A new approach for treatment of hypertension: modifying D1 dopamine receptor function

Essential hypertension is a major factor for myocardial infarction, heart failure and kidney failure. Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and vasodilatation directly or indirectly with other hormones and humoral factors, such as reactive oxygen species and the renin-angiotensin system. Dopamine receptors are classified into five subtypes based on their structure and pharmacology. Among those dopamine receptor subtypes, D(1) receptor is the most important one, during conditions of moderate sodium intake, more than 50% of renal sodium excretion is regulated by D(1)-like receptors. Decreased renal dopamine production and/or impaired D(1) receptor function have been reported in hypertension. Disruption of D(1) receptor results in hypertension. In this paper, we review the mechanisms by which hypertension develops when D(1) receptor function is perturbed. We also discuss possible new approaches developing anti-hypertensive medicine by increasing renal dopamine production, enhancing D(1) receptor function, or modifying its interactions with other blood pressure-regulating systems.     

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.     

A-412997 is a selective dopamine D4 receptor agonist in rats

A-412997 (2-(3',4',5',6'-tetrahydro-2'H-[2,4'] bipyridinyl-1'-yl)-N-m-tolyl-acetamide) is a highly selective dopamine D4 receptor agonist that binds with high affinity to rat dopamine D4 and human dopamine D4.4 receptors (Ki=12.1 and 7.9 nM, respectively). In contrast to the dopamine D4 receptor agonists PD168077 and CP226269, A-412997 showed a better selectivity profile and no affinity <1000 nM for other dopamine receptors or any other proteins in a panel of seventy different receptors and channels. In functional assays using calcium flux, A-412997 was a potent full agonist at rat dopamine D4 receptors (28.4 nM, intrinsic activity=0.83) and did not activate rat dopamine D2L receptors, unlike CP226269. Dopamine D4 receptor selective agonists have been shown to induce penile erection in rats by central mechanisms. A-412997 induces penile erection in a conscious rat model (effective dose=0.1 micromol/kg, s.c.) with comparable efficacy as the nonselective D2-like agonist, apomorphine. When dosed systemically, A-412997 crossed the blood brain barrier rapidly and achieved significantly higher levels than PD168077. A-412997 is a highly selective dopamine D4 receptor agonist and a useful tool to understand the role of dopamine D4 receptors in rat models of central nervous system processes and disease.     

Functional activation by central monoamines of human dopamine D(4) receptor polymorphic variants coupled to GIRK channels in Xenopus oocytes

We studied the functional activation of different polymorphic variants of the human dopamine D(4) receptors by the three major central monoamines, dopamine, noradrenaline and serotonin. Dopamine D(4) receptors carrying two (D4.2), four (D4.4) or seven (D4.7) repeats within the third intracellular domain were co-expressed with G protein-regulated inwardly rectifying potassium channels (GIRK1) in frog oocytes. All the dopamine D(4) receptor variants coupled to oocyte G(i/o) proteins and modulated co-expressed GIRK1 channels. Monoamine-induced responses were detected as increases in voltage-clamp recorded GIRK1 currents. Dopamine, noradrenaline as well as serotonin stimulated dopamine D(4) receptors. Dose-response analysis showed that dopamine and noradrenaline are full agonists whereas serotonin acted as partial agonist. Dopamine was 5-fold more potent on D4.2 and D4.7 (EC(50)=1 nM) than on D4.4 (EC(50)=5 nM) suggesting that the actions of dopamine and therapeutic drugs on dopamine D(4) receptors might vary among individuals depending on their repertoire of expressed alleles. In contrast, noradrenaline and serotonin did not discriminate among dopamine D(4) receptor variants (EC(50 NA)=50 nM, EC(50 5-HT)=1.5 microM). All monoamine effects were blocked by the specific dopaminergic D(4) antagonist (S)-(-)-4-[4-[2-(Isochroman-1-yl)ethyl]piperazin-1-yl]benzenesulfonamide (PNU101387). Sequence analyses of dopamine D(4) receptors and related monoamine receptors revealed that dopamine D(4) receptors have most aminoacidic residues necessary for binding of dopamine, noradrenaline and serotonin. Our data indicate that dopamine D(4) receptors can be pharmacologically stimulated by any the three major central monoamines.     

DOPAMINE RECEPTORS: CLASSIFICATION, PROPERTIES AND DRUG DEVELOPMENT

1. The pharmacology, biochemistry and structure-activity relationships pertinent to the D1 and D2 subtypes of dopamine receptors are reviewed. 2. Recent advances in receptor purification and our understanding of the secondary messenger systems involved are presented. 3. D1 and D2 receptors of central nervous, endocrine and peripheral systems are identical.     

Dopamine and serotonin regulate tumor behavior by affecting angiogenesis

The biogenic amines dopamine and serotonin are neurotransmitters and hormones, which are mainly produced in the central nervous system and in the gastro-intestinal tract. They execute local and systemic functions such as intestinal motility and tissue repair. Dopamine and serotonin are primarily stored in and transported by platelets. This review focuses on the recently recognized role of dopamine and serotonin in the regulation of tumor behavior by affecting angiogenesis and tumor cell proliferation. Preclinical studies demonstrate that dopamine inhibits tumor growth via activation of dopamine receptor D2 on endothelial and tumor cells. Serotonin stimulates tumor growth via activation of serotonin receptor 2B on endothelial cells and serotonin receptors on tumor cells. Drugs that stimulate dopamine receptor D2 or inhibit serotonin receptors are available and therefore clinical intervention studies for cancer patients are within reach.     

Dopamine receptor immunohistochemistry in the rat choroid plexus

1. Earlier studies have demonstrated a high density of dopamine D1-like receptor binding in the choroid plexus by light microscope autoradiography, but the dopaminergic specificity of this binding was questioned. 2. In this study the localization of dopamine receptor subtypes was investigated in the rat choroid plexus by Western blot analysis and immunohistochemistry using antibodies raised against dopamine D1-D5 receptor protein. 3. Western blot analysis revealed reactivity with immune bands of approximately 50 and 51 KDa corresponding to dopamine D1 and D5 receptors, respectively. Dopamine D1-like (D1 and D5) receptor protein immunoreactivity insensitive to superior cervical ganglionectomy was located in smooth muscle of choroid arteries and to a larger extent within choroid plexus epithelium. 4. Western blot analysis revealed reactivity with immune bands of approximately 53 KDa and 40-42 KDa corresponding to dopamine D2 and D4 receptors, respectively, and no dopamine D3 receptor reactivity. Dopamine D2-like receptor protein immunoreactivity displayed a distribution similar to that of tyrosine-hydroxylase (TH)-immunoreactive sympathetic fibres and disappeared after superior cervical ganglionectomy. It consisted in the expression of dopamine D2 and to a lesser extent of D4 receptor protein immunoreactivity perivascularly and associated with choroid epithelium. No D3 receptor protein immunoreactivity was found in rat choroid plexus. 5. The above results indicate that rat choroid plexus expresses dopamine receptor protein, being dopamine D1-like receptors predominant in epithelium and arterial smooth muscle and D2-like receptors in sympathetic nerve fibres supplying choroid plexus epithelium and vasculature. 6. These findings suggests that dopamine receptors with a different anatomical localization may modulate production of cerebrospinal fluid.     

Dopamine receptor subtypes in the human pulmonary arterial tree

Dopamine induces vasorelaxation of pulmonary artery primarily through an endothelium-dependent mechanism, but dopamine receptor subtypes involved in these mechanisms have not been identified yet. The expression and localization of dopamine D1-like (D1 and D5) and D2-like (D2, D3 and D4) receptors were investigated in hilar, lobar and intrapulmonary branches of human pulmonary artery by immunoblotting and immunohistochemistry. Pulmonary artery expresses dopamine D1, D2, D4 and D5 receptor subtypes, but not the D3 receptor subtype. Dopamine D1 and to a lesser extent D5 receptors were accumulated primarily in the endothelium of extrapulmonary branches of pulmonary artery. A faint dopamine D1 and D5 receptor immunoreactivity was found in the inner media of extrapulmonary and of large sized intrapulmonary branches of pulmonary artery, but not in medium- or small-sized intrapulmonary artery branches. Dopamine D2 and to a lesser extent D4 receptor immunoreactivity co-localized with the tyrosine hydroxylase-immunoreactive sympathetic plexus supplying pulmonary artery was found in the adventitia and in the adventitia-media of both extra- and different-sized intrapulmonary branches of pulmonary artery. These findings suggest the possible role of dopamine receptors in the pulmonary endothelium-dependent vasorelaxing activity. The D1 receptor subtype seems to be the most involved in this mechanism. Dopamine D2-like receptors are prejunctional and are located at the level of sympathetic neuroeffector plexus. The heterogeneous distribution and density of dopamine receptor subtypes along the human pulmonary arterial tree may be related to the different functional roles of dopamine at various levels of the pulmonary circulation.     

Regulation of dopamine D2 receptors in a novel cell line (SUP1).

A prolactin-secreting cell line, SUP1, has been established from rat pituitary tumor 7315a. In radioligand binding experiments, the D2 receptor antagonist (S)-(-)-3-[125I]iodo-2-hydroxy-6-methoxy-N-[(1-ethyl-2- pyrrolidinyl)methyl]benzamide ([125I]IBZM) labeled a single class of sites in homogenates of SUP1 cells (Kd = 0.6 nM; Bmax = 45 fmol/mg of protein). The sites displayed a pharmacological profile consistent with that of D2 receptors. Inhibition of the binding of [125I]IBZM by dopamine was sensitive to GTP, suggesting that D2 receptors in SUP1 cells are coupled to guanine nucleotide-binding protein(s). In the presence of isobutylmethylxanthine, dopamine decreased the level of cAMP accumulation in SUP1 cells. Dopamine also inhibited prolactin secretion from SUP1 cells. Both the inhibition of cAMP accumulation and the inhibition of prolactin secretion were blocked by D2 receptor antagonists, suggesting that these effects of dopamine were mediated by an interaction with D2 receptors. The regulation of D2 receptors in SUP1 cells by D2 receptor agonists was investigated. Exposure of SUP1 cells to dopamine or to the D2 receptor agonist N-propylnorapomorphine led to increased expression of D2 receptors, with no change in the affinity of the receptors for [125I]IBZM. An increase in the density of D2 receptors in SUP1 cells was evident within 7 hr of exposure to dopamine. Spiroperidol, a D2 receptor antagonist, blocked the effect of dopamine on receptor density. These results suggest that exposure of D2 receptors in SUP1 cells to agonists leads to an up-regulation of D2 receptors. Dopamine retained the ability to inhibit cAMP accumulation in SUP1 cells exposed to dopamine for 24 hr, suggesting that D2 receptors in SUP1 cells are not desensitized by prolonged exposure to agonist. SUP1 cells should be a useful model system for future studies of the regulation of the expression and function of D2 receptors in cultured cells.     

Glycoprotein nature of dopamine D1 receptors in the brain and parathyroid gland

Dopamine D1 receptors can be covalently labeled with the photo-affinity ligand (+-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4-azidophenyl)-2,3,4,5-tetrah yd ro-1H-3-benzazepine ([125I]IMAB) and visualized following sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. In brain membranes, [125I]IMAB labels a polypeptide of apparent Mr approximately equal to 74,000 as the major ligand binding subunit of D1 receptors and two minor polypeptides of Mr approximately equal to 64,000 and 52,000. In contrast, [125I]IMAB labels a single polypeptide of apparent Mr approximately equal to 64,000 in bovine parathyroid glands. In this study, the carbohydrate nature of dopamine D1 receptors from the brain and parathyroid gland were examined using specific exo- and endoglycosidases and lectin affinity chromatography. [125I]IMAB-labeled brain and parathyroid D1 receptors were sensitive to treatment with the exoglycosidases neuraminidase or alpha-mannosidase, suggestive of the existence of terminal sialic acid and oligomannose residues. Photolabeled D1 receptor polypeptides are not however, associated with distinct populations of complex-type or high mannose-containing carbohydrate chains because 1) wheat germ agglutinin and concanavalin A lectin chromatography of solubilized and photolabeled neuronal D1 receptors followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed no differences in the electrophoretic mobility of column pass-through and specifically eluted [125I]IMAB-labeled polypeptides, and 2) [125I]IMAB-labeled D1 receptors specifically bound to and eluted from concanavalin A-Sepharose were neuraminidase sensitive, indicative of the colocalization of oligomannose- and complex-type glycans. Removal of these terminal glycan residues did not affect the binding of [3H]SCH 23390 to dopamine D1 receptors. Complete N-linked deglycosylation of photolabeled D1 receptors from both the brain and parathyroid with peptide N-glycosidase F resulted in the migration of a single major labeled polypeptide of apparent Mr approximately equal to 46,000. These data suggest that, despite differences observed in the electrophoretic mobility and glycosylation patterns of brain and parathyroid D1 receptor polypeptides, the protein backbones of central and peripheral dopamine D1 receptors display similar if not identical molecular weights.     

Pharmacological agents affecting emesis. A review (Part I).

The availability of radiolabelled ligands selective for various putative neurotransmitter receptor sites and the development of quantitative autoradiography has led to a greater understanding of the neuronal pathway and receptor subtypes involved in the vomiting reflex induced by various mechanisms both within the central nervous system and the periphery. Receptors for acetylcholine, dopamine, histamine and serotonin have been detected in a number of brain regions associated with the vomiting reflex, and provide a rational basis for the antiemetic action of drugs that inhibit receptor subtypes for these neurotransmitters. The basis of the antiemetic action of other drugs such as dexamethasone and the cannabinoids is still obscure. Some drugs act on more than 1 receptor subtype. Metoclopramide may inhibit both dopamine D2- and 5-HT3 receptors in producing its antiemetic effect. Both metoclopramide and domperidone appear to have additional peripheral actions that contribute to their effectiveness. The cannabinoids are effective in cytotoxic-induced vomiting, perhaps acting via endorphin receptors or by inhibiting prostaglandin synthesis. The effectiveness of 5-HT3 receptor antagonists may depend on the block of both central and peripheral neuronal 5-HT3 receptors. Vomiting constitutes a major disadvantage to the use of many drugs; vomiting induced by aminoglycoside antibiotics appears to be due to ototoxicity and is relieved by histamine H1-receptor antagonists. The protracted vomiting associated with the use of some cytotoxics in cancer chemotherapy may involve psychic components, the chemoreceptor trigger zone and peripheral sensory neurons. Both 5-HT3 and dopamine D2-receptor antagonists exert some control, the former being more effective with cytotoxics of high emetogenic potential, such as cisplatin. Serotonin 5-HT3 receptor antagonists or high doses of metoclopramide in combination with anxiolytics and steroids as well as greater attention to pharmacokinetic profiles of the drugs involved would appear to offer improved control. The use of dopamine receptor antagonists in controlling emesis induced by dopamine agonists used in Parkinson's disease poses theoretical problems which can be overcome by using drugs with selectivity for the chemoreceptor trigger zone, such as domperidone or metoclopramide. However, higher doses of these drugs may produce some impairment of therapeutic responses to the agonists. Muscarinic and nicotinic agonists currently under investigation in Alzheimer's disease pose another therapeutic dilemma as emesis is due to a central action of these compounds. Several sites may be involved including the chemoreceptor trigger zone and frontal lobes. Opiates may act through dopamine receptors or mu-receptors on dopaminergic nerves, but serotonergic mechanisms may also be involved in the action of some opiates.(ABSTRACT TRUNCATED AT 400 WORDS)     

3H] A-369508 ([2-[4-(2-cyanophenyl)-1-piperazinyl]-N-(3-methylphenyl) acetamide): an agonist radioligand selective for the dopamine D4 receptor

Tritiation of the dopamine D(4) receptor selective agonist A-369508 ([2-[4-(2-cyanophenyl)-1-piperazinyl]-N-(3-methylphenyl) acetamide) has provided a radioligand for the characterization of dopamine D(4) receptors. [(3)H] A-369508 binds with high affinity to the major human dopamine D(4) receptor variants D(4.2), D(4.4) and D(4.7) (K(d)=1.7, 4, and 1.2 nM, respectively). It also binds to the rat dopamine D(4) receptor, (K(d)=4.4 nM), implying similar binding affinity across human and rat receptors. A-369508 shows >400-fold selectivity over D(2L), >350-fold selectivity over 5-HT(1A) and >700-1,000-fold selectivity over all other receptors tested. Agonist activity determined by inhibition of forskolin-induced cAMP in Chinese hamster ovary cells transfected with the human dopamine D(4.4) receptor (EC(50)=7.5 nM, intrinsic activity=0.71) indicates that A-369508 is a potent agonist at the human dopamine D(4) receptor. Similar data was observed in other functional assays. [(3)H] A-369508 binds to a single, high affinity site on membranes containing the human dopamine D(4.4) receptor. When compared to the D(2)-like antagonist [(3)H] spiperone, competition binding for agonists like dopamine and apomorphine were 2-10-fold more potent with [(3)H] A-369508, while the antagonists clozapine, haloperidol and L-745870 bind with similar affinity to both ligands. Binding to rat brain regions demonstrated that the most abundant area was cerebral cortex (51.2 fmol/mg protein) followed by hypothalamus, hippocampus, striatum and cerebellum. [(3)H] A-369508 is a useful tool to define the localization and physiological role of dopamine D(4) receptors in central nervous system and can facilitate measuring accurate affinities (K(i)) for structure/activity relationship studies designed to identify dopamine D(4) receptor selective agonists.     

Dopamine receptor agonists in cardiovascular therapy

Dopamine receptor agonists exert marked cardiovascular and renal actions that result from activation of specific dopamine receptors located at various sites within the cardiovascular system. The presence of at least two distinct subtypes of dopamine receptors is reported. (1) Neurotropic dopamine receptors, also referred to as δ₂ or DA₂ receptors, are located at various sites within the sympathetic nervous system, activation of which leads to inhibition of sympathetic nervous system activity. (2) Postsynaptic dopamine receptors, also referred to as δ₁ or DA₁ are located on vascular smooth muscles of blood vessels as well as at various sites in the kidney, and their activation causes, vasodilatation, inhibition of aldosterone release, and sodium and water reabsorption. The pharmacological responses to activation of neurotropic and postsynaptic dopamine receptors consist of hypotension, bradycardia, diuresis, and natriuresis. Since an elevated sympathetic nervous system activity and retention of salt and water appear to be responsible for several cardiovascular disorders, it is proposed that dopamine receptor agonists may represent a novel class of therapeutic agents for the treatment of certain cardiovascular diseases. Further research efforts in this area are required to understand better the role of endogenous dopamine and dopamine receptors in the maintenance of cardiovascular homeostasis and to develop more selective compounds for experimental and clinical studies in different disease states.     

大鼠心脏和肾脏多巴胺D_1A受体mRNA水平与大鼠年龄的关系

目的　观察了幼年、成年及老年大鼠心、肾组织中的多巴胺 D1 A受体 m RNA水平的变化 ,借以阐明外周多巴胺 D1 A受体与年龄变化的关系。方法　采用了反转录 -多聚酶链反应 (RT- PCR)方法。结果　以D1 A受体与 β- actin吸光度值之比表示组织中 D1 A受体 m RNA水平 ,以 3个月龄大鼠 m RN A水平为 10 0 % ,10 d龄大鼠 m RNA水平为 (6 1± 6 ) % (P<0 .0 5 ) ,18个月龄为 (6 9± 11) % (P<0 .0 5 ) ,10 d龄大鼠肾脏 m RNA为 (49± 8) % (P<0 .0 5 ) ,18个月龄大鼠为 (5 2± 7) % (P<0 .0 5 )。提示 10 d龄及 18个月龄大鼠心脏及肾脏 D1 A受体m RNA水平较 3个月龄大鼠明显下降 (P<0 .0 5 )。结论　 1外周 D1 A受体与中枢多巴胺受体相似 ,也有明显的年龄依赖性特征。2外周多巴胺受体的年龄相关性影响 ,提示多巴胺类药物对不同年龄段患者的效果可能有所不同 ,应引起临床用药的注意     

In vitro pharmacological profile of YM-43611, a novel D2-like receptor antagonist with high affinity and selectivity for dopamine D3 and D4 receptors.

1. We investigated some neurochemical properties of a novel benzamide, YM-43611, [(S)-N-(1-benzyl-3-pyrrolidinyl)-5-chloro-4-cyclopropylcarbonylamino+ ++-2- methoxybenzamide] in comparison with putative D2-like receptor antagonists using both rat and human cloned dopamine D2-like receptors in vitro. 2. Receptor binding studies revealed that YM-43611 had appropriately potent affinities for both rat and human D2-like receptors, with moderate selectivity for D3 receptors and high selectivity for D4 receptors over D2 receptors (Ki values (nM) for rat receptors: D2, 165; D3, 35.5; D4, 1.85, and for human receptors: D2, 42.9; D3, 11.2; D4, 2.10). 3. YM-43611 displayed weak or negligible affinity for other neurotransmitter receptors, namely D1, D5, alpha(1), alpha(2), beta, 5-HT1A, 5-HT2A, 5-HT3, H1, M1 and M2 receptors. 4. Dopamine stimulated low-Km GTPase activity on membranes from Chinese hamster ovary (CHO) cells expressing the human D2-like receptor subtype. This response to dopamine of low-Km GTPase activity was inhibited by use of putative D2-like receptor antagonists. YM-43611 showed a moderate selectivity for D3 receptors (Ki = 45.5 nM) and a high selectivity for D4 receptors (Ki = 3.28 nM) over D2 receptors (Ki = 70.6 nM). 5. Dopamine inhibited forskolin-stimulated adenylate cyclase in intact CHO cells expressing the human D2-like receptor subtype. YM-43611 shifted the inhibition curve of dopamine on respective D2-like receptor subtype-mediated cyclic AMP formation to the right in a parallel fashion, showing a pA2 value of 7.42 (38.1 nM) for D2 receptors, a pKB value of 8.06 (8.68 nM) for D3 receptors, and a pA2 value of 8.42 (3.77 nM) for D4 receptors. 6. YM-43611 but not the other D2-like receptor antagonists exhibited good selectivity with respect to dual antagonism for D3 and D4 receptors in both receptor binding and functional assays. 7. These results indicate that YM-43611 is a novel D2-like receptor antagonist with high potency and selectivity for both D3 and D4 receptors. YM-43611 is therefore expected to be valuable in exploration of the physiological role of D3 and D4 receptors.     

PRESYNAPTIC REGULATION OF NEUROTRANSMITTER RELEASE IN HYPERTENSION

1. The present review discusses evidence in support of the concept that alterations in sympathetic neurotransmitter release might contribute to the pathogenesis of hypertension. 2. Studies suggest that changes in sympathetic nervous activity in both the central and peripheral nervous systems participate in blood pressure control. 3. In the periphery increased release of norepinephrine from vascular adrenergic neurons might lead to the enhanced vasoconstrictor responses and thus to an elevation in systemic blood pressure. 4. The amount of neurotransmitter release from sympathetic nerve endings can be regulated by autoregulatory systems by presynaptic receptors located on nerve terminals. 5. It has been proposed that alterations to sympathetic nervous activity of hypertension might be partially due to abnormalities in presynaptic modulation of neurotransmitter release in central and peripheral tissues. 6. This article summarizes the results of studies to evaluate presynaptic receptor functions and sympathetic neurotransmitter release in hypertension.     

Dihydrexidine--the first full dopamine D1 receptor agonist

The functional role of dopamine D(1) receptors is still controversial. One reason for this controversy is that for a long time the only available agonists for in vivo characterization of dopamine D(1) receptors were benzazepines. Among them was the prototype dopamine D(1) receptor partial agonist, SKF 38393. The lack of a selective and fully efficacious dopamine D(1) receptor agonist hampered basic research on dopamine D(1) receptors and left the potential clinical utility of dopamine D(1) receptor agonists elusive. The research situation improved when the first potent full dopamine D(1) receptor agonist dihydrexidine, a phenanthridine, was introduced in the late 1980s. In contrast to SKF 38393, dihydrexidine was shown to stimulate cyclic AMP synthesis just as well or better than dopamine, and potently displaced [(3)H]SCH 23390 from rat and monkey striatal membranes. Also, dihydrexidine was the first dopamine D(1) receptor agonist that had potent antiparkinsonian activity in a primate model of Parkinson's disease. This finding suggested clinical utility for dopamine D(1) receptor agonists in Parkinson's disease and that this utility might be critically dependent on the intrinsic efficacy of the drug. Clinical utility for dopamine D(1) receptor agonists in other central nervous disorders might also be dependent on the intrinsic efficacy of the drug. However, even though studies with dihydrexidine as a pharmacological tool have pointed to the clinical use for dopamine D(1) receptor agonists, dihydrexidine's unfavorable pharmacokinetic profile and various adverse effects are likely to restrict or even preclude its use in humans. This review article provides an updated overview of the pharmacology of dihydrexidine and discusses possible clinical utility of dopamine D(1) receptor agonists in various central nervous system disorders.     

A photoaffinity ligand for dopamine D2 receptors: azidoclebopride

In order to label D2 dopamine receptors selectively and covalently by means of a photosensitive compound, azidoclebopride was synthesized directly from clebopride. The dissociation constant (KD) of clebopride for the D2 dopamine receptor (canine brain striatum) was 1.5 nM, while that for azidoclebopride was 21 nM. The affinities of both clebopride and azidoclebopride were markedly reduced in the absence of sodium chloride. In the presence of ultraviolet light, azidoclebopride inactivated D2 dopamine receptors irreversibly, as indicated by the inability of the receptors to bind [3H]spiperone. Maximal photoinactivation of about 60% of the D2 dopamine receptors occurred at 1 microM azidoclebopride; 30% of the receptors were inactivated at 80 nM azidoclebopride (pseudo-IC50). Dopamine agonists selectively protected the D2 receptors from being inactivated by azidoclebopride, the order of potency being (-)-N-n-propylnorapomorphine greater than apomorphine greater than (+/-)-6,7-dihydroxy-2-aminotetralin greater than (+)-N-n-propylnorapomorphine greater than dopamine greater than noradrenaline greater than serotonin. Similarly, dopaminergic antagonists prevented the photoinactivation of D2 receptors by azidoclebopride with the following order of potency: spiperone greater than (+)-butaclamol greater than haloperidol greater than clebopride greater than (-)-sulpiride greater than (-)-butaclamol. The degree of D2 dopamine receptor photoinduced inactivation by azidoclebopride was not significantly affected by scavengers such as p-aminobenzoic acid and dithiothreitol. Furthermore, irradiation of striatal membranes with a concentration of azidoclebopride sufficient to inactivate dopamine D2 receptors by 60% did not significantly reduce dopamine D1, serotonin (S2), benzodiazepine, alpha 1- or beta-noradrenergic receptors. This study describes the use of a novel and selective photoaffinity ligand for brain dopamine D2 receptors. The molecule, in radiolabeled form, may aid in the molecular characterization of these receptors.     

Relevance of dopamine D(2)/neurotensin NTS1 and NMDA/neurotensin NTS1 receptor interaction in psychiatric and neurodegenerative disorders

The existence of functional NT/dopamine interactions in the central nervous system has been extensively documented. Among others, a possible molecular mechanism underlying the NT-induced modulation of dopamine release is a direct antagonistic NTS(1)/D(2) receptor interaction. More recently, neurochemical experiments also supported the existence of a possible interaction between NT and N-methyl-d-aspartate (NMDA) receptors. In particular, it has been suggested that NT, by amplifying NMDA receptor signaling, could be involved in neurodegeneration. The present article attempts to provide a summary of current knowledge, mainly emerging from our studies, on the existence of receptor-receptor interactions between NT receptor subtype 1 (NTS1) and dopamine D(2) or NMDA receptors in the brain. Special emphasis is placed on the pre and post-synaptic neurochemical mechanisms possibly underlying the involvement of these interactions in the physiopathology of schizophrenia and acute neurodegenerative disorders.     

The in vitro pharmacology of the beta-adrenergic receptor pet ligand (s)-fluorocarazolol reveals high affinity for cloned beta-adrenergic receptors and moderate affinity for the human 5-HT1A receptor

RATIONALE: s-Fluorocarazolol [(S)-FCZ] is the major positron emission tomography (PET) ligand currently used to visualize central beta-adrenergic receptors in vivo, although its pharmacology is incompletely known. OBJECTIVE: Our objective was to comprehensively characterize the in vitro pharmacology of (S)- and (R)-FCZ to determine its suitability for study of central and peripheral beta-adrenergic receptors. METHODS: We characterized the in vitro pharmacology of (S)-FCZ at 42 biogenic amine receptors and transporters in vitro using the resources of the National Institute of Mental Health Psychoactive Drug Screening Program. RESULTS: As expected (R)- and (S)-FCZ had high affinities for beta-adrenergic receptors (Ki values=0.08-0.45 nM) and negligible affinities (Ki values>100 nM) for nearly all other tested receptors and transporters with the exception of the h5-HT1A receptor for which (S)-FCZ had high affinity (Ki=34 nM). Interestingly, (R)-FCZ had low affinity for the h5-HT1A receptor (Ki=342 nM). CONCLUSION: The high affinity of (S)-FCZ for the h5-HT1A receptor is not likely to interfere with studies of peripheral beta-adrenergic receptors, since 5-HT1A receptors are expressed at very low levels outside the central nervous system. Indeed, computer simulations predict that even at low ligand concentrations, 5-HT1A binding in brain regions like hippocampus are likely to be substantial. Thus, (S)-FCZ may not be a suitable PET ligand for studies of central nervous system beta-adrenergic receptors unless the contribution by 5-HT1A sites can be shown to be negligible.