Cocaine Inhibits Dopamine D2 Receptor Signaling via Sigma-1-D2 Receptor Heteromers

Under normal conditions the brain maintains a delicate balance between inputs of reward seeking controlled by neurons containing the D₁-like family of dopamine receptors and inputs of aversion coming from neurons containing the D₂-like family of dopamine receptors. Cocaine is able to subvert these balanced inputs by altering the cell signaling of these two pathways such that D₁ reward seeking pathway dominates. Here, we provide an explanation at the cellular and biochemical level how cocaine may achieve this. Exploring the effect of cocaine on dopamine D₂ receptors function, we present evidence of σ₁ receptor molecular and functional interaction with dopamine D₂ receptors. Using biophysical, biochemical, and cell biology approaches, we discovered that D₂ receptors (the long isoform of the D₂ receptor) can complex with σ₁ receptors, a result that is specific to D₂ receptors, as D₃ and D₄ receptors did not form heteromers. We demonstrate that the σ₁-D₂ receptor heteromers consist of higher order oligomers, are found in mouse striatum and that cocaine, by binding to σ₁ -D₂ receptor heteromers, inhibits downstream signaling in both cultured cells and in mouse striatum. In contrast, in striatum from σ₁ knockout animals these complexes are not found and this inhibition is not seen. Taken together, these data illuminate the mechanism by which the initial exposure to cocaine can inhibit signaling via D₂ receptor containing neurons, destabilizing the delicate signaling balance influencing drug seeking that emanates from the D₁ and D₂ receptor containing neurons in the brain.     

Dopamine-induced functional activation of Gαq mediated by dopamine D1-like receptor in rat cerebral cortical membranes

Although multiple roles of dopamine through D₁-like (D₁ and D₅) and D₂-like (D₂, D₃, and D₄) receptors are initiated primarily through stimulation or inhibition of adenylyl cyclase via G_s/olf or G_i/o, respectively, there have been many reports indicating diverse signaling mechanisms that involve alternative G protein coupling. In this study, dopamine-induced Gα_q activation in rat brain membranes was investigated. Agonist-induced Gα_q activation was assessed by increase in guanosine-5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding to Gα_q determined by [³⁵S]GTPγS binding/immunoprecipitation assay in rat brain membranes. Dopamine-stimulated Gα_q functionality was highest in cortex as compared to hippocampus or striatum. In cerebral cortical membranes, this effect was mimicked by benzazepine derivatives with agonist properties at dopamine D₁-like receptors, that is, SKF83959, SKF83822, R(+)-SKF81297, R(+)-SKF38393, and SKF82958, but not by the compounds with dopamine D₂-like receptor agonist properties except for aripiprazole. Against expectation, stimulatory effects were also induced by SKF83566, R(+)-SCH23390, and pergolide. The pharmacological profiling by using a series of antagonists indicated that dopamine-induced response was mediated through dopamine D₁-like receptor, which was distinct from the receptor involved in 5-HT-induced response (5-HT_2A receptor). Conversely, the responses induced by SKF83566, R(+)-SCH23390, and pergolide were most likely mediated by 5-HT_2A receptor, but not by dopamine D₁-like receptor. Caution should be paid when interpreting the experimental data, especially in behavioral pharmacological research, in which SKF83566 or R(+)-SCH23390 is used as a standard selective dopamine D₁-like receptor antagonist. Also, possible clinical implications of the agonistic effects of pergolide on 5-HT_2A receptor has been mentioned.     

Amphioxus expresses both vertebrate-type and invertebrate-type dopamine D1 receptors

The cephalochordate amphioxus (Branchiostoma floridae) has recently been placed as the most basal of all the chordates, which makes it an ideal organism for studying the molecular basis of the evolutionary transition from invertebrates to vertebrates. The biogenic amine, dopamine regulates many aspects of motor control in both vertebrates and invertebrates, and in both cases, its receptors can be divided into two main groups (D1 and D2) based on sequence similarity, ligand affinity and effector coupling. A bioinformatic study shows that amphioxus has at least three dopamine D1-like receptor sequences. We have recently characterized one of these receptors, AmphiD1/β, which was found to have high levels of sequence similarity to both vertebrate D1 receptors and to β-adrenergic receptors, but functionally appeared to be a vertebrate-type dopamine D₁ receptor. Here, we report on the cloning of two further dopamine D₁ receptors (AmphiAmR1 and AmphiAmR2) from adult amphioxus cDNA libraries and their pharmacological characterisation subsequent to their expression in cell lines. AmphiAmR1 shows closer structural similarities to vertebrate D₁-like receptors but shows some pharmacological similarities to invertebrate “DOP1” dopamine D₁-like receptors. In contrast, AmphiAmR2 shows closer structural and pharmacological similarities to invertebrate “INDR”-like dopamine D₁-like receptors.     

Drastic decrease in dopamine receptor levels in the striatum of acetylcholinesterase knock-out mouse

BackgroundThe acetylcholinesterase knock-out mouse lives to adulthood despite 60-fold elevated acetylcholine concentrations in the brain that are lethal to wild-type animals. Part of its mechanism of survival is a 50% decrease in muscarinic and nicotinic receptors and a 50% decrease in adrenoceptor levels.HypothesisThe hypothesis was tested that the dopaminergic neuronal system had also adapted.MethodsRadioligand binding assays measured dopamine receptor level and binding affinity in the striatum. Immunohistochemistry of brain sections with specific antibodies visualized dopamine transporter. Effects on the intracellular compartment were measured as cAMP content, PI-phospholipase C activity.ResultsDopamine receptor levels were decreased 28-fold for the D₁-like, and more than 37-fold for the D₂-like receptors, though binding affinity was normal. Despite these huge changes in receptor levels, dopamine transporter levels were not affected. The intracellular compartment had normal levels of cAMP and PI-phospholipase C activity.ConclusionSurvival of the acetylcholinesterase knock-out mouse could be linked to adaptation of many neuronal systems during development including the cholinergic, adrenergic and dopaminergic. These adaptations balance the overstimulation of cholinergic receptors caused by high acetylcholine concentrations and thus maintain homeostasis inside the cell, allowing the animal to live.     

D1-Like Dopamine Receptor-Mediated Function in Congenic Mutants with D1 vs. D5 Receptor “Knockout”

Current understanding of the functional roles of individual dopamine D₁-like [D₁, D₅] and D₂-like [D_2L/S, D₃, D₄] receptor subtypes remains incomplete. In particular, the lack of pharmacological agonists and antagonists able to distinguish between D₁ and D₅ receptors means that any differential roles in the regulation of behavior are poorly understood. Mutant mice with targeted gene deletion (“knockout”) of individual dopamine receptor subtypes offer an important alternative approach to resolving these functional roles. In congenic D₁ mutants examined ethologically, progressive increases in specific topographies of behavior over wildtypes were considerably greater than those in D₁ mutants on a mixed genetic background; D₁ knockout appears to influence the neuronal substrate(s) of habituation to disrupt sculpture of the changing topography of behavior from initial exploration through to quiescence. Similarly, the D₁ receptor appears to regulate specific topographies of orofacial movement in the mouse as these are “sculpted” in a time-dependent manner. Although the well-recognized role of the D₁-like family in regulating several aspects of behavioral topography has been assumed to involve primarily D₁ receptors, this presumption may require modification to accommodate a subtle but not negligible role for their D₅ counterparts as evidenced in the phenotype of congenic D₅ mutants.     

Molecular and pharmacological characterization of two D(1)-like dopamine receptors in the Lyme disease vector, Ixodes scapularis

Advancements in tick neurobiology may impact the development of acaricides to control those species that transmit human and animal diseases. Here, we report the first cloning and pharmacological characterization of two neurotransmitter binding G protein-coupled receptors in the Lyme disease (blacklegged) tick, Ixodes scapularis. The genes IscaGPRdop1 and IscaGPRdop2 were identified in the I. scapularis genome assembly and predicted as orthologs of previously characterized D₁-like dopamine receptors in the fruit fly Drosophila melanogaster and honeybee Apis mellifera. Heterologous expression in HEK 293 cells demonstrated that each receptor functioned as a D₁-like dopamine receptor because significant increases in levels of intracellular cyclic adenosine monophosphate (cAMP) were detected following dopamine treatment. Importantly, the receptors were distinct in their pharmacological properties regarding concentration-dependent response to dopamine, constitutive activity, and response to other biogenic amines. Exposure to a variety of dopamine receptor agonists and antagonists further demonstrated a D₁-like pharmacology of these dopamine receptors and highlighted their differential activities in vitro.     

Dopamine receptor localization in the mammalian retina

After a short history of dopamine receptor discovery in the retina and a survey on dopamine receptor types and subtypes, the distribution of dopamine receptors in the retinal cells is described and correlated with their possible role in cell and retinal physiology. All the retinal cells probably bear dopamine receptors. For example, the recently discovered D_1B receptor has a possible role in modulating phagocytosis by the pigment epithelium and a D₄ receptor is likely to be involved in the inhibition of melatonin synthesis in photoreceptors. Dopamine uncouples horizontal and amacrine cell-gap junctions through D₁-like receptors. Dopamine modulates the release of other transmitters by subpopulations of amacrine cells, including that of dopamine through a D₂ autoreceptor. Ganglion cells express dopamine receptors, the role of which is still uncertain. Müller cells also are affected by dopamine. A puzzling action of dopamine is observed in the ciliary retina, in which D₁- and D₂-like receptors are likely to be involved in the cyclic regulation of intraocular pressure. Most of the dopaminergic actions appears to be extrasynaptic and the signaling pathways remain uncertain. Further studies are needed to better understand the multiple actions of dopamine in the retina, especially those that implicate rhythmic regulations.     

Systemic Blockade of Dopamine D2-Like Receptors Increases High-Voltage Spindles in the Globus Pallidus and Motor Cortex of Freely Moving Rats

High-voltage spindles (HVSs) have been reported to appear spontaneously and widely in the cortical–basal ganglia networks of rats. Our previous study showed that dopamine depletion can significantly increase the power and coherence of HVSs in the globus pallidus (GP) and motor cortex of freely moving rats. However, it is unclear whether dopamine regulates HVS activity by acting on dopamine D₁-like receptors or D₂-like receptors. We employed local-field potential and electrocorticogram methods to simultaneously record the oscillatory activities in the GP and primary motor cortex (M1) in freely moving rats following systemic administration of dopamine receptor antagonists or saline. The results showed that the dopamine D₂-like receptor antagonists, raclopride and haloperidol, significantly increased the number and duration of HVSs, and the relative power associated with HVS activity in the GP and M1 cortex. Coherence values for HVS activity between the GP and M1 cortex area were also significantly increased by dopamine D₂-like receptor antagonists. On the contrary, the selective dopamine D₁-like receptor antagonist, {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390, had no significant effect on the number, duration, or relative power of HVSs, or HVS-related coherence between M1 and GP. In conclusion, dopamine D₂-like receptors, but not D₁-like receptors, were involved in HVS regulation. This supports the important role of dopamine D₂-like receptors in the regulation of HVSs. An siRNA knock-down experiment on the striatum confirmed our conclusion.     

D5 Dopamine Receptor Knockout Mice and Hypertension

Abnormalities in dopamine production and receptor function have been described in human essential hypertension and rodent models of genetic hypertension. All of the five dopamine receptor genes (D₁, D₂, D₃, D₄, and D₅) expressed in mammals and some of their regulators are in loci linked to hypertension in humans and in rodents. Under normal conditions, D₁-like receptors (D₁ and D₅) inhibit sodium transport in the kidney and the intestine. However, in the Dahl salt-sensitive and spontaneously hypertensive rats, and humans with essential hypertension, the D₁-like receptor-mediated inhibition of sodium transport is impaired because of an uncoupling of the D₁-like receptor from its G protein/effector complex. The uncoupling is genetic, and receptor-, organ-, and nephron segment-specific. In human essential hypertension, the uncoupling of the D₁ receptor from its G protein/effector complex is caused by an agonist-independent serine phosphorylation/desensitization by constitutively active variants of the G protein-coupled receptor kinase type 4. The D₅ receptor is also important in blood pressure regulation. Disruption of the D₅ or the D₁ receptor gene in mice increases blood pressure. However, unlike the D₁ receptor, the hypertension in D₅ receptor null mice is caused by increased activity of the sympathetic nervous system, apparently due to activation of oxytocin, V₁ vasopressin, and non-N-methyl D-aspartate receptors in the central nervous system. The cause of the activation of these receptors remains to be determined.     

Pharmacological Characterization of Mammalian D1 and D2 Dopamine Receptors Expressed in Drosophila Schneider‐2 Cells

Abstract

Mammalian D₁ and D₂ dopamine receptors were stably expressed in Drosophila Schneider‐2 (S2) cells and screened for their pharmacological properties. Saturable, dose‐dependent, high affinity binding of the D₁‐selective antagonist [³H]SCH‐23390 was detected only in membranes from S2 cells induced to express rat dopamine D₁ receptors, while saturable, dose‐dependent, high affinity binding of the D₂‐selective antagonist [³H]methylspiperone was detected only in membranes from S2 cells induced to express rat dopamine D₂ receptors. No specific binding of either radioligand could be detected in membranes isolated from uninduced or untransfected S2 cells. Both dopamine D₁ and D₂ receptor subtypes displayed the appropriate stereoselective binding of enantiomers of the nonselective antagonist butaclamol. Each receptor subtype also displayed the appropriate agonist stereoselectivities. The dopamine D₁ receptor bound the (+)‐enantiomer of the D₁‐selective agonist SKF38393 with higher affinity than the (?)‐enantiomer, while the dopamine D₂ receptor bound the (?)‐enantiomer of the D₂‐selective agonist norpropylapomorphine with higher affinity than the (+)‐enantiomer. At both receptor subtypes, dopamine binding was best characterized as occurring to a single low affinity site. In addition, the low affinity dopamine binding was also found to be insensitive to GTPγS and magnesium ions. Overall, the pharmacological profiles of mammalian dopamine D₁ and D₂ receptors expressed in Drosophila S2 cells is comparable to those observed for these same receptors when they are expressed in mammalian cell lines. A notable distinction is that there is no evidence for the coupling of insect G proteins to mammalian dopamine receptors. These results suggest that the S2 cell insect G system may provide a convenient source of pharmacologically active mammalian D₁ and D₂ dopamine receptors free of promiscuous G protein contaminants.     

Dopamine Receptors Antagonistically Regulate Behavioral Choice between Conflicting Alternatives in C. elegans

Caenorhabditis elegans is a useful model to study the neuronal or molecular basis for behavioral choice, a specific form of decision-making. Although it has been implied that both D1-like and D2-like dopamine receptors may contribute to the control of decision-making in mammals, the genetic interactions between D1-like and D2-like dopamine receptors in regulating decision-making are still largely unclear. In the present study, we investigated the molecular control of behavioral choice between conflicting alternatives (diacetyl and Cu²⁺) by D1-like and D2-like dopamine receptors and their possible genetic interactions with C. elegans as the assay system. In the behavioral choice assay system, mutation of dop-1 gene encoding D1-like dopamine receptor resulted in the enhanced tendency to cross the Cu²⁺ barrier compared with wild-type. In contrast, mutations of dop-2 or dop-3 gene encoding D2-like dopamine receptor caused the weak tendency to cross the Cu²⁺ barrier compared with wild-type. During the control of behavioral choice, DOP-3 antagonistically regulated the function of DOP-1. The behavioral choice phenotype of dop-2; dop-1dop-3 triple mutant further confirmed the possible antagonistic function of D2-like dopamine receptor on D1-like dopamine receptor in regulating behavioral choice. The genetic assays further demonstrate that DOP-3 might act through Gα_o signaling pathway encoded by GOA-1 and EGL-10, and DOP-1 might act through Gα_q signaling pathway encoded by EGL-30 and EAT-16 to regulate the behavioral choice. DOP-1 might function in cholinergic neurons to regulate the behavioral choice, whereas DOP-3 might function in GABAergic neurons, RIC, and SIA neurons to regulate the behavioral choice. In this study, we provide the genetic evidence to indicate the antagonistic relationship between D1-like dopamine receptor and D2-like dopamine receptor in regulating the decision-making of animals. Our data will be useful for understanding the complex functions of dopamine receptors in regulating decision-making in animals.     

Differential voltage-sensitivity of D2-like dopamine receptors

Agonist potency at some neurotransmitter receptors has been shown to be regulated by transmembrane voltage, a mechanism which has been suggested to play a crucial role in the regulation of neurotransmitter release by autoreceptors and in synaptic plasticity. We have recently described the voltage-sensitivity of the dopamine D_2L receptor and we now extend our studies to include the other members of the D₂-like receptor subfamily; the D_2S, D₃, and D₄ dopamine receptors. Electrophysiological recordings were performed on Xenopus oocytes coexpressing human dopamine D_2S, D₃, or D₄ receptors with G protein-coupled potassium (GIRK) channels. Comparison of concentration-response relationships at −80 mV and at 0 mV for dopamine-mediated GIRK activation revealed significant rightward shifts for both D_2S and D₄ upon depolarization. In contrast, the concentration-response relationships for D₃-mediated GIRK activation were not appreciably different at the two voltages. Our findings provide new insight into the functional differences of these closely related receptors.     

Dopamine Preferentially Inhibits NMDA Receptor-Mediated EPSCs by Acting on Presynaptic D1 Receptors in Nucleus Accumbens during Postnatal Development

Nucleus accumbens (nAcb), a major site of action of drugs of abuse and dopamine (DA) signalling in MSNs (medium spiny neurons), is critically involved in mediating behavioural responses of drug addiction. Most studies have evaluated the effects of DA on MSN firing properties but thus far, the effects of DA on a cellular circuit involving glutamatergic afferents to the nAcb have remained rather elusive. In this study we attempted to characterize the effects of dopamine (DA) on evoked glutamatergic excitatory postsynaptic currents (EPSCs) in nAcb medium spiny (MS) neurons in 1 to 21 day-old rat pups. The EPSCs evoked by local nAcb stimuli displayed both AMPA/KA and NMDA receptor-mediated components. The addition of DA to the superfusing medium produced a marked decrease of both components of the EPSCs that did not change during the postnatal period studied. Pharmacologically isolated AMPA/KA receptor-mediated response was inhibited on average by 40% whereas the isolated NMDA receptor-mediated EPSC was decreased by 90%. The effect of DA on evoked EPSCs were mimicked by the D₁-like receptor agonist SKF 38393 and antagonized by the D₁-like receptor antagonist SCH 23390 whereas D₂-like receptor agonist or antagonist respectively failed to mimic or to block the action of DA. DA did not change the membrane input conductance of MS neurons or the characteristics of EPSCs produced by the local administration of glutamate in the presence of tetrodotoxin. In contrast, DA altered the paired-pulse ratio of evoked EPSCs. The present results show that the activation D₁-like dopaminergic receptors modulate glutamatergic neurotransmission by preferentially inhibiting NMDA receptor-mediated EPSC through presynaptic mechanisms.     

α<Subscript>2A</Subscript>- and α<Subscript>2C</Subscript>-Adrenoceptors as Potential Targets for Dopamine and Dopamine Receptor Ligands

The poor norepinephrine innervation and high density of Gi/o-coupled α_2A- and α_2C-adrenoceptors in the striatum and the dense striatal dopamine innervation have prompted the possibility that dopamine could be an effective adrenoceptor ligand. Nevertheless, the reported adrenoceptor agonistic properties of dopamine are still inconclusive. In this study, we analyzed the binding of norepinephrine, dopamine, and several compounds reported as selective dopamine D₂-like receptor ligands, such as the D₃ receptor agonist 7-OH-PIPAT and the D₄ receptor agonist RO-105824, to α₂-adrenoceptors in cortical and striatal tissue, which express α_2A-adrenoceptors and both α_2A- and α_2C-adrenoceptors, respectively. The affinity of dopamine for α₂-adrenoceptors was found to be similar to that for D₁-like and D₂-like receptors. Moreover, the exogenous dopamine receptor ligands also showed high affinity for α_2A- and α_2C-adrenoceptors. Their ability to activate Gi/o proteins through α_2A- and α_2C-adrenoceptors was also analyzed in transfected cells with bioluminescent resonance energy transfer techniques. The relative ligand potencies and efficacies were dependent on the Gi/o protein subtype. Furthermore, dopamine binding to α₂-adrenoceptors was functional, inducing changes in dynamic mass redistribution, adenylyl cyclase activity, and ERK1/2 phosphorylation. Binding events were further studied with computer modeling of ligand docking. Docking of dopamine at α_2A- and α_2C-adrenoceptors was nearly identical to its binding to the crystallized D₃ receptor. Therefore, we provide conclusive evidence that α_2A- and α_2C-adrenoceptors are functional receptors for norepinephrine, dopamine, and other previously assumed selective D₂-like receptor ligands, which calls for revisiting previous studies with those ligands.     

Cloning and characterization of a Caenorhabditis elegans D2-like dopamine receptor

The neurotransmitter dopamine plays an important role in the regulation of behavior in both vertebrates and invertebrates. In mammals, dopamine binds and activates two classes of dopamine receptors, D1-like and D2-like receptors. However, D2-like dopamine receptors in Caenorhabditis elegans have not yet been characterized. We have cloned a cDNA encoding a putative C. elegans D2-like dopamine receptor. The deduced amino acid sequence of the cloned cDNA shows higher sequence similarities to vertebrate D2-like dopamine receptors than to D1-like receptors. Two splice variants that differ in the length of their predicted third intracellular loops were identified. The receptor heterologously expressed in cultured cells showed high affinity binding to [125I]iodo-lysergic acid diethylamide. Dopamine showed the highest affinity for this receptor among several amine neurotransmitters tested. Activation of the heterologously expressed receptor led to the inhibition of cyclic AMP production, confirming that this receptor has the functional property of a D2-like receptor. We have also analyzed the expression pattern of this receptor and found that the receptor is expressed in several neurons including all the dopaminergic neurons in C. elegans.     

Three amino acids in the D2 dopamine receptor regulate selective ligand function and affinity

The D₂ dopamine receptor is an important therapeutic target for the treatment of psychotic, agitated, and abnormal behavioral states. To better understand the specific interactions of subtype‐selective ligands with dopamine receptor subtypes, seven ligands with high selectivity (>120‐fold) for the D₄ subtype of dopamine receptor were tested on wild‐type and mutant D₂ receptors. Five of the selective ligands were observed to have 21‐fold to 293‐fold increases in D₂ receptor affinity when three non‐conserved amino acids in TM2 and TM3 were mutated to the corresponding D₄ amino acids. The two ligands with the greatest improvement in affinity for the D₂ mutant receptor [i.e., 3‐{[4‐(4‐iodophenyl) piperazin‐1‐yl]methyl}‐1H‐pyrrolo[2,3‐b]pyridine (L‐750,667) and 1‐[4‐iodobenzyl]‐4‐[N‐(3‐isopropoxy‐2‐pyridinyl)‐N‐methyl]‐aminopiperidine (RBI‐257)] were investigated in functional assays. Consistent with their higher affinity for the mutant than for the wild‐type receptor, concentrations of L‐750,667 or RBI‐257 that produced large reductions in the potency of quinpirole’s functional response in the mutant did not significantly reduce quinpirole’s functional response in the wild‐type D₂ receptor. In contrast to RBI‐257 which is an antagonist at all receptors, L‐750,667 is a partial agonist at the wild‐type D₂ but an antagonist at both the mutant D₂ and wild‐type D₄ receptors. Our study demonstrates for the first time that the TM2/3 microdomain of the D₂ dopamine receptor not only regulates the selective affinity of ligands, but in selected cases can also regulate their function. Utilizing a new docking technique that incorporates receptor backbone flexibility, the three non‐conserved amino acids that encompass the TM2/3 microdomain were found to account in large part for the differences in intermolecular steric contacts between the ligands and receptors. Consistent with the experimental data, this model illustrates the interactions between a variety of subtype‐selective ligands and the wild‐type D₂, mutant D₂, or wild‐type D₄ receptors.     

The dopamine D1 receptor is expressed and facilitates relaxation in airway smooth muscle

Background

Dopamine signaling is mediated by G_s protein-coupled “D₁-like” receptors (D₁ and D₅) and G_i-coupled “D₂-like” receptors (D_2-4). In asthmatic patients, inhaled dopamine induces bronchodilation. Although the G_i-coupled dopamine D₂ receptor is expressed and sensitizes adenylyl cyclase activity in airway smooth muscle (ASM) cells, the G_s-coupled dopamine D₁-like receptor subtypes have never been identified on these cells. Activation of G_s-coupled receptors stimulates cyclic AMP (cAMP) production through the stimulation of adenylyl cyclase, which promotes ASM relaxation. We questioned whether the dopamine D₁-like receptor is expressed on ASM, and modulates its function through G_s-coupling.

Methods

The mRNA and protein expression of dopamine D₁-like receptor subtypes in both native human and guinea pig ASM tissue and cultured human ASM (HASM) cells was measured. To characterize the stimulation of cAMP through the dopamine D₁ receptor, HASM cells were treated with dopamine or the dopamine D₁-like receptor agonists ({"type":"entrez-nucleotide","attrs":{"text":"A68930","term_id":"4759850","term_text":"A68930"}}A68930 or {"type":"entrez-protein","attrs":{"text":"SKF38393","term_id":"1157151916","term_text":"SKF38393"}}SKF38393) before cAMP measurements. To evaluate whether the activation of dopamine D₁ receptor induces ASM relaxation, guinea pig tracheal rings suspended under isometric tension in organ baths were treated with cumulatively increasing concentrations of dopamine or {"type":"entrez-nucleotide","attrs":{"text":"A68930","term_id":"4759850","term_text":"A68930"}}A68930, following an acetylcholine-induced contraction with or without the cAMP-dependent protein kinase (PKA) inhibitor Rp-cAMPS, the large-conductance calcium-activated potassium (BK_Ca) channel blocker iberiotoxin, or the exchange proteins directly activated by cAMP (Epac) antagonist NSC45576.

Results

Messenger RNA encoding the dopamine D₁ and D₅ receptors were detected in native human ASM tissue and cultured HASM cells. Immunoblots confirmed the protein expression of the dopamine D₁ receptor in both native human and guinea pig ASM tissue and cultured HASM cells. The dopamine D₁ receptor was also immunohistochemically localized to both human and guinea pig ASM. The dopamine D₁-like receptor agonists stimulated cAMP production in HASM cells, which was reversed by the selective dopamine D₁-like receptor antagonists {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 or {"type":"entrez-protein","attrs":{"text":"SCH39166","term_id":"1052842517","term_text":"SCH39166"}}SCH39166. {"type":"entrez-nucleotide","attrs":{"text":"A68930","term_id":"4759850","term_text":"A68930"}}A68930 relaxed acetylcholine-contracted guinea pig tracheal rings, which was attenuated by Rp-cAMPS but not by iberiotoxin or NSC45576.

Conclusions

These results demonstrate that the dopamine D₁ receptors are expressed on ASM and regulate smooth muscle force via cAMP activation of PKA, and offer a novel target for therapeutic relaxation of ASM.     

D1- and D2-like receptors differentially mediate the effects of dopaminergic transmission on cost–benefit evaluation and motivation in monkeys

It has been widely accepted that dopamine (DA) plays a major role in motivation, yet the specific contribution of DA signaling at D₁-like receptor (D₁R) and D₂-like receptor (D₂R) to cost–benefit trade-off remains unclear. Here, by combining pharmacological manipulation of DA receptors (DARs) and positron emission tomography (PET) imaging, we assessed the relationship between the degree of D₁R/D₂R blockade and changes in benefit- and cost-based motivation for goal-directed behavior of macaque monkeys. We found that the degree of blockade of either D₁R or D₂R was associated with a reduction of the positive impact of reward amount and increasing delay discounting. Workload discounting was selectively increased by D₂R antagonism. In addition, blocking both D₁R and D₂R had a synergistic effect on delay discounting but an antagonist effect on workload discounting. These results provide fundamental insight into the distinct mechanisms of DA action in the regulation of the benefit- and cost-based motivation, which have important implications for motivational alterations in both neurological and psychiatric disorders.

Using quantitatively controlled pharmacological manipulations, this study teases apart the role of D1- and D2-like dopamine receptors in motivation and goal-directed behavior in monkeys, revealing complementary roles of two dopamine receptor subtypes in the computation of the cost/benefit trade-off to guide action.     

Dopamine Receptor Subtype Agonists and Feeding Behavior

Stimulation or blockade of various dopamine receptor subtypes is associated with reduced feeding. For example, D₂ receptor agonists suppress feeding in food-deprived and free-feeding rats, and in rats given access to a highly palatable diet Similarly, reduced food intake is associated with the actions of diverse D₁ receptor agonists, and these compounds can interact synergistically with D₂ receptor agonists to potentiate reductions in feeding. Using micro-structural analysis to compare D₁ and D₂ agonist effects, specific differences emerge in their modes of action. D₁ agonists reduce the duration of feeding, primarily by decreasing the frequency of feeding bouts, whereas D₂ agonists reduce the local rate of eating. However, since D₁ agonists uniquely reduce feeding in the absence of other behavioral impairments and are less disruptive of the pattern of feeding behavior, it has been suggested that D₁ agonists are more likely than D₂ agonists to act on central mechanisms regulating food intake. Moreover, only D₁ agonists are effective in suppressing sucrose sham-feeding, suggesting that D₁ receptor stimulation may promote satiety. Nevertheless, many questions remain. For example, antagonist studies have implicated 5-HT receptor stimulation in the anorectic effects of D₁ agonists, suggesting that further pharmacological and behavioral analyses of receptor-subtype agonist effects are required. Above all, recent developments in the classification of dopamine receptor subtypes reveal the need for new studies examining the involvement of D₃, D₄ and D₅ receptors in feeding.