Dopamine D2-Like Receptors and Behavioral Economics of Food Reinforcement

Previous studies suggest dopamine (DA) D₂-like receptor involvement in the reinforcing effects of food. To determine contributions of the three D₂-like receptor subtypes, knockout (KO) mice completely lacking DA D₂, D₃, or D₄ receptors (D₂R, D₃R, or D₄R KO mice) and their wild-type (WT) littermates were exposed to a series of fixed-ratio (FR) food-reinforcement schedules in two contexts: an open economy with additional food provided outside the experimental setting and a closed economy with all food earned within the experimental setting. A behavioral economic model was used to quantify reinforcer effectiveness with food pellets obtained as a function of price (FR schedule value) plotted to assess elasticity of demand. Under both economies, as price increased, food pellets obtained decreased more rapidly (ie, food demand was more elastic) in DA D₂R KO mice compared with WT littermates. Extinction of responding was studied in two contexts: by eliminating food deliveries and by delivering food independently of responding. A hyperbolic model quantified rates of extinction. Extinction in DA D₂R KO mice occurred less rapidly compared with WT mice in both contexts. Elasticity of food demand was higher in DA D₄R KO than WT mice in the open, but not closed, economy. Extinction of responding in DA D₄R KO mice was not different from that in WT littermates in either context. No differences in elasticity of food demand or extinction rate were obtained in D₃R KO mice and WT littermates. These results indicate that the D₂R is the primary DA D₂-like receptor subtype mediating the reinforcing effectiveness of food.     

Loss of Alternative Non-Drug Reinforcement Induces Relapse of Cocaine-Seeking in Rats: Role of Dopamine D1 Receptors

Animal models of relapse to drug seeking have focused primarily on relapse induced by exposure to drugs, drug-associated cues or contexts, and foot-shock stress. However, relapse in human drug abusers is often precipitated by loss of alternative non-drug reinforcement. The present experiment used a novel ‘resurgence'' paradigm to examine relapse to cocaine seeking of rats as a result of loss of an alternative source of non-drug reinforcement. Rats were first trained to press a lever for intravenous infusions of cocaine. Next, cocaine deliveries were omitted and food pellets were provided for an alternative nose-poke response. Once cocaine seeking was reduced to low levels, food pellets for the alternative response were also omitted. Cocaine seeking increased with the loss of the alternative non-drug reinforcer (ie, resurgence occurred) despite continued extinction conditions. The increase in cocaine seeking did not occur in another group of rats injected with SCH 23390 before the loss of the alternative reinforcer. These results suggest that removal of an alternative source of reinforcement may induce relapse of cocaine seeking and that the dopamine D₁ receptor may have a role in this effect.     

Effects of Remoxipride's Metabolites on Dopamine D2 Receptors and Receptor Functions in the Rat

Abstract: The main metabolites of remoxipride formed in rat and man were examined for their affinities for the [³H] SCH 23390-labelled DA D₁ and [³H]-raclopride-labelled D₂ receptors in rat striatal homogenates. In addition, their effectiveness in blocking postsynaptic DA receptor activity in vivo was measured by the use of several different test models in the male rat. Phenolic metabolites formed mainly in the rat retained (similar to remoxipride) their selectivity for the D₂ receptor with very low affinities for the D₁ receptor. The pyrrolidone metabolites formed mainly in man showed very low affinities for both the D₁ and D₂ receptors. The ability of the metabolites to block postsynaptic DA receptor activity in vivo correlated with their affinities for the D₂ receptor. Among the metabolites tested, the phenolic compounds FLA 797 (–) and FLA 908 (–) were much more effective than remoxipride in inducing catalepsy, which is consistent with a higher affinity for [³H] raclopride labelled striatal D₂ receptors. However, analysis of the effectiveness of the DA receptor blockade (blockade of d-amphetamine locomotion or DA agonist hypothermia) after intraperitoneal or subcutaneous administration suggested that FLA 797 (–)/FLA 908 (–) may only contribute marginally to the D₂ receptor-blocking activity of remoxipride in the rat. This conclusion was further supported by the observation that the atypical antipsychotic profile of remoxipride was not mimicked by the active metabolites. The weak DA D₂ blocking effect of the pyrrolidone metabolites indicated that remoxipride is responsible for the clinical action.     

In Vivo Imaging of Cerebral Dopamine D3 Receptors in Alcoholism

Animal studies support the role of the dopamine D3 receptor (DRD3) in alcohol reinforcement or liking. Sustained voluntary alcohol drinking in rats has been associated with an upregulation of striatal DRD3 gene expression and selective blockade of DRD3 reduces ethanol preference, consumption, and cue-induced reinstatement. In vivo measurement of DRD3 in the living human brain has not been possible until recently owing to a lack of suitable tools. In this study, DRD3 status was assessed for the first time in human alcohol addiction. Brain DRD3 availability was compared between 16 male abstinent alcohol-dependent patients and 13 healthy non-dependent age-matched males using the DRD3-preferring agonist positron emission tomography (PET) radioligand [¹¹C]PHNO with and without blockade with a selective DRD3 antagonist (GSK598809 60 mg p.o.). In striatal regions of interest, where the [¹¹C]PHNO PET signal represents primarily DRD2 binding, no differences were seen in [¹¹C]PHNO binding between the groups at baseline. However, baseline [¹¹C]PHNO binding was higher in alcohol-dependent patients in hypothalamus (V_T: 16.5±4 vs 13.7±2.9, p=0.040), a region in which the [¹¹C]PHNO signal almost entirely reflects DRD3 availability. The reductions in regional receptor binding (V_T) following a single oral dose of GSK598809 (60 mg) were consistent with those observed in previous studies across all regions. There were no differences in regional changes in V_T following DRD3 blockade between the two groups, indicating that the regional fractions of DRD3 are similar in the two groups, and the increased [¹¹C]PHNO binding in the hypothalamus in alcohol-dependent patients is explained by elevated DRD3 in this group. Although we found no difference between alcohol-dependent patients and controls in striatal DRD3 levels, increased DRD3 binding in the hypothalamus of alcohol-dependent patients was observed. This may be relevant to the development of future therapeutic strategies to treat alcohol abuse.     

Chronic Fluoxetine Selectively Upregulates Dopamine D1-Like Receptors in the Hippocampus

The dentate gyrus of the hippocampus has been implicated in mechanisms of action of selective serotonin reuptake inhibitors (SSRIs). We have recently demonstrated that the SSRI fluoxetine can reverse the state of maturation of the adult dentate granule cells and enhances serotonin 5-HT₄ receptor-mediated synaptic potentiation at the synapses formed by their mossy fiber axons. Here, we show that fluoxetine can induce long-lasting enhancement of dopaminergic modulation at the mossy fiber synapse. Synaptic responses arising from the mossy fiber-CA3 pyramidal cell synapse were recorded using acute mouse hippocampal slices. Dopamine potentiates mossy fiber synaptic transmission by activating D₁-like receptors. Chronic fluoxetine treatment induced a prominent increase in the magnitude of dopamine-induced synaptic potentiation, and this effect was maintained at least up to 1 month after withdrawal of fluoxetine. Quantitative autoradiography revealed that binding of the D₁-like receptor ligand [³H]{"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 was selectively increased in the dentate gyrus and along the mossy fiber in fluoxetine-treated mice. However, binding of the 5-HT₄ receptor ligand [³H]{"type":"entrez-nucleotide","attrs":{"text":"GR113808","term_id":"238362519","term_text":"GR113808"}}GR113808 was not significantly changed. These results suggest that chronic fluoxetine enhanced the dopaminergic modulation at least in part by upregulating expression of D₁-like receptors, while the enhanced serotonergic modulation may be mediated by modifications of downstream signaling pathways. These enhanced monoaminergic modulations would greatly increase excitatory drive to the hippocampal circuit through the dentate gyrus. The highly localized upregulation of D₁-like receptors further supports the importance of the dentate gyrus in the mechanism of action of SSRIs.     

Role of Dopamine Receptors on Electroencephalographic Changes Produced by Repetitive Apomorphine Treatments in Rats

Repeated psychostimulants induce electroencephalographic (EEG) changes, which reflect adaptation of the neural substrate related to dopaminergic pathways. To study the role of dopamine receptors in EEG changes, we examined the effect of apomorphine, the dopamine D1 receptor antagonist, SCH-23390, and the D2 receptor antagonist, haloperidol, on EEG in rats. For single and repeated apomorphine treatment groups, the rats received saline or apomorphine for 4 days followed by a 3-day withdrawal period and then apomorphine (2.5 mg/kg, i.p.) challenge after pretreatment with saline, SCH-23390, or haloperidol on the day of the experiment. EEGs from the frontal and parietal cortices were recorded. On the frontal cortex, apomorphine decreased the power of all the frequency bands in the single treatment group, and increased the theta (4.5~8 Hz) and alpha (8~13 Hz) powers in the repeated treatment group. Changes in both groups were reversed to the control values by SCH-23390. On the parietal cortex, single apomorphine treatment decreased the power of some frequency bands, which were reversed by haloperidol but not by SCH-23390. Repeated apomorphine treatment did not produce significant changes in the power profile. These results show that adaptation of dopamine pathways by repeated apomorphine treatment could be identified with EEG changes such as increases in theta and alpha power of the frontal cortex, and this adaptation may occur through changes in the D1 receptor and/or the D2 receptor.     

Blockade of Dopamine D4 Receptors Attenuates Reinstatement of Extinguished Nicotine-Seeking Behavior in Rats

Since cloning of the dopamine receptor D4 (DRD4), its role in the brain has remained unclear. It has been reported that polymorphism of the DRD4 gene in humans is associated with reactivity to cues related to tobacco smoking. However, the role of DRD4 in animal models of nicotine addiction has seldom been explored. In our study, male Long-Evans rats learned to intravenously self-administer nicotine under a fixed-ratio (FR) schedule of reinforcement. Effects of the selective DRD4 antagonist L-745,870 were evaluated on nicotine self-administration behavior and on reinstatement of extinguished nicotine-seeking behavior induced by nicotine-associated cues or by priming injections of nicotine. L-745,870 was also tested on reinstatement of extinguished food-seeking behavior as a control. In addition, the selective DRD4 agonist PD 168,077 was tested for its ability to reinstate extinguished nicotine-seeking behavior. Finally, L-745,870 was tested in Sprague Dawley rats trained to discriminate administration of 0.4 mg/kg nicotine from vehicle under an FR schedule of food delivery. L-745,870 significantly attenuated reinstatement of nicotine-seeking induced by both nicotine-associated cues and nicotine priming. In contrast, L-745,870 did not affect established nicotine self-administration behavior or reinstatement of food-seeking behavior induced by food cues or food priming. L-745,870 did not produce nicotine-like discriminative-stimulus effects and did not alter discriminative-stimulus effects of nicotine. PD 168,077 did not reinstate extinguished nicotine-seeking behavior. As DRD4 blockade by L-745,870 selectively attenuated both cue- and nicotine-induced reinstatement of nicotine-seeking behavior, without affecting cue- or food-induced reinstatement of food-seeking behavior, DRD4 antagonists are potential therapeutic agents against tobacco smoking relapse.     

Knockdown of Dopamine D2 Receptors in the Nucleus Accumbens Core Suppresses Methamphetamine-Induced Behaviors and Signal Transduction in Mice

Background:

Addictive drugs lead to reinforcing properties by increasing dopamine in the nucleus accumbens, which is composed of a core and shell regions. Neurons in the nucleus accumbens are divided into 2 subtypes based on the differential gene expression of the dopamine D₁ receptors and D₂ receptors.

Methods:

In the present study, we investigated the role of D₂ receptors in the nucleus accumbens core in behaviors and signal transduction induced by psychostimulant methamphetamine in mice that were microinjected with adeno-associated virus vectors containing a microRNA (miRNA) sequence for D₂ receptor (adeno-associated virus-miD₂r vectors) in the nucleus accumbens core. The adeno-associated virus vectors containing a miRNA sequence for D₂ receptor-treated mice (miD₂r mice) were assessed at a reduction in D₂ receptor, but at no change in dopamine D₁ receptor, in the nucleus accumbens core compared with the adeno-associated virus-Mock vectors-treated mice (Mock mice).

Results:

miD₂r mice exhibited a reduction in hyperlocomotion that was induced by a single treatment with methamphetamine. The development of locomotor sensitization induced by repeated treatment with methamphetamine exhibited less extension in miD₂r mice. In a place conditioning paradigm, the preferred effects of methamphetamine were significantly weaker in miD₂r mice than in Mock mice. Furthermore, the single treatment with methamphetamine-induced phosphorylation of extracellular signal regulated kinase and cyclic adenosine monophosphate response element-binding protein in the nucleus accumbens core of miD₂r mice was decreased compared with that in Mock mice. Repeated treatment with methamphetamine-induced delta FBJ murine osteosarcoma viral oncogene homolog B accumulation in the nucleus accumbens core of miD₂r mice was also attenuated.

Conclusions:

These findings suggest that a D₂ receptor-mediated neuronal pathway from the nucleus accumbens core plays an inhibitory role in the development of reinforcing properties.     

Novel Bivalent Ligands for D2/D3 Dopamine Receptors: Significant Cooperative Gain in D2 Affinity and Potency

相似文献   

Blockade of Spinal Dopamine D2 Receptors Enhances the Pressor Effect of Intravenous Quinpirole in Normotensive,Conscious Rats

Abstract: The present investigation was undertaken to examine whether in conscious intact rats blockade of spinal dopamine D₂ receptors enhances the pressor effect of intravenous quinpirole. In saline‐pretreated rats, intravenous quinpirole (1 mg/kg) induced a significant pressor effect, which reached a maximum (17.71±0.60 mmHg) within the first min. after injection. Pretreatment with intravenous (0.5 mg/kg) or intrathecal (40 μg/rat at T₉‐T₁₀) domperidone, a dopamine D₂ receptor antagonist that does not cross the blood‐brain barrier, significantly enhanced the maximal pressor response to quinpirole (25.60±1.52 and 24.00±1.72 mmHg, respectively). The pressor effect of quinpirole was also significantly enhanced after combined pretreatment with intravenous and intrathecal domperidone, and its maximum (31.60±2.31 mmHg) was significantly higher than that recorded in animals pretreated with intrathecal or intravenous domperidone alone. Intravenous pretreatment with metoclopramide (5 mg/kg) fully abolished the quinpirole‐induced pressor effect. These results show that in conscious intact rats, blockade of spinal dopamine D₂ receptors enhances the pressor response to systemic quinpirole, suggesting that this agonist can decrease blood pressure through a spinal dopaminergic mechanism. Thus, our previous hypothesis that the entire effect of intravenous quinpirole on blood pressure in conscious rats can be composed of a central pressor action, a peripheral sympathoinhibitory depressor effect and also a spinal depressor effect is strongly supported by the present findings.     

Role of Dopamine D1 Receptors in the Activation of Nucleus Accumbens Extracellular Signal-Regulated Kinase (ERK) by Cocaine-Paired Contextual Cues

Exposure to drug-paired cues can trigger addicts to relapse into drug seeking. Although the molecular mechanisms underlying cue-elicited cocaine seeking are incompletely understood, the protein kinase extracellular signal-regulated kinase (ERK) is known to have an important role. Psychostimulants and their associated cues can activate ERK in medium spiny neurons of the nucleus accumbens core (AcbC). These medium spiny neurons can be classified according to their projections (to ventral pallidum and/or substantia nigra) and by their mRNA expression. The present experiments were designed to determine which distinct set of AcbC projection neurons expresses phosphorylated ERK (pERK) in response to cocaine-paired contextual cues. Combined use of the retrograde label Flurogold with immunohistochemical staining of pERK was used to show that the AcbC pERK accompanying preference for cocaine-paired contexts occurs in both the accumbens (Acb)-nigral and Acb-pallidal projections. The gene expression characteristics of the neurons expressing pERK in response to cocaine-paired cues was further investigated using combined in situ hybridization and immunocytochemistry to show that AcbC pERK+ cells correspond to D1, but not preproenkephalin, mRNA+ cells. Furthermore, intra-AcbC infusion of the D1-antagonist {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 attenuated cue-induced AcbC pERK expression. In aggregate, these results indicate that (i) the D1-expressing AcbC neurons evidence long-term plasticity related to drug-cue memories and (ii) local dopamine D1 receptors are necessary for the expression of cocaine-paired cue-induced pERK in these AcbC neurons.     

Molecular Characteristics of Mammalian Dopamine Receptors

Abstract: Abstract: Dopamine receptors belong to a large super-gene family of receptors which are linked to their signal transduction pathways through heterotrimeric G proteins. A variety of signalling events are known to be regulated by dopamine receptors including adenylate cyclase and phospholipase activities and various ion channels. Prior to the advent of molecular cloning technology, dopamine receptors were believed to belong to two subtypes, D₁ and D₂. This distinction was based on both pharmacological and functional criteria. We now know that at least five different dopamine receptors exist although they can still be described as to belonging within “D₁” and “D₂” subfamilies. The D₁ subfamily consists of two receptors—the D₁ and D₅, whereas the D₂, D₃ and D₄ receptors comprise the D₂ subfamily. The cloning and molecular characteristics of these five receptors are described in this review.     

Erratum to: Translational Modeling in Schizophrenia: Predicting Human Dopamine D2 Receptor Occupancy

Pharmaceutical Research -     

Chronic Dopamine D1, Dopamine D2 and Combined Dopamine D1 and D2 Antagonist Treatment in Cebus Apella Monkeys: Antiamphetamine Effects and Extrapyramidal Side Effects

To determine: (1) whether the apparent lack of efficacy of dopamine D1 (D1) antagonists in the clinic might be attributable to development of tolerance to antipsychotic effects; and (2) whether combined D1 and D2 antagonism might contribute to clozapine’s clinical profile, eight Cebus apella monkeys were chronically treated with a D1 antagonist (NNC 756) ((+)-8-chloro-7-hydroxy-3-methyl-5-(7-(2,3-dihydrobenzofuranyl)-2,3,4,5-tetrahydro-1H-3-benzazepine), a D2 antagonist (raclopride) or a combination of the two antagonists. Prior neuroleptic exposure had resulted in oral dyskinesia in seven monkeys and sensitization to dystonia in all, yielding a model for acute and chronic extrapyramidal side effects (EPS). Dextroamphetamine-induced motoric unrest and stereotypies were used as a psychosis model. We found tolerance toward dystonic symptoms during D1 and D1 + D2 treatments but not during D2 treatment. D2 but not D1 or D1 + D2 antagonism caused exacerbation of dyskinesia. Both D1 and D1 + D2 antagonism were superior to D2 antagonism alone in counteracting the amphetamine-induced behaviors, with no tolerance to antiamphetamine effects. These findings suggest: (1) reasons other than tolerance (e.g., differences among antagonists) may explain the lack of efficacy in clinical trials with D1 antagonists; and (2) that D1 antagonism alone or combined and modest D1 and D2 antagonism offers the potential of antipsychotic efficacy with a lower risk of EPS than traditional D2 antagonism. Further clinical trials with D1 or combined D1 and D2 antagonists are, therefore, recommended.     

Role of Dorsal Medial Prefrontal Cortex Dopamine D1-Family Receptors in Relapse to High-Fat Food Seeking Induced by the Anxiogenic Drug Yohimbine

In humans, relapse to maladaptive eating habits during dieting is often provoked by stress. In rats, the anxiogenic drug yohimbine, which causes stress-like responses in both humans and nonhumans, reinstates food seeking in a relapse model. In this study, we examined the role of medial prefrontal cortex (mPFC) dopamine D1-family receptors, previously implicated in stress-induced reinstatement of drug seeking, in yohimbine-induced reinstatement of food seeking. We trained food-restricted rats to lever press for 35% high-fat pellets every other day (9–15 sessions, 3 h each); pellet delivery was accompanied by a discrete tone-light cue. We then extinguished operant responding for 10–16 days by removing the pellets. Subsequently, we examined the effect of yohimbine (2 mg/kg, i.p.) on reinstatement of food seeking and Fos (a neuronal activity marker) induction in mPFC. We then examined the effect of systemic injections of the D1-family receptor antagonist {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 (10 μg/kg, s.c.) on yohimbine-induced reinstatement and Fos induction, and that of mPFC {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 (0.5 and 1.0 μg/side) injections on this reinstatement. Yohimbine-induced reinstatement was associated with strong Fos induction in the dorsal mPFC and with weaker Fos induction in the ventral mPFC. Systemic {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 injections blocked both yohimbine-induced reinstatement and mPFC Fos induction. Dorsal, but not ventral, mPFC injections of {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 decreased yohimbine-induced reinstatement of food seeking. In addition, dorsal mPFC {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390 injections decreased pellet-priming-induced reinstatement, but had no effect on ongoing high-fat pellet self-administration or discrete-cue-induced reinstatement. Results indicate a critical role of dorsal mPFC dopamine D1-family receptors in stress-induced relapse to palatable food seeking, as well as relapse induced by acute re-exposure to food taste, texture, and smell.     

Upregulation of Dopamine D2 Receptors in the Nucleus Accumbens Indirect Pathway Increases Locomotion but Does Not Reduce Alcohol Consumption

Brain imaging studies performed in humans have associated low striatal dopamine release and D2R binding with alcohol dependence. Conversely, high striatal D2R binding has been observed in unaffected members of alcoholic families suggesting that high D2R function may protect against alcohol dependence. A possible protective role of increased D2R levels in the striatum is further supported by preclinical studies in non-human primates and rodents. Here, we determined whether there is a causal relationship between D2R levels and alcohol intake. To this end, we upregulated D2R expression levels in the nucleus accumbens of the adult mouse, but selectively restricted the upregulation to the indirect striatal output pathway, which endogenously expresses D2Rs. After overexpression was established, mice were tested in two models of free-choice alcohol drinking: the continuous and intermittent access two-bottle choice models. As anticipated, we found that D2R upregulation leads to hyperactivity in the open field. Contrary to our expectation, D2R upregulation did not reduce alcohol intake during continuous or intermittent access or when alcohol drinking was tested in the context of aversive outcomes. These data argue against a protective role of accumbal indirect pathway D2Rs in alcohol consumption but emphasize their importance in promoting locomotor activity.     

Occupancy of Brain Dopamine D3 Receptors and Drug Craving: A Translational Approach

Selective dopamine D₃ receptor (D₃R) antagonists prevent reinstatement of drug-seeking behavior and decrease the rewarding effects of contextual cues associated with drug intake preclinically, suggesting that they may reduce drug craving in humans. GSK598809 is a selective D₃R antagonist recently progressed in Phase I trials. The aim of this study was to establish a model, based on the determination of the occupancy of brain D₃Rs (O^D₃R) across species, to predict the ability of GSK598809 to reduce nicotine-seeking behavior in humans, here assessed as cigarette craving in smokers. Using ex vivo [¹²⁵I](R)-trans-7-hydroxy-2-[N-propyl-N-(3′-iodo-2′-propenyl)amino] tetralin ([¹²⁵I]7OH-PIPAT) autoradiography and [¹¹C]PHNO positron emission tomography, we demonstrated a dose-dependent occupancy of the D₃Rs by GSK598809 in rat, baboon, and human brains. We also showed a direct relationship between O^D₃R and pharmacokinetic exposure, and potencies in line with the in vitro binding affinity. Likewise, GSK598809 dose dependently reduced the expression of nicotine-induced conditioned place preference (CPP) in rats, with an effect proportional to the exposure and O^D₃R at every time point, and 100% effect at O^D₃R values ⩾72%. In humans, a single dose of GSK598809, giving submaximal levels (72–89%) of O^D₃R, transiently alleviated craving in smokers after overnight abstinence. These data suggest that either higher O^D₃R is required for a full effect in humans or that nicotine-seeking behavior in CPP rats only partially translates into craving for cigarettes in short-term abstinent smokers. In addition, they provide the first clinical evidence of potential efficacy of a selective D₃R antagonist for the treatment of substance-use disorders.