Dopamine D2 Receptor Levels Research Articles

Activity-dependent reduction of dopamine D2 receptors during a postnatal critical period of plasticity in rat striatum is not affected by prenatal haloperidol treatment

Motor activity induced in the Circling Training test (CT) during a postnatal (PN) critical period of plasticity (PN30–37) produces a long-lasting decrease in the number of binding sites and mRNA expression levels of the dopamine D2 receptor (D2R) in rat striatum. Prenatal exposure to the antipsychotic haloperidol also decreases postnatal levels of the striatal D2R in the offspring. We examined whether such fetal exposure to haloperidol could affect the activity-dependent reduction of the D2R system during the critical period. Half of the male offspring exposed to either haloperidol (2.5 mg/kg/day), i.p.) or saline during gestational days 5–18 were subjected to the CT during the critical period, while the remaining represented CT control animals. The adult number of binding sites and mRNA expression levels of the striatal D2R at PN90 were not changed by prenatal haloperidol treatment alone. On the other hand, only pups subjected to the CT during the critical period showed decreases in both studied parameters, regardless the prenatal treatment. These findings indicated that the postnatal reduction of the striatal D2R binding induced prenatally by haloperidol does not affect long-lasting activity-dependent plastic changes on the same receptor system elicited by motor activity in an ontogenetic critical period of plasticity in rat striatum.

Genetic Dissection of Behavioral Flexibility: Reversal Learning in Mice

Behavioral inflexibility is a feature of schizophrenia, attention-deficit/hyperactivity disorder, and behavior addictions that likely results from heritable deficits in the inhibitory control over behavior. Here, we investigate the genetic basis of individual differences in flexibility, measured using an operant reversal learning task. We quantified discrimination acquisition and subsequent reversal learning in a cohort of 51 BXD strains of mice (2-5 mice/strain, n = 176) for which we have matched data on sequence, gene expression in key central nervous system regions, and neuroreceptor levels. Strain variation in trials to criterion on acquisition and reversal was high, with moderate heritability (∼.3). Acquisition and reversal learning phenotypes did not covary at the strain level, suggesting that these traits are effectively under independent genetic control. Reversal performance did covary with dopamine D2 receptor levels in the ventral midbrain, consistent with a similar observed relationship between impulsivity and D2 receptors in humans. Reversal, but not acquisition, is linked to a locus on mouse chromosome 10 with a peak likelihood ratio statistic at 86.2 megabase (p < .05 genome-wide). Variance in messenger RNA levels of select transcripts expressed in neocortex, hippocampus, and striatum correlated with the reversal learning phenotype, including Syn3, Nt5dc3, and Hcfc2. This work demonstrates the clear trait independence between, and genetic control of, discrimination acquisition and reversal and illustrates how globally coherent data sets for a single panel of highly related strains can be interrogated and integrated to uncover genetic sources and molecular and neuropharmacological candidates of complex behavioral traits relevant to human psychopathology.

Loss of dendrite stabilization by the Abl-related gene (Arg) kinase regulates behavioral flexibility and sensitivity to cocaine

Adolescence is characterized by increased vulnerability to developing neuropsychiatric disorders and involves a period of prefrontal cortical dendritic refinement and synaptic pruning that culminates in cytoskeletal stabilization in adulthood. The Abl-related gene (Arg) acts through p190RhoGAP to inhibit the RhoA GTPase and stabilize cortical dendritic arbors beginning in adolescence. Cortical axons, dendrites, and synapses develop normally in Arg-deficient (arg(-/-)) mice, but adult dendrites destabilize and regress; thus, arg(-/-) mice present a model of adolescent-onset dendritic simplification. We show that arg(-/-) mice are impaired in a reversal task and that deficits are grossly exacerbated by low-dose cocaine administration. Although ventral prefrontal dopamine D2 receptor levels predict "perseverative" error counts in wild-type mice, no such relationship is found in arg(-/-) mice. Moreover, arg(-/-) mice are insensitive to the disruptive effects of the D2/D3 antagonist haloperidol in reversal but show normal sensitivity to its locomotor-depressant actions. Arg deficiency and orbitofrontal cortical Arg inhibition via STI-571 infusion also enhance the psychomotor stimulant actions of cocaine. These findings provide evidence that stabilization of dendritic structure beginning in adolescence is critical for the development of adaptive and flexible behavior after cocaine exposure.

Effects of long-term ouabain treatment on blood pressure, sodium excretion, and renal dopamine D1 receptor levels in rats

To examine the effects of chronic ouabain treatment on blood pressure (BP), sodium excretion, and renal dopamine D(1) receptor level, male Sprague-Dawley (SD) rats were treated with ouabain (27.8 microg kg(-1) d(-1)) intraperitoneally for 5 weeks, and systolic blood pressure (SBP) were recorded weekly. After 5 weeks, sodium excretion and dopamine D(1) receptor agonist fenoldopam-mediated natriuresis were measured, and the expression and phosphorylation levels of the renal cortical dopamine D(1) receptor were confirmed by Western blot analysis. The effects of ouabain on fenoldopam-mediated inhibition of Na(+)-K(+)-ATPase activity were determined by colorimetric assays in human proximal tubular epithelial cells (HK-2 cells). After 5 weeks, the SBP in ouabain group was significantly higher than that in the control group (P < 0.01), but the sodium excretion and renal cortical D(1) receptor expression levels were reduced, and D(1) receptor phosphorylation levels were increased after ouabain treatment. Intravenous administration of fenoldopam caused an increased sodium excretion in control rats, but failed to induce natriuresis in ouabain-treated rats. In addition, fenoldopam induced a dose-respone (10(-9) to 10(-6) M) inhibition of Na(+)-K(+)-ATPase activity in HK-2 cells,but these effects were significantly diminished in HK-2 cells pretreated with nanomolar concentration of ouabain for 5 days (P < 0.01). We propose that the ouabain-induced reduction of the renal dopamine D(1) receptor function serves as a mechanism responsible for sodium retention, and this contributes to the hypertension induced by chronic ouabain treatment.

Amphetamine-Induced Changes in Dopamine Receptors in Early Postnatal Rat Brain

Amphetamines are among the most widely abused drugs. The user population includes a large proportion of women of child-bearing age. The early ontogeny of the axons in the neocortex and other neural structures positions them to influence the development and connectivity of non-aminergic dendrites and axons in these structures. A cascade of abnormalities in neural circuitry may result from the effects of amphetamines on the dopaminergic system. An attempt has been made to investigate the possible changes in the dopaminergic system in neonatal rats (a human third trimester equivalent model) following chronic D-amphetamine exposure. Neonatal rats were administered 5–15 mg/kg D-amphetamine subcutaneously daily from postnatal day 4 to day 10. Several parameters related to the dopaminergic system were measured. The results showed that tyrosine hydroxylase enzyme levels were significantly decreased in the prefrontal cortex, dorsal striatum and nucleus accumbens. Dopamine D1 receptor (DRD1) levels increased in the dorsal striatum whereas dopamine D2 receptor (DRD2) levels significantly decreased in both the prefrontal cortex and the dorsal striatum but significantly increased in the nucleus accumbens. In order to investigate whether these changes occurred at the transcriptional level, DRD1 and DRD2 mRNAs were detected. The results showed that DRD1 mRNA levels were significantly increased in the dorsal striatum whereas DRD2 mRNA levels were significantly increased in all three brain regions. These results indicate that early D-amphetamine exposure altered the dopaminergic system in the developing rat brain. This change may lead to abnormal perinatal stimulation that may yield long-term consequences.

Neonatal caffeine induces sex-specific developmental plasticity of the hypoxic respiratory chemoreflex in adult rats

Caffeine is widely used to treat apneas of prematurity during the neonatal period; however, the potential consequences of administering a neonatal caffeine treatment (NCT) during a critical period for respiratory control development are unknown. The present study therefore determined whether NCT in rats alters the hypoxic respiratory chemoreflex measured at adulthood. Newborn rats received either caffeine (15 mg/kg) or water (control) each day from postnatal day 3 to 12. The ventilatory response to a hypoxic challenge (inspired O(2) fraction = 0.12) was first evaluated in awake adult female and male rats using whole body plethysmography. Results showed that NCT increased the initial phase of the breathing frequency response to hypoxia in males only. This result was confirmed in anesthetized and artificially ventilated adult male rats where NCT also increased the phrenic burst frequency response to hypoxia. RT-PCR assessment of mRNA encoding for adenosine A(1A) and A(2A) receptors, dopamine D(2) receptors, and tyrosine hydroxylase in the rat carotid bodies showed that NCT enhanced mRNA expression levels of adenosine A(2A), dopamine D(2) receptors, and tyrosine hydroxylase of males but not females. Subsequent experiments on awake male rats showed that injection of the adenosine A(2A) receptor antagonist ZM2413855 (1 mg/kg ip) before ventilatory measurements abolished, in NCT rats, the enhanced respiratory frequency response observed during the early phase of hypoxia. We propose that NCT elicits a sex-specific increase in the hypoxic respiratory chemoreflex, which is related, at least partially, to an enhancement in adenosine A(2A) receptors in the rat carotid body.

Heroin Abuse Is Characterized by Discrete Mesolimbic Dopamine and Opioid Abnormalities and Exaggerated Nuclear Receptor-Related 1 Transcriptional Decline with Age

Dysfunction of mesocorticolimbic dopaminergic neurons is considered a common feature of all drugs of abuse, yet few investigations have evaluated the dopamine (DA) system in nonstimulant human abusers. We examined mRNA expression levels of DA transporter (DAT), tyrosine hydroxylase (TH), dopamine D2 receptor, alpha-synuclein, and nuclear receptor-related 1 (Nurr1) in discrete mesocorticolimbic and nigrostriatal subpopulations of heroin users and control subjects. The chronic use of heroin was significantly associated with decreased DAT mRNA expression localized to the paranigral nucleus (PN) and the mesolimbic division of the ventral tegmental area (VTA) with no alterations in nigrostriatal populations. Consistently, the density of DAT immunoreactivity was significantly reduced in the nucleus accumbens but not in dorsal striatum, mesolimbic and nigrostriatal efferent targets, respectively. Significant alteration of the mRNA expression of Nurr1, a transcription factor that regulates DAT expression, was also confined to the PN. Moreover, the results revealed an exaggerated reduction of Nurr1 expression with age in heroin users (r = -0.8268, p < 0.001 vs controls, r = -0.6204, p = 0.0746). TH and alpha-synuclein mRNA levels were, in contrast, elevated in the VTA PN in heroin users with no change of the D2 receptor. Evaluating midbrain mu- and kappa-opioid receptors, relevant for the action of heroin and regulation of DA neurons, revealed dysregulation of G-protein coupling selective to the VTA PN. Altogether the current findings provide direct neurobiological evidence that midbrain reward circuits have the most prominent DA and opioid impairments in human heroin abusers and that abnormal Nurr1 transcription with opiate use may exacerbate limbic dysfunction with age.

Evidence That the BLOC-1 Protein Dysbindin Modulates Dopamine D2Receptor Internalization and Signaling But Not D1Internalization

The schizophrenia susceptibility gene dystrobrevin-binding protein 1 (DTNBP1) encodes dysbindin, which along with its binding partner Muted is an essential component of the biogenesis of lysosome-related organelles complex 1 (BLOC-1). Dysbindin expression is reduced in schizophrenic brain tissue, but the molecular mechanisms by which this contributes to pathogenesis and symptomatology are unknown. We studied the effects of transfection of DTNBP1 siRNA on cell surface levels of dopamine D2 receptor (DRD2) in human SH-SY5Y neuroblastoma cells and in rat primary cortical neurons. DTNBP1 siRNA decreased dysbindin protein, increased cell surface DRD2 and blocked dopamine-induced DRD2 internalization. MUTED siRNA produced similar effects. In contrast, decreased dysbindin did not change dopamine D1 receptor (DRD1) levels, or its basal or dopamine-induced internalization. The DRD2 agonist quinpirole reduced phosphorylation of CREB (cAMP response element-binding protein) in dysbindin downregulated cells, demonstrating enhanced intracellular signaling caused by the upregulation of DRD2. This is the first demonstration of a schizophrenia susceptibility gene exerting a functional effect on DRD2 signaling, a pathway that has long been implicated in the illness. We propose a molecular mechanism for pathogenesis in which risk alleles in DTNBP1, or other factors that also downregulate dysbindin, compromise the ability of BLOC-1 to traffic DRD2 toward degradation, but has little effect on DRD1 trafficking. Impaired trafficking of DRD2 decreases dopamine-induced internalization, and with more receptors retained on the cell surface, dopamine stimulation produces excess intracellular signaling. Such an increase in DRD2 signaling relative to DRD1 would contribute to the imbalances in dopaminergic neurotransmission characteristic of schizophrenia.

Food restriction markedly increases dopamine D2 receptor (D2R) in a rat model of obesity as assessed with in‐vivo μPET imaging ([11C] raclopride) and in‐vitro ([3H] spiperone) autoradiography

Dopamine (DA) regulates food intake by modulating food reward and motivation but its involvement in obesity is much less understood. Recent evidence points to the involvement of leptin in the DA-related modulation of food intake. Here we assess DA D2 receptors (D2R) in a genetic rodent obesity model characterized by leptin-receptor deficiency and assess the influence of food restriction on these receptors. We compared D2R levels between Zucker Obese (fa/fa) and Lean (Fa/Fa) rats at 1 and 4 months of age and in two different feeding conditions (restricted and unrestricted food access) using in-vivo muPET imaging ([11C] raclopride, which is a method sensitive to competition with endogenous DA) and in-vitro ([3H] spiperone washed to ensure no competition with endogenous DA) autoradiography (ARG). Both ARG and muPET showed that D2R were higher at 1 month than at 4 months of age and that food restricted animals had higher D2R than unrestricted animals. However there were significant differences in the results obtained at 4 months between ARG and muPET. ARG showed that at 1 month and at 4 months unrestricted lean rats (Le U) had significantly higher D2R binding than obese unrestricted rats (Ob U) but showed no differences between restricted obese (Ob R) and restricted lean rats (Le R). It also showed that D2R decline between 1 and 4 months of age was significantly attenuated in food restricted rats [both obese and lean]. In contrast, muPET showed that at 4 months of age, Ob U showed greater D2R availability than Le U rats but like ARG showed no differences between Ob R and Le R rats. The lower D2R binding in Ob U than Le U rats observed with ARG most likely reflects decreases in striatal D2 receptors levels whereas the increased availability observed with muPET is likely to reflect reduced DA release (resulting in decreased competition with endogenous DA). Lack of a significant difference between Ob R and Le R suggests that the differences in dopamine activity and D2R levels between Ob and Le Zucker rats are modulated by access to food. The ARG finding of an attenuation of the age-related loss of D2R binding corroborates previous studies of the salutary effects of food restriction in the aging process. Because [11C] raclopride is sensitive to competition with endogenous DA, the higher D2R binding in obese rats with raclopride despite the lower D2R levels shown with spiperone could reflect lower extracellular DA in the Ob rats and merits further investigation.

Isolated Flinders Sensitive Line rats have decreased dopamine D2 receptor mRNA

Social isolation has profound effects on animal behavior and dopamine systems. We investigated the effect of social isolation on the dopamine receptor and neuropeptide mRNAs in the brain reward system in an animal model of depression, the Flinders Sensitive Line rats and Sprague-Dawley controls. We demonstrate that socially isolated but not group housed Flinders sensitive line rats had lower dopamine D2 receptor mRNA levels compared with Sprague-Dawley rats. Isolated and group housed Flinders Sensitive Line rats had higher levels of dopamine D1 receptor and substance P and enkephalin but not dynorphin mRNAs when compared with Sprague-Dawley rats. Our findings of decreased dopamine D2 receptor levels in socially isolated Flinders Sensitive Line rats suggest that low D2 receptor expression may play a role in pathophysiology of depression.

Estimation of Baseline Dopamine D2 Receptor Occupancy in Striatum and Extrastriatal Regions in Humans with Positron Emission Tomography with [18F] Fallypride

This study examined whether positron emission tomography (PET) studies with [18F] fallypride performed before and after alpha-methyl-para-tyrosine (AMPT) administration can be used to estimate baseline dopamine (DA) D2 receptor occupancy in striatal and extrastriatal regions. Six normal subjects underwent PET with [18 F] fallypride before and after administration of AMPT. The DA D2 receptor binding potentials (bp) were calculated with the reference region method. Percent changes in bp in striatal and extrastriatal regions were calculated with both region-of-interest analysis and on a voxel by voxel basis with parametric images of DA D2 receptor levels. The results of the current study indicate that AMPT treatment significantly increased the bp in the caudate, putamen, ventral striatum, and substantia nigra. A trend level increase was seen in the medial thalamus. This study demonstrates that PET with [18F] fallypride can be used to estimate baseline DA D2 receptor occupancy in striatal and extrastriatal regions.

Effects of chronic oral methylphenidate on cocaine self-administration and striatal dopamine D2 receptors in rodents

Background Methylphenidate (MP) and amphetamine, which are the mainstay for the treatment of ADHD, have raised concerns because of their reinforcing effects and the fear that their chronic use during childhood or adolescence could induce changes in the brain that could facilitate drug abuse in adulthood. Methods Here we measured the effects of chronic treatment (8 months) with oral MP (1 or 2 mg/kg), which was initiated in periadolescent rats (postnatal day 30). Following this treatment, rats were tested on cocaine self-administration. In addition at 2 and 8 months of treatment we measured dopamine D2 receptor (D2R) availability in the striatum using [ 11C]raclopride microPET (μPET) imaging. Results Animals treated for 8 months with 2 mg/kg of MP showed significantly reduced rates of cocaine self-administration at adulthood than vehicle treated rats. D2R availability in the striatum was significantly lower in rats after 2 months of treatment with MP (1 and 2 mg/kg) but significantly higher after 8 months of MP treatment than in the vehicle treated rats. In vehicle treated rats D2R availability decreased with age whereas it increased in rats treated with MP. Because low D2R levels in the striatum are associated with a propensity for self-administration of drugs both in laboratory animals and in humans, this effect could underlie the lower rates of cocaine self-administration observed in the rats given 8 months of treatment with MP. Conclusions Eight month treatment with oral MP beginning in adolescence decreased cocaine-self administration (1 mg/kg) during adulthood which could reflect the increases in D2R availability observed at this life stage since D2R increases are associated with reduced propensity for cocaine self administration. In contrast, two month treatment with MP started also at adolescence decreased D2R availability, which could raise concern that at this life stage short treatments could possibly increase vulnerability to drug abuse during adulthood. These findings indicate that MP effects on D2R expression in the striatum are sensitive not only to length of treatment but also to the developmental stage at which treatment is given. Future studies evaluating the effects of different lengths of treatment on drug self-administration are required to assess optimal duration of treatment regimes to minimize adverse effects on the propensity for drug self administration.

Increased dopamine D2High receptors in knockouts of the dopamine transporter and the vesicular monoamine transporter may contribute to spontaneous hyperactivity and dopamine supersensitivity

INTRODUCTIONDopamine transporters (DA T) on nerve terminalshelp to control the concentration of dopamine in thesynaptic space by transporting extracellular dopamineback into the presynaptic nerve terminals. Concentra-tions within the nerve terminals are divided into twocompartments, vesicular and cytoplasmic. The relativeconcentrations of neurotransmitters within these cellu-lar compartments are regulated, in part, by actions ofthe brain vesicular monoamine transporter (VMA T2)localized on the cellular membranes of neurotransmit-ter vesicles. Amphetamine, cocaine, and some antide-pressants act on these transporters. Their abilities toincrease the extracellular concentrations of dopaminethrough these actions contributes to both psychostimu-lant and rewarding effects in animals and humans.Most of these drugs also act at other monoamine trans-porters so that combin ed a ction s a t th ese tra n sp ortersma y a lso con trib u te to the se effects (S ora et al ., 20 01 ;Uh l et a l.,2 00 1, 2 0 0 2 ;X u et a l.,2 0 0 0 ).Genetic deletion of the dopamine T results in mice(DA T ! /! ) that are spontaneously hyperactive, butinsensitive to the motor- stimulating actions of cocaineand amphetamine (Gainetdinov et al., 1999; Giroset al., 1996; Jones et al., 1998; Sora et al., 1998,2001). Although the striata of these altered mice havea 95% reduction in dopamine content, dopamine iscleared very slowly from extracellular spaces, result-ing in dopamine concentrations that are as much asÞve times higher than normal (Jones et al., 1998).A lth ou g h th e sp on ta n eou s h y p era ctiv ity of th e D A T ! /!mouse may be explained by these elevated extracellu-lar dopamine levels, these mice have been describedto have other differences from wild type animals. Inparticular , dopamine D2 receptor levels have beenreported to be decreased (Giros et al., 1996) in waysthat reopens questions about the cause of the sponta-neous hyperactivity found in these mice. The data inthe present study show that the proportion of dopa-mine D2 receptors in the functional high-afÞnitystate, D2

Dopamine and Dependence in Families with Alcoholism

Various studies suggest that striatal levels of dopamine D2 receptors are low in alcoholics. These researchers hypothesized that high D2 receptor

High Levels of Dopamine D2 Receptors in Unaffected Members of Alcoholic Families

Predisposition to alcoholism is likely an interaction between genetic and environmental factors that confer vulnerability and protection. Alcoholic subjects have low levels of dopamine D(2) receptors in striatum, and increasing D(2) receptor levels in laboratory animals reduces alcohol consumption. To test whether high levels of D(2) receptors may be protective against alcoholism and whether this is mediated by their modulation of activity in orbitofrontal cortex and cingulate gyrus (regions involved in salience attribution, emotional reactivity, and inhibitory control). Research (nonalcoholic subjects with a family history of alcoholism) and comparison (nonalcoholic subjects with a negative family history) sample. Outpatient setting. Fifteen nonalcoholic subjects who had an alcoholic father and at least 2 other first- or second-degree relatives who were alcoholics (family-positive group) and 16 nonalcoholic controls with no family history of alcoholism (family-negative group). Results of positron emission tomography with raclopride C 11 to assess D(2) receptors and with fludeoxyglucose F 18 to assess brain glucose metabolism (marker of brain function). Personality measures were obtained with the Multidimensional Personality Questionnaire. Availability of D(2) receptors was significantly higher in caudate and ventral striatum in family-positive than family-negative subjects. In family-positive but not family-negative subjects, striatal D(2) receptors were associated with metabolism in anterior cingulate (Brodmann area 24/25) and orbitofrontal (Brodmann area 11) and prefrontal (Brodmann area 9/10) cortices, and with personality scores of positive emotionality. The higher-than-normal D(2) receptor availability in nonalcoholic members of alcoholic families supports the hypothesis that high levels of D(2) receptors may protect against alcoholism. The significant associations between D(2) receptors and metabolism in frontal regions involved with emotional reactivity and executive control suggest that high levels of D(2) receptors could protect against alcoholism by regulating circuits involved in inhibiting behavioral responses and in controlling emotions.

P1-024: Altered dopamine receptor immunoreactivity in CNS of transgenic mice overexpressing alpha-synuclein

Transgenic mice overexpressing wild–type human alpha–synuclein (α–syn) exhibit α–syn immunopositive inclusions and neurites in multiple regions of the brain including the hippocampus, cerebellum and cortex. Drawing on the methods of multi–scale imaging, behavioral pharmacology and bioinformatics, we are using a multi–disciplinary approach to characterize a transgenic mouse overexpressing human α–syn. We previously reported alterations in cognitive and motor behaviors, and increased metabotropic glutamate, subtype 5 (mGluR5) immunoreactivity in the CNS of α–syn transgenic mice. In addition to dopamine (DA) receptor agonists, mGluR5 antagonists have received interest as a therapeutic target for Parkinsonian motor dysfunction via restoration of dopamine–glutamatergic homeostasis. While the basis for mGluR5 antagonist induced amelioration of PD symptoms is not clearly understood, most PD researchers appreciate that the complexity of clinical cognitive and motor symptoms are the result of complex changes in many regions and neurotransmitter systems. Accordingly, alterations in adenosine, serotonin, GABA, and dopamine receptors have been reported in PD patients. Of these, alterations in dopamine D1 and D2 receptors have been observed in patients, and have been investigated in PD animal models. Assessment of dopamine D1 and D2 receptor expressions is a necessary first step towards validation of any animal model for preclinical assessment of dopamine D1/D2 receptor agonists. With this consideration, we conducted multi–scale imaging studies of dopamine D1 and D2 receptor immunolabeling in the α–syn transgenic model of PD, using both laser scanning confocal and multi–photon microscopy. Here we report our findings of PD–like increases in striatal dopamine D1 receptors in α–syn transgenic animals. Studies of dopamine D2 receptor levels and distribution are underway. Taken together, these results suggest that PD–like alterations in glutamatergic and dopaminergic neurotransmission are recapitulated in the α–syn transgenic model of PD, and further that this animal model is an appropriate system for preclinical assessment of mGluR5 and DA receptor ligands.

Cocaine-induced dendritic spine formation in D1 and D2 dopamine receptor-containing medium spiny neurons in nucleus accumbens.

Psychostimulant-induced alteration of dendritic spines on dopaminoceptive neurons in nucleus accumbens (NAcc) has been hypothesized as an adaptive neuronal response that is linked to long-lasting addictive behaviors. NAcc is largely composed of two distinct subpopulations of medium-sized spiny neurons expressing high levels of either dopamine D1 or D2 receptors. In the present study, we analyzed dendritic spine density after chronic cocaine treatment in distinct D1 or D2 receptor-containing medium-sized spiny neurons in NAcc. These studies made use of transgenic mice that expressed EGFP under the control of either the D1 or D2 receptor promoter (Drd1-EGFP or Drd2-EGFP). After 28 days of cocaine treatment and 2 days of withdrawal, spine density increased in both Drd1-EGFP- and Drd2-EGFP-positive neurons. However, the increase in spine density was maintained only in Drd1-EGFP-positive neurons 30 days after drug withdrawal. Notably, increased DeltaFosB expression also was observed in Drd1-EGFP- and Drd2-EGFP-positive neurons after 2 days of drug withdrawal but only in Drd1-EGFP-positive neurons after 30 days of drug withdrawal. These results suggest that the increased spine density observed after chronic cocaine treatment is stable only in D1-receptor-containing neurons and that DeltaFosB expression is associated with the formation and/or the maintenance of dendritic spines in D1 as well as D2 receptor-containing neurons in NAcc.

Effects of Chronic Alcohol and Repeated Deprivations on Dopamine D1 and D2 Receptor Levels in the Extended Amygdala of Inbred Alcohol‐Preferring Rats

Dopaminergic (DA) activity in the extended amygdala (EA) has been known to play a pivotal role in mediating drug and alcohol addiction. Alterations of DA activity within the EA after chronic exposure to alcohol or substances of abuse are considered a major mechanism for the development of alcoholism and addiction. To date, it is not clear how different patterns of chronic alcohol drinking affect DA receptor levels. Therefore, the current studies investigated the effects of chronic ethanol consumption, with or without deprivations, on D1 and D2 receptor densities within the EA. Inbred alcohol-preferring (iP) rats were divided into 3 groups with the following treatments: (1) water for 14 weeks; (2) continuous alcohol (C-Alc) for 14 weeks [24-hour concurrent access to 15 and 30% (v/v) ethanol]; or (3) repeatedly deprived of alcohol (RD-Alc) (24-hour concurrent access to 15 and 30% ethanol for 6 weeks, followed by 2 cycles of 2 weeks of deprivation of and 2 weeks of reexposure to ethanol access). At the end of 14 weeks, the rats were killed for autoradiographic labeling of D1 and D2 receptors. Compared with the water control group, both the C-Alc and the RD-Alc groups displayed increases in D1 receptor binding density in the anterior region of the Acb core, whereas the RD-Alc group displayed additional increases in D1 receptor binding density in anterior regions of the lateral and intercalated nuclei of the amygdala. Additionally, both C-Alc and RD-Alc rats displayed increases in D2 receptor binding density in anterior regions of the Acb shell and core, whereas RD-Alc rats displayed additional increases in D2 receptor binding density in the dorsal striatum. The results of this study indicate that 14-week extended alcohol drinking with continuous chronic or repeated deprivations increase binding sites of D1 and D2 receptors in specific regions of the EA with greater sensitivity in the anterior regions. The repeated deprivation has greater effect on altering D1 and D2 receptor binding sites in the Acb, dorsal striatum, and subamygdala regions. The current result indicates that the two drinking paradigms may have common as well as differential mechanisms on alteration of dopamine receptor-binding sites in specific regions of the EA.

Chronic nicotine and smoke treatment modulate dopaminergic activities in ventral tegmental area and nucleus accumbens and the γ‐aminobutyric acid type B receptor expression of the rat prefrontal cortex

Dopaminergic afferents from the mesencephalic areas, such as ventral tegmental area (VTA), synapse with the gamma-aminobutyric acid (GABA)-ergic interneurons in the prefrontal cortex (PFC). Pharmacological and electrophysiological data show that the reinforcement, the dependence-producing properties, as well as the psychopharmacologic effects of nicotine depend to a great extent on activation of nicotinic receptors within the mesolimbocortical dopaminergic projection. To explore further the relationship between the mesencephalic dopaminergic neurons and PFC GABAergic neurons, we investigated the effects of nicotine and passive exposure to cigarette smoke on the regulation of tyrosine hydroxylase (TH) in VTA and substantia nigra (SNC) and dopamine (DA) D1 receptor levels in nucleus accumbens (NAc) and caudate-putamen (CPu). Also, the simultaneous changes in GABAB receptors mRNAs in the PFC were studied. The results showed that chronic nicotine and smoking treatment differentially changed the levels of TH protein in VTA and SNC and DA D1 receptor levels in Nac and CPu. GABAB1 and GABAB2 receptor mRNA levels also showed different change patterns. Ten and thirty minutes of smoke exposure increased GABAB1 receptor mRNA to a greater extent than that of GABAB2, whereas GABAB2 was greatly enhanced after 1 hr of smoke exposure. The TH levels in VTA were closely related to DA D1 receptor levels in NAc and with GABAB receptor mRNA changes in PFC. These results suggest that the mesolimbic pathway and GABAB receptor mRNA in PFC are modulated by nicotine and cigarette smoke, implying an important role in nicotine's psychopharmacological effects.

Similarity Between Obesity and Drug Addiction as Assessed by Neurofunctional Imaging

SUMMARY Overeating in obese individuals shares similarities with the loss of control and compulsive drug taking behavior observed in drug-addicted subjects. The mechanism of these behaviors is not well understood. Our prior studies with positron emission tomography (PET) in drug-addicted subjects documented reductions in striatal dopamine (DA) D2 receptors. In pathologically obese subjects, we found reductions in striatal DA D2 receptors similar to that in drug-addicted subjects. Moreover, DA D2 receptor levels were found to have an inverse relationship to the body mass index of the obese subjects. We postulated that decreased levels of DA D2 receptors predisposed subjects to search for reinforcers; in the case of drug-addicted subjects for the drug and in the case of the obese subjects for food as a means to temporarily compensate for a decreased sensitivity of DA D2 regulated reward circuits. Understanding the mechanism in food intake will help to suggest strategies for the treatment of obesity.