D2-like Receptor Family Research Articles

Dopamine and Dopamine-Related Ligands Can Bind Not Only to Dopamine Receptors.

The dopaminergic system is one of the most important neurotransmitter systems in the central nervous system (CNS). It acts mainly by activation of the D1-like receptor family at the target cell. Additionally, fine-tuning of the signal is achieved via pre-synaptic modulation by the D2-like receptor family. Some dopamine drugs (both agonists and antagonists) bind in addition to DRs also to α2-ARs and 5-HT receptors. Unfortunately, these compounds are often considered subtype(s) specific. Thus, it is important to consider the presence of these receptor subtypes in specific CNS areas as the function virtually elicited by one receptor type could be an effect of other—or the co-effect of multiple receptors. However, there are enough molecules with adequate specificity. In this review, we want to give an overview of the most common off-targets for established dopamine receptor ligands. To give an overall picture, we included a discussion on subtype selectivity. Molecules used as antipsychotic drugs are reviewed too. Therefore, we will summarize reported affinities and give an outline of molecules sufficiently specific for one or more subtypes (i.e., for subfamily), the presence of DR, α2-ARs, and 5-HT receptors in CNS areas, which could help avoid ambiguous results.

A test of context- and sex-dependent dopaminergic effects on the behavior of a gregarious bird, the common waxbill, Estrilda astrild.

The dopaminergic (DAergic) system has well-known influences on behavioral and cognitive functions. Previous work with common waxbills (Estrilda astrild) reported context-specific DAergic effects that could have been due to social environment. Manipulating the dopamine D2-like receptor family (D2R) pathways had opposing effects on behavior depending on whether waxbills were tested alone or in a small cage with a mirror as a social stimulus. As waxbills are highly gregarious, it was hypothesized that being alone or perceiving that they have a companion might explain this context dependence. To test context-dependent DAergic effects, we compared behavioral effects of D2R manipulation in waxbills in the same familiar environment, but either alone or with a familiar, same-sex companion. We found that D2R agonism decreased movement and feeding, similar to previous results when testing waxbills alone. However, contrary to the hypothesis of dependence on social context, we found that the behavioral effects of the D2R agonist were unchanged when waxbills were tested with a companion. The context dependence reported earlier might thus be due to other factors, such as the stress of being in a novel environment (small cage) or with an unfamiliar social stimulus (mirror image). In tests with a companion, we also found a sex-specific social effect of D2R manipulation: D2R blocking tended to decrease aggression in males but to increase it in females. Together with past work, our results suggest that DAergic effects on behavior involve different types of context or sex dependence.

Altered Pain Processing Associated with Administration of Dopamine Agonist and Antagonist in Healthy Volunteers.

Striatal dopamine dysfunction is associated with the altered top-down modulation of pain processing. The dopamine D2-like receptor family is a potential substrate for such effects due to its primary expression in the striatum, but evidence for this is currently lacking. Here, we investigated the effect of pharmacologically manipulating striatal dopamine D2 receptor activity on the anticipation and perception of acute pain stimuli in humans. Participants received visual cues that induced either certain or uncertain anticipation of two pain intensity levels delivered via a CO2 laser. Rating of the pain intensity and unpleasantness was recorded. Brain activity was recorded with EEG and analysed via source localisation to investigate neural activity during the anticipation and receipt of pain. Participants completed the experiment under three conditions, control (Sodium Chloride), D2 receptor agonist (Cabergoline), and D2 receptor antagonist (Amisulpride), in a repeated-measures, triple-crossover, double-blind study. The antagonist reduced an individuals’ ability to distinguish between low and high pain following uncertain anticipation. The EEG source localisation showed that the agonist and antagonist reduced neural activations in specific brain regions associated with the sensory integration of salient stimuli during the anticipation and receipt of pain. During anticipation, the agonist reduced activity in the right mid-temporal region and the right angular gyrus, whilst the antagonist reduced activity within the right postcentral, right mid-temporal, and right inferior parietal regions. In comparison to control, the antagonist reduced activity within the insula during the receipt of pain, a key structure involved in the integration of the sensory and affective aspects of pain. Pain sensitivity and unpleasantness were not changed by D2R modulation. Our results support the notion that D2 receptor neurotransmission has a role in the top-down modulation of pain.

The Dopamine D5 receptor contributes to activation of cholinergic interneurons during L-DOPA induced dyskinesia

The dopamine D5 receptor (D5R) is a Gαs-coupled dopamine receptor belonging to the dopamine D1-like receptor family. Together with the dopamine D2 receptor it is highly expressed in striatal cholinergic interneurons and therefore is poised to be a positive regulator of cholinergic activity in response to L-DOPA in the dopamine-depleted parkinsonian brain. Tonically active cholinergic interneurons become dysregulated during chronic L-DOPA administration and participate in the expression of L-DOPA induced dyskinesia. The molecular mechanisms involved in this process have not been elucidated, however a correlation between dyskinesia severity and pERK expression in cholinergic cells has been described. To better understand the function of the D5 receptor and how it affects cholinergic interneurons in L-DOPA induced dyskinesia, we used D5R knockout mice that were rendered parkinsonian by unilateral 6-OHDA injection. In the KO mice, expression of pERK was strongly reduced indicating that activation of these cells is at least in part driven by the D5 receptor. Similarly, pS6, another marker for the activity status of cholinergic interneurons was also reduced. However, mice lacking D5R exhibited slightly worsened locomotor performance in response to L-DOPA and enhanced LID scores. Our findings suggest that D5R can modulate L-DOPA induced dyskinesia and is a critical activator of CINs via pERK and pS6.

Comparison of Dopamine D3 and D2 Receptor Occupancies by a Single Dose of Blonanserin in Healthy Subjects: A Positron Emission Tomography Study With [11C]-(+)-PHNO

BackgroundBlockade of D3 receptor, a member of the dopamine D2-like receptor family, has been suggested as a possible medication for schizophrenia. Blonanserin has high affinity in vitro for D3 as well as D2 receptors. We investigated whether a single dose of 12 mg blonanserin, which was within the daily clinical dose range (i.e., 8–24 mg) for the treatment of schizophrenia, occupies D3 as well as D2 receptors in healthy subjects.MethodsSix healthy males (mean 35.7±7.6 years) received 2 positron emission tomography scans, the first prior to taking blonanserin, and the second 2 hours after the administration of a single dose of 12 mg blonanserin. Dopamine receptor occupancies by blonanserin were evaluated by [11C]-(+)-PHNO.ResultsOccupancy of each region by 12 mg blonanserin was: caudate (range 64.3%–81.5%; mean±SD, 74.3±5.6%), putamen (range 60.4%–84.3%; mean±SD, 73.3%±8.2%), ventral striatum (range 40.1%–88.2%; mean±SD, 60.8%±17.1%), globus pallidus (range 65.8%–87.6%; mean±SD, 75.7%±8.6%), and substantia nigra (range 56.0%–88.7%; mean±SD, 72.4%±11.0%). Correlation analysis between plasma concentration of blonanserin and receptor occupancy in D2-rich (caudate and putamen) and D3-rich (globus pallidus and substantia nigra) regions showed that EC50 for D2-rich region was 0.39 ng/mL (r=0.43) and EC50 for D3-rich region was 0.40 ng/mL (r=0.79).ConclusionsA single dose of 12 mg blonanserin occupied D3 receptor to the same degree as D2 receptor in vivo. Our results were consistent with previous studies that reported that some of the pharmacological effect of blonanserin is mediated via D3 receptor antagonism.

Palmitoylation of the carboxyl-terminal tail of dopamine D4 receptor is required for surface expression, endocytosis, and signaling

The amino acid sequences and signaling pathways of D2-like dopamine receptors (D2R, D3R, and D4R) are highly conserved. D4R has been suggested to be associated with novelty seeking, and binds with high affinity to an atypical antipsychotic drug with fewer motor function-related side effects than typical neuroleptics. A study comparing D2R and D3R reported that palmitoylation is important for the proper functioning of D3R. Although D4R is a member of the D2-like receptor family, its palmitoylation status and the functional significance of any such posttranslational modification are unknown. In this study, it was found that D4-4, an alternatively spliced form of D4R, was palmitoylated on Cys467, the terminal amino acid residue of the receptor. When palmitoylation of D4R was inhibited, by either mutation of the consensus site or by treatment with the palmitoylation inhibitor 2-bromopalmitate (2BP), D4R cell surface expression, signaling, and endocytosis were all impaired. Exchanging the carboxyl-terminal tails of D2R and D4R resulted in a switching of the palmitoylation phenotype, as well as concomitant changes in functionality, such as receptor surface expression, endocytosis, and signaling. Despite the high degree of homology in the carboxyl-terminal tails of D2-like receptors, palmitoylation occurs exclusively in the D3R and D4R family members, with the D4R tail being sufficient to endow D2R with palmitoylation-associated functionality. Thus, this study provides new insights into a consensus sequence for palmitoylation, as well as possible strategies for the regulation of D4R, which has implications for the treatment of various neurological and psychiatric disorders.

Evidence for Noncanonical Neurotransmitter Activation: Norepinephrine as a Dopamine D2-Like Receptor Agonist

The Gαi/o-coupled dopamine D2-like receptor family comprises three subtypes: the D2 receptor (D2R), with short and long isoform variants (D2SR and D2LR), D3 receptor (D3R), and D4 receptor (D4R), with several polymorphic variants. The common overlap of norepinephrine innervation and D2-like receptor expression patterns prompts the question of a possible noncanonical action by norepinephrine. In fact, previous studies have suggested that norepinephrine can functionally interact with D4R. To our knowledge, significant interactions between norepinephrine and D2R or D3R receptors have not been demonstrated. By using radioligand binding and bioluminescent resonance energy transfer (BRET) assays in transfected cells, the present study attempted a careful comparison between dopamine and norepinephrine in their possible activation of all D2-like receptors, including the two D2R isoforms and the most common D4R polymorphic variants. Functional BRET assays included activation of G proteins with all Gαi/o subunits, adenylyl cyclase inhibition, and β arrestin recruitment. Norepinephrine acted as a potent agonist for all D2-like receptor subtypes, with the general rank order of potency of D3R > D4R ≥ D2SR ≥ D2L. However, for both dopamine and norepinephrine, differences depended on the Gαi/o protein subunit involved. The most striking differences were observed with Gαi2, where the rank order of potencies for both dopamine and norepinephrine were D4R > D2SR = D2LR >> D3R. Furthermore the results do not support the existence of differences in the ability of dopamine and norepinephrine to activate different human D4R variants. The potency of norepinephrine for adrenergic α2A receptor was only about 20-fold higher compared with D3R and D4R across the three functional assays.

Dopamine Modulates the Activity of Sensory Hair Cells.

The neurotransmitter dopamine acts in a paracrine fashion (diffusion over a short distance) in several tissues and bodily organs, influencing and regulating their activity. The cellular target and mechanism of the action of dopamine in mechanosensory organs, such as the inner ear and lateral-line organ, is not clearly understood. Here we demonstrate that dopamine receptors are present in sensory hair cells at synaptic sites that are required for signaling to the brain. When nearby neurons release dopamine, activation of the dopamine receptors increases the activity of these mechanosensitive cells. The mechanism of dopamine activation requires voltage-gated calcium channels that are also present at hair-cell synapses.

Dopamine D3 Receptor Modulates l-DOPA-Induced Dyskinesia by Targeting D1 Receptor-Mediated Striatal Signaling.

The dopamine D3 receptor (D3R) belongs to the dopamine D2-like receptor family and is principally located in the ventral striatum. However, previous studies reported D3R overexpression in the dorsal striatum following l-DOPA treatment in parkinsonian animals. This fact has drawn attention in the importance of D3R in l-DOPA-induced dyskinesia (LID). Here, we used D3R knockout mice to assess the role of D3R in LID and rotational sensitization in the hemiparkinsonian model. Mice lacking D3R presented a reduction in dyskinesia without interfering with the antiparkinsonian l-DOPA effect and were accompanied by a reduction in the l-DOPA-induced rotations. Interestingly, deleting D3R attenuated important molecular markers in the D1R-neurons such as FosB, extracellular signal-regulated kinase, and histone-3 (H3)-activation. Colocalization studies in D1R-tomato and D2R-green fluorescent protein BAC-transgenic mice indicated that l-DOPA-induced D3R overexpression principally occurs in D1R-containing neurons although it is also present in the D2R-neurons. Moreover, D3R pharmacological blockade with PG01037 reduced dyskinesia and the molecular markers expressed in D1R-neurons. In addition, this antagonist further reduced dyskinetic symptoms in D1R heterozygous mice, indicating a direct interaction between D1R and D3R. Together, our results demonstrate that D3R modulates the development of dyskinesia by targeting D1R-mediated intracellular signaling and suggest that decreasing D3R activity may help to ameliorate LID.

Towards a Better Understanding of Disordered Gambling: Efficacy of Animal Paradigms in Modelling Aspects of Gambling Behaviour

The parallels between gambling and substance addiction are increasingly acknowledged; despite this, legislation surrounding gambling is being progressively relaxed and opportunities to gamble continue to increase. There are currently no dedicated treatment options for gambling disorder, and as such, it may come to represent a considerable public health concern. Gamblers represent a heterogeneous population; yet, deficits have been repeatedly observed in regard to impulse control and decision making. Additionally, gamblers typically demonstrate greater endorsement of irrational or distorted beliefs regarding gambling in comparison to healthy controls. In this review, we suggest that animal models may offer a valuable opportunity to elucidate the neurobiological basis underlying these three major areas of dysfunction. We present a number of rodent behavioural paradigms that indicate a somewhat common involvement of the dopamine D2-like receptor family. Yet, importantly, these tasks show that these behaviours appear to be both pharmacologically and neurologically dissociable. Use of these animal models could therefore theoretically inform more symptom-directed pharmacotherapies based on areas of dysfunction seen in individuals.

A D2-like receptor family agonist produces analgesia in mechanonociception but not in thermonociception at the spinal cord level in rats

The administration of dopaminergic drugs produces analgesia in individuals experiencing different types of pain. Analgesia induced by these drugs at the spinal cord level is mediated by D2-like agonists, which specifically inhibit the detection of nociceptive stimuli by sensory afferents. The extent of the analgesia provided by spinal dopamine agonists remains controversial, and the cellular mechanism of this analgesic process is poorly understood. The objective of this study was to evaluate the analgesic effect of quinpirole, a D2-like agonist, based on two nociceptive tests and at various doses that were selected to specifically activate dopamine receptors. We found that intrathecal quinpirole administration produces analgesia of mechanical but not thermal nociception and that the analgesic effect of quinpirole is reversed by a mix of D2, D3, and D4 receptor-specific antagonists, suggesting that the activation of all D2-like receptors is involved in the analgesia produced by intrathecal quinpirole. The differential effect on thermal and mechanical nociception was also tested upon the activation of μ-opioid receptors. As reported previously, low doses of the μ-opioid receptor agonist DAMGO produced analgesia of only thermonociception. This evidence shows that a D2-like receptor agonist administered at the spinal cord level produces analgesia specific to mechanonociception but not thermonociception.

Applications of the Neonatal Quinpirole Model to Psychosis and Convergence upon the Dopamine D2 Receptor.

This mini review focuses on the importance of the dopamine D2-like receptor family and its importance in psychosis. Past findings from this laboratory along with collaborators have been that neonatal quinpirole (a dopamine D2-like receptor agonist) results in increases in dopamine D2 receptor sensitivity that persists throughout the animal's lifetime. Findings from this model have been shown to have particular application and validity to schizophrenia, but may have broader implications toward other psychoses, which is reviewed in the present manuscript. In the present review, we also highlight other models of psychoses that have been centered on the subchronic administration of quinpirole to rats in order to model certain psychoses, which has uncovered some interesting and valid behavioral findings. This review highlights the importance of the combination of behavioral findings and neurobiological mechanisms focusing on neural plasticity in discovering underlying pathologies in these disorders that may lead to treatment discoveries, as well as the value of animal models across all psychoses.

What is the role of the D3 receptor in addiction? A mini review of PET studies with [11C]-(+)-PHNO

The chronic use of drugs, including psychostimulants such as cocaine and amphetamine, has been associated with low D2/3 dopamine receptor availability, which in turn has been linked to poor clinical outcome. In contrast, recent studies focused on the D3 receptor (a member of the D2-like receptor family) suggest that chronic exposure to stimulant drugs can up-regulate this receptor subtype, which, in preclinical models, is linked to dopamine system sensitization - a process hypothesized to contribute to relapse in addiction. In this mini review we present recent human data suggesting that the D3 receptor may contribute to core features of addiction, and discuss the usefulness of the PET imaging probe [(11)C]-(+)-PHNO in investigating this question.

395 INTRARENAL ANGIOTENSIN AND DOPAMINE RECEPTOR INTERACTIONS IN THE CONTROL OF SODIUM EXCRETION AND BLOOD PRESSURE

The cardiovascular and renal actions of type-2 angiotensin (Ang) receptors (AT2Rs) generally oppose those mediated by Ang type-1 receptors (AT1Rs). At the level of cell signaling, AT2Rs inhibit MAP kinases ERK-1 and -2 reducing or eliminating AT1R-mediated increases in ERK activity. The bradykinin (BK) - nitric oxide (NO) - cyclic GMP (cGMP) pathway is now recognized as the extracellular signaling pathway relevant to the majority of AT2R actions. From a functional standpoint, AT2Rs induce systemic vasodilation and natriuresis, the latter of which is mediated by des-aspartyl1-Ang II (Ang III) rather than Ang II. AT2R inhibition of renal tubule Na+ transport occurs largely in renal proximal tubule cells (RPTCs) and also to some extent in the thick ascending limb of Henle. The renal dopaminergic system is crucial in the control of renal Na+ excretion and blood pressure. Dopamine (DA) is synthesized from filtered dihydroxyphenylalanine (DOPA) taken up into RPTCs. DA is exported from RPTCs largely into the tubule lumen, where it binds to the D1-like receptor family [D1R (D1 and D5 subtypes)]. D1R activation inhibits Na+ reabsorption through an adenylate cyclase-cyclic AMP (cAMP)-dependent mechanism. The importance of the renal dopaminergic system is underscored by the fact that renal D1Rs account for about 50% of basal Na+ excretion under normal to high Na+ intake conditions. Surprisingly, natriuresis induced by D1R activation in proximal tubules is abolished by concurrent AT2R blockade. We have found that DA-induced natriuresis requires cAMP-stimulated translocation and activation of AT2Rs at the apical plasma membranes of RPTCs. Renal interstitial accumulation of cAMP via direct activation of adenylyl cyclase with forskolin and selective inhibition of cAMP degradation with 3-isobutyl-1-methylxanthine (IBMX)] caused a significant and sustained increase in AT2R-mediated natriuresis. Direct agonist stimulation of D1Rs was not necessary for AT2R-mediated natriuresis since forskolin+IBMX-induced natriuresis persisted in the presence of D1R blockade with SCH-23390. These findings indicate that cAMP is a key mediator of the interaction between renal D1Rs and AT2Rs to induce natriuresis during high-salt states. Targeting the common trafficking pathways of these receptors would affect the natriuretic capacity of two of the most powerful systems governing Na+ homeostasis. Such endeavors hold significant clinical implications in cardiovascular medicine and hypertension where the focus has predominantly been on AT1R blockade and inhibition of the RAS.

Plasticity in D1-Like Receptor Expression Is Associated with Different Components of Cognitive Processes

Dopamine D1-like receptors consist of D1 (D1A) and D5 (D1B) receptors and play a key role in working memory. However, their possibly differential contribution to working memory is unclear. We combined a working memory training protocol with a stepwise increase of cognitive subcomponents and real-time RT-PCR analysis of dopamine receptor expression in pigeons to identify molecular changes that accompany training of isolated cognitive subfunctions. In birds, the D1-like receptor family is extended and consists of the D1A, D1B, and D1D receptors. Our data show that D1B receptor plasticity follows a training that includes active mental maintenance of information, whereas D1A and D1D receptor plasticity in addition accompanies learning of stimulus-response associations. Plasticity of D1-like receptors plays no role for processes like response selection and stimulus discrimination. None of the tasks altered D2 receptor expression. Our study shows that different cognitive components of working memory training have distinguishable effects on D1-like receptor expression.

D3 DOPAMINE RECEPTOR REGULATION OF D5 RECEPTOR EXPRESSION AND FUNCTION IN RENAL PROXIMAL TUBULE CELLS: PP.9.348

Disruption of the D3 dopamine receptor gene in mice produces hypertension that is associated with a decreased ability to excrete an acute sodium load. The mechanism of the decreased ability of the D3 receptor knockout mice to excrete a sodium load is not known. However, D3 receptors, with or without interaction with D1 receptors, can inhibit sodium transporter/pump activities (NHE3, Na+-K+ ATPase). We have reported positive interactions between D3 and D1 receptors in renal proximal tubule (RPT) cells; stimulation of one receptor increases the expression and function of the other receptor. Because both D1 and D5 receptors belong to the D1-like receptor family, we presume that there is also an interaction between D3 and D5 receptors in RPT cells. We now report that a D3 receptor agonist, PD128907, increased the expression of D5 receptors in a concentration- (10–9–10–6 M) [control = 0.6±0.2, 10–7 M/24hrs PD128907 = 1.3±0.2 density units (DU); P < 0.01, n = 6] and time (2 hrs-30 hrs)-dependent (control = 0.6±0.1, 10–7 M/24hrs PD128907 = 1.3±0.2 DU; P < 0.01, n = 4) manner in RPT cells from Wistar-Kyoto (WKY) rats. The effect of PD128907 (10–7 M) was blocked by the D3 receptor antagonist, U99194A (10–5 M/24hrs) (control = 0.8±0.07, PD128907 = 1.5±0.1, U99194A = 0.8±0.09, PD128907 + U99194A = 0.8±0.08 DU, P < 0.01, n = 7). The stimulatory effect of the D3 receptor on D5 receptor expression was impaired in RPT cells from spontaneously hypertensive rats (SHRs) (D5 receptor protein: WKY: control = 1±0.05, PD128907 = 1.9±0.1; SHR: control = 0.7±0.05, PD128907 = 0.5±0.08 DU, n = 6). Stimulation of the D5 receptor in D5 receptor-transfected HEK 293 (D5-HEK293) cells inhibited Na+-K+ ATPase activity. Pretreatment with PD128907 (10–7 M) for 24 hours increased the D5 receptor expression and the D5 receptor-mediated inhibitory effect on Na+-K+ ATPase activity in D5-HEK293 cells (control = 0.15±0.006, fenoldopam alone = 0.12±0.01, PD128907 pretreatment + fenoldopam = 0.09± 0.007, n = 8). We suggest that D3 and D5 receptors positively interact with each other. Altered interaction between D3 and D5 receptors may play a role in the pathogenesis of hypertension.

D1-like receptors inhibit insulin-induced vascular smooth muscle cell proliferation via down-regulation of insulin receptor expression

Vascular smooth muscle cell (VSMC) proliferation is central to the development of vascular diseases, including hypertension, which is regulated by numerous hormones and humoral factors. Our previous study showed that the stimulatory effect of norepinephrine on VSMC proliferation is inhibited by D1-like receptors and the D3 dopamine receptor, a member of the D2-like receptor family. Insulin is a proliferative hormone but it is not known if there is any interaction between insulin and D1-like receptors. We hypothesized that Dl-like receptors may have an inhibitory effect on the insulin-induced VSMC proliferation; aberrant insulin and Dl-like receptor functions could be involved in the pathogenesis of essential hypertension. VSMC proliferation was determined by [H]-thymidine incorporation; insulin receptor mRNA and protein expressions were determined by RT-PCR, immunoblotting, and immunohistochemistry. Insulin increased VSMC proliferation in immortalized aortic A10 cells, determined by [H]-thymidine incorporation. Although the D1-like receptor, by itself, had no effect on VSMC proliferation, stimulation with fenoldopam, a D1-like receptor agonist, inhibited the stimulatory effect of insulin. The inhibitory effect of fenoldopam on insulin-mediated VSMC proliferation was receptor specific, because its effect could be blocked by SCH23390, a D1-like receptor antagonist. Fenoldopam also inhibited insulin receptor mRNA and protein expression, which was time dependent and concentration dependent. A PKC or MAP kinase inhibitor blocked the inhibitory effect of fenoldopam on insulin receptor expression, indicating that PKC and MAP kinase were involved in the signaling pathway. The inhibitory effect of D1-like receptors on insulin-mediated VSMC proliferation may play an important role in the regulation of blood pressure.

Differential D 1 and D 5 Receptor Regulation and Degradation of the Angiotensin Type 1 Receptor

Renal sodium transport is increased by the angiotensin type 1 receptor (AT(1)R), which is counterregulated by dopamine via unknown mechanisms involving either the dopamine type 1 (D(1)R) or dopamine type 5 receptor (D(5)R) that belong to the D(1)-like receptor family of dopamine receptors. We hypothesize that the D(1)R and D(5)R differentially regulate AT(1)R protein expression and signaling, which may have important implications in the pathogenesis of essential hypertension. D(1)R and D(5)R share the same agonists and antagonists; therefore, the selective effects of either D(1)R or D(5)R stimulation on AT(1)R expression in human renal proximal tubule cells were determined using antisense oligonucleotides selective to either D(1)R or D(5)R. We also determined the role of receptor tyrosine kinase and the proteosome on the D(1)R/D(5)R-mediated effects on AT(1)R expression and internalization. In renal proximal tubule cells, D(5)R (not D(1)R) decreased AT(1)R expression (half-life: 0.47+/-0.18 hours) and AT(1)R-mediated extracellular signal-regulated kinase 1/2 phosphorylation (232+/-18.9 U with angiotensin II [10(-7) mol/L] versus 81+/-8.9 U with angiotensin II [10(-7) mol/L] and fenoldopam [D(1)R/D(5)R agonist; 10(-6) mol/L; P<0.05; n=6). The fenoldopam-induced decrease in AT(1)R expression was reversed by 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo (3,4-d) pyrimidine (c-Src tyrosine-kinase inhibitor) and clasto-lactacystin beta-lactone (proteasome inhibitor), demonstrating that the fenoldopam-mediated decrease in total cell AT(1)R expression is a result of a c-Src- and proteasome-dependent process. D(5)R stimulation decreases AT(1)R expression and is c-Src and proteasome dependent. The discovery of differential regulation by D(1)R and D(5)R opens new avenues for the development of agonists selective to either receptor subtype as targeted antihypertensive agents that can decrease AT(1)R-mediated antinatriuresis.

Dopamine D2-like Receptors Are Expressed in Pancreatic Beta Cells and Mediate Inhibition of Insulin Secretion

Dopamine signaling is mediated by five cloned receptors, grouped into D1-like (D1 and D5) and D2-like (D2, D3 and D4) families. We identified by reverse transcription-PCR the presence of dopamine receptors from both families in INS-1E insulin-secreting cells as well as in rodent and human isolated islets. D2 receptor expression was confirmed by immunodetection revealing localization on insulin secretory granules of INS-1E and primary rodent and human beta cells. We then tested potential effects mediated by the identified receptors on beta cell function. Dopamine (10 microM) and the D2-like receptor agonist quinpirole (5 microM) inhibited glucose-stimulated insulin secretion tested in several models, i.e. INS-1E beta cells, fluorescence-activated cell-sorted primary rat beta cells, and pancreatic islets of rat, mouse, and human origin. Insulin exocytosis is controlled by metabolism coupled to cytosolic calcium changes. Measurements of glucose-induced mitochondrial hyperpolarization and ATP generation showed that dopamine and D2-like agonists did not inhibit glucose metabolism. On the other hand, dopamine decreased cell membrane depolarization as well as cytosolic calcium increases evoked by glucose stimulation in INS-1E beta cells. These results show for the first time that dopamine receptors are expressed in pancreatic beta cells. Dopamine inhibited glucose-stimulated insulin secretion, an effect that could be ascribed to D2-like receptors. Regarding the molecular mechanisms implicated in dopamine-mediated inhibition of insulin release, our results point to distal steps in metabolism-secretion coupling. Thus, the role played by dopamine in glucose homeostasis might involve dopamine receptors, expressed in pancreatic beta cells, modulating insulin release.

Dopamine D2-Like Receptor Agonists Accelerate Barrier Repair and Inhibit the Epidermal Hyperplasia Induced by Barrier Disruption

Two families of G protein-coupled receptors of the neurotransmitter dopamine, i.e., dopamine 1-like family (D1-like receptor) and dopamine 2-like family (D2-like receptor), show dopaminergic activity in nerve cells. The D2-like receptor family, composed of D2, D3, and D4 receptors, downregulates the intracellular cAMP signaling pathway, and dopamine receptor agonists reduce the cAMP level in neurons. We previously demonstrated that the cAMP level in epidermal keratinocytes is related to epidermal barrier homeostasis. Since keratinocytes are known to carry various neurotransmitter receptors, we hypothesized that D2-like receptors on keratinocytes might be related to skin barrier homeostasis. In this study, we examined the effect of topical application of receptor agonists and antagonists on skin barrier recovery after barrier disruption. Application of D2-like receptor agonists accelerated barrier recovery, whereas D2-like receptor antagonists delayed it. D2-like receptor agonists also reduced the epidermal hyperplasia induced by barrier disruption under low environmental humidity. Immunohistochemical study and RT-PCR analysis revealed the expression of the D2 receptor in the basal epidermis and the D4 receptor in the uppermost layer of the epidermis. These results suggest that dopaminergic receptors are involved in epidermal barrier homeostasis.