Dopamine D1A receptor regulation of phospholipase C isoform

Dopamine induces release of calcium from internal stores in layer II lateral entorhinal cortex fan cells

The entorhinal cortex plays an important role in temporal lobe processes including learning and memory, object recognition, and contextual information processing. The alteration of the strength of synaptic inputs to the lateral entorhinal cortex may therefore contribute substantially to sensory and mnemonic functions. The neuromodulatory transmitter dopamine exerts powerful effects on excitatory glutamatergic synaptic transmission in the entorhinal cortex. Interestingly, inputs from midbrain dopamine neurons appear to specifically target clusters of excitatory cells located in the superficial layers of the entorhinal cortex. We have previously demonstrated that dopamine facilitates synaptic transmission through the activation of D₁-like receptors. This facilitation of synaptic transmission is dependent on both activation of classical D₁-like-receptors, and upon activation of dopamine receptors linked to increases in phospholipase C, inositol triphosphate (IP₃), and intracellular calcium. In the present study we combined electrophysiological recordings of evoked excitatory postsynaptic currents with imaging of intracellular calcium using the fluorescent indicator fluo-4 to monitor calcium transients evoked by dopamine in electrophysiologically identified putative fan and pyramidal cells of the lateral entorhinal cortex. Bath application of dopamine (1 μM), or the phosphatidylinositol (PI)-linked D₁-like-receptor agonist SKF83959 (5 μM), induced reliable and reversible increases in fluo-4 fluorescence and excitatory postsynaptic currents in fan cells, but not in pyramidal cells. In contrast, application of the classical D₁-like-receptor agonist SKF38393 (10 μM) did not result in significant increases in fluorescence. Blocking release of calcium from internal stores by loading cells with the IP₃ receptor blocker heparin (1 mM) or the ryanodine receptor blocker dantrolene (20 μM) abolished both the calcium transients and the facilitation of evoked synaptic currents induced by dopamine. Dopamine also induced calcium transients in fan cells when calcium was excluded from the extracellular medium, further indicating that the calcium transients are linked to release from internal stores. These results indicate that following D₁-like-receptor binding, dopamine selectively induces transient elevations in intracellular calcium via activation of IP₃ and ryanodine receptors, and that these elevations are linked to the facilitation of synaptic responses in putative layer II entorhinal cortex fan cells.

Chronic hyperammonemia alters in opposite ways membrane expression of GluA1 and GluA2 AMPA receptor subunits in cerebellum. Molecular mechanisms involved

Hyperammonemia contributes to altered neurotransmission and cognition in patients with hepatic encephalopathy. Hyperammonemia in rats affects differently high- and low-affinity AMPA receptors (AMPARs) in cerebellum. We hypothesized that hyperammonemia would alter differently membrane expression of AMPARs GluA1 and GluA2 subunits by altering its phosphorylation. This work aims were: 1) assess if hyperammonemia alters GluA1 and GluA2 subunits membrane expression in cerebellum and 2) analyze the underlying mechanisms. Hyperammonemia reduces membrane expression of GluA2 and enhances membrane expression of GluA1 in vivo. We show that changes in GluA2 and GluA1 membrane expression in hyperammonemia would be due to enhanced NMDA receptors activation which reduces cGMP levels and phosphodiesterase 2 (PDE2) activity, resulting in increased cAMP levels. This leads to increased protein kinase A (PKA) activity which activates phospholipase C (PLC) and protein kinase C (PKC) thus increasing phosphorylation of GluA2 in Ser880, which reduces GluA2 membrane expression, and phosphorylation of GluA1 in Ser831, which increases GluA1 membrane expression. Blocking NMDA receptors or inhibiting PKA, PLC or PKC normalizes GluA2 and GluA1 phosphorylation and membrane expression in hyperammonemic rats. Altered GluA2 and GluA1 membrane expression would alter signal transduction which may contribute to cognitive and motor alterations in hyperammonemia and hepatic encephalopathy.

Age-Dependent D1-D2 Receptor Coactivation in the Lateral Orbitofrontal Cortex Potentiates NMDA Receptors and Facilitates Cognitive Flexibility

The orbitofrontal cortex (OFC) integrates information about the environment to guide decision-making. Glutamatergic synaptic transmission mediated through N-methyl-d-aspartate receptors is required for optimal functioning of the OFC. Additionally, abnormal dopamine signaling in this region has been implicated in impulsive behavior and poor cognitive flexibility. Yet, despite the high prevalence of psychostimulants prescribed for attention deficit/hyperactivity disorder, there is little information on how dopamine modulates synaptic transmission in the juvenile or the adult OFC. Using whole-cell patch-clamp recordings in OFC pyramidal neurons, we demonstrated that while dopamine or selective D2-like receptor (D2R) agonists suppress excitatory synaptic transmission of juvenile or adult lateral OFC neurons; in juvenile lateral OFC neurons, higher concentrations of dopamine can target dopamine receptors that couple to a phospholipase C (PLC) signaling pathway to enhance excitatory synaptic transmission. Interfering with the formation of a putative D1R-D2R interaction blocked the potentiation of excitatory synaptic transmission. Furthermore, targeting the putative D1R-D2R complex with a biased agonist, SKF83959, not only enhanced excitatory synaptic transmission in a PLC-dependent manner, but also improved the performance of juvenile rats on a reversal-learning task. Our results demonstrate that dopamine signaling in the lateral OFC differs between juveniles and adults, through potential crosstalk between dopamine receptor subtypes.

Dopamine D1 receptor signaling: does GαQ-phospholipase C actually play a role?

Despite numerous studies showing therapeutic potential, no central dopamine D1 receptor ligand has ever been approved, because of potential limitations, such as hypotension, seizures, and tolerance. Functional selectivity has been widely recognized as providing a potential mechanism to develop novel therapeutics from existing targets, and a highly biased, functionally selective D1 ligand might overcome some of the past limitations. SKF-83959 [6-chloro-3-methyl-1-(m-tolyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepine-7,8-diol] is reported to be a highly biased D1 ligand, having full agonism at D1-mediated activation of phospholipase C (PLC) signaling (via GαQ) and antagonism at D1-mediated adenylate cyclase signaling (via GαOLF/S). For this reason, numerous studies have used this compound to elucidate the physiologic role of D1-PLC signaling, including a novel molecular mechanism (GαQ-PLC activation via D1-D2 heterodimers). There is, however, contradictory literature that suggests that SKF-83959 is actually a partial agonist at both D1-mediated adenylate cyclase and β-arrestin recruitment. Moreover, the D1-mediated PLC stimulation has also been questioned. This Minireview examines 30 years of relevant literature and proposes that the data strongly favor alternate hypotheses: first, that SKF-83959 is a typical D1 partial agonist; and second, that the reported activation of PLC by SKF-83959 and related benzazepines likely is due to off-target effects, not actions at D1 receptors. If these hypotheses are supported by future studies, it would suggest that caution should be used regarding the role of PLC and downstream pathways in D1 signaling.

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.

Generation and characterization of hD5 and C-terminal Mutant hD(5m) transgenic rats

Dopamine D1-like receptors play important roles in many brain activities such as cognition and emotion. We have generated human hD5 and mutant human hD5 (hD(5m)) transgenic rats. The C-terminal juxtamembrane domain of mutant hD5 was identical to that of hD5 pseudogenes. The transgenes were driven by the CAMKII promoter that led the expression mainly in the cerebral cortex and hippocampus. We have used different dopamine receptor agonists to compare the pharmacological profiles of the human hD5 and hD(5m) receptors. The results showed that they exhibited distinct pharmacological properties. Our results of pharmacological studies indicated that the C-terminal of D5 receptor could play important roles in agonist binding affinity. Hippocampal long-term potentiation (LTP) evoked by tetanic stimulation was significantly reduced in both transgenic rats. In addition, we found that the overexpression of dopamine hD5 and hD(5m) receptors in the rat brain resulted in memory impairments. Interestingly, an atypical D1-like receptor agonist, SKF83959, could induce anxiety in hD(5m) receptor transgenic rats but had no effect on the anxiety-like behavior in D5 receptor transgenic and wild-type rats.

PLCgamma2 Activates CREB-dependent Transcription in PC12 Cells Through Phosphorylation of CREB at Serine 133

The cAMP and Ca(2+) signaling pathways activate the transcription factor CREB through its phosphorylation at Serine 133. Activation of CREB is involved in the regulation of various biological phenomena. To understand further the mechanisms of the regulation of CREB activity in response to activation of the cAMP and Ca(2+) signaling pathways, we examined the roles of PLCgammas in CREB activation in PC12 cells. siRNA-mediated reduction of the expression of PLCgamma2, but not PLCgamma1, inhibited both the phosphorylation of CREB at S133 and the activation of CREB-dependent transcription following treatment of cells with forskolin or ionomycin, which increases the intracellular concentrations of cAMP or Ca(2+), respectively. Importantly, the siRNA targeting PLCgamma2 completely abolished CREB activation by Ca(2+) signaling but not by cAMP signaling. These results suggest that PLCgamma2 functions as an essential signal transducer leading to CREB activation in response to activation of the Ca(2+) signaling pathway and that the cAMP signaling pathway might activate CREB through phosphorylation of CREB by PKA and another signaling pathway mediated by PLCgamma2.

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.

Dopamine D1 receptor-mediated inhibition of NADPH oxidase activity in human kidney cells occurs via protein kinase A-protein kinase C cross talk

Dopamine cellular signaling via the D(1) receptor (D(1)R) involves both protein kinase A (PKA) and protein kinase C (PKC), but the PKC isoform involved has not been determined. Therefore, we tested the hypothesis that the D(1)R-mediated inhibition of NADPH oxidase activity involves cross talk between PKA and a specific PKC isoform(s). In HEK-293 cells heterologously expressing human D(1)R (HEK-hD(1)), fenoldopam, a D(1)R agonist, and phorbol 12-myristate 13-acetate (PMA), a PKC activator, inhibited oxidase activity in a time- and concentration-dependent manner. The D(1)R-mediated inhibition of oxidase activity (68.1±3.6%) was attenuated by two PKA inhibitors, H89 (10μmol/L; 88±8.1%) and Rp-cAMP (10μmol/L; 97.7±6.7%), and two PKC inhibitors, bisindolylmaleimide I (1μmol/L; 94±6%) and staurosporine (10nmol/L; 93±8%), which by themselves had no effect (n=4-8/group). The inhibitory effect of PMA (1μmol/L) on oxidase activity (73±3.2%) was blocked by H89 (100±7.8%; n=5 or 6/group). The PMA-mediated inhibition of NADPH oxidase activity was accompanied by an increase in PKCθ(S676), an effect that was also blocked by H89. Fenoldopam (1μmol/L) also increased PKCθ(S676) in HEK-hD(1) and human renal proximal tubule (RPT) cells. Knockdown of PKCθ with siRNA in RPT cells prevented the inhibitory effect of fenoldopam on NADPH oxidase activity. Our studies demonstrate for the first time that cross talk between PKA and PKCθ plays an important role in the D(1)R-mediated negative regulation of NADPH oxidase activity in human kidney cells.

Activation of the dopamine 1 and dopamine 5 receptors increase skeletal muscle mass and force production under non-atrophying and atrophying conditions

BACKGROUND

Control of skeletal muscle mass and force production is a complex physiological process involving numerous regulatory systems. Agents that increase skeletal muscle cAMP levels have been shown to modulate skeletal muscle mass and force production. The dopamine 1 receptor and its closely related homolog, the dopamine 5 receptor, are G-protein coupled receptors that are expressed in skeletal muscle and increase cAMP levels when activated. Thus we hypothesize that activation of the dopamine 1 and/or 5 receptor will increase skeletal muscle cAMP levels thereby modulating skeletal muscle mass and force production.

METHODS

We treated isolated mouse tibialis anterior (TA) and medial gastrocnemius (MG) muscles in tissue bath with the selective dopamine 1 receptor and dopamine 5 receptor agonist SKF 81297 to determine if activation of skeletal muscle dopamine 1 and dopamine 5 receptors will increase cAMP. We dosed wild-type mice, dopamine 1 receptor knockout mice and dopamine 5 receptor knockout mice undergoing casting-induced disuse atrophy with SKF 81297 to determine if activation of the dopamine 1 and dopamine 5 receptors results in hypertrophy of non-atrophying skeletal muscle and preservation of atrophying skeletal muscle mass and force production.

RESULTS

In tissue bath, isolated mouse TA and MG muscles responded to SKF 81297 treatment with increased cAMP levels. Treating wild-type mice with SKF 81297 reduced casting-induced TA and MG muscle mass loss in addition to increasing the mass of non-atrophying TA and MG muscles. In dopamine 1 receptor knockout mice, extensor digitorum longus (EDL) and soleus muscle mass and force was not preserved during casting with SKF 81297 treatment, in contrast to significant preservation of casted wild-type mouse EDL and soleus mass and EDL force with SKF 81297 treatment. Dosing dopamine 5 receptor knockout mice with SKF 81297 did not significantly preserve EDL and soleus muscle mass and force although wild-type mouse EDL mass and force was significantly preserved SKF 81297 treatment.

CONCLUSIONS

These data demonstrate for the first time that treatment with a dopamine 1/5 receptor agonist results in (1) significant preservation of EDL, TA, MG and soleus muscle mass and EDL muscle force production during periods of atrophy and (2) hypertrophy of TA and MG muscle. These effects appear to be mainly mediated by both the dopamine 1 and dopamine 5 receptors.

A molecular dissociation between cued and contextual appetitive learning

In appetitive Pavlovian learning, animals learn to associate discrete cues or environmental contexts with rewarding outcomes, and these cues and/or contexts can potentiate an ongoing instrumental response for reward. Although anatomical substrates underlying cued and contextual learning have been proposed, it remains unknown whether specific molecular signaling pathways within the striatum underlie one form of learning or the other. Here, we show that while the striatum-enriched isoform of adenylyl cyclase (AC5) is required for cued appetitive Pavlovian learning, it is not required for contextual appetitive learning. Mice lacking AC5 (AC5KO) could not learn an appetitive Pavlovian learning task in which a discrete signal light predicted reward delivery, yet they could form associations between context and either natural or drug reward, which could in turn elicit Pavlovian approach behavior. However, unlike wild-type (WT) mice, AC5KO mice could not use these Pavlovian conditioned stimuli to potentiate ongoing instrumental behavior in a Pavlovian-to-instrumental transfer paradigm. These data suggest that AC5 is specifically required for learning associations between discrete cues and outcomes in which the temporal relationship between conditioned stimulus (CS) and unconditioned stimulus (US) is essential, while alternative signaling mechanisms may underlie the formation of associations between context and reward. In addition, loss of AC5 compromises the ability of both contextual and discrete cues to modulate instrumental behavior.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

The d5 dopamine receptor mediates large-conductance, calcium- and voltage-activated potassium channel activation in human coronary artery smooth muscle cells

Large-conductance, calcium- and voltage-activated potassium (BK(Ca)) channels hyperpolarize coronary artery smooth muscle cells, causing vasorelaxation. Dopamine activates BK(Ca) channels by stimulating D(1)-like receptor-mediated increases in cAMP in porcine coronary artery myocytes. There are two D(1)-like receptors (R), D(1)R and D(5)R. We hypothesize that the specific D(1)-like receptor involved in BK(Ca) channel activation in human coronary artery smooth muscle cells (HCASMCs) is the D(5)R and that activation occurs via cAMP cross-activation of cGMP-dependent protein kinase (PKG), rather than cAMP-dependent protein kinase (PKA). The effects of D(1)-like receptor agonists and antagonists on BK(Ca) channel opening in HCASMCs were examined in the presence and absence of PKG/PKA inhibition by cell-attached patch clamp. In the absence of commercially available ligands specific for D(1)R or D(5)R, D(1)R or D(5)R protein was down-regulated by transfecting HCASMCs with human D(1)R or D(5)R antisense oligonucleotides, respectively: cells transfected with scrambled oligonucleotides and nontransfected HCASMCs served as controls. The predominant ion channel conducting outward currents in nontransfected HCASMCs was identified as the large-conductance, calcium- and voltage-activated potassium (BK(Ca)) channel, which was activated by D(1)-like receptor agonists despite PKA inhibition with (9R,10S,12S)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid (KT 5720) (300 nM), but was abolished by inhibiting PKG with 9-methoxy-9-methoxycarbonyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b-11a-triazadibenzo(a,g) cycloocta(cde)-trinden-1-one (KT 5823) (300 nM). D(1)-like receptor agonists activated BK(Ca) channels in all transfected cells except those transfected with D(5)R antisense oligonucleotides. Thus, the dopamine (D(1)-like) receptor mediates activation of BK(Ca) channels in HCASMCs by D(5)R, not D(1)R, and via PKG, not PKA. This is the first report of differential D(1)-like receptor regulation of vascular smooth muscle function in human cells.

The role of PKA and PKCepsilon pathways in prostaglandin E2-mediated hypernociception

BACKGROUND AND PURPOSE

Protein kinase (PK) A and the epsilon isoform of PKC (PKCepsilon) are involved in the development of hypernociception (increased sensitivity to noxious or innocuous stimuli) in several animal models of acute and persistent inflammatory pain. The present study evaluated the contribution of PKA and PKCepsilon to the development of prostaglandin E(2) (PGE(2))-induced mechanical hypernociception.

EXPERIMENTAL APPROACH

Prostaglandin E(2)-induced mechanical hypernociception was assessed by constant pressure rat paw test. The activation of PKA or PKCepsilon was evaluated by radioactive enzymic assay in the dorsal root ganglia (DRG) of sensory neurons from the hind paws.

KEY RESULTS

Hypernociception induced by PGE(2) (100 ng) by intraplantar (i.pl.) injection, was reduced by i.pl. treatment with inhibitors of PKA [A-kinase-anchoring protein St-Ht31 inhibitor peptide (AKAPI)], PKCepsilon (PKCepsilonI) or adenylyl cyclase. PKA activity was essential in the early phase of the induction of hypernociception, whereas PKC activity was involved in the maintenance of the later phase of hypernociception. In the DRG (L4-L5), activity of PKA increased at 30 min after injection of PGE(2) but PKC activity increased only after 180 min. Moreover, i.pl. injection of the catalytic subunit of PKA induced hypernociception which was markedly reduced by pretreatment with an inhibitor of PKCepsilon, while the hypernociception induced by paw injection of PKCepsilon agonist was not affected by an inhibitor of PKA (AKAPI).

CONCLUSIONS AND IMPLICATIONS

Taken together, these findings are consistent with the suggestion that PKA activates PKCepsilon, which is a novel mechanism of interaction between these kinases during the development of PGE(2)-induced mechanical hypernociception.

Activation of phosphatidylinositol-linked novel D1 dopamine receptor contributes to the calcium mobilization in cultured rat prefrontal cortical astrocytes

Recent evidences indicate the existence of an atypical D(1) dopamine receptor other than traditional D(1) dopamine receptor in the brain that mediates PI hydrolysis via activation of phospholipase C(beta) (PLC(beta)). To further understand the basic physiological function of this receptor in brain, the effects of a selective phosphoinositide (PI)-linked D(1) dopamine receptor agonist SKF83959 on cytosolic free calcium concentration ([Ca(2+)](i)) in cultured rat prefrontal cortical astrocytes were investigated by calcium imaging. The results indicated that SKF83959 caused a transient dose-dependent increase in [Ca(2+)](i). Application of D(1) receptor, but not D(2), alpha(1) adrenergic, 5-HT receptor, or cholinergic antagonist prevented SKF83959-induced [Ca(2+)](i) rise, indicating that activation of the D(1) dopamine receptor was essential for this response. Increase in [Ca(2+)](i) was a two-step process characterized by an initial increase in [Ca(2+)](i) mediated by release from intracellular stores, supplemented by influx through voltage-gated calcium channels, receptor-operated calcium channels, and capacitative Ca(2+) entry. Furthermore, SKF83959-stimulated increase in [Ca(2+)](i) was abolished following treatment with a PLC inhibitor. Overall, these results suggested that activation of D(1) receptor by SKF83959 mediates a dose-dependent mobilization of [Ca(2+)](i) via the PLC signaling pathway in cultured rat prefrontal cortical astrocytes.

Quantification of D1 and D5 dopamine receptor localization in layers I, III, and V of Macaca mulatta prefrontal cortical area 9: coexpression in dendritic spines and axon terminals

D1 family receptors (D1R) in prefrontal cortex (PFC) are critical for normal cognition and are implicated in pathological states such as schizophrenia. The two D1R subtypes, D1 and D5, cannot be pharmacologically distinguished but have important functional differences. To understand their contributions to cortical function, we quantified their localization in the neuropil of primate PFC. We identified different patterns of distribution for the two receptors that showed variation across cortical laminae. Although D1 was enriched in spines and D5 in dendrites, there was considerable overlap in their distribution within neuronal compartments. To determine whether the D1 and D5 receptors are localized to separate populations of synapses, we employed double-labeling methods. We found the two receptors colocalized and quantified the overlap of their distribution in spines and axon terminals of prefrontal cortical area 9 in the Macaca mulatta monkey. The two receptors are found in partially overlapping populations, such that the D5 receptor is found in a subpopulation of those spines and terminals that contain D1. These results indicate that dopamine activation of the two D1R subtypes does not modulate disparate populations of synapses onto dendritic spines in prefrontal cortical area 9; rather, dopamine can activate D1 and D5 receptors on the same spines, plus an additional group of spines that contains only D1. The implications of these results for the dose-dependent relationship between D1R activation and PFC function are discussed.

Dopamine and ethanol cause translocation of epsilonPKC associated with epsilonRACK: cross-talk between cAMP-dependent protein kinase A and protein kinase C signaling pathways

We found previously that neural responses to ethanol and the dopamine D2 receptor (D2) agonist 2,10,11-trihydroxy-N-propylnorapomorphine hydrobromide (NPA) involve both epsilon protein kinase C (epsilonPKC) and cAMP-dependent protein kinase A (PKA). However, little is known about the mechanism underlying ethanol- and D2-mediated activation of epsilonPKC and the relationship to PKA activation. In the present study, we used a new epsilonPKC antibody, 14E6, that selectively recognized active epsilonPKC when not bound to its anchoring protein epsilonRACK (receptor for activated C-kinase), and PKC isozyme-selective inhibitors and activators to measure PKC translocation and catalytic activity. We show here that ethanol and NPA activated epsilonPKC and induced translocation of both epsilonPKC and its anchoring protein, epsilonRACK to a new cytosolic site. The selective epsilonPKC agonist, pseudo-epsilonRACK, activated epsilonPKC but did not cause translocation of the epsilonPKC/epsilonRACK complex to the cytosol. These data suggest a step-wise activation and translocation of epsilonPKC after NPA or ethanol treatment, where epsilonPKC first translocates and binds to its RACK and subsequently the epsilonPKC/epsilonRACK complex translocates to a new subcellular site. Direct activation of PKA by adenosine-3',5'-cyclic monophosphorothioate, Sp-isomer (Sp-cAMPS), prostaglandin E1, or the adenosine A2A receptor is sufficient to cause epsilonPKC translocation to the cytosolic compartment in a process that is dependent on PLC activation and requires PKA activity. These data demonstrate a novel cross-talk mechanism between epsilonPKC and PKA signaling systems. PKA and PKC signaling have been implicated in alcohol rewarding properties in the mesolimbic dopamine system. Cross-talk between PKA and PKC may underlie some of the behaviors associated with alcoholism.

Dopamine D1-like receptor-mediated inhibition of Cl/HCO3- exchanger activity in rat intestinal epithelial IEC-6 cells is regulated by G protein-coupled receptor kinase 6 (GRK 6)

The present study investigated the effect of dopamine D1-like receptor stimulation on the Cl-/HCO3- exchange activity in rat intestinal epithelial IEC-6 cells. The Cl-/HCO3- exchange activity was found to be a chloride-dependent, DIDS-sensitive and niflumate-insensitive process. The presence of the SLC26A6 anion exchanger was detected by both RT-PCR and immunoblotting analysis in IEC-6 cells, in which three different small interfering RNAs (siRNAs) targeting SLC26A6 markedly inhibited Cl-/HCO3- exchange. Activation of dopamine D1-like receptors with SKF 38393 inhibited Cl-/HCO3- exchanger activity, this being antagonized by the D1 selective antagonist SKF 83566. However, effects of SKF 38393 were maximal at 5 min of exposure to the agonist and rapidly diminished with no effect at 15 min, suggestive of agonist-induced desensitization of D1-like receptors. Pretreatment of cells with heparin, a non-selective inhibitor of G protein-coupled receptor kinases (GRKs), prevented the observed attenuation of SKF 38393-induced inhibition of Cl-/HCO3- exchange. Overnight pretreatment with anti-GRK6A and anti-GRK6B, but not with anti-GRK4 antibodies, prevented the loss of SKF 38393-mediated effects. Both PKA and PKC signaling pathways participate in SKF 38393-mediated inhibition of Cl-/HCO3- exchange. These findings suggest that SLC26A6 is at least one of the anion exchanger's family members responsible for Cl-/HCO3- exchange in IEC-6 cells. Dopamine D1 receptors in IEC-6 rapidly desensitize to D1-like agonist stimulation and GRK 6, but not GRK 4, appear to be involved in agonist-mediated responsiveness and desensitization.

Multiple genes and factors associated with bipolar disorder converge on growth factor and stress activated kinase pathways controlling translation initiation: implications for oligodendrocyte viability

Famine and viral infection, as well as interferon therapy have been reported to increase the risk of developing bipolar disorder. In addition, almost 100 polymorphic genes have been associated with this disease. Several form most of the components of a phosphatidyl-inositol signalling/AKT1 survival pathway (PIK3C3, PIP5K2A, PLCG1, SYNJ1, IMPA2, AKT1, GSK3B, TCF4) which is activated by growth factors (BDNF, NRG1) and also by NMDA receptors (GRIN1, GRIN2A, GRIN2B). Various other protein products of genes associated with bipolar disorder either bind to or are affected by phosphatidyl-inositol phosphate products of this pathway (ADBRK2, HIP1R, KCNQ2, RGS4, WFS1), are associated with its constituent elements (BCR, DUSP6, FAT, GNAZ) or are downstream targets of this signalling cascade (DPYSL2, DRD3, GAD1, G6PD, GCH1, KCNQ2, NOS3, SLC6A3, SLC6A4, SST, TH, TIMELESS). A further pathway relates to endoplasmic reticulum-stress (HSPA5, XBP1), caused by problems in protein glycosylation (ALG9), growth factor receptor sorting (PIK3C3, HIP1R, SYBL1), or aberrant calcium homoeostasis (WFS1). Key processes relating to these pathways appear to be under circadian control (ARNTL, CLOCK, PER3, TIMELESS). DISC1 can also be linked to many of these pathways. The growth factor pathway promotes protein synthesis, while the endoplasmic reticulum stress pathway, and other stress pathways activated by viruses and cytokines (IL1B, TNF, Interferons), oxidative stress or starvation, all factors associated with bipolar disorder risk, shuts down protein synthesis via control of the EIF2 alpha and beta translation initiation complex. For unknown reasons, oligodendrocytes appear to be particularly prone to defects in the translation initiation complex (EIF2B) and the convergence of these environmental and genomic signalling pathways on this area might well explain their vulnerability in bipolar disorder.

Dopamine D1/5 receptor-mediated long-term potentiation of intrinsic excitability in rat prefrontal cortical neurons: Ca2+-dependent intracellular signaling

Prefrontal cortex (PFC) dopamine D1/5 receptors modulate long- and short-term neuronal plasticity that may contribute to cognitive functions. Synergistic to synaptic strength modulation, direct postsynaptic D1/5 receptor activation also modulates voltage-dependent ionic currents that regulate spike firing, thus altering the neuronal input-output relationships in a process called long-term potentiation of intrinsic excitability (LTP-IE). Here, the intracellular signals that mediate this D1/5 receptor-dependent LTP-IE were determined using whole cell current-clamp recordings in layer V/VI rat pyramidal neurons from PFC slices. After blockade of all major amino acid receptors (V(hold) = -65 mV) brief tetanic stimulation (20 Hz) of local afferents or application of the D1 agonist SKF81297 (0.2-50 microM) induced LTP-IE, as shown by a prolonged (>40 min) increase in depolarizing pulse-evoked spike firing. Pretreatment with the D1/5 antagonist SCH23390 (1 microM) blocked both the tetani- and D1/5 agonist-induced LTP-IE, suggesting a D1/5 receptor-mediated mechanism. The SKF81297-induced LTP-IE was significantly attenuated by Cd(2+), [Ca(2+)](i) chelation, by inhibition of phospholipase C, protein kinase-C, and Ca(2+)/calmodulin kinase-II, but not by inhibition of adenylate cyclase, protein kinase-A, MAP kinase, or L-type Ca(2+) channels. Thus this form of D1/5 receptor-mediated LTP-IE relied on Ca(2+) influx via non-L-type Ca(2+) channels, activation of PLC, intracellular Ca(2+) elevation, activation of Ca(2+)-dependent CaMKII, and PKC to mediate modulation of voltage-dependent ion channel(s). This D1/5 receptor-mediated modulation by PKC coexists with the previously described PKA-dependent modulation of K(+) and Ca(2+) currents to dynamically regulate overall excitability of PFC neurons.

Activation of D 3 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 D 3 dopamine receptor gene in mice produces renin-dependent hypertension. In rats, D 2 -like receptors reduce angiotensin II binding sites in renal proximal tubules (RPTs). Because the major D 2 -like receptor in RPTs is the D 3 receptor, we examined whether D 3 receptors regulate angiotensin II type 1 (AT 1 ) receptors in rat RPT cells. The effect of D 3 receptors on AT 1 receptors was studied in vitro and in vivo. The D 3 receptor agonist PD128907 decreased AT 1 receptor protein and mRNA in WKY RPT cells and increased it in SHR cells. PD128907 increased D 3 receptors in WKY cells but had no effect in SHR cells. D 3 /AT 1 receptors colocalized in RPT cells; D 3 receptor stimulation decreased the percent amount of D 3 receptors that coimmunoprecipitated with AT 1 receptors to a greater extent in WKY than in SHR cells. However, D 3 receptor stimulation did not change the percent amount of AT 1 receptors that coimmunoprecipitated with D 3 receptors in WKY cells and markedly decreased the coimmunoprecipitation in SHR cells. The D 3 receptor also regulated the AT 1 receptor in vivo because AT 1 receptor expression was increased in kidneys of D 3 receptor–null mice compared with wild type littermates. D 3 receptors may regulate AT 1 receptor function by direct interaction with and regulation of AT 1 receptor expression. One mechanism of hypertension may be related to increased renal expression of AT 1 receptors due decreased D 3 receptor regulation.

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.

Gialpha3 protein-coupled dopamine D3 receptor-mediated inhibition of renal NHE3 activity in SHR proximal tubular cells is a PLC-PKC-mediated event

This study evaluated the transduction pathway associated with type 3 Na(+)/H(+) exchanger (NHE3) activity-induced inhibition during dopamine D(3) receptor activation in immortalized renal proximal tubular epithelial cells from the spontaneously hypertensive rat. The dopamine D(3) receptor agonist 7-OH-DPAT decreased NHE3 activity, which was prevented by the D(2)-like receptor antagonist S-sulpiride, pertussis toxin (PTX; overnight treatment), and the PKC inhibitor chelerythrine, but not by cholera toxin (overnight treatment), the MAPK inhibitor PD-098059, or the p38 inhibitor SB-203580. The PKA inhibitor H-89 abolished the inhibitory effects of forskolin on NHE3 activity, but not that of 7-OH-DPAT. The phospholipase C (PLC) inhibitor U-73122 prevented the inhibitory effects of 7-OH-DPAT, whereas PDBu and 7-OH-DPAT increased PLC activity and reduced NHE3 activity; downregulation of PKC abolished the inhibitory effects of both PDBu and 7-OH-DPAT on NHE activity. The inhibition of NHE3 activity by GTPgammaS and the prevention of the effect of 7-OH-DPAT by PTX suggest an involvement of a G(i/o) protein coupled to the dopamine D(3) receptor. Indeed, the 7-OH-DPAT-induced decrease in NHE3 activity was abolished in cells treated overnight with the anti-G(i)alpha3 antibody, but not in cells treated with antibodies against G(q/11), G(s)alpha, G(beta), and G(i)alpha1,2 proteins. The calcium ionophore A-23187 and the Ca(2+)-ATPase inhibitor thapsigargin increased intracellular Ca(2+) but did not affect NHE3 activity. However, the inhibitory effects of PDBu and 7-OH-DPAT on NHE3 activity were completely abolished by A-23287 and thapsigargin. It is concluded that inhibition of NHE3 activity by dopamine D(3) receptors coupled to G(i)alpha3 proteins is a PLC-PKC-mediated event, modulated by intracellular Ca(2+).

Dopamine D1 and D2 receptor Co-activation generates a novel phospholipase C-mediated calcium signal

Although dopamine D1 and D2 receptors belong to distinct subfamilies of dopamine receptors, several lines of evidence indicate that they are functionally linked. However, a mechanism for this linkage has not been elucidated. In this study, we demonstrate that agonist stimulation of co-expressed D1 and D2 receptors resulted in an increase of intracellular calcium levels via a signaling pathway not activated by either receptor alone or when only one of the co-expressed receptors was activated by a selective agonist. Calcium signaling by D1-D2 receptor co-activation was abolished following treatment with a phospholipase C inhibitor but not with pertussis toxin or inhibitors of protein kinase A or protein kinase C, indicating coupling to the G(q) pathway. We also show, by co-immunoprecipitation from rat brain and from cells co-expressing the receptors, that D1 and D2 receptors are part of the same heteromeric protein complex and, by immunohistochemistry, that these receptors are co-expressed and co-localized within neurons of human and rat brain. This demonstration that D1 and D2 receptors have a novel cellular function when co-activated in the same cell represents a significant step toward elucidating the mechanism of the functional link observed between these two receptors in brain.

Dopamine D 1 Receptor Augmentation of D 3 Receptor Action in Rat Aortic or Mesenteric Vascular Smooth Muscles

Dopamine is an important modulator of blood pressure, in part, by regulating vascular resistance. To test the hypothesis that D 1 and D 3 receptors interact in vascular smooth muscle cells, we studied A10 cells, a rat aortic smooth muscle cell line, and rat mesenteric arteries that express both dopamine receptor subtypes. Fenoldopam, a D 1 -like receptor agonist, increased both D 1 and D 3 receptor protein in a time-dependent and a concentration-dependent manner in A10 cells. The effect of fenoldopam was specific because a D 1 -like receptor antagonist, SCH23390 (10 −7 M/24 h), completely blocked the stimulatory effect of fenoldopam (10 −7 M/24 h) (D 3 receptor: control=21±1 density units [DU]); SCH23390=23±2 DU; fenoldopam=33±2 DU; fenoldopam+SCH23390=23±2 DU; n=10). D 1 and D 3 receptors physically interacted with each other because fenoldopam (10 −7 M/24 h) increased D 1 /D 3 receptor coimmunoprecipitation (35±5 versus 65±5 DU; n=8). A D 3 receptor agonist, PD128907, relaxed mesenteric arterial rings independent of the endothelium, effects that were blocked by a D 3 receptor antagonist, U99194A. Costimulation of D 1 and D 3 receptors led to additive vasorelaxation. We conclude that the D 1 receptor regulates the D 3 receptor by physical interaction and receptor expression. D 1 receptor stimulation augments D 3 receptor vasorelaxant effects. An interaction of D 1 and D 3 receptors may be involved in the regulation of blood pressure.

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.

SKF83959 selectively regulates phosphatidylinositol-linked D1 dopamine receptors in rat brain

Previously a distinct D1-like dopamine receptor (DAR) that selectively couples to phospholipase C/phosphatidylinositol (PLC/PI) was proposed. However, lack of a selective agonist has limited efforts aimed at characterizing this receptor. We characterized the in vitro and in vivo effects of SKF83959 in regulating PI metabolism. SKF83959 stimulates (EC50, 8 micro m) phosphatidylinositol 4,5-biphosphate hydrolysis in membranes of frontal cortex (FC) but not in membranes from PC12 cells expressing classical D1A DARs. Stimulation of FC PI metabolism was attenuated by the D1 antagonist, SCH23390, indicating that SKF83959 activates a D1-like DAR. The PI-linked DAR is located in hippocampus, cerebellum, striatum and FC. Most significantly, administration of SKF83959 induced accumulations of IP3 in striatum and hippocampus. In contrast to other D1 DAR agonists, SKF83959 did not increase cAMP production in brain or in D1A DAR-expressing PC12 cell membranes. However, SKF83959 inhibited cAMP elevation elicited by the D1A DAR agonist, SKF81297, indicating that the compound is an antagonist of the classical D1A DAR. Lastly, we demonstrated that SKF83959 enhances [35S]guanosine 5'-O-(3-thiotriphosphate) binding to membrane Galphaq and Galphai proteins, suggesting that PI stimulation is mediated by activation of these guanine nucleotide-binding regulatory proteins. Results indicate that SKF83959 is a selective agonist for the PI-linked D1-like DAR, providing a unique tool for investigating the functions of this brain D1 DAR subtype.

Angiotensin II regulation of AT1 and D3 dopamine receptors in renal proximal tubule cells of SHR

Dopamine and angiotensin II negatively interact to regulate sodium excretion and blood pressure. D3 dopamine receptors downregulate angiotensin type 1 (AT1) receptors in renal proximal tubule cells from normotensive Wistar-Kyoto rats. We determined whether AT1 receptors regulate D3 receptors and whether the regulation is different in cultured renal proximal tubule cells from normotensive and spontaneously hypertensive rats. Angiotensin II (10(-8)M/24 hours) decreased D3 receptors in both normotensive (control, 36+/-3; angiotensin II, 24+/-3 U) and hypertensive (control, 30+/-3; angiotensin II, 11+/-3 U; n=9 per group) rats; effects that were blocked by the AT1 receptor antagonist, losartan (10(-8)M/24 hours). However, the reduction in D3 expression was greater in hypertensive (60+/-10%) than in normotensive rats (32+/-9%). In normotensive rats, angiotensin II (10(-8)M/24hr) also decreased AT1 receptors. In contrast, in cells from hypertensive rats, angiotensin II increased AT1 receptors. AT1 and D3 receptors co-immunoprecipitated in renal proximal tubule cells from both strains. Angiotensin II decreased D3/AT1 receptor co-immunoprecipitation similarly in both rat strains, but basal D3/AT1 co-immunoprecipitation was 6 times higher in normotensive than in hypertensive rats. Therefore, AT1 and D3 receptor interaction is qualitatively and quantitatively different between normotensive and hypertensive rats; angiotensin II decreases AT1 expression in normotensive but increases it in hypertensive rats. In addition, angiotensin II decreases D3 expression to a greater extent in hypertensive than in normotensive rats. Aberrant interactions between D3 and AT1 receptors may play a role in the pathogenesis of hypertension.

Activation of presynaptic D1 dopamine receptors by dopamine increases the frequency of spontaneous excitatory postsynaptic currents through protein kinase A and protein kinase C in pyramidal cells of rat prelimbic cortex

To determine the effect of dopamine on the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in pyramidal cells of layers V-VI of the prelimbic cortex, whole-cell patch-clamp recordings were made from 92 pyramidal cells of layers V-VI of the rat prelimbic cortex. In normal buffer, dopamine 100 microM apparently increased the frequency of spontaneous EPSCs. Decreasing the concentration of dopamine from 100 to 50 microM was accompanied by a decreased effect of dopamine. Further decreasing the dopamine concentration to 10 and 1 microM had no effects on the frequency of spontaneous EPSCs. In the presence of tetrodotoxin or cadmium, the increasing effect of dopamine was eliminated. The increasing effect of dopamine was blocked by the dopamine D1 receptor antagonist SCH23390, but not by the dopamine D2 receptor antagonist sulpiride. The D1 receptor agonist SKF38393 partially mimicked the increasing effect, but the D2 receptor agonist quinpirole did not. The alpha(1)-adrenoceptor antagonist prazosin could not block the increasing effect of dopamine on the frequency of spontaneous EPSCs in most cells tested. The protein kinase A inhibitor H-89 and the protein kinase C inhibitor chelerythrine could antagonize the effect of dopamine. The protein kinase A activator forskolin and the protein kinase C activator phorbol 12,13-dibutyrate could mimic the effect of dopamine. These results indicate that dopamine, presynaptically acting on dopamine D1 receptors, increases the frequency of spontaneous EPSCs via intracellular protein kinase A and protein kinase C signaling pathways in pyramidal cells of layers V-VI of the prelimbic cortex.

Intracellular sodium modulates the state of protein kinase C phosphorylation of rat proximal tubule Na+,K+-ATPase

The natriuretic hormone dopamine and the antinatriuretic hormone noradrenaline, acting on alpha-adrenergic receptors, have been shown to bidirectionally modulate the activity of renal tubular Na+,K+-adenosine triphosphate (ATPase). Here we have examined whether intracellular sodium concentration influences the effects of these bidirectional forces on the state of phosphorylation of Na+,K+-ATPase. Proximal tubules dissected from rat kidney were incubated with dopamine or the alpha-adrenergic agonist, oxymetazoline, and transiently permeabilized in a medium where sodium concentration ranged between 5 and 70 mM. The variations of sodium concentration in the medium had a proportional effect on intracellular sodium. Dopamine and protein kinase C (PKC) phosphorylate the catalytic subunit of rat Na+,K+-ATPase on the Ser23 residue. The level of PKC induced Na+,K+-ATPase phosphorylation was determined using an antibody that only recognizes Na+,K+-ATPase, which is not phosphorylated on its PKC site. Under basal conditions Na+,K+-ATPase was predominantly in its phosphorylated state. When intracellular sodium was increased, Na+,K+-ATPase was predominantly in its dephosphorylated state. Phosphorylation of Na+,K+-ATPase by dopamine was most pronounced when intracellular sodium was high, and dephosphorylation by oxymetazoline was most pronounced when intracellular sodium was low. The oxymetazoline effect was mimicked by the calcium ionophore A23187. An inhibitor of the calcium-dependent protein phosphatase, calcineurin, increased the state of Na+,K+-ATPase phosphorylation. The results imply that phosphorylation of renal Na+,K+-ATPase activity is modulated by the level of intracellular sodium and that this effect involves PKC and calcium signalling pathways. The findings may have implication for the regulation of salt excretion and sodium homeostasis.

Role of cAMP-PKA-PLC signaling cascade on dopamine-induced PKC-mediated inhibition of renal Na(+)-K(+)-ATPase activity

We studied the molecular events set into motion by stimulation of D(1)-like receptors downstream of Na(+)-K(+)-ATPase, while measuring apical-to-basal ouabain-sensitive, amphotericin B-induced increases in short-circuit current in opossum kidney (OK) cells. The D(1)-like receptor agonist SKF-38393 decreased Na(+)-K(+)-ATPase activity (IC(50), 130 nM). This effect was prevented by the D(1)-like receptor antagonist SKF-83566, overnight cholera toxin treatment, the protein kinase A (PKA) antagonist H-89, or the PKC antagonist chelerythrine, but not the mitogen-activated PK inhibitor PD-098059 or phosphatidylinositol 3-kinase inhibitors wortmannin and LY-294002. Dibutyryl cAMP (DBcAMP) and phorbol 12,13-dibutyrate (PDBu) both effectively reduced Na(+)-K(+)-ATPase activity. PKA downregulation abolished the inhibitory effects of SKF-38393 and DBcAMP but not those of PDBu. PKC downregulation abolished inhibition by PDBu, SKF-38393, and DBcAMP. The phospholipase C (PLC) inhibitor U-73122 prevented inhibition by SKF-38393 and DBcAMP. However, DBcAMP increased PLC activity. Although OK cells express both G(s)alpha and G(q/11)alpha proteins, D(1)-like receptors are coupled to G(s)alpha proteins only, as evidenced by studies in cells treated overnight with specific antibodies raised against G(s)alpha and G(q/11)alpha proteins. We conclude that PLC and Na(+)-K(+)-ATPase are effector proteins for PKA and PKC, respectively, after stimulation of D(1)-like receptors coupled to G(s)alpha proteins, in a sequence of events that begins with adenylyl cyclase-PKA system activation followed by PLC-PKC system activation.

D1/D5 dopamine receptors stimulate intracellular calcium release in primary cultures of neocortical and hippocampal neurons

D1/D5 dopamine receptors in basal ganglia, hippocampus, and cerebral cortex modulate motor, reward, and cognitive behavior. Previous work with recombinant proteins revealed that in cells primed with heterologous G(q/11)-coupled G-protein-coupled receptor (GPCR) agonists, the typically G(s)-linked D1/D5 receptors can stimulate robust release of calcium from internal stores when coexpressed with calcyon. To learn more about the intracellular signaling mechanisms underlying these D1/D5 receptor regulated behaviors, we explored the possibility that endogenous receptors stimulate internal release of calcium in neurons. We have identified a population of neurons in primary cultures of hippocampus and neocortex that respond to D1/D5 dopamine receptor agonists with a marked increase in intracellular calcium (Ca) levels. The D1/D5 receptor stimulated responses occurred in the absence of extracellular Ca(2+) indicating the rises in Ca involve release from internal stores. In addition, the responses were blocked by D1/D5 receptor antagonists. Further, the D1/D5 agonist-evoked responses were state dependent, requiring priming with agonists of G(q/11)-coupled glutamate, serotonin, muscarinic, and adrenergic receptors or with high external K(+) solution. In contrast, D1/D5 receptor agonist-evoked Ca(2+) responses were not detected in neurons derived from striatum. However, D1/D5 agonists elevated cAMP levels in striatal cultures as effectively as in neocortical and hippocampal cultures. Further, neither forskolin nor 8-Br-cAMP stimulation following priming was able to mimic the D1/D5 agonist-evoked Ca(2+) response in neocortical neurons indicating that increased cAMP levels are not sufficient to stimulate Ca release. Our data suggest that D1-like dopamine receptors likely modulate neocortical and hippocampal neuronal excitability and synaptic function via Ca(2+) as well as cAMP-dependent signaling.

Dopamine, hypertension and obesity

Dopamine, a neurotransmitter, precursor of noradrenaline, is responsible for cardiovascular and renal actions, such as increase in myocardial contractility and cardiac output, without changes in heart rate, producing passive and active vasodilatation, diuresis and natriuresis. These cardiovascular and renal actions take place through the interaction with dopamine receptors, D(1), D(2), D(3), D(4), and D(5). Recent findings point to the possibility of D(6) and D(7)receptors. Dopamine is known to influence the control of arterial pressure by influencing the central and peripheral nervous system and target organs such as kidneys and adrenal glands, in some types of hypertension. Although dopamine and its derivatives have been shown to have antihypertensive effects, these are still being studied; therefore it is important to explain some physiological and pharmacological aspects of dopamine, its receptors, and the clinical uses it could have in the treatment of arterial hypertension and more recently in obesity, based on evidence proving a clear association between obesity and the decrease in the expression of D(2) receptors in the brain of obese persons.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Dopamine-induced translocation of protein kinase C isoforms visualized in renal epithelial cells

Short-term regulation of sodium metabolism is dependent on the modulation of the activity of sodium transporters by first and second messengers. In understanding diseases associated with sodium retention, it is necessary to identify the coupling between these messengers. We have examined whether dopamine, an important first messenger in tubular cells, activates and translocates various protein kinase C (PKC) isoforms. We used a proximal tubular-like cell line, LLCPK-1 cells, in which dopamine was found to inhibit Na(+)-K(+)-ATPase in a PKC-dependent manner. Translocation of PKC isoforms was studied with both subcellular fractionation and confocal microscopy. Both techniques revealed a dopamine-induced translocation from cytosol to plasma membrane of PKC-alpha and -epsilon, but not of PKC-delta, -gamma, and -zeta. The process of subcellular fractionation resulted in partial translocation of PKC-epsilon. This artifact was eliminated in confocal studies. Confocal imaging permitted detection of translocation within 20 s. Translocation was abolished by a phospholipase C inhibitor and by an antagonist against the dopamine 1 subtype (D(1)) but not the 2 subtype of receptor (D(2)). In conclusion, this study visualizes in renal epithelial cells a very rapid activation of the PKC-alpha and -epsilon isoforms by the D(1) receptor subtype.

Phosphorylation of protein phosphatase-1 inhibitors, inhibitor-1 and DARPP-32, in renal medulla

Inhibitor-1 and DARPP-32 (dopamine and cAMP-regulated phosphoprotein, Mr 32 kDa) are each phosphorylated by cAMP-dependent protein kinase, resulting in their conversion to potent inhibitors of protein phosphatase-1. Protein phosphatase-1 is involved in the regulation of Na(+) reabsorption from renal tubule by modulating the activity of Na(+),K(+)-ATPase. In this study, we have investigated the regulation of inhibitor-1 and DARPP-32 phosphorylation in slices of renal medulla. Activation of cAMP-dependent protein kinase by forskolin and 8-bromo-cAMP increased the level of phosphorylated inhibitor-1. Okadaic acid (1 microM), used to inhibit protein phosphatase-2A, increased the level of phosphorylated inhibitor-1, but cyclosporin A had no effect. DARPP-32, like inhibitor-1, was phosphorylated by cAMP-dependent protein kinase and dephosphorylated only by protein phosphatase-2A. These data demonstrate that the phosphorylation of inhibitor-1 and DARPP-32 is regulated by the balance of phosphorylation by cAMP-dependent protein kinase and dephosphorylation by protein phosphatase-2A in renal medulla. Furthermore, the phosphorylation step is regulated by pharmacological stimuli such as activation of beta(1)-adrenoceptors and dopamine D1 receptors.

Intrarenal dopamine: a key signal in the interactive regulation of sodium metabolism

The kidney regulates sodium metabolism with extraordinary precision and sensitivity. This is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between anti-natriuretic and natriuretic factors. Dopamine, produced in renal proximal tubule cells, plays a central role in this interactive network. Natriuretic hormones that are released from extrarenal sources, such as atrial natriuretic peptide, mediate some of their effects via renal dopamine receptors. On the level of the tubules, dopamine acts by opposing the effects of anti-natriuretic factors, such as angiotensin II and alpha-adrenergic receptors. Sodium retention leads to an increase in renal dopamine tonus, and the natriuretic effects of dopamine are more prominent under this condition. Inhibition or down-regulation of dopamine receptors significantly attenuates the natriuretic response to salt loading. Renal dopamine is modulated by the supply of filtered L-DOPA and the metabolism of dopamine via catechol-O-methyldopamine. The importance of dopamine as a natriuretic hormone is reflected by its capacity to inhibit the majority of renal tubule sodium transporters. Notably, the activity of Na+, K+ ATPase is inhibited in most tubule segments by dopamine. Recent studies have elucidated many of the signaling pathways for renal dopamine receptors. Novel principles for homologous and heterologous sensitization of dopamine receptors have been detected that may explain some of the interaction between dopamine and other first messengers that modulate renal tubule sodium transport. A broad understanding of the renal dopamine system has become increasingly important, since there is now strong evidence from both clinical and experimental studies that dysregulation of the renal dopamine system plays a role in many forms of multigenetic hypertension.

Dopamine(1) receptor, G(salpha), and Na(+)-H(+) exchanger interactions in the kidney in hypertension

The ability of dopamine(1) (D(1)) receptors to inhibit luminal Na(+)-H(+) exchanger (NHE) activity in renal proximal tubules and induce a natriuresis is impaired in spontaneously hypertensive rats (SHR). However, it is not clear whether the defect is at the level of the D(1) receptor, G(salpha), or effector proteins. The coupling of the D(1) receptor to G(salpha) and NHE3 was studied in renal brush border membranes (BBM), devoid of cytoplasmic second messengers. D(1) receptor, G(salpha), and NHE3 expressions were similar in SHR and their normotensive controls, Wistar-Kyoto rats (WKY). Guanosine-5'-O:-(3-thiotriphosphate) (GTPgammaS) decreased NHE activity and increased NHE3 linked with G(salpha) similarly in WKY and SHR, indicating normal G(salpha) and NHE3 regulation in SHR. However, D(1) agonists increased NHE3 linked with G(salpha) in WKY but not in SHR, and the inhibitory effects of D(1) agonists on NHE activity were less in SHR than in WKY. Moreover, GTPgammaS enhanced the inhibitory effect of D(1) agonist on NHE activity in WKY but not in SHR, suggesting an uncoupling of the D(1) receptor from G(salpha)/NHE3 in SHR. Similar results were obtained with the use of immortalized renal proximal tubule cells from WKY and SHR. We conclude that the defective D(1) receptor function in renal proximal tubules in SHR is proximal to G(salpha)/effectors and presumably at the receptor level. The mechanism(s) responsible for the uncoupling of the D(1) receptor from G proteins remains to be determined. Because the primary structure of the D(1) receptor is not different between normotensive and hypertensive rats, differences in D(1) receptor posttranslational modification are possible.

Transcriptional regulation of a receptor protein tyrosine phosphatase gene hPTP-J by PKC-mediated signaling pathways in Jurkat and Molt-4 T lymphoma cells

The recently cloned type II receptor protein tyrosine phosphatase (RPTP) gene hPTP-J is a new member of the MAM (meprin, A5, PTPmicro) domain subfamily. We previously reported that hPTP-J mRNA was detected significantly in Jurkat T lymphoma cells and its expression was completely down-regulated by phorbol myristate acetate (PMA). In this study, we investigated what signaling pathways/molecules are involved in the transcriptional regulation of hPTP-J expression in Jurkat and Molt-4 T cell lines. The hPTP-J transcription was transiently up-regulated 20 min after the addition of PMA (20 ng/ml) to the Jurkat culture, followed by the complete down-regulation in 8 h after PMA addition. The transient up-regulation and the complete down-regulation induced by PMA was blocked by a PKC-specific inhibitor, GF109203X, suggesting that the regulatory effect of PMA on the hPTP-J transcription depends on protein kinase C activation. hPTP-J transcription was down-regulated not only by PMA but also by several signaling modulators including 1-oleoyl-2-acetylglycerol, forskolin, orthovanadate, manumycin and okadaic acid. Therefore, several signaling molecules such as protein tyrosine phosphatases, PP2A/CaMKIV and Ras are required for hPTP-J transcription in Jurkat and Molt-4 cells.

Opposite modulation of cortical N-methyl-D-aspartate receptor-mediated responses by low and high concentrations of dopamine

To examine whether dopamine modulates cortical N-methyl-D-aspartate receptor-mediated glutamate transmission, whole-cell recordings were made from identified pyramidal cells located in layers V and VI of the medial prefrontal cortex of the rat using a slice preparation. In the presence of tetrodotoxin and the absence of Mg2+, a brief local application of N-methyl-D-aspartate evoked an inward current which was blocked by the N-methyl-D-aspartate antagonist dizocilpine maleate but not affected by the non-N-methyl-D-aspartate antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline, suggesting that the observed current is mediated by N-methyl-D-aspartate receptors located on recorded cells. Bath application of dopamine produced opposite effects on the N-methyl-D-aspartate current depending on the concentrations of dopamine applied. At low concentrations (<50 microM), dopamine enhanced the N-methyl-D-aspartate current, whereas at higher concentrations, dopamine suppressed the current. The same concentrations of dopamine did not significantly affect the inward current induced by the non-N-methyl-D-aspartate agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. The enhancing effect of dopamine on the N-methyl-D-aspartate response was mimicked by the D1 agonist SKF38393 and blocked by the D1 antagonist SCH31966, whereas the suppressing effect was mimicked by the D2 agonist quinpirole and blocked by the D2 antagonist eticlopride. The above results suggest that dopamine at low concentrations acts preferentially on D1-like receptors to promote N-methyl-D-aspartate receptor-mediated transmission, while at high concentrations dopamine also activates D2-like receptors, leading to a suppression of the N-methyl-D-aspartate function. This differential modulation of N-methyl-D-aspartate function may have significant implications for understanding behaviors and disorders involving both cortical dopamine- and glutamate-mediated neurotransmission.

New insights into dopaminergic receptor function using antisense and genetically altered animals

Dopaminergic receptors are widespread throughout the central and peripheral nervous systems, where they regulate a variety of physiological, behavioral, and endocrine functions. These receptors are also clinically important drug targets for the treatment of a number of disorders, such as Parkinson's disease, schizophrenia, and hyperprolactinemia. To date, five different dopamine receptor subtypes have been cloned and characterized. Many of these subtypes are pharmacologically similar, making it difficult to selectively stimulate or block a specific receptor subtype in vivo. Thus, the assignment of various physiological or behavioral functions to specific dopamine receptor subtypes using pharmacological tools is difficult. In view of this, a number of investigators have--in order to elucidate functional roles--begun to use highly selective genetic approaches to alter the expression of individual dopamine receptor subtypes in vivo. This review discusses recent studies involving the use of genetic approaches for the study of dopaminergic receptor function.

Selective inhibition of the renal dopamine subtype D1A receptor induces antinatriuresis in conscious rats

Both dopamine D1-like (D1A and D1B) and D2-like (D2, D3, and D4) receptor subfamilies are present in the kidney. Blockade of the intrarenal D1-like receptor family is associated with natriuresis and diuresis. Because the D1A and D1B receptor subtypes are not distinguishable by currently available dopaminergic agents, their functional role remains undefined. In the present study, the effect of selective inhibition of the renal D1A receptor with phosphorothioated antisense oligodeoxynucleotide (AS-ODN) was investigated in conscious uninephrectomized rats. After renal interstitial administration of Texas red-labeled D1A receptor AS-ODN, intense fluorescent signal was localized in the renal tubular epithelium and vasculature. In rats on normal salt intake, AS-ODN injected interstitially into the kidney reduced daily urinary sodium excretion (1.4+/-0.04 versus 0.8+/-0.2 mEq/d, n=5, P<0.05) and urine output (16.9+/-3.8 versus 12.5+/-3.6 mL/d, n=5, P<0.05). In rats on high sodium intake, continuous renal interstitial administration of D1A receptor AS-ODN transiently decreased daily urinary sodium excretion (5.4+/-0.5 versus 4.2+/-0.3 mEq/d, n=7, P<0.01) and urine output (27.6+/-4.5 versus 18.1+/-1.8 mL/d, n=7, P<0.01). Neither vehicle nor sense oligodeoxynucleotide had significant effects. Systolic blood pressure remained unchanged. The renal D1A receptor protein was significantly decreased by 35% and 46% at the end of the study in AS-ODN-treated rats on normal and high salt intake, respectively, whereas the D1B receptor and beta-actin were not affected. These results provide the first direct evidence that the renal D1A receptor subtype plays an important role in the control of sodium excretion.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Differential routes of Ca2+ influx in Swiss 3T3 fibroblasts in response to receptor stimulation

Ca2+ influx into cells in response to stimulation of various receptors was studied with Swiss 3T3 fibroblasts. The mechanisms involved were found to be so diverse that they were classified into four groups, Type I to IV. Type-I influx occurred, via pertussis toxin-susceptible G-proteins, immediately after internal Ca2+ mobilization by bradykinin, thrombin, endothelin, vasopressin or angiotensin II. Type-II influx induced by bombesin differed from Type I in its insusceptibility to pertussis toxin treatment. Ca2+ influx induced by prostaglandin E1, referred to as Type-III influx, was unique in that phospholipase C was apparently not activated without extracellular Ca2+, strongly suggesting that the Ca2+ influx preceded and was responsible for InsP3 generation and internal Ca2+ mobilization. More Ca2+ entered the cells more slowly via the Type-IV route opened by platelet-derived and other growth factors. These types of Ca2+ influx could be differentiated by their different susceptibilities to protein kinase C maximally activated by 1 h of exposure of cells to PMA, which inhibited phospholipase Cbeta coupled to receptors involved in Type-I and -II influx but did not inhibit growth-factor-receptor-coupled phospholipase Cgamma. Type-I and -II Ca2+ influxes, together with store-operated influx induced by thapsigargin, were not directly inhibited by exposure of cells to PMA, but Type-III and -IV influxes were completely inhibited. In addition, stimulation of receptors involved in Type-I and -IV Ca2+ influx, but not Type-II and -III influx, led to phospholipase A2 activation in the presence of extracellular Ca2+. Inhibition of Type-I and -IV Ca2+ influxes by their respective inhibitors, diltiazem and nifedipine, resulted in abolition of phospholipase A2 activation induced by the respective receptor agonists, in agreement with the notion that Ca2+ influx via these routes is responsible for receptor-mediated phospholipase A2 activation.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Dopamine D1A receptors and renin release in rat juxtaglomerular cells

Two dopamine D1-like receptors have been cloned from mammals, the D1 and D5 receptors, also known as D1A and D1B receptors, respectively, in rodents. Although D1-like receptors are known to stimulate renin release, the receptor subtype mediating this action has not been determined. We investigated D1 receptor subtype expression in rat juxtaglomerular cells obtained after enzymatic dispersion of kidney cortex and differential centrifugation. Juxtaglomerular cells in primary culture were immunocytochemically 85% to 95% renin positive. These cells expressed the D1A but not the D1B receptor (mRNA and protein). D1-like receptor function was demonstrated by a concentration-dependent stimulation of cAMP production by dopamine (n = 5-9 per group). Fenoldopam, a D1-like receptor agonist, also caused a concentration-dependent increase in cAMP production and renin secretion that was blocked by the selective D1-like receptor antagonist SCH23390 (n = 4-13 per group). Although the D1 ligands do not distinguish between the cloned D1-like receptors, the actions of fenoldopam were due to occupancy of the D1A receptor: (1) the D1B receptor, the only other mammalian D1-like receptor, is not expressed in juxtaglomerular cells; (2) antisense but not sense D1A oligonucleotides completely blocked the stimulatory effect of fenoldopam on cAMP production and renin secretion. We conclude that there is selective dopamine receptor gene expression in juxtaglomerular cells; the dopamine receptor subtype linked to the stimulation of cAMP and renin secretion in juxtaglomerular cells is the D1A subtype.