RBI-257: a highly potent dopamine D4 receptor-selective ligand.

RBI-257 (1-[4-iodobenzyl]-4-[N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperid ine), the p-iodobenzyl analog of U-101,958 (1-benzyl-4-[N-(3-isopropoxy-2-pyridinyl)-N-methyl]-aminopiperidine) had a lower dissociation constant (Ki = 0.3 vs. 2.7 nM) and higher selectivity than U-101,958 at dopamine D4 receptors, over dopamine D2 and D3 receptors in transfected cell membranes and D2-like sites in rat forebrain. Dopamine D4 receptor affinity of iodo-isomers of RBI-257 ranked: para > meta > ortho. RBI-257 had much lower affinity at D1 and D5 dopamine receptors in transfected cells, as well as dopamine D1-like receptors, alpha1, alpha2 or beta(1,2) adrenoceptors, sigma(1,2) receptors and 5-HT1A or 5-HT2A receptors, and transporters for dopamine, norepinephrine or serotonin in rat forebrain tissue. RBI-257 may be a useful probe or radioligand for brain dopamine D4 receptors.

A phosphorylation cascade in the basal ganglia of the mammalian brain: regulation by the D-1 dopamine receptor. A mathematical model of known biochemical reactions.

Stimulation of the dopamine D-1 receptor in the corpus striatum initiates a cascade of biochemical events. These events include: activation of adenylate cyclase, stimulation of cAMP-dependent protein kinase, protein phosphorylation and inhibition of phosphoprotein phosphotase-1. This article presents and discusses a mathematical model of these biochemical events (and their dependence upon the concentration of cytosolic calcium). According to this model, the activity of calcineurin (which is regulated by the concentration of cytosolic calcium ions) counterbalances the activity of the "D-1 cascade". The combined activity of the "D-1 cascade" and calcineurin can regulate the activity of calcium- and calmodulin-dependent protein kinase II.

The D-1 dopamine receptor: past, present and future an idiosyncratic review.

1. Key events in the experimental investigation of the D-1 dopamine receptor are reviewed. 2. The efficacy of D-1 receptor agonists in the treatment of experimental parkinsonism in MPTP-treated primates is demonstrated. The diminished dyskinetic liability of D-1 agonists is discussed. 3. The significance of the dopa-induced dyskinesias is discussed from the perspective of deterministic chaos. The unpredictibility and irreproducibility of dyskinetic movements is highlighted and compared with features of the logistic equation. 4. The authors propose that the dopa-induced dyskinesias should be considered to be a manifestation of a chaotic process within the basal ganglia. The loss of the dopaminergic innervation and the subsequent repeated exposure to dopamine (derived from the exogenous dopa administered to the subjects) alters the response properties of the basal ganglia circuitry so that stimulation of dopamine receptors now elicits the dyskinetic movements.

S-(+)-aporphines are not selective for human D3 dopamine receptors.

1. Our aim was to test the hypothesis that selectivity for D3 dopamine (DA) receptors may contribute to limbic anti-DA selectivity of S-(+)-aporphine DA partial agonists. 2. Affinity was tested with 3H-emonapride, using human D3 receptors in mouse fibroblasts and D2 receptors in rat striatal tissue. 3. D3 receptors showed a picomolar affinity for 3H-emonapride, Na+ dependence, and reversible saturability, as well as stereoselectivity. Confirmatory or novel D3/D2 pharmacologic selectivity was found with several benzamides, thioxanthenes, buspirone, GBR-12909, and DA agonists including hydroxyaminotetralins [ADTN, (+)-7-OH-DPAT, (-)-PPHT and its fluorescein derivative], (-)-N-propylnorapomorphine, (-)-3-PPP, (-)-quinpirole, and SDZ-205-502, but neither aminoergoline nor (+)-aporphine partial agonists. 4. The results extend pharmacologic characterization of D3-transfected cell membranes but fail to account for the high limbic anti-DA selectivity of S-(+)-aporphines.

Differential effect of selective D-1 and D-2 dopamine receptor agonists on levodopa-induced dyskinesia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine- exposed monkeys.

The motor effects of selective D-1 dopamine receptor stimulation in Parkinson's disease have been explored in a limited number of studies with partial D-1 agonists only and the results were unsatisfactory. Four 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-exposed parkinsonian monkeys already exhibiting levodopa- and dopamine agonist-induced dyskinesia received selective D-1 agonists ([2,3,4,5-tetrahydro-7-8-dihydroxy-1-phenyl-1-H-3-benzazepine- HCI] (SKF 38393), [(+-)6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro- 1H-3-benzazepine hydrobromide] (SKF 82958), [(1R, 3S)3-(1'-adamantyl)-1-aminomethyl-3,4-dihydro-5,6- dihydroxy-1H-2-benzopyran hydrochloride] (A-77636) and [(-)-(6aR)(12bR)-4,6,6a,7,8,12b-hexahydro-7-methyli ndolo (4,3-ab)-phenanthridine] (CY 208-243)) to compare these drugs with selective D-2 agonists (LY 171555, (+)-4-propyl-9- hydroxynaphthoxazine and bromocriptine) and levodopa in terms of antiparkinsonian efficacy and side effects. The D-1 class of compounds was as efficacious as the D-2 agents in alleviating parkinsonism in these animals. However, D-1 agonists were, in general, less likely to reproduce dyskinesia. In addition, D-1 agonists occasionally improved motor symptoms without concomitant dyskinesia, unlike D-2 agonists or levodopa (which always produced some dyskinesia with improvement in motor function). These preliminary results do not support the hypothesis that preferential D-1 receptor stimulation facilitates dyskinesia in primates.(ABSTRACT TRUNCATED AT 250 WORDS)

Further characterization of the D2 dopamine receptor expressed in MMQ cells.

The D2 dopamine receptor expressed in the MMQ cell line was characterized by saturation binding using the D2 dopamine radioligand [3H]spiperone. The KD for spiperone was 41 pM and the Bmax for these sites was 34 fmol/mg protein. Inhibition of forskolin-stimulated cAMP accumulation occurred in response to a variety of D2 agonists, and the agonist effects were reversed by D2 antagonists. Pertussis toxin pretreatment abolished agonist inhibition of cAMP accumulation. In addition, the alpha 2-adrenergic agonist UK 14304 inhibited cAMP accumulation; this effect was reversed by an alpha 2-adrenergic antagonist but not by a D2 antagonist, indicating the presence of alpha 2-adrenergic receptors on these cells. Specific oligonucleotide primers were used in the polymerase chain reaction to determine, by restriction enzyme analysis and Southern blotting, that the long form of the two alternatively spliced variants of the D2 dopamine receptor was the predominant variant expressed in these cells.

Isomeric selectivity at dopamine D3 receptors.

Racemic 7-hydroxy-N,N-dipropylaminotetralin (7-OH-DPAT) shows greater affinity for limbic-selective dopamine D3 receptors than for more ubiquitous dopamine D2 receptors. R(+)-7-OH-DPAT was prepared and evaluated in radioreceptor assays using membranes of fibroblasts expressing the human dopamine D3 receptor as well as rat striatal membranes containing dopamine D2 receptors. This enantiomer had 2-fold greater D3 affinity than the racemate and similarly greater D3 vs. D2 selectivity (64-fold). The results may facilitate development of D3 selective agents and evaluation of functions of these receptors.

Brain dopamine receptors: 20 years of progress.

I present a brief overview of the contributions of Paul Greengard's laboratory to the field of dopamine receptor research. I show that the work on the biochemical pharmacology of dopamine receptors was part of the intellectual foundation for the division of dopamine receptors into two general pharmacological categories.

A-77636: a potent and selective dopamine D1 receptor agonist with antiparkinsonian activity in marmosets.

A-77636, ((1R,3S) 3-(1'-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1H-2-benz opyran hydrochloride), is a selective dopamine D1 receptor agonist. In a battery of receptor binding assays, A-77636 shows the highest affinity (pKi = 7.40 +/- 0.09; Ki = 39.8 nM) for the dopamine D1 receptor. A-77636 is an agonist at the dopamine D1 receptors in the fish retina (pEC50 = 8.13; EC50 = 1.1 nM; intrinsic activity = 102% of dopamine) and the rat caudate-putamen (pEC50 = 8.97; intrinsic activity = 134% of dopamine). The compound is functionally inactive at dopamine D2 receptors (EC50 > 10 microM). In rats with unilateral 6-OHDA (6-hydroxydopamine) lesions of the nigro-striatal dopaminergic pathway, A-77636 elicits prolonged (> 20 h) contralateral turning that is blocked by SCH 23390, a D1 receptor antagonist, but not by haloperidol at doses selective for the dopamine D2 receptor. Higher doses of A-77636 produce forelimb clonus in rats and mice. When tested in marmosets treated with MPTP to induce a parkinsonian-like state, A-77636 increases locomotor activity and decreases the severity of the parkinsonian-like symptoms: the compound is active after either subcutaneous or oral administration. A-77641, the optical antipode of A-77636, has a lower affinity towards the dopamine D1 receptor (pKi = 5.14, Ki = 7200 nM), is less potent as a dopamine D1 receptor agonist (pEC50 = 5.65; EC50 = 2200 nM), fails to elicit turning in the 6-OHDA-lesioned rat, and lacks antiparkinsonian efficacy in the MPTP-treated marmoset.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of DARPP-32 and protein phosphatase inhibitor-1 in rat choroid plexus: regulation by factors other than dopamine.

The molecular mechanisms underlying regulation of fluid production by secretory epithelia such as the choroid plexus are poorly understood. Two cAMP-regulated inhibitors of protein phosphatase-1, inhibitor-1 (I1) and a dopamine- and cAMP-regulated phosphoprotein, M(r) = 32,000 (DARPP-32), are enriched in the choroid plexus. We show here that these two phosphoproteins are colocalized in choroid plexus epithelial cells. We have developed a novel method for studying the phosphorylation state of DARPP-32 and I1 in intact cells, using a phosphorylation state-specific monoclonal antibody. Several drugs and hormones that are known to alter fluid secretion and that increase cAMP levels (forskolin, isoproterenol, vasoactive intestinal peptide) or cGMP levels (atrial natriuretic peptide) or that may use additional second messenger pathways (5-HT), increase the phosphorylation of I1 and DARPP-32 in rat choroid plexus. In contrast, dopamine does not alter cAMP and cGMP levels, or I1 and DARPP-32 phosphorylation. Our results indicate that DARPP-32, known to be regulated by dopamine in a number of tissues, can be phosphorylated in response to non-dopaminergic factors, including hormones acting through non-cAMP-dependent pathways. Our results also raise the possibility that inhibition of phosphatase-1, as a result of I1 and DARPP-32 phosphorylation, might be part of a final common pathway in the action of several factors that are known or thought to alter cerebrospinal fluid production.

Multiple dopamine receptors and their implications in medicine.

Two categories of dopamine receptor were identified with classical pharmacological and biochemical techniques. Five different molecular species of dopamine receptor have been identified with molecular biological techniques. Each of the these five receptors can be assigned to one of the two classically identified dopamine receptor categories. The biology of the dopamine receptors can be investigated with molecular biological techniques. Recent pharmacological investigations show that the D1 dopamine receptor may be an important site of action for antiparkinsonian drugs.

A68930: a potent agonist specific for the dopamine D1 receptor.

A68930, [1R, 3S] 1-aminomethyl-5,6-dihydroxy-3-phenylisochroman HCl, is a potent, partial agonist in the dopamine-sensitive adenylate cyclase model of the D1 dopamine receptor in fish retina. In the rat caudate-putamen model of the D1 dopamine receptor, A68930 is a potent (EC50 2.1 nM) full agonist. In contrast, A68930 is a much weaker (EC50 = 3,920 nM) full agonist in a biochemical model of the D2 dopamine receptor. A68930 also displays weak 2 agonist activity but the molecule is virtually inactive at the 1 and beta-adrenoceptors. When tested in rats bearing a unilateral 6-OHDA lesion of the nigro-neostriatal neurons, A68930 elicits prolonged (> 20 hr) contralateral turning.

Activation of the 5-HT1C receptor expressed in Xenopus oocytes by the benzazepines SCH 23390 and SKF 38393.

1. A cloned 5-HT1C receptor expressed in Xenopus laevis oocytes was used to characterize the action of four dopamine D1-selective benzazepines at the 5-HT1C receptor. Additionally, the apparent binding of the D1-selective benzazepines to 5-HT1C receptors was measured in the choroid plexus of the pig. 2. In voltage-clamped oocytes expressing the cloned 5-HT1C receptor, 5-hydroxytryptamine (5-HT) elicited a characteristic inward current response with an EC50 of 13 nM. SCH 23390 acted as a stereoselective agonist (or partial agonist) with an EC50 of about 550 nM. SKF 38393 (1 microM-1 mM), SKF 77434 (100 microM), and SKF 82958 (100 microM) also acted as agonists (or partial agonists) at the cloned 5-HT1C receptor. SKF 38393 was not stereoselective at the 5-HT1C receptor. 3. The response to SCH 23390 activated slowly and, although the response contained many oscillations characteristic of the activation of the phosphatidylinositol signal transduction system, SCH 23390 rarely elicited the rapid spike-like response seen routinely in response to 5-HT. However, the responses to SKF 38393, SKF 77434, and SKF 82958 were identical in appearance to the response to 5-HT, except that the responses to the benzazepines were smaller. These comparisons were made by applying both a benzazepine and 5-HT to each individual oocyte expressing the cloned 5-HT1C receptor. 4. Consistent with the responses measured in oocytes, SCH 23390 bound stereoselectively to 5-HT1C receptors in the choroid plexus of the pig (Ki = 6.3 nM), and SKF 38393 bound non-stereoselectively with lower affinity (Ki = 2.0-2.2 microM).5. It is concluded that while these benzazepines demonstrate selectivity for the dopamine D1 receptor, they also can act as agonists or partial agonists at the 5-HT1c receptor in situ and as expressed in Xenopus oocytes. The oocyte expression system is useful for studies of the functional pharmacology of these 5-HTic receptors. Information about the pharmacological actions and variations in stereoselectivity among dopamine and 5-HT receptors should be of interest in modelling the interactions of ligands with these G-protein coupled receptors, and in the testing of such models through receptor mutagenesis.

Synthesis and pharmacological evaluation of 1-(aminomethyl)-3,4-dihydro-5-hydroxy-1H-2-benzopyrans as dopamine D1 selective ligands.

A series of 3-substituted 1-(aminomethyl)-3,4-dihydro-5-hydroxy-1H-2- benzopyrans were prepared as potential D1 selective antagonists. The compounds were evaluated for their affinity and selectivity for the D1 receptor as well as for their functional antagonism of D1-mediated pharmacological events. The compounds show potent D1 antagonist properties in vitro. The optimum nitrogen substitution was found to be the primary amine and the observed order of potency for substitution at the 6-position is OH greater than Br greater than H greater than OMe. Two representative compounds, the 6-methyl and 6-bromo analogues, were also evaluated in vivo for dopaminergic activity. Interestingly, both compounds behave as potent in vivo agonists.

Synthesis and dopaminergic activity of 3-substituted 1-(aminomethyl)-3,4-dihydro-5,6-dihydroxy-1H-2-benzopyrans: characterization of an auxiliary binding region in the D1 receptor.

The synthesis and dopaminergic activity of a series of C3 and nitrogen-substituted 1-(aminomethyl)-3,4-dihydro-5,6-dihydroxy-1H-2-benzopyrans (isochromans) is described. The synthesis of the compounds was stereospecific for the 1,3 cis isomer, and the enantioselective synthesis of both enantiomers of one of the analogues (20) was achieved. It was determined that all of the dopaminergic activity resides in the [1R,3S] isomer. Generally, substitution at the C3 position provided compounds with very high potency (less than 10 nm EC50) and selectivity for the D1 receptor, with a wide range of intrinsic activities (60-160%). Analogues containing C3 substituents including aryl, arylalkyl, and cyclic and acyclic alkyl groups showed a marked enhancement of dopaminergic activity compared to the unsubstituted compound. As a class, the drugs were orally active in the rat rotation model with a very long duration of action.

Evidence for involvement of both D1 and D2 receptors in maintaining cocaine self-administration.

Rats trained to self-administer cocaine (0.75 mg/kg/infusion) on an FR-5 schedule were treated with selective D1 or D2 antagonists. A69045, a D1 antagonist with no appreciable affinity for 5-HT receptors increased cocaine self-administration to 147, 172 and 167% of baseline at doses of 2.5, 5.0 or 10.0 mumol/kg, SC respectively. SCH-23390 (0.007, 0.015 and 0.030 mumol/kg, SC) increased self-administration to 116, 147 and 165% of baseline, respectively. Both D1 antagonists decreased responding in some animals at the highest dose tested. The D2 antagonist YM-09151-2 showed a similar profile, increasing cocaine self-administration at 0.01 and 0.016 mumol/kg, SC and suppressing responding by most animals at the dose of 0.03 mumol/kg, SC. These data give further support to the hypothesis that both D1 and D2 receptors are involved in maintaining cocaine self-administration.

Rapid reversal of denervation supersensitivity of dopamine D1 receptors by l-dopa or a novel dopamine D1 receptor agonist, A68930.

The present report describes the effects of sub-chronic treatment with l-dopa or with a recently characterized, selective dopamine D1 receptor agonist (A68930) on the denervation-induced behavioral supersensitivity of the dopamine D1 receptor. Rats with unilateral 6-OHDA lesions of the nigrostriatal pathway, when treated for four successive days with l-dopa + carbidopa show robust contralateral rotation on each day. However, after three days of l-dopa + carbidopa treatment lesioned animals show a significant loss of behavioral supersensitivity to the dopamine D1-selective agonists, A68930 and SKF38393. When lesioned animals were treated daily with A68930, by the second day they showed a virtually complete loss of responsiveness to a dose of the dopamine D1 agonist which previously produced near maximal rotation. In contrast, locomotor hyperactivity to A68930 by intact rats was undiminished over five successive treatment days. These data demonstrate rapid and substantial diminution of the supersensitivity of the denervated dopamine D1 receptor following treatment with l-dopa + carbidopa or with a selective dopamine D1 agonist, while normosensitive dopamine D1 receptor-mediated locomotion in non-lesioned rats is unaltered.

A68930: a potent and specific agonist for the D-1 dopamine receptor.

A68930 (5,6-dihydroxy-3-phenyl-1-aminomethyl-isochroman) is a potent (EC50 = 2.5 nmol/L) partial agonist at the D-1 dopamine receptor. In contrast, A68930 is a much weaker agonist (EC50 = 3,920 nmol/L) at the D-2 dopamine receptor. The orientation of the 3-phenyl substituent in the molecule is critical for the affinity and selectivity of the molecule towards the D-1 receptor in vitro and in vivo. The role of the D-1 receptor in the functioning of the basal ganglia is discussed.

A-69024: a non-benzazepine antagonist with selectivity for the dopamine D-1 receptor.

A-69024 HBr, 1-(2-bromo-4,5-dimethoxybenzyl)-7-hydroxy-6-methoxy-2-methyl-1,2,3,4- tetrahydroisoquinoline hydrobromide, is a selective antagonist of the dopamine D-1 receptor. A-69024 HBr shows an apparent affinity toward the D-1 receptor (identified using [125I]SCH 23390) of 12.6 (4.15-38.3) nM (mean (90% CL), n = 3); the apparent affinity toward the D-2 receptor (identified using [3H]spiroperidol is 1 290 (1,200-1,380) nM (n = 3); using [125I]lysergic acid diethylamine to identify the 5-HT1C receptor gives apparent affinity of 17,800 (9,700-32,600) nM (n = 3). In assays of adenylate cyclase activity, A-69024 HBr antagonizes the D-1 receptor with a calculated affinity of 43.9 (17.5-110) nM (n = 5), while the molecule antagonizes the D-2 receptor with a calculated affinity greater than 400 nM. Behavioral studies demonstrate that A-69024 HBr (5 mg/kg s.c.) is able to block both amphetamine-induced locomotor activity and apomorphine-induced stereotypy. Furthermore, A-69024 HBr blocks SF&F 38393-, but not quinpirole-, induced rotation in rats having unilateral 6-hydroxydopamine lesions of the substantia nigra. When administered at behaviorally effective doses. A-69024 HBr neither increases the concentration of serum prolactin nor potentiates dihydroxyphenylalanine (DOPA) accumulation in the caudate-putamen of rats pretreated with the DOPA decarboxylase inhibitor NSD 1015. Because A-69024 is a dopamine receptor antagonist discriminating between the D-1 and D-2 receptors, it may be a useful research tool.