Dopamine D(2) receptor activation causes mitogenesis via p44/42 mitogen-activated protein kinase in opossum kidney cells

Protective Role of Sarpogrelate in Combination with Bromocriptine and Cabergoline for Treatment of Diabetes in Alloxan-induced Diabetic Rats

BACKGROUND

Although dopamine D2 receptor agonists, bromocriptine and cabergoline, are notable medications in the treatment of Parkinsonism, hyperprolactinemia, and hyperglycemia, there is an identified relationship between the utilization of D2-like R agonists and the progress of myocardial injury, especially in the early phase of therapy.

OBJECTIVE

This investigation aimed to examine the potential activity of sarpogrelate (a 5-hydroxytryptamine 2A [5-HT2A] receptor blocker) in reducing myocardial injury prompted by extended haul utilization of D2 receptor agonists in a model of diabetic rats.

METHODS

In the in vivo studies, both bromocriptine and cabergoline were managed independently and combined with sarpogrelate for about a month in diabetic nephropathy rats. Blood glucose level and other myocardial biochemical parameters were estimated. The probable mechanism for insulin secretagogue action was evaluated through in vitro isolated islets study. Sodium/potassium-adenosine triphosphatase activity was assayed in an isolated microsomal fraction of the renal cortex. Isolated perfused rat hearts were treated with different doses of dopamine before and after being subjected to the tested drugs, dose response of heart rate, and heart contractility were recorded.

RESULTS

Bromocriptine and cabergoline created a significant reduction in blood glucose level without any action on insulin secretagogues. Bromocriptine prevented the loss of sodium/potassium-adenosine triphosphatase activity in the cortex of an ischemic kidney. Treatment of bromocriptine or cabergoline with sarpogrelate altogether decreased the levels of the elevated myocardial biomarkers in serum. Administration of different doses of dopamine in presence of bromocriptine or capergoline resulted in significantly rising in the heart rate percentage comparing to dopamine alone. A mix of bromocriptine or cabergoline with sarpogrelate diminished both heart rate and contractility, respectively.

CONCLUSIONS

The examination demonstrated that the combined use of sarpogrelate with bromocriptine or cabergoline decreased the potential adverse effects of these 2 drugs on myocardial tissues.

Selective blockade of spinal D2DR by levo-corydalmine attenuates morphine tolerance via suppressing PI3K/Akt-MAPK signaling in a MOR-dependent manner

Morphine tolerance remains a challenge in the management of chronic pain in the clinic. As shown in our previous study, the dopamine D2 receptor (D2DR) expressed in spinal cord neurons might be involved in morphine tolerance, but the underlying mechanisms remain to be elucidated. In the present study, selective spinal D2DR blockade attenuated morphine tolerance in mice by inhibiting phosphatidylinositol 3 kinase (PI3K)/serine–threonine kinase (Akt)-mitogen activated protein kinase (MAPK) signaling in a μ opioid receptor (MOR)-dependent manner. Levo-corydalmine (l-CDL), which exhibited micromolar affinity for D2DR in D2/CHO-K1 cell lines in this report and effectively alleviated bone cancer pain in our previous study, attenuated morphine tolerance in rats with chronic bone cancer pain at nonanalgesic doses. Furthermore, the intrathecal administration of l-CDL obviously attenuated morphine tolerance, and the effect was reversed by a D2DR agonist in mice. Spinal D2DR inhibition and l-CDL also inhibited tolerance induced by the MOR agonist DAMGO. l-CDL and a D2DR small interfering RNA (siRNA) decreased the increase in levels of phosphorylated Akt and MAPK in the spinal cord; these changes were abolished by a PI3K inhibitor. In addition, the activated Akt and MAPK proteins in mice exhibiting morphine tolerance were inhibited by a MOR antagonist. Intrathecal administration of a PI3K inhibitor also attenuated DAMGO-induced tolerance. Based on these results, l-CDL antagonized spinal D2DR to attenuate morphine tolerance by inhibiting PI3K/Akt-dependent MAPK phosphorylation through MOR. These findings provide insights into a more versatile treatment for morphine tolerance.

By blocking dopamine receptors located in the spinal cord, a compound found in a traditional Chinese herbal medicine may help mitigate tolerance to morphine, a common problem among cancer patients who regularly take the opioid painkiller. A team led by Ji-Hua Liu and Bo-Yang Yu from China Pharmaceutical University in Nanjing had previously showed that inhibiting dopamine D2 receptors in spinal neurons prevented mice from developing morphine tolerance, but it wasn’t clear why. They have now demonstrated that blocking D2 receptors prevents the relay of cellular signals from morphine-binding “μ-opioid” receptors to mediators of drug tolerance. Levo-corydalmine, a compound isolated from the Asian Corydalis plant, binds and inhibits D2 receptors. When administered directly into the spinal cords of mice and rats, it blocked downstream signaling, reducing morphine tolerance.

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.

Static magnetic field exposure reproduces cellular effects of the Parkinson's disease drug candidate ZM241385

Background

This study was inspired by coalescing evidence that magnetic therapy may be a viable treatment option for certain diseases. This premise is based on the ability of moderate strength fields (i.e., 0.1 to 1 Tesla) to alter the biophysical properties of lipid bilayers and in turn modulate cellular signaling pathways. In particular, previous results from our laboratory (Wang et al., BMC Genomics, 10, 356 (2009)) established that moderate strength static magnetic field (SMF) exposure altered cellular endpoints associated with neuronal function and differentiation. Building on this background, the current paper investigated SMF by focusing on the adenosine A_2A receptor (A_2AR) in the PC12 rat adrenal pheochromocytoma cell line that displays metabolic features of Parkinson's disease (PD).

Methodology and Principal Findings

SMF reproduced several responses elicited by ZM241385, a selective A_2AR antagonist, in PC12 cells including altered calcium flux, increased ATP levels, reduced cAMP levels, reduced nitric oxide production, reduced p44/42 MAPK phosphorylation, inhibited proliferation, and reduced iron uptake. SMF also counteracted several PD-relevant endpoints exacerbated by A_2AR agonist CGS21680 in a manner similar to ZM241385; these include reduction of increased expression of A_2AR, reversal of altered calcium efflux, dampening of increased adenosine production, reduction of enhanced proliferation and associated p44/42 MAPK phosphorylation, and inhibition of neurite outgrowth.

Conclusions and Significance

When measured against multiple endpoints, SMF elicited qualitatively similar responses as ZM241385, a PD drug candidate. Provided that the in vitro results presented in this paper apply in vivo, SMF holds promise as an intriguing non-invasive approach to treat PD and potentially other neurological disorders.

The dopamine D4 receptor: biochemical and signalling properties

Dopamine is an important neurotransmitter that regulates several key functions in the brain, such as motor output, motivation and reward, learning and memory, and endocrine regulation. Dopamine does not mediate fast synaptic transmission, but rather modulates it by triggering slow-acting effects through the activation of dopamine receptors, which belong to the G-protein-coupled receptor superfamily. Besides activating different effectors through G-protein coupling, dopamine receptors also signal through interaction with a variety of proteins, collectively termed dopamine receptor-interacting proteins. We focus on the dopamine D4 receptor, which contains an important polymorphism in its third intracellular loop. This polymorphism has been the subject of numerous studies investigating links with several brain disorders, such as attention-deficit hyperactivity disorder and schizophrenia. We provide an overview of the structure, signalling properties and regulation of dopamine D4 receptors, and briefly discuss their physiological and pathophysiological role in the brain.

Reactive oxygen species and dopamine receptor function in essential hypertension

Essential hypertension is a major risk factor for stroke, myocardial infarction, and heart and kidney failure. Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones and humoral factors. However, the mechanisms leading to impaired dopamine receptor function in hypertension states are not clear. Compelling experimental evidence indicates a role of reactive oxygen species (ROS) in hypertension, and there are increasing pieces of evidence showing that in conditions associated with oxidative stress, which is present in hypertensive states, dopamine receptor effects, such as natriuresis, diuresis, and vasodilation, are impaired. The goal of this review is to present experimental evidence that has led to the conclusion that decreased dopamine receptor function increases ROS activity and vice versa. Decreased dopamine receptor function and increased ROS production, working in concert or independent of each other, contribute to the pathogenesis of essential hypertension.

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.

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.

Inhibitory effect of D1-like and D3 dopamine receptors on norepinephrine-induced proliferation in vascular smooth muscle cells

The sympathetic nervous system plays an important role in the regulation of blood pressure. There is increasing evidence for positive and negative interactions between dopamine and adrenergic receptors; the activation of the alpha-adrenergic receptor induces vasoconstriction, whereas the activation of dopamine receptor induces vasorelaxation. We hypothesize that the D1-like receptor and/or D3 receptor also inhibit alpha1-adrenergic receptor-mediated proliferation in vascular smooth muscle cells (VSMCs). In this study, VSMC proliferation was determined by measuring [3H]thymidine incorporation, cell number, and uptake of 3-(4,5-dimethylthiazol-2-yl)-diphenyltetrazolium bromide (MTT). Norepinephrine increased VSMC number and MTT uptake, as well as [3H]thymidine incorporation via the alpha1-adrenergic receptor in aortic VSMCs from Sprague-Dawley rats. The proliferative effects of norepinephrine were attenuated by the activation of D1-like receptors or D3 receptors, although a D1-like receptor agonist, fenoldopam, and a D3 receptor agonist, PD-128907, by themselves, at low concentrations, had no effect on VSMC proliferation. Simultaneous stimulation of both D1-like and D3 receptors had an additive inhibitory effect. The inhibitory effect of D3 receptor was via protein kinase A, whereas the D1-like receptor effect was via protein kinase C-zeta. The interaction between alpha1-adrenergic and dopamine receptors, especially D1-like and D3 receptors in VSMCs, could be involved in the pathogenesis of hypertension.

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.

Effect of pre-treatment with catecholamines on cold preservation and ischemia/reperfusion-injury in rats

Treatment of organ donors with catecholamines reduces acute rejection episodes and improves long-term graft survival after renal transplantation. The aim of this study was to investigate the effect of catecholamine pre-treatment on ischemia/reperfusion (I/R)- and cold preservation injury in rat kidneys. I/R-injury was induced by clamping the left kidney vessels for 60 min along with a contralateral nephrectomy. Cold preservation injury was induced by storage of the kidneys for 24 h at +4 degrees Celsius in University of Wisconsin solution, followed by syngeneic transplantation. Rats were pre-treated with either dopamine (DA), dobutamine (DB), or norepinephrine (2, 5, and 10 microg/kg/min, each group) intravenously via an osmotic minipump for 24 h before I/R- and cold preservation injury. Pre-treatment with DA (2 or 5 microg/kg/min) and DB (5 microg/kg/min) improved recovery of renal function after I/R-injury and dose dependently reduced mononuclear and major histocompatibility complex class II-positive cells infiltrating the kidney after I/R-injury. One day after I/R-injury, upregulation of transforming growth factor (TGF)-beta 1 and 2 and phosphorylation of p42/p44 mitogen-activated protein kinases was observed in kidneys of animals treated with DA or DB. DA (5 microg/kg/min) and DB (5 microg/kg/min) pre-treatment reduced endothelial cell damage after 24 h of cold preservation. Only DA pre-treatment improved renal function and reduced renal inflammation after 24 h of cold preservation and syngeneic transplantation. Our results demonstrate a protective effect of pre-treatment with catecholamines on renal inflammation and function after I/R- or cold preservation injury. This could help to explain the potent organoprotective effects of catecholamine pre-treatment observed in human kidney transplantation.

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.

Rat strain effects of AT1 receptor activation on D1 dopamine receptors in immortalized renal proximal tubule cells

The dopaminergic and renin-angiotensin systems regulate blood pressure, in part, by affecting sodium transport in renal proximal tubules (RPTs). We have reported that activation of a D1-like receptor decreases AT1 receptor expression in the mouse kidney and in immortalized RPT cells from Wistar-Kyoto (WKY) rats. The current studies were designed to test the hypothesis that activation of the AT1 receptor can also regulate the D1 receptor in RPT cells, and this regulation is aberrant in spontaneously hypertensive rats (SHRs). Long-term (24 hours) stimulation of RPT cells with angiotensin II, via AT1 receptors increased total cellular D1 receptor protein in a time- and concentration-dependent manner in WKY but not in SHR cells. Short-term stimulation (15 minutes) with angiotensin II did not affect total cellular D1 receptor protein in either rat strain. However, in the short-term experiments, angiotensin II decreased cell surface membrane D1 receptor protein in WKY but not in SHR cells. D1 and AT1 receptors colocalized (confocal microscopy) and their coimmunoprecipitation was greater in WKY than in SHRs. However, AT1/D1 receptor coimmunoprecipitation was decreased by angiotensin II (10(-8) M/24 hours) to a similar extent in WKY (-22+/-8%) and SHRs (-22+/-12%). In summary, these studies show that AT1 and D1 receptors interact differently in RPT cells from WKY and SHRs. It is possible that an angiotensin II-mediated increase in D1 receptors and dissociation of AT1 from D1 receptors serve to counter regulate the long-term action of angiotensin II in WKY rats; different effects are seen in SHRs.

Functional genomics of the dopaminergic system in hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. Under normal conditions, D(1)-like receptors (D(1) and D(5)) inhibit sodium transport in the kidney and intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats (SHRs) and in humans with essential hypertension, the D(1)-like receptor-mediated inhibition of epithelial sodium transport is impaired because of an uncoupling of the D(1)-like receptor from its G protein/effector complex. The uncoupling is receptor specific, organ selective, nephron-segment specific, precedes the onset of hypertension, and cosegregates with the hypertensive phenotype. The defective transduction of the renal dopaminergic signal is caused by activating variants of G protein-coupled receptor kinase type 4 (GRK4: R65L, A142V, A486V). The GRK4 locus is linked to and GRK4 gene variants are associated with human essential hypertension, especially in salt-sensitive hypertensive subjects. Indeed, the presence of three or more GRK4 variants impairs the natriuretic response to dopaminergic stimulation in humans. In genetically hypertensive rats, renal inhibition of GRK4 expression ameliorates the hypertension. In mice, overexpression of GRK4 variants causes hypertension either with or without salt sensitivity according to the variant. GRK4 gene variants, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic factors (e.g., angiotensin II type 1 receptor) to predominate, may be responsible for salt sensitivity. Subclasses of hypertension may occur because of additional perturbations caused by variants of other genes, the quantitative interaction of which may vary depending upon the genetic background.

Insulin treatment enhances AT1receptor function in OK cells

Increased renal sodium retention is considered a major risk factor contributing to hypertension associated with chronic hyperinsulinemia and obesity. However, the molecular mechanism involved is not understood. The present study investigates the effect of insulin treatment on AT1receptor expression and ANG II-induced stimulation of Na/H exchanger (NHE) and Na-K-ATPase (NKA) in opossum kidney (OK) cells, a proximal tubule cell line. The presence of the AT1receptors in OK cells was confirmed by the specific binding of125I-sar-ANG II and by detecting ∼43-kDa protein on Western blot analysis with AT1receptor antibody and blocking peptide as well as by expression of AT1receptor mRNA as determined by RT-PCR. Insulin treatment (100 nM for 24 h) caused an increase in125I-sar-ANG II binding, AT1receptor protein content, and mRNA levels. The whole cell lysate and membrane showed similar insulin-induced increase in the AT1receptor protein expression, which was blocked by genistein (100 nM), a tyrosine kinase inhibitor, and cycloheximide (1.5 μg/ml), a protein synthesis inhibitor. Determination of ethyl isopropyl amiloride-sensitive22Na+uptake, a measure of the NHE activity, revealed that ANG II (1–100 pM)-induced stimulation of NHE in insulin-treated cells was significantly greater than in the control cells. Similarly, ANG II (1–100 pM)-induced stimulation of ouabain-sensitive86Rb+uptake, a measure of NKA activity in insulin-treated cells, was significantly greater than in the control cells. ANG II stimulation of both the transporters was blocked by AT1receptor antagonist losartan, suggesting the involvement of AT1receptors. Thus chronic insulin treatment causes upregulation of AT1receptors, which evoked ANG II-induced stimulation of NHE and NKA. We propose that insulin-induced increase in the renal AT1receptor function serves as a mechanism responsible for the increased renal sodium reabsorption and thus may contribute to development of hypertension in conditions associated with hyperinsulinemia.

Dopamine D2-like receptor agonist bromocriptine protects against ischemia/reperfusion injury in rat kidney

BACKGROUND

Dopamine, via activation of D1-like and D2-like receptors, plays an important role in the regulation of renal sodium excretion. Recently, we demonstrated that dopamine D2-like receptor agonist (bromocriptine) stimulates p44/42 mitogen-activated protein kinases (MAPKs) and Na+,K(+)ATPase (NKA) activity in proximal tubular epithelial cells. Since both these parameters are compromised in ischemia/reperfusion (I/R) injury to the kidney, we investigated whether bromocriptine protects against the injury.

METHODS

In this study we used unilateral rat model of renal I/R injury. The Sprague-Dawley rats were divided into vehicle and bromocriptine groups. The vehicle and bromocriptine group was treated with vehicle and bromocriptine (500 microg/kg intravenously), respectively, 15 minutes before the induction of unilateral ischemia followed by 24- or 48-hour reperfusion. At the end of 24 or 48 hours the animals were sacrificed to collect control and ischemic kidney cortices, in which necrosis, apoptosis, NKA activity, NKA alpha1 subunit expression, and p44/42 MAPK phosphorylation were measured.

RESULTS

We found extensive necrosis, apoptosis, and decreased NKA activity (with no change in alpha1 subunit) in the ischemic kidney cortex compared to the nonischemic cortex from the vehicle-treated rats as early as 24 hours post-reperfusion. In contrast, I/R injury-induced necrotic, apoptotic, and decrease in NKA activity were absent in the outer cortex of bromocriptine-treated rats after 24 or 48 hours. Interestingly, we detected significantly higher phosphorylation of p44/42 MAPKs in control and ischemic kidneys of bromocriptine-treated rats compared to those of vehicle-treated rats.

CONCLUSION

Therefore, bromocriptine, a D1-like receptor agonist, may protect against I/R injury to proximal tubules of the kidney, via p44/42 MAPK activation.

Opioids as modulators of cell death and survival--unraveling mechanisms and revealing new indications

Opioids are powerful analgesics but also drugs of abuse. Because opioid addicts are susceptible to certain infections, opioids have been suspected to suppress the immune response. This was supported by the finding that various immune-competent cells express opioid receptors and undergo apoptosis when treated with opioid alkaloids. Recent evidence suggests that opioids may also effect neuronal survival and proliferation or migrating properties of tumor cells. A multitude of signaling pathways has been suggested to be involved in these extra-analgesic effects of opioids. Growth-promoting effects were found to be mediated through Akt and Erk signaling cascades. Death-promoting effects have been ascribed to inhibition of nuclear factor-kappaB, increase of Fas expression, p53 stabilization, cytokine and chemokine release, and activation of nitric oxide synthase, p38, and c-Jun-N-terminal kinase. Some of the observed effects were inhibited with opioid receptor antagonists or pertussis toxin; others were unaffected. It is still unclear whether these properties are mediated through typical opioid receptor activation and inhibitory G-protein-signaling. The present review tries to unravel controversial findings and provides a hypothesis that may help to integrate diverse results.

Aberrant D1 and D3 dopamine receptor transregulation in hypertension

Dopamine plays a role in the regulation of blood pressure by inhibition of sodium transport in renal proximal tubules (RPTs) and relaxation of vascular smooth muscles. Because dopamine receptors can regulate and interact with each other, we studied the interaction of D(1) and D(3) receptors in immortalized RPT cells and mesenteric arteries from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHRs), and in human coronary artery smooth muscle cells (CASMCs). In WKY rats, the D(1)-like agonist, fenoldopam, increased D(3) receptor protein in a time-dependent and concentration-dependent manner (EC(50)=4.5x10(-9) M, t(1/2)=15.8 hours). In SHRs, fenoldopam (10(-5) M) actually decreased the expression of D(3) receptors. D(1) and D(3) receptor co-immunoprecipitation was increased by fenoldopam (10(-7) M/24 h) in WKY rats but not in SHRs. The effects of fenoldopam in CASMCs were similar as those in WKY RPT cells (ie, fenoldopam increased D(1) and D(3) receptor proteins). Both D(3) (PD128907, Emax=80%+/-6%, pED(50)=5+/-0.1) and D(1)-like receptor (fenoldopam, Emax=81%+/-8%, pED(50)=5+/-0.2, n=12) agonists relaxed mesenteric arterial rings. Co-stimulation of D(1) and D(3) receptors led to additive vasorelaxation in WKY rats, but not in SHRs. D(1) and D(3) receptors interact differently in WKY and SHRs. Altered interactions between D(1) and D(3) receptors may play a role in the pathogenesis of genetic hypertension, including human hypertension, because these receptors also interact in human vascular smooth muscle cells.

Repeated amphetamine treatment induces neurite outgrowth and enhanced amphetamine-stimulated dopamine release in rat pheochromocytoma cells (PC12 cells) via a protein kinase C- and mitogen activated protein kinase-dependent mechanism

Repeated intermittent treatment with amphetamine (AMPH) induces both neurite outgrowth and enhanced AMPH-stimulated dopamine (DA) release in PC12 cells. We investigated the role of protein kinases in the induction of these AMPH-mediated events by using inhibitors of protein kinase C (PKC), mitogen activated protein kinase (MAP kinase) or protein kinase A (PKA). PKC inhibitors chelerythrine (100 nm and 300 nm), Ro31-8220 (300 nm) and the MAP kinase kinase inhibitor, PD98059 (30 micro m) inhibited the ability of AMPH to elicit both neurite outgrowth and the enhanced AMPH-stimulated DA release. The direct-acting PKC activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA, 250 nm) mimicked the ability of AMPH to elicit neurite outgrowth and enhanced DA release. On the contrary, a selective PKA inhibitor, 100 micro m Rp-8-Br-cAMPS, blocked only the development of AMPH-stimulated DA release but not the neurite outgrowth. Treatment of the cells with acute AMPH elicited an increase in the activity of PKC and MAP kinase but not PKA. These results demonstrated that AMPH-induced increases in MAP kinase and PKC are important for induction of both the enhancement in transporter-mediated DA release and neurite outgrowth but PKA was only required for the enhancement in AMPH-stimulated DA release. Therefore the mechanisms by which AMPH induces neurite outgrowth and the enhancement in AMPH-stimulated DA release can be differentiated.

Dopamine-mediated inhibition of renal Na,K-ATPase is reduced by insulin

Recently we have reported that rosiglitazone treatment of obese Zucker rats reduced plasma insulin and restored the ability of dopamine to inhibit Na,K-ATPase (NKA) in renal proximal tubules. The present study was performed to test the hypothesis that a chronic increase in levels of insulin causes a decrease in expression of the D1 receptor and its uncoupling from G proteins, which may account for the diminished inhibitory effect of dopamine on NKA in obese Zucker rats. We conducted experiments in primary proximal tubule epithelial cells obtained from Sprague-Dawley rat kidneys. These cells at 80% to 90% confluence were pretreated with insulin (100 nmol/L for 24 hours) in growth factor-/serum-free medium. SKF-38393, a D1 receptor agonist, inhibited NKA activity in untreated cells, but the agonist failed to inhibit enzyme activity in insulin-pretreated cells. Basal NKA activity was similar in untreated and insulin-pretreated cells. Measurement of D1 receptors in the plasma membranes revealed that [3H]SCH-23390 binding, a D1 receptor ligand, as well as D1 receptor protein abundance, was significantly reduced in insulin-pretreated cells compared with untreated cells. SKF-38393 (10 micromol/L) elicited significant stimulation of [35S]GTPgammaS binding in the membranes from control cells, suggesting that the D1 receptor-G protein coupling was intact. However, the stimulatory effect of SKF-38393 was absent in membranes from insulin-pretreated cells. We suggest that chronic exposure of cells to insulin causes both the reduction in D1 receptor abundance and its uncoupling from G proteins. These phenomena might account for the diminished inhibitory effect of dopamine on NKA activity in hyperinsulinemic rats.

Role of tyrosine kinase and p44/42 MAPK in D(2)-like receptor-mediated stimulation of Na(+), K(+)-ATPase in kidney

Our laboratory has shown that dopamine D(2)-like receptor activation causes stimulation of Na(+), K(+)-ATPase (NKA) activity in the proximal tubules of the rat kidney. The present study was designed to investigate the cellular signaling mechanisms mediating this response to D(2)-like receptor activation. We measured the stimulation of NKA activity by bromocriptine (D(2)-like receptor agonist) in the absence and presence of PD-98059 [p44/42 mitogen-activated protein kinase (MAPK) kinase inhibitor] and genistein (tyrosine kinase inhibitor) in renal proximal tubules. Both agents inhibited bromocriptine-mediated stimulation of NKA, suggesting the involvement of p44/42 MAPK and tyrosine kinase in this response. Additionally, we found that bromocriptine increased the phosphorylation of p44/42 MAPK in the proximal tubules, which was blocked by PD-98059 and genistein. These results show that D(2)-like receptor activation causes stimulation of NKA activity by means of a tyrosine kinase-p44/42 MAPK pathway in the proximal tubules of the kidney.