Characterization of 3,4-methylenedioxypyrovalerone discrimination in female Sprague-Dawley rats

3,4-Methylenedioxypyrovalerone (MDPV), one of several synthetic cathinones, is a popular constituent of illicit 'bath salts'. In preclinical studies utilizing drug discrimination methods with male rodents, MDPV has been characterized as similar to both cocaine and 3,4-methylenedioxymethamphetamine-hydrochloride (MDMA). Whereas few drug discrimination studies have utilized female rats, the current study evaluated the discriminative stimulus effects of MDPV in 12 adult female Sprague-Dawley rats trained to discriminate 0.5 mg/kg MDPV from saline under a fixed ratio 20 schedule of food reinforcement. Stimulus substitution was assessed with MDPV and its enantiomers, other synthetic cathinones [alpha pyrrolidinopentiophenone-hydrochloride(α-PVP), 4-methylmethcathinone (4-MMC)], other dopamine agonists (cocaine, [+)-methamphetamine] and serotonin agonists [MDMA, lysergic acid diethylamide (LSD)] Stimulus antagonism was assessed with the dopamine D1 receptor antagonist, Sch 23390 and the D2 receptor antagonist, haloperidol. Cocaine and (+)-methamphetamine engendered full stimulus generalization to MDPV with minimal effects on response rate. LSD produced partial substitution, whereas MDMA and 4-MMC produced complete substitution, and all these serotonergic compounds produced dose-dependent response suppression. (S)-MDPV and α-PVP engendered full substitution with similar potency to the racemate, while (R)-MDPV failed to substitute up to 5 mg/kg. Both Sch 23390 and haloperidol attenuated the discrimination of low MDPV doses and essentially shifted the dose-response curve to the right but failed to block discrimination of the training dose. These findings are generally consistent with previous reports based exclusively on male rodents. Moreover, they confirm the contribution of dopaminergic mechanisms but do not rule out the possible contribution of other neurotransmitter actions to the interoceptive stimulus effects of MDPV.

A Split Luciferase Complementation Assay for the Quantification of β-Arrestin2 Recruitment to Dopamine D2-Like Receptors

Investigations on functional selectivity of GPCR ligands have become increasingly important to identify compounds with a potentially more beneficial side effect profile. In order to discriminate between individual signaling pathways, the determination of β-arrestin2 recruitment, in addition to G-protein activation, is of great value. In this study, we established a sensitive split luciferase-based assay with the ability to quantify β-arrestin2 recruitment to D_2long and D₃ receptors and measure time-resolved β-arrestin2 recruitment to the D_2long receptor after agonist stimulation. We were able to characterize several standard (inverse) agonists as well as antagonists at the D_2longR and D₃R subtypes, whereas for the D_4.4R, no β-arrestin2 recruitment was detected, confirming previous reports. Extensive radioligand binding studies and comparisons with the respective wild-type receptors confirm that the attachment of the Emerald luciferase fragment to the receptors does not affect the integrity of the receptor proteins. Studies on the involvement of GRK2/3 and PKC on the β-arrestin recruitment to the D_2longR and D₃R, as well as at the D₁R using different kinase inhibitors, showed that the assay could also contribute to the elucidation of signaling mechanisms. Its broad applicability, which provides concentration-dependent and kinetic information on receptor/β-arrestin2 interactions, renders this homogeneous assay a valuable method for the identification of biased agonists.

Assessment of the Association of D2 Dopamine Receptor Gene and Reported Allele Frequencies With Alcohol Use Disorders: A Systematic Review and Meta-analysis

IMPORTANCE

The association between the D2 dopamine receptor gene (DRD2) Taq1A locus (rs1800497) and alcohol use disorder (AUD) is enduring but the subject of long-standing controversy; meta-analysis of studies across 3 decades shows an association between rs1800497 and AUD, but genome-wide analyses have detected no role for rs1800497 in any phenotype. No evidence has emerged that rs1800497, which is located in ANKK1, perturbs the expression or function of DRD2.

OBJECTIVE

To resolve contradictions in previous studies by identifying hidden confounders and assaying for functional effects of rs1800497 and other loci in the DRD2 region.

DATA SOURCES

PubMed (882 studies), Embase (1056 studies), and Web of Science (501 studies) databases were searched through August 2018. Three clinical populations-Finnish, Native American, and African American participants-were genotyped for 208 to 277 informative single-nucleotide polymorphisms (SNPs) across the DRD2 region to test the associations of SNPs in this region with AUD.

STUDY SELECTION

Eligible studies had diagnosis of AUD made by accepted criteria, reliable genotyping methods, sufficient genotype data to calculate odds ratios and 95% CIs, and availability of control allele frequencies or genotype frequencies.

DATA EXTRACTION AND SYNTHESIS

After meta-analysis of 62 studies, metaregression was performed to detect between-study heterogeneity and to explore the effects of moderators, including deviations of cases and controls from allele frequencies in large population databases (ExAC and 1000 Genomes). Linkage to AUD and the effect on gene expression of rs1800497 were evaluated in the context of other SNPs in the DRD2 region. Data analysis was performed from August 2018 to March 2019. This study follows the Preferred Reporting Items for Systematic Reviews and Meta-analyses reporting guideline.

MAIN OUTCOMES AND MEASURES

The effects of rs1800497 and other SNPs in the DRD2 region on gene expression were measured in human postmortem brain samples via differential allelic expression and evaluated in other tissues via publicly available expression quantitative locus data.

RESULTS

A total of 62 studies of DRD2 and AUD with 16 294 participants were meta-analyzed. The rs1800497 SNP was associated with AUD (odds ratio, 1.23; 95% CI, 1.14-1.31; P < .001). However, the association was attributable to spuriously low allele frequencies in controls in positive studies, which also accounted for some between-study heterogeneity (I2 = 43%; 95% CI, 23%-58%; Q61 = 107.20). Differential allelic expression of human postmortem brain and analysis of expression quantitative loci in public data revealed that a cis-acting locus or loci perturb the DRD2 transcript level; however, rs1800497 does not and is not in strong disequilibrium with such a locus. Across the DRD2 region, other SNPs are more strongly associated with AUD than rs1800497, although no DRD2 SNP was significantly associated in these 3 clinical samples.

CONCLUSIONS AND RELEVANCE

In this meta-analysis, the significant association of DRD2 with AUD was reassessed. The DRD2 association was attributable to anomalously low control allele frequencies, not function, in positive studies. For genetic studies, statistical replication is not verification.

High-Frequency Activation of Nucleus Accumbens D1-MSNs Drives Excitatory Potentiation on D2-MSNs

Subtypes of nucleus accumbens medium spiny neurons (MSNs) promote dichotomous outcomes in motivated behaviors. However, recent reports indicate enhancing activity of either nucleus accumbens (NAc) core MSN subtype augments reward, suggesting coincident MSN activity may underlie this outcome. Here, we report a collateral excitation mechanism in which high-frequency, NAc core dopamine 1 (D1)-MSN activation causes long-lasting potentiation of excitatory transmission (LLP) on dopamine receptor 2 (D2)-MSNs. Our mechanistic investigation demonstrates that this form of plasticity requires release of the excitatory peptide substance P from D1-MSNs and robust cholinergic interneuron activation through neurokinin receptor stimulation. We also reveal that D2-MSN LLP requires muscarinic 1 receptor activation, intracellular calcium signaling, and GluR2-lacking AMPAR insertion. This study uncovers a mechanism for shaping NAc core activity through the transfer of excitatory information from D1-MSNs to D2-MSNs and may provide a means for altering goal-directed behavior through coordinated MSN activity.

Cigarette Use and Striatal Dopamine D2/3 Receptors: Possible Role in the Link between Smoking and Nicotine Dependence

BACKGROUND

Cigarette smoking induces dopamine release in the striatum, and smoking- or nicotine-induced ventral striatal dopamine release is correlated with nicotine dependence. Smokers also exhibit lower dopamine D2/3 receptor availability in the dorsal striatum than nonsmokers. Negative correlations of striatal dopamine D2/3 receptor availability with smoking exposure and nicotine dependence, therefore, might be expected but have not been tested.

METHODS

Twenty smokers had positron emission tomography scans with [¹⁸F]fallypride to measure dopamine D2/3 receptor availability in ventral and dorsal regions of the striatum and provided self-report measures of recent and lifetime smoking and of nicotine dependence.

RESULTS

As reported before, lifetime smoking was correlated with nicotine dependence. New findings were that ventral striatal dopamine D2/3 receptor availability was negatively correlated with recent and lifetime smoking and also with nicotine dependence.

CONCLUSION

The results suggest an effect of smoking on ventral striatal D2/3 dopamine receptors that may contribute to nicotine dependence.

Relationship of Alexithymia Ratings to Dopamine D2-type Receptors in Anterior Cingulate and Insula of Healthy Control Subjects but Not Methamphetamine-Dependent Individuals

BACKGROUND

Individuals with substance-use disorders exhibit emotional problems, including deficits in emotion recognition and processing, and this class of disorders also has been linked to deficits in dopaminergic markers in the brain. Because associations between these phenomena have not been explored, we compared a group of recently abstinent methamphetamine-dependent individuals (n=23) with a healthy-control group (n=17) on dopamine D2-type receptor availability, measured using positron emission tomography with [(18)F]fallypride.

METHODS

The anterior cingulate and anterior insular cortices were selected as the brain regions of interest, because they receive dopaminergic innervation and are thought to be involved in emotion awareness and processing. The Toronto Alexithymia Scale, which includes items that assess difficulty in identifying and describing feelings as well as externally oriented thinking, was administered, and the scores were tested for association with D2-type receptor availability.

RESULTS

Relative to controls, methamphetamine-dependent individuals showed higher alexithymia scores, reporting difficulty in identifying feelings. The groups did not differ in D2-type receptor availability in the anterior cingulate or anterior insular cortices, but a significant interaction between group and D2-type receptor availability in both regions, on self-report score, reflected significant positive correlations in the control group (higher receptor availability linked to higher alexithymia) but nonsignificant, negative correlations (lower receptor availability linked to higher alexithymia) in methamphetamine-dependent subjects.

CONCLUSIONS

The results suggest that neurotransmission through D2-type receptors in the anterior cingulate and anterior insular cortices influences capacity of emotion processing in healthy people but that this association is absent in individuals with methamphetamine dependence.

Imaging dopaminergic dysfunction as a surrogate marker of neuropathology in a small-animal model of HIV

The dopaminergic system is especially vulnerable to the effects of human immunodeficiency virus (HIV) infection, rendering dopaminergic deficits early surrogate markers of HIV-associated neuropathology. We quantified dopamine D2/3 receptors in young HIV-1 transgenic (Tg) (n  =  6) and age-matched control rats (n  =  7) and adult Tg (n  =  5) and age-matched control rats (n  =  5) using [18F]fallypride positron emission tomography (PET). Regional uptake was quantified as binding potential (BPND) using the two-tissue reference model with the cerebellum as the reference. Time-activity curves were generated for the ventral striatum, dorsal striatum, thalamus, and cerebellum. Whereas BPND values were significantly lower in the ventral striatum (p < .001) and dorsal striatum (p  =  .001) in the adult Tg rats compared to controls rats, they were significantly lower only in the dorsal striatum (p < .05) in the young rats. Tg rats had smaller striatal volumes on magnetic resonance imaging. We also found lower expression levels of tyrosine hydroxylase on immunohistochemistry in the Tg animals. Our findings suggest that progressive striatal D2/3 receptor deficits occur in Tg rats as they age and can be detected using small-animal PET imaging. The effectiveness of various approaches in preventing or halting this dopaminergic loss in the Tg rat can thus be measured preclinically using [18F]fallypride PET as a molecular imaging biomarker of HIV-associated neuropathology.

T-cell subpopulations express a different pattern of dopaminergic markers in intra- and extra-thymic compartments

An involvement of dopamine in regulation of the immune function has been assessed and dopaminergic system has been found widely represented in thymus. Nevertheless detail on the characterization of dopaminergic system in assisting thymocytes development and lymphocytes mature physiology are still lacking. The present study was designed to characterize dopamine plasma membrane transporter (DAT), vesicular dopamine transporters (VMAT)-1 and -2, and dopamine D1-like and D2-like receptors in rat thymocytes, splenocytes and peripheral blood mononuclear cells. Western blot and RT-PCR analyses, performed on these cells, showed an expression of dopamine transporters and receptors during thymocyte development (when of CD4 and CD8 markers are differently expressed). Furthermore FACS analysis, indicates that DAT and dopamine D1-like receptors are expressed at high levels in thymocytes, splenocytes, and peripheral lymphocytes. The percentage of CD4+ CD8+ (double-positive) thymocytes expressing dopaminergic markers was significantly higher compared to the percentage of double-negative ones. The percentage of CD8+ single positive cells expressing dopaminergic markers was significantly higher than that of CD4+ cells. The results suggest that the dopaminergic system plays a role in the thymus microenvironment during T-cell development. The more pronounced expression of dopaminergic markers in CD8+ subsets suggests that dopamine plays a role in development of cytotoxic T-cells. Our findings indicate dopaminergic system to have a role during the maturation and selection of lymphocytes, and support its involvement in the active phases of immune response.

Dopaminergic receptors in the human skin

Dopamine is a neurotransmitter which plays an important role in many human organs including the skin. In this study we will examine the presence and the distribution of D1 and D2 dopamine receptors in a particular zone of the human skin. Samples of the human plantar skin were harvested during autopsies after the consent of relatives of the dead donors. In this study the following experimental procedures were performed: 1) drawing of the human plantar skin; 2) cutting of tissues; 3) staining of tissues; 4) staining of the nerve fibres; 5) radio-binding methods for labelling D1 and D2 dopamine receptors; 6) light microscope autoradiography; 7) quantitative analysis of images and 8) statistical analysis of data. The dopamine receptors D1 are distributed particularly in the dermis layer of the human plantar skin. They are numerous in lower epidermal layers (with exclusion of the corneal layer) and few in subcutaneous tissue. On the contrary D2 dopamine receptors are prominent in the subcutaneous tissue near the vessels. Quantitative analysis of images and statistical analysis of the data confirm all our results. The specific distribution of D1 and D2 dopamine receptors in the human plantar skin is in close relation with the functions of a particular zone of the human skin that supports the weight of all the body. Moreover the character of dopamine receptors distribution is very important for further understanding the role of these receptors in the human skin.

[123I]Epidepride neuroimaging of dopamine D2/D3 receptor in chronic MK-801-induced rat schizophrenia model

PURPOSE

[(123)I]Epidepride is a radio-tracer with very high affinity for dopamine D(2)/D(3) receptors in brain. The importance of alteration in dopamine D(2)/D(3) receptor binding condition has been wildly verified in schizophrenia. In the present study we set up a rat schizophrenia model by chronic injection of a non-competitive NMDA receptor antagonist, MK-801, to examine if [(123)I]epidepride could be used to evaluate the alterations of dopamine D(2)/D(3) receptor binding condition in specific brain regions.

METHOD

Rats were given repeated injection of MK-801 (dissolved in saline, 0.3mg/kg) or saline for 1month. Afterwards, total distance traveled (cm) and social interaction changes were recorded. Radiochemical purity of [(123)I]epidepride was analyzed by Radio-Thin-Layer Chromatography (chloroform: methanol, 9:1, v/v) and [(123)I]epidepride neuroimages were obtained by ex vivo autoradiography and small animal SPECT/CT. Data obtained were then analyzed to determine the changes of specific binding ratio.

RESULT

Chronic MK-801 treatment for a month caused significantly increased local motor activity and induced an inhibition of social interaction. As shown in [(123)I]epidepride ex vivo autoradiographs, MK-801 induced a decrease of specific binding ratio in the striatum (24.01%), hypothalamus (35.43%), midbrain (41.73%) and substantia nigra (37.93%). In addition, [(123)I]epidepride small animal SPECT/CT neuroimaging was performed in the striatum and midbrain. There were statistically significant decreases in specific binding ratio in both the striatum (P<.01) and midbrain (P<.05) between the saline and MK-801 group.

CONCLUSION

These results suggest that [(123)I]epidepride is a useful radio-tracer to reveal the alterations of dopamine D(2)/D(3) receptor binding in a rat schizophrenia model and is also helpful to evaluate therapeutic effects of schizophrenia in the future.

Somatostatin and dopamine receptor interaction in prostate and lung cancer cell lines

Somatostatin analogues inhibit in vitro cell proliferation via specific membrane receptors (SSTRs). Recent studies on transfected cell lines have shown a ligand-induced formation of receptor dimers. The aim of this study is 1) to evaluate the role of specific ligands in modulating receptor interactions in the androgen-dependent prostate cancer cell line, LNCaP, and in the non-small cell lung cancer line, Calu-6, by co-immunoprecipitation and immunoblot; and 2) to correlate the antiproliferative effect of these compounds with their ability in modulating receptor interactions. In LNCaP, we have demonstrated the constitutive presence of sstr₁/sstr₂, sstr₂/sstr₅, sstr₅/dopamine (DA) type 2 receptor (D₂R), and sstr₂/D₂R dimers. BIM-23704 (sstr₁- and sstr₂-preferential compound) increased the co-immunoprecipitation of sstr₁/sstr₂ and significantly inhibited proliferation (-30.98%). BIM-23244 (sstr₂-sstr₅ selective agonist) significantly increased the co-immunoprecipitation of sstr₂/sstr₅, and induced a -41.36% inhibition of proliferation. BIM-23A760, a new somatostatin/DA chimeric agonist with a high affinity for sstr₂ and D₂R and a moderate affinity for sstr₅, significantly increased the sstr₅/D₂R and sstr₂/D₂R complexes and was the most powerful in inhibiting proliferation (-42.30%). The chimeric compound was also the most efficient in modulating receptor interaction in Calu-6, increasing the co-immunoprecipitation of D₂R/sstr₅ and inhibiting cell proliferation (-30.54%). However, behind BIM-23A760, BIM-53097 (D₂R-preferential compound) also significantly inhibited Calu-6 proliferation (-17.71%), suggesting a key role for D₂R in receptor cross talk and in controlling cell growth. Indeed, activation of monomeric receptors did not affect receptor co-immunoprecipitation, whereas cell proliferation was significantly inhibited when the receptors were synergistically activated. In conclusion, our data show a dynamic ligand-induced somatostatin and DA receptor interaction, which may be crucial for the antiproliferative effects of the new analogues.

Altered thyrotropic and lactotropic axes regulation in Huntington's disease

BACKGROUND

Recently, a loss of hypothalamic dopamine D(2) receptors was demonstrated in Huntington's disease (HD). Activation of dopamine D(2) receptors is known to inhibit the function of both thyrotropic and lactotropic axes.

OBJECTIVE

To assess whether the activity of the thyrotropic and lactotropic axes is disturbed in patients with HD, contributing to symptoms such as unintended weight loss.

PARTICIPANTS AND METHODS

In nine medication-free patients with early-stage HD (six men, three women) and nine age-, sex- and body mass index-matched controls, we measured serum levels of thyroid-stimulating hormone (TSH) and prolactin (men only) every 10 min for 24 h. Multiparameter auto-deconvolution and approximate entropy analysis were applied to quantify basal, pulsatile and total TSH and prolactin secretion rates as well as the regularity of hormone release.

RESULTS

Compared with controls, TSH and prolactin secretion tended to be slightly, but not significantly, higher in patients with HD (TSH: 1.13 ± 0.14 vs 0.91 ± 0.19 mU/l, P = 0.40; prolactin: 213 ± 18 vs 209 ± 11 pmol/l, P = 0.87). However, in patients with HD, total T(3) levels were significantly higher (1.60 ± 0.05 vs 1.35 ± 0.09, P = 0.045), while T(4) levels tended to be higher as well (91.9 ± 3.9 vs 81.3 ± 3.1, P = 0.085). Prolactin secretion was significantly more irregular in patients with HD (Approximate entropy (ApEn): 1.06 ± 0.08 vs 0.80 ± 0.09, P = 0.037). Total T(3) levels were negatively associated with motor impairment (r = -0.72, P = 0.030), whereas increasing free T(4) levels were associated with a larger mutant cytosine-adenine-guanine (CAG) repeat size (r = +0.68, P = 0.044).

CONCLUSION

Our findings indicate a mild hyperactivity of the thyrotropic axis and a disturbed regulation of the lactotropic axis in patients with early-stage HD.

Dopamine receptor alterations in female rats with diet-induced decreased brain docosahexaenoic acid (DHA): interactions with reproductive status

Decreased tissue levels of n-3 (omega-3) fatty acids, particularly docosahexaenoic acid (DHA), are implicated in the etiologies of non-puerperal and postpartum depression. This study examined the effects of a diet-induced loss of brain DHA content and concurrent reproductive status on dopaminergic parameters in adult female Long-Evans rats. An alpha-linolenic acid-deficient diet and breeding protocols were used to produce virgin and parous female rats with cortical phospholipid DHA levels 20-22% lower than those fed a control diet containing adequate alpha-linolenic acid. Decreased brain DHA produced a significant main effect of decreased density of ventral striatal D(2)-like receptors. Virgin females with decreased DHA also exhibited higher density of D(1)-like receptors in the caudate nucleus than virgin females with normal DHA. These receptor alterations are similar to those found in several rodent models of depression, and are consistent with the proposed hypodopaminergic basis for anhedonia and motivational deficits in depression.

Striatal medium-sized spiny neurons: identification by nuclear staining and study of neuronal subpopulations in BAC transgenic mice

Precise identification of neuronal populations is a major challenge in neuroscience. In the striatum, more than 95% of neurons are GABAergic medium-sized spiny neurons (MSNs), which form two intermingled populations distinguished by their projections and protein content. Those expressing dopamine D₁-receptors (D1Rs) project preferentially to the substantia nigra pars reticulata (SNr), whereas those expressing dopamine D₂- receptors (D2Rs) project preferentially to the lateral part of the globus pallidus (LGP). The degree of segregation of these populations has been a continuous subject of debate, and the recent introduction of bacterial artificial chromosome (BAC) transgenic mice expressing fluorescent proteins driven by specific promoters was a major progress to facilitate striatal neuron identification. However, the fraction of MSNs labeled in these mice has been recently called into question, casting doubt on the generality of results obtained with such approaches. Here, we performed an in-depth quantitative analysis of striatal neurons in drd1a-EGFP and drd2-EGFP mice. We first quantified neuronal and non-neuronal populations in the striatum, based on nuclear staining with TO-PRO-3, and immunolabeling for NeuN, DARPP-32 (dopamine- and cAMP-regulated phosphoprotein Mr∼32,000), and various markers for interneurons. TO-PRO-3 staining was sufficient to identify MSNs by their typical nuclear morphology and, with a good probability, interneuron populations. In drd1a-EGFP/drd2-EGFP double transgenic mice all MSNs expressed EGFP, which was driven in about half of them by drd1a promoter. Retrograde labeling showed that all MSNs projecting to the SNr expressed D1R and very few D2R (<1%). In contrast, our results were compatible with the existence of some D1R-EGFP-expressing fibers giving off terminals in the LGP. Thus, our study shows that nuclear staining is a simple method for identifying MSNs and other striatal neurons. It also unambiguously confirms the degree of segregation of MSNs in the mouse striatum and allows the full exploitation of results obtained with BAC-transgenic mice.

Direct effects of prolactin and dopamine on the gonadotroph response to GnRH

The intrapituitary mechanisms underlying the inhibitory actions of hyperprolactinaemia on the reproductive axis remain unclear. Previous work on primary pituitary cultures revealed combined suppressive effects of prolactin (PRL) and dopamine on the gonadotrophin response to GnRH. However, whether these effects occur directly at the level of the gonadotroph and are accompanied by changes in gene expression is still unresolved. Here, alphaT(3)-1 and LbetaT2 cells were used to investigate the effects of PRL and dopamine on gonadotrophin synthesis and release in gonadotroph monocultures under basal and GnRH-stimulated conditions. PRL receptor and dopamine receptor mRNA expressions were first determined by RT-PCR in both cell lines. Then, PRL and the dopamine agonist bromocriptine (Br), alone or in combination, were shown to block the maximal alpha-subunit and LHbeta-subunit mRNA responses to a dose-range of GnRH. The LH secretory response was differentially affected by treatments. GnRH dose-dependently stimulated LH release, with a 4-5 fold increase at 10(-8) M GnRH. Unexpectedly, PRL or Br stimulated basal LH release, with PRL, but not Br, enhancing the LH secretory response to GnRH. This effect was, however, completely blocked by Br. These results reveal direct effects of PRL and dopamine at the level of the gonadotroph cell, and interactions between these two hormones in the regulation of gonadotrophin secretion. Moreover, uncoupling between LH synthesis and release in both the basal and the GnRH-stimulated responses to PRL and dopamine was clearly apparent.

Correlation of in vitro and in vivo somatotropic adenoma responsiveness to somatostatin analogs and dopamine agonists with immunohistochemical evaluation of somatostatin and dopamine receptors and electron microscopy

OBJECTIVE AND PATIENTS

Twenty-four pituitary adenomas from acromegalic patients (13 females, 11 males; age range 19-65 yr) were characterized for somatostatin receptor subtype 2A (sst(2A)), dopamine D(2) receptor (D(2)R), GH, and prolactin (PRL) expression by immunohistochemistry, and results correlated with the in vitro and in vivo hormone responses to octreotide and quinagolide. In nine cases, GH and PRL content was further studied by immunoelectron microscopy.

RESULTS

Immunoreactivity was semiquantitatively scored as 2 (>50% stained cells), 1 (10-50% stained cells), and 0 (<10% stained cells). Sst(2A) was scored as 2 in 13 cases, 1 in 10, and 0 in one; D(2)R was scored as 2 in 13 cases, 1 in nine, and 0 in 2; GH was 2 in 15 cases and 1 in nine; PRL was 2 in six cases, 1 in 13, and 0 in 5. Sst(2A) was positively correlated with in vitro (P = 0.003) and in vivo (P = 0.006) percent GH suppression by octreotide and with the chronic suppression of IGF-I by somatostatin analogs (P =0.008). D(2)R was positively correlated with in vitro percent GH (P =0.000) and PRL (P =0.005) suppression by quinagolide. Electron microscopy revealed two pure somatotroph adenomas, five somatomammotrophs with a variable coexpression of GH and PRL in the same cells, and two tumors consisting of mixed cell types, which were less sensitive to quinagolide and octreotide.

CONCLUSION

Sst(2A) and D(2)R are frequently coexpressed in adenomas from acromegalic patients, and immunohistochemistry may be helpful in characterizing receptor expression in pituitary adenomas to select patients responsive to different treatments.

[The effect of dopaminergic nigrostriatal system on sleep deprivation in rats]

The analysis of the electrophysiological features of sleep-wakefulness cycle in Wistar rats for 9h after a 6h sleep deprivation was carried out. The delay of sleep rebound (since 2.5-3 h after deprivation) was found in the form of moderate increasing of slow-wave sleep and fast-wave sleep phases. According to these sleep-wakefulness cycle changes, a quantitative immunohistochemical study of tyrosine hydroxylase: a key enzyme of dopamine synthesis--and D1 and D2 receptors in nigro-striatal projections has been performed. After sleep, an elevation of D1 receptors immunoreactivity in caudate nucleus and reduction of tyrosine hydroxylase immunoreactivity in compact part of substancia nigra was found. After postdeprivation sleep, a decrease of D1 receptors immunoreactivity and increase of D2 receptors immunoreactivity in caudate nucleus together some increase of tyrosine hydroxilase immunoreactivity in substancia nigra compacta has been observed. These data can testify about active role of dopaminergic nigrostriatal system which provide at the same time with another neurotransmitters of the central nervous system the telencephalo-diencephalic interaction in sleep-wakefulness-sleep cycle.

[Distribution and function of dopamine D1, D2 receptor]

Positron emission tomography (PET) techniques have made it possible to measure changes in target molecular in living human brain. PET can be used to investigate various brain functions such as receptors, transporters, enzymes and various biochemical pathways; therefore, it could be a powerful tool for molecular imaging of functional neurotransmission. Since dopamine is an important molecule for pathophysiology of Parkinson's disease and schizophrenia, we reviewed in vivo imaging studies focusing on dopaminergic transmission. Dopamine D2 receptor occupancy by antipsychotics and it's time-course have been measured using PET. This approach can provide in vivo pharmacological evidences of antipsychotics and establish the rational therapeutic strategy. PET is a powerful tool not only in the field of brain research but also drug discovery. On the other hand Dopamine D1 receptor is highly expressed in the prefrontal cortex, has been implicated in the control of working memory, seeking, craving, reward. We propose that dysfunction of Dopamine D1 receptor signalling in the prefrontal cortex may contribute to the negative symptoms and cognitive deficits seen in schizophrenia and we suggest that Dopamine D1 receptor binding and cerebral blood flow changes in ventral striatum play the important role of cigarette craving.

In vitro affinities of various halogenated benzamide derivatives as potential radioligands for non-invasive quantification of D2-like dopamine receptors

Benzamide derivatives as radiotracers have played an important role in diagnosing malfunction in dopaminergic neurotransmission. A variety of halogenated and two unsubstituted benzamide derivatives were synthesised and their in vitro affinities to dopaminergic, serotonergic and adrenergic receptors and their lipophilicities were determined. As references IBZM (3), raclopride (4) and FLB457 (5) were tested as well. The two iodinated compounds NAE (27) and NADE (28) displayed K(i) values of 0.68 and 14 nM for the D(2) receptor. The well-established radiotracers FP (1) and DMFP (2) showed affinities in the same range as did the brominated compounds NABrE (29) and NABrDE (30). The log D(7.4) values of 2.91 for NAE (27) and of 2.81 for NADE (28) are in the range of those found for IBZM (3), FP (1) and DMFP (2). These facts allow to expect good properties for the two iodinated compounds NAE (27) and NADE (28) regarding in vivo imaging with SPECT.

Multitracer positron emission tomographic imaging of exogenous gene expression mediated by a universal herpes simplex virus 1 amplicon vector

To develop efficient and safe gene therapy approaches, the herpes simplex virus type 1 thymidine kinase gene (HSV-1-tk) has been shown to function as a marker gene for the direct noninvasive in vivo localization of thymidine kinase (TK) expression by positron emission tomography (PET) using radiolabeled nucleoside analogues as specific TK substrates. Moreover, the gene encoding dopamine type 2 receptor (d2r) could be used as a PET marker gene using specific radiolabeled receptor binding compounds. Here we describe the quantitative colocalization of d2r and HSV-1-tk gene expression mediated from a universal HSV-1 amplicon vector in a subcutaneous human Gli36dEGFR glioma model by PET. The HSV-1 amplicon vector was constructed using a bicistronic gene cassette to contain (1) the d2r80A mutant, which is able to bind its ligand racloprid but unable to activate downstream signal transduction pathways, and (2) the tk39 mutant with enhanced enzymatic activity toward guanosine analogues fused to the green fluorescent protein gene (tk39gfp) serving as a marker gene in cell culture. After infection of human Gli36dEGFR glioma cells with the HSV-d2r80AIREStk39gfp (HSV-DITG) amplicon vector in cell culture, D2 receptor expression and its targeting to the cell surface were determined by Western blotting and immunolabeling. Vector application in vivo served for quantitative colocalization of d2r80A- and tk39gfp-derived PET signals employing the specific D2 receptor binding compound [(11)C]racloprid and the specific TK39 substrate 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl)guanine. Our results demonstrate that for the range of gene expression studied in vivo, both enzymatic and receptor binding assays give comparable quantitative information on the level of vector-mediated gene expression in vivo. The d2r80A in combination with a specific binding compound passing the intact blood-brain barrier might be an alternative marker gene for the noninvasive assessment of vector-mediated gene expression in the brain using PET.

Frontal dopaminergic abnormality in Tourette syndrome: a postmortem analysis

Frontal-subcortical abnormalities have been implicated in the pathophysiology of Tourette syndrome (TS). The goal of this study was to more extensively evaluate a possible underlying neurochemical abnormality in frontal cortex. Postmortem brain tissue from frontal and occipital regions (Brodmann's areas 4, 6, 9, 10, 11, 12, and 17) from three TS patients and three age-and sex-matched controls were analyzed by semiquantitative immunoblotting. Relative densities were measured for a variety of neurochemical markers including dopamine (D1, D2), serotonin (5HT-1A), and alpha-adrenergic (alpha-2A) receptors, the dopamine transporter (DAT), a monoamine terminal marker (vesicular monoamine transporter type 2, VMAT-2), and vesicular docking and release proteins (VAMP-2, synaptotagmin, SNAP-25, syntaxin, synaptophysin). Data from each TS sample, corrected for actin content, was expressed as a percentage value of its control. Results identified consistent increases of DAT and D2 receptor density in five of six frontal regions in all three TS subjects. D1 and alpha-2A receptor density were increased in a few frontal regions. These results support the hypothesis of a dopaminergic dysfunction in the frontal lobe and a likely role in the pathophysiology of TS.

Strain-specific steroidal control of pituitary function

We have previously shown that 7B2 null mice on the 129/SvEvTac (129) genetic background die at 5 weeks of age with hypercorticosteronemia due to a Cushing's-like disease unless they are rescued by adrenalectomy; however, 7B2 nulls on the C57BL/6NTac (B6) background remain healthy, with normal steroid levels. Since background exerts such a profound influence on the phenotype of this mutation, we have evaluated whether these two different mouse strains respond differently to high circulating steroids by chronically treating wild-type 129 and B6 mice with the synthetic steroid dexamethasone (Dex). Dex treatment decreased the dopamine content of the neurointermediate lobes (NIL) of 129 mice, leading to NIL enlargement and increased total D(2)R mRNA in the 129, but not the B6, NIL. Despite the decrease in this inhibitory transmitter, Dex-treated 129 mice exhibited reduced circulating alpha-melanocyte-stimulating hormone (alpha-MSH) along with reduced POMC-derived peptides compared with controls, possibly due to reduced POMC content in the NIL. In contrast, Dex-treated B6 mice showed lowered cellular ACTH, unchanged alpha-MSH and beta-endorphin, and increased circulating alpha-MSH, most likely due to increased cleavage of NIL ACTH by increased PC2. Dex-treated 129 mice exhibited hyperinsulinemia and lowered blood glucose, whereas Dex-treated B6 mice showed slightly increased glucose levels despite their considerably increased insulin levels. Taken together, our results suggest that the endocrinological response of 129 mice to chronic Dex treatment is very different from that of B6 mice. These strain-dependent differences in steroid sensitivity must be taken into account when comparing different lines of transgenic or knockout mice.

[Molecular imaging of psychiatric disorders]

Positron emission tomography (PET) techniques have made it possible to measure changes in target molecular in living human brain. PET can be used to investigate various brain functions such as receptors, transporters, enzymes and various biochemical pathways; therefore, it could be a powerful tool for molecular imaging of psychiatric disorders. Since dopamine is an important molecule for pathophysiology of schizophrenia, we reviewed in vivo imaging studies focusing on dopaminergic transmission in schizophrenia. Dopamine D2 receptor occupancy by antipsychotics and it' s time-course have been measured using PET. This approach can provide in vivo pharmacological evidences of antipsychotics and establish the rational therapeutic strategy. PET is a powerful tool not only in the field of brain research but also drug discovery.

Altered mesolimbocortical and thalamic dopamine in Tourette syndrome

We used [F-18]fallypride PET in six adults with Tourette syndrome and age-matched controls to assess extrastriatal dopamine 2 (D2) receptors. D2 receptor availability was significantly lower in the orbitofrontal cortex, primary motor cortex, anterior cingulate gyrus, mediodorsal nucleus of thalamus, and hippocampus, areas important for motivation and reward, sensory gating, movement, and attention. Altered dopaminergic function in mesolimbocortical systems and thalamus may contribute to increased motivational salience of tics.

Influence of the antipsychotic drug pipamperone on the expression of the dopamine D4 receptor

The dopamine D4 receptor is a G protein-coupled receptor that binds with high affinity various antipsychotics. The receptor may be involved in attention/cognition, and in genetic studies a polymorphic repeat sequence in its coding sequence has been associated with attention deficit/hyperactivity disorder. We developed an inducible episomal expression system based on the reverse tetracycline transactivator and Epstein-Barr viral sequences. In HEK293rtTA cells expressing the dopamine D4 receptor from this episomal expression vector, addition of doxycycline in combination with sodium butyrate and trichostatin A induces high levels of receptor expression, resulting in 1970 +/- 20 fmol/mg membrane protein. Addition of the dopamine D4 receptor and serotonin 5-HT2A receptor antagonist pipamperone to these cells further increased the expression of the dopamine receptor, reaching 3800 +/- 60 fmol/mg membrane protein. This up-regulation was not restricted to the dopamine D4 receptor but was also found for the serotonin 5-HT2A receptor. We further provide evidence that the increase in receptor expression is not due to increased mRNA synthesis. As pipamperone could rescue the expression of a folding mutant of the dopamine D4 receptor (M345), we propose that pipamperone acts as a pharmacological chaperone for correct receptor folding thereby resulting in an increased dopamine D4 receptor expression. This study describes a strong and inducible expression system for proteins, difficult to express in other heterologous expression systems. This study also demonstrates that pipamperone, an antipsychotic, acts as a pharmacological chaperone and by doing so, increases the expression level of the dopamine D4 receptor. The fact that ligands can also act as pharmacological chaperones is a fairly new additional element in the regulation of receptor expression levels with potential great impact in drug treatment.

Dopamine agonist therapy of clinically non-functioning pituitary macroadenomas. Is there a role for 123I-epidepride dopamine D2 receptor imaging?

OBJECTIVE

Clinically non-functioning pituitary adenomas (NFPAs) can express functional dopamine D2 receptors. Therapy with dopamine (DA) agonists may result in a NFPA size reduction. However, DA agonist-sensitive and -resistant NFPAs are clinically indistinguishable. We have studied the correlation between in vivo imaging of D2 receptors using (123)I-epidepride and the radiological response of NFPA to DA in 18 patients.

METHODS

Patients were treated with either cabergoline (1-2 mg/week) or quinagolide (150-300 mug/day) for a mean period of 89.7 months (range, 34-187 months).

RESULTS

Pituitary uptake of (123)I-epidepride varied from slight uptake classified as grade 0 to very high classified as grade 3. Grade 0 uptake was found in four patients; grade 1 in three; grade 2 in ten, and grade 3 in one. NFPA stabilization or shrinkage with DA agonist therapy showed no significant difference between grade 0, 1, and 2 tumors (mean tumor stabilization or shrinkage: 31, 30, and 36% respectively). However, when we considered a decrease in tumor size ranging from 0 to 20% as tumor stabilization and >20% decrease in tumor size as true shrinkage, one out of four NFPAs with grade 1 uptake, two out of three with grade 1 uptake, and eight out of ten with grade 2 uptake showed tumor shrinkage.

CONCLUSION

In conclusion, there is limited clinical usefulness of dopamine D2 receptor imaging for predicting the clinical efficacy of DA agonist in selected patients with NFPAs. DA agonist therapy in NFPAs can result in tumor stabilization and shrinkage.

Low-dose dopamine agonist administration blocks vascular endothelial growth factor (VEGF)-mediated vascular hyperpermeability without altering VEGF receptor 2-dependent luteal angiogenesis in a rat ovarian hyperstimulation model

No specific treatment is available for ovarian hyperstimulation syndrome (OHSS), the most important complication in infertile women treated with gonadotropins. OHSS is caused by increased vascular permeability (VP) through ovarian hypersecretion of vascular endothelial growth factor (VEGF)-activating VEGF receptor 2 (VEGFR-2). We previously demonstrated in an OHSS rodent model that increased VP was prevented by inactivating VEGFR-2 with a receptor antagonist (SU5416). However, due to its toxicity (thromboembolism) and disruption of VEGFR-2-dependent angiogenic processes critical for pregnancy, this kind of compound cannot be used clinically to prevent OHSS. Dopamine receptor 2 (Dp-r2) agonists, used in the treatment of human hyperprolactinemia including pregnancy, inhibit VEGFR-2-dependent VP and angiogenesis when administered at high doses in animal cancer models. To test whether VEGFR-2-dependent VP and angiogenesis could be segregated in a dose-dependent fashion with the Dp-r2 agonist cabergoline, a well-established OHSS rat model supplemented with prolactin was used. A 100 microg/kg low-dose Dp-r2 agonist cabergoline reversed VEGFR-2-dependent VP without affecting luteal angiogenesis through partial inhibition of ovarian VEGFR-2 phosphorylation levels. No luteolytic effects (serum progesterone levels and luteal apoptosis unaffected) were observed. Cabergoline administration also did not affect VEGF/VEGFR-2 ovarian mRNA levels. Results in the animal model and the safe clinical profile of Dp-r2 agonists encouraged us to administer cabergoline to oocyte donors at high risk for developing the syndrome. Prophylactic administration of cabergoline (5-10 microg/kg x d) decreased the occurrence of OHSS from 65% (controls) to 25% (treatment). Therefore, a specific, safe treatment for OHSS is now available.

Effects of chronic alcohol and repeated deprivations on dopamine D1 and D2 receptor levels in the extended amygdala of inbred alcohol-preferring rats

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

What has been learnt from study of dopamine receptors in Parkinson's disease?

Since the introduction of dopamine replacement therapy using L-3,4-dihydroxyphenyalanine (L-DOPA) to treat Parkinson's disease and the recognition of the problems associated with L-DOPA use, numerous studies have investigated dopamine receptor regulation and function in Parkinson's disease. These studies have provided insight into the pathological process of the disorder and the molecular consequences of chronic dopaminergic treatment, but they have been less successful in identifying new pharmacological targets or treatment regimes that are as effective as L-DOPA at alleviating the symptoms of Parkinson's disease. This review will present a summary of the reported changes in dopamine receptor regulation and function that occur in Parkinson's disease and will discuss their contribution to the current pharmacological management of Parkinson's disease.

In vivo characterization of the pharmacokinetics and pharmacological properties of [11C]-(+)-PHNO in rats using an intracerebral beta-sensitive system

This study reports on the binding kinetics and pharmacological characterization of [11C]-(+)-PHNO ((+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol), a promising agonist radiotracer for in vivo evaluation of the D2-receptor. Its in vivo kinetics were monitored in rat striatum and cerebellum using a beta-sensitive Microprobe system. Control studies showed that [11C]-(+)-PHNO binding was reversible and reached a peak time equilibrium of specific binding in striatum 30 min after radiotracer injection. The binding potential (BP) calculated by the simplified reference tissue model was 3-fold higher than that measured with [11C]-(-)-NPA (2.14 +/- 0.50 vs. 0.66 +/- 0.01, respectively). In contrast, the methyl analog of (+)-PHNO, [11C]-(+)-MHNO, which displayed promising D2-agonist properties in vitro, showed no specific binding in the striatum in vivo. [11C]-(+)-PHNO binding was totally blocked by raclopride (1 mg/kg; i.v.) and 97% displaced by NPA (2 mg/kg; i.v.) suggesting that [11C]-(+)-PHNO was specific for the high affinity states of D2/D3-receptors. However, (+)-PHNO (1 mg/kg; i.v.) totally blocked and displaced [11C]-raclopride binding in striatum. Thus, (+)-PHNO at high concentrations might be able to bind to the low affinity states of D2/D3-receptors. After an amphetamine pretreatment (2 mg/kg; i.v.), a 69% decrease in BP value (P < 0.05) was observed for [11C]-(+)-PHNO indicating that its binding was highly sensitive to variations of endogenous DA. These results substantiate the use of [11C]-(+)-PHNO as an agonist radiotracer for D2-imaging. The sensitivity of its binding to competition with endogenous DA suggests an association with the subset of high affinity state D2-receptors.

Moderate-level prenatal alcohol exposure alters striatal dopamine system function in rhesus monkeys

BACKGROUND

Moderate prenatal alcohol exposure can cause impairments even in the absence of gross morphological defects associated with fetal alcohol syndrome. The basal ganglia, which include the dopamine-rich striatum, are sensitive to fetal alcohol-induced injury. In this study, we manipulated the timing of moderate-level alcohol exposure and compared the risk of adverse effects on striatal dopamine (DA) system function in rhesus monkeys.

METHODS

Thirty-five young adult rhesus monkeys (Macaca mulatta) from four groups of females were assessed: (1) an early alcohol-exposed group (n=9), in which mothers voluntarily consumed 0.6 g/kg alcohol solution on gestational days 0 through 50; (2) a middle-to-late gestation alcohol-exposed group (n=7), in which mothers voluntarily consumed 0.6 g/kg alcohol solution on gestational days 50 through 135; (3) a continuous-exposure group (n=9), in which mothers voluntarily consumed 0.6 g/kg alcohol solution on days 0 through 135; and (4) controls (n=10), in which mothers voluntarily consumed an isocaloric control solution on gestational days 0 through 50, 50 through 135, or 0 through 135. We studied striatal DA system function by positron emission tomography in separate scans for trapping of [(18)F]fallypride and 6-[(18)F]fluoro-m-tyrosine to assess striatal DA D2 receptor (D2R) binding and DA synthesis, respectively, via dopadecarboxylase activity.

RESULTS

Moderate-level alcohol exposure during early gestation and continuous exposure throughout gestation (early + middle-to-late exposure) reduced the striatal D2R binding to DA synthesis ratio, whereas middle-to-late alcohol gestation exposure increased the striatal D2R binding to DA synthesis ratio. The continuous-exposure group showed the largest effect. Moreover, the D2R binding/DA synthesis ratio was related to neonatal neurobehavior measures in control monkeys, but these relationships were disrupted in the fetal alcohol-exposed monkeys.

CONCLUSION

These results suggest that the vulnerability of the DA system to the effects of moderate doses of alcohol during gestation depend on the timing of the alcohol exposure. Early-gestation moderate alcohol exposure resulted in a reduction or blunting of dopaminergic function in adulthood, whereas middle to late exposure (without early exposure) either induced the opposite pattern or heightened dopaminergic function. Continuously exposed monkeys showed the largest effect, suggesting that the sooner women stop drinking, the better it is for the fetus.

Assessment of methylphenidate-induced changes in binding of continuously infused [(11)C]-raclopride in healthy human subjects: correlation with subjective effects

RATIONALE

The dopaminergic system has been implicated in the pathogenesis and treatment of a variety of neuropsychiatric disorders. It has been shown that information on endogenous dopamine (DA) release can be obtained noninvasively by combining positron emission tomography with a dopaminergic challenge. This approach is based on the assumption that an injected radiolabeled ligand competes with the neurotransmitter for the same receptor. Increases in DA release will therefore result in a decreased binding of the radioligand.

OBJECTIVES

We investigated the effect of the DA reuptake blocker methylphenidate (MP) on the binding of the D(2) receptor ligand [(11)C]-raclopride (RAC).

METHODS

The effect of a 0.25 mg/kg intravenous dose of MP was studied in six healthy volunteers. RAC was administered as a bolus followed by constant infusion, and subjective effects were assessed using verbal rating scales.

RESULTS

Control scans without MP administration showed that the mean RAC binding reached stable values approximately 30 min after start of the infusion. MP administration induced a 24% decrease in RAC binding in the total striatum. Correlations were found between the MP-induced change in euphoria and the percent change in binding potential (DeltaBP) in the dorsal striatum and between baseline anxiety and DeltaBP in the dorsal and middle striatum. We also found a negative correlation between baseline BP in the dorsal striatum and change in euphoria.

CONCLUSIONS

Our results comply with previous findings, indicating the feasibility of the bolus infusion design combined with a relatively low MP dose to study dopaminergic (dys)function.

Measurement of the proportion of D2 receptors configured in state of high affinity for agonists in vivo: a positron emission tomography study using [11C]N-propyl-norapomorphine and [11C]raclopride in baboons

Dopamine D2 receptors are configured in interconvertible states of high (D(2 high)) or low (D(2 low)) affinity for agonists. The in vivo proportion of sites in high-affinity state remains poorly documented. Previous studies have established the D2 agonist [11C]N-propyl-norapomorphine (NPA) as a suitable positron emission tomography radiotracer for imaging D(2 high) in the living brain. To elucidate the proportion of D2 receptors configured in D(2 high) states in vivo, imaging studies were conducted in three baboons with both [11C]NPA and the D2 receptor antagonist [11C]raclopride. These studies were performed under noncarrier- and carrier-added conditions, to compare the Bmax of [11C]NPA and [11C]raclopride in the same animals. [11C]raclopride in vivo KD and Bmax were 1.59 +/- 0.28 nM (n = 3) and 27.3 +/- 3.9 nM (n = 3), respectively. The in vivo KD of [11C]NPA was 0.16 +/- 0.01 nM (n = 3), consistent with its affinity for D(2 high) reported in vitro. The maximal density of sites for [11C]NPA was 21.6 +/- 2.8 nM (n = 3), i.e., 79% of the [11C]raclopride Bmax. This result suggested that 79% of D2 receptors are configured as D(2 high) in vivo. This large proportion of D(2 high) sites might explain the vulnerability of D2 radiotracers to competition by endogenous dopamine, and is consistent with a previous report that the in vivo binding of agonist radiotracer [11C]NPA is more vulnerable to competition by endogenous dopamine than that of antagonist radiotracer [11C]raclopride.

A positron emission tomography (PET) study of cerebral dopamine D2 and serotonine 5-HT2A receptor occupancy in patients treated with cyamemazine (Tercian)

RATIONALE

Cyamemazine (Tercian) is an antipsychotic drug with anxiolytic properties. Recently, an in vitro study showed that cyamemazine possesses high affinity for serotonin 5-HT(2A) receptors, which was fourfold higher than its affinity for dopamine D(2) receptors (Hameg et al. 2003).

OBJECTIVES

The aim of this study is to confirm these previous data in vivo in patients treated with clinically relevant doses of Tercian.

METHODS

Eight patients received 37.5, 75, 150 or 300 mg/day of Tercian depending on their symptomatology. Dopamine D(2) and serotonin 5-HT(2A) receptor occupancies (RO) were assessed at steady-state plasma levels of cyamemazine with positron emission tomography (PET), using [(11)C]raclopride and [(11)C]N-methyl-spiperone, respectively. The effective plasma level of the drug leading to 50% of receptor occupancy was estimated by fitting RO with plasma levels of cyamemazine at the time of the PET scan.

RESULTS

Cyamemazine induced near saturation of 5-HT(2A) receptors (RO=62.1-98.2%) in the frontal cortex even at low plasma levels of the drug. On the contrary, occupancy of striatal D(2) receptors increased with plasma levels, and no saturation was obtained even at high plasma levels (RO=25.2-74.9%). The effective plasma level of cyamemazine leading to 50% of D(2) receptor occupancy was fourfold higher than that for 5-HT(2A) receptors. Accordingly, individual 5-HT(2A)/D(2) RO ratios ranged from 1.26 to 2.68. No patients presented relevant increased prolactin levels, and only mild extrapyramidal side effects were noticed on Simpson and Angus Scale.

CONCLUSION

This in vivo binding study conducted in patients confirms previous in vitro findings indicating that cyamemazine has a higher affinity for serotonin 5-HT(2A) receptors compared to dopamine D(2) receptors. In the dose range 37.5-300 mg, levels of dopamine D(2) occupancy remained below the level for motor side effects observed with typical antipsychotics and is likely to explain the low propensity of the drug to induce extrapyramidal side effects.

Investigation of the use of positron emission tomography for neuroreceptor imaging in rabbit eyes

To determine whether positron emission tomography (PET) can be used for imaging of neuroreceptors in eyes of rabbits. PET imaging of dopamine D(2) receptor, dopamine transporter, serotonin(1A) receptor and sigma(1) receptor in the eyes and brain was performed using corresponding positron-emitting ligands in baseline, pretreatment and displacement conditions. The 4 radioligands outlined the eyes and brain in the baseline. Pretreatment resulted in a slight reduction (26-28%) in the uptake in the anterior segments of eyes. The binding of each radioligand in the iris-ciliary body and retina was confirmed by ex vivo autoradiography. However, the PET signal in the eyes was unexpectedly higher than the autoradiography signal. The identification of radioligand-neuroreceptor binding by PET in the rabbit eyes is not specific enough.

Sex differences and repeated intravenous nicotine: behavioral sensitization and dopamine receptors

The present study examined the sex-dependent expression of behavioral sensitization as well as changes of dopamine (DA) transporters and D1, D2, and D3 receptors following repeated intravenous nicotine administration. Male and female Sprague-Dawley rats were implanted with indwelling jugular catheters, equipped with subcutaneous intravenous injection ports. Rats were habituated to activity chambers for 3 days and were subsequently administered 15-s bolus injections of intravenous nicotine (50 microg/kg/ml) 1/day for 21 days. Animals were placed in activity chambers for 60 min immediately after the 1st and 21st nicotine injection. Observational time sampling was also performed. Brains were subsequently removed and frozen for autoradiographic DA transporter/DA receptor analysis on the afternoon females were in proestrus. With one exception, no robust sex differences were observed for locomotor activity or any rearing measures either during baseline or after initial nicotine injection. Females exhibited markedly more behavioral sensitization of locomotor activity, rearing, duration of rearing, and incidence of observed rearing. There were no sex differences in the number of D1 or D2 receptors. Females exhibited an increased number of DA transporters and decreased D3 receptors in the NAcc, relative to males. Multiple regression analyses suggest that D3 receptors and DA transporters in various striatal and NAcc subregions differentially predicted nicotine-induced behaviors for males and females. Collectively, these findings demonstrate that repeated intravenous nicotine produces sex differences in the expression of behavioral sensitization, and suggest that nicotine-induced changes of DA transporters and D3 receptors are partly responsible for increased behavioral sensitization in female rats.

Pharmacologic resistance in prolactinoma patients

Pharmacologic resistance to dopamine agonists is defined here as failure to normalize PRL levels and failure to decrease macroprolactinoma size by >or=50%. Failure to normalize PRL levels is found in about one-quarter of patients treated with bromocriptine and 10-15% of those treated with pergolide or cabergoline. Failure to achieve at least a 50% reduction in tumor size occurs in about one-third of those treated with bromocriptine and 10-15% of those treated with pergolide or cabergoline. The cause of dopamine resistance is primarily a decrease in D(2) receptors but the receptors have normal affinity for dopamine. Treatment approaches for patients resistant to dopamine agonists include changing to another dopamine agonist and increasing the dose of the drug as long as there is continued response to the dose increases and no adverse effects with higher doses. Transsphenoidal surgery is also an option. Clomiphene, gonadotropins, and GnRH can be used if fertility is desired. For those not desiring fertility, estrogen replacement may be used unless there is a macroadenoma, in which case control of tumor growth is also an issue and dopamine agonists are generally necessary. In many patients modest or even no reduction in tumor size may be acceptable as long as there is not tumor growth. Hormone replacement (estrogen or testosterone) may cause a decrease in efficacy of the dopamine agonist so that it must be carried out cautiously. Reduction of endogenous estrogen, use of selective estrogen receptor modulators, and aromatase inhibitors are potential experimental approaches.

Effect of cabergoline treatment on Cushing's disease caused by aberrant adrenocorticotropin-secreting macroadenoma

The present case involves a 47-yr-old woman with Cushing's disease due to pituitary macroadenoma. The patient had suffered from hypertension and obesity for two yr. Her serum cortisol levels were moderately elevated throughout the observation period, and dexamethasone failed to suppress the cortisol secretion. Plasma ACTH levels were markedly high (>100 pg/ml) and did not respond to CRH provocation. Gel filtration analysis of the patient's plasma detected the existence of big ACTH molecules, which eluted with a peak of authentic 1-39 ACTH. Cranial magnetic resonance imaging (MRI) revealed a 3 cm pituitary tumor occupying the sellar region and right cavernous sinus with diffuse enhancement by gadolinium. The pituitary mass was removed by transsphenoidal surgery, and was pathologically identified as compatible to ACTH-producing pituitary adenoma by immunohistochemistry. RT-PCR analysis of total cellular RNA extracted from the resected adenoma revealed a relatively high expression level of dopamine D2 receptor (D2R) mRNA. Therefore, a long-acting D2R agonist, cabergoline (0.25 to 0.5 mg/week), was administered for the remnant adenoma, which gradually reduced ACTH levels in 90 days. In addition, cranial MRI exhibited shrinkage of the remnant pituitary mass after a 6-month treatment with cabergoline. This case demonstrates the efficacy of cabergoline to treat Cushing's disease caused by pituitary macroadenoma secreting aberrant ACTH molecules.

Correlation between dopamine D(2) receptors in the ventral striatum and central processing of alcohol cues and craving

OBJECTIVE

Alcohol and other drugs of abuse stimulate dopamine release in the ventral striatum, which includes the nucleus accumbens, a core region of the brain reward system, and reinforce substance intake. Chronic alcohol intake is associated with down-regulation of central dopamine D(2) receptors, and delayed recovery of D(2) receptor sensitivity after detoxification is positively correlated with high risk for relapse. Prolonged D(2) receptor dysfunction in the ventral striatum may interfere with a dopamine-dependent error detection signal and bias the brain reward system toward excessive attribution of incentive salience to alcohol-associated stimuli.

METHOD

Multimodal imaging, with the radioligand [(18)F]desmethoxyfallypride and positron emission tomography as well as functional magnetic resonance imaging (fMRI), was used to compare 11 detoxified male alcoholics with 13 healthy men. The authors measured the association of D(2)-like dopamine receptors in the ventral striatum with alcohol craving and central processing of alcohol cues.

RESULTS

Activation of the medial prefrontal cortex and striatum by alcohol-associated stimuli, relative to activation by neutral visual stimuli, was greater in the detoxified alcoholics than in the healthy men. The alcoholics displayed less availability of D(2)-like receptors in the ventral striatum, which was associated with alcohol craving severity and with greater cue-induced activation of the medial prefrontal cortex and anterior cingulate as assessed with fMRI.

DISCUSSION

In alcoholics, dopaminergic dysfunction in the ventral striatum may attribute incentive salience to alcohol-associated stimuli, so that alcohol cues elicit craving and excessive activation of neural networks associated with attention and behavior control.

Molecular, chemical, and anatomical characterization of globus pallidus dopamine D2 receptor mRNA-containing neurons

Essential for normal movement, the globus pallidus (GP) is a prominent nucleus whose neurons project to all other basal ganglia nuclei. The GP is composed of at least two distinct neuron populations. GP neurons of the rodent contain either the calcium-binding protein parvalbumin (PV) or preproenkephalin (PPE) mRNA, differentially innervate several basal ganglia structures, and have distinct immediate early gene responses to dopamine agonists or antagonists. Recent research has revealed that dopamine directly influences GP neurons, with D2 receptors contributing to both pre- and postsynaptic effects of dopaminergic agents. The existence of D2 mRNA-expressing (D2+) GP neurons has been established, but little is known concerning their numbers, regional distribution, or relationship to pallidal subpopulations identified on the basis of PV immunocytochemistry, PPE mRNA, or axonal targets. Detection of pallidal D2 mRNA with a 35S-cRNA probe revealed that D2+ neurons are found throughout the GP, comprising approximately one-half of pallidal neurons, but they are most dense within a dorsoventral band in lateral GP. While a substantial proportion (42-51%) of all chemically and anatomically labeled pallidal neuron subpopulations expressed D2 transcript, the D2+ neurons exhibited both population-based and regional heterogeneities. Overall, the pallidostriatal cells had a greater density of D2 mRNA than did pallidosubthalamic cells. Also, compared to other pallidal regions, the ventromedial GP contained fewer D2+ cells, and the PPE mRNA-expressing cells in this region had lower densities of D2 mRNA per neuron. These results reveal heterogeneous chemical and anatomical properties of the extensive population of D2+ GP neurons, a potential cellular substrate for dopamine's effects in pallidum.

Significance of non-presynaptic SPECT tracer methods in Parkinson's disease

Tropane-related tracers for SPECT and PET scanning have shown their value in assessing the in vivo striatal presynaptic status of the dopaminergic neurotransmitter system in man. Previously, there were a few other tracers suggested to be useful in the study of in vivo brain biochemistry in humans, particularly in patients afflicted with Parkinson's disease (PD) or parkinsonian syndromes. We summarize the work concerning the two main tracers applied in that context using single photon emission tomography (SPECT) scanning in patients with PD: IBZM and HMPAO. IBZM binds specifically to striatal dopamine D(2) receptors and HMPAO uptake is a measure of cerebral tissue perfusion.

A cell-based microarrayed compound screening format for identifying agonists of G-protein-coupled receptors

The identification of agonist and antagonist leads for G-protein-coupled receptors (GPCRs) is of critical importance to the pharmaceutical and biotechnology industries. We report on the utilization of a novel, high-density, well-less screening platform known as microarrayed compound screening microARCS) that tests 8640 compounds in the footprint of a standard microtiter plate for the identification of novel agonists for a specific G-protein-coupled receptor. Although receptors coupled to the G alpha(q) protein can readily be assessed by fluorescence-based Ca(2+) release measurements, many GPCRs that are coupled to G alpha(s) or G alpha(i/o) proteins are not amenable to functional evaluation in such a high-throughput manner. In this study, the human dopamine D(4.4) receptor, which normally couples through the G alpha(i/o) protein to inhibit adenylate cyclase and to reduce levels of intracellular cAMP, was coupled to intracellular Ca(2+) release by stably coexpressing this receptor with a chimeric G(alpha qo5) protein in HEK-293 cells. In microARCS format, the cells expressing D(4.4) receptor and G alpha(qo5) protein were preloaded with fluo-4, cast into a 1% agarose gel, placed above the compound sheets, and imaged successively using a ViewLux charge-coupled device imaging system. Dopamine and other agonists evoked an increase in fluorescence response that appeared as bright spots in a time- and concentration-dependent manner. Utilizing this technology, a library of 260,000 compounds was rapidly screened and led to the identification of several novel agonists. These agonists were further characterized using a fluorometric imaging plate reader assay. Excellent confirmation rates coupled with enhanced efficiency and throughput enable microARCS to serve as an alternative platform for the screening and identification of novel GPCR agonists.

Pharmacological analysis of a dopamine D(2Short):G(alphao) fusion protein expressed in Sf9 cells

A dopamine D(2Short) receptor:G(alphao) fusion protein was expressed in Sf9 cells using the baculovirus expression system. [(3)H]Spiperone bound to D(2Short):G(alphao) with a pK(d) approximately 10. Dopamine stimulated the binding of [(35)S]guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) to D(2Short):G(alphao) expressed with Gbeta(1)gamma(2) (E(max)>460%; pEC(50) 5.43+/-0.06). Most of the putative D(2) antagonists behaved as inverse agonists (suppressing basal [(35)S]GTPgammaS binding) at D(2Short):G(alphao)/Gbeta(1)gamma(2) although (-)-sulpiride and ziprasidone were neutral antagonists. Competition of [(3)H]spiperone binding by dopamine and 10,11-dihydroxy-N-n-propylnorapomorphine revealed two binding sites of different affinities, even in the presence of GTP (100 micro M). The D(2Short):G(alphao) fusion protein is therefore a good model for characterising D(2) receptors.

[Induced expression of dopamine D2 receptor in human neuro-progenitor cell line HNG1210-E]

OBJECTIVE

To investigate whether the dopamine receptor D2 can express or can be induced to express in HNG1210-E, a monoclonal, telomerase-immortalized, human neural progenitor cell line.

METHODS

By means of RT-PCR, immuno-fluorescent staining, and fluo3 Ca2+ imaging to the expression of D2 mRNA and protein as well as the reaction to dopamine were demonstrated.

RESULTS

HNG1210-E cells could be induced to express D2 mRNA and its proteins. The induced cells also reacted to dopamine (5 mmol.L-1), which caused rapid rising of cytoplasm Ca2+.

CONCLUSION

HNG1210-E cells can be induced to express D2 mRNA and functional proteins.