Zinc modulates antagonist interactions with D2-like dopamine receptors through distinct molecular mechanisms.
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Recently, zinc has been shown to modulate antagonist drug interactions with the D1 dopamine receptor (Schetz and Sibley, 1997) and the dopamine transporter (Norregaard et al., 1998). We now demonstrate that zinc also reversibly and dose-dependently modulates the specific binding of the butyrophenone antagonist [3H]methylspiperone to all D2-like dopamine receptors: D2L, D3, and D4. The molecular mechanisms of zinc regulation of these D2-like receptor subtypes are distinct because zinc inhibition of [3H]methylspiperone binding to the D4 receptor is noncompetitive by both equilibrium and kinetic measures (lower Bmax and essentially no change in koff), whereas the corresponding inhibition of zinc at D2L and D3 receptors is primarily characterized by competitive allosterism (increases in KD and koff). Interestingly, thermodynamic measurements reveal that the macroscopic properties of zinc binding are entropy-driven for all receptor subtypes, despite their having distinct molecular mechanisms. Zinc also reduces the binding affinity of the D2L receptor for [3H]raclopride, a structurally different antagonist of the substituted benzamide class. Sodium ions negatively modulate zinc inhibition of both sodium-insensitive [3H]methylspiperone binding and sodium-sensitive [3H]raclopride binding. In addition to its demonstrated effects on antagonist binding in membrane preparations, zinc also retards the functional effects of antagonist at the D2L receptor in intact cells. These findings suggest that synaptic zinc may be a factor influencing the effectiveness of therapies that rely on dopamine receptor antagonists. (+info)
Dopamine D4 receptor polymorphism and idiopathic Parkinson's disease.
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Patients with idiopathic Parkinson's disease (IPD) are described as having markedly decreased novelty seeking characteristics. Since recent publications suggest an association between the dopamine D4 receptor polymorphism and novelty seeking, we investigated this polymorphism in a group of 122 patients with IPD and 127 healthy control subjects. We found similar allele and genotype frequencies in both groups and no association with the age of onset of symptoms. Therefore, the dopamine D4 receptor polymorphism does not confer genetic susceptibility to IPD and cannot explain the decreased novelty seeking in IPD patients. (+info)
The dopamine D2, but not D3 or D4, receptor subtype is essential for the disruption of prepulse inhibition produced by amphetamine in mice.
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Brain dopamine (DA) systems are involved in the modulation of the sensorimotor gating phenomenon known as prepulse inhibition (PPI). The class of D2-like receptors, including the D2, D3, and D4 receptor subtypes, have all been implicated in the control of PPI via studies of DA agonists and antagonists in rats. Nevertheless, the functional relevance of each receptor subtype remains unclear because these ligands are not specific. To determine the relevance of each receptor subtype, we used genetically altered strains of "knock-out" mice lacking the DA D2, D3, or D4 receptors. We tested the effects of each knock-out on both the phenotypic expression of PPI and the disruption of PPI produced by the indirect DA agonist d-amphetamine (AMPH). No phenotypic differences in PPI were observed at baseline. AMPH significantly disrupted PPI in the D2 (+/+) mice but had no effect in the D2 (-/-) mice. After AMPH treatment, both DA D3 and D4 receptor (+/+) and (-/-) mice had significant disruptions in PPI. These findings indicate that the AMPH-induced disruption of PPI is mediated via the DA D2 receptor and not the D3 or D4 receptor subtypes. Uncovering the neural mechanisms involved in PPI will further our understanding of the substrates of sensorimotor gating and could lead to better therapeutics to treat gating disorders, such as schizophrenia. (+info)
Dopamine D4 receptor gene: novelty or nonsense?
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Although the role of genetics in personality has been studied extensively at a phenomenological level, only lately has the investigation of specific genes been performed. Recent reports suggest that DNA variants of the dopamine D4 receptor gene (DRD4) are associated with the personality trait of novelty seeking; however, others fail to replicate this finding. Such conflicting results suggest either a weak effect, an association only in certain populations, or a false-positive resulting from population stratification. We provide a critical analysis of genetic studies of DRD4 variants with novelty seeking, alcoholism, drug abuse, and attention deficit hyperactivity disorder. Evidence for the role of DRD4 in novelty seeking is inconclusive, with a number of methodological concerns. Use of more conservative statistical criteria for significance, employing gene haplotypes, as well as linkage disequilibrium studies, are recommended. The molecular biology of the D4 gene is also reviewed. (+info)
High-voltage-activated calcium current and its modulation by dopamine D4 and pituitary adenylate cyclase activating polypeptide receptors in cerebellar granule cells.
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Cerebellar granule cells were a good mold for electrophysiologic studies at the single neuron level. Two distinct types of high-voltage-activated Ca2+ channels were present in cerebellar granule cells. These calcium channels change their expression, gating, and pharmacological properties during development, suggesting that calcium channel must be related to the processes of granule cell maturation and excitability. Dopamine inhibited L-type calcium current by activating D4 receptor, and this effect might involve another signaling system with the exception of cAMP system. The functional D4 receptor discovered in cerebellum not only gave a possibility to find other antipsychotics, but also supported the existence of a dopaminergic system in the granule cell involving the D4 receptor. Pituitary adenylate cyclase activating polypeptide (PACAP) could increase intracellular Ca2+ content by activation of Ca2+ channel and mobilization of intracellular Ca2+ stores. The effects were also cAMP-independent. Activating Ca2+ currents might be an important and necessary role of PACAP as a neurotropic factor involved in the control of multiplication, differentiation, and migration of granule cells. (+info)
Pharmacological characterization of extracellular acidification rate responses in human D2(long), D3 and D4.4 receptors expressed in Chinese hamster ovary cells.
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This study characterized pharmacologically the functional responses to agonists at human dopamine D2(long) (hD2), D3 (hD3) and D4.4 (hD4) receptors separately expressed in cloned cells using the cytosensor microphysiometer. Dopaminergic receptor agonists caused increases in extracellular acidification rate in adherent Chinese hamster ovary (CHO) clones expressing hD2, hD3 or hD4 receptors. Acidification rate responses to agonists in other cell lines expressing these receptors were smaller than those in adherent CHO cells. The time courses and maximum increases in acidification rate of the agonist responses in adherent CHO cells were different between the three dopamine receptor clones. Responses were blocked by pretreatment of cells with pertussis toxin or amiloride analogues. Most agonists had full intrinsic activity at each of the dopamine receptor subtypes, as compared to quinpirole, however both enantiomers of UH-232 and (-)3-PPP were partial agonists in this assay system. The functional potency of full agonists at each of the three receptors expressed in CHO cells was either higher than, or similar to, the apparent inhibition constants (Ki) determined in [125I]-iodosulpride competition binding studies. Functional selectivities of the agonists were less than radioligand binding selectivities. The rank orders of agonist potencies and selectivities were similar, but not identical, to the rank orders of radioligand binding affinities and selectivities. The dopamine receptor antagonists, iodosulpride and clozapine, had no effect on basal acidification rates but inhibited acidification responses in CHO cells to quinpirole in an apparently competitive manner. Antagonist potencies closely matched their radioligand binding affinities in these cells. (+info)
Comparison of the functional potencies of ropinirole and other dopamine receptor agonists at human D2(long), D3 and D4.4 receptors expressed in Chinese hamster ovary cells.
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1. The aim of the present study was to characterize functional responses to ropinirole, its major metabolites in man (SKF-104557 (4-[2-(propylamino)ethyl]-2-(3H) indolone), SKF-97930 (4-carboxy-2-(3H) indolone)) and other dopamine receptor agonists at human dopamine D2(long) (hD2), D3 (hD3) and D4.4 (hD4) receptors separately expressed in Chinese hamster ovary cells using microphysiometry. 2. All the receptor agonists tested (ropinirole, SKF-104557, SKF-97930, bromocriptine, lisuride, pergolide, pramipexole, talipexole, dopamine) increased extracellular acidification rate in Chinese hamster ovary clones expressing the human D2, D3 or D4 receptor. The pEC50s of ropinirole at hD2, hD3 and hD4 receptors were 7.4, 8.4 and 6.8, respectively. Ropinirole is therefore at least 10 fold selective for the human dopamine D3 receptor over the other D2 receptor family members. 3. At the hD2 and hD3 dopamine receptors all the compounds tested were full agonists as compared to quinpirole. Talipexole and the ropinirole metabolite, SKF-104557, were partial agonists at the hD4 receptor. 4. Bromocriptine and lisuride had a slow onset of agonist action which precluded determination of EC50s. 5. The rank order of agonist potencies was dissimilar to the rank order of radioligand binding affinities at each of the dopamine receptor subtypes. Functional selectivities of the dopamine receptor agonists, as measured in the microphysiometer, were less than radioligand binding selectivities. 6. The results show that ropinirole is a full agonist at human D2, D3 and D4 dopamine receptors. SKF-104557 the major human metabolite of ropinirole, had similar radioligand binding affinities to, but lower functional potencies than, the parent compound. (+info)
Receptor density as a factor governing the efficacy of the dopamine D4 receptor ligands, L-745,870 and U-101958 at human recombinant D4.4 receptors expressed in CHO cells.
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1. The relationships between the density of dopamine D4.4 receptors and the agonist efficacies of L-745,870 (3-(4-[4-chlorophhenyl]piperazin-1-yl)-methyl-1H-pyrrolo [2, 3-b]pyridine) and U-101958 ((1-benzyl-piperidin-4-yl)-(3-isopropoxy-pyridin-2-yl)-methyl-a min e) were investigated in Chinese hamster ovary (CHO) cells, after treatment with the gene expression enhancer, sodium butyrate. 2. In CHO cells expressing D4.4 receptors (CHO/D4 cells), dopamine inhibited forskolin-stimulated cyclic AMP accumulation (Emax 56+/-1% inhibition, pEC50 7.4+/-0.1, n=10). U-101958 behaved as a partial agonist (39+/-7% the efficacy of dopamine, pEC50 8.1+/-0.3, n=4), whereas L-745,870 had no detectable agonist effect. 3. Receptor density, as estimated by [3H]-spiperone saturation binding was 240+/-30 fmol mg-1 protein (n=8) in CHO/D4 cell homogenates. It reached 560+/-150 (n=6), 1000+/-190 (n=4) and 840+/-120 (n=4) fmol mg-1 protein after treatment with sodium butyrate (5 mM) for 6, 18 and 48 h, respectively. 4. The increase in receptor density was associated with a gradual enhancement of the agonist effects (increased Emax and pEC50 values) of dopamine. The efficacy of U-101958 (relative to dopamine) doubled and L-745,870 was turned into a partial agonist (efficacy 49% relative to dopamine, pEC50 8. 6+/-0.2, n=6, after 48 h treatment with sodium butyrate). These agonist effects of U-101958 and L-745,870 could be antagonized by spiperone (0.1 microM) but not by raclopride (10 microM). 5. The results show that U-101958 and L-745,870 are partial agonists at human dopamine D4.4 receptors expressed in CHO cells. Their efficacy is governed by receptor density. Agonist effects of these two compounds in vivo cannot be excluded under circumstances of increased receptor levels. (+info)