFICATION OF ADRENALINE NEURONS IN RAT BRAIN

4-Chloro- and 4,5-dichloro-1-aminoindans, like other noradrenaline N-methyltrans-ferase (NMT) inhibitors, decreased adrenaline but not noradrenaline or dopamine concentration in rat brain. Reserpine, Ro 4-1284, 6-hydroxydopamine, H77/77, 1-cyclohexyl-2-mercapto-imidazole and α-methyl-m-tyrosine all lowered adrenaline concentration in rat brain, and pargyline increased it. Imipramine and desipramine antagonized the lowering of adrenaline and noradrenaline by α-methyl-m-tyrosine. Thus many drugs previously known to affect noradrenaline and/or dopamine neurons in brain also affect adrenaline neurons. NMT inhibitors, the only agents reported so far to affect adrenaline neurons specifically, appear to be useful for studying physiologic functions of adrenaline neurons and for estimating adrenaline turnover in brain.

Inhibition of rat brain norepinephrine N-methyltransferase by 2,3,4,5-tetrahydro-1H-indeno [1,2-c]pyridine hydrochloride (LY87130).

Abstract 2,3,4,5-Tetrahydro-1H-indeno[1,2-c]pyridine hydrochloride (LY87130) was a potent inhibitor of rat brain stem norepinephrine N-methyltransferase (NMT) in vitro. The inhibition was competitive with l -norepinephrine as the variable substrate and non-competitive with S-adenosyl- l -methionine as the variable substrate. The K i value for LY87130 calculated from a Dixon plot was 2 × 10 −7 M. In rats, LY87130 inhibited NMT in vivo when injected i.p. at 10–40 mg/kg doses. Epinephrine concentration in hypothalamus was lowered within 1 h after LY87130 injection; the lowering was maximum at 6 h and persisted at 24 but not 48 h after a 40 mg/kg dose of LY87130. The lowering of epinephrine at 6 h was dose-related over a 10–40 mg/kg dose range. Norepinephrine and dopamine concentrations were increased transiently; by 6 h the concentrations of these other catecholamines were normal whereas epinephrine was maximally depleted. MOPEG sulfate, a major metabolite of norepinephrine in rat brain, was markedly elevated by LY87130. The effects of low doses of LY87130 (5 and 10 mg/kg) were enhanced by iprindole, which increased LY87130 levels in brain, suggesting that ring hydroxylation is a major pathway of LY87130 metabolism. The ability of LY87130 to produce selective depletion of epinephrine in rat brain makes it a useful tool for manipulating epinephrine-forming neurons.

Selective in vivo inhibition of monoamine oxidase in rat tissues by N-[2-(O-chlorophenoxy)-ethyl]-cyclopropylamine.

Summary Both at 1 and at 24 hr, 51641 inhibited serotonin oxidation to a greater extent than phenylethylamine oxidation in brain, heart and liver when it was injected at different dose levels into rats. After a 30 mg/kg dose, 51641 produced a long-lasting inhibition of serotonin oxidation in all three tissues (half-life 3-6 days). In contrast, the initial half-life of inhibition of phenylethylamine oxidation was only 0.3-0.4 days in brain and liver (4 days in heart). Apparently 51641 does not inhibit type B MAO irreversibly in vivo.

LOWERING OF BRAIN EPINEPHRINE BY INHIBITION OF NOREPINEPHRINE N-METHYLTRANSFERASE IN RATS

Rat brain norepinephrine N-methyltransferase was inhibited in vitro and hypothalamic epinephrine was lowered in vivo in rats by 2,3-dichloro- and 2-chloro-3-trifluoromethyl-α-methylbenzylamines, by 4-chloro-1-aminoindan, and by 8-chloro- and 7,8-dichloro-2,3,4,5-tetrahydro-1H-2-benzazepines. The lowering of hypothalamic epinephrine by 2,3-dichloro-α-methylbenzylamine was enhanced by piperoxan and antagonized by clonidine.

Inhibition of norepinephrine N-methyltransferase by 1-aminoindans, conformationally rigid analogs of benzylamines.

Abstract Bicyclic analogs of benzylamine (with the α carbon connected by one or more methylene units to the ortho position of the benzene ring) inhibited rabbit adrenal norepinephrine N-methyltransferase (NMT) in vitro . Inhibition was greater when the second ring contained five carbons (1-aminoindan) than when it contained four, six, or seven carbons. Substitution of chlorines on the benzene ring further enhanced the inhibition by 1-aminoindan. The most active inhibitor, 4,5-dichloro- 1-aminoindan, showed competitive kinetics with l-norepinephrine as the variable substrate, and the Ki for this compound as an inhibitor of adrenal NMT was 0.22 μM. 4,5-Dichloro-1-aminoindan significantly decreased epinephrine concentration in the adrenal glands of exercised rats, suggesting that it was effective in inhibiting NMT in vivo .

Properties of norepinephrine N-methyltransferase from rat, cat and chicken brain

Abstract 1. Kinetic properties of norepinephrine N-methyltransferase (NMT) in ammonium sulfate fractions from rat, cat and chicken brain were studied. 2. For all three enzymes, Km values for S-adenosyl-methionine—the methyl-donating substrate—were lower than those for l -norepinephrine—the methyl-accepting substrate. For both substrates, Km values were lowest for cat, intermediate for rat, and highest for chicken brain NMT. 3. The enzymes from all three species were inhibited by excess concentrations of l -norepinephrine. 4. The enzymes were inhibited by various ring-substituted benzylamines and α-methylbenzylamines, and the structure-activity relationships and stereospecificity of this inhibition were similar for the enzymes from all three species. 5. Though quantitative differences exist, NMT from rat, cat and chicken brain seems to be qualitatively similar both in substrate kinetics and susceptibility to inhibitors.