Phenotypic analysis of dopamine receptor knockout mice; recent insights into the functional specificity of dopamine receptor subtypes

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

Exploring the structural basis and atomistic binding mechanistic of the selective antagonist blockade at D3 dopamine receptor over D2 dopamine receptor

More recently, there has been a paradigm shift toward selective drug targeting in the treatment of neurological disorders, including drug addiction, schizophrenia, and Parkinson's disease mediated by the different dopamine receptor subtypes. Antagonists with higher selectivity for D₃ dopamine receptor (D3DR) over D₂ dopamine receptor (D2DR) have been shown to attenuate drug-seeking behavior and associated side effects compared to non-subtype selective antagonists. However, high conservations among constituent residues of both proteins, particularly at the ligand-binding pockets, remain a challenge to therapeutic drug design. Recent studies have reported the discovery of two small-molecules R-VK4-40 and Y-QA31 which substantially inhibited D3DR with >180-fold selectivity over D2DR. Therefore, in this study, we seek to provide molecular and structural insights into these differential binding mechanistic using meta-analytic computational simulation methods. Findings revealed that R-VK4-40 and Y-QA31 adopted shallow binding modes and were more surface-exposed at D3DR while on the contrary, they exhibited deep hydrophobic pocket binding at D2DR. Also, two non-conserved residues; Tyr36^1.39 and Ser182^45.51 were identified in D3DR, based on their crucial roles and contributions to the selective binding of R-VK4-40 and Y-QA31. Importantly, both antagonists exhibited high affinities in complex with D3DR compared to D2DR, while van der Waals energies contributed majorly to their binding and stability. Structural analyses also revealed the distinct stabilizing effects of both compounds on D3DR secondary architecture relative to D2DR. Therefore, findings herein pinpointed the origin and mechanistic of selectivity of the compounds, which may assist in the rational design of potential small molecules of the D₂ -like dopamine family receptor subtype with improved potency and selectivity.

Dopamine D4 receptor subtype activation reduces the rat cardiac parasympathetic discharge

The dopaminergic system influences the heart rhythm by inhibiting the rat cardiac sympathetic and parasympathetic neurotransmissions through activation of D₂-like receptors (encompassing the D₂, D₃, and D₄ subtypes). Whereas D₂ receptor subtype activation results in cardiac sympatho-inhibition, the dopamine receptor subtypes involved in rat cardiac vago-inhibition remain unknown. Hence, this study investigated the specific functional role of the D₂-like receptor subtypes (D₂, D₃, and/or D₄) inhibiting the rat heart cholinergic drive. For this purpose, male Wistar rats were pithed and prepared for cardiac vagal stimulation. Bradycardic responses were obtained by electrical stimulation of vagal fibres (3, 6, 9 Hz; n = 100) or i.v. acetylcholine (ACh; 1, 5, 10 μg/kg; n = 15). Expression of D₂, D₃, and D₄ receptors was studied in left and right atrium samples by PCR (n = 4). Intravenous injections of quinpirole (D₂-like agonist; 1-30 μg/kg), but not of SFK-38393 (D₁-like agonist; 1-30 μg/kg), dose-dependently inhibited the vagally induced bradycardia. The vago-inhibition induced by quinpirole (which failed to affect the bradycardia to i.v. ACh) was unchanged after i.v. injections of the antagonists L-741,626 (D₂; 100 μg/kg) or SB-277011-A (D₃; 100 μg/kg), but it was abolished by L-745,870 (D₄; 100 μg/kg). mRNA levels of D₂, D₃, and D₄ receptor subtype were detected in the left and right rat atria. Our results suggest that the quinpirole-induced vagolytic effect involves prejunctional D₄ receptor subtypes, located in the left and right atria. This provides new evidence on the relevance of D₄ receptor modulating the heart parasympathetic control.

Dopamine D1 Receptors Regulate Spines in Striatal Direct-Pathway and Indirect-Pathway Neurons

BACKGROUND

Dopamine transmission is involved in the maintenance of the structural plasticity of direct-pathway and indirect-pathway striatal projection neurons (d-SPNs and i-SPNs, respectively). The lack of dopamine in Parkinson's disease produces synaptic remodeling in both types of SPNs, reducing the length of the dendritic arbor and spine density and increasing the intrinsic excitability. Meanwhile, the elevation of dopamine levels by levodopa recovers these alterations selectively in i-SPNs. However, little is known about the specific role of the D1 receptor (D1R) in these alterations.

METHODS

To explore the specific role of D1R in the synaptic remodeling of SPNs, we used knockout D1R mice (D1R^-/- ) and wild-type mice crossed with drd2-enhanced green fluorescent protein (eGFP) to identify d-SPNs and i-SPNs. Corticostriatal slices were used for reconstruction of the dendritic arbors after Lucifer yellow intracellular injection and for whole-cell recordings in naïve and parkinsonian mice treated with saline or levodopa.

RESULTS

The genetic inactivation of D1R reduces the length of the dendritic tree and the spine density in all SPNs, although more so in d-SPNs, which also increases their spiking. In parkinsonian D1R^-/- mice, the spine density decreases in i-SPNs, and this spine loss recovers after chronic levodopa.

CONCLUSIONS

D1R is essential for the maintenance of spine plasticity in d-SPNs but also affects i-SPNs, indicating an important crosstalk between these 2 types of neurons. © 2020 International Parkinson and Movement Disorder Society.

Constitutive activity of a G protein-coupled receptor, DRD1, contributes to human cerebral organoid formation

The intricate balance of neural stem cell (NSC) amplification and neurogenesis is central to nervous system development. Dopamine D1 receptor (DRD1) is a typical G protein‐coupled receptor (GPCR) mainly expressed in neurogenic area, with high constitutive activity. The receptor appears in the embryonic period before the formation of mature synaptic contacts, which indicates that dopamine receptor and its constitutive activity play crucial roles in the embryonic brain development. Here, we found that DRD1 was enriched in human NSCs. Inhibition of the receptor activity by its inverse agonists promoted human NSCs proliferation and impeded its differentiation. These results were also mimicked by genetic knockdown of DRD1, which also blocked the effects of inverse agonists, suggesting a receptor‐dependent manner. More interestingly, knock‐in A229T mutant with reduced DRD1 constitutive activity by CRISPR‐Cas9 genome editing technology resulted into increased endogenous human NSCs proliferation. These results were well reproduced in human cerebral organoids, and inhibition of the DRD1 constitutive activity by its inverse agonists induced the expansion and folding of human cerebral organoids. The anatomic analysis uncovered that decreasing the constitutive activity of DRD1 by its inverse agonists promoted the NSCs proliferation and maintenance that led to hindered cortical neurogenesis. Further mechanistic studies revealed that the PKC‐CBP pathway was involved in the regulation by DRD1. Thus, our findings indicate that the constitutive activity of DRD1 and possibly other GPCRs plays an important role in the development of human nervous system.

Structure-based development of caged dopamine D2/D3 receptor antagonists

Dopamine is a neurotransmitter of great physiological relevance. Disorders in dopaminergic signal transduction are associated with psychiatric and neurological pathologies such as Parkinson’s disease, schizophrenia and substance abuse. Therefore, a detailed understanding of dopaminergic neurotransmission may provide access to novel therapeutic strategies for the treatment of these diseases. Caged compounds with photoremovable groups represent molecular tools to investigate a biological target with high spatiotemporal resolution. Based on the crystal structure of the D₃ receptor in complex with eticlopride, we have developed caged D₂/D₃ receptor ligands by rational design. We initially found that eticlopride, a widely used D₂/D₃ receptor antagonist, was photolabile and therefore is not suitable for caging. Subtle structural modification of the pharmacophore led us to the photostable antagonist dechloroeticlopride, which was chemically transformed into caged ligands. Among those, the 2-nitrobenzyl derivative 4 (MG307) showed excellent photochemical stability, pharmacological behavior and decaging properties when interacting with dopamine receptor-expressing cells.

Dopamine D1 and muscarinic acetylcholine receptors in dorsal striatum are required for high speed running

Dopamine (DA) signaling in the basal ganglia plays important roles in motor control. Motor deficiencies were previously reported in dopamine receptor D1 (D1R) and D2 (D2R) knockout mice. While these results indicate the involvement of DA receptors in motor execution, the null knockout (KO) mouse lacks the specificity necessary to determine when and where in the brain D1R and D2R function in motor execution. To address these questions, we restricted the loss of function temporally and spatially by using D1R conditional knockdown (cKD) mice and mice injected with antagonists against DA receptors directly into the dorsal striatum. In addition, we address the DA and acetylcholine (ACh) balance hypothesis by using antagonists against ACh receptors. We tested the motor ability of the mice with a newly devised task named the accelerating step-wheel. In this task, the maximum running speed was measured in a situation where the wheel rotation speed was gradually accelerated in one trial. We found significant decreases in the maximum running speed of D1R cKD mice and the mice injected with the antagonist against D1R or muscarinic ACh receptor. These results indicated that D1R and muscarinic ACh receptor in the dorsal striatum play pivotal roles in the execution of walking/running.

Sensitization-dependent nicotine place preference in the adult zebrafish

Sensitization of motor activity is a behavioural test to evaluate the effects of psychostimulants. Conditioned place preference (CPP) is an associative learning procedure to examine the rewarding properties of drugs. We aimed to assess whether motor sensitization to drugs of abuse can make zebrafish more vulnerable to establishing drug-induced CPP. We first evaluated sensitization of locomotor activity of zebrafish to repeated administrations of nicotine and cocaine during 5 days and after 5 days of withdrawal. After withdrawal, when zebrafish were re-exposed to the same dose of nicotine or cocaine locomotor activity was increased by 103% and 166%, respectively. Different groups of zebrafish were sensitized to nicotine or cocaine and trained on a nicotine-CPP task the day after withdrawal. The nicotine dose selected for sensitization was not effective for developing CPP in naïve zebrafish whereas it elicited CPP in zebrafish that were previously sensitized to nicotine or cocaine. Levels of nicotinic acetylcholine receptor β₂, α₆ and α₇ subunit, Pitx3, and tyrosine hydroxylase 1 (TH1) mRNAs were increased in the brain of nicotine- and cocaine-sensitized zebrafish. Nicotine-CPP performed with drug-sensitized zebrafish provoked further enhancements in the expression of α₆ and α₇ subunit, Pitx3, and TH1 mRNAs suggesting that the expression of these molecules in the reward pathway is involved in both processes. Our findings indicate that repeated exposures to low doses of drugs of abuse can increase subject's sensitivity to the rewarding properties of the same or different drugs. This further suggests that casual drug intake increases the probability of becoming addict.

Modulating Dopamine Signaling and Behavior with Chemogenetics: Concepts, Progress, and Challenges

For more than 60 years, dopamine (DA) has been known as a critical modulatory neurotransmitter regulating locomotion, reward-based motivation, and endocrine functions. Disturbances in DA signaling have been linked to an array of different neurologic and psychiatric disorders, including Parkinson's disease, schizophrenia, and addiction, but the underlying pathologic mechanisms have never been fully elucidated. One major obstacle limiting interpretation of standard pharmacological and transgenic interventions is the complexity of the DA system, which only appears to widen as research progresses. Nonetheless, development of new genetic tools, such as chemogenetics, has led to an entirely new era for functional studies of neuronal signaling. By exploiting receptors that are engineered to respond selectively to an otherwise inert ligand, so-called Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), chemogenetics enables pharmacological remote control of neuronal activity. Here we review the recent, extensive application of this technique to the DA field and how its use has advanced the study of the DA system and contributed to our general understanding of DA signaling and related behaviors. Moreover, we discuss the challenges and pitfalls associated with the chemogenetic technology, such as the metabolism of the DREADD ligand clozapine N-oxide (CNO) to the D2 receptor antagonist clozapine. We conclude that despite the recent concerns regarding CNO, the chemogenetic toolbox provides an exceptional approach to study neuronal function. The huge potential should promote continued investigations and additional refinements to further expound key mechanisms of DA signaling and circuitries in normal as well as maladaptive behaviors.

An investigation of the single and combined effects of hypogravity and ionizing radiation on brain monoamine metabolism and rats' behavior

BACKGROUND

Ionizing radiation and hypogravity can cause central nervous system (CNS) dysfunctions. This is a key limiting factor for deep space missions. Up until now, the mechanisms through which they affect the neural tissue are not completely understood.

OBJECTIVES

We studied how the combination of hypogravity (antiorthostatic suspension model, AS) and ionizing radiations (γ-quanta and ¹H⁺ together, R) affects the CNS.

METHODS

We applied separately and in combination AS and R to determine the influence of these factors on behavior and metabolism of monoamines in Wistar rat's brain.

RESULTS

We found out that R has a slight effect on both the behavior and metabolism of monoamines. However, when applied in combination with AS the former was able to reduce the negative effects of the latter. The combined effect of ionizing radiation and hypogravity led to the recovery of locomotor activity, orientation and exploratory behavior, and long-term context memory impaired under the impact of hypogravity only. These changes came together with an increase in the serotonin and dopamine turnover in all of the brain structures that were studied.

CONCLUSIONS

We received the first evidence of interferential interaction between the effects of ionizing radiation and hypogravity factors with regard to a behavior and monoamine turnover in the brain. Further studies with heavy nuclei at relevant doses (<0.5 Gy) are needed.

Dopamine D1 receptor activation improves adult hippocampal neurogenesis and exerts anxiolytic and antidepressant-like effect via activation of Wnt/β-catenin pathways in rat model of Parkinson's disease

Parkinson's disease (PD) is primarily characterized by midbrain dopamine depletion. Dopamine acts through dopamine receptors (D1 to D5) to regulate locomotion, motivation, pleasure, attention, cognitive functions and formation of newborn neurons, all of which are likely to be impaired in PD. Reduced hippocampal neurogenesis associated with dopamine depletion has been demonstrated in patients with PD. However, the precise mechanism to regulate multiple steps of adult hippocampal neurogenesis by dopamine receptor(s) is still unknown. In this study, we tested whether pharmacological agonism and antagonism of dopamine D1 and D2 receptor regulate nonmotor symptoms, neural stem cell (NSC) proliferation and fate specification and explored the cellular mechanism(s) underlying dopamine receptor (D1 and D2) mediated adult hippocampal neurogenesis in rat model of PD-like phenotypes. We found that single unilateral intra-medial forebrain bundle administration of 6-hydroxydopamine (6-OHDA) reduced D1 receptor level in the hippocampus. Pharmacological agonism of D1 receptor exerts anxiolytic and antidepressant-like effects as well as enhanced NSC proliferation, long-term survival and neuronal differentiation by positively regulating Wnt/β-catenin signaling pathway in hippocampus in PD rats. shRNA lentivirus mediated knockdown of Axin-2, a negative regulator of Wnt/β-catenin signaling potentially attenuated D1 receptor antagonist induced anxiety and depression-like phenotypes and impairment in adult hippocampal neurogenesis in PD rats. Our results suggest that improved nonmotor symptoms and hippocampal neurogenesis in PD rats controlled by D1-like receptors that involve the activation of Wnt/β-catenin signaling.

Adolescent Nicotine Exposure Induces Dysregulation of Mesocorticolimbic Activity States and Depressive and Anxiety-like Prefrontal Cortical Molecular Phenotypes Persisting into Adulthood

Considerable evidence demonstrates strong comorbidity between nicotine dependence and mood and anxiety disorders. Nevertheless, the neurobiological mechanisms linking adolescent nicotine exposure to mood and anxiety disorders are not known. Disturbances in the mesocorticolimbic dopamine (DA) system, comprising the prefrontal cortex (PFC), ventral tegmental area (VTA), and nucleus accumbens (NAc), are correlates of mood and anxiety-related symptoms and this circuitry is strongly influenced by acute or chronic nicotine exposure. Using a combination of behavioral pharmacology, in vivo neuronal electrophysiology and molecular analyses, we examined and compared the effects of chronic nicotine exposure in rats during adolescence versus adulthood to characterize the mechanisms by which adolescent nicotine may selectively confer increased risk of developing mood and anxiety-related symptoms in later life. We report that exposure to nicotine, selectively during adolescence, induces profound and long-lasting neuronal, molecular and behavioral disturbances involving PFC DA D1R and downstream extracellular-signal-related kinase 1-2 (ERK 1-2) signaling. Remarkably, adolescent nicotine induced a persistent state of hyperactive DA activity in the ventral tegmental area (VTA) concomitant with hyperactive neuronal activity states in the PFC. Our findings identify several unique neuronal and molecular biomarkers that may serve as functional risk mechanisms for the long-lasting neuropsychiatric effects of adolescent smoking behaviors.

Dopamine D1 receptor subtype mediates acute stress-induced dendritic growth in excitatory neurons of the medial prefrontal cortex and contributes to suppression of stress susceptibility in mice

Dopamine in prefrontal cortices is implicated in cognitive and emotional functions, and the dysfunction of prefrontal dopamine has been associated with cognitive and emotional deficits in mental illnesses. These findings have led to clinical trials of dopamine-targeting drugs and brain imaging of dopamine receptors in patients with mental illnesses. Rodent studies have suggested that dopaminergic pathway projecting to the medial prefrontal cortex (mPFC) suppresses stress susceptibility. Although various types of mPFC neurons express several dopamine receptor subtypes, previous studies neither isolated a role of dopamine receptor subtype nor identified the site of its action in mPFC. Using social defeat stress (SDS) in mice, here we identified a role of dopamine D1 receptor subtype in mPFC excitatory neurons in suppressing stress susceptibility. Repeated social defeat stress (R-SDS) reduces the expression of D1 receptor subtype in mPFC of mice susceptible to R-SDS. Knockdown of D1 receptor subtype in whole neuronal populations or excitatory neurons in mPFC facilitates the induction of social avoidance by SDS. Single social defeat stress (S-SDS) induces D1 receptor-mediated extracellular signal-regulated kinase phosphorylation and c-Fos expression in mPFC neurons. Whereas R-SDS reduces dendritic lengths of mPFC layer II/III pyramidal neurons, S-SDS increases arborization and spines of apical dendrites of these neurons in a D1 receptor-dependent manner. Collectively, our findings show that D1 receptor subtype and related signaling in mPFC excitatory neurons mediate acute stress-induced dendritic growth of these neurons and contribute to suppression of stress susceptibility. Therefore, we propose that D1 receptor-mediated dendritic growth in mPFC excitatory neurons suppresses stress susceptibility.

Dop1 enhances conspecific olfactory attraction by inhibiting miR-9a maturation in locusts

Dopamine receptor 1 (Dop1) mediates locust attraction behaviors, however, the mechanism by which Dop1 modulates this process remains unknown to date. Here, we identify differentially expressed small RNAs associated with locust olfactory attraction after activating and inhibiting Dop1. Small RNA transcriptome analysis and qPCR validation reveal that Dop1 activation and inhibition downregulates and upregulates microRNA-9a (miR-9a) expression, respectively. miR-9a knockdown in solitarious locusts increases their attraction to gregarious volatiles, whereas miR-9a overexpression in gregarious locusts reduces olfactory attraction. Moreover, miR-9a directly targets adenylyl cyclase 2 (ac2), causing its downregulation at the mRNA and protein levels. ac2 responds to Dop1 and mediates locust olfactory attraction. Mechanistically, Dop1 inhibits miR-9a expression through inducing the dissociation of La protein from pre-miR-9a and resulting in miR-9a maturation inhibition. Our results reveal a Dop1–miR-9a–AC2 circuit that modulates locust olfactory attraction underlying aggregation. This study suggests that miRNAs act as key messengers in the GPCR signaling.

Migratory locusts shift between aggregating together during gregarious phases and living individually during solitary phases. Here, the authors find that the D1-like dopamine receptor regulates the olfactory attraction underlying this behavioral switch via microRNA-9a and adenylyl cyclase.

Translating advances in the molecular basis of schizophrenia into novel cognitive treatment strategies

The presence and severity of cognitive symptoms, including working memory, executive dysfunction and attentional impairment, contributes materially to functional impairment in schizophrenia. Cognitive symptoms have proved to be resistant to both first- and second-generation antipsychotic drugs. Efforts to develop a consensus set of cognitive domains that are both disrupted in schizophrenia and are amenable to cross-species validation (e.g. the National Institute of Mental Health Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia and Research Domain Criteria initiatives) are an important step towards standardization of outcome measures that can be used in preclinical testing of new drugs. While causative genetic mutations have not been identified, new technologies have identified novel genes as well as hitherto candidate genes previously implicated in the pathophysiology of schizophrenia and/or mechanisms of antipsychotic efficacy. This review comprises a selective summary of these developments, particularly phenotypic data arising from preclinical genetic models for cognitive dysfunction in schizophrenia, with the aim of indicating potential new directions for pro-cognitive therapeutics. Linked Articles This article is part of a themed section on Pharmacology of Cognition: a Panacea for Neuropsychiatric Disease? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.19/issuetoc.

Recent Advances in Structure-Based Drug Design Targeting Class A G Protein-Coupled Receptors Utilizing Crystal Structures and Computational Simulations

G protein-coupled receptors (GPCRs) represent the largest and most physiologically important integral membrane protein family, and these receptors respond to a wide variety of physiological and environmental stimuli. GPCRs are among the most critical therapeutic targets for numerous human diseases, and approximately one-third of the currently marketed drugs target this receptor family. The recent breakthroughs in GPCR structural biology have significantly contributed to our understanding of GPCR function, ligand binding, and pharmacological action as well as to the design of new drugs. This perspective highlights the latest advances in GPCR structures with a focus on the receptor-ligand interactions of each receptor family in class A nonrhodopsin GPCRs as well as the structural features for their activation, biased signaling, and allosteric mechanisms. The current state-of-the-art methodologies of structure-based drug design (SBDD) approaches in the GPCR research field are also discussed.

Transcriptome analysis of zebrafish embryos exposed to deltamethrin

Deltamethrin (DTM), a type II pyrethroid, is one of the most commonly used insecticides. The increased use of pyrethroid leads to potential adverse effects, particularly in sensitive populations such as children and pregnant women. None of the related studies was focused on the transcriptome responses in zebrafish embryos after treatment with DTM; therefore, RNA-seq, a high-throughput method, was performed to analyze the global expression of differential expressed genes (DEGs) in zebrafish embryos treated with DTM (40 and 80 μg/L) from fertilization to 48 h postfertilization (hpf) as compared with that in the control group (without DTM treatment). Two cDNA libraries were generated from treated embryos and one cDNA library from nontreated embryos, respectively. Over 92% of reads mapped to the reference in these three libraries. It was observed that many differential genes were expressed in comparison with embryos before and after DTM. The 20 most differentially expressed upregulated or downregulated genes were majorly involved in the signaling transduction. Validation of selected nine genes expression using qRT-PCR confirmed RNA-seq results. The transcriptome sequences were further subjected to gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, showing G-protein-coupled receptor signaling pathway and neuroactive ligand-receptor interaction, respectively, were most enriched. The data from this study contributed to a better understanding of the potential consequences of fish exposed to DTM, to an evaluation of the potential threat of DTM to fish populations in aquatic environments. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 1548-1557, 2017.

Effects of D1 receptor knockout on fear and reward learning

Dopamine signaling is involved in a variety of neurobiological processes that contribute to learning and memory. D1-like dopamine receptors (including D1 and D5 receptors) are thought to be involved in memory and reward processes, but pharmacological approaches have been limited in their ability to distinguish between D1 and D5 receptors. Here, we examine the effects of a specific knockout of D1 receptors in associative learning tasks involving aversive (shock) or appetitive (cocaine) unconditioned stimuli. We find that D1 knockout mice show similar levels of cued and contextual fear conditioning to WT controls following conditioning protocols involving one, two, or four shocks. D1 knockout mice show increased generalization of fear conditioning and extinction across contexts, revealed as increased freezing to a novel context following conditioning and decreased freezing to an extinguished cue during a contextual renewal test. Further, D1 knockout mice show mild enhancements in extinction following an injection of SKF81297, a D1/D5 receptor agonist, suggesting a role for D5 receptors in extinction enhancements induced by nonspecific pharmacological agonists. Finally, although D1 knockout mice show decreased locomotion induced by cocaine, they are able to form a cocaine-induced conditioned place preference. We discuss these findings in terms of the role of dopamine D1 receptors in general learning and memory processes.

Molecular genetic models related to schizophrenia and psychotic illness: heuristics and challenges

Schizophrenia is a heritable disorder that may involve several common genes of small effect and/or rare copy number variation, with phenotypic heterogeneity across patients. Furthermore, any boundaries vis-à-vis other psychotic disorders are far from clear. Consequently, identification of informative animal models for this disorder, which typically relate to pharmacological and putative pathophysiological processes of uncertain validity, faces considerable challenges. In juxtaposition, the majority of mutant models for schizophrenia relate to the functional roles of a diverse set of genes associated with risk for the disorder or with such putative pathophysiological processes. This chapter seeks to outline the evidence from phenotypic studies in mutant models related to schizophrenia. These have commonly assessed the degree to which mutation of a schizophrenia-related gene is associated with the expression of several aspects of the schizophrenia phenotype or more circumscribed, schizophrenia-related endophenotypes; typically, they place specific emphasis on positive and negative symptoms and cognitive deficits, and extend to structural and other pathological features. We first consider the primary technological approaches to the generation of such mutants, to include their relative merits and demerits, and then highlight the diverse phenotypic approaches that have been developed for their assessment. The chapter then considers the application of mutant phenotypes to study pathobiological and pharmacological mechanisms thought to be relevant for schizophrenia, particularly in terms of dopaminergic and glutamatergic dysfunction, and to an increasing range of candidate susceptibility genes and copy number variants. Finally, we discuss several pertinent issues and challenges within the field which relate to both phenotypic evaluation and a growing appreciation of the functional genomics of schizophrenia and the involvement of gene × environment interactions.

Altered Sleep Homeostasis in Rev-erbα Knockout Mice

STUDY OBJECTIVES

The nuclear receptor REV-ERBα is a potent, constitutive transcriptional repressor critical for the regulation of key circadian and metabolic genes. Recently, REV-ERBα's involvement in learning, neurogenesis, mood, and dopamine turnover was demonstrated suggesting a specific role in central nervous system functioning. We have previously shown that the brain expression of several core clock genes, including Rev-erbα, is modulated by sleep loss. We here test the consequences of a loss of REV-ERBα on the homeostatic regulation of sleep.

METHODS

EEG/EMG signals were recorded in Rev-erbα knockout (KO) mice and their wild type (WT) littermates during baseline, sleep deprivation, and recovery. Cortical gene expression measurements after sleep deprivation were contrasted to baseline.

RESULTS

Although baseline sleep/wake duration was remarkably similar, KO mice showed an advance of the sleep/wake distribution relative to the light-dark cycle. After sleep onset in baseline and after sleep deprivation, both EEG delta power (1-4 Hz) and sleep consolidation were reduced in KO mice indicating a slower increase of homeostatic sleep need during wakefulness. This slower increase might relate to the smaller increase in theta and gamma power observed in the waking EEG prior to sleep onset under both conditions. Indeed, the increased theta activity during wakefulness predicted delta power in subsequent NREM sleep. Lack of Rev-erbα increased Bmal1, Npas2, Clock, and Fabp7 expression, confirming the direct regulation of these genes by REV-ERBα also in the brain.

CONCLUSIONS

Our results add further proof to the notion that clock genes are involved in sleep homeostasis. Because accumulating evidence directly links REV-ERBα to dopamine signaling the altered homeostatic regulation of sleep reported here are discussed in that context.

New insights into sodium transport regulation in the distal nephron: Role of G-protein coupled receptors

The renal handling of Na⁺ balance is a major determinant of the blood pressure (BP) level. The inability of the kidney to excrete the daily load of Na⁺ represents the primary cause of chronic hypertension. Among the different segments that constitute the nephron, those present in the distal part (i.e., the cortical thick ascending limb, the distal convoluted tubule, the connecting and collecting tubules) play a central role in the fine-tuning of renal Na⁺ excretion and are the target of many different regulatory processes that modulate Na⁺ retention more or less efficiently. G-protein coupled receptors (GPCRs) are crucially involved in this regulation and could represent efficient pharmacological targets to control BP levels. In this review, we describe both classical and novel GPCR-dependent regulatory systems that have been shown to modulate renal Na⁺ absorption in the distal nephron. In addition to the multiplicity of the GPCR that regulate Na⁺ excretion, this review also highlights the complexity of these different pathways, and the connections between them.

Dopamine receptors - IUPHAR Review 13

The variety of physiological functions controlled by dopamine in the brain and periphery is mediated by the D1, D2, D3, D4 and D5 dopamine GPCRs. Drugs acting on dopamine receptors are significant tools for the management of several neuropsychiatric disorders including schizophrenia, bipolar disorder, depression and Parkinson's disease. Recent investigations of dopamine receptor signalling have shown that dopamine receptors, apart from their canonical action on cAMP-mediated signalling, can regulate a myriad of cellular responses to fine-tune the expression of dopamine-associated behaviours and functions. Such signalling mechanisms may involve alternate G protein coupling or non-G protein mechanisms involving ion channels, receptor tyrosine kinases or proteins such as β-arrestins that are classically involved in GPCR desensitization. Another level of complexity is the growing appreciation of the physiological roles played by dopamine receptor heteromers. Applications of new in vivo techniques have significantly furthered the understanding of the physiological functions played by dopamine receptors. Here we provide an update of the current knowledge regarding the complex biology, signalling, physiology and pharmacology of dopamine receptors.

Developmental Deltamethrin Exposure Causes Persistent Changes in Dopaminergic Gene Expression, Neurochemistry, and Locomotor Activity in Zebrafish

Pyrethroids are commonly used insecticides that are considered to pose little risk to human health. However, there is an increasing concern that children are more susceptible to the adverse effects of pesticides. We used the zebrafish model to test the hypothesis that developmental exposure to low doses of the pyrethroid deltamethrin results in persistent alterations in dopaminergic gene expression, neurochemistry, and locomotor activity. Zebrafish embryos were treated with deltamethrin (0.25-0.50 μg/l), at concentrations below the LOAEL, during the embryonic period [3-72 h postfertilization (hpf)], after which transferred to fresh water until the larval stage (2-weeks postfertilization). Deltamethrin exposure resulted in decreased transcript levels of the D1 dopamine (DA) receptor (drd1) and increased levels of tyrosine hydroxylase at 72 hpf. The reduction in drd1 transcripts persisted to the larval stage and was associated with decreased D2 dopamine receptor transcripts. Larval fish, exposed developmentally to deltamethrin, had increased levels of homovanillic acid, a DA metabolite. Since the DA system is involved in locomotor activity, we measured the swim activity of larval fish following a transition to darkness. Developmental exposure to deltamethrin significantly increased larval swim activity which was attenuated by concomitant knockdown of the DA transporter. Acute exposure to methylphenidate, a DA transporter inhibitor, increased swim activity in control larva, while reducing swim activity in larva developmentally exposed to deltamethrin. Developmental exposure to deltamethrin causes locomotor deficits in larval zebrafish, which is likely mediated by dopaminergic dysfunction. This highlights the need to understand the persistent effects of low-dose neurotoxicant exposure during development.

Role of nuclear factor erythroid 2-related factor 2 in the oxidative stress-dependent hypertension associated with the depletion of DJ-1

Renal dopamine 2 receptor dysfunction is associated with oxidative stress and high blood pressure (BP). We have reported that DJ-1, an oxidative stress response protein, is positively regulated by dopamine 2 receptor in the kidney. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) regulates the expression of several antioxidant genes. We tested the hypothesis that Nrf2 is involved in the renal DJ-1-mediated inhibition of reactive oxygen species production. We have reported that silencing dopamine 2 receptor in mouse renal proximal tubule cells decreases the expression of DJ-1. We now report that silencing DJ-1 or dopamine 2 receptor in mouse proximal tubule cells and mouse kidneys decreases Nrf2 expression and activity and increases reactive oxygen species production; BP is also increased in mice in which renal DJ-1 or dopamine 2 receptor is silenced. DJ-1(-/-) mice have decreased renal Nrf2 expression and activity and increased nitro-tyrosine levels and BP. Silencing Nrf2 in mouse proximal tubule cells does not alter the expression of DJ-1 or dopamine 2 receptor, indicating that Nrf2 is downstream of dopamine 2 receptor and DJ-1. An Nrf2 inducer, bardoxolone, normalizes the systolic BP and renal malondialdehyde levels in DJ-1(-/-) mice without affecting them in their wild-type littermates. Because Nrf2 ubiquitination is increased in DJ-1(-/-) mice, we conclude that the protective effect of DJ-1 on renal oxidative stress is mediated, in part, by preventing Nrf2 degradation. Moreover, renal dopamine 2 receptor and DJ-1 are necessary for normal Nrf2 activity to keep a normal redox balance and BP.

Novel roles for β-arrestins in the regulation of pharmacological sequestration to predict agonist-induced desensitization of dopamine D3 receptors

BACKGROUND AND PURPOSE

In addition to typical GPCR kinase (GRK)-/β-arrestin-dependent internalization, dopamine D3 receptor employed an additional GRK-independent sequestration pathway. In this study, we investigated the molecular mechanism of this novel sequestration pathway.

EXPERIMENTAL APPROACH

Radioligand binding, flow cytometry and cell surface biotinylation assay were used to characterize trafficking properties of D2 and D3 receptors. Serine/threonine and N-linked glycosylation mutants of the D3 receptor were utilized to locate receptor regions involved in pharmacological sequestration and desensitization. Various point mutants of the D2 and D3 receptors, whose sequestration and desensitization properties were altered, were combined with knockdown cells of GRKs or β-arrestins to functionally correlate pharmacological sequestration and desensitization.

KEY RESULTS

The D3 receptor, but not the D2 receptor, showed characteristic trafficking behaviour in which receptors were shifted towards the more hydrophobic domains within the plasma membrane without translocation into other intracellular compartments. Among various amino acid residues tested, S145/S146, C147 and N12/19 were involved in pharmacological sequestration and receptor desensitization. Both pharmacological sequestration and desensitization of D3 receptor required β-arrestins, and functional relationship was observed between two processes when it was tested for D3 receptor variants and agonists.

CONCLUSIONS AND IMPLICATIONS

Pharmacological sequestration of D3 receptor accompanies movement of cell surface receptors into a more hydrophobic fraction within the plasma membrane and renders D3 receptor inaccessible to hydrophilic ligands. Pharmacological sequestration is correlated with desensitization of the D3 receptor in a Gβγ- and β-arrestin-dependent manner. This study provides new insights into molecular mechanism governing GPCR trafficking and desensitization.

Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders

The burden of anxiety disorders is growing, but the efficacy of available anxiolytic treatments remains inadequate. Cognitive behavioural therapy for anxiety disorders focuses on identifying and modifying maladaptive patterns of thinking and behaving, and has a testable analogue in rodents in the form of fear extinction. A large preclinical literature has amassed in recent years describing the neural and molecular basis of fear extinction in rodents. In this review, we discuss how this work is being harnessed to foster translational research on anxiety disorders and facilitate the search for new anxiolytic treatments. We begin by summarizing the anatomical and functional connectivity of a medial prefrontal cortex (mPFC)-amygdala circuit that subserves fear extinction, including new insights from optogenetics. We then cover some of the approaches that have been taken to model impaired fear extinction and associated impairments with mPFC-amygdala dysfunction. The principal goal of the review is to evaluate evidence that various neurotransmitter and neuromodulator systems mediate fear extinction by modulating the mPFC-amygdala circuitry. To that end, we describe studies that have tested how fear extinction is impaired or facilitated by pharmacological manipulations of dopamine, noradrenaline, 5-HT, GABA, glutamate, neuropeptides, endocannabinoids and various other systems, which either directly target the mPFC-amygdala circuit, or produce behavioural effects that are coincident with functional changes in the circuit. We conclude that there are good grounds to be optimistic that the progress in defining the molecular substrates of mPFC-amygdala circuit function can be effectively leveraged to identify plausible candidates for extinction-promoting therapies for anxiety disorders.

Distinct motor impairments of dopamine D1 and D2 receptor knockout mice revealed by three types of motor behavior

Both D1R and D2R knock out (KO) mice of the major dopamine receptors show significant motor impairments. However, there are some discrepant reports, which may be due to the differences in genetic background and experimental procedures. In addition, only few studies directly compared the motor performance of D1R and D2R KO mice. In this paper, we examined the behavioral difference among N10 congenic D1R and D2R KO, and wild type (WT) mice. First, we examined spontaneous motor activity in the home cage environment for consecutive 5 days. Second, we examined motor performance using the rota-rod task, a standard motor task in rodents. Third, we examined motor ability with the Step-Wheel task in which mice were trained to run in a motor-driven turning wheel adjusting their steps on foothold pegs to drink water. The results showed clear differences among the mice of three genotypes in three different types of behavior. In monitoring spontaneous motor activities, D1R and D2R KO mice showed higher and lower 24 h activities, respectively, than WT mice. In the rota-rod tasks, at a low speed, D1R KO mice showed poor performance but later improved, whereas D2R KO mice showed a good performance at early days without further improvement. When first subjected to a high speed task, the D2R KO mice showed poorer rota-rod performance at a low speed than the D1R KO mice. In the Step-Wheel task, across daily sessions, D2R KO mice increased the duration that mice run sufficiently close to the spout to drink water, and decreased time to touch the floor due to missing the peg steps and number of times the wheel was stopped, which performance was much better than that of D1R KO mice. These incongruent results between the two tasks for D1R and D2R KO mice may be due to the differences in the motivation for the rota-rod and Step-Wheel tasks, aversion- and reward-driven, respectively. The Step-Wheel system may become a useful tool for assessing the motor ability of WT and mutant mice.

A single glycine in extracellular loop 1 is the critical determinant for pharmacological specificity of dopamine D2 and D3 receptors

Subtype-selective agents for the dopamine D3 receptor (D3R) have been considered as potential medications for drug addiction and other neuropsychiatric disorders. Medicinal chemistry efforts have led to the discovery of 4-phenylpiperazine derivatives that are >100-fold selective for the dopamine D3 receptor over dopamine D2 receptor (D2R), despite high sequence identity (78% in the transmembrane domain). Based on the recent crystal structure of D3R, we demonstrated that the 4-phenylpiperazine moiety in this class of D3R-selective compounds binds to the conserved orthosteric binding site, whereas the extended aryl amide moiety is oriented toward a divergent secondary binding pocket (SBP). In an effort to further characterize molecular determinants of the selectivity of these compounds, we modeled their binding modes in D3R and D2R by comparative ligand docking and molecular dynamics simulations. We found that the aryl amide moiety in the SBP differentially induces conformational changes in transmembrane segment 2 and extracellular loop 1 (EL1), which amplify the divergence of the SBP in D3R and D2R. Receptor chimera and site-directed mutagenesis studies were used to validate these binding modes and to identify a divergent glycine in EL1 as critical to D3R over D2R subtype selectivity. A better understanding of drug-dependent receptor conformations such as these is key to the rational design of compounds targeting a specific receptor among closely related homologs, and may also lead to discovery of novel chemotypes that exploit subtle differences in protein conformations.

Dissociable effects of Sry and sex chromosome complement on activity, feeding and anxiety-related behaviours in mice

Whilst gonadal hormones can substantially influence sexual differentiation of the brain, recent findings have suggested that sex-linked genes may also directly influence neurodevelopment. Here we used the well-established murine ‘four core genotype’ (FCG) model on a gonadally-intact, outbred genetic background to characterise the contribution of Sry-dependent effects (i.e. those arising from the expression of the Y-linked Sry gene in the brain, or from hormonal sequelae of gonadal Sry expression) and direct effects of sex-linked genes other than Sry (‘sex chromosome complement’ effects) to sexually dimorphic mouse behavioural phenotypes. Over a 24 hour period, XX and XY gonadally female mice (lacking Sry) exhibited greater horizontal locomotor activity and reduced food consumption per unit bodyweight than XX and XY gonadally male mice (possessing Sry); in two behavioural tests (the elevated plus and zero mazes) XX and XY gonadally female mice showed evidence for increased anxiety-related behaviours relative to XX and XY gonadally male mice. Exploratory correlational analyses indicated that these Sry-dependent effects could not be simply explained by brain expression of the gene, nor by circulating testosterone levels. We also noted a sex chromosome complement effect on food (but not water) consumption whereby XY mice consumed more over a 24hr period than XX mice, and a sex chromosome complement effect in a third test of anxiety-related behaviour, the light-dark box. The present data suggest that: i) the male-specific factor Sry may influence activity and feeding behaviours in mice, and ii) dissociable feeding and anxiety-related murine phenotypes may be differentially modulated by Sry and by other sex-linked genes. Our results may have relevance for understanding the molecular underpinnings of sexually dimorphic behavioural phenotypes in healthy men and women, and in individuals with abnormal sex chromosome constitutions.

Upregulation of renal D5 dopamine receptor ameliorates the hypertension in D3 dopamine receptor-deficient mice

D3 dopamine receptor (D3R)-deficient mice have renin-dependent hypertension associated with sodium retention, but the hypertension is mild. To determine whether any compensatory mechanisms in the kidney are involved in the regulation of blood pressure with disruption of Drd3, we measured the renal protein expression of all dopamine receptor subtypes (D1R, D2R, D4R, and D5R) in D3R homozygous (D3(-/-)) and heterozygous (D3(+/-)) knockout mice and their wild-type (D3(+/+)) littermates. The renal immunohistochemistry and protein expression of D5R were increased (n=5/group) in D3(-/-) mice; renal D4R protein expression was decreased, whereas renal protein expressions of D1R and D2R were similar in both groups. Renal D5R protein expression was also increased in D3(+/-) (n=5/group) relative to D3(+/+) mice, whereas D1R, D2R, and D4R protein expressions were similar in D3(+/-) and D3(+/+) mice. The increase in renal D5R protein expression was abolished when D3(-/-) mice were fed a high-salt diet. Treatment with the D1-like receptor antagonist, SCH23390, increased the blood pressure in anesthetized D3(-/-) but not D3(+/+) mice (n=4/group), suggesting that the renal upregulation of D5R may have minimized the hypertension in D3(-/-) mice. The renal D5R protein upregulation was not caused by increased transcription because renal mRNA expression of D5R was similar in D3(-/-) and D3(+/+) mice. Our findings suggest that the renal upregulation of D5R may have minimized the hypertension that developed in D3(-/-) mice.

Dopamine D(3) receptor deletion or blockade attenuates cocaine-induced conditioned place preference in mice

The dopamine (DA) D3 receptor (D3R) has received much attention in medication development for treatment of addiction. However, the functional role of the D3R in drug reward and addiction has been a matter of debate. We recently reported that D3 receptor-knockout (D3(-/-)) mice display increased vulnerability to cocaine self-administration, which we interpret as a compensatory response to attenuated cocaine reward after D3R deletion. Here we report that D3(-/-) mice displayed attenuated cocaine-induced conditioned place response (CPP) compared to wild-type mice. Similarly, blockade of brain D3Rs by YQA-14, a novel DA D3 receptor antagonist, significantly and dose-dependently inhibits acquisition and expression of cocaine-induced CPP in WT mice, but not in D3(-/-) mice. These findings suggest that: 1) D3Rs play an important role in mediating cocaine's rewarding effects; and 2) YQA-14 is a highly potent and selective D3R antagonist in vivo, which deserves further study as a candidate for treatment of cocaine addiction.

Methods for the study of dopamine receptors within lipid rafts of kidney cells

There is increasing evidence that G protein-coupled receptor (GPCR) signaling is regulated in lipid raft microdomains. GPCRs and GPCR-signaling molecules, including G proteins and protein kinases, have been reported to compartmentalize in these microdomains. Dopamine D(1)-like receptors (D(1)R and D(5)R) belong to a family of GPCRs that are important in the regulation of renal function. These receptors are not only localized and regulated in caveolae that contains caveolin-1 but are also distributed in non--caveolar lipid rafts which do not contain caveolin-1. This chapter describes detergent- and non-detergent-based methods to obtain lipid raft fractions from renal proximal tubule cells.

Deletion of GSK3β in D2R-expressing neurons reveals distinct roles for β-arrestin signaling in antipsychotic and lithium action

Several studies in rodent models have shown that glycogen synthase kinase 3 β (GSK3β) plays an important role in the actions of antispychotics and mood stabilizers. Recently it was demonstrated that GSK3β through a β-arrestin2/protein kinase B (PKB or Akt)/protein phosphatase 2A (PP2A) signaling complex regulates dopamine (DA)- and lithium-sensitive behaviors and is required to mediate endophenotypes of mania and depression in rodents. We have previously shown that atypical antipsychotics antagonize DA D2 receptor (D2R)/β-arrestin2 interactions more efficaciously than G-protein-dependent signaling, whereas typical antipsychotics inhibit both pathways with similar efficacy. To elucidate the site of action of GSK3β in regulating DA- or lithium-sensitive behaviors, we generated conditional knockouts of GSK3β, where GSK3β was deleted in either DA D1- or D2-receptor-expressing neurons. We analyzed these mice for behaviors commonly used to test antipsychotic efficacy or behaviors that are sensitive to lithium treatment. Mice with deletion of GSK3β in D2 (D2GSK3β(-/-)) but not D1 (D1GSK3β(-/-)) neurons mimic antipsychotic action. However, haloperidol (HAL)-induced catalepsy was unchanged in either D2GSK3β(-/-) or D1GSK3β(-/-) mice compared with control mice. Interestingly, genetic stabilization of β-catenin, a downstream target of GSK3β, in D2 neurons did not affect any of the behaviors tested. Moreover, D2GSK3β(-/-) or D1GSK3β(-/-) mice showed similar responses to controls in the tail suspension test (TST) and dark-light emergence test, behaviors which were previously shown to be β-arrestin2- and GSK3β-dependent and sensitive to lithium treatment. Taken together these studies suggest that selective deletion of GSK3β but not stabilization of β-catenin in D2 neurons mimics antipsychotic action without affecting signaling pathways involved in catalepsy or certain mood-related behaviors.

Sulpiride, but not SCH23390, modifies cocaine-induced conditioned place preference and expression of tyrosine hydroxylase and elongation factor 1α in zebrafish

Finding genetic polymorphisms and mutations linked to addictive behavior can provide important targets for pharmaceutical and therapeutic interventions. Forward genetic approaches in model organisms such as zebrafish provide a potentially powerful avenue for finding new target genes. In order to validate this use of zebrafish, the molecular nature of its reward system must be characterized. We have previously reported the use of cocaine-induced conditioned place preference (CPP) as a reliable method for screening mutagenized fish for defects in the reward pathway. Here we test if CPP in zebrafish involves the dopaminergic system by co-treating fish with cocaine and dopaminergic antagonists. Sulpiride, a potent D2 receptor (DR2) antagonist, blocked cocaine-induced CPP, while the D1 receptor (DR1) antagonist SCH23390 had no effect. Acute cocaine exposure also induced a rise in the expression of tyrosine hydroxylase (TH), an important enzyme in dopamine synthesis, and a significant decrease in the expression of elongation factor 1α (EF1α), a housekeeping gene that regulates protein synthesis. Cocaine selectively increased the ratio of TH/EF1α in the telencephalon, but not in other brain regions. The cocaine-induced change in TH/EF1α was blocked by co-treatment with sulpiride, but not SCH23390, correlating closely with the action of these drugs on the CPP behavioral response. Immunohistochemical analysis revealed that the drop in EF1α was selective for the dorsal nucleus of the ventral telencephalic area (Vd), a region believed to be the teleost equivalent of the striatum. Examination of TH mRNA and EF1α transcripts suggests that regulation of expression is post-transcriptional, but this requires further examination. These results highlight important similarities and differences between zebrafish and more traditional mammalian model organisms.

Identification of two functionally distinct endosomal recycling pathways for dopamine D₂ receptor

Dopamine D₂ receptor (DRD2) is important for normal function of the brain reward circuit. Lower DRD2 function in the brain increases the risk for substance abuse, obesity, attention deficit/hyperactivity disorder, and depression. Moreover, DRD2 is the target of most antipsychotics currently in use. It is well known that dopamine-induced DRD2 endocytosis is important for its desensitization. However, it remains controversial whether DRD2 is recycled back to the plasma membrane or targeted for degradation following dopamine stimulation. Here, we used total internal reflection fluorescent microscopy (TIRFM) to image DRD2 with a superecliptic pHluorin tagged to its N terminus. With these technical advances, we were able to directly visualize vesicular insertion events of DRD2 in cultured mouse striatal medium spiny neurons. We showed that insertion of DRD2 occurs on neuronal somatic and dendritic surfaces. Lateral diffusion of DRD2 was observed following its insertion. Most importantly, using our new approach, we uncovered two functionally distinct recycling pathways for DRD2: a constitutive recycling pathway and a dopamine activity-dependent recycling pathway. We further demonstrated that Rab4 plays an important role in constitutive DRD2 recycling, while Rab11 is required for dopamine activity-dependent DRD2 recycling. Finally, we demonstrated that the two DRD2 recycling pathways play distinct roles in determining DRD2 function: the Rab4-sensitive constitutive DRD2 recycling pathway determines steady-state surface expression levels of DRD2, whereas the Rab11-sensitive dopamine activity-dependent DRD2 recycling pathway is important for functional resensitization of DRD2. Our findings underscore the significance of endosomal recycling in regulation of DRD2 function.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Specific knockdown of the D2 long dopamine receptor variant

Dopamine signaling in the nucleus accumbens is critical in mediating the effects of cocaine. There are two splice variants of dopamine D2 receptors, D2L and D2S, which are believed to have different functional roles. Here, we show, that knocking down D2L selectively using viral-mediated short-hairpin RNA led to a slight but significant decrease in basal locomotor activity with no significant change in cocaine-induced stimulation of locomotion. The knockdown appears to produce a trend of reduced conditioned place preference to cocaine but the difference was not statistically significant. Our results demonstrated that the splice variants of D2 receptors can be selectively manipulated in vivo in specific brain regions allowing more specific studies of each D2 receptor isoform.

Selective blockade of dopamine D3 receptors enhances while D2 receptor antagonism impairs social novelty discrimination and novel object recognition in rats: a key role for the prefrontal cortex

Dopamine D(3) receptor antagonists exert pro-cognitive effects in both rodents and primates. Accordingly, this study compared the roles of dopamine D(3) vs D(2) receptors in social novelty discrimination (SND), which relies on olfactory cues, and novel object recognition (NOR), a visual-recognition task. The dopamine D(3) receptor antagonist, S33084 (0.04-0.63 mg/kg), caused a dose-related reversal of delay-dependent impairment in both SND and NOR procedures in adult rats. Furthermore, mice genetically deficient in dopamine D(3) receptors displayed enhanced discrimination in the SND task compared with wild-type controls. In contrast, acute treatment with the preferential dopamine D(2) receptor antagonist, L741,626 (0.16-5.0 mg/kg), or with the dopamine D(3) agonist, PD128,907 (0.63-40 μg/kg), caused a dose-related impairment in performance in rats in both tasks after a short inter-trial delay. Bilateral microinjection of S33084 (2.5 μg/side) into the prefrontal cortex (PFC) of rats increased SND and caused a dose-related (0.63-2.5 μg/side) improvement in NOR, while intra-striatal injection (2.5 μg/side) had no effect on either. In contrast, bilateral microinjection of L741,626 into the PFC (but not striatum) caused a dose-related (0.63-2.5 μg/side) impairment of NOR. These observations suggest that blockade of dopamine D(3) receptors enhances both SND and NOR, whereas D(3) receptor activation or antagonism of dopamine D(2) receptor impairs cognition in these paradigms. Furthermore, these actions are mediated, at least partly, by the PFC. These data have important implications for exploitation of dopaminergic mechanisms in the treatment of schizophrenia and other CNS disorders, and support the potential therapeutic utility of dopamine D(3) receptor antagonism.

Mutant mouse models in evaluating novel approaches to antipsychotic treatment

In this review we consider the application of mutant mouse phenotypes to the study of psychotic illness in general and schizophrenia in particular, as they relate to behavioral, psychopharmacological, and cellular phenotypes of putative import for antipsychotic drug development. Mutant models appear to be heuristic at two main levels; firstly, by indicating the functional roles of neuronal components thought to be of relevance to the putative pathobiology of psychotic illness, they help resolve overt behavioral and underlying cellular processes regulated by those neuronal components; secondly, by indicating the functional roles of genes associated with risk for psychotic illness, they help resolve overt behavioral and underlying cellular processes regulated by those risk genes. We focus initially on models of dopaminergic and glutamatergic dysfunction. Then, we consider advances in the genetics of schizophrenia and mutant models relating to replicable risk genes. Lastly, we extend this discussion by exemplifying two new variant approaches in mutant mice that may serve as prototypes for advancing antipsychotic drug development. There is continuing need not only to address numerous technical challenges but also to develop more "real-world" paradigms that reflect the milieu of gene × environment and gene × gene interactions that characterize psychotic illness and its response to antipsychotic drugs.

DRD2 Taq1A polymorphism modulates prolactin secretion induced by atypical antipsychotics in healthy volunteers

Hyperprolactinemia mediated by antagonism of dopaminergic neurotransmission in the pituitary gland is a common adverse effect of antipsychotics. Recent studies have suggested that polymorphisms of dopamine receptors can affect the therapeutic response to antipsychotics. Thus, our aim was to evaluate whether 2 such polymorphisms (DRD2 Taq1A and DRD3 Ser9Gly) modulate prolactin release in healthy volunteers (n = 119) receiving a single dose of quetiapine (25 mg, n = 26), olanzapine (5 mg, n = 57), or risperidone (1 mg, n = 36). The increases in maximum concentration and in area under the curve were calculated from plasma prolactin levels after subtraction of pretreatment levels. Multiple regression analyses revealed that prolactin increases in maximum concentration and in area under the curve depended on drug (quetiapine < olanzapine < risperidone; P < 0.001), sex (women > men; P < 0.001), and Taq1A polymorphism (A1⁺ > A2/A2; P < 0.05). Analysis of the individual drugs revealed that prolactin secretion was modulated by sex and Taq1A polymorphism in olanzapine and risperidone (P < 0.05); however, these factors were not linked to prolactin secretion in quetiapine.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Maternal separation as a model of brain-gut axis dysfunction

RATIONALE

Early life stress has been implicated in many psychiatric disorders ranging from depression to anxiety. Maternal separation in rodents is a well-studied model of early life stress. However, stress during this critical period also induces alterations in many systems throughout the body. Thus, a variety of other disorders that are associated with adverse early life events are often comorbid with psychiatric illnesses, suggesting a common underlying aetiology. Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that is thought to involve a dysfunctional interaction between the brain and the gut. Essential aspects of the brain-gut axis include spinal pathways, the hypothalamic pituitary adrenal axis, the immune system, as well as the enteric microbiota. Accumulating evidence suggest that stress, especially in early life, is a predisposing factor to IBS.

OBJECTIVE

The objective of this review was to assess and compile the most relevant data on early life stress and alterations at all levels of the brain gut axis.

RESULTS

In this review, we describe the components of the brain-gut axis individually and how they are altered by maternal separation. The separated phenotype is characterised by alterations of the intestinal barrier function, altered balance in enteric microflora, exaggerated stress response and visceral hypersensitivity, which are all evident in IBS.

CONCLUSION

Thus, maternally separated animals are an excellent model of brain-gut axis dysfunction for the study of disorders such as IBS and for the development of novel therapeutic interventions.

Role of dopamine D3 and serotonin 5-HT 1A receptors in L: -DOPA-induced dyskinesias and effects of sarizotan in the 6-hydroxydopamine-lesioned rat model of Parkinson's disease

Sarizotan, a 5-HT(1A) agonist with additional affinity for D(3) and D(4) receptors, has been demonstrated to have anti-dyskinetic effects. The mechanism by which these effects occur is not clear. Using unilateral 6-hydroxydopamine-lesioned rats that received chronic intraperitoneal (ip) administration of L: -3,4-dihydroxyphenylalanine (L: -DOPA) we investigated the involvement of D(3) and 5-HT(1A) receptors in the effects of sarizotan on contraversive circling and abnormal involuntary movements (AIMs). Before sensitization by chronic L: -DOPA treatment (12.5 with 3.25 mg/kg benserazide ip, twice daily for 21 days), no effect of the selective D(3) agonist, PD128907 (1 or 3 mg/kg ip), or the selective D(3) antagonist, GR103691 (0.5 or 1.5 mg/kg ip), was observed. Treatment with sarizotan (1 or 5 mg/kg ip) dose-dependently inhibited the L: -DOPA-induced contraversive turning and AIMs. In co-treatment with the 5-HT(1A) antagonist, WAY100635 (1 mg/kg ip), sarizotan failed to affect this behaviour, confirming the prominent 5-HT(1A) receptor-mediated mechanism of action. In the presence of PD128907 (3 mg/kg ip), the effects of sarizotan on contraversive turning, locomotive dyskinesia and axial dystonia, but not on orolingual and forelimb dyskinesia, were blocked. On its own, PD128907 had no effect on the behavioural effects of L: -DOPA except that it tended to reduce orolingual and forelimb dyskinesia. GR103691 had no effect on its own or in combination with sarizotan. These data identify an involvement of D(3) receptors in the action of sarizotan on some, but not all L: -DOPA-induced motor side effects. This selective involvement is in contrast to the more general involvement of 5-HT(1A) receptors in the anti-dyskinetic effects of sarizotan.

Susceptibility genes for schizophrenia: mutant models, endophenotypes and psychobiology

Schizophrenia is characterised by a multifactorial aetiology that involves genetic liability interacting with epigenetic and environmental factors to increase risk for developing the disorder. A consensus view is that the genetic component involves several common risk alleles of small effect and/or rare but penetrant copy number variations. Furthermore, there is increasing evidence for broader, overlapping genetic-phenotypic relationships in psychosis; for example, the same susceptibility genes also confer risk for bipolar disorder. Phenotypic characterisation of genetic models of candidate risk genes and/or putative pathophysiological processes implicated in schizophrenia, as well as examination of epidemiologically relevant gene × environment interactions in these models, can illuminate molecular and pathobiological mechanisms involved in schizophrenia. The present chapter outlines both the evidence from phenotypic studies in mutant mouse models related to schizophrenia and recently described mutant models addressing such gene × environment interactions. Emphasis is placed on evaluating the extent to which mutant phenotypes recapitulate the totality of the disease phenotype or model selective endophenotypes. We also discuss new developments and trends in relation to the functional genomics of psychosis which might help to inform on the construct validity of mutant models of schizophrenia and highlight methodological challenges in phenotypic evaluation that relate to such models.

Neurogenetics and pharmacology of learning, motivation, and cognition

Many of the individual differences in cognition, motivation, and learning-and the disruption of these processes in neurological conditions-are influenced by genetic factors. We provide an integrative synthesis across human and animal studies, focusing on a recent spate of evidence implicating a role for genes controlling dopaminergic function in frontostriatal circuitry, including COMT, DARPP-32, DAT1, DRD2, and DRD4. These genetic effects are interpreted within theoretical frameworks developed in the context of the broader cognitive and computational neuroscience literature, constrained by data from pharmacological, neuroimaging, electrophysiological, and patient studies. In this framework, genes modulate the efficacy of particular neural computations, and effects of genetic variation are revealed by assays designed to be maximally sensitive to these computations. We discuss the merits and caveats of this approach and outline a number of novel candidate genes of interest for future study.

Structure of the human dopamine D3 receptor in complex with a D2/D3 selective antagonist

Dopamine modulates movement, cognition, and emotion through activation of dopamine G protein-coupled receptors in the brain. The crystal structure of the human dopamine D3 receptor (D3R) in complex with the small molecule D2R/D3R-specific antagonist eticlopride reveals important features of the ligand binding pocket and extracellular loops. On the intracellular side of the receptor, a locked conformation of the ionic lock and two distinctly different conformations of intracellular loop 2 are observed. Docking of R-22, a D3R-selective antagonist, reveals an extracellular extension of the eticlopride binding site that comprises a second binding pocket for the aryl amide of R-22, which differs between the highly homologous D2R and D3R. This difference provides direction to the design of D3R-selective agents for treating drug abuse and other neuropsychiatric indications.