Presynaptic dopamine D(4) receptor localization in the rat nucleus accumbens shell

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

The central blockade of the dopamine DR4 receptor decreases sucrose consumption by modifying the microstructure of drinking behavior in male rats

Sugar solutions promote hedonic feeding and increase the risk of obesity and binge-type behavior. In rodents, ingestion of sugar solutions enhances dopamine release to mesolimbic regions, suggesting changes in hedonic intake and brain reward processes. Moreover, dopaminergic D2R/D3R receptors contribute to the hedonic intake of palatable solutions. Although the experimental evidence indicate that the dopaminergic D4 receptor (D4R) modulates feeding at homeostatic levels, it is currently unknown whether D4R also regulate the hedonic intake of sugar solutions. In this study, we evaluated the effect of the central blockade of D4R on the consumption of a 20% sucrose solution, the drinking microstructure parameters, and levels of locomotor activity in sated rats. In the first experiment, male Wistar rats were daily exposed to a 20% sugar solution in the first hour of the light phase of the light:dark cycle. On day 10, rats received i.c.v injections of the D4R antagonist, L-745870 (0, 1 or 2 µg/5 µl) and sucrose consumption and drinking microstructure parameters (latency to start drinking, bouts, drinking duration, bout size, inter-bout interval, time in activity and time in resting) were evaluated. In the second experiment, rats were trained to receive the 20% sucrose solution as described in experiment 1. On day 10, after the 1 h of sucrose access, the rats were placed in the open field for 5-min (habituation phase). Then, rats received i.c.v injections of L-745870 (0, 1 or 2 µg/ 5 µl), and were placed again in the open-field test for 10-min (pharmacological phase). The number or crosses trough squares and number of rears were scored for both the habituation and pharmacological phase. Here we found that administration of L-745870 decreased the consumption of sucrose in a dose-depended manner. Moreover, L-745870-treated rats displayed microstructural changes, including greater number of bouts and reduced drinking duration, bout size and inter-bout intervals. Furthermore, the number of crosses and number of rears in the open field test remained unchanged for habituation and pharmacological phase. Finally, present findings suggest that D4R modulates the consumption of sugar solutions by alteration of hedonic responses, but the contribution of homeostatic systems is discussed. These results open perspectives for the potential use of the D4R antagonists for treating obesity or binge-eating behavior.

Early life stress influences basal ganglia dopamine receptors and novel object recognition of adolescent and adult rats

Environmental stimuli in early life are recognized to affect brain development and behavior. Mother-pup interaction constitutes a determinant stimulus during this critical period. It is known that the dopaminergic system undergoes significant reorganization during adolescence and that dopamine receptors are involved in recognition memory. Based on the above, we examined the effects of brief and prolonged maternal separation during the neonatal period (15 or 180 min daily) on basal ganglia dopamine receptors and on the behavior in the novel object recognition task of adolescent and adult male rats. Using the NOR task, we observed that the discrimination index (DI) was decreased in rats with brief maternal separations independent of age. Using receptor autoradiography, we observed that brief maternal separation induced decreases in D1, D2 and D4 receptor binding levels in adult basal ganglia nuclei, while prolonged maternal separation induced increases in D1 receptor binding levels in caudate - putamen (CPu) of adolescent rats. With immunoblotting experiments, we found decreases in D1 and increases in D2 total protein levels in CPu of adult rats with prolonged maternal separations. Α positive correlation was observed between DI and D1 binding levels in CPu, internal globus pallidus and substantia nigra, and D2 binding levels in nucleus accumbens core in adult rats, using the Pearson correlation coefficient. Our results indicate that the long-lasting effects of neonatal mother-offspring separation on dopamine receptors depend on the duration of maternal separation and age and that this early life experience impairs recognition memory in adolescent and adult rats. Furthermore, the present results suggest that modulation of striatal dopamine receptors might underlie the reduced recognition memory of adult rats with brief neonatal maternal separations.

Implications for impulsive-control disorders

Polymorphic alleles of the human dopamine D₄ receptor gene (DRD4) have been consistently associated with individual differences in personality traits and neuropsychiatric disorders, particularly between the gene encoding dopamine D_4.7 receptor variant and attention deficit hyperactivity disorder (ADHD). The α_2A adrenoceptor gene has also been associated with ADHD. In fact, drugs targeting the α_2A adrenoceptor (α_2AR), such as guanfacine, are commonly used in ADHD treatment. In view of the involvement of dopamine D₄ receptor (D₄R) and α_2AR in ADHD and impulsivity, their concurrent localization in cortical pyramidal neurons and the demonstrated ability of D₄R to form functional heteromers with other G protein-coupled receptors, in this study we evaluate whether the α_2AR forms functional heteromers with D₄R and weather these heteromers show different properties depending on the D₄R variant involved. Using cortical brain slices from hD_4.7R knock-in and wild-type mice, here, we demonstrate that α_2AR and D₄R heteromerize and constitute a significant functional population of cortical α_2AR and D₄R. Moreover, in cortical slices from wild-type mice and in cells transfected with α_2AR and D_4.4R, we detect a negative crosstalk within the heteromer. This negative crosstalk is lost in cortex from hD_4.7R knock-in mice and in cells expressing the D_4.7R polymorphic variant. We also show a lack of efficacy of D₄R ligands to promote G protein activation and signaling only within the α_2AR-D_4.7R heteromer. Taken together, our results suggest that α_2AR-D₄R heteromers play a pivotal role in catecholaminergic signaling in the brain cortex and are likely targets for ADHD pharmacotherapy.

Time-Dependent Effects of Acute Handling on the Brain Monoamine System of the Salmonid Coregonus maraena

The immediate stress response involves the activation of the monoaminergic neurotransmitter systems including serotonin, dopamine and noradrenaline in particular areas of the fish brain. We chose maraena whitefish as a stress-sensitive salmonid species to investigate the influence of acute and chronic handling on the neurochemistry of monoamines in the brain. Plasma cortisol was quantified to assess the activation of the stress axis. In addition, we analyzed the expression of 37 genes related to the monoamine system to identify genes that could be used as markers of neurophysiological stress effects. Brain neurochemistry responded to a single handling (1 min netting and chasing) with increased serotonergic activity 3 h post-challenge. This was accompanied by a modulated expression of monoaminergic receptor genes in the hindbrain and a significant increase of plasma cortisol. The initial response was compensated by an increased monoamine synthesis at 24 h post-challenge, combined with the modulated expression of serotonin-receptor genes and plasma cortisol concentrations returning to control levels. After 10 days of repeated handling (1 min per day), we detected a slightly increased noradrenaline synthesis and a down-regulated expression of dopamine-receptor genes without effect on plasma cortisol levels. In conclusion, the changes in serotonergic neurochemistry and selected gene-expression profiles, together with the initial plasma cortisol variation, indicate an acute response and a subsequent recovery phase with signs of habituation after 10 days of daily exposure to handling. Based on the basal expression patterns of particular genes and their significant regulation upon handling conditions, we suggest a group of genes as potential biomarkers that indicate handling stress on the brain monoamine systems.

Dopamine D4 Receptor-Selective Compounds Reveal Structure-Activity Relationships that Engender Agonist Efficacy

The dopamine D4 receptor (D4R) plays important roles in cognition, attention, and decision making. Novel D4R-selective ligands have promise in medication development for neuropsychiatric conditions, including Alzheimer's disease and substance use disorders. To identify new D4R-selective ligands, and to understand the molecular determinants of agonist efficacy at D4R, we report a series of eighteen novel ligands based on the classical D4R agonist A-412997 (1, 2-(4-(pyridin-2-yl)piperidin-1-yl)- N-( m-tolyl)acetamide). Compounds were profiled using radioligand binding displacement assays, β-arrestin recruitment assays, cyclic AMP inhibition assays, and molecular dynamics computational modeling. We identified several novel D4R-selective ( Ki ≤ 4.3 nM and >100-fold vs other D2-like receptors) compounds with diverse partial agonist and antagonist profiles, falling into three structural groups. These compounds highlight receptor-ligand interactions that control efficacy at D2-like receptors and may provide insights into targeted drug discovery, leading to a better understanding of the role of D4Rs in neuropsychiatric disorders.

Dopamine receptor D4 (DRD4) polymorphisms with reduced functional potency intensify atrophy in syndrome-specific sites of frontotemporal dementia

OBJECTIVE

We aimed to understand the impact of dopamine receptor D4 (DRD4) polymorphisms on neurodegeneration in patients with dementia. We hypothesized that DRD4dampened-variants with reduced functional potency would be associated with greater atrophy in regions with higher receptor density. Given that DRD4 is concentrated in anterior regions of the limbic and cortical forebrain we anticipated genotype effects in patients with a more rostral pattern of neurodegeneration.

METHODS

337 subjects, including healthy controls, patients with Alzheimer's disease (AD) and frontotemporal dementia (FTD) underwent genotyping, structural MRI, and cognitive/behavioral testing. We conducted whole-brain voxel-based morphometry to examine the relationship between DRD4 genotypes and brain atrophy patterns within and across groups. General linear modeling was used to evaluate relationships between genotype and cognitive/behavioral measures.

RESULTS

DRD4 dampened-variants predicted gray matter atrophy in disease-specific regions of FTD in anterior cingulate, ventromedial prefrontal, orbitofrontal and insular cortices on the right greater than the left. Genotype predicted greater apathy and repetitive motor disturbance in patients with FTD. These results covaried with frontoinsular cortical atrophy. Peak atrophy patterned along regions of neuroanatomic vulnerability in FTD-spectrum disorders. In AD subjects and controls, genotype did not impact gray matter intensity.

CONCLUSIONS

We conclude that DRD4 polymorphisms with reduced functional potency exacerbate neuronal injury in sites of higher receptor density, which intersect with syndrome-specific regions undergoing neurodegeneration in FTD.

Key role of the dopamine D4 receptor in the modulation of corticostriatal glutamatergic neurotransmission

Polymorphic variants of the dopamine D₄ receptor gene (DRD4) have been repeatedly associated with numerous neuropsychiatric disorders. Yet, the functional role of the D₄ receptor and the functional differences of the products of DRD4 polymorphic variants remained enigmatic. Immunohistochemical and optogenetic-microdialysis experiments were performed in knock-in mice expressing a D₄ receptor with the long intracellular domain of a human DRD4 polymorphic variant associated with attention deficit hyperactivity disorder (ADHD). When compared with the wild-type mouse D₄ receptor, the expanded intracellular domain of the humanized D₄ receptor conferred a gain of function, blunting methamphetamine-induced cortical activation and optogenetic and methamphetamine-induced corticostriatal glutamate release. The results demonstrate a key role of the D₄ receptor in the modulation of corticostriatal glutamatergic neurotransmission. Furthermore, these data imply that enhanced D₄ receptor-mediated dopaminergic control of corticostriatal transmission constitutes a vulnerability factor of ADHD and other neuropsychiatric disorders.

Expression of a Novel D4 Dopamine Receptor in the Lamprey Brain. Evolutionary Considerations about Dopamine Receptors

Numerous data reported in lampreys, which belong to the phylogenetically oldest branch of vertebrates, show that the dopaminergic system was already well developed at the dawn of vertebrate evolution. The expression of dopamine in the lamprey brain is well conserved when compared to other vertebrates, and this is also true for the D2 receptor. Additionally, the key role of dopamine in the striatum, modulating the excitability in the direct and indirect pathways through the D1 and D2 receptors, has also been recently reported in these animals. The moment of divergence regarding the two whole genome duplications occurred in vertebrates suggests that additional receptors, apart from the D1 and D2 previously reported, could be present in lampreys. We used in situ hybridization to characterize the expression of a novel dopamine receptor, which we have identified as a D4 receptor according to the phylogenetic analysis. The D4 receptor shows in the sea lamprey a more restricted expression pattern than the D2 subtype, as reported in mammals. Its main expression areas are the striatum, lateral and ventral pallial sectors, several hypothalamic regions, habenula, and mesencephalic and rhombencephalic motoneurons. Some expression areas are well conserved through vertebrate evolution, as is the case of the striatum or the habenula, but the controversies regarding the D4 receptor expression in other vertebrates hampers for a complete comparison, especially in rhombencephalic regions. Our results further support that the dopaminergic system in vertebrates is well conserved and suggest that at least some functions of the D4 receptor were already present before the divergence of lampreys.

Neuregulin-ERBB signaling in the nervous system and neuropsychiatric diseases

Neuregulins (NRGs) comprise a large family of growth factors that stimulate ERBB receptor tyrosine kinases. NRGs and their receptors, ERBBs, have been identified as susceptibility genes for diseases such as schizophrenia (SZ) and bipolar disorder. Recent studies have revealed complex Nrg/Erbb signaling networks that regulate the assembly of neural circuitry, myelination, neurotransmission, and synaptic plasticity. Evidence indicates there is an optimal level of NRG/ERBB signaling in the brain and deviation from it impairs brain functions. NRGs/ERBBs and downstream signaling pathways may provide therapeutic targets for specific neuropsychiatric symptoms.

Dopamine D4 receptor, but not the ADHD-associated D4.7 variant, forms functional heteromers with the dopamine D2S receptor in the brain

Polymorphic variants of the dopamine D(4) receptor have been consistently associated with attention-deficit hyperactivity disorder (ADHD). However, the functional significance of the risk polymorphism (variable number of tandem repeats in exon 3) is still unclear. Here, we show that whereas the most frequent 4-repeat (D(4.4)) and the 2-repeat (D(4.2)) variants form functional heteromers with the short isoform of the dopamine D(2) receptor (D(2S)), the 7-repeat risk allele (D(4.7)) does not. D(2) receptor activation in the D(2S)-D(4) receptor heteromer potentiates D(4) receptor-mediated MAPK signaling in transfected cells and in the striatum, which did not occur in cells expressing D(4.7) or in the striatum of knockin mutant mice carrying the 7 repeats of the human D(4.7) in the third intracellular loop of the D(4) receptor. In the striatum, D(4) receptors are localized in corticostriatal glutamatergic terminals, where they selectively modulate glutamatergic neurotransmission by interacting with D(2S) receptors. This interaction shows the same qualitative characteristics than the D(2S)-D(4) receptor heteromer-mediated mitogen-activated protein kinase (MAPK) signaling and D(2S) receptor activation potentiates D(4) receptor-mediated inhibition of striatal glutamate release. It is therefore postulated that dysfunctional D(2S)-D(4.7) heteromers may impair presynaptic dopaminergic control of corticostriatal glutamatergic neurotransmission and explain functional deficits associated with ADHD.

Regulation of striatal dopamine release by presynaptic auto- and heteroreceptors

Striatal dopamine neurotransmission is critical for normal voluntary movement, affect and cognition. Dysfunctions of its regulation are implicated in a broad range of behaviors and disorders including Parkinson's disease, schizophrenia and drug abuse. Extracellular dopamine levels result from a dynamic equilibrium between release and reuptake by dopaminergic terminals. Both processes are regulated by multiple mechanisms. Here we review data characterizing how dopamine levels are regulated by presynaptic autoreceptors and heteroreceptors, an area intensively investigated due to advances in real time electrochemical detection of extracellular dopamine, i.e., fast-scan cyclic voltammetry and amperometry, and the development of mutant mouse lines with deletions for specific receptors.

Blockade of dopamine d4 receptors attenuates reinstatement of extinguished nicotine-seeking behavior in rats

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

The dopamine D Receptor (DRD4) gene exon III polymorphism, problematic alcohol use and novelty seeking: direct and mediated genetic effects

The present study sought to integrate convergent lines of research on the associations among the dopamine D(4) receptor (DRD4) gene, novelty seeking and drinking behaviors with the overall goal of elucidating genetic influences on problematic drinking in young adulthood. Specifically, this study tested a model in which novelty seeking mediated the relationship between DRD4 variable number of tandem repeats (VNTR) genotype and problematic alcohol use. Participants (n = 90, 40 females) were heavy-drinking college students. Analyses using a structural equation modeling framework suggested that the significant direct path between DRD4 VNTR genotype and problematic alcohol use was reduced to a trend level in the context of a model that included novelty seeking as a mediator, thereby suggesting that the effects of DRD4 VNTR genotype on problematic alcohol use among heavy-drinking young adults were partially mediated by novelty seeking. Cross-group comparisons indicated that the relationships among the model variables were not significantly different in models for men versus women. These results extend recent findings of the association between this polymorphism of the DRD4 receptor gene, problematic alcohol use and novelty seeking. These findings may also help elucidate the specific pathways of risk associated with genetic influences on alcohol use and abuse phenotypes.

Absence of the 7-repeat variant of the DRD4 VNTR is associated with drifting sustained attention in children with ADHD but not in controls

Many genetic studies have demonstrated an association between the 7-repeat (7r) allele of a 48-base pair variable number of tandem repeats (VNTR) in exon 3 of the DRD4 gene and the phenotype of attention deficit hyperactivity disorder (ADHD). Previous studies have shown inconsistent associations between the 7r allele and neurocognitive performance in children with ADHD. We investigated the performance of 128 children with and without ADHD on the Fixed and Random versions of the Sustained Attention to Response Task (SART). We employed time-series analyses of reaction-time data to allow a fine-grained analysis of reaction time variability, a candidate endophenotype for ADHD. Children were grouped into either the 7r-present group (possessing at least one copy of the 7r allele) or the 7r-absent group. The ADHD group made significantly more commission errors and was significantly more variable in RT in terms of fast moment-to-moment variability than the control group, but no effect of genotype was found on these measures. Children with ADHD without the 7r allele made significantly more omission errors, were significantly more variable in the slow frequency domain and showed less sensitivity to the signal (d') than those children with ADHD the 7r and control children with or without the 7r. These results highlight the utility of time-series analyses of reaction time data for delineating the neuropsychological deficits associated with ADHD and the DRD4 VNTR. Absence of the 7-repeat allele in children with ADHD is associated with a neurocognitive profile of drifting sustained attention that gives rise to variable and inconsistent performance.

Dopaminergic signaling in dendritic spines

Dopamine regulates movement, motivation, reward, and learning and is implicated in numerous neuropsychiatric and neurological disorders. The action of dopamine is mediated by a family of seven-transmembrane G protein-coupled receptors encoded by at least five dopamine receptor genes (D1, D2, D3, D4, and D5), some of which are major molecular targets for diverse neuropsychiatric medications. Dopamine receptors are present throughout the soma and dendrites of the neuron, but accumulating ultrastructural and biochemical evidence indicates that they are concentrated in dendritic spines, where most of the glutamatergic synapses are established. By modulating local channels, receptors, and signaling modules in spines, this unique population of postsynaptic receptors is strategically positioned to control the excitability and synaptic properties of spines and mediate both the tonic and phasic aspects of dopaminergic signaling with remarkable precision and versatility. The molecular mechanisms that underlie the trafficking, targeting, anchorage, and signaling of dopamine receptors in spines are, however, largely unknown. The present commentary focuses on this important subpopulation of postsynaptic dopamine receptors with emphases on recent molecular, biochemical, pharmacological, ultrastructural, and physiological studies that provide new insights about their regulatory mechanisms and unique roles in dopamine signaling.

Dopamine and cyclic-AMP regulated phosphoprotein-32-dependent modulation of prefrontal cortical input and intercellular coupling in mouse accumbens spiny and aspiny neurons

The roles of dopamine and cyclic-AMP regulated phosphoprotein-32 (DARPP-32) in mediating dopamine (DA)-dependent modulation of corticoaccumbens transmission and intercellular coupling were examined in mouse accumbens (NAC) neurons by both intracellular sharp electrode and whole cell recordings. In wild-type (WT) mice bath application of the D2-like agonist quinpirole resulted in 73% coupling incidence in NAC spiny neurons, compared with baseline (9%), whereas quinpirole failed to affect the basal coupling (24%) in slices from DARPP-32 knockout (KO) mice. Thus, D2 stimulation attenuated DARPP-32-mediated suppression of coupling in WT spiny neurons, but this modulation was absent in KO mice. Further, whole cell recordings revealed that quinpirole reversibly decreased the amplitude of cortical-evoked excitatory postsynaptic potentials (EPSPs) in spiny neurons of WT mice, but this reduction was markedly attenuated in KO mice. Bath application of the D1/D5 agonist SKF 38393 did not alter evoked EPSP amplitude in WT or KO spiny neurons. Therefore, DA D2 receptor regulation of both cortical synaptic (chemical) and local non-synaptic (dye coupling) communications in NAC spiny neurons is critically dependent on intracellular DARPP-32 cascades. Conversely, in fast-spiking interneurons, blockade of D1/D5 receptors produced a substantial decrease in EPSP amplitude in WT, but not in KO mice. Lastly, in putative cholinergic interneurons, cortical-evoked disynaptic inhibitory potentials (IPSPs) were attenuated by D2-like receptor stimulation in WT but not KO slices. These data indicate that DARPP-32 plays a central role in 1) modulating intercellular coupling, 2) cortical excitatory drive of spiny and aspiny GABAergic neurons, and 3) local feedforward inhibitory drive of cholinergic-like interneurons within accumbens circuits.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Lu 35-138 ((+)-(S)-3-{1-[2-(1-acetyl-2,3-dihydro-1H-indol-3-yl)ethyl]-3,6-dihydro-2H-pyridin-4-yl}-6-chloro-1H-indole), a dopamine D4 receptor antagonist and serotonin reuptake inhibitor: Characterisation of its in vitro profile and pre-clinical antipsychotic potential

The present study describes the pharmacological profile of the putative antipsychotic drug Lu 35-138 ((+)-(S)-3-{1-[2-(1-acetyl-2,3-dihydro-1H-indol-3-yl)ethyl]-3,6-dihydro-2H-pyridin-4-yl}-6-chloro-1H-indole). The in vitro receptor profile of Lu 35-138 revealed high affinity (K(i)=5 nM) and competitive antagonism (K(b)=8 nM) at dopamine D(4) receptors combined with potent 5-HT uptake inhibition (IC(50)=3.2 nM) and moderate alpha(1)-adrenoceptor affinity (K(i)=45 nM). In vivo, Lu 35-138 selectively counteracted hyperlocomotion induced by d-amphetamine (0.5 mg/kg; ED(50)=4.0 mg/kg, s.c.) in rats and phencyclidine (PCP; 2.5 mg/kg; ED(50)=13 mg/kg, s.c.) in mice. Lu 35-138 was unable to affect hyperlocomotion induced by a high dose of d-amphetamine (2.0 mg/kg), which indicates a preferential action on limbic versus striatal structures. A similar limbic selectivity of Lu 35-138 was indicated in voltammetric measure of dopamine output in the core and shell subdivisions of the nucleus accumbens in rats. Furthermore, a relatively large dose of Lu 35-138 (18 mg/kg, s.c.) counteracted d-amphetamine-induced disruption of pre-pulse inhibition in rats and repeated administration of Lu 35-138 (0.31 or 1.25 mg/kg, p.o. once daily for 3 weeks) reduced the number of spontaneously active dopamine neurones in the ventral tegmental area, underlining its antipsychotic-like profile. Lu 35-138 failed to induce catalepsy in rats or dystonia in Cebus apella monkeys and did not deteriorate spatial memory in rats as assessed by water maze performance. Collectively, these results suggest that Lu 35-138 possesses antipsychotic activity combined with a low extrapyramidal and cognitive side effect liability.

Dopamine D4 receptor knockout mice exhibit neurochemical changes consistent with decreased dopamine release

Dopamine D4 receptor (D4R) knockout mice (D4R-/-) provided for unique neurochemical studies designed to understand D4R contributions to dopamine (DA) regulation. In this study, post-mortem brain tissue content of DA did not differ between D4R+/+ and D4R-/- mice in the striatum (Str) or nucleus accumbens core (NAc). However, there was a significant decrease (82%) in the content of 3,4-dihydoxyphenylacetic acid (DOPAC), a major metabolite of DA, in the NAc of D4R-/- mice. Microdialysis studies performed in a region of brain spanning of the dorsal Str and NAc showed lower baseline levels of DA and a significant reduction in KCl-evoked overflow of DA in the D4R-/- mice. Baseline extracellular levels of DOPAC and homovanillic acid were also significantly lower in the D4R-/- mice. In vivo chronoamperometric recordings of KCl-evoked release of DA also showed decreased release of DA in the Str and NAc of the D4R-/- mice. These studies demonstrate a role of D4Rs in presynaptic DA regulation and support the hypothesis that alterations in D4Rs may lead to diminished DA function.

Dopamine receptor genes (DRD2, DRD3 and DRD4) and gene-gene interactions associated with smoking-related behaviors

Cigarette smoking, like many addictive behaviors, has a genetic component, and the dopamine D2-like receptor genes (DRD2, DRD3 and DRD4) are candidates for contributing to these behaviors. Phenotypic information concerning smoking-related behaviors from a nationally representative sample of research volunteers was analyzed for association with polymorphisms in these genes. Genotype status at the DRD2 intron 2 simple tandem repeat was related to cigarettes per day (P = 0.035) and heaviness of smoking index (P = 0.049). The presence of the glycine allele at the S9G polymorphism of the DRD3 gene was associated with frequency/quantity measures of smoking [log-transformed time to first cigarette (P = 0.031) and heaviness of smoking index (P = 0.035)]. There was a trend for DRD4 long alleles of the variable number of tandem repeats polymorphism to be associated with reduced severity of three withdrawal symptoms [desire/craving (P = 0.054); anger/irritability (P = 0.10); and trouble sleeping (P = 0.068)]. Interactions between genotypes at all three genes were associated with nervousness (P = 0.020) and trouble sleeping (P = 0.015). An interaction between DRD2 and DRD3 was found for trouble concentrating (P = 0.020). These relationships present possible dopamine-related responses to nicotine that warrant further study.

Effects of craving and DRD4 VNTR genotype on the relative value of alcohol: an initial human laboratory study

BACKGROUND

Craving for alcohol is a highly controversial subjective construct and may be clarified by Loewenstein's visceral theory, which emphasizes craving's behavioral effects on the relative value of alcohol. Based on the visceral theory, this study examined the effects of a craving induction on the relative value of alcohol as measured by a behavioral choice task. In addition, based on previous evidence of its role in the expression of craving, the influence of DRD4 VNTR genotype (DRD4-L vs. DRD4-S) was also examined.

METHODS

Thirty-five heavy drinkers (54% male; 31% DRD4-L) were randomly assigned to receive either a craving induction (exposure to personally relevant alcohol cues) or a control induction (exposure to neutral cues), which was followed by an alcohol-money choice task. Participants were assessed for craving and positive/negative affect throughout the procedure, and relative value of alcohol was derived from participant choices for alcohol versus money. DRD4 VNTR status was assessed retrospectively via buccal samples using previously established protocols.

RESULTS

Factorial analysis of the craving induction revealed that it was associated with significant increase in craving (p < .001), but not greater relative value of alcohol. Factorial analyses including DRD4 VNTR genotype of did not suggest an influence on reactivity to the craving induction, although this analysis was substantially compromised by small cell sample sizes. Continuous analyses revealed that craving was significantly associated with the relative value of alcohol (p < .05) and possession of the DRD4-L allele further amplified this relationship (p < .001).

CONCLUSION

These results are interpreted as generally supporting Loewenstein's visceral theory of craving and evidence of a functional role of DRD4 VNTR genotype in the expression of craving for alcohol. Methodological limitations, mechanisms underlying these findings, and future directions are discussed.

Distinct roles of presynaptic dopamine receptors in the differential modulation of the intrinsic synapses of medium-spiny neurons in the nucleus accumbens

Background

In both schizophrenia and addiction, pathological changes in dopamine release appear to induce alterations in the circuitry of the nucleus accumbens that affect coordinated thought and motivation. Dopamine acts principally on medium-spiny GABA neurons, which comprise 95% of accumbens neurons and give rise to the majority of inhibitory synapses in the nucleus. To examine dopamine action at single medium-spiny neuron synapses, we imaged Ca²⁺ levels in their presynaptic varicosities in the acute brain slice using two-photon microscopy.

Results

Presynaptic Ca²⁺ rises were differentially modulated by dopamine. The D1/D5 selective agonist SKF81297 was exclusively facilitatory. The D2/D3 selective agonist quinpirole was predominantly inhibitory, but in some instances it was facilitatory. Studies using D2 and D3 receptor knockout mice revealed that quinpirole inhibition was either D2 or D3 receptor-mediated, while facilitation was mainly D3 receptor-mediated. Subsets of varicosities responded to both D1 and D2 agonists, showing that there was significant co-expression of these receptor families in single medium-spiny neurons. Neighboring presynaptic varicosities showed strikingly heterogeneous responses to DA agonists, suggesting that DA receptors may be differentially trafficked to individual varicosities on the same medium-spiny neuron axon.

Conclusion

Dopamine receptors are present on the presynaptic varicosities of medium-spiny neurons, where they potently control GABAergic synaptic transmission. While there is significant coexpression of D1 and D2 family dopamine receptors in individual neurons, at the subcellular level, these receptors appear to be heterogeneously distributed, potentially explaining the considerable controversy regarding dopamine action in the striatum, and in particular the degree of dopamine receptor segregation on these neurons. Assuming that post-receptor signaling is restricted to the microdomains of medium-spiny neuron varicosities, the heterogeneous distribution of dopamine receptors on individual varicosities is likely to encode patterns in striatal information processing.

Dopamine receptor regulation of Ca2+ levels in individual isolated nerve terminals from rat striatum: comparison of presynaptic D1-like and D2-like receptors

We have directly observed the effects of activating presynaptic D1-like and D2-like dopamine receptors on Ca2+ levels in isolated nerve terminals (synaptosomes) from rat striatum. R-(+)-SKF81297, a selective D1-like receptor agonist, and (-)-quinpirole, a selective D2-like receptor agonist, induced increases in Ca2+ levels in different subsets of individual striatal synaptosomes. The SKF81297- and quinpirole-induced effects were blocked by R-(+)-SCH23390, a D1-like receptor antagonist, and (-)-sulpiride, a D2-like receptor antagonist, respectively. SKF81297- or quinpirole-induced Ca2+ increases were inhibited following blockade of voltage-gated calcium channels or sodium channels. In a larger subset of synaptosomes, quinpirole decreased baseline Ca2+. Quinpirole also inhibited veratridine-induced increases in intrasynaptosomal Ca2+ level. Immunostaining confirmed the presynaptic expression of D1, D5, D2 and D3 receptors, but not D4 receptors. The array of neurotransmitter phenotypes of the striatal nerve endings expressing D1, D5, D2 or D3 varied for each receptor subtype. These results suggest that presynaptic D1-like and D2-like receptors induce increases in Ca2+ levels in different subsets of nerve terminals via Na+ channel-mediated membrane depolarization, which, in turn, induces the opening of voltage-gated calcium channels. D2-like receptors also reduce nerve terminal Ca2+ in a different but larger subset of synaptosomes, consistent with the predominant presynaptic action of dopamine in the striatum being inhibitory.

The effect of olanzapine on craving and alcohol consumption

Previous studies have indicated that olanzapine decreases craving after a priming dose of alcohol, that craving after a priming dose of alcohol is greater among individuals with the seven-repeat allele of the DRD4 variable number of tandem repeats (VNTR) polymorphism, and that the effect of olanzapine (a D2/D4 antagonist) is more pronounced among individuals with this allele. The present study tested the hypothesis that olanzapine may be differentially effective at reducing cue-elicited craving and differentially effective as a treatment for alcohol dependence over the course of a 12-week, randomized, placebo-controlled trial among individuals with and without the seven-repeat allele. Participants who met DSM IV criteria for alcohol dependence were randomly assigned to receive olanzapine (5 mg) or a placebo over the course of the trial. After 2 weeks of treatment, participants completed a cue reactivity assessment. The results suggested that participants who were homozygous or heterozygous for the seven (or longer)-repeat allele of the DRD4 VNTR responded to olanzapine with reductions in cue-elicited craving as well as reductions in alcohol consumption over the course of the 12-week trial, whereas individuals with the shorter alleles did not respond favorably to olanzapine.

Anatomy and pharmacology of cocaine priming-induced reinstatement of drug seeking

Cocaine addiction in human addicts is characterized by a high rate of relapse following successful detoxification. Relapse to drug taking/seeking can be precipitated by several stimuli including, but not limited to, re-exposure to cocaine itself. In order to understand the mechanisms underlying cocaine craving, a substantial effort has been devoted to elucidating the anatomical and neurochemical bases underlying cocaine priming-induced reinstatement, an animal model of relapse. Here, we review evidence that changes in dopaminergic and glutamatergic transmission in limbic/basal ganglia circuits of interconnected nuclei including the medial prefrontal cortex, nucleus accumbens, ventral pallidum, amygdala, hippocampus, orbitofrontal cortex, neostriatum and thalamus underlie cocaine priming-induced reinstatement of cocaine seeking. Maladaptive changes in the processing of motivationally relevant stimuli by these circuits following cocaine self-administration result in drug craving and compulsive drug seeking upon re-exposure to cocaine.

Multiple dopamine receptor subtypes in the medial prefrontal cortex of the rat regulate set-shifting

Dopamine (DA) input to the prefrontal cortex (PFC), acting on D1 receptors, plays an essential role in mediating working memory functions. In comparison, less is known about the importance of distinct PFC DA receptor subtypes in mediating executive functions such as set-shifting. The present study assessed the effects of microinfusion of D2 and D4 receptor antagonists, and D1, D2, and D4 receptor agonists into the PFC on performance of a maze-based set-shifting task. In Experiment 1, rats were trained on a response discrimination task, and then on a visual-cue discrimination task requiring rats to suppress the use of the response strategy and approach the previously irrelevant cue to locate food. In Experiment 2, the order of training was reversed. Infusions of the D2 antagonist eticlopride, or the D4 agonist PD-168,077, impaired shifting from a response to a visual-cue discrimination strategy and vice versa, and caused a selective increase in perseverative errors. In contrast, infusions of the D4 antagonist L-745,870 improved set-shifting. Infusions of the D1 agonist SKF81297 or the D2 agonist quinpirole caused no reliable effect. These data, in combination with previous reports of impaired set-shifting following D1 receptor blockade, suggest that multiple receptors in the PFC are essential for set-shifting and that the mechanisms by which PFC DA mediates behavioral flexibility may be different from those underlying working memory. These findings may have important implications for developing novel treatments for cognitive deficits observed in disorders such as attentional deficit and hyperactivity disorder and schizophrenia.

Comparing sertindole to other new generation antipsychotics on preferential dopamine output in limbic versus striatal projection regions: Mechanism of action

The effects of acute administration of sertindole on DA output were examined in the shell part of the nucleus accumbens (NACS) and the striatum (STR), areas which are associated with limbic functions and motor control, respectively, by using in vivo differential normal pulse voltammetry in rats. The effect of sertindole was compared to those obtained with the reference antipsychotic drugs clozapine and haloperidol, new generation antipsychotics represented by risperidone, olanzapine, ziprasidone, quetiapine, and aripiprazole, as well as, with those of preferential D2/3, D4, 5-HT1A, 5-HT2A, 5-HT2C, alpha1, and alpha2 receptor ligands. In similarity with the new generation antipsychotics, sertindole preferentially increase DA output in the NACS as compared to the STR whereas the opposite was true for haloperidol. The regional specific effect of the partial D2 receptor agonist aripiprazole was mainly driven by a decrease in striatal rather that by an increase in accumbal DA output. The selective 5-HT2A and D4 receptor antagonists MDL100,151 and Lu 38-012, respectively, both preferentially increased DA output in the NACS. Thus, the present results are in line with the hypothesis that 5-HT2A receptor antagonism is of importance for the observed limbic selectivity of new generation antipsychotics and, in turn, to their favorable clinical profile especially as regards extrapyramidal side effects (EPS) liability. For some compounds, blockade of D4 receptors may also play a role in this regard.

Cocaine-induced alterations in dopamine receptor signaling: Implications for reinforcement and reinstatement

The transition from casual drug use to addiction, and the intense drug craving that accompanies it, has been postulated to result from neuroadaptations within the limbic system caused by repeated drug exposure. This review will examine the implications of cocaine-induced alterations in mesolimbic dopamine receptor signaling within the context of several widely used animal models of addiction. Extensive evidence indicates that dopaminergic mechanisms critically mediate behavioral sensitization to cocaine, cocaine-induced conditioned place preference, cocaine self-administration, and the drug prime-induced reinstatement of cocaine-seeking behavior. The propagation of the long-term neuronal changes associated with recurring cocaine use appears to occur at the level of postreceptor signal transduction. Repeated cocaine treatment causes an up-regulation of the 3',5'-cyclic adenosine monophosphate (cAMP)-signaling pathway within the nucleus accumbens, resulting in a dys-regulation of balanced D1/D2 dopamine-like receptor signaling. The intracellular events arising from enhanced D1-like postsynaptic signaling mediate both facilitatory and compensatory responses to the further reinforcing effects of cocaine.

Regulation of working memory by dopamine D4 receptor in rats

Working memory is regulated by neurotransmitters in prefrontal cortex (PFC), including dopamine and norepinephrine. Previous studies of dopamine function in working memory have focused on the D1 and D2 receptors, with most evidence suggesting a dominant role for the D1 receptor. Since the dopamine D4 receptor is highly expressed in PFC, we hypothesize that it may also contribute to working memory. To test this hypothesis, we examined behavioral effects of L-745,870, a highly selective, centrally active, D4 antagonist, using a delayed alternation task in rats. Task performance was dose-dependently affected by the D4 antagonist, depending on individual baseline functional status of working memory. In rats with good baseline performance, the D4 antagonist had no effects at low doses, whereas high doses disrupted working memory. In rats with poor baseline working memory, the D4 antagonist significantly improved working memory at low doses, and higher doses were not distinguishable from vehicle controls. Effects of the D4 antagonist among poor performers were most robust when task demand for working memory was high, with lesser effects at lower demand level, suggesting that such effects were selective for working memory. The present findings indicate a significant role of the D4 receptor in working memory, and suggest innovative, D4-based, treatment of cognitive deficits associated with neuropsychiatric disorders.

Genetics of tobacco use

The worldwide prevalence of tobacco use is widespread, resulting in nearly 4.5 million deaths every year. Nicotine is addictive and has psychopharmacological effects that maintain the use of tobacco products. Several studies have documented a strong hereditary component to tobacco use. The present article reviews results from twin and adoption studies and proceeds to present association studies of specific genes that may be involved in tobacco use. Cholinergic receptor nicotinic beta polypeptide 2, serotonin receptor and transporter genes, dopamine receptor and transporter genes, and the cytochrome P450A6 gene are reviewed. Linkage studies help to identify regions of the genome that may be worth further investigation. The paper concludes with a discussion of the limitations of genetic research and the future of genetic epidemiology in this domain.

Dopamine receptor and transporter levels are altered in the brain of Purkinje Cell Degeneration mutant mice

The Purkinje Cell Degeneration (Nna1pcd, pcd) mutant mouse is mainly characterized by the complete, primary loss of the Purkinje cells and the secondary, partial, retrograde loss of the granule and inferior olive neurons and is considered a model of human degenerative ataxia. We determined, by in vitro quantitative autoradiography and in situ hybridization, the effects of the Purkinje cell deprivation on the dopaminergic system of the Nna1pcd mutant mouse. The dopamine transporters, as determined by [3H]WIN35428 binding, were increased compared with wild-type mice in the ventral mesencephalic dopaminergic nuclei and in the lateral striatum, motor cortex and septum. In the cerebellum of Nna1pcd mice, the dopamine transporters showed a significant increase in the deep cerebellar nuclei, but were significantly decreased in the molecular layer. The D1-like receptors, as determined by [3H]SCH23390 binding, increased significantly in the Nna1pcd substantia nigra. The D2/D3 receptors, as determined by [3H]raclopride binding, exhibited a significant decrease in lateral divisions of the striatum. Significant increases in D2-like receptors, as determined by [3H]nemonapride binding, were observed in most divisions of the striatum as well as in septum, hippocampus, and piriform cortex. This D2-like fraction most probably corresponds to the D4 receptor subtype. In the cerebellum of Nna1pcd mice, D2-like receptors were significantly decreased in the molecular layer. The results suggest an increased excitatory input on the dopaminergic mesencephalic neurons and an alteration of the dopaminergic neurotransmission in basal ganglia, cortical and limbic regions of the Nna1pcd mutant mouse. In the cerebellum, the significant downregulation of the dopamine transporters and D2-like receptors in the mutant cerebellar molecular layer is possibly due to the absence of the Purkinje cells.

Dopamine D4 receptor-deficient mice, congenic on the C57BL/6J background, are hypersensitive to amphetamine

Mice lacking the dopamine D4 receptor subtype (D4R-/-) are supersensitive to methamphetamine and cocaine. We sought to expand and refine earlier experiments performed on F2 generation D4R-/- mice by lengthening the behavioral session, utilizing an N10 D4R-/- incipient congenic C57BL/6J line (D4R-/- mice backcrossed with wildtype C57BL/6J mice for 10 successive generations), and investigating whether dopamine D4Rs are necessary for the expression of behavioral sensitization to amphetamine. The D4R-/- mice demonstrated an enhanced and dose-dependent increase in amphetamine-stimulated activity compared to wildtype mice following acute administrations of amphetamine. For the behavioral sensitization experiments, separate groups of mice received either repeated administrations of the same dose of amphetamine or a subthreshold dose of amphetamine (2 mg/kg) 28 days following pretreatment with either saline, 1.0, 3.0, or 10.0 mg/kg amphetamine. The D4R-/- mice displayed an enhanced dose-dependent sensitized response to repeated amphetamine administrations compared to their wildtype littermates in both behavioral sensitization paradigms. Our present results further support the importance of dopamine D4Rs in psychostimulant-mediated locomotion and neural plasticity.

Synaptology of the rostral reticular thalamic nucleus of absence epileptic WAG/Rij rats

The adult WAG/Rij rat is a well-established animal model for human absence epilepsy characterized by the presence of spike-wave discharges (SWDs). The pacemaking activity of the rostral reticular thalamic nucleus (rRTN) has been demonstrated to be essential for SWD maintenance. We investigated if SWD maintenance can be related to the synaptic organization of the rRTN, by studying the ultrastructure of the rRTN of absence epileptic WAG/Rij rats in comparison with that of non-epileptic, age-matched ACI control rats. In WAG/Rij rats, D-, L- and F-type terminals constitute the synaptic organization of the rRTN. D-type synapses, especially axo-dendritic ones, occur frequently. L- and F-type terminals are common but less frequent than D-type terminals. Semi-quantitative observations indicate that all terminal types are present on different parts of the postsynaptic neuron, but in different numbers: they are frequent on dendrites, common on somata and axons, and occur occasionally on dendritic spines. In addition, occasionally an F-type terminal was observed on the axon hillock. The three terminal types are also involved in multiple synaptic configurations, convergent as well as divergent, with dendrites, somata, axon hillocks and axons as postsynaptic structures. Convergent synaptic configurations outnumber divergent ones. The synaptic organization of the rRTN of the non-epileptic ACI rat appears to be very similar to that of the epileptic WAG/Rij rat. This indicates that SWD maintenance in the WAG/Rij rat does not depend on a different synaptic organization of the rRTN.

Distribution of mGluR1? and mGluR5 immunolabeling in primate prefrontal cortex

Metabotropic glutamate receptors (mGluRs) mediate important modulatory glutamatergic influences throughout the brain. However, the specific localization and functions of group I mGluR subtypes (mGluR1alpha and mGluR5) in cortical neurotransmission are not well known, particularly in primates. To address this issue, we used immunoelectron microscopy to compare the subcellular localizations of mGluR1alpha and mGluR5 in the prefrontal cortex of macaque monkeys. Both receptor subtypes were found in a variety of subcellular compartments, including spines, dendrites, preterminal axons, axon terminals, and glia; however, quantitative differences were found in the relative abundance of labeled elements for each receptor. The mGluR1alpha-immunoreactive (-IR) elements were overwhelmingly the spines and dendrites, with labeled terminals, axons, and glia seen more rarely. The mGluR5-IR elements were also mostly spines and dendrites, but the proportion of labeled unmyelinated axons, terminals, and glia was higher than for mGluR1alpha-IR elements. Double labeling with SMI-32 and parvalbumin confirmed that both receptors were found in pyramidal cell and interneuron dendrites. The localization of mGluR1alpha to pyramidal cells in primate cortex contrasts with reports that mGluR1alpha is found almost exclusively in interneurons in rodent cortex. By using double labeling, we found no evidence for mGluR1alpha or mGluR5 in dopaminergic afferents to prefrontal cortex. The data presented here provide an anatomical substrate for a differential role of mGluR1alpha and mGluR5 in post-and presynaptic actions of glutamate in primate prefrontal cortex. They further suggest differences in the cortical distribution of group I mGluRs between primates and rodents.

The dopamine D4 receptors and mechanisms of antipsychotic atypicality

The dopamine D4 receptor (D4) is a target for most common neuroleptic medications. After its initial discovery, it was found to possess the highest affinity of all dopamine receptor subtypes for the archetypical, atypical, antipsychotic clozapine. Nevertheless, initial clinical trials have not provided evidence that this receptor is a primary target for antipsychotic drugs. Considering the accumulated in vivo evidence that at least a subgroup of psychotic patients have altered dopamine signaling, all dopamine receptor subtypes likely contribute to the phenotypic expression of schizophrenia. New insights into the function of this receptor and its role in the modulation of excitatory signaling support the view that this dopamine receptor may affect attention and cognition. In this review, the authors outline some recent developments that provide insight into D4 receptor physiology, function and its possible relationship to schizophrenia treatment.

Presynaptic regulation of dopaminergic neurotransmission

The development of electrochemical recordings with small carbon-fiber electrodes has significantly advanced the understanding of the regulation of catecholamine transmission in various brain areas. Recordings in vivo or in slice preparations monitor diffusion of catecholamine following stimulated synaptic release into the surrounding tissue. This synaptic 'overflow' is defined by the amount of release, by the activity of reuptake, and by the diffusion parameters in brain tissue. Such studies have elucidated the complex regulation of catecholamine release and uptake, and how psychostimulants and anti-psychotic drugs interfere with it. Moreover, recordings with carbon-fiber electrodes from cultured neurons have provided analysis of catecholamine release and its plasticity at the quantal level.

Dopamine D4 receptor expression in rat kidney: evidence for pre- and postjunctional localization

Dopamine D4 receptors mediate inhibition of vasopressin-dependent sodium reabsorption by dopamine in collecting tubules. At present, the distribution of D4 receptors in other renal districts remains an open issue. The renal distribution of D4 receptor was assessed in normally innervated and denervated male Sprague-Dawley rats by quantitative immunohistochemistry using an anti-dopamine D4 receptor rabbit polyclonal antibody. D4 receptor protein immunoreactivity was observed perivascularly in the adventitia and the adventitia-media border. The density of perivascular dopamine D4 receptor was higher in afferent and efferent arterioles than in other segments of the renal vascular tree. Renal denervation abolished perivascular dopamine D4 receptor protein immunoreactivity. In renal tubules, the epithelium of collecting tubules showed the highest dopamine D4 receptor protein immunoreactivity, followed by the epithelium of proximal and distal tubules. No dopamine D4 receptor protein immunoreactivity was observed in the epithelium of the loop of Henle. Denervation did not change dopamine D4 receptor protein immunoreactivity in renal tubules. These results indicate that rat kidney expresses dopamine D4 receptors located both prejunctionally and nonprejunctionally in collecting, proximal, and distal tubules. This suggests that the dopamine D4 receptor may be involved in the control of neurotransmitter release and in renal hemodynamic and tubule function.

The DRD4 VNTR polymorphism influences reactivity to smoking cues

Recent research has indicated that craving for tobacco can be reliably elicited by exposure to smoking cues, suggesting that cue-elicited craving for tobacco may be a useful phenotype for research on genetic factors related to nicotine dependence. Given the potential role of dopamine in cue-elicited craving, the authors examined whether the DRD4 VNTR polymorphism is associated with cue-elicited craving for tobacco. Participants who were homozygous or heterozygous for the 7 repeat (or longer) allele were classified as DRD4 L, and all other participants were classified as DRD4 S. Participants were exposed to smoking cues before smoking either high-nicotine cigarettes or control cigarettes. Analyses suggested that participants in the L group demonstrated significantly greater craving, more arousal, less positive affect, and more attention to the smoking cues than did the participants in the S group.

Dopamine D4 receptor activation inhibits presynaptically glutamatergic neurotransmission in the rat supraoptic nucleus

Oxytocin and vasopressin release from magnocellular neurons of the supraoptic nucleus is under the control of glutamate-dependent excitation. The supraoptic nucleus also receives a generalized dopaminergic input from hypothalamic sources. To determine if dopamine can influence this excitatory drive onto the magnocellular neurons, we used whole-cell patch clamp to record the effect of dopamine on evoked and miniature excitatory postsynaptic currents in rat hypothalamic slices. Dopamine exposure (30 microM to 1 mM) induced a large and reversible reduction in the amplitude of evoked excitatory postsynaptic current in nearly all magnocellular cells tested. D4 receptors appeared to mediate dopamine's activity, based on inhibition of the response with 50 microM clozapine, but not by SCH 23390 or sulpiride, and mimicry of dopamine's action with the D4 specific agonist, PD 168077. Analysis of paired-pulse experiments and miniature postsynaptic currents indicated that dopamine's action involved a presynaptic mechanism, since the frequency of miniature postsynaptic currents was reduced with dopamine exposure without any change in current kinetics or amplitude, while the paired-pulse ratio increased. We therefore have demonstrated for the first time a role for dopamine D4 receptors in the supraoptic nucleus in the presynaptic inhibition of glutamatergic neurotransmission onto magnocellular neurons.