N-n-propylnorapomorphine Research Articles

New Dopamine D2 Receptor Agonist, [3H]MCL-536, for Detecting Dopamine D2high Receptors in Vivo.

Increases in the D2 receptor high affinity state are associated with certain neurological disorders. We synthesized and characterized the high-affinity D2high ligand [3H]MCL-536 in competition binding against the D2/3 agonist R-(-)- N- n-propylnorapomorphine (NPA) and the D2/3 antagonist raclopride. The total binding of [3H]MCL-536 (minus that in the presence of 100 nM NPA) was measured by saturation binding in CHO cells expressing human D2long; the data yielded separable, nonsaturable nonspecific, and saturable specific components. The former represents an aporphine site common to NPA and [3H]MCL-536. The latter indicated specific binding to the total D2 receptors (both high and low-affinity states). [3H]MCL-536 had a Kd of 0.8 nM. In competition binding, NPA had a Ki of 0.16 nM, and raclopride had a Ki of 0.9 nM. Co-incubation with guanylylimidodiphosphate abolished binding to D2high. This unique profile makes radiolabeled MCL-536 a versatile tool for diagnostics and therapeutics, and may quantify D2high sites in schizophrenia and PD patients in vivo.

Dopamine D2 Receptor Relies upon PPM/PP2C Protein Phosphatases to Dephosphorylate Huntingtin Protein

Striatal dopamine D2 receptor (D2R) relies upon G protein- and β-arrestin-dependent signaling pathways to convey its action on motor control and behavior. Considering that D2R activation inhibits Akt in the striatum and that huntingtin physiological functions are affected by Akt phosphorylation, we sought to investigate whether D2R-mediated signaling could regulate huntingtin phosphorylation. We demonstrate that D2R activation decreases huntingtin phosphorylation on its Akt site. This dephosphorylation event depends upon the Gαi-dependent engagement of specific members of the protein phosphatase metallo-dependent (PPM/PP2C) family and is independent of β-arrestin 2. These observations identify the PPM/PP2C family as a mediator of G protein-coupled receptor signaling and thereby suggest a novel mechanism of dopaminergic signaling.

Regional distribution of dopamine D4 receptors in rat forebrain.

Binding of the D2-like (D2/D3/D4) radioligand [3H]nemonapride under selective conditions (with 300 nM S[-]-raclopride and other masking agents to occlude D2/D3 receptors and non-specific binding sites) revealed a subset of raclopride-insensitive binding sites considered D4-like receptors. These sites were stereoselective to R(-)-N-n-propylnorapomorphine (NPA) over its S(+)-NPA in a similar fashion to cloned D4 receptors expressed in cell lines. In addition, the highly D4-selective agent L-745,870 displaced 74-83% of these sites in rat brain regions, suggesting that most were D4 receptors. These apparent D4 receptors represented a relatively high proportion of D2-like receptors in hippocampus, dorsolateral frontal, medial prefrontal and entorhinal cortex, but fewer in caudate-putamen and nucleus accumbens.

A high dose of EEDQ reduces pituitary dopamine D 2 receptor density and the prolactin suppressive potency of agonists

In male rats, a high dose of the alkylating compound N-ethoxycarbonyl-2-ethoxyl-1,2-dihydroquinoline (EEDQ, 30 mg/kg s.c., 24 h) cause a reduction of pituitary dopamine D 2 receptor density by 27% as measured by means of in vivo radioligand binding (using a single dose of the ligand [ 3H]spiperone). The same dose of EEDQ reduced the potency, but not the maximal response, of the dopamine D 2 receptor agonists R-(-)- N-n-propylnorapomorphine (NPA), (−)-3(33-pydroxyphenyl)- N-n-propylpiperidine((+)-3-PPP), and 6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5]azepine (B-HT 920) with respect to suppression of prolactin release after pretreatment with γ-butyrolactone. The measured reduction in dopamine D 2 receptor density after EEDQ was of the same magnitude as the reduction in receptor number predicted from the EEDQ induced shift in the dose-response curve of the full dopamine D 2 receptor agonist NPA. The findings are discussed in relation to our previous observation that a somewhat lower dose of EEDQ (20 mg/kg s.c., 24 h) effectively reduces the efficacy of partial dopamine D 2 receptor agonists while affecting neither the prolactin response to full dopamine D 2 receptor agonists nor the density of pituitary dopamine D 2 receptors.

Altered spontaneous behavior and sensitivity to apomorphine in rats following pretreatment with S(+)-aporphines or fluphenazine.

Rats were pretreated for 2 weeks with similarly effective doses of the typical neuroleptic fluphenazine (FPZ) or the experimental weak partial D2 agonists S(+)N-n-propylnorapomorphine (NPA) and S(+)11-hydroxy-N-n-propylnoraporphine (11-OH-NPa). Spontaneous and dopamine (DA) agonist (apomorphine; APO) stimulated stereotyped behaviors or locomotion, and interactions with APO were evaluated over the following 2 weeks. While FPZ induced marked supersensitivity in APO stereotype, (+)NPA showed no significant change, and (+)11-OH-NPa produced only a small, transient increase in response; NPA also lacked a supersensitizing effect on locomotor arousal induced by APO. The time-course of stereotyped responses to APO following pretreatment with FLZ included a marked increase following FPZ that became maximal at day 5 and normalized by day 9; there was a parallel reduction of acute antisteotypy efficacy of FPZ. (+)11-OH-NPa had similar, but much lesser and shorter-lived effects. Spontaneous locomotion was markedly depressed following FPZ, recovered in 1 week, exceeded controls at day 9, and returned to baseline by day 11; (+)11-OH-NPa, again, had similar but smaller effects. Acute effects of FPZ to reduce spontaneous or APO-induced locomotion were greater after FPZ pretreatment and normalized within a week; (+)11-OH-NPa had a similar but smaller effect. Locomotor arousal by APO was altered inconsistently in the week after pretreatment with FPZ or (+)11-OH-NPa. Thus, FPZ appeared to induce tolerance and supersensitivity in central DA systems, most clearly seen following a several-day period to eliminate the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine D 1 and D 2 receptors in the caudate nucleus of spontaneously hypertensive rats and normotensive Wistar-Kyoto rats

A series of studies was carried out to characterize the binding properties of dopamine D 1 and D 2 receptors in membrane homogenates of the caudate nucleus of spontaneously hypertensive rats (SHR). Binding in SHR was studied at the age of 4 weeks when the rats were still in the prehypertensive phase, and at the age of 8 weeks, during the phase in which blood pressure is increasing dramatically; age-matched normotensive Wistar-Kyoto rats (WKY) were used as controls. Binding to dopamine D 1 receptors was studied using [ 3H]SCH 23390. Antagonist binding of dopamine D 2 receptors was performed with [ 3H]spiperone. At both ages no differences were found between SHR and WKY in affinity ( K d ) or concentration ( B max ) of dopamine D 1 and D 2 receptzrs. Binding to the high affinity state of the dopamine D 2 receptor was measured using the agonist [ 3H] N-n-propylnorapomorphine (NPA). No differences in B max or K d were found between SHR and WKY at both ages studied. indicating that the ratio between dopamine D 2 receptors in the high and in the low affinity state is not altered in spontaneous hypertension. Although the results do not reveal differences in affinities or concentrations of dopamine D 1 or D 2 receptors in the caudate nucleus between SHR and WKY, a role in the development of hypertension for the here described lack of receptor up-regulation in connection with our previous observation of lower release of dopamine in the caudate nucleus of SHR, cannot be excluded.

Synaptic Concentration of Dopamine in the Mouse Striatum in Relationship to the Kinetic Properties of the Dopamine Receptors and Uptake Mechanism

The concentration of dopamine (DA) in the synaptic cleft in the mouse striatum in vivo was estimated from the competition between the synaptic DA and the 3H-labelled DA D2 receptor agonists N-n-propylnorapomorphine (NPA) or N,N-diethyl-N'-[(3 alpha, 4a alpha, 10 beta)-1,2,3,4,4a,5,10,10a-octahydro- 7-hydroxyl-1-propyl-3-benzo (g) quinolinyl]sulfamide (Sandoz 205-501) injected intravenously in tracer doses. Knowing the inhibitor constant for DA in inhibiting the binding of these receptor agonists in vitro, attempts were made to calculate the changes in the synaptic DA concentration from the changes in the in vivo binding of the receptor agonists evoked by various pharmacological agents. Inhibiting the firing of the dopaminergic neurons by gamma-butyrolactone (GBL) increased the binding of the receptor agonists corresponding to a decrease in the synaptic DA concentration of 55 +/- 2 nM in the experiments with [3H]Sandoz 205-501 and 48 +/- 3 nM in the experiments with tracer doses of [3H]NPA. These values may therefore approximate the normal DA concentration in the synaptic cleft in the mouse striatum. With this technique it was also possible to determine the synaptic concentration of NPA by its competition with [3H]Sandoz 205-501 for the DA D2 receptors in the striatum of GBL-treated mice in vivo. To compare the estimated synaptic concentration of DA with the affinity of DA to D1 and D2 receptors and to the DA transporter in the mouse striatum the kinetic parameters were determined at 37 degrees C in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of isomers of hydroxyaporphines on dopamine metabolism in rat brain regions

The effects of isomers of di- and monohydroxyaporphines on cerebral dopamine (DA) metabolism were evaluated in representative extrapyramidal (corpus striatum) and limbic (nucleus accumbens septi) tissues of rat brain by three methods: (1) changes in the ratio of homovanillic acid (HVA) to DA, (2) accumulation of l-dihydroxyphenylalanine (DOPA) after inhibiting its decarboxylation to DA under “open-loop” conditions, as well as (3) after γ-butyrolactone (GBL) pretreatment to provide selective effects at presynaptic DA autoreceptors. The DA-agonist R(−) isomers of the aporphines apomorphine (APO), N-n-propylnorapomorphine (NPA), and 11-hydroxy- N-n-propyl-noraporphine (11-OH-NPa) showed consistent dose-dependent inhibition of DA synthesis in both brain regions with all models; the neuroleptic haloperidol had the opposite effect in the first two models only, as expected. The S(+) isomers of NPA and 11-OH-NPa have shown behavioral evidence of antidopaminergic activity, especially in the limbic system. Unlike the neuroleptic, 5(+)NPA did not show DA-synthesis enhancing actions in accumbens or striatal tissue but, instead, inhibited DA synthesis like its R(−) antipode in all three test paradigms. S(+)11-OH-NPa given alone produced minor changes in the HVA/DA ratio and did not antagonize R(−)11-OH-NPa, weakly increased accumulation of DOPA in the second model, and had no effect in the third—all without regional selectivity. In the test of autoreceptor functioning, the dihydroxyaporphine S(+)NPA, but not S(+)11-OH-NPa, inhibited DA synthesis and this effect, in turn, was largely reversed by haloperidol, as were the inhibitory effects of the three R(−)aporphines tested. In this model, however, neither S(+)NPA nor S(+)11-OH-NPa antagonized the DA-synthesis inhibiting effect of R(−)APO as haloperidol did. Overall, these results are consistent with evidence that R(−)NPA and 11-OH-NPa have high affinity at D-2 receptor sites in rat brain and show behavioral effects of typical DA agonists. The non-stereoselective inhibitory effects of NPA on DA synthesis may reflect its activity as a weak DA agonist with very low intrinsic activity, but may also include a direct “catechol-effect” on tyrosine hydroxylase. In contrast, R(−)11-OH-NPa appears to be a stereoselective D-2 agonist, active at autoreceptors as well as postsynaptic receptors, that lacks the nonstereospecific effects on DA metabolism of its catechol-aporphine congener. It may be a useful probe for the further characterization of dopamine receptors and autoreceptors.

The role of multiple dopamine receptors in apomorphine and N-n-propylnorapomorphine-induced climbing and hypothermia

Apomorphine and N-n-propylnorapomorphine (NPA) were compared for their ability to induce stereotyped cage climbing and hypothermia in mice. Climbing behaviour was produced by similar doses of apomorphine and NPA (0.625–2.5 mg/kg s.c.), whereas NPA was 43 times more potent than apomorphine in inducing a hypothermic response. SKF38393 caused a shift to the left in the dose-response curve for NPA-induced climbing, the ED 50 changing from 0.98 to 0.014 mg/kg. SKF38393 had no effect on apomorphine-induced climbing behaviour. The climbing response produced by apomorphine was antagonised by both D-1 and D-2 antagonists. Climbing behaviour induced by NPA (2.5 mg/kg) could be antagonised by SCH23390 but not by clebopride, however climbing behaviour induced by a low dose of NPA (0.06 mg/kg) plus SKF38393 could be blocked by both D-1 and D-2 receptor antagonists. The hypothermic responses produced by either apomorphine or NPA could only be reversed by the selective D-2 antagonist, clebopride. These results demonstrate that dopamine agonist-induced stereotyped cage climbing requires both D-1 and D-2 receptor stimulation, whereas the hypothermic response is D-2-mediated. The results also show that it is possible to assess the relative activity of a dopamine agonist at D-1 or D-2 receptors in vivo by comparing the ability of the compound to induce hypothermia and climbing behaviour.

Determination of O-dealkylation products of R-(−)- and S-(+)-10,11-methylenedioxy- N-n-propylnoraporphine in Cunninghamella elegans cultures by liquid chromatography

A rapid and selective liquid chromatographic method is described for the simultaneous determination of 10,11-methylenedioxy- N-n-propylnoraporphine (MDO-APO) and N-n-propylnorapomorphine (NPA) in Cunninghamella elegans (ATCC 9245) cultures. Isolation of substrate R-(−)- or S-(+)-MDO-NPA, metabolite R-(−)- or S-(+)-NPA and internal standard R-(−)-10,11-methylenedioxyaporphine (MDO-APO) was achieved via liquid-liquid extraction from buffered fermentation samples (pH 7.5), and were confirmed by the addition of chemically pure compounds. Aliquots were separated on a 3-μm Supelcosil LC-CN column using acetonitrile-phosphate (pH 3.0) (19+81) as eluent and quantified by monitoring the ultraviolet absorbance at 280 nm. A single chromatographic run could be completed in ca. 13 min; no interference from other metabolites or endogenous compounds was noted. The detection limit for aporphines was 0.33 nmol ml −1; the within-assay and day-to-day variation were consistently lower than ca. 7%. The time course of MDO-NPA metabolism by the fungus was also studied. The bioconversion studies showed that (+)-MDO-NPA was O-dealkylated to a slightly higher extent to (+)-NPA by C. elegans compared with the (−)-enantiomer; after 4 days of incubation the (+)-NPA/(−)-NPA product ratio was calculated to be 3.2:1.

Kinetic properties of the in vivo accumulation of 3H-(?)-N-n-propylnorapomorphine in mouse brain

(1) The influence of various dopamine (DA) receptor agonists and antagonists on the kinetic properties of the specific binding of 3H(-)-N-n-propylnorapomorphine (NPA) in the mouse striatum in vivo was studied. The specific binding of 3H-NPA, defined as the difference between the radioactivity in the striatum and cerebellum, was completely antagonized by the selective D-2 receptor antagonist raclopride but not by the selective D-1 antagonist SCH 23390, showing that the binding occurs exclusively to the D-2 receptors. (2) The selective D-2 receptor agonists pergolide and quinpirole inhibited the 3H-NPA binding biphasically at low doses, indicating that these DA receptor agonists have high affinities for a subfraction (10 to 30%) of the NPA binding sites. (3) Increasing the synaptic DA concentration by DA release [(+)-amphetamine] or uptake blockade (amfonelic acid and methylphenidate) inhibited the 3H-NPA binding in a competitive manner (unchanged Bmax, increased KD). Depletion of the DA in the synapses by gamma-butyrolactone or reserpine decreased the apparent KD value. (4) The possibility of estimating changes in the synaptic DA concentration from changes in the apparent KD is discussed. According to the results obtained, the normal concentration of DA in the synaptic cleft in mouse striatum in vivo is about 40 nmol/l and this concentration is increased 2 to 3 times by (+)-amphetamine and amfonelic acid in doses which evoke hyperactivity and stereotypic behaviour.

Hypothermia in mice: D2 dopamine receptor mediation and absence of spare receptors

The nonselective dopamine (DA) receptor agonists R(-)apomorphine (APO) and R(-)-N-n-propylnorapomorphine (NPA) elicited dose- and time-dependent hypothermia in mice with ED50 values of 300 and 18 micrograms/kg, respectively. The selective D2 agonist quinpirole (LY 171555) also elicited dose-dependent hypothermia, whereas the selective D1 agonist SKF 38393 had no effect. The selective D1 and D2 antagonists SCH 23390 (1 mg/kg) and sulpiride (200 mg/kg), respectively, did not significantly alter body temperature. The hypothermic effect of a maximal dose of NPA (0.2 mg/kg) was not blocked by SCH 23390 (1 mg/kg) but was significantly attenuated (p less than 0.001) by pretreatment with sulpiride (200 mg/kg). Pretreatment with sulpiride (200 mg/kg) produced a parallel, 40-fold shift to the right of the dose-response curve for NPA. Partial, irreversible DA receptor inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (2 mg/kg) reduced the maximal hypothermic effect of NPA (to 49% of control) without altering its ED50. Analysis of the data indicated a linear relationship between DA receptor occupancy and hypothermic response. The results demonstrate that DA agonist-induced hypothermia in mice is mediated by D2 receptors and that there is no receptor reserved for this response.

In vivo labelling of rat brain dopamine D-2 receptors. Stereoselective blockade by the D-2 antagonist raclopride and its enantiomer of 3H-spiperone, 3H-N,N-propylnorapomorphine and 3H-raclopride binding in the rat brain.

The stereospecific blockade by raclopride and FLB472 (the R enantiomer of raclopride) of the specific in vivo binding of [3H]-spiperone, [3H]-N,N-propylnorapomorphine (NPA) and [3H]-raclopride was studied in seven brain regions (e.g., caudate nucleus, olfactory tubercle, septum, hippocampus, frontal cortex, substantia nigra, pituitary gland) of the male albino rat. The binding of all three ligands was dose-dependently blocked by raclopride and FLB472. The blockade by FLB472 occurred at doses 50-100 times higher than that obtained by raclopride. The maximal blockade by raclopride of [3H]-spiperone binding differed between brain areas. Thus, the largest blockade was obtained in the substantia nigra (95%), septum (90%), caudate nucleus (60%) and olfactory tubercle (60%), while the blockade of [3H]-spiperone binding in the frontal cortex and pituitary gland did not exceed 30% and 50%, respectively. In contrast to [3H]-spiperone, the in vivo binding of [3H]-NPA and [3H]-raclopride was prevented by 90-100% in all brain areas examined. Taken together, the present findings indicate that the in vivo binding of three radioactive ligands to a central dopamine D-2 receptor can be stereoselectively blocked by the enantiomers of raclopride. The findings suggest that, under in vivo conditions, [3H]-raclopride and [3H]-NPA may label a closely related receptor site. However only some of the [3H]-spiperone binding sites may be identical to the [3H]-raclopride binding sites. The findings indicate furthermore that the relative overlap of D-2 sites shared by [3H]-spiperone and [3H]-raclopride may vary between brain regions.

Neuropharmacology and Stereochemistry of Dopamine Receptor Agonist and Antagonist Enantiomeric Pairs

Neuropharmacological evaluation of the R and S isomers of 11-hydroxy-N-n-propylnoraporphine (11-OH-NPa) supports the impression that the 11-OH group in aporphines (analogous to the meta hydroxyl of dopamine, DA) is sufficient to confer high affinity and activity at DA receptors. As in the case of the catechol congeners, (R)-apomorphine (APO) and (R)-N-n-propylnorapomorphine (NPA), (R)-11-OH-NPa is a potent DA agonist while, like (S)-NPA, (S)-11-OH-NPa is a DA antagonist. Thus, (R) and (S)-11-OH-NPa are an additional pair of compounds in which one enantiomer is a DA agonist and the other an antagonist. Other analogous pairs are the enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP), and cis-1-methyl-5-hydroxy-2-(di-n-propylamino)tetralin (5-OH-MDAT). All contain a meta hydroxyphenyl, an N-n-propyl, and a phenethylamine moiety which can be superimposed in a consistent way to discriminate the DA agonists from the antagonists, with the key feature in this discrimination being the direction of the ammonium hydrogen. An energy penalty must be incurred by 3-PPP to assume the required conformations and it may account for the relatively low potency of the 3-PPP enantiomers. This analysis supports the view that rigid analogs of flexible compounds when "frozen" in their biologically active conformation exhibit higher affinity interactions with the receptor.

S(+)methylenedioxy-N-n-propylnoraporphine: an orally active inhibitor of dopamine selective for rat limbic system

The 10–11-methylenedioxy (MDO) derivative of S(+)N-n-propylnorapomorphine (NPA) was prepared and tested as a possible active prodrug to S(+)NPA, which we have recently found to exert in vivo activity suggestive of selective antagonism of dopamine receptors in the limbic forebrain but not the extrapyramidal basal ganglia. Like S(+)NPA, S(+)MDO-NPA inhibited the behavioral arousal induced by dopamine injected into nucleus accumbens of the rat, but not the head-turning response to dopamine injected into the corpus striatum. However, only MDO-NPA was orally active and it was somewhat longer-acting than NPA. The activity of S(+)MDO-NPA was prevented by pretreatment with the oxidase inhibitor SKF-525A. These properties are analogous to those of R(−)MDO-NPA, which we had previously reported as an orally active prodrug of the dopamine agonist R(−)NPA. Thus the methylenedioxy derivatives of the two entantiomers of NPA have properties desirable in a potentially clinically useful dopamine agonist and limbic dopamine antagonist, respectively.

Tracer and maximal specific binding of tritiated spiperone or N-n-propylnorapomorphine to quantify dopamine receptors in rat brain regions in vivo

Specific tracer and maximal specific binding (B max) were determined in rat brain regions from radioactivity accumulation after intravenous administration of 3H N-n-propylnorapomorphine (NPA) or 3H spiperone at various specific activities. With NPA the highest B max-values (expressed in pmol.g −1 tissue) were found in the striatum (26 pmol.g −1) nucleus accumbens (about 27 pmol.g −1) and the olfactory tubercle (11 pmol.g −1). Saturable NPA binding was also found in the amygdaloid complex, medulla oblongata and inferior colliculi, but not in the frontal cortex. B max values for spiperone were high in the striatum (73 pmol.g −1), the nucleus accumbens (48 pmol.g −1), the olfactory tubercle (34 pmol.g −1) and the frontal cortex (18 pmol.g −1). A similar order was found for the tracer contents in these regions. There was no linear relationship between these contents and B max values. The possible implcations of these findings and usefulness of NPA for brain imaging are discussed.

Two automated locomotor activity tests for dopamine autoreceptor agonists

In the first test (exploratory activity), pretreated rats explored a novel environment in the dark. The potential autoreceptor agonists apomorphine HCl, N-n-propylnorapomorphine (NPA), and N-n-propyl-3-(3-hydroxyphenyl)-piperidine (3-PPP) and its enantiomers decreased the total distance travelled while at the same time paradoxiacally increasing the number of discrete movements. This is a very different pattern from that of the typical antipsychotic drugs haloperidol HCl and chlorpromazine HCl, and the atypical antipsychotic drug clozapine, which also decreased the total distance travelled but decreased the number of movements. Both groups decreased the distance/movement. In the second test, rats were habituated to the monitors in the light and then treated with test drug and stimulant (d-amphetamine sulfate or apomorphine HCl). Apomorphine HCl, NPA, and (+)3-PPP antagonized amphetamine-stimulated locomotor behavior (total distance) without antagonizing apomorphine-stimulated behavior, suggesting a presynaptic dopamine autoreceptor agonism. EMD 23448 gave equivocal activity. On the other hand, haloperidol HCl, chlorpromazine HCl, and clozapine decreased both amphetamine- and apomorphine-stimulated behavior, suggesting a postsynaptic dopamine antagonism. 3-PPP and (−)3-PPP showed neither pattern in this test.

ChemInform Abstract: RESOLUTION OF 5,6‐DIHYDROXY‐2‐(N,N‐DI‐N‐PROPYLAMINO)TETRALIN IN RELATION TO THE STRUCTURAL AND STEREOCHEMICAL REQUIREMENTS FOR CENTRALLY ACTING DOPAMINE AGONISTS

The enantiomers of 5,6-dimethoxy-2-(N,N-dipropylamino)tetralin were prepared with use of (+)- and (-)-dibenzoyltartaric acid as the resolving agent. Ether cleavage with BBr3 gave the enantiomers of the dihydroxy compound 5,6-dihydroxy-2-(N,N-dipropylamino)tetralin (5,6-(OH)2-DPATN). The in vitro activities of (+)- and (-)-5,6-(OH)2-DPATN were evaluated in binding studies with rat striatal tissue with use of [3H]-N-n-propylnorapomorphine (NPA) as the ligand. IC50 (nM) values for (-)- and (+)-5,6-(OH)2-DPATN were 2.5 and 400, respectively. The in vivo efficacy of the enantiomers was evaluated by examining their effects on the metabolism of dopamine in rat striatum. After a 0.5 mumol/kg ip injection of the (-) enantiomer, the concentrations of the metabolites HVA and DOPAC were reduced to 50% of control values, whereas at this dose the (+) isomer was inactive. On the basis of these findings together with the stereochemical data of previously described DA agonists, a dopamine-receptor model has been developed which consists of two binding sites for the amine nitrogen of DA agonists in addition to a major binding sit for the m-hydroxy group. The relevance of this model with its accessory features is discussed in relation to the structure and pharmacological data of different DA agonists.

Heterogeneous distribution of dopamine D 2 receptors within the rat striatum as revealed by autoradiography of [formula omitted] binding sites

The precise topographical distribution of dopamine D 2 receptors in the rat striatum has been studied autoradiographically through the use of [ 3 H] N-n- propylnorapomorphine (NPA) as a ligand. The highest densities of [ 3H]NPA binding sites were found in the ventrolateral aspect and medial margin of the striatum, while sparse grain densities were present in the medial and latero-dorsal striatum. Moreover, a clear gradient of [ 3H]NPA binding was observed along the rostro-caudal axis of the striatum. These heterogeneities in the distribution of dopamine receptors provide further evidence for a functional compartmentalization of the striatum.

Resolution of 5,6-dihydroxy-2-(N,N-di-n-propylamino)tetralin in relation to the structural and stereochemical requirements for centrally acting dopamine agonists.

The enantiomers of 5,6-dimethoxy-2-(N,N-dipropylamino)tetralin were prepared with use of (+)- and (-)-dibenzoyltartaric acid as the resolving agent. Ether cleavage with BBr3 gave the enantiomers of the dihydroxy compound 5,6-dihydroxy-2-(N,N-dipropylamino)tetralin (5,6-(OH)2-DPATN). The in vitro activities of (+)- and (-)-5,6-(OH)2-DPATN were evaluated in binding studies with rat striatal tissue with use of [3H]-N-n-propylnorapomorphine (NPA) as the ligand. IC50 (nM) values for (-)- and (+)-5,6-(OH)2-DPATN were 2.5 and 400, respectively. The in vivo efficacy of the enantiomers was evaluated by examining their effects on the metabolism of dopamine in rat striatum. After a 0.5 mumol/kg ip injection of the (-) enantiomer, the concentrations of the metabolites HVA and DOPAC were reduced to 50% of control values, whereas at this dose the (+) isomer was inactive. On the basis of these findings together with the stereochemical data of previously described DA agonists, a dopamine-receptor model has been developed which consists of two binding sites for the amine nitrogen of DA agonists in addition to a major binding sit for the m-hydroxy group. The relevance of this model with its accessory features is discussed in relation to the structure and pharmacological data of different DA agonists.