Betty W. Roush

The effects of quipazine on serotonin metabolism in rat brain.

Quipazine, 2-(1-piperazinyl)-quinoline, is a drug that has been reported to stimulate serotonin receptors in brain. We therefore studied the effect of quipazine on several parameters of serotonin metabolism in rat brain. Quipazine caused a slight, dose-related elevation of serotonin levels and decrease in 5-hydroxyindoleacetic acid levels for 2–4 hrs after it was administered. The decrease in 5-hydroxyindoleacetic acid levels was probably due primarily to a depression of 5-hydroxyindole synthesis, since quipazine also decreased the rate of 5-hydroxytryptophan accumulation after NSD 1015, the rate of serotonin decline after α-propyldopacetamide, and the rate of 5-hydroxyindoleacetic acid accumulation after probenecid. The elevation of serotonin was probably due to weak inhibition of monoamine oxidase. Quipazine reversibly inhibited the oxidation of serotonin by rat brain monoamine oxidase invitro and protected against the irreversible inactivation of the enzyme invivo. Quipazine also was a potent inhibitor of serotonin uptake into brain synaptosomes invitro and attained concentrations in brain higher than the invitro IC50. However, quipazine did not prevent the depletion of brain serotonin by p-chloroamphetamine invivo. In addition to stimulating serotonin receptors in brain, quipazine may inhibit monoamine oxidase and serotonin reuptake invivo.

BINDING OF TRYPTOPHAN TO PLASMA PROTEINS IN SEVERAL SPECIES

Abstract 1. 1. The concentration of tryptophan was measured fluorometrically in whole plasma from several species and again after ultrafiltration to remove tryptophan bound to plasma proteins. 2. 2. The major portion of the tryptophan was protein-bound in all of the species, although the degree of binding decreased in the order: dog, rabbit > rat, cat, rhesus monkey > sheep > human > horse > green monkey > marmoset, cow > pig > guinea pig > mouse > turkey > chicken. 3. 3. Thus tryptophan binding to plasma proteins seems to be a characteristic of many warm-blooded animals, and the extent of the binding may have physiologic significance in all of these species.

Further structure-activity studies on the lowering of brain 5-hydroxyindoles by 4-chloroamphetamine

Abstract A single injection of 4-chloroamphetamine lowered brain 5-hydroxytryptamine (5-HT) in rats, with the maximum effect at 16 hr. Multiple injections were no more effective than a single injection, and 20–40% of the 5-HT in brain persisted. No other 4-substituted amphetamines studied lowered 5-HT as much as did 4-chloroamphetamine, though 5-hydroxyindoleacetic acid (5-HIAA) levels were decreased as much by 4-trifluoromethylamphetamine. The addition of a β-hydroxyl to 4-chloroamphetamine diminished the lowering of 5-HT despite the presence of the β-hydroxyl compound in brain at levels comparable to those of 4-chloroamphetamine. When metabolic inactivation by para-hydroxylation was not a factor (as in guinea pigs or in desmethylimipramine-treated rats), 3-chloro but not 2-chloroamphetamine lowered brain 5-HT as 4-chloroamphetamine did. 2-Chloroamphetamine in desmethylimipramine-treated rats was present in brain at levels comparable to those of 4-chloroamphetamine, but the 2-chloro compound raised instead of lowered 5-HT levels. Addition of a 2-chloro substituent to 4-chloroamphetamine reduced the effect on 5-HT, whereas the decrease in 5-HIAA was at least as great with 2,4-dichloroamphetamine as with 4-chloroamphetamine. The dichloro compound was present in brain at slightly lower levels than 4-chloroamphetamine and caused slightly less CNS stimulation and hyperthermia; both compounds were found primarily in the paniculate fraction after high speed centrifugation of brain homogenates. The ability of some 4-chloroamphetamine derivatives to lower 5-HIAA more than 5-HT suggests that inhibition of tryptophan hydroxylation cannot completely account for their effects on brain 5-hydroxyindole levels.

Drug disposition as a factor in the lowering of brain serotonin by chloroamphetamines in the rat

Abstract Brain serotonin content of rats treated 6 hr earlier was markedly reduced by 4-chloroamphetamine, slightly reduced by 3-chloroamphetamine, and not changed by 2-chloroamphetamine or amphetamine. In rats pretreated with desmethylimipramine (DMI), the reduction of serotonin caused by 4-chloroamphetamine was unchanged, but 3-chloroamphetamine was now equally as effective as 4-chloroamphetamine; amphetamine still had no effect, and 2-chloroamphetamine caused a slight elevation of serotonin. Levels of amphetamine, 2-chloroamphetamine and 3-chloroamphetamine were too low to detect in rat brain at 6 hr except in the rats pretreated with DMI to block para-hydroxylation. 4-Chloroamphetamine levels in brain were the same in controls and in DMI-pretreated rats. 4-Chloroamphetamine and 3-chloroamphetamine, which lowered serotonin equally in DMI-pretreated rats, were both present mainly in the particulate fraction after high speed centrifugation of brain homogenates, whereas 2-chloroamphetamine was evenly distributed between the supernatant and particulate fractions and amphetamine was present mainly in the supernatant fraction. The intrinsic ability of these compounds to lower serotonin is better demonstrated in DMI-pretreated rats in which metabolic differences between them are minimized; thus 3-chloro- and 4-chloroamphetamine are equally active. The association of those two drugs with brain particulate material is suggested to be related to their reduction of serotonin content.

Regulation of phenylethanolamine N-methyl transferase

Phenylethanolamine N-methyl transferase (PNMT) catalyzes the methylation of norepinephrine, forming epinephrine in the adrenal medulla. In vitro, PNMT accepts a variety of phenylethanolamines and phenylethylenediamines as substrates. It is inhibited by phenethylamines and other compounds structurally similar to its substrates, as well as by agents that bind to sulfhydryl groups. Specific inhibitors of PNMT may be useful pharmacologic tools and potentially drugs. A group of benzylamines, including 2,3-dichloro-α-methylbenzylamine, are among the most potent inhibitors presently known. In vivo, maintenance of PNMT levels in the adrenal medulla is dependent upon an adequate supply of glucocorticoids from the adrenal cortex. Neural input to the medulla may also influence the enzyme level. The possibility that product inhibition may influence PNMT activity in vivo requires further study.

Substrates and inhibitors of phenyletholamine n-methyl transferase from human adrenal glands

Abstract 1. 1. Phenylethanolamine N-methyl transferase (PNMT) from human adrenal glands uses 3,4-dichlorophenylethanolamine, 3,4-dichlorophenylethylenediamine, norepinephrine, phenylethanolamine and normetanephrine as substrates to about the same extent as PNMT from the rabbit and other animal species. 2. 2. Calculated Km values for the physiologically occurring substrates, norepinephrine, phenylethanolamine and normetanephrine, were. parallel for the human and rabbit enzymes. 3. 3. Inhibition of human PNMT by ring-substituted α-methylphenylethylamines was similar to the inhibition of the rabbit enzyme. 4. 4. Human PNMT was also inhibited by its product, epinephrine. Among the species studied so far, PNMT from human adrenal glands has a relatively high Km for norepinephrine as a substrate and Ki for epinephrine as an inhibitor.

Disposition and behavioral effects of amphetamine and β,β-difluoroamphetamine in mice

Abstract β, β-Difluoroamphetamine, which has a lower p K value than amphetamine and so exists at physiological pH predominantly as a neutral molecule rather than as a cation, localized in epididymal fat to a greater degree than in brain in mice, in contrast to amphetamine. Difluoroamphetamine had a much shorter half-life in brain (19 min) than did amphetamine (51 min), and the half-life was not affected by a 4-chloro substituent (18 min), in contrast to that of amphetamine (261 min for 4-chloroamphetamine). The half-life of difluoroamphetamine and of 4-chloro-difluoroamphetamine was essentially the same in fat as in brain (16 and 20 min respectively). Due to its different distribution and its shorter half-life, difluoroamphetamine had to be given at higher doses to maintain brain levels comparable to those of amphetamine. Likewise, higher doses of the difluoroamphetamine were required for equivalent degrees of central nervous system (CNS) stimulation as measured by increased locomotor activity. The duration of CNS stimulation was shorter for difluoroamphetamine than for amphetamine, correlating with the more rapid removal of the former compound from brain. Two inhibitors of microsomal drug-metabolizing enzymes, 2,4-dichloro-6-phenylphenoxyethylamine (DPEA) and β-diethylaminoethyldiphenylpropylacetate (SKF 525-A), caused increased brain levels of difluoroamphetamine but not amphetamine, and desmethylimipramine (DMI) did not affect brain levels of either drug. The results suggest an alteration in metabolism as well as in tissue distribution resulting from the decreased basicity of the β, β-difluoro compound.

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 .

Inhibition of monoamine oxidase by 3-amino-2-oxazolidinone and 2-hydroxy-ethylhydrazine.

Die MAO-hemmende Aktivität von 3-Amino-2-Oxazolidinon und von 2-Hydroxy-Äthylhydrazin wird beschrieben.

Procaine hydrochloride as a monoamine oxidase inhibitor: implications for Geriatric therapy.

The results of in vitro experiments showed that inhibition by procaine hydrochloride of monoamine oxidase (MAO) from either rat brain or liver was substrate‐dependent. Procaine was more effective in inhibiting serotonin oxidation than phenylethylamine oxidation and had an intermediate effect on tryptamine oxidation. MAO activity in tissue homogenates from rats treated with procaine (150 mg/kg intraperitoneally) was inhibited most in liver, less in heart, and only very slightly in brain for a duration of up to 8 hours. Procaine injected in that dose did not alter brain norepinephrine levels and elevated only slightly the brain serotonin levels. It did not protect against the degradation of exogenous radioactive tryptamine in brain. These data confirm and extend prior observations on in vitro inhibition of MAO by procaine and suggest that in high doses procaine may inhibit MAO weakly in vivo. If the reported usefulness of procaine preparations in treating geriatric patients indeed depends upon MAO inhibition, more effective inhibitors would seem to be available.