Brain levels and metabolism of the dopaminergic agonist 2-amino-6,7-dihydroxytetrahydronaphthalene after administration of various prodrugs.

The synthesis of four prodrug diesters (diacetyl, diisobutyryl, dipivaloyl, and dibenzoyl) of the potent dopaminergic agonist 2-amino-6,7-dihydroxytetrahydronaphthalene (6,7-ADTN) is described. The effects of prodrug structure on the levels of 6,7-ADTN in the rat corpus striatum and cerebellum, as well as the levels of the metabolite, 6-hydroxy-7-methoxy-2-aminotetralin, in the corpus striatum, have been determined after intraperitoneal administration. In addition, the striatal levels of 6,7-ADTN after administration of the dibenzoyl analogue via intraperitoneal, subcutaneous, and oral routes have been measured. These prodrugs produce a significant improvement in the penetration and accumulation of 6,7-ADTN in the brain.

Synthesis and toxicity toward nigrostriatal dopamine neurons of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) analogues.

Six compounds having structural features in common with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were synthesized and tested in mice for the ability to produce a prolonged decrease in nigrostriatal dopamine (DA) and DA metabolites. The compounds that were prepared and tested include the ester elimination products of the analgetic drugs alpha-prodine and trimeperidine. None of the compounds in this study, except for MPTP, produced significant neurotoxic effects in the mouse model. The study shows that minor changes in the tetrahydropyridine ring of MPTP result in a marked decrease in neurotoxicity.

Facile syntheses of potent dopaminergic argonists and their effect on neurotransmitter release.

The facile syntheses of important intermediates used in the preparation of the two potent dopaminergic argonists, 2-amino-6,7-dihydroxytetrahydronaphthalene (11) (referred to by some authors as ADTN) and its 5,6-dihydroxyl isomer 12, are described. Thus 6,7-dimethyoxy-2-tetralone has been prepared in two steps and 5,6-dimethoxy-2-tetralone in three steps both from commercially available materials. The effects of 11, 12, and the noncatechol analogue, 2-aminotetrahydronaphthalene (ATN), on radioactive neurotransmitter release have been studied in vitro using rat brain slices. It has been shown that both 11 and 12, at a concentraiton of 2 micron, cause a release of [3H]-DA and NA, 11 being more potent than 12 in releasing [3H]-DA. ATN (2 micron) was found to be inactive in these experiments which shows the importance of the catechol function in this uptake--release process.

2-Amido-8-methoxytetralins: a series of nonindolic melatonin-like agents.

A series of unsubstituted and methoxy-substituted 2-amidotetralins (4a-q) was prepared and evaluated for their ability to compete for 2-[125I]iodomelatonin binding to chicken retinal membranes and for their potency to inhibit the calcium-dependent release of [3H]dopamine from rabbit retina. The lead compound, 2-acetamido-8-methoxytetralin (4j), showed a moderate affinity (Ki = 46 nM) and potency (IC50 = 1.4 nM) at the melatonin receptor. The structural requirements necessary for optimal agonistic activity at the melatonin receptor are as follows. First, the amido group, which should have a small, nonbranched alkyl group, is essential for affinity, and second, the methoxy substituent at the 8-position of the 2-amidotetralin ring is essential for optimal agonistic activity at the melatonin receptor. We concluded that this series of unsubstituted and methoxy-substituted 2-amidotetralins constitutes a class of nonindolic melatonin-like agents that can be used as pharmacological tools to further characterize melatonin receptors and to elucidate the mode of action of melatonin.

6,7-Dihydroxy-3-chromanamine: synthesis and pharmacological activity of an oxygen isostere of the dopamine agonist 6,7-dihydroxy-2-aminotetralin.

6,7-Dihydroxy-3-chromanamine, the oxygen isostere of 6,7-dihydroxy-2-aminotetralin (6,7-ADTN), has been synthesized and its dopaminergic activity in various test systems determined. Following bilateral injection into the rat nucleus accumbens, a pattern of locomotor activity similar to that produced by 6,7-ADTN was observed. Its ability to displace N-n-propyl[3H]norapomorphine binding to homogenates of rat brain corpus striatum was found to be about 15 times weaker than 6,7-ADTN and apomorphine. Like 6,7-ADTN it failed to influence dopamine metabolism following an intraperitoneal injection. It is suggested that in addition to the 2-aminotetralins, the 3-chromanamines may be a potential source of new dopamine receptor agonists.

Conformationally restricted phenothiazine neuroleptics. 1. 3-(Dimethylamino)-1,2,3,4-tetrahydroazepino[3,2,1-kl]phenothiazine.

A rigid analogue of promazine, 3-(dimethylamino)-1,2,3,4-tetrahydroazepino[3,2,1-kl]phenothiazine (1), was prepared by reductive amination of the corresponding ketone 4. An X-ray crystallographic study revealed that the seven-membered ring of the hydrochloride salt of 1 exists as a half-chair-like form with the dimethylammonium group in an equatorial-like conformation. Compound 1 was approximately one-half as active as promazine as an inhibitor of [3H]spiperone binding in rat corpus striatal homogenates. In homogenates obtained from calf caudate tissue, however, 1 was only about one-twentieth as active as promazine as an inhibitor of [3H] spiperone binding. As a stimulator of homovanilic acid (HVA) synthesis in rat corpus striatum in vivo, it was about one-tenth as active as promazine.

Synthesis and pharmacology of trans-4-n-propyl-3,4,4a,10b-tetrahydro-2H,5H-1-benzopyrano[4,3-b ]-1,4-oxazin-7- and -9-ols: the significance of nitrogen pKa values for central dopamine receptor activation.

The 6-oxa analogues of potent dopamine agonists, hexahydronaphthoxazines (4a,4b), have been tested for dopamine receptor binding and stimulating activity and were found to be almost inactive. pKa value determinations indicated that these compounds are protonated to approximately 2%, while potent compounds are protonated to a much greater extent. These results strongly support the assumption that the protonated form of DA agonists is the active species at the receptor.

Structure-activity relations for the inhibition of catecholamine uptake into synaptosomes from noradrenaline and dopaminergic neurones in rat brain homogenates.

1. The effects of various phenylethylamine analogues on the inhibition of (3)H-noradrenaline and (3)H-dopamine uptake into homogenates of rat hypothalamus and corpus striatum respectively, were examined.2. Phenolic hydroxyl groups and alpha-methylation of the side chain were both found to enhance the affinity for the neuronal uptake sites.3. Methoxylation, beta-hydroxylation and N-methylation were all found to reduce the ability of a compound to inhibit catecholamine transport.4. The noradrenaline and dopamine transport systems responded in a quantitatively different manner to the various phenylethylamine analogues. It was found that, in general, the noradrenaline uptake process was more sensitive to structural changes, both positive and negative, than the dopamine system.

Structure-activity relations for the inhibition of 5-hydroxytryptamine uptake by tricyclic antidepressants into synaptosomes from serotoninergic neurones in rat brain homogenates.

1 The inhibitory effects of various analogues of imipramine on [(3)H]-5-hydroxytryptamine (5-HT) uptake into homogenates of rat hypothalamus were examined.2 For structures with a three carbon side chain the tertiary amine derivative was more potent than the compound with a secondary amine function.3 Potency was reduced by increasing or decreasing the length of the three carbon side chain by one carbon atom.4 Substitution of a methyl group in the alpha or beta position in the side chain reduced potency.5 Replacement of the dimethylene bridge in imipramine by a sulphur atom or substitution of a C=C double bond for the exocyclic N-C bond of imipramine both led to a fall in potency.6 3-Chlorimipramine was the most potent inhibitor of [(3)H]-5-hydroxytryptamine uptake of the compounds tested.

A comparison of the facilitatory actions of 4-aminopyridine methiodide and 4-aminopyridine on neuromuscular transmission.

1 4-Aminopyridine methiodide (4-APMI), a quaternary analogue of aminopyridine (4-AP), was tested for neuromuscular facilitatory actions on the chick biventer cervicis and frog sartorius nerve-muscle preparations. 2 In the chick, 4-APMI (10(-4) to 10(-2) M) augmented indirectly elicited twitches and reversed tubocurarine-induced neuromuscular block. Reversal of tubocurarine block was observed after treatment of the muscle with an anticholinesterase. 4-APMI did not itself produce contracture but augmented responses to added acetylcholine. 3 4-APMI (10(-4) M) prolonged the time courses of endplate potentials (e.p.ps) and miniature endplate potentials (m.e.p.ps) in the frog. 4 4-APMI (10(-4) M) increased e.p.p. quantal content. 4-AP was about 100 times more active than 4-APMI in increasing quantal content. Both compounds prolonged muscle action potentials at similar concentrations. 5 4-APMI (10(-3) to 3 X 10(-3) M) possessed anticholinesterase activity in homogenates of chick biventer cervicis muscle. 6 It is concluded that 4-APMI increases evoked acetylcholine release and also possesses a weak anticholinesterase action. The greater action of 4-AP on quantal content is probably due to an intracellular action, possibly involving the release of calcium ions.

Evidence for the existence of at least two different binding sites for 5HT-reuptake inhibitors within the 5HT-reuptake system from human platelets.

Chemical modification procedures have been used to study the interaction of tricyclic and non-tricyclic 5HT-reuptake inhibitors with the [3H]imipramine binding site (IBS). N-Ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) induced a pronounced loss in [3H]imipramine binding due to a reduction in Bmax. Preincubation with reuptake inhibitors and subsequent inactivation by EEDQ revealed that imipramine and 5HT prevented the EEDQ-induced inhibition, but citalopram and fluoxetine did not. Thiol modification studies demonstrated that reduction by dithiothreitol (DTT) enhanced the binding of [3H]imipramine by increasing the Bmax. The thioselective reagents 1,1-diazobis- (N,N-dimethylformamide) (diamide), phenyl-arsineoxide (PAO) and N-ethylmaleimide (NEM) attenuated the binding capacity by lowering the Bmax. PAO, a reversible thiol reagent, prevented NEM alkylation indicating that dithiols are involved in the NEM-induced inactivation. Binding of tricyclics or non-tricyclics prior to PAO inactivation revealed that tricyclics provide complete protection against thiol modification, while the non-tricyclics do not. The results support the hypothesis that the 5HT-reuptake system of human platelets possesses at least two distinguishable binding sites.

Evidence concerning the anatomical location of the dopamine stimulated adenylate cyclase in the substantia nigra.

The dopamine (DA)-sensitive adenylate cyclase in the substantia nigra was assayed in rats which had been subjected to 3 different kinds of brain lesion: (1) unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle; (2) unilateral lesions of the descending strio-nigral and pallido-nigral projections; (3) total lesions of the serotoninergic raphe-nigral pathway. Lesions of the medial forebrain bundle causing 97% depletion of striatal DA, 72% depletion of nigral tyrosine hydroxylase, and no change in nigral glutamate decarboxylase (GAD), resulted in no change in basal or DA-stimulated cyclic AMP production ipsilateral to the injection. Lesions of the globus pallidus, causing 70% and 79% reductions in GAD and substance P respectively in the ipsilateral nigra, produced a reduction in basal cyclic AMP production and abolished the normal increase in cyclic AMP produced by DA on the side of the lesion. Lesions to the dorsal and median raphe nuclei did not affect the normal DA-sensitive adenylate cyclase response in the nigra. The results suggest that one of the neurotransmitter functions of DA in this brain region may be to modulate the release of psi-aminobutyric acid (GABA) or substance P from synaptic terminals afferent to the nigra.

Do neuroleptics prevent the penetration of dopamine agonists into the brain?

A new, specific and highly sensitive method for the determination of apomorphine based on high performance liquid chromatography on a C18 reverse-phase column, coupled with electrochemical detection has been developed. The limit of detection of the assay is approximately 0.5 ng/sample (2 pmol). Haloperidol, cis-flupenthixol, metoclopramide and reserpine prevented the accumulation of apomorphine equally in "dopaminergic" as well as in "non-dopaminergic" brain areas. The non-neuroleptic trans-isomer of flupenthixol was without effect. Suppression of the accumulation of the dopamine agonist 6,7-ADTN (administered as the prodrug dibenzoyl-6,7-ADTN) was seen after combined treatment with haloperidol or reserpine, whereas cis- and trans-flupenthixol were without effect. The results imply that certain behavioural, biochemical and neuropharmacological studies, based on apomorphine in combination with other drugs, may need reinterpretation.

A noradren aline sensitive adenylate cyclase in the rat limbic forebrain: preparation, properties and the effects of agonists, adrenolytics and neuroleptic drugs.

A method for the preparation of a noradrenaline sensitive adenylate cyclase from homogenates of the rat 'limbic' forebrain is described using Krebs--Ringer as the homogenising medium. Some of its properties resemble those reported previously by other workers, using slices. Its response to agonists show that it has the characteristics of a beta1-receptor i.e. the potency of 1-isoprenaline exceeds that of 1-noradrenaline which exceeds that of 1-adrenaline. Structure--activity analysis of the response of the adenylate cyclase to a range of adrenergic agonists shows a strict requirement for a catechol moiety and a beta-hydroxyl group. The activation of the enzyme by 1-noradrenaline is sensitive to stereoselective inhibition by 1-propranolol. The effect of a number of neuroleptic drugs was examined. Promazine was the most effective agent tested in antagonising the stimulation produced by 50 muM 1-noradrenaline, whilst the potent dopamine receptor antagonist, alpha-flupenthixol was only weakly active. Furthermore, there was no stereoselectivity in the antagonism produced by alpha- and beta-siomers of flupenthixol. Pimozide was not found to be a potent antagonist. Thus the spectrum of antagonism produced by neuroleptic drugs was quite different from that seen in the dopamine sensitive adenylate cyclase of the rat corpus striatum.

Comparison of the pharmacological actions of some new 4-aminopyridine derivatives.

The pharmacological actions of 2,4-diaminopyridine (2,4-DAP) and 3-[(dimethylamino)-carbonyl] amino 4-aminopyridine (LF-14) were examined and compared with those of 4-aminopyridine (4-AP) in anaesthetized rats and on isolated rat and guinea-pig tissues. Both compounds were more potent than 4-AP in reversing the neuromuscular block caused by pancuronium bromide. The ED50S of LF-14, 2,4-DAP and 4-AP were 100 micrograms/kg, 140 micrograms/kg and 450 micrograms/kg, respectively. LF-14 and 2,4-DAP were also more potent in their in vitro actions on the neuroeffector junctions in the ileum and the isolated heart. 2,4-DAP and LF-14 either did not facilitate or only slightly facilitated the recovery time from xylazine/ketamine anaesthesia which was used as a test for their central action; 4-AP significantly reduced the recovery time. We therefore conclude that both 2,4-DAP and LF-14 are stronger peripherally acting compounds with less central action, and that they may be possible replacements for 4-AP as antagonists of non-depolarizing muscle relaxants.

Desenkephalin-gamma-endorphin and neuroleptics counteract the DP-7-ATN-induced hypomotility after intra-accumbens treatment.

Injection of the selective presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin (DP-7-ATN, 1 fg to 1 microgram) into the nucleus accumbens decreased motor activity. The reduction of motor activity was reversed by pretreatment with haloperidol (10 pg), (-)-sulpiride (10 pg) or desenkephalin-gamma-endorphin (DE gamma E) (100 pg). These results support the hypothesis that DE gamma E may interfere with presynaptically located D-2 dopamine receptor systems in the nucleus accumbens. Furthermore, the use of DP-7-ATN in the described test procedure might be a useful model for testing novel neuroleptic compounds in vivo.

A comparison of the pharmacological actions of 4-aminopyridine and two of its derivatives in the monkey.

The neuromuscular, cardiovascular and central nervous system stimulating effects of 4-aminopyridine (4-AP), 2,4-diaminopyridine (2,4-DAP) and LF-14 were investigated in the monkey. All these compounds were shown to reverse the stable neuromuscular blockade produced by the intravenous infusion of pancuronium bromide. The doses producing 50% antagonism (ED50) of the pancuronium-induced neuromuscular block were 0.50, 0.54 and 0.71 mg/kg for LF-14, 2,4-DAP and 4-AP respectively. The compounds had only slight cardiovascular effects. In contrast to 4-AP, LF-14 and 2,4-DAP did not reduce the duration of ketamine/diazepam-induced anesthesia, suggesting minimal if any central nervous system effects of these two compounds.

Activation of presynaptic alpha-noradrenaline receptors in rat brain by the potent dopamine-mimetic N,N-dipropyl-5,6-ADTN.

Rat cerebral cortex slices labeled with 3H-noradrenaline (NA) were superfused and 3H-NA release was induced with 20 mM K+. The release of 3H-NA was not affected by DA (1 micro M), but was inhibited by the 2-aminotetralins, 5,6-ADTN and N,N-dipropyl-5,6-ADTN (at a concentration of 1 micro M) by about 27% and 65%, respectively. The inhibitory effect of N,N-dipropyl-5,6-ADTN was concentration-dependent (3 x 10(-8) to 3 x 10(-7) M) and was antagonized by phentolamine but not by fluphenazine. The data indicate that, in addition to activating dopamine receptors, N,N-dipropyl-5,6-ADTN stimulates presynaptic alpha-NA receptors.

A prodrug of ADTN: selectivity of dopaminergic action and brain levels of ADTN.

The effects of administration of the prodrug dibenzoyl ADTN (DBADTN) on ADTN concentrations in rat brain and on behaviour in rats having a unilateral 6-hydroxydopamine lesion in the corpus striatum have been studied. Using a combination of HPLC and electrochemical detection as assay method it was found that there was a more selective accumulation of ADTN in the corpus striatum than in the cerebellum. In addition the accumulation of ADTN in the corpus striatum was slow in onset yet long in duration. The peak concentration of ADTN was relatively low and although it was sufficient to cause a strong stimulation of presynaptic DA receptors it did not cause significant rotation in the unilaterally lesioned rat. Prodrug methodology may thus prove useful in designing new selectively acting DA agonists.

Involvement of both dopaminergic and alpha-adrenergic receptors in the hypomotility induced by dibenzoyl-6,7-ADTN.

The dopaminergic prodrug dibenzoyl-6,7-ADTN (DB-6,7-ADTN) can enter the brain following intraperitoneal injection and be hydrolysed to produce low concentrations of the dopamine agonist 6,7-ADTN. Intraperitoneal injections of DB-6,7-ADTN produce a decrease in motor activity and in the present study this response has been characterised, and the underlying mechanisms examined. Doses of 10-100 mumol/kg DB-6,7-ADTN elicit a strong hypomotive response, which is dose dependent. Treated animals are significantly less active than controls. DB-6,7-ADTN hypomotility was significantly attenuated by the non-sedative dopamine receptor antagonist sulpiride (62 mumol/kg, i.p.), but haloperidol (0.3 mumol/kg, i.p.) and cis-flupenthixol (0.45 mumol/kg, i.p.) were without effect. The hypomotility due to DB-6,7-ADTN was also antagonised by yohimbine (13 mumol/kg, i.p.) and piperoxane (21 mumol/kg i.p.), drugs which act mainly by blocking presynaptic (alpha 2) adrenergic receptors. Prazosin (1.5 mumol/kg, i.p.), drugs which act mainly by blocking presynaptic (alpha 2) adrenergic receptors. Prazosin (1.5 mumol/kg, i.p.), a postsynaptic (alpha 1) adrenergic blocker, did not affect the hypomotility, and nor did a range of other neurotransmitter antagonists. DB-6,7-ADTN (50 mumol/kg, i.p.) was also found to antagonise the alpha-methyltyrosine (alpha-MT, 1.02 mmol/kg, i.p.) induced fall in noradrenaline and dopamine levels in brain and spinal cord. Inhibition of the effects of DB-6,7-ADTN on noradrenaline and dopamine turnover by yohimbine and sulpiride, respectively, suggests that 6,7-ADTN (derived from the prodrug) has alpha adrenergic as well as dopaminergic activity. The results are discussed in connection with the hypomotive effects of other dopamine agonists.