Prejunctional adrenoceptor activity of N-0437: a relatively selective DA2 dopamine receptor agonist

Prejunctional adrenoceptor activity of N-0437, 2[N-n-propyl-N-2-(thienylethyl-amino)-5-hydroxytetralin], was investigated by means of the cat nictitating membrane (CNM) preparation. Intra-arterial (i.a.) administration of N-0437 produced a dose-related inhibition (ED50 = 14 micrograms) of the CNM contractions elicited by electrical stimulation of pre- and postganglionic sympathetic nerves of the superior cervical ganglion. Pretreatment with domperidone i.a., a relatively selective DA2 receptor antagonist, markedly attenuated the CNM response to racemic N-0437 (ED50 = 6.7 x 10(2) micrograms). However, pretreatment with rauwolscine i.a., a relatively selective alpha 2-receptor antagonist, did not alter the CNM responses to racemic N-0437. Evaluation of the (R)-(+) and (S)-(-) enantiomers showed that only the (S)-(-) enantiomer was active in suppressing the contractions in the CNM preparation. These results demonstrate that N-0437 is a potent agonist for prejunctional DA2 dopamine receptors on peripheral sympathetic nerves in the CNM and that these peripheral DA2 receptors appear to be enantioselective.     

Synthesis and radioreceptor binding activity of N-0437, a new,extremely potent and selective D2 dopamine receptor agonist

The synthesis of a new, potent and selective D₂ dopamine receptor agonist, N-0437, of the 2-aminotetralin group is described. The results of a radioreceptor binding assay using a homogenate of porcine anterior pituitary as a tissue source for D₂ dopamine receptors and³H-spiperone as radioligand demonstrate that this compound is one of the most potent compounds so far evaluated in this test system.     

N-0437: a selective D-2 dopamine receptor agonist in in vitro and in vivo models

The selectivity of the potent dopamine D-2 agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) was examined in a series of in vivo and in vitro pharmacological models. In radioligand binding assays, N-0437 showed high potency (Ki = 0.69 nM) and selectivity for D-2 receptors as compared to its potency and selectivity at various other neuronal receptors (Ki in nM): D-1 (678) dopamine, alpha 1-(534) and alpha 2-(195) adrenoceptor, S1-(6940) and S2-(5900) serotonin and muscarine (2660). Very low activity (Ki greater than 10(-5) M) was seen at the beta-adrenoceptor, A1-adenosine, GABAA and benzodiazepine receptors. Furthermore, N-0437 inhibited the calcium-dependent release of [3H]dopamine (IC50: 4 nM) and [3H]acetylcholine (IC50: 6.3 nM) from rabbit striatal slices in the nanomolar range. These effects of N-0437 were mediated through activation of D-2 dopamine autoreceptors and D-2 dopamine heteroreceptors, respectively. Presynaptic dopaminergic activity in vivo was measurable as an inhibition of the locomotor activity of mice, and in this model N-0437 was more effective than apomorphine. Moreover, the effect of N-0437 could be antagonized by sulpiride but not by yohimbine. N-0437 was equipotent with apomorphine in inducing circling behaviour in 6-OHDA-lesioned rats. N-0437 had almost no serotonergic activity in vivo. The results show that N-0437 is a selective dopamine D-2 agonist, and thus, that it is a new ligand of choice for studies on the D-2 receptor.     

Sucrose sham-feeding in the rat after administration of the selective dopamine D2 receptor agonist N-0437, d-amphetamine or cocaine

Drugs which act as agonists at dopamine receptors, or which increase dopamine release (e.g., d-amphetamine, cocaine) are known to reduce food intake. The present experiments investigated, for the first time, the effects of a highly selective dopamine D2 receptor agonist, N-0437 (0.3-3.0 mg/kg, IP), on 5% sucrose sham-feeding in gastric fistulated rats, and compared these results with those of d-amphetamine (0.1-3.0 mg/kg, IP) and cocaine (3.0-10.0 mg/kg, IP). The results showed that sucrose sham-feeding was resistant to the effects of N-0437, even though the D2 agonist dose-relatedly reduced sucrose real-feeding in intact animals. The two psychomotor stimulants, d-amphetamine and cocaine, produced some reductions in sham-feeding, although in the case of the highest dose of d-amphetamine, the pronounced reduction in the consumption of sucrose was probably secondary to induced behavioral stereotypy. The results suggest that D2 receptor stimulation may interact with satiety cues to reduce ingestion of sucrose, but that in the absence of potent satiety stimuli D2 receptor stimulation is ineffective. Furthermore, N-0437 appeared not to be equivalent to either d-amphetamine or cocaine in their effects to reduce sucrose sham-feeding.     

Synthesis and radioreceptor binding activity of N-0437, a new, extremely potent and selective D2 dopamine receptor agonist

The synthesis of a new, potent and selective D₂ dopamine receptor agonist, N-0437, of the 2-aminotetralin group is described. The results of a radioreceptor binding assay using a homogenate of porcine anterior pituitary as a tissue source for D₂ dopamine receptors and³H-spiperone as radioligand demonstrate that this compound is one of the most potent compounds so far evaluated in this test system.     

The anorectic effect of the selective dopamine D1-receptor agonist A-77636 determined by meal pattern analysis in free-feeding rats

Free-feeding rats meet much of their daily energy requirements by consuming food in meals during the nocturnal phase of the night/day cycle. Meal pattern analysis methodology has been developed to record the patterns of meal taken over a 24-h period, and to provide detailed information on a number of meal-related parameters. Previous work indicates that selective dopamine D1-receptor agonists reduce food intake in short-term feeding tests under the control of homeostatic or hedonic factors. In the present study, our aim was to investigate the effects of the dopamine D1-receptor agonist, A-77636 (0.1-1.0 mg/kg, s.c.), administered just prior to the start of the night period, on the free-feeding and drinking patterns of rats maintained on a standard ad libitum diet. The results indicate that A-77636 exerted a suppressant effect on food intake, due principally to a reduction in meal size and duration. We conclude that there is a dopamine D1-receptor involvement in the normal controls of meal size, and that selective D1-receptor agonists may act to limit meal size.     

The enantiomers of the D-2 dopamine receptor agonist N-0437 discriminate between pre- and postsynaptic dopamine receptors

The (+) and (-) enantiomers of the substituted 2-aminotetralin, N-0437 were evaluated in vivo for their dopaminergic activity, using biochemical as well as behavioural models. In presynaptic models, i.e. antagonism of gamma-butyrolactone-induced dihydroxyphenylalanine elevations and the induction of hypomotility, both enantiomers exhibited a similar high degree of potency. Following postsynaptic stimulation, (-)N-0437 was able to induce stereotypy in rats in a dose-dependent manner. Moreover this compound was able to produce rotation in 6-hydroxy-dopamine-lesioned rats. In contrast, at the doses tested (i.e. 1 and 10 mumol/kg, i.p.), (+)N-0437 displayed virtually no activity at all in either of these postsynaptic models. From in vitro evoked release studies of [3H]acetylcholine it became clear that (-)N-0437 is a postsynaptic dopamine agonist, while (+)N-0437 is a weak antagonist at these receptors. Taken together, the results indicate that (-)N-0437 is very selective for the stimulation of postsynaptic dopamine receptors, while (+)N-0437 stimulates presynaptic dopamine receptors and blocks postsynaptic receptors. These properties make (+)- and (-)N-0437 very promising candidates for psychotherapeutic use.     

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.     

The effects of the enantiomers of the dopamine agonist N-0437 on food consumption and yawning behaviour in rats

The enantiomers of the potent and selective dopamine (DA) D-2 receptor agonist 2-(N-propyl-N-thienylethyl-amino)-5-hydroxytetralin, N-0437, were tested for their effects on palatable food consumption and yawning behaviour in rats. (-)-N-0437 (1.0 and 5.0 mumols/kg). This confirms the agonistic action of (-)-N-0437 on postsynaptic receptors as food consumption is considered to be related to stimulation of postsynaptic DA receptors. Yawning behaviour was stimulated by (-)-N-0437 (0.5 mumol/kg) and could be antagonized by the autoreceptor-selective antagonist (+)-UH 232 (25 mumols/kg), which suggests an agonistic action on DA autorecptors. (+)-N-0437 (5.0 and 10.0 mumols/kg) also reduced food consumption and the effect could be antagonized by YM 09151-2 (0.03 mumol/kg). The weaker effect of (+)-N-0437 on food intake in comparison to that induced by (-)-N-0437 can be explained if it assumed that (+)-N-0437 is a partial agonist. (+)-N-0437 did not induce yawning behaviour in rats, suggesting that autoreceptors mediating the release of DA may be involved in stimulating yawning by DA agonists.     

The potential antipsychotic activity of the partial dopamine receptor agonist (+)N-0437

The (+) enantiomer of the very potent and selective dopamine D-2 agonist, 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437), displays partial agonistic activity at dopamine D-2 receptors. In this study (+)N-0437 was investigated for its antagonistic activity at postsynaptic DA receptors in four behavioural tests which are commonly used to evaluate potential neuroleptic activity, i.e. d-amphetamine-induced stereotypy, passive avoidance responding, intracranial self-stimulation behaviour, and catalepsy. (+)N-0437 (25-50 mumol/kg) was active in the first three models, but did not cause catalepsy. Haloperidol, which was used as a reference compound for classical DA antagonists, showed clear activity in all four models at low doses (0.5-1.0 mumol/kg). (-)N-0437, a full D-2 agonist, displayed no activity in these behavioural models. These results suggest that (+)N-0437 could be used to examine the hypothesis that the use of partial agonists could provide a new treatment for schizophrenia.     

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

Summary 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-methoxyN-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 (3 H)-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. Because of the in vivo formation of N-0437 after N-0724 and N-0953 administration, it is hypothesized that this dopaminergic activity is the result of metabolic activation. The cyclopentoxy group (present in N-0953) then appears to be an interesting prodrug moiety because it seems to give rise to small (but sufficient) and constant N-0437 levels in the brain.Deceased January 2, 1990Send offprint requests to J. M. Jansen at the above addressPart of this work was presented at a satellite meeting of the XIth international congress of pharmacology: Dopamine '90 in Como, Italy (July 1990).     

Orally active carbamate prodrugs of the selective dopamine agonist N-0437: in-vivo activities in the 6-OHDA turning model and in-vitro activities.

The in-vivo activities of eight carbamate prodrugs of the D2-agonist N-0437 were determined by examining the effects of the prodrugs, after their oral administration in rats with unilateral 6-OHDA lesions of the striatum. The resulting contralateral turning was used as an index of the activity of the compounds. A comparison of the area under the curve of the time-effect curves of the prodrugs, revealed a significantly improved duration of action compared with N-0437 during the period 11-15 h after administration, for the propylcarbamate and the dimethoxyphenylcarbamate derivatives. The 2,4-dimethylphenylcarbamate showed a significantly enhanced turning behaviour over the whole 15 h time interval in comparison with N-0437. Three of the nine carbamates were virtually unhydrolysed in rat serum at 37 degrees C, while the other test compounds were hydrolysed relatively slowly, with t1/2 values ranging from 1.5-6 h. The test compounds differed greatly in partition coefficients, which were estimated by RP-HPLC (1-12 times more lipophilic than N-0437). The potential cholinesterase inhibiting properties of the carbamate prodrugs were assessed by a simple in-vitro incubation assay, which showed that only two carbamates were very weak cholinesterase inhibitors.     

PHNO, a selective dopamine D2 receptor agonist, does not reduce prepulse inhibition of the startle reflex in rats

RATIONALE: Dopamine agonists nonselective for dopamine receptor subtypes, such as apomorphine, reduce prepulse inhibition of the startle reflex. It has been suggested that either D2 or D3 dopamine receptors mediate this action of apomorphine. OBJECTIVE: The present study investigated whether a selective D2 agonist with relatively low affinity for D3 receptors can reduce prepulse inhibition. METHODS: Rats (n=48) were treated with vehicle or one of three doses ( 15, 30 or 60 microg/kg, s.c.) of the specific dopamine D2 receptor agonist (+)-4-propyl-9-hydroxynaphthoxazine (PHNO) for 11 days. On days 1, 6 and 11 of treatment, the rats (n=12 in each group) were tested for their acoustic startle reflexes (105-dB, 40-ms white noise) and for prepulse inhibition (5-kHz tone, 5 dB above a 65 dB background white noise). Prepulses were presented with a range of stimulus onset asynchronies (SOAs: 5-160 ms) or lead times between the onset of the prepulse and the onset of the startle stimulus. In a second experiment, two groups of rats (n=10 in each group) were tested in a similar manner after vehicle or apomorphine (0.8 mg/kg, s.c.) to verify the sensitivity of the present procedure to agonist-induced reductions in prepulse inhibition. RESULTS: At doses that increased motor activity, PHNO increased prepulse inhibition at SOAs less than 80 ms and had no effect on prepulse inhibition at SOAs of 80 ms or above. However, all doses decreased startle amplitudes on trials in which only the startle-eliciting stimulus was presented. Apomorphine reduced prepulse inhibition under the same conditions. CONCLUSIONS: These findings with PHNO are in contrast to the less-specific D2 agonist, quinpirole, which has been reported to decrease prepulse inhibition. It is concluded that activation of D2 dopamine receptors alone is not sufficient to attenuate prepulse inhibition of the startle reflex.     

Aripiprazole acts as a selective dopamine D2 receptor partial agonist

Aripiprazole has made a significant contribution to the treatment of schizophrenia and related disorders with an improved safety and tolerability profile, which has been attributed to its unique pharmacological profile. It has been claimed that partial agonism of the dopamine D(2) and 5-HT(1A) receptors and antagonism of the 5-HT(2) receptor contribute to the clinical profile of aripiprazole, a so-called dopamine- and 5-HT stabiliser. However, recent studies have questioned the role of the 5-HT-mediated systems in the mechanism of action of aripiprazole. This report reviews published and unpublished data that suggest that aripiprazole acts as a selective partial agonist at the dopamine D(2) receptor and does not affect 5-HT receptors at therapeutic doses.     

Aripiprazole acts as a selective dopamine D2 receptor partial agonist

Aripiprazole has made a significant contribution to the treatment of schizophrenia and related disorders with an improved safety and tolerability profile, which has been attributed to its unique pharmacological profile. It has been claimed that partial agonism of the dopamine D₂ and 5-HT_1A receptors and antagonism of the 5-HT₂ receptor contribute to the clinical profile of aripiprazole, a so-called dopamine- and 5-HT stabiliser. However, recent studies have questioned the role of the 5-HT-mediated systems in the mechanism of action of aripiprazole. This report reviews published and unpublished data that suggest that aripiprazole acts as a selective partial agonist at the dopamine D₂ receptor and does not affect 5-HT receptors at therapeutic doses.     

Improvement of the oral bioavailability of the selective dopamine agonist N-0437 in rats: the in vitro and in vivo activity of eight ester prodrugs

Summary The potent and selective D2-agonist N-0437 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin undergoes considerable first-pass metabolism due to glucuronidation of the phenolic group after oral administration. In an attempt to improve the bioavailability, eight ester prodrugs of N-0437 were synthesized, i.e. the acetyl, isobutyryl, pivaloyl, benzoyl, 2-methylbenzoyl, 2-methoxybenzoyl, 2,4-dimethylbenzoyl and 2-aminobenzoyl analogues. To examine the hydrolysis rates of these compounds in vitro studies were performed in rat serum. The prodrugs showed a very diverse pattern of hydrolysis rates. The in vivo activities were determined by testing the prodrugs in rats with unilateral 6-OHDA lesions of the striatum. The resulting contralateral turning was used to measure the activity of the compounds. By calculating the area under the curve (AUC), of the time-effect curves of the prodrugs, a significantly improved duration of action was found for those prodrugs which have a slow in vitro hydrolysis rate. However no significant differences in total activity of these slowly hydrolysing prodrugs compared with N-0437 could be demonstrated, although the 2-aminobenzoyl and the 2,4-dimethylbenzoyl derivatives show interesting behavioural profiles. In contrast the isobutyryl ester, a prodrug with a relatively rapid hydrolysis rate, gave an improvement of turning behaviour over the whole time course in comparison with N-0437. Send offprint request to I. den Daas at the above addressDeceased January 2, 1990     

Neurotensin attenuates dopamine D2 agonist quinpirole-induced inhibition of midbrain dopamine neurons

The effects of intracerebroventricular (i.c.v.) administration of NT on the activity of midbrain DA neurons were studied in rats using single unit recording techniques. NT (i.c.v., 50 micrograms) was found to have no significant effect on the spontaneous activity of DA cells. On the other hand, NT treatment significantly attenuated the inhibitory effect of quinpirole (i.v.), a specific D2 agonist, on a subpopulation of DA cells. This result is consistent with previous behavioral and biochemical studies suggesting that NT may produce some of its effects through modulation of central DA systems.     

Rotation following intranigral injections of a selective D1 or a selective D2 dopamine receptor agonist in rats

Injections of various nonselective dopamine agonists into the substantia nigra, pars reticulata (SNpr), have been reported to produce contralateral rotation in rats. Since a number of recent dopamine receptor distribution studies have indicated a preponderance of D1 compared to D2 dopamine receptor subtypes within the SNpr, we examined the relative behavioral functions of these two subtypes within the nigra by studying rotation following unilateral, local injections of a D1 (SKF38393) and D2 (quinpirole) agonist, Significant, dose-dependent contralateral rotation was observed following injections of R,S-SKF38393. This effect was found to be stereoselective to the R- enantiomer, suggesting that the effect is receptor mediated. In contrast, quinpirole (LY171555) produced significant, dose-dependent ipsilateral rotation following nigral injection. These results suggest that the rotation seen following intranigral injections of nonselective dopamine agonists is due to the simulation of the D1 dopamine receptor, and that nigral D1 and D2 dopamine receptors may play opposite roles in the control of behavior.     

Microdialysis and striatal dopamine release: stereoselective actions of the enantiomers of N-0437

An intracerebral dialysis method was used to test both enantiomers of the very potent and selective dopamine (DA) D-2 agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin, N-0437, for their actions on DA receptors in the striatum of the rat. (-)N-0437 induced a 60% decrease in DA release, which was independent of the presence or absence of a kainic acid lesion placed unilaterally in the striatum. Stereotyped behaviour was apparent following administration of the (-) enantiomer. Thus, (-)N-0437 displayed an agonistic action on both pre- and postsynaptic D-2 receptors. (+)N-0437 did not induce any effect in the release model after peripheral administration nor did it induce any form of stereotypy. A comparison between the effects of (-)N-0437 after oral (10 mumol/kg) and transdermal (10 mumol/kg) administration showed the advantages of the latter mode of administration. Transdermal application induced a much longer duration of action of the drug (13 h) in comparison with the oral mode (5 h). Thus, transdermal administration may be a very useful method of drug application for therapeutic use.