Radioreceptor binding reveals the potencies of N,N-disubstituted 2-aminotetralins as D2 dopamine agonists

Summary The affinity of a series of N,N-disubstituted 2-aminotetralins for the rat striatal D₂ dopamine receptor labelled by [³H]spiperone has been determined. Displacement data for the more potent 2-aminotetralins were better described by a model where the compounds competed for [³H]spiperone at two sites. The high affinity component accounted for approximately 80% of the total sites. Displacement curves for all 2-aminotetralins were shifted to the right by 100 M guanosine-5-triphosphate; a result attributable to the redistribution of 13–47% of the sites to a low affinity form. These data are consistent with the N,N-disubstituted 2-aminotetralins being agonists at the D₂ dopamine receptor. In particular, the affinities of the 5-hydroxy-2-aminotetralins were as high as those of traditional dopamine agonists. Send offprint requests to P. M. Beart     

Dopaminergic 2-aminotetralins: affinities for dopamine D2-receptors, molecular structures, and conformational preferences

A combination of X-ray crystallography, NMR spectroscopy, and molecular mechanics (MMP2) calculations was used to determine the three-dimensional structures and conformational preferences of the enantiomers of 5-hydroxy-2-(di-n-propylamino)tetralin and their C1-methylated derivatives. In addition, the affinities of the compounds for striatal 3H-spiroperidol- and 3H-N-n-propylnorapomorphine-binding sites were determined. In the present series, the dopamine D2-receptor agonists have the S-configuration at the nitrogen-bearing carbon (C2), whereas the only established D2-receptor antagonist, 1S,2R-5-hydroxyl-1-methyl-2-(di-n-propylamino)tetralin (1S,2R-UH-242), has the opposite absolute configuration at C2. Two conformational parameters, the tetralin inversion angle (phi) and the dihedral angle tau(C1, C2, N, N-H or electron pair) (tau N), are shown to be critical for D2-receptor agonism; phi values around 0 degrees and tau N values around 60 degrees appear to be optimal. The low D2-affinity of 1S,2S-5-hydroxyl-1-methyl-2-(di-n-propylamino)tetralin seems to be related to its inability to assume a low-energy "D2-receptor agonistic conformation." It is noted that the common structural denominator between the D2-receptor antagonists 1S,2R-UH-242 and 6aS-apomorphine is their inability to assume "dopamine D2-receptor agonistic nitrogen electron pair orientations."     

The deamination of dopamine by human brain monoamine oxidase

The deamination of dopamine has been studied in seven regions of human brain. Both A and B forms of the enzyme were found to be active towards this substrate. The ratio of activities of MAO-A: MAO-B was found to vary considerably from brain region to brain region, from about 1:1 for the cerebral and cerebellar cortex to about 1:2 for the pons and medulla oblongata. Enzyme titration studies and comparisons of the substrate specificities of MAO-A and MAO-B across the brain indicated that dopamine was metabolised by the same MAO active centres as other monoamines. In the cerebral cortex, the Km values of MAO-A and -B towards dopamine were found to be 210 and 230 microM, respectively, indicating that the relative contributions of these two forms towards the oxidation of this substrate will not be significantly affected by changes in its concentration.     

Central and peripheral dopamine beta hydroxylase: Responses to long term treatment with monoamine oxidase inhibitors

The effect of long-term (3 weeks) treatment with monoamine oxidase inhibitors on dopamine beta hydroxylase (DBH) in cats was studied. The DBH was measured in blood and CSF drawn before treatment began and after 2, 8, 14, and 22 days of injections. Animals were killed after 24 days of injections, and DBH was measured in brain cortex and pons-medulla. Both clorgyline and pargyline decreased DBH activity in CSF after 22 days of treatment. These drugs decreased plasma DBH after 8 days of treatment, and the decrease persisted throughout the study. Brain DBH levels in the drug-treated animals were not significantly different from enzyme levels in saline-treated controls. The decrease in CSF DBH occurred 2 weeks after the decrease in the plasma enzyme, suggesting that CSF DBH levels do not simply reflect plasma DBH levels. The decrease in DBH in CSF was not caused by a large-scale depletion of the enzyme in brain. It is likely that the observed change in CSF DBH is due to a decrease in central noradrenergic activity, which has been demonstrated in rats. It is of interest that this change is seen only after three weeks of drug treatment, at the time when the therapeutic effects of these drugs become evident in patients.     

RS-2232, a compound with a reversible and specific type-A monoamine oxidase inhibiting property in mouse brain

Effects of RS-2232 on monoamine oxidase (MAO) activities in mouse brain and liver were investigated with 5-hydroxytryptamine (5-HT), beta-phenyl-ethylamine (PEA), and in some cases, kynuramine as substrate. IC50s of RS-2232 for 5-HT (100 microM) and PEA (20 microM) deaminations in brain mitochondrial preparations were 0.14 microM and 52 microM, respectively. RS-2232 was found to be a competitive inhibitor of 5-HT deamination in the preparation, and its Ki was 0.054 microM. The inhibitions of MAO in both brain and liver homogenate by RS-2232 in vitro measured with kynuramine (100 microM) were independent of the prolonged preincubation. 5-HT deaminations in the brain homogenates of mice treated with RS-2232 were decreased significantly by 15% and 59% at 10 and 30 mg/kg (p.o.) of the compound, respectively. On the other hand, PEA deaminations were not changed at the same doses. Pressor responses induced by intravenous tyramine (0.1-1.0 mg/kg) in anesthetized rats was little affected by oral administration of RS-2232 (3-30 mg/kg) once daily for two weeks. These results reveal that RS-2232 has a reversible and specific type-A MAO inhibiting property in mouse brain, and they suggest that RS-2232 is relatively safe in tyramine-potentiation.     

Inhibition of rat brain monoamine oxidase type A by 2- aminotetralin and tetrahydroisoquinoline analogues of dopamine

Thein vitro inhibition of rat brain monoamine oxidase type A (Mao-a) by various dopamine analogues is reported and compared to some established inhibitors of the enzyme. The estimated ic50's were found to be in the range 10^?5–10^?3 mol/1. This makes these compounds more than 10 000 times less potent than the selectiveMao-a inhibitor harmaline and more than 10 times less potent than the selectiveMao-b inhibitors pargyline and deprenyl. When the brain concentrations that are reached after peripheral administration of these drugs are taken into account it is unlikely that inhibition ofMao is relevant to their effects as has been suggested. Also the endogenous brain concentrations of some tetrahydroisoquinolines are probably too low to produce an inhibition of the enzyme.     

BRL 13776: a novel antihypertensive agent with interesting monoamine depleting properties.

1. Oral doses of 10-100 mg/kg of BRL 13776 lowered the blood pressure of both deoxycorticosterone acetate (DOCA)/NaCl-treated hypertensive rats and untreated normotensive rats. 2. BRL 13776 (100 mg/kg, orally) also reduced the blood pressure of renal hypertensive cats (cellophane perinephritis model). 3. No tolerance developed to the blood-pressure lowering action of BRL 13776 when an oral daily dose of 100 mg/kg was administered repeatedly for up to 15 days to hypertensive rats and cats. 4. The fall in blood pressure to BRL 13776 in rats was associated with a reduction of tissue catecholamines. 5. The catecholamine depletion occurred in all the peripheral tissues examined but in the brain was restricted to certain regions, these being the hind-brain on single dosing and the hind-brain, hypothalamus and mid-brain on repeated dosing. Catecholamine levels in the cerebral hemispheres were not affected by either single or repeated doses of BRL 13776. 6. BRL 13776 caused some reduction of the 5-hydroxytryptamine content of the heart but not of whole brain or any brain region. 7. Neither single doses (up to 900 mg/kg orally) nor repeated doses (100-300 mg/kg orally) of BRL 13776 produced any significant behavioural effects in animals. 8. BRL 13776 is a new type of agent to display both antihypertensive and monoamine-depleting properties. The reduction of noradrenaline in certain brain regions may be a cause of the antihypertensive response but depletion in the periphery could contribute in a major or minor way. The differential action on noradrenaline in the brain together with the lack of effect on 5-hydroxytryptamine might also explain the apparent absence of behavioural effects.     

Discriminative stimulus properties of the dopamine D3 receptor agonists, PD128,907 and 7-OH-DPAT: a comparative characterization with novel ligands at D3 versus D2 receptors

Rats were trained to recognize a discriminative stimulus (DS) elicited by the preferential dopamine D3 receptor agonists, PD128,907 (0.16 mg/kg, i.p.) and 7-OH-DPAT (0.16 mg/kg, i.p.). PD128,907 and 7-OH-DPAT showed "full" (> or = 80%) and mutual generalization. Chemically-diverse, preferential D3 versus D2 agonists, quinelorane, CGS15855A, pramipexole, ropinirole and piribedil, generalized to PD128,907 (and 7-OH-DPAT) in this order of potency, which correlated more strongly with affinity/activity at cloned human (h)D3 (r=0.68/0.81, n=7) than hD2 (0.27/0.64, n=7) receptors. Further, generalization potency strongly correlated with potency for suppression of response rates (0.86), induction of hypothermia (0.92), reduction of striatal dopamine turnover (0.92) and diminution of immobility in a forced-swim procedure (0.97). Nafadotride, UH232 and AJ76, which show a mild preference for D3 versus D2 sites, blocked the PD128,907 DS, and the modestly-selective D3 antagonist, U99194A, was partially effective. Both nafadotride and U99194A blocked the 7-OH-DPAT DS. However, antagonist potency (n=4) versus PD128,907 correlated better with affinity at D2 (0.89) versus D3 (0.27) sites. Further, whereas the preferential D2 versus D3 antagonist, L741,626, antagonized the PD128,907 DS, the selective D3 antagonists, S11566, S14297 (its eutomer) and GR218,231 were ineffective against PD128907 and 7-OH-DPAT DS. S11566 and GR218,231 likewise did not generalize to PD128,907. In conclusion, under the present conditions, D2 receptors are principally implicated in the DS properties of PD128,907 and 7-OH-DPAT.     

Increase of brain endogenous monoamine oxidase inhibitory activity (tribulin) in experimental audiogenic seizures in rats: evidence for a monoamine oxidase A inhibiting component of tribulin.

Brain tribulin activity in rats with an inherited predisposition to audiogenic epilepsy was studied after seizures of different intensity were induced by an electric bell. Weak seizures (from 0 to 2 arbitrary units) did not produce any changes in endogenous inhibitory activity towards either monoamine oxidase (MAO) A or B. Moderate seizures were characterized by increases in both MAO A and MAO B inhibitory activity (up to 1.9-fold). Complete tonic epileptiform seizures with total areflexia (4 arbitrary units) induced further augmentation (up to 2.5-fold) of MAO A but not of MAO B inhibitory activity. This dissociation between the two inhibitory activities points to the existence of a separate MAO A-inhibiting component of brain tribulin which is different from isatin.     

Comparison of the monoamine oxidase inhibiting properties of two reversible and selective monoamine oxidase-A inhibitors moclobemide and toloxatone, and assessment of their effect on psychometric performance in healthy subjects.

1. The effects of two reversible, predominantly monoamine oxidase-A (MAO-A) inhibitors, moclobemide (150 mg three times daily) and toloxatone (400-200-400 mg day-1) on monoamine metabolites and psychometric performance were compared in a double-blind placebo controlled crossover study in 12 healthy subjects. 2. After 7 days of moclobemide/toloxatone/placebo administration subjects were hospitalized for 24 h on day 8. Blood samples were drawn every 2 h for determination of plasma noradrenaline (NA), 3,4-dihydroxyphenylglycol (DHPG), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA). Urine was collected for measurements of normetanephrine and 3-methoxytyramine excretion. Psychometric performance (short- and long-term memory, critical flicker fusion frequency, choice reaction time) and subjective feelings were assessed before each drug intake (in the morning, at noon, in the evening). 3. Compared with placebo, both reversible monoamine oxidase inhibitors decreased the plasma concentration of DHPG and HVA. The overall fall in DHPG (AUC from 0 to 24 h) was 44% during moclobemide and 12% during toloxatone (P less than 0.001) and the overall decrease in HVA was 38% and 20% (P less than 0.005) on moclobemide and toloxatone, respectively. 4. Before the next drug intake, MAO-A inhibition, as judged by the decrease of plasma DHPG concentration, was significantly different from placebo with moclobemide but not with toloxatone. 5. Moclobemide, but not toloxatone, exerted a moderate, but significant inhibition of the deamination of 5-hydroxytryptamine (5-HT) as judged by the fall in plasma 5-HIAA concentration. Neither drug influenced plasma NA concentration. 6. A significant rise in urinary excretion of normetanephrine was observed on moclobemide and to a lesser extent on toloxatone. The urinary excretion of 3-methoxytyramine was significantly raised by moclobemide but not by toloxatone. 7. Neither moclobemide nor toloxatone altered memory function, vigilance, subjective feelings or sleep characteristics of the subjects.     

Effect of D2 dopamine agonists on tuberoinfundibular dopamine neurons.

The effects of piribedil and the selective D2 dopamine agonists, quinpirole and quinelorane, on the synthesis and metabolism of dopamine, within tuberoinfundibular neurons, were studied. The synthesis and metabolism of dopamine within these hypothalamic neurons were assessed by measuring the accumulation of DOPA after inhibition of DOPA decarboxylase and the concentration of DOPAC in the median eminence. Quinpirole (0.1-2.5 mg/kg, i.p.) produced a dose-related increase in accumulation of DOPA and concentrations of DOPAC in the median eminence. The increased accumulation of DOPA after administration of quinpirole was evident for at least 4 hr. The accumulation of DOPA in the median eminence also was enhanced after the administration of quinelorane (0.025 mg/kg, i.p.) and piribedil (50 mg/kg, i.p.). The stimulatory effect of quinpirole on accumulation of DOPA in the median eminence was antagonized by haloperidol (1 mg/kg, i.p.) but not by SCH 23390 (0.5 mg/kg, i.p.). Although D2 agonists have been shown to acutely suppress the synthesis and metabolism of dopamine in nigrostriatal and mesocorticolimbic dopamine neurons, it is apparent that activation of D2 receptors enhanced the synthesis and metabolism of dopamine within tuberoinfundibular neurons in the hypothalamus.     

The acetylenic monoamine oxidase inhibitors clorgyline,deprenyl, pargyline and J-508: their properties and applications

The article presents a short review of some of the properties of the acetylenic inhibitors of monoamine oxidase currently under investigation: clorgyline, (—)-deprenyl, pargyline and J-508. Their substrate-selective inactivation, mechanism of inhibition, titration and pharmacology with respect to monoamine oxidase are critically discussed.