Dopamine D2 Receptors In Brain Research Articles

Rapid-eye-movement sleep in jittery infants

The pathogenesis of neonatal jitteriness (JT) remains unknown. Neonatal JT could be one of the symptoms associated with drug withdrawal syndrome due to maternal medication. To study the influence of chemicals and environment on brain development in the fetal period, Swaab and Mirmiran proposed "behavioral teratology". JT could be one of the targets for this concept. Swaab and Mirmiran postulated that rapid-eye-movement sleep (REM sleep) is useful for studying behavioral teratology. We aimed to determine the neurophysiological alterations in infants with JT by investigating REM sleep. Sleep records were obtained three times for each of six jittery infants. The mothers of three infants had been on medication (either alpha-methyl dopa, reserpine or haloperidol) during pregnancy, whereas the mothers of the other three infants took no medication during pregnancy. REM sleep parameters (the amount of REM sleep against the total sleep time, newly designated indices-tonic inhibition index (TII) and phasic inhibition index--, and the incidences of gross movements, phasic chin muscle activity (PCMA), and bursts of rapid eye movements) in the six jittery infants were compared with those in age-matched controls. Regardless of maternal medication, TIIs, which represent the shortening of PCMA during REM sleep, were higher in the jittery infants than in the controls. No other REM sleep parameters showed constant differences between the patients and controls. Release of the blockade of dopamine D2 receptors in the fetal brain is considered to be critical for the occurrence of neonatal JT with elevation of TII. Since the abnormal elevation of TII continued after 6 months of age when our patients did not show JT anymore, we have to keep monitoring jittery infants from the standpoint of "behavioral teratology".

Error Analysis for Quantification of [11C]FLB 457 Binding to Extrastriatal D2 Dopamine Receptors in the Human Brain

To estimate receptor binding of ligand by positron emission tomography (PET) without an arterial input function, several quantitative approaches based on the use of a reference region have been proposed. We compared three approaches for quantifying extrastriatal D2 dopamine receptors using [11C]FLB 457. The PET measurements were performed on seven healthy men. Binding potential (BP) of [11C]FLB 457 was calculated by the reference tissue model method, transient equilibrium method, and late time method. The reference tissue model describes the time-activity curve in a brain region in terms of that in the reference region, assuming that the levels of nondisplaceable radioligand binding in both regions are the same. The transient equilibrium theoretically occurs when the derivative for specific binding is zero. With the late time method, BP is calculated by integrating a late part of the time-activity curve. BP values obtained by all methods were in good agreement with those obtained by the kinetic approach, and the highest coefficient of correlation was observed in the reference tissue model method. In the simulation study, the error of BP calculated by the reference tissue model method was smallest. Moreover, the effect of the difference in the influx rate constant K1 between the brain and the reference regions on BP was nearly avoided as theoretically predicted. We concluded that the reference tissue model method is most suitable for calculating BP of extrastriatal D2 dopamine receptors with [11C]FLB 457.

Brain dopamine and obesity

The cerebral mechanisms underlying the behaviours that lead to pathological overeating and obesity are poorly understood. Dopamine, a neurotransmitter that modulates rewarding properties of food, is likely to be involved. To test the hypothesis that obese individuals have abnormalities in brain dopamine activity we measured the availability of dopamine D2 receptors in brain. Brain dopamine D2 receptor availability was measured with positron emission tomography (PET) and [C-11]raclopride (a radioligand for the dopamine D2 receptor). Bmax/Kd (ratio of the distribution volumes in striatum to that in cerebellum minus 1) was used as a measure of dopamine D2 receptor availability. Brain glucose metabolism was also assessed with 2-deoxy-2[18F]fluoro-D-glucose (FDG). Striatal dopamine D2 receptor availability was significantly lower in the ten obese individuals (2.47 [SD 0.36]) than in controls (2.99 [0.41]; p < or = 0.0075). In the obese individuals body mass index (BMI) correlated negatively with the measures of D2 receptors (r=0.84; p < or = 0.002); the individuals with the lowest D2 values had the largest BMI. By contrast, neither whole brain nor striatal metabolism differed between obese individuals and controls, indicating that striatal reductions in D2 receptors were not due to a systematic reduction in radiotracer delivery. The availability of dopamine D2 receptor was decreased in obese individuals in proportion to their BMI. Dopamine modulates motivation and reward circuits and hence dopamine deficiency in obese individuals may perpetuate pathological eating as a means to compensate for decreased activation of these circuits. Strategies aimed at improving dopamine function may be beneficial in the treatment of obese individuals.

Cellular colocalization of dopamine D1 mRNA and D2 receptor in rat brain using a D2 dopamine receptor specific polyclonal antibody

1. 1. The main objective of this work was to investigate the extent of cellular colocalization of dopamine D1 and D2 receptors in the rat brain. A double labeling technique, that combined immunocytochemical labeling of the D2 receptor using polyclonal antibodies raised against the third intracellular loop of the short isoform of the human D2 receptor in combination with in situ hybridization detecting D1 mRNA expression, was designed to accomplish this goal. 2. 2. The specificity of the antisera obtained was confirmed by immunoprecipitation assay, Western blot analysis, and immunocytochemistry on D2R transfected cells and murine brain tissue. Western blot using the D2 receptor antibody revealed a specific broad band centered at 67 kDa in transfected cells and a major protein of 88 kDa corresponding to D2R expressed in the caudate-putamen, to a lesser extent in the cortex, and not at all detected in the hypothalamic region. 3. 3. The content of neurons double-labeled for D1 D2 receptors was observed at in differing intensities in the dorsal endopiroform nucleus, the intercalated nucleus of amygdala, the anterior part of the cortical nucleus amygdala, the nucleus of the lateral olfactory tract, the piriform cortex, the parabrachial nucleus, the supraoptic nucleus and the parabigeminal nucleus. All other regions of the brain revealed neurons expressing either D1 or D2 dopamine receptors but not both at that same time. 4. 4. These results clearly demonstrated that specific neurons expressed both receptors D1 and D2, and that this colocalization was restricted to particular regions of the rat brain.

The human dopamine D2 Longer receptor has a high-affinity state and inhibits adenylyl cyclase

Brain dopamine D2 receptors are the main targets for antipsychotic and anti-Parkinsonian drugs. The dopamine D2 receptor has three forms, D2 Short, D2 Long and D2 Longer. D2 Longer is a newly found splice variant which contains two additional amino acids (valine and glutamine) in the third cytoplasmic loop of the receptor. To determine whether D2 Longer was functional, the cDNA was transfected into CHO cells. D2 Longer revealed a high-affinity state for dopamine (∼1.5 nM), and mediated dopamine-inhibited adenylyl cyclase.

Gamma-Hydroxybutyrate and Cocaine Administration Increases mRNA Expression of Dopamine D1 and D2 Receptors in Rat Brain

The effects of acute and repeated gamma-hydroxybutyrate (GHB) and cocaine administration on D1 and D2 dopamine receptor mRNA expression were examined using in situ hybridization histochemistry in different rat brain structures rich in GHB receptors. Six hours after a single GHB administration (500 mg/kg IP), an increase in D1 and D2 mRNA expression was observed in almost all regions examined; whereas, acute cocaine injection (20 mg/kg IP) had no effect. Repeated exposure to GHB (500 mg/kg IP twice daily) for 10 days, followed by a 14-h withdrawal period, induced increasing effects on D1 and D2 dopamine receptor mRNA expression, similar to those caused by chronic treatment with cocaine (20 mg/kg IP once a day). These effects of GHB and cocaine on dopamine receptor mRNA expression could be a consequence, for both compounds, of the modulation of dopaminergic activity; thus, supporting the benefit of GHB in cocaine substitution therapy.

Vulnerability to Addiction: A Study Looks at Neuroreceptors and MPH

Are brain dopamine D2 receptor levels predictive of an individual's response to psychostimulants? This study tracked differences in self-reported

Free-Wilson-Type Analysis of Non-Additive Substituent Effects on THPB Dopamine Receptor Affinity Using Artificial Neural Networks

Recently published brain dopamine D2 receptor affinity data of 15 tetrahydroprotoberberine (THPB) derivatives acting as dopamine receptor antagonists have been analyzed by two different QSAR techniques. The following main results were obtained by this analysis: In contrast to an unsuccessful Free-Wilson/Fujita-Ban analysis the investigated receptor binding data could be described by a neural network approach using only binary substructural indicator variables. The artificial/computational neural network was able to recognize that the affinity depends significantly on the simultaneous presence or absence of two or more substituents. A 4-D plot demonstrates the non-additivity/-variability of substituent effects on D2 receptor affinity.

Brain dopamine D2 receptor mRNA levels are elevated in young spontaneously hypertensive rats.

Levels of brain dopamine D2 receptor expression were compared between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) controls by quantitative in situ hybridisation, using a complementary RNA probe for D2 receptor mRNA. In SHR which were 6 weeks of age, significantly higher levels of D2 receptor mRNA were found in the caudate-putamen (42%), nucleus accumbens (23%), olfactory tubercle (17%) and substantia nigra (38%) compared to age-matched WKY controls. D2 receptor mRNA levels were also higher in the substantia nigra (27%) of 12-14-week old SHR compared to WKY. The increased levels of dopamine D2 receptor gene expression displayed in young prehypertensive SHR could implicate altered central dopaminergic activity in the pathogenesis of hypertension.

Prediction of reinforcing responses to psychostimulants in humans by brain dopamine D2 receptor levels.

This study assessed whether brain dopamine D2 receptor levels, which show significant intersubject variability, predict reinforcing responses to psychostimulants in humans. [11C]Raclopride and positron emission tomography were used to measure D2 receptor levels in 23 healthy men (mean age = 34 years, SD = 7) who had no drug abuse histories in order to assess if there were differences between the subjects who liked and those who disliked the effects of intravenous methylphenidate (0.5 mg/kg). Subjects who liked the effects of methylphenidate had significantly lower D2 receptor levels (mean = 2.72 Bmax/Kd, SD = 0.3) than subjects who disliked its effects (mean = 3.16, SD = 0.3). Moreover, the higher the D2 levels found, the more intense were methylphenidate's unpleasant effects. These results provide preliminary evidence that D2 receptor levels predict response to psychostimulants in humans and that low D2 receptors may contribute to psychostimulant abuse by favoring pleasant response.

Analysis of neuroleptic binding affinities and potencies for the different human D2 dopamine receptor missense variants.

Neuroleptics, or antipsychotics, are widely used for the treatment of psychotic symptoms such as hallucinations and delusions in schizophrenia and other psychiatric disorders. Pharmacotherapy of these diseases is frequently complicated by a great variability in the clinical response to neuroleptics and by the development of serious and potentially life-threatening side-effects. Brain D2 dopamine receptors are one of the major targets of neuroleptic treatment. The human D2 dopamine receptor (DRD2) gene has three variants predicting the amino acid substitutions Ser311Cys, Pro310Ser and Val96Ala in the receptor protein. We show that several typical and atypical neuroleptics commonly used in the treatment of psychotic disorders have differences in binding affinities and potencies for the D2 dopamine receptor variants. Functional differences between dopamine receptor variants might be related to genetically determined differences in response to neuroleptic treatment.

Use of positron emission tomography to measure the effects of nalmefene on D1 and D2 dopamine receptors in rat brain

Positron emission tomography (PET) has been used in humans and in non-human primates to image and measure radioligand binding to neuroreceptors. The present study evaluated the feasibility of performing high-resolution PET experiments in a rodent model to measure receptor kinetics. The effects of acute and chronic administration of the opioid antagonist, nalmefene, on the binding activity of [11C]SCH23390 and [11C]N-methylspiperone at D1 and D2 dopamine receptors, respectively, was investigated in the rat. The interaction between central opioid and dopaminergic systems has been the focus of much attention due to their interactive role in mediating reinforcement and locomotor activity. In the present study, adult male Sprague–Dawley rats received either a single injection of 10 mg/kg of nalmefene or control vehicle solution 1 h prior to the PET scan or were chronically administered 10 mg/kg/day of nalmefene or vehicle for 7 days by an osmotic minipump. Following acute administration of nalmefene, the binding potential of [11C]SCH23390 in the striatum was significantly increased. No changes in [11C]N-methylspiperone binding were found. Following chronic nalmefene administration, no significant change in either [11C]SCH23390 binding potential or [11C]N-methylspiperone binding was detected. These results suggest that nalmefene administration produces transient changes in the binding potential of D1-receptors in the striatum that are normalized after 1 week of steady-state administration.

Repeated administration of antidepressant drugs affects the levels of mRNA coding for D1 and D2 dopamine receptors in the rat brain.

The present study examined the effects of acute and repeated administration of three antidepressant drugs (imipramine, citalopram and (+)-oxaprotiline) on the levels of mRNA coding for dopamine D1 and D2 receptors in the rat brain. Quantitive in situ hybridization with 35S-labelled oligonucleotide probes has been utilised. The level of mRNA coding for dopamine D1 receptor (D1 mRNA) is decreased following repeated administration of imipramine, both in the nucleus accumbens and in the striatum. On the other hand, the repeated administration of citalopram, the selective inhibitor of serotonin reuptake, resulted in an increase in the level of D1 mRNA in the striatum and in the core region of nucleus accumbens. A similar tendency, i.e.: an increase in the level of D1 mRNA was observed after repeated administration of (+)-oxaprotiline, a selective inhibitor of noradrenaline reuptake. The level of mRNA coding for dopamine D2 receptors (D2 mRNA) was increased in all the brain regions studied, both after administration of imipramine and citalopram. (+)-Oxaprotiline did not produce any statistically significant changes in the level of D2 mRNA. The results obtained in this study indicate that the levels of mRNA coding for dopamine D1 and D2 receptors are regulated by the antidepressant drugs. The changes concerning the dopamine D2 receptors are more consistent and fit in with the previously described binding and behavioral effects and seem to be important for the mechanism of action of antidepressant drugs.

Clozapine occupies high levels of dopamine D2 receptors.

An important discrepancy has been noted concerning the number of dopamine D2 receptors which must be occupied in patients by clozapine, in contrast to other antipsychotic drugs, in order to achieve an antipsychotic effect. For example, when D2 receptors are labelled by radioactive raclopride or spiperone congeners in patients, psychosis-controlling doses of all antipsychotic drugs occupy about 70% or more of the D2 receptors in patients. However, equi-effective psychosis-controlling doses of clozapine (approximately 400 mg/day; approximately 70-130 nM in spinal fluid) only occupy between 20% and 50% of the D2 receptors in patients. This discrepancy of a consistently lower occupancy of D2 by psychosis-controlling doses of clozapine may be resolved when one considers that the neuroleptic concentration for half-occupancy of D2 receptors in vitro (i.e. the inhibition constant) depends on the radioligand used to label the receptor. Radioligands with higher tissue/buffer partition coefficients are less displaced by clozapine. This principle applies both in vitro and in vivo. Thus, allowing for this principle, psychosis-controlling doses of clozapine can be shown to occupy over 70% of the brain dopamine D2 receptors in patients, as found with other neuroleptics.

P-9-15 - Clozapine-like behavioral effects of ‘Seroquel‘ (LCI 204,636) in squirrel monkeys

This chapter discusses the strategies for developing new antipsychotic drugs, as well as the problems in clinical schizophrenia research. Reports from clinical trials suggest possible improved activity profiles for several new dopamine (DA) antagonists. Remoxipride has been efficacious in the treatment of schizophrenia with fewer extrapyramidal side effects. A pharmacophore for DA D2 antagonist activity has been developed, based on the conformational properties of semi-rigid antipsychotic drugs. The biological mechanisms underlying the effects of clozapine continued to be an important area of research. The involvement of brain serotonin (5-hydroxytryptamine, 5-HT) systems in the etiology and drug therapy of schizophrenia is reviewed in the chapter. Zotepine, a potent 5-HT and DA antagonist, has been efficacious in clinical trials with paranoid, resistant, and negative schizophrenia. Umespirone has been found to be active in preclinical tests for antipsychotic and anxiolytic activity. Results from early clinical trials on DA autoreceptor agonists in schizophrenia have provided mixed results. While sigma ligands have been proposed as potential antipsychotics, the functional role of brain sigma binding sites remained an enigma. Studies of the brain DA D2 receptor from schizophrenics revealed no structural changes. A review considered the hypothesis that an imbalance between GABA and glutamate contributes to psychosis. Another review proposed that schizophrenia might result from alterations in the regulation of phasic and tonic components of brain DA release. Further studies on prepulse inhibition of acoustic startle have supported the selectivity of this procedure for identifying antipsychotic drugs. Amphetamine-induced release of DA from brain slices has been proposed as a biochemical model for psychosis.

P-9-12 - Pharmacological profile of PR-000608, a novel dopamine uptake inhibitor

This chapter discusses the strategies for developing new antipsychotic drugs, as well as the problems in clinical schizophrenia research. Reports from clinical trials suggest possible improved activity profiles for several new dopamine (DA) antagonists. Remoxipride has been efficacious in the treatment of schizophrenia with fewer extrapyramidal side effects. A pharmacophore for DA D2 antagonist activity has been developed, based on the conformational properties of semi-rigid antipsychotic drugs. The biological mechanisms underlying the effects of clozapine continued to be an important area of research. The involvement of brain serotonin (5-hydroxytryptamine, 5-HT) systems in the etiology and drug therapy of schizophrenia is reviewed in the chapter. Zotepine, a potent 5-HT and DA antagonist, has been efficacious in clinical trials with paranoid, resistant, and negative schizophrenia. Umespirone has been found to be active in preclinical tests for antipsychotic and anxiolytic activity. Results from early clinical trials on DA autoreceptor agonists in schizophrenia have provided mixed results. While sigma ligands have been proposed as potential antipsychotics, the functional role of brain sigma binding sites remained an enigma. Studies of the brain DA D2 receptor from schizophrenics revealed no structural changes. A review considered the hypothesis that an imbalance between GABA and glutamate contributes to psychosis. Another review proposed that schizophrenia might result from alterations in the regulation of phasic and tonic components of brain DA release. Further studies on prepulse inhibition of acoustic startle have supported the selectivity of this procedure for identifying antipsychotic drugs. Amphetamine-induced release of DA from brain slices has been proposed as a biochemical model for psychosis.

3H]YM-09151-2 (nemonapride), a potent radioligand for both sigma 1 and sigma 2 receptor subtypes.

Using K+ phosphate buffer with 25 nM spiperone, [3H]YM-09151-2 binding showed a high affinity for sigma receptors but no affinity for D2 dopamine or 5-HT1A receptors in rat brain. The order of pKi values of various sigma compounds at [3H]YM-09151-2 binding sites and stereoisomer selectivity were consistent with previous studies using other sigma ligands such as (+)-[3H]SKF-10047, [3H]DTG and (+)-[3H]3-PPP. Although Scatchard analysis fitted a one-site model, competition between [3H]YM-09151-2 and (+)-pentazocine revealed two sites, sigma 1 and sigma 2 receptors, at which the Ki values of YM-09151-2 were 8.4 nM and 9.6nM, respectively. Autoradiography using [3H]YM-09151-2 also showed a characteristic distribution of sigma receptors in rat brain. [3H]YM-09151-2 is, therefore, a potent and useful radioligand for sigma 1/sigma 2 receptor subtypes.

Inhibition of adenylyl cyclase activity by a homogeneous population of dopamine receptors: selective blockade by antisera directed against Gi1 and/or Gi2.

The 7315c pituitary tumor cell expresses a homogeneous population of dopamine receptors that are functionally similar to brain dopamine D2 receptors. [3H]-Sulpiride binding to 7315c cell homogenates was specific and saturable, and Ki values for compounds to compete for these sites were highly correlated with values for the same compounds at D2 receptors in brain. Dopamine maximally inhibited approximately 65% of forskolin-stimulated cyclase activity in cell membranes. Some D2 agonists had lower efficacies, suggesting that some compounds are partial agonists at this receptor. Removal of GTP from the assay buffer or pretreatment of the tissue with pertussis toxin abolished the inhibition of adenylyl cyclase by dopamine. Immunodetection of most of the known G alpha subunits revealed that Gi1, Gi2, Gi3, Go, Gq, and Gs are present in the 7315c membrane. Pretreatment with the AS antibody (which recognizes the C-terminal regions of G alpha i and G alpha i2) significantly attenuated the inhibition of adenylyl cyclase activity by dopamine, whereas antibodies to C-terminal regions of the other G alpha subunits had no effect. These findings suggest that the dopamine D2 receptor regulates cyclase inhibition predominantly via Gi1 and/or Gi2 and that the 7315c tumor cells provide a useful model for studying naturally expressed dopamine D2 receptors in the absence of other dopamine receptor subtypes.

D2 dopamine receptor response to endophyte-infected tall fescue and an antagonist in the rat.

Effects on rat brain D2 dopamine receptors by endophyte-infected tall fescue seed consumption and antagonist injection were characterized. Forty-eight male Wistar rats (225 g) in three separate trials were exposed to either 22 or 32 degrees C. Diets, to maintain similar concentrations of ergovaline, contained 10% (Trial 1) or 15% (Trials 2 and 3) endophyte-infected (E+; 325 average ppb of ergovaline) or uninfected (E-; 0 ppb of ergovaline) tall fescue seed. Rats were injected i.p. daily with either placebo (PL) or an experimental D2 dopamine antagonist (DA, .0375 mg/kg BW). No effects (P > .10) on diet DM intake by E+ ingestion or DA injection were detected at 22 degrees C. However, ingestion of E+ reduced (P < .01) and injection of DA improved (P < .05) DM intake of rats housed in 32 degrees C (11.1 vs 15.4 g of DM/d for E+ vs E-, respectively). Whole brain D2 dopamine receptor density (Bmax) and mRNA were reduced (P < .05) by E+ and increased (P < .05) by DA in Trial 1. No treatment effects (P > .10) on cerebral cortex alpha 1- and alpha 2-adrenergic or striatal D2 dopamine receptor Bmax were measured in Trials 2 and 3. In summary, dietary E+ reduced whole brain D2 dopamine mRNA and Bmax, whereas injection of DA increased D2 dopamine mRNA. Thus, long-term regulation of monoamine receptors seems to be affected by E+ ingestion or DA injection.

Localization of D1 and D2 dopamine receptors in brain with subtype-specific antibodies.

Five or more dopamine receptor genes are expressed in brain. However, the pharmacological similarities of the encoded D1-D5 receptors have hindered studies of the localization and functions of the subtypes. To better understand the roles of the individual receptors, antibodies were raised against recombinant D1 and D2 proteins and were shown to bind to the receptor subtypes specifically in Western blot and immunoprecipitation studies. Each antibody reacted selectively with the respective receptor protein expressed both in cells transfected with the cDNAs and in brain. By immunocytochemistry, D1 and D2 had similar regional distributions in rat, monkey, and human brain, with the most intense staining in striatum, olfactory bulb, and substantia nigra. Within each region, however, the precise distributions of each subtype were distinct and often complementary. D1 and D2 were differentially enriched in striatal patch and matrix compartments, in selective layers of the olfactory bulb, and in either substantia nigra pars compacta or reticulata. Electron microscopy demonstrated that D1 and D2 also had highly selective subcellular distributions. In the rat neostriatum, the majority of D1 and D2 immunoreactivity was localized in postsynaptic sites in subsets of spiny dendrites and spine heads in rat neostriatum. Presynaptic D1 and D2 receptors were also observed, indicating both subtypes may regulate neurotransmitter release. D1 was also present in axon terminals in the substantia nigra. These results provide a morphological substrate for understanding the pre- and postsynaptic functions of the genetically defined D1 and D2 receptors in discrete neuronal circuits in mammalian brain.