Dopamine receptors in canine caudate nucleus

Shared GABA transmission pathology in dopamine agonist- and antagonist-induced dyskinesia

Dyskinesia is involuntary movement caused by long-term medication with dopamine-related agents: the dopamine agonist 3,4-dihydroxy-L-phenylalanine (L-DOPA) to treat Parkinson's disease (L-DOPA-induced dyskinesia [LID]) or dopamine antagonists to treat schizophrenia (tardive dyskinesia [TD]). However, it remains unknown why distinct types of medications for distinct neuropsychiatric disorders induce similar involuntary movements. Here, we search for a shared structural footprint using magnetic resonance imaging-based macroscopic screening and super-resolution microscopy-based microscopic identification. We identify the enlarged axon terminals of striatal medium spiny neurons in LID and TD model mice. Striatal overexpression of the vesicular gamma-aminobutyric acid transporter (VGAT) is necessary and sufficient for modeling these structural changes; VGAT levels gate the functional and behavioral alterations in dyskinesia models. Our findings indicate that lowered type 2 dopamine receptor signaling with repetitive dopamine fluctuations is a common cause of VGAT overexpression and late-onset dyskinesia formation and that reducing dopamine fluctuation rescues dyskinesia pathology via VGAT downregulation.

Interpreting the role of the striatum during multiple phases of motor learning

The synaptic pathways in the striatum are central to basal ganglia functions including motor control, learning and organization, action selection, acquisition of motor skills, cognitive function, and emotion. Here, we review the role of the striatum and its connections in motor learning and performance. The development of new techniques to record neuronal activity and animal models of motor disorders using neurotoxin, pharmacological, and genetic manipulations are revealing pathways that underlie motor performance and motor learning, as well as how they are altered by pathophysiological mechanisms. We discuss approaches that can be used to analyze complex motor skills, particularly in rodents, and identify specific questions central to understanding how striatal circuits mediate motor learning.

Abundance Compensates Kinetics: Similar Effect of Dopamine Signals on D1 and D2 Receptor Populations

The neuromodulator dopamine plays a key role in motivation, reward-related learning, and normal motor function. The different affinity of striatal D1 and D2 dopamine receptor types has been argued to constrain the D1 and D2 signaling pathways to phasic and tonic dopamine signals, respectively. However, this view assumes that dopamine receptor kinetics are instantaneous so that the time courses of changes in dopamine concentration and changes in receptor occupation are basically identical. Here we developed a neurochemical model of dopamine receptor binding taking into account the different kinetics and abundance of D1 and D2 receptors in the striatum. Testing a large range of behaviorally-relevant dopamine signals, we found that the D1 and D2 dopamine receptor populations responded very similarly to tonic and phasic dopamine signals. Furthermore, because of slow unbinding rates, both receptor populations integrated dopamine signals over a timescale of minutes. Our model provides a description of how physiological dopamine signals translate into changes in dopamine receptor occupation in the striatum, and explains why dopamine ramps are an effective signal to occupy dopamine receptors. Overall, our model points to the importance of taking into account receptor kinetics for functional considerations of dopamine signaling.SIGNIFICANCE STATEMENT Current models of basal ganglia function are often based on a distinction of two types of dopamine receptors, D1 and D2, with low and high affinity, respectively. Thereby, phasic dopamine signals are believed to mostly affect striatal neurons with D1 receptors, and tonic dopamine signals are believed to mostly affect striatal neurons with D2 receptors. This view does not take into account the rates for the binding and unbinding of dopamine to D1 and D2 receptors. By incorporating these kinetics into a computational model we show that D1 and D2 receptors both respond to phasic and tonic dopamine signals. This has implications for the processing of reward-related and motivational signals in the basal ganglia.

Cell-Type-Specific Afferent Innervation of the Nucleus Accumbens Core and Shell

The nucleus accumbens (NAc) is clearly implicated in reward processing and drug addiction, as well as in numerous neurological and psychiatric disorders; nevertheless, the circuit mechanisms underlying the diverse functions of the NAc remain poorly understood. Here, we characterized the whole-brain and monosynaptic inputs to two main projection cell types – D1 dopamine receptor expressing medium spiny neurons (D1R-MSNs) and D2 dopamine receptor expressing medium spiny neurons (D2R-MSNs) – within the NAc core and NAc shell by rabies-mediated trans-synaptic tracing. We discovered that D1R-MSNs and D2R-MSNs in both NAc subregions receive similar inputs from diverse sources. Inputs to the NAc core are broadly scattered, whereas inputs to the NAc shell are relatively concentrated. Furthermore, we identified numerous brain areas providing important contrasting inputs to different NAc subregions. The anterior cortex preferentially innervates the NAc core for both D1R-MSNs and D2R-MSNs, whereas the lateral hypothalamic area (LH) preferentially targets D1R-MSNs in the NAc shell. Characterizing the cell-type-specific connectivity of different NAc subregions lays a foundation for studying how diverse functions of the NAc are mediated by specific pathways.

Prenatal exposure to methylphenidate affects the dopamine system and the reactivity to natural reward in adulthood in rats

BACKGROUND

Methylphenidate (MPH) is a commonly-used medication for the treatment of children with Attention-Deficit/Hyperactivity Disorders (ADHD). However, its prescription to adults with ADHD and narcolepsy raises the question of how the brain is impacted by MPH exposure during pregnancy. The goal of this study was to elucidate the long-term neurobiological consequences of prenatal exposure to MPH using a rat model.

METHODS

We focused on the effects of such treatment on the adult dopamine (DA) system and on the reactivity of animals to natural rewards.

RESULTS

This study shows that adult male rats prenatally exposed to MPH display elevated expression of presynaptic DA markers in the DA cell bodies and the striatum. Our results also suggest that MPH-treated animals could exhibit increased tonic DA activity in the mesolimbic pathway, altered signal-to-noise ratio after a pharmacological stimulation, and decreased reactivity to the locomotor effects of cocaine. Finally, we demonstrated that MPH rats display a decreased preference and motivation for sucrose.

CONCLUSIONS

This is the first preclinical study reporting long-lasting neurobiological alterations of DA networks as well as alterations in motivational behaviors for natural rewards after a prenatal exposure to MPH. These results raise concerns about the possible neurobiological consequences of MPH treatment during pregnancy.

Distinct dopaminergic control of the direct and indirect pathways in reward-based and avoidance learning behaviors

The nucleus accumbens (NAc) plays a pivotal role in reward and aversive learning and learning flexibility. Outputs of the NAc are transmitted through two parallel routes termed the direct and indirect pathways and controlled by the dopamine (DA) neurotransmitter. To explore how reward-based and avoidance learning is controlled in the NAc of the mouse, we developed the reversible neurotransmission-blocking (RNB) technique, in which transmission of each pathway could be selectively and reversibly blocked by the pathway-specific expression of transmission-blocking tetanus toxin and the asymmetric RNB technique, in which one side of the NAc was blocked by the RNB technique and the other intact side was pharmacologically manipulated by a transmitter agonist or antagonist. Our studies demonstrated that the activation of D1 receptors in the direct pathway and the inactivation of D2 receptors in the indirect pathway are key determinants that distinctly control reward-based and avoidance learning, respectively. The D2 receptor inactivation is also critical for flexibility of reward learning. Furthermore, reward and aversive learning is regulated by a set of common downstream receptors and signaling cascades, all of which are involved in the induction of long-term potentiation at cortico-accumbens synapses of the two pathways. In this article, we review our studies that specify the regulatory mechanisms of each pathway in learning behavior and propose a mechanistic model to explain how dynamic DA modulation promotes selection of actions that achieve reward-seeking outcomes and avoid aversive ones. The biological significance of the network organization consisting of two parallel transmission pathways is also discussed from the point of effective and prompt selection of neural outcomes in the neural network.

Aversive behavior induced by optogenetic inactivation of ventral tegmental area dopamine neurons is mediated by dopamine D2 receptors in the nucleus accumbens

Dopamine (DA) transmission from the ventral tegmental area (VTA) is critical for controlling both rewarding and aversive behaviors. The transient silencing of DA neurons is one of the responses to aversive stimuli, but its consequences and neural mechanisms regarding aversive responses and learning have largely remained elusive. Here, we report that optogenetic inactivation of VTA DA neurons promptly down-regulated DA levels and induced up-regulation of the neural activity in the nucleus accumbens (NAc) as evaluated by Fos expression. This optogenetic suppression of DA neuron firing immediately evoked aversive responses to the previously preferred dark room and led to aversive learning toward the optogenetically conditioned place. Importantly, this place aversion was abolished by knockdown of dopamine D2 receptors but not by that of D1 receptors in the NAc. Silencing of DA neurons in the VTA was thus indispensable for inducing aversive responses and learning through dopamine D2 receptors in the NAc.

Decreased striatal dopamine release underlies increased expression of long-term synaptic potentiation at corticostriatal synapses 24 h after 3-nitropropionic-acid-induced chemical hypoxia

The striatum is particularly sensitive to the irreversible inhibitor of succinate dehydrogenase 3-nitropropionic acid (3-NP). In the present study, we examined early changes in behavior and dopamine and glutamate synaptic physiology created by a single systemic injection of 3-NP in Fischer 344 rats. Hindlimb dystonia was seen 2 h after 3-NP injections, and rats performed poorly on balance beam and rotarod motor tests 24 h later. Systemic 3-NP increased NMDA receptor-dependent long-term potentiation (LTP) at corticostriatal synapses over the same time period. The 3-NP-induced corticostriatal LTP was not attributable to increased NMDA receptor number or function, because 3-NP did not change MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine] binding or NMDA/AMPA receptor current ratios. The LTP seen 24 h after 3-NP was D(1) receptor dependent and reversed by exogenous addition of dopamine or a D(2) receptor agonist to brain slices. HPLC and fast-scan cyclic voltammetry revealed a decrease in dopamine content and release in rats injected 24 h earlier with 3-NP, and much like the enhanced LTP, dopamine changes were reversed by 48 h. Tyrosine hydroxylase expression was not changed, and there was no evidence of striatal cell loss at 24-48 h after 3-NP exposure. Sprague Dawley rats showed similar physiological responses to systemic 3-NP, albeit with reduced sensitivity. Thus, 3-NP causes significant changes in motor behavior marked by parallel changes in striatal dopamine release and corticostriatal synaptic plasticity.

Limbic and cortical information processing in the nucleus accumbens

The nucleus accumbens regulates goal-directed behaviors by integrating information from limbic structures and the prefrontal cortex. Here, we review recent studies in an attempt to provide an integrated view of the control of information processing in the nucleus accumbens in terms of the regulation of goal-directed behaviors and how disruption of these functions might underlie the pathological states in drug addiction and other psychiatric disorders. We propose a model that could account for the results of several studies investigating limbic-system interactions in the nucleus accumbens and their modulation by dopamine and provide testable hypotheses for how these might relate to the pathophysiology of major psychiatric disorders.

Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder--the spontaneously hypertensive rat

RUSSELL, V.A. Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder-the spontaneously hypertensive rat. NEUROSCI. BIOBEHAV. REV.27(2003). Disturbances in glutamate, dopamine and norepinephrine function in the brain of a genetic animal model for attention-deficit hyperactivity disorder (ADHD), the spontaneously hypertensive rat (SHR), and information obtained from patients with ADHD, suggest a defect in neuronal circuits that are required for reward-guided associative learning and memory formation. Evidence derived from (i). the neuropharmacology of drugs that are effective in treating ADHD symptoms, (ii). molecular genetic and neuroimaging studies of ADHD patients, as well as (iii). the behaviour and biochemistry of animal models, suggests dysfunction of dopamine neurons. SHR have decreased stimulation-evoked release of dopamine as well as disturbances in the regulation of norepinephrine release and impaired second messenger systems, cAMP and calcium. In addition, evidence supports a selective deficit in the nucleus accumbens shell of SHR which could contribute to impaired reinforcement of appropriate behaviour.

Inhibition of histone deacetylases by chlamydocin induces apoptosis and proteasome-mediated degradation of survivin

The naturally occurring cyclic tetrapeptide chlamydocin is a very potent inhibitor of cell proliferation. Here we show that chlamydocin is a highly potent histone deacetylase (HDAC) inhibitor, inhibiting HDAC activity in vitro with an IC(50) of 1.3 nM. Like other HDAC inhibitors, chlamydocin induces the accumulation of hyperacetylated histones H3 and H4 in A2780 ovarian cancer cells, increases the expression of p21(cip1/waf1), and causes an accumulation of cells in G(2)/M phase of the cell cycle. In addition, chlamydocin induces apoptosis by activating caspase-3, which in turn leads to the cleavage of p21(cip1/waf1) into a 15-kDa breakdown product and drives cells from growth arrest into apoptosis. Concomitant with the activation of caspase-3 and cleavage of p21(cip1/waf1), chlamydocin decreases the protein level of survivin, a member of the inhibitor of apoptosis protein family that is selectively expressed in tumors. Although our data indicate a potential link between degradation of survivin and activation of the apoptotic pathway induced by HDAC inhibitors, stable overexpression of survivin does not suppress the activation of caspase-3 or cleavage of p21(cip1/waf1) induced by chlamydocin treatment. The decrease of survivin protein level is mediated by degradation via proteasomes since it can be inhibited by specific proteasome inhibitors. Taken together, our results show that induction of apoptosis by chlamydocin involves caspase-dependent cleavage of p21(cip1/waf1), which is strikingly associated with proteasome-mediated degradation of survivin.

Mechanism for fetal globin gene expression: Role of the soluble guanylate cyclase-cGMP-dependent protein kinase pathway

Despite considerable concerns with pharmacological stimulation of fetal hemoglobin (Hb F) as a therapeutic option for the beta-globin disorders, the molecular basis of action of Hb F-inducing agents remains unclear. Here we show that an intracellular pathway including soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase (PKG) plays a role in induced expression of the gamma-globin gene. sGC, an obligate heterodimer of alpha- and beta-subunits, participates in a variety of physiological processes by converting GTP to cGMP. Northern blot analyses with erythroid cell lines expressing different beta-like globin genes showed that, whereas the beta-subunit is expressed at similar levels, high-level expression of the alpha-subunit is preferentially observed in erythroid cells expressing gamma-globin but not those expressing beta-globin. Also, the levels of expression of the gamma-globin gene correlate to those of the alpha-subunit. sGC activators or cGMP analogs increased expression of the gamma-globin gene in erythroleukemic cells as well as in primary erythroblasts from normal subjects and patients with beta-thalassemia. Nuclear run-off assays showed that the sGC activator protoporphyrin IX stimulates transcription of the gamma-globin gene. Furthermore, increased expression of the gamma-globin gene by well known Hb F-inducers such as hemin and butyrate was abolished by inhibiting sGC or PKG activity. Taken together, these results strongly suggest that the sGC-PKG pathway constitutes a mechanism that regulates expression of the gamma-globin gene. Further characterization of this pathway should permit us to develop new therapeutics for the beta-globin disorders.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

Epigenetic silencing of RNA polymerase I transcription: a role for DNA methylation and histone modification in nucleolar dominance

Nucleolar dominance is an epigenetic phenomenon that describes nucleolus formation around rRNA genes inherited from only one progenitor of an interspecific hybrid or allopolyploid. The phenomenon is widespread, occurring in plants, insects, amphibians, and mammals, yet its molecular basis remains unclear. We have demonstrated nucleolar dominance in three allotetraploids of the plant genus Brassica. In Brassica napus, accurately initiated pre-rRNA transcripts from one progenitor, Brassica rapa are detected readily, whereas transcripts from the approximately 3000 rRNA genes inherited from the other progenitor, Brassica oleracea, are undetectable. Nuclear run-on confirmed that dominance is controlled at the level of transcription. Growth of B. napus seedlings on 5-aza-2'-deoxycytidine to inhibit cytosine methylation caused the normally silent, under-dominant B. oleracea rRNA genes to become expressed to high levels. The histone deacetylase inhibitors sodium butyrate and trichostatin A also derepressed silent rRNA genes. These results reveal an enforcement mechanism for nucleolar dominance in which DNA methylation and histone modifications combine to regulate rRNA gene loci spanning tens of megabase pairs of DNA.

Selection of a highly tumorigenic breast cancer cell line sensitive to estradiol to evidence in vivo the tumor-inhibitory effect of butyrate derivative Monobut-3

To increase butyric acid mean residence time in vivo, we have produced a stable butyric acid derivative. Monobut-3. Recently, we have described that Monobut-3 is able to induce phenotypic changes in human mammary tumor cells in vitro. In this study, we explore the in vivo effect of Monobut-3. Human breast tumor cell-lines did not easily produce in vivo xenografts, thus, MCF-7 cells required exogenous 17 beta-estradiol to grow and to form in vivo xenografts. To evaluate in vivo and anti-tumor effects of monobut-3 without exogenous 17 beta-estradiol addition, we have established MCF-7 variant cells, highly tumorigenic MCF-7vht, in which transfection of ras oncogene induced a bypass of estrogen requirement but did not delete the presence of functional estrogen receptor (ER). Monobut-3 inhibited growth of this variant by about 90% at 4 mM and reduced 17 beta-estradiol cell growth stimulation. In vivo, in absence of 17 beta-estradiol, 2 mg per mouse monobut-3 decreased tumor take by about 25% and tumor growth by about 50% in nude mice. This is the first experimental demonstration of an in vivo antitumoral effect of a butyric acid derivative alone on a solid human tumor. These data suggest that this compound does not only act by reducing of 17 beta-estradiol stimulation but it also has an 17 beta-estradiol-independent effect. Absence of toxicity and its antiproliferative effects could suggest its use in clinical treatment of well differentiated carcinoma.