Dopaminergic mouse mutants: investigating the roles of the different dopamine receptor subtypes and the dopamine transporter

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

A rapid introduction to neurological biochemistry using Drosophila melanogaster

Short, cost-effective teaching activities are a useful way of providing an integrated view on biological processes. Here we describe a brief, hands-on workshop that allows pre-university students to explore their understanding of a neurological pathway from its chemical bases to phenotype. The workshop effectively introduces the students to data collection and analysis in an enjoyable way and at an appropriate level, determined by an end of session feedback survey. The design of the workshop can be adapted and scaled to generate diverse sessions such as university teaching practicals or summer school training workshops.

Catecholamines and cognition after traumatic brain injury

Cognitive problems are one of the main causes of ongoing disability after traumatic brain injury. The heterogeneity of the injuries sustained and the variability of the resulting cognitive deficits makes treating these problems difficult. Identifying the underlying pathology allows a targeted treatment approach aimed at cognitive enhancement. For example, damage to neuromodulatory neurotransmitter systems is common after traumatic brain injury and is an important cause of cognitive impairment. Here, we discuss the evidence implicating disruption of the catecholamines (dopamine and noradrenaline) and review the efficacy of catecholaminergic drugs in treating post-traumatic brain injury cognitive impairments. The response to these therapies is often variable, a likely consequence of the heterogeneous patterns of injury as well as a non-linear relationship between catecholamine levels and cognitive functions. This individual variability means that measuring the structure and function of a person’s catecholaminergic systems is likely to allow more refined therapy. Advanced structural and molecular imaging techniques offer the potential to identify disruption to the catecholaminergic systems and to provide a direct measure of catecholamine levels. In addition, measures of structural and functional connectivity can be used to identify common patterns of injury and to measure the functioning of brain ‘networks’ that are important for normal cognitive functioning. As the catecholamine systems modulate these cognitive networks, these measures could potentially be used to stratify treatment selection and monitor response to treatment in a more sophisticated manner.

Dopamine D2 receptor-mediated Akt/PKB signalling: initiation by the D2S receptor and role in quinpirole-induced behavioural activation

The short and long isoforms of the dopamine D2 receptor (D2S and D2L respectively) are highly expressed in the striatum. Functional D2 receptors activate an intracellular signalling pathway that includes a cAMP-independent route involving Akt/GSK3 (glycogen synthase kinase 3). To investigate the Akt/GSK3 response to the seldom-studied D2S receptor, we established a rat D2S receptor-expressing cell line [HEK (human embryonic kidney)-293/rD2S]. We found that in HEK-293/rD2S cells, the D2/D3 agonists bromocriptine and quinpirole significantly induced Akt and GSK3 phosphorylation, as well as ERK1/2 (extracellular-signal-regulated kinase 1/2) activation. The D2S receptor-induced Akt signals were profoundly inhibited by the internalization blockers monodansyl cadaverine and concanavalin A. Activation of the D2S receptor in HEK-293/rD2S cells appeared to trigger Akt/phospho-Akt translocation to the cell membrane. In addition to our cell culture experiments, we studied D2 receptor-dependent Akt in vivo by systemic administration of the D2/D3 agonist quinpirole. The results show that quinpirole evoked Akt-Ser⁴⁷³ phosphorylation in the ventral striatum. Furthermore, intra-accumbens administration of wortmannin, a PI3K (phosphoinositide 3-kinase) inhibitor, significantly suppressed the quinpirole-evoked behavioural activation. Overall, we demonstrate that activation of the dopamine D2S receptor stimulates Akt/GSK3 signalling. In addition, in vivo Akt activity in the ventral striatum appears to play an important role in systemic D2/D3 agonist-induced behavioural activation.

Dopamine-mediated autocrine inhibitory circuit regulating human insulin secretion in vitro

We describe a negative feedback autocrine regulatory circuit for glucose-stimulated insulin secretion in purified human islets in vitro. Using chronoamperometry and in vitro glucose-stimulated insulin secretion measurements, evidence is provided that dopamine (DA), which is loaded into insulin-containing secretory granules by vesicular monoamine transporter type 2 in human β-cells, is released in response to glucose stimulation. DA then acts as a negative regulator of insulin secretion via its action on D2R, which are also expressed on β-cells. We found that antagonism of receptors participating in islet DA signaling generally drive increased glucose-stimulated insulin secretion. These in vitro observations may represent correlates of the in vivo metabolic changes associated with the use of atypical antipsychotics, such as increased adiposity.

Drosophila Dopamine2-like receptors function as autoreceptors

Dopaminergic signaling pathways are conserved between mammals and Drosophila and D2 receptors have been identified in Drosophila. However, it has not been demonstrated whether Drosophila D2 receptors function as autoreceptors and regulate the release of dopamine. The goal of this study was to determine if Drosophila D2 receptors act as autoreceptors by probing the extent to which D2 agonists and antagonists affect evoked dopamine release. Fast-scan cyclic voltammetry was used to measure stimulated dopamine release at a carbon-fiber microelectrode implanted in an intact, larval Drosophila nervous system. Dopamine release was evoked using 5-second blue light stimulations that open Channelrhodopsin-2, a blue light activated cation channel that was specifically expressed in dopaminergic neurons. In mammals, administration of a D2 agonist decreases evoked dopamine release by increasing autoreceptor feedback. Similarly, we found that the D2 agonists bromocriptine and quinpirole decreased stimulated dopamine release in Drosophila. D2 antagonists were expected to increase dopamine release and the D2 antagonists flupenthixol, butaclamol, and haloperidol did increase stimulated release. Agonists did not significantly modulate dopamine uptake although the modulatory effects of D2 drugs on release were affected by prior administration of the uptake inhibitor nisoxetine. These results demonstrate that the D2 receptor functions as an autoreceptor in Drosophila. The similarities in dopamine regulation validate Drosophila as a model system for studying the basic neurobiology of dopaminergic signaling.

Antisense-mediated RNA targeting: versatile and expedient genetic manipulation in the brain

A limiting factor in brain research still is the difficulty to evaluate in vivo the role of the increasing number of proteins implicated in neuronal processes. We discuss here the potential of antisense-mediated RNA targeting approaches. We mainly focus on those that manipulate splicing (exon skipping and exon inclusion), but will also briefly discuss mRNA targeting. Classic knockdown of expression by mRNA targeting is only one possible application of antisense oligonucleotides (AON) in the control of gene function. Exon skipping and inclusion are based on the interference of AONs with splicing of pre-mRNAs. These are powerful, specific and particularly versatile techniques, which can be used to circumvent pathogenic mutations, shift splice variant expression, knock down proteins, or to create molecular models using in-frame deletions. Pre-mRNA targeting is currently used both as a research tool, e.g., in models for motor neuron disease, and in clinical trials for Duchenne muscular dystrophy and amyotrophic lateral sclerosis. AONs are particularly promising in relation to brain research, as the modified AONs are taken up extremely fast in neurons and glial cells with a long residence, and without the need for viral vectors or other delivery tools, once inside the blood brain barrier. In this review we cover (1). The principles of antisense-mediated techniques, chemistry, and efficacy. (2) The pros and cons of AON approaches in the brain compared to other techniques of interfering with gene function, such as transgenesis and short hairpin RNAs, in terms of specificity of the manipulation, spatial, and temporal control over gene expression, toxicity, and delivery issues. (3) The potential applications for Neuroscience. We conclude that there is good evidence from animal studies that the central nervous system can be successfully targeted, but the potential of the diverse AON-based approaches appears to be under-recognized.

Dopamine D4 receptors modulate brain metabolic activity in the prefrontal cortex and cerebellum at rest and in response to methylphenidate

Methylphenidate (MP) is widely used to treat attention deficit hyperactivity disorder (ADHD). Variable number of tandem repeats polymorphisms in the dopamine D4 receptor (D(4)) gene have been implicated in vulnerability to ADHD and the response to MP. Here we examined the contribution of dopamine D4 receptors (D4Rs) to baseline brain glucose metabolism and to the regional metabolic responses to MP. We compared brain glucose metabolism (measured with micro-positron emission tomography and [(18)F]2-fluoro-2-deoxy-D-glucose) at baseline and after MP (10 mg/kg, i.p.) administration in mice with genetic deletion of the D(4). Images were analyzed using a novel automated image registration procedure. Baseline D(4)(-/-) mice had lower metabolism in the prefrontal cortex (PFC) and greater metabolism in the cerebellar vermis (CBV) than D(4)(+/+) and D(4)(+/-) mice; when given MP, D(4)(-/-) mice increased metabolism in the PFC and decreased it in the CBV, whereas in D(4)(+/+) and D(4)(+/-) mice, MP decreased metabolism in the PFC and increased it in the CBV. These findings provide evidence that D4Rs modulate not only the PFC, which may reflect the activation by dopamine of D4Rs located in this region, but also the CBV, which may reflect an indirect modulation as D4Rs are minimally expressed in this region. As individuals with ADHD show structural and/or functional abnormalities in these brain regions, the association of ADHD with D4Rs may reflect its modulation of these brain regions. The differential response to MP as a function of genotype could explain differences in brain functional responses to MP between patients with ADHD and healthy controls and between patients with ADHD with different D(4) polymorphisms.

Enhanced latent inhibition in dopamine receptor-deficient mice is sex-specific for the D1 but not D2 receptor subtype: implications for antipsychotic drug action

Latent inhibition (LI) is reduced learning to a stimulus that has previously been experienced without consequence. It is an important model of abnormal allocation of salience to irrelevant information in patients with schizophrenia. In rodents LI is abolished by psychotomimetic drugs and in experimental conditions where LI is low in controls, its expression is enhanced by antipsychotic drugs with activity at dopamine (DA) receptors. It is however unclear what the independent contributions of DA receptor subtypes are to these effects. This study therefore examined LI in congenic DA D1 and D2 receptor knockout (D1 KO and D2 KO) mice. Conditioned suppression of drinking was used as the measure of learning in the LI procedure. Both male and female DA D2 KO mice showed clear enhancement of LI reproducing antipsychotic drug effects in the model. Unexpectedly, enhancement was also seen in D1 KO female mice but not in D1 KO male mice. This sex-specific pattern was not replicated in locomotor or motor coordination tasks nor in the effect of DA KOs on baseline learning in control groups indicating some specificity of the effect to LI. These data suggest that the dopaminergic mechanism underlying LI potentiation and possibly antipsychotic action may differ between the sexes, being mediated by D2 receptors in males but by both D1 and D2 receptors in females. These data suggest that the DA D1 receptor may prove an important target for understanding sex differences in the mechanisms of action of antipsychotic drugs and in the aetiology of aberrant salience allocation in schizophrenia.

Persistent cognitive dysfunction after traumatic brain injury: A dopamine hypothesis

Traumatic brain injury (TBI) represents a significant cause of death and disability in industrialized countries. Of particular importance to patients the chronic effect that TBI has on cognitive function. Therapeutic strategies have been difficult to evaluate because of the complexity of injuries and variety of patient presentations within a TBI population. However, pharmacotherapies targeting dopamine (DA) have consistently shown benefits in attention, behavioral outcome, executive function, and memory. Still it remains unclear what aspect of TBI pathology is targeted by DA therapies and what time-course of treatment is most beneficial for patient outcomes. Fortunately, ongoing research in animal models has begun to elucidate the pathophysiology of DA alterations after TBI. The purpose of this review is to discuss clinical and experimental research examining DAergic therapies after TBI, which will in turn elucidate the importance of DA for cognitive function/dysfunction after TBI as well as highlight the areas that require further study.

Getting specialized: presynaptic and postsynaptic dopamine D2 receptors

Dopamine (DA) signaling controls many physiological functions ranging from locomotion to hormone secretion, and plays a critical role in addiction. DA elevation, for instance in response to drugs of abuse, simultaneously activates neurons expressing different DA receptors; how responses from diverse neurons/receptors are orchestrated in the generation of behavioral and cellular outcomes, is still not completely defined. Signaling from D2 receptors (D2Rs) is a good example to illustrate this complexity. D2Rs have presynaptic and postsynaptic localization and functions, which are shared by two isoforms in vivo. Recent results from knockout mice are clarifying the role of site and D2 isoform-specific effects thereby increasing our understanding of how DA modulates neuronal physiology.

Getting specialized: presynaptic and postsynaptic dopamine D2 receptors

Dopamine (DA) signaling controls many physiological functions ranging from locomotion to hormone secretion, and plays a critical role in addiction. DA elevation, for instance in response to drugs of abuse, simultaneously activates neurons expressing different DA receptors; how responses from diverse neurons/receptors are orchestrated in the generation of behavioral and cellular outcomes, is still not completely defined. Signaling from D2 receptors (D2Rs) is a good example to illustrate this complexity. D2Rs have presynaptic and postsynaptic localization and functions, which are shared by two isoforms in vivo. Recent results from knockout mice are clarifying the role of site and D2 isoform-specific effects thereby increasing our understanding of how DA modulates neuronal physiology.

Loss of erbB signaling in oligodendrocytes alters myelin and dopaminergic function, a potential mechanism for neuropsychiatric disorders

Several psychiatric disorders are associated with white matter defects, suggesting that oligodendrocyte (OL) abnormalities underlie some aspects of these diseases. Neuregulin 1 (NRG1) and its receptor, erbB4, are genetically linked with susceptibility to schizophrenia and bipolar disorder. In vitro studies suggest that NRG1-erbB signaling is important for OL development. To test whether erbB signaling contributes to psychiatric disorders by regulating the structure or function of OLs, we analyzed transgenic mice in which erbB signaling is blocked in OLs in vivo. Here we show that loss of erbB signaling leads to changes in OL number and morphology, reduced myelin thickness, and slower conduction velocity in CNS axons. Furthermore, these transgenic mice have increased levels of dopamine receptors and transporters and behavioral alterations consistent with neuropsychiatric disorders. These results indicate that defects in white matter can cause alterations in dopaminergic function and behavior relevant to neuropsychiatric disorders.

The genetics of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a highly heritable, disruptive, childhood-onset condition, the aetiology and pathogenesis of which is poorly understood. There have been relatively few genome-wide linkage studies, and no chromosomal region has yet been unequivocally implicated. In contrast, evidence from pharmacological, neuroimaging and animal studies has suggested the involvement of specific neurotransmitter systems, notably dopaminergic pathways, in ADHD and these aetiological clues have inspired a fruitful application of the candidate gene association approach. Meta-analyses or pooled data analyses have supported association between ADHD and polymorphisms in DRD4, DRD5 and SLC6A3 which encode dopamine D4 and D5 receptors and the dopamine transporter, respectively. A weaker, but nevertheless replicated, body of evidence also supports associations with SNAP-25 (synaptosomal-associated protein, 25 kDa) and SLC6A4 (serotonin transporter). There is increasing research interest in gene-phenotype links, clinical phenotypic markers of heterogeneity and gene--environment interaction, which are likely to be important in the next generation of genetic studies.

Physiological modulation of intestinal motility by enteric dopaminergic neurons and the D2 receptor: analysis of dopamine receptor expression, location, development, and function in wild-type and knock-out mice

Dopaminergic neurons are present in both plexuses of the murine bowel and are upregulated after extrinsic denervation but play unknown roles in enteric nervous system (ENS) physiology. Transcripts encoding dopamine (DA) receptors D1-D5 were analyzed by reverse transcription-PCR in stomach approximately duodenum approximately ileum approximately proximal > > distal colon. Dissected muscle and myenteric plexus contained transcripts encoding D1-D3 and D5, whereas mucosa contained D1 and D3-D5. D1-D5 expression began in fetal gut [embryonic day 10 (E10)], before the appearance of neurons (E12), and was sustained without developmental regulation through postnatal day 1. In situ hybridization revealed that subsets of submucosal and myenteric neurons contained mRNA encoding D2 or D3. Immunoblots confirmed that D1, D2, and D5 receptor proteins were present from stomach through distal colon. Subsets of submucosal and myenteric neurons were also D1, D2, or D3 immunoreactive. When double labeled by in situ hybridization, these neurons contained mRNA encoding the respective receptors. Total gastrointestinal transit time (TGTT) and colonic transit time (CTT) were measured in mice lacking D2, D3, or D2 plus D3. Both TGTT and CTT were decreased significantly (motility increased) in D2 and D2 plus D3, but not D3, knock-out animals. Mice lacking D2 and D2 plus D3 but not D3 were smaller than wild-type littermates, yet ate significantly more and had greater stool frequency, water content, and mass. Because motility is abnormal when D2 is absent, the net inhibitory DA effect on motility is physiologically significant. The early expression of DA receptors is also consistent with the possibility that DA affects ENS development.

Dopamine in neurotoxicity and neuroprotection: what do D2 receptors have to do with it?

Accurate control of dopamine levels and/or the resulting dopamine-receptor interaction is essential for brain function. Indeed, several human neurological and psychiatric disorders are characterized by dysfunctions of the dopaminergic system. Dopamine has been reported to exert either protective or toxic effects on neurons, yet it is unclear whether these effects are receptor-dependent and, if so, which dopamine receptor could be involved. The D(2) dopamine receptor occupies a privileged position because its signalling might be neuroprotective in human diseases, such as Parkinson's disease, ischaemia and epilepsy. Unravelling the role of D(2) receptors in neuronal death and survival might be central to understanding the mechanisms that underlie several neuropathologies.

Phenotypic studies on dopamine receptor subtype and associated signal transduction mutants: insights and challenges from 10 years at the psychopharmacology-molecular biology interface

BACKGROUND

Mutants with targeted gene deletion ('knockout') or insertion (transgenic) of D1, D2, D3, D4 and D5 dopamine (DA) receptor subtypes are complemented by an increasing variety of double knockout and transgenic-'knockout' models, together with knockout of critical components of DA receptor signalling cascades such as G alpha(olf)[G gamma7], adenylyl cyclase type 5, PKA [RIIbeta] and DARPP-32. However, it is increasingly recognised that these molecular techniques have a number of inherent limitations. Furthermore, there are poorly understood methodological factors that contribute to inconsistent phenotypic findings between laboratories.

OBJECTIVE

This review seeks to document the impact of DA receptor subtype and related transduction mutants on our understanding of the behavioural roles of these entities, primarily at the level of unconditioned psychomotor behaviour.

METHODS

It includes ethologically based and orofacial movement studies in our own laboratories, since these are the only studies to systematically compare each of the D1, D2, D3, D4 and D5 receptor and DARPP-32 signal transduction 'knockouts'.

DISCUSSION

There is a particular emphasis on identifying methodological factors that might influence phenotypic effects and account for inconsistencies. The findings are offered empirically to (1) specify the extent of phenotypic diversity among individual DA receptor subtypes and transduction components and (2) indicate relationships between D1, D2, D3, D4 and D5 receptor subtype proteins, associated G alpha(i)/G alpha(s)/G alpha(olf)[G gamma7]-adenylyl cyclase type 5-PKA [RIIbeta]-DARPP-32 signalling cascades and behaviour. The findings are also offered heuristically as a base for such phenotypic comparisons at additional levels of behaviour so that a yet more complete phenotypic profile might emerge.

D1-like dopamine receptor-mediated function in congenic mutants with D1 vs. D5 receptor "knockout"

Current understanding of the functional roles of individual dopamine D1-like [D1, D5] and D2-like [D2L/s, D3, D4] receptor subtypes remains incomplete. In particular, the lack of pharmacological agonists and antagonists able to distinguish between D1 and D5 receptors means that any differential roles in the regulation of behavior are poorly understood. Mutant mice with targeted gene deletion ("knockout") of individual dopamine receptor subtypes offer an important alternative approach to resolving these functional roles. In congenic D1 mutants examined ethologically, progressive increases in specific topographies of behavior over wildtypes were considerably greater than those in D1 mutants on a mixed genetic background; D1 knockout appears to influence the neuronal substrate(s) of habituation to disrupt sculpture of the changing topography of behavior from initial exploration through to quiescence. Similarly, the D1 receptor appears to regulate specific topographies of orofacial movement in the mouse as these are "sculpted" in a time-dependent manner. Although the well-recognized role of the D1-like family in regulating several aspects of behavioral topography has been assumed to involve primarily D1 receptors, this presumption may require modification to accommodate a subtle but not negligible role for their D5 counterparts as evidenced in the phenotype of congenic D5 mutants.

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

The functional specificity of dopamine receptor subtypes remains incompletely understood, in part due to the absence of highly selective agonists and antagonists. Phenotypic analysis of dopamine receptor knockout mice has been instrumental in identifying the role of dopamine receptor subtypes in mediating dopamine's effects on motor function, cognition, reward, and emotional behaviors. In this article, we provide an update of recent studies in dopamine receptor knockout mice and discuss the limitations and future promise of this approach.

Simultaneous absence of dopamine D1 and D2 receptor-mediated signaling is lethal in mice

Dopamine (DA) controls a wide variety of physiological functions in the central nervous system as well as in the neuroendocrine and gastrointestinal systems. DA signaling is mediated by five cloned receptors named D1-D5. Knockout mouse models for the five receptors have been generated, and, albeit impaired for some important DA-mediated functions, they are viable and can reproduce. D1 and D2 receptors are the most abundant and widely expressed DA receptors. Cooperative/synergistic effects mediated by these receptors have been suggested, in particular, in the control of motor behaviors. To analyze the extent of such interrelationship, we have generated double D1/D2 receptor mutants. Interestingly, in contrast to single knockouts, we found that concurrent ablation of the D1 and D2 receptors is lethal during the second or third week after birth. This dramatic phenotype is likely to be related to altered feeding behavior and dysfunction of the gastrointestinal system, especially because major anatomical changes were not identified in the brain. Similarly, in the absence of functional D1, heterozygous D2 mutants (D1r(-/-);D2r(+/-)) showed severe growth retardation and did not survive their postweaning period. The analysis of motor behavior in D1r/D2r compound mutants showed that loss of D2-mediated functions reduces motor abilities, whereas the effect of D1r ablation on locomotion strongly depends on the experimental paradigms used. These studies highlight the interrelationship between D1 and D2 receptor-mediated control of motor activity, food intake, and gastrointestinal functions, which has been elusive in the single-gene ablation studies.