Differential expression of D2-like dopamine receptors in the kidney of the spontaneously hypertensive rat

NLRP3 Inflammasome: A central player in renal pathologies and nephropathy

The cytoplasmic oligomer NLR Family Pyrin Domain Containing 3 (NLRP3) inflammasome has been implicated in most inflammatory and autoimmune diseases. Here, we highlight the significance of NLRP3 in diverse renal disorders, demonstrating its activation in macrophages and non-immune tubular epithelial and mesangial cells in response to various stimuli. This activation leads to the release of pro-inflammatory cytokines, contributing to the development of acute kidney injury (AKI), chronic renal injury, or fibrosis. In AKI, NLRP3 inflammasome activation and pyroptotic renal tubular cell death is driven by contrast and chemotherapeutic agents, sepsis, and rhabdomyolysis. Nevertheless, inflammasome is provoked in disorders such as crystal and diabetic nephropathy, obesity-related renal fibrosis, lupus nephritis, and hypertension-induced renal damage that induce chronic kidney injury and/or fibrosis. The mechanisms by which the inflammatory NLRP3/ Apoptosis-associated Speck-like protein containing a Caspase recruitment domain (ASC)/caspase-1/interleukin (IL)-1β & IL-18 pathway can turn on renal fibrosis is also comprehended. This review further outlines the involvement of dopamine and its associated G protein-coupled receptors (GPCRs), including D1-like (D1, D5) and D2-like (D2-D4) subtypes, in regulating this inflammation-linked renal dysfunction pathway. Hence, we identify D-related receptors as promising targets for renal disease management by inhibiting the functionality of the NLRP3 inflammasome.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Inverse Salt Sensitivity of Blood Pressure Is Associated with an Increased Renin-Angiotensin System Activity

High and low sodium diets are associated with increased blood pressure and cardiovascular morbidity and mortality. The paradoxical response of elevated BP in low salt diets, aka inverse salt sensitivity (ISS), is an understudied vulnerable 11% of the adult population with yet undiscovered etiology. A linear relationship between the number of single nucleotide polymorphisms (SNPs) in the dopamine D2 receptor (DRD2, rs6276 and 6277), and the sodium myo-inositol cotransporter 2 (SLC5A11, rs11074656), as well as decreased expression of these two genes in urine-derived renal proximal tubule cells (uRPTCs) isolated from clinical study participants suggest involvement of these cells in ISS. Insight into this newly discovered paradoxical response to sodium is found by incubating cells in low sodium (LS) conditions that unveil cell physiologic differences that are then reversed by mir-485-5p miRNA blocker transfection and bypassing the genetic defect by DRD2 re-expression. The renin-angiotensin system (RAS) is an important counter-regulatory mechanism to prevent hyponatremia under LS conditions. Oversensitive RAS under LS conditions could partially explain the increased mortality in ISS. Angiotensin-II (AngII, 10 nmol/L) increased sodium transport in uRPTCs to a greater extent in individuals with ISS than SR. Downstream signaling of AngII is verified by identifying lowered expression of nuclear factor erythroid 2-related factor 2 (NRF2), CCCTC-binding factor (CTCF), and manganese-dependent mitochondrial superoxide dismutase (SOD2) only in ISS-derived uRPTCs and not SR-derived uRPTCs when incubated in LS conditions. We conclude that DRD2 and SLC5A11 variants in ISS may cause an increased low sodium sensitivity to AngII and renal sodium reabsorption which can contribute to inverse salt-sensitive hypertension.

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

Renal Dopamine Receptors and Oxidative Stress: Role in Hypertension

Significance: The kidney plays an important role in the long-term control of blood pressure. Oxidative stress is one of the fundamental mechanisms responsible for the development of hypertension. Dopamine, via five subtypes of receptors, plays an important role in the control of blood pressure by various mechanisms, including the inhibition of oxidative stress. Recent Advances: Dopamine receptors exert their regulatory function to decrease the oxidative stress in the kidney and ultimately maintain normal sodium balance and blood pressure homeostasis. An aberration of this regulation may be involved in the pathogenesis of hypertension. Critical Issues: Our present article reviews the important role of oxidative stress and intrarenal dopaminergic system in the regulation of blood pressure, summarizes the current knowledge on renal dopamine receptor-mediated antioxidation, including decreasing reactive oxygen species production, inhibiting pro-oxidant enzyme nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and stimulating antioxidative enzymes, and also discusses its underlying mechanisms, including the increased activity of G protein-coupled receptor kinase 4 (GRK4) and abnormal trafficking of renal dopamine receptors in hypertensive status. Future Directions: Identifying the mechanisms of renal dopamine receptors in the regulation of oxidative stress and their contribution to the pathogenesis of hypertension remains an important research focus. Increased understanding of the role of reciprocal regulation between renal dopamine receptors and oxidative stress in the regulation of blood pressure may give us novel insights into the pathogenesis of hypertension and provide a new treatment strategy for hypertension.

Dopamine Receptors and the Kidney: An Overview of Health- and Pharmacological-Targeted Implications

The dopaminergic system can adapt to the different physiological or pathological situations to which the kidneys are subjected throughout life, maintaining homeostasis of natriuresis, extracellular volume, and blood pressure levels. The role of renal dopamine receptor dysfunction is clearly established in the pathogenesis of essential hypertension. Its associations with other pathological states such as insulin resistance and redox balance have also been associated with dysfunction of the dopaminergic system. The different dopamine receptors (D1-D5) show a protective effect against hypertension and kidney disorders. It is essential to take into account the various interactions of the dopaminergic system with other elements, such as adrenergic receptors. The approach to therapeutic strategies for essential hypertension must go through the blocking of those elements that lead to renal vasoconstriction or the restoration of the normal functioning of dopamine receptors. D1-like receptors are fundamental in this role, and new therapeutic efforts should be directed to the restoration of their functioning in many patients. More studies will be needed to allow the development of drugs that can be targeted to renal dopamine receptors in the treatment of hypertension.

The Role of the Renal Dopaminergic System and Oxidative Stress in the Pathogenesis of Hypertension

The kidney is critical in the long-term regulation of blood pressure. Oxidative stress is one of the many factors that is accountable for the development of hypertension. The five dopamine receptor subtypes (D₁R–D₅R) have important roles in the regulation of blood pressure through several mechanisms, such as inhibition of oxidative stress. Dopamine receptors, including those expressed in the kidney, reduce oxidative stress by inhibiting the expression or action of receptors that increase oxidative stress. In addition, dopamine receptors stimulate the expression or action of receptors that decrease oxidative stress. This article examines the importance and relationship between the renal dopaminergic system and oxidative stress in the regulation of renal sodium handling and blood pressure. It discusses the current information on renal dopamine receptor-mediated antioxidative network, which includes the production of reactive oxygen species and abnormalities of renal dopamine receptors. Recognizing the mechanisms by which renal dopamine receptors regulate oxidative stress and their degree of influence on the pathogenesis of hypertension would further advance the understanding of the pathophysiology of hypertension.

Renal rescue of dopamine D2 receptor function reverses renal injury and high blood pressure

Dopamine D2 receptor (DRD2) deficiency increases renal inflammation and blood pressure in mice. We show here that long-term renal-selective silencing of Drd2 using siRNA increases renal expression of proinflammatory and profibrotic factors and blood pressure in mice. To determine the effects of renal-selective rescue of Drd2 expression in mice, the renal expression of DRD2 was first silenced using siRNA and 14 days later rescued by retrograde renal infusion of adeno-associated virus (AAV) vector with DRD2. Renal Drd2 siRNA treatment decreased the renal expression of DRD2 protein by 55%, and DRD2 AAV treatment increased the renal expression of DRD2 protein by 7.5- to 10-fold. Renal-selective DRD2 rescue reduced the expression of proinflammatory factors and kidney injury, preserved renal function, and normalized systolic and diastolic blood pressure. These results demonstrate that the deleterious effects of renal-selective Drd2 silencing on renal function and blood pressure were rescued by renal-selective overexpression of DRD2. Moreover, the deleterious effects of 45-minute bilateral ischemia/reperfusion on renal function and blood pressure in mice were ameliorated by a renal-selective increase in DRD2 expression by the retrograde ureteral infusion of DRD2 AAV immediately after the induction of ischemia/reperfusion injury. Thus, 14 days after ischemia/reperfusion injury, the renal expression of profibrotic factors, serum creatinine, and blood pressure were lower in mice infused with DRD2 AAV than in those infused with control AAV. These results indicate an important role of renal DRD2 in limiting renal injury and preserving normal renal function and blood pressure.

Inhibition of D4 Dopamine Receptors on Insulin Receptor Expression and Effect in Renal Proximal Tubule Cells

BACKGROUND

Ion transport in the renal proximal tubule (RPT), which is increased in essential hypertension, is regulated by numerous hormones and humoral factors, including insulin and dopamine. Activation of dopamine receptor inhibits sodium reabsorption, whereas activation of insulin receptor increases sodium reabsorption in RPTs, and hyperinsulinemic animals and patients have defective renal dopaminergic system. We presume that there is an inhibition of D4 receptor on insulin receptor expression and effect, and the regulation is lost in spontaneously hypertensive rats (SHRs).

METHODS AND RESULTS

Insulin receptor expression was determined by immunoblotting, and Na(+)-K(+)-ATPase activity was detected in both Wistar-Kyoto (WKY) and SHR RPT cells. Stimulation of D4 receptor with PD168077 decreased expression of insulin receptors, which was blocked in the presence of the calcium-channel blocker, nicardipine (10(-6) mol/L per 24 hours), in cell culture medium without calcium or in the presence of inositol 1,4,5-trisphosphate (IP3) receptor blocker (2-aminoethyl diphenylborinate [2-ADB]; 10(-6) mol/L per 24 hours), indicating that extracellular calcium entry and calcium release from the endoplasmic reticulum were involved in the signal pathway. Stimulation of the insulin receptor stimulated Na(+)-K(+)-ATPase activity, whereas pretreatment with PD168077 for 24 hours decreased the inhibitory effects of insulin receptor on Na(+)-K(+)-ATPase activity in WKY cells. However, in SHR cells, inhibition of D4 receptor on insulin receptor expression and effect were lost.

CONCLUSIONS

Activation of D4 receptor inhibits insulin receptor expression in RPT cells from WKY rats. The aberrant inhibition of D4 receptor on insulin receptor expression and effect might be involved in the pathogenesis of essential hypertension.

Activation of D4 dopamine receptor decreases angiotensin II type 1 receptor expression in rat renal proximal tubule cells

The dopaminergic and renin-angiotensin systems interact to regulate blood pressure. Disruption of the D4 dopamine receptor gene in mice produces hypertension that is associated with increased renal angiotensin type 1 (AT1) receptor expression. We hypothesize that the D4 receptor can inhibit AT1 receptor expression and function in renal proximal tubule cells from Wistar-Kyoto (WKY) rats, but the D4 receptor regulation of AT1 receptor is aberrant in renal proximal tubule cells from spontaneously hypertensive rats (SHRs). The D4 receptor agonist, PD168077, decreased AT1 receptor protein expression in a time- and concentration-dependent manner in WKY cells. By contrast, in SHR cells, PD168077 increased AT1 receptor protein expression. The inhibitory effect of D4 receptor on AT1 receptor expression in WKY cells was blocked by a calcium channel blocker, nicardipine, or calcium-free medium, indicating that calcium is involved in the D4 receptor-mediated signaling pathway. Angiotensin II increased Na(+)-K(+) ATPase activity in WKY cells. Pretreatment with PD168077 decreased the stimulatory effect of angiotensin II on Na(+)-K(+) ATPase activity in WKY cells. In SHR cells, the inhibitory effect of D4 receptor on angiotensin II-mediated stimulation of Na(+)-K(+) ATPase activity was aberrant; pretreatment with PD168077 augmented the stimulatory effect of AT1 receptor on Na(+)-K(+) ATPase activity in SHR cells. This was confirmed in vivo; pretreatment with PD128077 for 1 week augmented the antihypertensive and natriuretic effect of losartan in SHRs but not in WKY rats. We suggest that an aberrant interaction between D4 and AT1 receptors may play a role in the abnormal regulation of sodium excretion in hypertension.

Renal dopamine receptors, oxidative stress, and hypertension

Dopamine, which is synthesized in the kidney, independent of renal nerves, plays an important role in the regulation of fluid and electrolyte balance and systemic blood pressure. Lack of any of the five dopamine receptor subtypes (D1R, D2R, D3R, D4R, and D5R) results in hypertension. D1R, D2R, and D5R have been reported to be important in the maintenance of a normal redox balance. In the kidney, the antioxidant effects of these receptors are caused by direct and indirect inhibition of pro-oxidant enzymes, specifically, nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase, and stimulation of anti-oxidant enzymes, which can also indirectly inhibit NADPH oxidase activity. Thus, stimulation of the D2R increases the expression of endogenous anti-oxidants, such as Parkinson protein 7 (PARK7 or DJ-1), paraoxonase 2 (PON2), and heme oxygenase 2 (HO-2), all of which can inhibit NADPH oxidase activity. The D5R decreases NADPH oxidase activity, via the inhibition of phospholipase D2, and increases the expression of HO-1, another antioxidant. D1R inhibits NADPH oxidase activity via protein kinase A and protein kinase C cross-talk. In this review, we provide an overview of the protective roles of a specific dopamine receptor subtype on renal oxidative stress, the different mechanisms involved in this effect, and the role of oxidative stress and impairment of dopamine receptor function in the hypertension that arises from the genetic ablation of a specific dopamine receptor gene in mice.

Paraoxonase 2 decreases renal reactive oxygen species production, lowers blood pressure, and mediates dopamine D2 receptor-induced inhibition of NADPH oxidase

The dopamine D(2) receptor (D(2)R) regulates renal reactive oxygen species (ROS) production, and impaired D(2)R function results in ROS-dependent hypertension. Paraoxonase 2 (PON2), which belongs to the paraoxonase gene family, is expressed in various tissues, acting to protect against cellular oxidative stress. We hypothesized that PON2 may be involved in preventing excessive renal ROS production and thus may contribute to maintenance of normal blood pressure. Moreover, D(2)R may decrease ROS production, in part, through regulation of PON2. D(2)R colocalized with PON2 in the brush border of mouse renal proximal tubules. Renal PON2 protein was decreased (-33±6%) in D(2)(-/-) relative to D(2)(+/+) mice. Renal subcapsular infusion of PON2 siRNA decreased PON2 protein expression (-55%), increased renal oxidative stress (2.2-fold), associated with increased renal NADPH oxidase expression (Nox1, 1.9-fold; Nox2, 2.9-fold; and Nox4, 1.6-fold) and activity (1.9-fold), and elevated arterial blood pressure (systolic, 134±5 vs 93±6mmHg; diastolic, 97±4 vs 65±7mmHg; mean 113±4 vs 75±7mmHg). To determine the relevance of the PON2 and D(2)R interaction in humans, we studied human renal proximal tubule cells. Both D(2)R and PON2 were found in nonlipid and lipid rafts and physically interacted with each other. Treatment of these cells with the D(2)R/D(3)R agonist quinpirole (1μM, 24h) decreased ROS production (-35±6%), associated with decreased NADPH oxidase activity (-32±3%) and expression of Nox2 (-41±7%) and Nox4 (-47±8%) protein, and increased expression of PON2 mRNA (2.1-fold) and protein (1.6-fold) at 24h. Silencing PON2 (siRNA, 10nM, 48h) not only partially prevented the quinpirole-induced decrease in ROS production by 36%, but also increased basal ROS production (1.3-fold), which was associated with an increase in NADPH oxidase activity (1.4-fold) and expression of Nox2 (2.1-fold) and Nox4 (1.8-fold) protein. Inhibition of NADPH oxidase with diphenylene iodonium (10μM/30 min) inhibited the increase in ROS production caused by PON2 silencing. Our results suggest that renal PON2 is involved in the inhibition of renal NADPH oxidase activity and ROS production and contributes to the maintenance of normal blood pressure. PON2 is positively regulated by D(2)R and may, in part, mediate the inhibitory effect of renal D(2)R on NADPH oxidase activity and ROS production.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Impaired stimulatory effect of ETB receptor on D₃ receptor in immortalized renal proximal tubule cells of spontaneously hypertensive rats

BACKGROUND

Activation of renal D₃ receptor induces natriuresis and diuresis in Wistar-Kyoto (WKY) rats; in the presence of ETB receptor antagonist, the natriuretic effect of D₃ receptor in WKY rats is reduced. We hypothesize that ETB receptor activation may regulate D₃ receptor expression in renal proximal tubule (RPT) cells from WKY rats, which is impaired in RPT cells from spontaneously hypertensive rats (SHRs).

METHODS

D₃ receptor expression was determined by immunoblotting; the D₃/ETB receptor linkage was checked by coimmunoprecipitation; Na(+)-K(+)-ATPase activity was determined as the rate of inorganic phosphate released in the presence or absence of ouabain.

RESULTS

In RPT cells from WKY rats, the ETB receptor agonist BQ3020 increased D₃ receptor protein. In contrast, in RPT cells from SHRs, BQ3020 did not increase D₃ receptor. There was coimmunoprecipitation between D₃ and ETB receptors in RPT cells from WKY and SHRs. Activation of ETB receptor increased D₃/ETB coimmunoprecipitation in RPT cells from WKY rats, but not from SHRs. The basal levels of D₃/ETB receptor coimmunoprecipitation were greater in RPT cells from WKY rats than in those from SHRs. Stimulation of D₃ receptor inhibited Na(+)-K(+)-ATPase activity, which was augmented by the pretreatment with the ETB receptor agonist BQ3020 in WKY RPT cells, but not in SHR RPT cells.

CONCLUSION

ETB receptors regulate and physically interact with D₃ receptors differently in WKY rats and SHRs. The impaired natriuretic effect in SHRs may be, in part, related to impaired ETB and D₃ receptor interactions.

Intrarenal dopamine attenuates deoxycorticosterone acetate/high salt-induced blood pressure elevation in part through activation of a medullary cyclooxygenase 2 pathway

Locally produced dopamine in the renal proximal tubule inhibits salt and fluid reabsorption, and a dysfunctional intrarenal dopaminergic system has been reported in essential hypertension and experimental hypertension models. Using catechol-O-methyl-transferase knockout (COMT(-/-)) mice, which have increased renal dopamine because of deletion of the major renal dopamine-metabolizing enzyme, we investigated the effect of intrarenal dopamine on the development of hypertension in the deoxycorticosterone acetate/high-salt (DOCA/HS) model. DOCA/HS led to significant increases in systolic blood pressure in wild-type mice (from 115+/-2 to 153+/-4 mm Hg), which was significantly attenuated in COMT(-/-) mice (from 114+/-2 to 135+/-3 mm Hg). In DOCA/HS COMT(-/-) mice, the D1-like receptor antagonist SCH-23390 increased systolic blood pressure (156+/-2 mm Hg). DOCA/HS COMT(-/-) mice also exhibited more urinary sodium excretion (COMT(-/-) versus wild-type: 3038+/-430 versus 659+/-102 micromol/L per 24 hours; P<0.01). Furthermore, DOCA/HS-induced renal oxidative stress was significantly attenuated in COMT(-/-) mice. COX-2-derived prostaglandins in the renal medulla promote sodium excretion, and dopamine stimulates medullary prostaglandin production. Renal medullary COX-2 expression and urinary prostaglandin E2 excretion were significantly higher in COMT(-/-) than in wild-type mice after DOCA/HS treatment. In DOCA/HS-treated COMT(-/-) mice, the COX-2 inhibitor SC-58236 reduced urinary sodium and prostaglandin E(2) excretion and increased systolic blood pressure (153+/-2 mm Hg). These studies indicate that an activated renal dopaminergic system attenuates the development of hypertension, at least in large part through activating medullary COX-2 expression/activity, and also decreases oxidative stress resulting from DOCA/HS.

Hypoxia-induced transcription of dopamine D3 and D4 receptors in human neuroblastoma and astrocytoma cells

Background

Dopaminergic pathways that influence mood and behaviour are severely affected in cerebral hypoxia. In contrast, hypoxia promotes the differentiation of dopaminergic neurons. In order to clarify the hypoxic sensitivity of key dopaminergic genes, we aimed to study their transcriptional regulation in the context of neuroblastoma and astrocytoma cell lines exposed to 1% hypoxia.

Results

Quantitative RT-PCR assays revealed that the transcription of both type D3 and D4 postsynaptic dopamine receptors (DRD3 and DRD4) was induced several fold upon 2-day hypoxia in a cell-specific manner, while the vascular endothelial growth factor gene was activated after 3-hr incubation in hypoxia. On the other hand, mRNA levels of type 2 dopamine receptor, dopamine transporter, monoamino oxidase and catechol-O-methyltransferase were unaltered, while those of the dopamine receptor regulating factor (DRRF) were decreased by hypoxia. Notably, 2-day hypoxia did not result in elevation of protein levels of DRD3 and DRD4.

Conclusion

In light of the relatively delayed transcriptional activation of the DRD3 and DRD4 genes, we propose that slow-reacting hypoxia sensitive transcription factors might be involved in the transactivation of DRD3 and DRD4 promoters in hypoxia.

G protein-coupled receptor kinase 4 (GRK4) regulates the phosphorylation and function of the dopamine D3 receptor

During conditions of moderate sodium excess, the dopaminergic system regulates blood pressure and water and electrolyte balance by engendering natriuresis. Dopamine exerts its effects on dopamine receptors, including the dopamine D(3) receptor. G protein-coupled receptor kinase 4 (GRK4), whose gene locus (4p16.3) is linked to essential hypertension, desensitizes the D(1) receptor, another dopamine receptor. This study evaluated the role of GRK4 on D(3) receptor function in human proximal tubule cells. D(3) receptor co-segregated in lipid rafts and co-immunoprecipitated and co-localized in human proximal tubule cells and in proximal and distal tubules and glomeruli of kidneys of Wistar Kyoto rats. Bimolecular fluorescence complementation and confocal microscopy revealed that agonist activation of the receptor initiated the interaction between D(3) receptor and GRK4 at the cell membrane and promoted it intracellularly, presumably en route to endosomal trafficking. Of the four GRK4 splice variants, GRK4-gamma and GRK4-alpha mediated a 3- and 2-fold increase in the phosphorylation of agonist-activated D(3) receptor, respectively. Inhibition of GRK activity with heparin or knockdown of GRK4 expression via RNA interference completely abolished p44/42 phosphorylation and mitogenesis induced by D(3) receptor stimulation. These data demonstrate that GRK4, specifically the GRK4-gamma and GRK4-alpha isoforms, phosphorylates the D(3) receptor and is crucial for its signaling in human proximal tubule cells.

Colocalization of dopamine receptor subtypes with dopamine and cAMP-regulated phosphoprotein (DARPP-32) in rat brain

In the present study using indirect immunofluorescence immunohistochemistry, co-immunoprecipitation and western blot analysis we determined the colocalization of dopamine receptors 1-5 and dopamine and cAMP-regulated phosphoprotein (DARPP-32) in rat brain cortex and striatum. All five DR subtypes and DARPP-32 were expressed in rat brain cortex and striatum. DARPP-32 positive neurons displayed comparative colocalization with DR1-5. In cingulate cortex, the colocalization of DR subtypes was greatly different from frontal or temporal cortex. D1R is one of the most predominant subtypes which colocalized with DARPP-32 in cortex as well as striatum and followed by D2R, D3R, D4R and D5R. Amongst all DR subtypes D5R was coexpressed the least with DARPP-32 positive neurons. Consistent with immunohistochemical data, western blot analysis also reveals comparable distribution of DR subtypes and DARPP-32 in cortex and striatum. Colocalization studies were also supported by using co-immunoprecipitate assay displaying DARPP-32 expression in DR immunoprecipitate from tissue lysate prepared from cortex and striatum. Taken together our data support receptor specific association of DARPP-32 with DR subtypes that might shed new information in drugs of abuse and pathophysiology of neurodegenerative diseases as well as neuropsychiatric disorders such as schizophrenia.

Dopamine, kidney, and hypertension: studies in dopamine receptor knockout mice

Dopamine is important in the pathogenesis of hypertension because of abnormalities in receptor-mediated regulation of renal sodium transport. Dopamine receptors are classified into D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), D(4)) subtypes, all of which are expressed in the kidney. Mice deficient in specific dopamine receptors have been generated to provide holistic assessment on the varying physiological roles of each receptor subtype. This review examines recent studies on these mutant mouse models and evaluates the impact of individual dopamine receptor subtypes on blood pressure regulation.

Dysregulation of dopamine-dependent mechanisms as a determinant of hypertension: studies in dopamine receptor knockout mice

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones/humoral factors, such as aldosterone, angiotensin, catecholamines, endothelin, oxytocin, prolactin pro-opiomelancortin, reactive oxygen species, renin, and vasopressin. Dopamine receptors are classified into D(1)-like (D(1) and D(5)) and D(2)-like (D(2), D(3), and D(4)) subtypes based on their structure and pharmacology. In recent years, mice deficient in one or more of the five dopamine receptor subtypes have been generated, leading to a better understanding of the physiological role of each of the dopamine receptor subtypes. This review summarizes the results from studies of various dopamine receptor mutant mice on the role of individual dopamine receptor subtypes and their interactions with other G protein-coupled receptors in the regulation of blood pressure.

The dopaminergic system in hypertension

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport, vascular smooth muscle contractility and production of reactive oxygen species and by interacting with the renin–angiotensin and sympathetic nervous systems. Dopamine receptors are classified into D1-like (D1 and D5) and D2-like (D2, D3 and D4) subtypes based on their structure and pharmacology. Each of the dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central and/or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. This review summarizes the physiology of the different dopamine receptors in the regulation of blood pressure, and the relationship between dopamine receptor subtypes and hypertension.

Immunohistochemical localization of dopamine receptor subtypes (D1R-D5R) in Alzheimer's disease brain

Among the neurotransmitter abnormalities that have been investigated in Alzheimer's disease (AD), deficits in the cholinergic system have been the most intensively studied. Another key neurotransmitter system involved with emotion and cognition is the dopaminergic system. Here we have investigated alterations in all five dopamine receptor subtypes in AD brain. Using antipeptide rabbit antibodies for each of the five dopamine receptors (D1-D5) we mapped the distribution of these receptors in postmortem AD and age-matched control brains in the frontal cortex, utilizing biotin-avidin immunocytochemistry. All five DR subtypes were expressed as cell surface and cytoplasmic proteins. Receptor-specific changes in control and AD brain were identified as follows: D4R and D3R were the predominant receptor subtypes in age-matched controls followed by D2R and D1R; D5R is the least expressed receptor subtype. In AD brain, D2R and D5R are well expressed in comparison to D1R, D3R and D4R. Expression of D1R, D3R and D4R was severely reduced in AD cortex. D2R expression is moderately reduced in the frontal cortex of AD brain. D5R is the only receptor subtype whose expression is increased in AD frontal cortex. Furthermore, in AD, we found comparable expression of D3R in astrocytes, whereas D5R-like immunoreactivity is significantly increased in astrocytes, in comparison to normal frontal cortex, where it was predominantly neuronal. These results demonstrate subtype-specific changes in dopamine receptors in AD that may be important in disease pathophysiology and that may also serve as potential targets for therapeutic intervention in AD.

D3 dopamine receptor directly interacts with D1 dopamine receptor in immortalized renal proximal tubule cells

D3 receptors act synergistically with D1 receptors to inhibit sodium transport in renal proximal tubules; however, the mechanism by which this occurs is not known. Because dopamine receptor subtypes can regulate and interact with each other, we studied the interaction of D3 and D1 receptors in rat renal proximal tubule (RPT) cells. The D3 agonist PD128907 increased the immunoreactive expression of D1 receptors in a concentration- and time-dependent manner; these effects were blocked by the D3 antagonist U99194A. PD128907 also transiently (15 minutes) increased the amount of cell surface membrane D1 receptors. Laser confocal immunofluorescence microscopy showed that D3 receptor and D1 receptor colocalized in RPT cells more distinctly in Wistar-Kyoto rats than in spontaneously hypertensive rats (SHRs). In addition, D3 and D1 receptors could be coimmunoprecipitated, and this interaction was increased after D3 receptor agonist stimulation for 24 hours in Wistar-Kyoto rats but not in SHRs. We propose that the synergistic effects of D3 and D1 receptors may be caused by a D3 receptor-mediated increase in total, as well as cell surface membrane D1 receptor expression, and direct D3 and D1 receptor interaction, both of which are impaired in SHRs.

Dopamine D3 receptor mRNA and renal response to D3 receptor activation in spontaneously hypertensive rats

Defective dopamine receptors may be involved in the development of hypertension. Recently, it has been shown that gene expression and function of the renal dopamine D3 receptor is impaired in salt-sensitive Dahl rats, a model of salt-dependent hypertension. Here, the functional response to D3 receptor activation was investigated in spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto rats (WKY). In addition, expression of the D3 receptor gene was studied in both rat strains. In clearance experiments, Ringer solution was infused at baseline in thiopental-anesthetized SHR and WKY (each n = 8), followed by an infusion of R(+)-7-hydroxy-dipropylaminotetralin (DPAT), a specific D3 receptor agonist. DPAT was infused in two consecutive doses of 0.01 and 0.1 microg/min per kg body weight. During the entire experiment mean arterial blood pressure was significantly higher (1.5-fold) in adult SHR when compared to age-matched WKY. In both groups DPAT infusion induced a similar dose-dependent increase in urinary flow rate and sodium excretion by a maximum of 2.3-fold and 3.5-fold, respectively. DPAT also increased the glomerular filtration rate in both SHR and WKY. Reverse transcription-polymerase chain reaction studies of whole kidney samples showed no significant differences between young prehypertensive and adult hypertensive SHR when compared to age-matched normotensive WKY. In summary, pharmacological dopamine D3 receptor activation induces a uniform renal response in SHR and WKY. Together with the similar D3 receptor gene expression in both rat strains, which is independent of age or blood pressure levels, the results do not support the notion that the dopamine D3 receptor system is involved in the pathogenesis of hypertension in the SHR model.