Induction of G protein-independent agonist high-affinity binding sites of D-1 dopamine receptors by beta-mercaptoethanol

Differential subcellular distribution of rat brain dopamine receptors and subtype-specific redistribution induced by cocaine

We investigated the subcellular distribution of dopamine D(1), D(2) and D(5) receptor subtypes in rat frontal cortex, and examined whether psychostimulant-induced elevation of synaptic dopamine could alter the receptor distribution. Differential detergent solubilization and density gradient centrifugation were used to separate various subcellular fractions, followed by semi-quantitative determination of the relative abundance of specific receptor proteins in each fraction. D(1) receptors were predominantly localized to detergent-resistant membranes, and a portion of these receptors also floated on sucrose gradients. These properties are characteristic of proteins found in lipid rafts and caveolae. D(2) receptors exhibited variable distribution between cytoplasmic, detergent-soluble and detergent-resistant membrane fractions, yet were not present in buoyant membranes. Most D(5) receptor immunoreactivity was distributed into the cytoplasmic fraction, failing to sediment at forces up to 300,000g, while the remainder was localized to detergent-soluble membranes in cortex. D(5) receptors were undetectable in detergent-resistant fractions or raft-like subdomains. Following daily cocaine administration for seven days, a significant portion of D(1) receptors translocated from detergent-resistant membranes to detergent-soluble membranes and the cytoplasmic fraction. The distributions of D(5) and D(2) receptor subtypes were not significantly altered by cocaine treatment. These data imply that D(5) receptors are predominantly cytoplasmic, D(2) receptors are diffusely distributed within the cell, whereas D(1) receptors are mostly localized to lipid rafts within the rat frontal cortex. Dopamine receptor subtype localization is susceptible to modulation by pharmacological manipulations that elevate synaptic dopamine, however the functional implications of such drug-induced receptor warrant further investigation.

Mitogen-activated protein kinase upregulation reduces renal D1 receptor affinity and G-protein coupling in obese rats

Reactive oxygen species play a key role in pathophysiology of cardiovascular diseases by modulating G-protein-coupled receptor signaling. We have shown that treatment of animal models of diabetes and aging with tempol decreases oxidative stress and restores renal dopamine D1 receptor (D1R) function. In present study, we determined whether oxidation of D1R and upregulation of mitogen-activated protein kinases (MAPK) were responsible for decreased D1R signaling in obese animals. Male lean and obese Zucker rats were supplemented with antioxidants tempol or lipoic acid for 2 weeks. Compared to lean, obese animals were hyperglycemic and hyperinsulinemic with increased oxidative stress, D1R oxidation and decreased glutathione levels. These animals had decreased renal D1R affinity and basal coupling to G-proteins. SKF-38393, a D1R agonist failed to stimulate G-proteins and adenylyl cyclase. Obese animals showed marked increase in renal MAPK activities. Treatment of obese rats with tempol or lipoic acid decreased blood glucose, reduced oxidative stress, and restored the basal D1R G-protein coupling. Antioxidants also normalized MAPK activities and restored D1R affinity and SKF-38393 induced D1R G-protein coupling and adenylyl cyclase stimulation. These studies show that D1R oxidation and MAPK upregulation contribute to D1R dysfunction in obese animals. Consequently, antioxidants while reducing the oxidative stress normalize the MAPK activities and restore D1R signaling.

Modulation of D1-like dopamine receptor function by aldehydic products of lipid peroxidation

Growing evidence indicates that aldehydic products of lipid peroxidation play an important role in the pathophysiology of neurodegenerative disorders such as Parkinson's disease. In the present study, modulation of D1-like receptor binding and function by saturated alkanals and unsaturated alkenals, 4-hydroxynonenal (4-HNE) and trans-2-nonenal (nonenal), was examined in rat striatal membranes. The 4-HNE and nonenal were most effective in modulating both the specific D1-like receptor binding and function as measured by adenylate cyclase activation. Inactivation of receptor binding and the depression of adenylate cyclase activity were partially prevented by protection of the D1/D5-receptor with the agonist (R)-SKF 38393 or the specific antagonist SCH 23390. 4-HNE inhibited adenylate cyclase activation by Gpp (NH)p and forskolin, indicating the modulation of Gsalpha and the catalytic subunit of adenylate cyclase, respectively. Our data suggests that aldehydic products of lipid peroxidation can directly modulate the binding and functional properties of D1/D5 receptors, as well as effector proteins within their signaling pathway.

Regulation of human D1 dopamine receptor function and gene expression in SK-N-MC neuroblastoma cells

SK-N-MC human neuroblastoma cells express functional D1, but not D5, dopaminergic receptors. Stimulating cells with dopamine or the D1-selective agonist, SKF R-38393, rapidly (t(1/2) = 1 h) resulted in > 95% attenuation of dopamine-mediated accumulation of cyclic AMP, without any change in D1 dopamine receptor levels. Prolonged (> 4 h) exposure of cells to dopamine attenuated D1 receptor levels to 45-50% of control (t(1/2) = 8 h) and was accompanied by a loss of high-affinity binding sites. At the molecular level, the expression of D1 receptor messenger RNA was bimodal: an initial increase (by approximately 60%) of receptor messenger RNA within 2 h of treatment of cells with dopamine was followed by a decline to 50% below control messenger RNA levels. Low concentrations (1-10 nM) of dopamine also potentiated D1 messenger RNA levels (up to 48%), resulting in a twofold increase in receptor levels. Transfection studies with the cloned human D1 promoter construct, pGL-D1P, indicated that the up-regulation of D1 messenger RNA was due to activation of promoter by dopamine. The dopamine-mediated up-regulation of both D1 receptor messenger RNA and promoter was prevented by the D1-selective antagonist, SCH 23390. The results suggest that dopamine regulates D1 receptor gene and protein expression in a bimodal manner, partly through activation of the receptor promoter. Moreover, the effects of dopamine are independent of the second messenger, cyclic AMP.

Increased oxidative stress in renal proximal tubules of the spontaneously hypertensive rat: a mechanism for defective dopamine D1A receptor/G-protein coupling

AIM

Defective dopamine D1A dopamine receptor/G-protein coupling has been demonstrated in renal proximal tubules of the spontaneously hypertensive rat (SHR). In the present study, we aimed to analyze the underlying mechanisms through which such defects are introduced into the D1A receptor protein of SHR.

MATERIALS AND METHODS

The oxidative state of SHR proximal tubules was analyzed by measuring lipid peroxidation. D1A receptor/G-protein coupling was measured following the induction of oxidative stress in normotensive Wistar-Kyoto (WKY) rats.

RESULTS

For the first time, an increased state of oxidative stress was demonstrated in SHR proximal tubules compared with those of normotensive controls, WKY and Sprague-Dawley rats. Lipid peroxidation levels in SHR were significantly higher by 66 and 79%, relative to WKY or Sprague-Dawley rats, respectively. Hydrogen peroxide treatment of proximal tubules from SHR, WKY and Sprague-Dawley rats induced an additional increase in lipid peroxidation in a dose-dependent manner, although the percentage induction was lower in SHR than in WKY and Sprague-Dawley rats. This induction of lipid peroxidation in WKY rats resulted in a loss of D1A/G-protein coupling, with no decrease in receptor protein. Treatment of WKY rat proximal tubules with an antioxidant, ascorbic acid, or a reducing agent, dithiothreitol, induced D1A receptor/G-protein coupling.

CONCLUSIONS

These data indicate that D1A receptor/G-protein coupling is modulated by changes in redox states. Therefore, the D1A receptor/G-protein coupling in SHR may have been damaged by reactive oxygen species released as a result of the elevated oxidative stress seen in the proximal tubules.

The human D1A dopamine receptor: heterologous expression in Saccharomyces cerevisiae and purification of the functional receptor

Functional human D1A dopamine receptor has been expressed in Saccharomyces cerevisiae. The primary sequence of the receptor was modified to include two affinity tags at the C-terminus of the protein, a FLAG tag (DYKDDDDK), and a His6 tag (HHHHHH). These modifications allowed for purification to near homogeneity using immobilized metal affinity chromatography and immunoaffinity chromatography. Radioligand binding demonstrated that the purified and reconstituted receptor binds the antagonist [3H]SCH23390 with an affinity (KD = 8.0 +/- 3.2 nM) comparable to that of the native receptor.

Differential regulation of melatonin receptors in sheep, chicken and lizard brains by cholera and pertussis toxins and guanine nucleotides

G-proteins define both the pharmacological characteristics and the signalling pathways of G-protein-coupled receptors. Melatonin receptors have been shown to belong to this class of receptors through their sensitivity to modulators of G-protein function. This study reveals that 2-125I-iodomelatonin (125I-MEL) binding to different target tissues is differentially affected by agents which disrupt the G-protein cycle. GTP gamma S, pertussis (PTX) and cholera (CTX) toxins each reduce 125I-MEL binding to ovine pars tuberalis (oPT) and lizard brain membranes, whereas chicken brain is affected only by GTP gamma S (guanosine 5'-O-(3-thiotriphosphate)) and CTX. In contrast, high affinity binding of 125I-MEL in the ovine hippocampus was not affected by any of these agents. This finding, together with the fact that neural binding sites of the sheep brain were found to have markedly lower molecular mass than those of the oPT on native gel electrophoresis (365 vs 525 kDa), suggests that the neural 125I-MEL binding sites in sheep may not be G-protein coupled. Pharmacologically, however, the binding sites in the hippocampus and oPT could not be distinguished using 11 analogues of melatonin. Therefore, these data support the notion not only of multiple forms of melatonin receptor/G-protein complex, but of high affinity binding sites for 125I-MEL which do not display sensitivity to guanine nucleotides.

Coupling of human D-1 dopamine receptors to different guanine nucleotide binding proteins. Evidence that D-1 dopamine receptors can couple to both Gs and G(o)

Coupling between D-1 dopamine receptors and G proteins in cell lines expressing human D-1 receptors and different G proteins was examined. Pertussis toxin (PTX) treatment of rat pituitary GH4C1 cells significantly reduced, but did not abolish, agonist high affinity binding sites of the D-1 dopamine receptor; in SK-N-MC neuroblastoma cells, PTX failed to have any effect on D-1 high affinity sites. Cholera toxin (CTX) treatment of GH4C1 cells reduced but did not abolish the high affinity sites of D-1 receptors, while in SK-N-MC cells, treatment with CTX abolished all the high affinity sites. Western blot analyses with specific antisera indicated that Gs alpha, Gi1 alpha, Gi3 alpha, and Gq alpha were expressed in both cell lines, while Gi2 alpha and G(o) alpha were expressed in GH4C1 but not SK-N-MC cells. Antisera NEI-805 (anti-Gs alpha) and 9072 (anti-G(o) alpha) immunoprecipitated 24 +/- 4.3 and 34.4 +/- 6.9%, respectively, of G protein-associated D-1 dopamine receptors. Antisera 3646 (anti-Gi1 alpha), 1521 (anti-Gi2 alpha), 1518 (anti-Gi3 alpha), and 0941 (anti-Gq alpha) failed to coimmunoprecipitate appreciable levels of soluble receptors. These data indicate that D-1 dopamine receptors are coupled to both Gs alpha and G(o) alpha but not to Gq alpha.