Kinetic and pharmacological profiles of the in vitro binding of the potent dopamine agonist [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin to rat striatal membranes

Pharmacological profile of non-hydroxylated and ether derivatives of the potent D2-selective agonist N-0437

Derivatives of the potent dopamine D2-selective agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) were designed, aimed at producing drugs with less sensitivity towards metabolic inactivation (in particular glucuronidation at the 5-OH position). Since aminotetralins with a 5-methoxy substituent or lacking the 5-hydroxy group have been reported to retain dopaminergic activity, the non-5-hydroxylated N-0437 (N-0918) and two ethers of N-0437 [5-methoxy-N-0437 (N-0724) and 5-cyclopentoxy-N-0437 (N-0953)] have been prepared and tested. Three indices for activity at central dopamine receptors are considered: (1) the displacement of (3H)-SCH-23390 and (3H)-spiperone from calf caudate membranes, (2) the effects on dopamine release and metabolism in the striatum of freely moving rats after systemic and intrastriatal administration as assessed by brain microdialysis, and (3) the ability to elicit contralateral turning in rats with a unilateral 6-OH-dopamine lesion of the nigrostriatal pathway. In order to differentiate between direct dopaminergic activity and metabolic activation, brain and plasma levels of N-0437 after administration of N-0724 and N-0953 were measured. The results show the necessity of the 5-OH group for direct dopaminergic activity: N-0918, N-0724 and N-0953 are all inactive after intrastriatal administration in the microdialysis model and all three drugs show a weak in vitro affinity for both D1 and D2 receptors. Although N-0918 is also inactive after systemic administration in the microdialysis and turning model, N-0724 and N-0953 do exhibit dopaminergic activity after systemic administration in these models.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological profile of a chromanamine analogue (DP-6OH-3CA) of the selective presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

The pharmacological profile of an oxygen isostere of the selective presynaptic dopamine agonist DP-7OH-AT, i.e. dipropyl-6-hydroxy-3-chromanamine (DP-6OH-3CA) has been evaluated in various receptor binding, neurobiochemical and behavioural experiments. The chromanamine displaced the 3H-labelled dopamine ligands, 5,6-DPAT and N-0437, with Ki values of 106 and 143 nM, respectively. In in-vivo biochemical models for presynaptic activity the chromanamine induced a half-maximal effect in the gamma-butyrolactone reversal test at 6.8 mumol kg-1 and had an ED70 value of 40 mumol kg-1 for HVA decrease in the striatum. In behavioural models for postsynaptic dopaminergic activity a half-maximal effect for the induction of stereotypy was reached at 100 mumol kg-1 and reversal of the effects of reserpine to a level of 200 counts was induced at 11 mumol kg-1. On comparison of these results with the results obtained with the carbon analogue of DP-6OH-3CA, i.e. DP-7OH-AT, it is apparent that the chromanamine has a reduced potency for dopamine D2 receptors in in-vitro and in-vivo models. The selectivity for presynaptic dopamine receptors was lower than with DP-7OH-AT and the isomeric chromanamine, DP-8OH-3CA, indicating that the optimal position of the hydroxyl group for presynaptic selectivity is in the 8 and not in the 6 position for the chromanamines.

In vivo receptor binding, neurochemical and functional studies with the dopamine D-1 receptor antagonist SCH23390

A series of in vivo experiments were undertaken, relating functional (motor activity, body temperature), dopamine (DA) receptor binding and neurochemical (catecholamine synthesis and utilization, DA release) aspects of the pharmacology of SCH23390 in the rat. The compound inhibited the locomotor hyperactivity, but not the hypothermia, induced by the potent DA stimulant DP-5,6-ADTN. Interstingly, SCH23390 simultaneously failed to displace DP-5,6-ADTN from its binding sites in the rat striatum--used as a direct in vivo biochemical index of DA (D-2) receptor interaction. The spontaneous locomotion in non-pretreated rats was likewise inhibited by SCH23390. The locomotor-suppressive action, but not the DP-5,6-ADTN-displacing capacity of the D-2 blocker haloperidol was significantly enhanced by SCH23390, suggesting that motility can be suppressed by either enhanced D-1 or D-2 (postsynaptic) receptor blockade, but also that the D-1 and D-2 sites involved may be physically distinct. SCH23390 only slightly altered in vivo neurochemical of DA synthesis, release and nerve-impulse flow, indicating that, while similar in suppressing dopaminergic behaviour, the D-1 antagonist is less effective than traditional neuroleptics as an activator of DA neuronal feedback mechanisms. The weak increases of DA synthesis and release nonetheless obtained were equal in magnitude (30-40%) in the limbic vs. striatal brain areas; also in this respect, SCH23390 thus differs from classical neuroleptics, which generally display more marked effects in the striatum than in limbic tissue. No major changes in the in vivo indices of NA synthesis and utilization (or in 5-HT synthesis) were found after SCH23390 administration, by and large supporting the DA receptor specificity of the compound. In summary, the studies demonstrated that SCH23390 can offset and accentuate, respectively, behavioural consequences of D-2 receptor stimulation and blockade. Importantly, at the same time no direct interaction at the level of D-2 DA receptor sites in the striatum was detected. Only slight, D-2 antagonist-like, changes in neurochemical indices of dopaminergic activity were observed after D-1 receptor blockade by means of SCH23390. With regard to DA agonist hypothermia, SCH23390 was without effect per se, but (at a high dose) attenuated the action of the D-2 antagonist haloperidol. The observations may indicate that the complex interactions between central D-1 and D-2 receptor-controlled mechanisms that influence behaviour, neurochemistry, and possibly autonomic nervous expression, are not identical.

In vivo dopamine (DA) receptor binding and behavioural effects of the putative DA autoreceptor antagonists (+)-AJ 76 and (+)-UH 232 in rats with a unilateral nigral 6-OH-DA lesion

The in vivo dopamine (DA) receptor binding and behavioural properties of the recently characterised putative preferential DA autoreceptor antagonists (+)-AJ 76 and (+)-UH 232 were studied in rats with a unilateral 6-OH-DA lesion of the substantia nigra. The main findings were a) that (+)-UH 232 and (+)-AJ 76 per se failed to produce significant turning behaviour, b) that both agents antagonised contralateral rotation caused by the DA agonist apomorphine, including a change of the characteristic two-peak apomorphine rotation pattern into a single peak, indicating that the DA antagonist properties of (+)-UH 232 and (+)-AJ 76 are retained also at denervation-sensitised postsynaptic DA receptors and--in support of this notion--c) that (+)-UH 232 and (+)-AJ 76 were able to displace the specific in vivo binding of the DA receptor agonist DP-5,6-ADTN in the denervated as well as in the intact striata of the 6-OH-DA-lesioned animals. Interestingly, in this regard (+)-UH 232 was significantly less efficient on the lesioned as compared to the intact side. The DP-5,6-ADTN-displacing effect of (+)-AJ 76 did not, however, differ between the intact and the denervated striatum. The implications of the present findings are discussed with particular reference to DA receptor sensitivity and adaptational phenomena.

Further in vitro and in vivo studies with the putative presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

The in vitro binding of the putative dopamine autoreceptor agonist [3H]DP-7-ATN to rat striatal membrane homogenates was investigated. The maximum number of binding sites Bmax was 497.5 +/- 50.2 fmol/mg protein and the affinity constant KD was 8.3 +/- 1.5 nM using 10 microM (+) butaclamol to define non-specific binding. Lesion of the left medium forebrain bundle by 6-hydroxydopamine resulted in an almost complete loss of dopamine in the striatum but did not affect the binding of [3H]DP-7-ATN. The binding of [3H]DP-7-ATN to the homogenates of the dopaminergic cell bodies in the substantia nigra revealed a Bmax of 542.4 +/- 40.1 fmol/mg protein and a KD of 11.1 +/- 1.3 nM. The pharmacological profile of the binding was characterized as being to D-2 receptors. No direct in vitro evidence could be found for a selective binding to DA autoreceptors. The dopamine uptake inhibitor GBR 12909 interacted in a noncompetitive manner with the in vitro binding of [3H]DP-7-ATN and the latter compounds uptake into isolated synaptosomes was not through the specific dopamine uptake system but rather through diffusion. GBR 12909 failed to reveal any agonistic or antagonistic activity in the GBL model but was able to antagonize the hypomotility in rats induced by 0.25 mg/kg DP-7-ATN. The inhibitory effect of DP-7-ATN on DA release was also demonstrated using in vivo brain dialysis in conscious rats. Based on the above results, the possibility is discussed that the release regulating DA autoreceptors, which might be coupled to the reuptake complex, and the DA biosynthesis regulating autoreceptors, are different entities.

In vitro binding of the very potent and selective D-2 dopamine agonist, [3H]N-0437 to calf caudate membranes

N-0437, a non-catecholic aminotetralin has recently been described as a very potent and selective dopamine D-2 agonist. In this study the in vitro binding of [3H]N-0437 (specific activity 80.6 Ci/mmol) to calf caudate membranes is described. It was found that [3H]N-0437 binds with a high affinity (KD = 0.17 nM) and a low proportion of non-specific binding. Moreover the binding was saturable with a high number of binding sites (Bmax = 703 +/- 28 fmol/mg protein) and reversible (dissociation half-time = 68 min). Pharmacological analysis of [3H]N-0437 binding showed that it was selective for dopamine receptors and that it was also stereoselective for D-2 receptors. Non-dopaminergic drugs were without exception very poor displacers. Taken together the results suggest that [3H]N-0437 labels dopamine D-2 receptors with a high selectivity in the calf brain, and thus, that it should be a useful tool in studies of central dopamine receptors.

Pharmacological profiles of three new, potent and selective dopamine receptor agonists: N-0434, N-0437 and N-0734

A series of new dopamine (DA) receptor agonists, of the 2-aminotetralin group, i.e. N-0434, N-0437 and N-0734 were investigated in both in vivo and in vitro pharmacological test systems. In vivo, the reversal of the gamma-butyrolactone-induced increase in rat central DOPA biosynthesis rate was taken as a measure of presynaptic activity. In addition, the homovanillic acid (HVA) decrease, after intraperitoneal and after oral administration of the drugs was also taken as a measure of presynaptic activity. Postsynaptic activity was measured in two behavioural models, i.e. reserpine reversal and stereotypy induction. The effects of these drugs on noradrenaline and dopamine turnover (alpha-MpT method) were studied in addition. The displacement of [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin [( 3H]DP-5,6-ADTN) binding to rat striatal homogenates was studied in vitro. The results indicate that all three compounds are potent and selective DA agonists that lack significant alpha 2 activity. Because of its long duration of action and high oral activity, N-0437 seems to be a most promising candidate for further evaluation for possible therapeutic use.

Neuropharmacological profile of a new series of dopamine agonists: N-n-propyl-hexahydronaphthoxazines

It has been demonstrated that within the series of hydroxylated (7-OH, 9-OH) and non-hydroxylated (N-0498) hexahydronaphthoxazines the 9-OH (N-0500) analogue is a very potent centrally acting DA receptor agonist. In in vitro [3H]DP-5,6-ADTN binding experiments, reflecting D-2 dopaminergic activity, N-0500 was equipotent with apomorphine and RU-29717, whereas both the 7-OH (N-0499) and N-0498 were much less effective. In in vivo tests related to DA receptor stimulation N-0500 was found to be the most active compound. In the gamma-butyrolactone model, a test for DA autoreceptor activation, N-0500 was 10 times as potent as apomorphine, but 3 times less active than RU-29717. The locomotor activity of mice was inhibited more strongly by N-0500 than by N-0499. Striatal concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid were rapidly reduced by N-0500 both after intraperitoneal and oral administration, indicating that this compound is well absorbed from the gastrointestinal tract and passes the blood-brain barrier to activate DA autoreceptors. In models for postsynaptic DA receptor stimulation (induction of stereotypy in rats, reversal of reserpine-induced immobility of mice) N-0500 was found to be as effective as RU-29717 in inducing stereotyped behaviors in rats, but was much less effective than RU-29717 in restoring the mobility of reserpinized mice, suggesting a selectivity for D-2 DA receptors by N-0500 in contrast to the mixed D-1/D-2 receptor activity of RU-29717. In in vitro binding experiments for evaluating the affinity towards other receptor types, N-0500 exhibited only a weak affinity towards 5-HT1 and alpha 2 binding sites and possessed a very weak affinity for 5-HT2 and alpha 1 receptor sites. It was concluded from these in vitro binding experiments that N-0500, has not only a very high affinity for D-2 DA receptors, but is more selective than RU-29717 and much more selective than the ergot bromocriptine. On the basis of its very potent in vivo central D-2 dopamine receptor activities and its in vitro selectivity, N-0500, being the most potent compound within the series, is a much more specifically acting drug than many of the dopaminergic ergolines and might therefore be a good candidate for the treatment of Parkinson's disease.

S(-)DP-5,6-ADTN as an in vivo dopamine receptor ligand: relation between displacement by dopamine agonists and their pharmacological effects

The use of (-)DP-5,6-ADTN as a non-radioactively labeled ligand for an in vivo DA receptor assay is described and compared with racemic DP-5,6-ADTN, previously used for that purpose. The effects of four DA agonists (NPA, bromocriptine, DP-7-OH-ATN and 3-PPP) on the specific (-)DP-5,6-ADTN binding are related to their potencies to decrease striatal HVA concentrations and to induce stereotypy in rats. NPA and DP-7-OH-ATN caused a maximal decrease in HVA levels, when only a fraction of the receptors were occupied, while the occurrence of stereotypy was associated with a high receptor occupation, reflecting the higher affinity of these agonists for presynaptic than for postsynaptic receptors. Bromocriptine did not show this effect, as the dose-response relationships for HVA decrease, for induction of stereotypy and for the decrease in specific (-)DP-5,6-ADTN binding were all virtually equal to each other. While NPA and bromocriptine behaved as full postsynaptic agonists, in that maximal stereotyped behavior was observed after high doses, DP-7-OH-ATN was found to be a partial postsynaptic agonist, as it did not induce maximal stereotypy at a maximal receptor occupation. Racemic 3-PPP only caused a state of hypoactivity, but did neither affect specific (-)DP-5,6-ADTN binding nor striatal HVA levels. Our results are discussed in view of theories on the relation between receptor occupation and pharmacological effects and it is concluded that the in vivo receptor binding method using (-)DP-5,6-ADTN is a very useful tool for such investigations.