Vesicular neurotransmitter transporters in Huntington's disease: initial observations and comparison with traditional synaptic markers

Markers of identified neuronal populations have previously suggested selective degeneration of projection neurons in Huntington's disease (HD) striatum. Interpretations are, however, limited by effects of compensatory regulation and atrophy. Studies of the vesicular monoamine transporter type-2 (VMAT2) and of the vesicular acetylcholine transporter (VAChT) in experimental animals indicate that they are robust markers of presynaptic integrity and are not subject to regulation. We measured dopamine and acetylcholine vesicular transporters to characterize the selectivity of degeneration in HD striatum. Brains were obtained at autopsy from four HD patients and five controls. Autoradiography was used to quantify radioligand binding to VMAT2, VAChT, the dopamine plasmalemmal transporter (DAT), benzodiazepine (BZ) binding sites, and D2-type dopamine receptors. The activity of choline acetyltransferase (ChAT) was determined as an additional marker of cholinergic neurons. Autoradiograms were analyzed by video-assisted densitometry and assessment of atrophy was made from regional structural areas in the coronal projection. Striatal VMAT2, DAT, and VAChT concentrations were unchanged or increased, while D2 and BZ binding and ChAT activity were decreased in HD. After atrophy correction, all striatal binding sites were decreased. However, the decrease in ChAT activity was 3-fold greater than that of VAChT binding. In addition to degeneration of striatal projection neurons, there are losses of extrinsic nigrostriatal projections and of striatal cholinergic interneurons in HD on the basis of vesicular transporter measures. There is also markedly reduced expression of ChAT by surviving cholinergic striatal interneurons.     

Dopaminhypothese der Schizophrenien Neue Befunde für eine alte Theorie

The dopamine hypothesis of schizophrenia was based on the neuroleptical blockade of central dopamine D2 receptors. Brain imaging studies, however, generally failed to demonstrate a significant increase in central D2 receptors among schizophrenic patients. Using a novel approach, the group of Laruelle and Abi-Dargham was now able to demonstrate that schizophrenic patients have increased synaptic dopamine concentrations in the striatum. Endogenous dopamine competes with a radioligand for binding at dopamine D2 receptors; compared to healthy control subjects, blockade of dopamine production in neuroleptic-naive schizophrenic patients induced a significantly higher increase in D2 binding of the infused radioligand, indicating higher endogenous synaptic dopamine. A similar increase in D2 binding was also observed in drug-free schizophrenics who had previously been treated with neuroleptics; these patients also showed an increased density of striatal D2 receptors, most likely due to counteradaptive upregulation of D2 receptors during neuroleptic medication. The Columbia study provides an important indication of hyperdopaminergic function in schizophrenia.     

Quantification of [11C]FLB 457 binding to extrastriatal dopamine receptors in the human brain.

Positron emission tomography (PET) has hitherto been used to examine D2 dopamine receptor binding in the striatum, a region with a high density of receptors. Research has been hampered by the lack of suitable radioligands for detection of the low-density D2 dopamine receptor populations in the limbic and cortical dopamine systems that are implicated in the pathophysiology of schizophrenia. [11C]FLB 457 is a new radioligand with the very high affinity of 20 pmol/L (K(i)) for the D2 and D3 dopamine receptor subtypes. This study in eight healthy subjects was designed to evaluate the suitability of [11C]FLB 457 for quantification of extrastriatal D2/D3 dopamine receptors. PET-data were acquired in the three-dimensional mode and the arterial input function was corrected for labeled metabolites. The standard three-compartment model and four derived approaches were applied to calculate and compare the binding potentials. Besides the striatum, conspicuous radioactivity was found in extrastriatal regions such as the thalamus, the anterior cinguli, and the temporal and frontal cortices. The time activity curves could be described by the three compartment model. The different approaches gave similar binding potential values and the rank order between regions was consistent with that found in vitro. The short time of a PET measurement using [11C]FLB 457 (63 minutes) seemed not to be sufficient for reliable determination of the high binding potential in the striatum. These results are of principal importance because they show the potential for PET quantification of minute receptor populations in the human brain.     

Striatal dopamine D1 and D2 receptors: widespread influences on methamphetamine-induced dopamine and serotonin neurotoxicity

Methamphetamine (mAMPH) is an addictive psychostimulant drug that releases monoamines through nonexocytotic mechanisms. In animals, binge mAMPH dosing regimens deplete markers for monoamine nerve terminals, for example, dopamine and serotonin transporters (DAT and SERT), in striatum and cerebral cortex. Although the precise mechanism of mAMPH-induced damage to monoaminergic nerve terminals is uncertain, both dopamine D1 and D2 receptors are known to be important. Systemic administration of dopamine D1 or D2 receptor antagonists to rodents prevents mAMPH-induced damage to striatal dopamine nerve terminals. Because these studies employed systemic antagonist administration, the specific brain regions involved remain to be elucidated. The present study examined the contribution of dopamine D1 and D2 receptors in striatum to mAMPH-induced DAT and SERT neurotoxicities. In this experiment, either the dopamine D1 antagonist, SCH23390, or the dopamine D2 receptor antagonist, sulpiride, was intrastriatally infused during a binge mAMPH regimen. Striatal DAT and cortical, hippocampal, and amygdalar SERT were assessed as markers of mAMPH-induced neurotoxicity 1 week following binge mAMPH administration. Blockade of striatal dopamine D1 or D2 receptors during an otherwise neurotoxic binge mAMPH regimen produced widespread protection against mAMPH-induced striatal DAT loss and cortical, hippocampal, and amygdalar SERT loss. This study demonstrates that (1) dopamine D1 and D2 receptors in striatum, like nigral D1 receptors, are needed for mAMPH-induced striatal DAT reductions, (2) these same receptors are needed for mAMPH-induced SERT loss, and (3) these widespread influences of striatal dopamine receptor antagonists are likely attributable to circuits connecting basal ganglia to thalamus and cortex.     

Striatal D1 and D2 dopamine receptor loss in asymptomatic mutation carriers of Huntington's disease

We have investigated striatal dopamine D1 and D2 receptor binding in asymptomatic subjects from Huntington's disease (HD) families using positron emission tomography. Nineteen adult subjects at risk of developing HD were scanned with ¹¹C-SCH 23390 and ¹¹C-raclopride to calculate the D1 and D2 receptor binding potential, respectively. Eight of the 19 were shown to have the HD mutation; of these, 4 subjects had significant reductions in striatal dopamine receptor binding. Abnormalities were more common in older subjects and were not correlated with the size of the HD mutation. There was a strong coefficient of correlation between individual levels of striatal D1 and D2 binding in subjects with the mutation. Of 6 other cases with a 50% risk of carrying the HD gene, 1 showed subclinical loss of caudate and putamen D2 binding. Our study suggests that both striatal D1 and D2 dopamine receptors are lost in parallel from both cauudate and putamen in presymptomatic HD and that dopamine receptor binding provides a sensitive means of detecting subclinical striatal dysfunction.     

11C]NNC 112 selectivity for dopamine D1 and serotonin 5-HT(2A) receptors: a PET study in healthy human subjects.

The dopamine D(1) receptor antagonist radioligand [(11)C]NNC 112 has previously been reported to have 100-fold selectivity for the D(1) receptor compared with the serotonin 5-HT(2A) receptor. In this study, we tested the selectivity by scanning seven healthy human research volunteers with [(11)C]NNC 112 before and after 2 mg of the antipsychotic drug risperidone, a dose that putatively blocks all 5-HT(2A) receptors with negligible effect on D(1) receptors. We found that specific binding in cortical regions was reduced by 20% to 30%, whereas the striatum showed no change. Based on the known relative densities of these receptors in humans, our results suggest 5- to 10-fold selectivity of [(11)C]NNC 112 for D(1) versus 5-HT(2A) as opposed to 100-fold selectivity. These results suggest caution in interpreting data from studies using this tracer to measure cortical D(1) receptors as well as the need for more selective radioligands to assess cortical D(1) receptors.     

The problem of antipsychotic treatment for functional imaging in Huntington's disease: receptor binding, gene expression and locomotor activity after sub-chronic administration and wash-out of haloperidol in the rat

It has been demonstrated that withdrawal from chronic treatment with haloperidol is associated with a long-lasting increase in the number of striatal dopamine D(2) receptors and variable changes in D(1) receptors. We have investigated the effect of withdrawal from sub-chronic administration of haloperidol on the density of dopamine receptors, dopamine receptor gene expression, and spontaneous locomotor activity. Following a 3-week treatment period with haloperidol (1.5 mg/kg, i.p.), spontaneous locomotor activity measurements, autoradiography of D(1) and D(2) receptors and in situ hybridisation histochemistry of D(1) and D(2) mRNA were performed. Using [3H]raclopride as the ligand, sub-chronic haloperidol administration produced a robust upregulation in D(2) binding in the striatum of rats which correlated with parallel increases in spontaneous locomotor activity from 24 h to 4 weeks. Using, [3H]SCH23390 as the ligand, D(1) binding was largely unaffected by the drug treatment. Non-significant changes were measured in the striatal expression of D(1) receptor mRNA or the nigral or striatal expression of D(2) receptor mRNA. Our findings have implications for the use of dopaminergic ligands in positron emission tomography (PET) imaging of patients under regimens of chronic neuroleptics in particular in the context of forthcoming trials of neural grafts in Huntington's disease (HD) striatum.     

Striatal dopamine D2 receptors in tardive dyskinesia: PET study.

The dopamine D2 receptors were investigated in vivo in eight neuroleptic-free patients with persistent tardive dyskinesia using positron emission tomography and 76Br-bromospiperone. The striatal receptor density, estimated by the striatum/cerebellum radioligand concentration ratio, was not elevated in patients as compared with age-matched controls but was positively correlated with the severity of orofacial dyskinesia assessed with the Abnormal Involuntary Movement Rating Scale. These results indicate that tardive dyskinesia is associated with normal levels of striatal D2 receptors but the severity of orofacial movements may depend on the relative density of striatal D2 receptors.     

Cortically driven Fos induction in the striatum is amplified by local dopamine D2-class receptor blockade

Dopamine D2-class receptors have been shown to control the excitability of striatal neurons in response to cortical activation. It has been unclear, however, whether such receptors could regulate the number of striatal neurons activated by cortical stimulation, and thus affect the population response of the striatum to its cortical inputs. We used Fos induction as a readout to measure the ensemble response of striatal neurons to localized stimulation of the frontal cortex and tested for the effects of D2-class dopamine receptor blockade on this response. In freely moving rats, we stimulated the frontal cortex by local epidural application of a dose of a GABAA receptor antagonist (picrotoxin) just threshold for inducing Fos in the striatum. We combined this treatment with D2-class dopamine receptor antagonist treatments at dose levels also just threshold for inducing Fos, using either (i) systemic haloperidol or (ii) intrastriatal (-)sulpiride. Both systemic and intrastriatal blockade of D2-class receptors sharply increased the numbers of striatal neurons exhibiting cortically evoked Fos induction. These findings suggest that local activation of intrastriatal D2-class dopamine receptors can regulate the number of striatal neurons responsive to cortical inputs, thus dynamically shaping the flow of information through the striatum.     

Development of D1 and D2 dopamine receptors and associated second messenger systems in fetal striatal transplants.

Stereotactic implantation of fetal brain regional anlage into adult host brain ("brain transplantation") appears to be an increasingly viable strategy for therapy of neurodegenerative diseases. We have studied implantation of fetal striatum into adult striatum, previously lesioned by neurotoxic amino acid injection, as a model for transplantation therapy of Huntington's disease. The beginning of behavioral recovery to apomorphine is not apparent until 6.5 months after implantation. By 4 months after implantation cerebral blood flow through the implants appears equal to that in the intact contralateral striatum. At this time, cerebral glucose utilization is reduced in the implants but increases following apomorphine treatment. The development of D1 and D2 dopamine (DA) receptors is markedly deficient in the striatal grafts at both 4 and 6.5 months after implantation. Very little D2 radioligand binding was observed in the grafts at either time point; D1 receptors appeared in a patchy fashion by 6.5 months at densities approaching normal striatum. In situ hybridization of D2 dopamine receptor mRNA demonstrated robust hybridization signal in normal striatum and accumbens but no signal in 6.5-month-old striatal grafts. Adenylate cyclase (AC) activity, examined with high-affinity [3H]forskolin binding, also appeared in patches similar to D1 receptors at 6.5 months. In contrast, protein kinase C activity, labeled with [3H]phorbol ester, was very apparent in the grafts at both time points. Higher and generally homogenous densities of muscarinic cholinergic receptors, assessed with [3H]QNB binding, develop in the grafts, but there appear to be few functioning cholinergic terminals, as measured by [3H]hemicholinium binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dopamine receptor properties in Parkinson''s disease and Huntington''s chorea evaluated by positron emission tomography using 11C-N-methyl-spiperone

Dopaminergic receptor properties in the striatum of patients with Parkinson's disease (PD) and Huntington's chorea (HD) were studied by positron emission tomography (PET), using 11C-N-methyl-spiperone as a dopamine D2 receptor ligand. The time-dependent regional radioactive uptake in the caudate nucleus and the putamen was measured and fitted to a 3-compartment pharmacokinetic model. The rate constant k3 for specific binding to the receptor compartment in the striatum was determined in relation to the binding in regions with a low density of specific binding sites, such as the cerebellum and the frontal cortex . k3, which is a measure of the receptor density, was reduced in one patient with HD but less affected in PD in comparison with healthy controls. The pattern of k3 values calculated from the 6 PD patients is discussed in relation to any side-to-side differences in dopamine receptor densities in hemiparkinsonism and to possible "hypersensitivity" of dopamine receptors in the early stage of the disease and down-regulation in more advanced disease.     

Cortical dopamine dysfunction in symptomatic and premanifest Huntington's disease gene carriers

We used ¹¹C-raclopride PET, a marker of D₂ dopamine receptor binding, and statistical parametric mapping (SPM) to localise cortical D₂ receptor dysfunction in individual Huntington's disease (HD) gene carriers (16 symptomatic and 11 premanifest subjects) and assess its clinical significance.62.5% of symptomatic HD patients and 54.5% of premanifest carriers showed cortical reductions in D₂ binding. The most frequent decreases in cortical binding in individual HD subjects were seen in temporal and frontal areas. Symptomatic HD subjects with decreased cortical D₂ binding had worse scores on neuropsychological tests assessing attention and executive functions than subjects without cortical dopamine dysfunction, notwithstanding comparable reduction in striatal D₂ binding and motor disability. Our results indicate that cortical dopaminergic dysfunction is common in both symptomatic and premanifest HD gene carriers. It is an early event in HD pathophysiology and could contribute to the impairment in neuropsychological performance in these patients.     

Alterations in dopamine and benzodiazepine receptor binding precede overt neuronal pathology in mice modelling early Huntington disease pathogenesis

Huntington disease (HD) is an inherited, late onset, progressive neurodegenerative disorder. Primary degeneration appears to selectively occur in striatal medium spiny neurones but this is most likely preceded by a period of neuronal dysfunction. Altered levels of neurotransmitter receptors may disrupt neuronal function and contribute to a toxic environment within the brain. In the present study, a knock-in HD mouse modelling early stages of the disease was used to determine whether alterations in neurotransmitter receptor densities occurred before overt neuronal loss. Receptor autoradiography demonstrated reduced dopamine D2 and increased benzodiazepine receptor binding in the striatum of HD animals compared to wild-type littermates. The density of benzodiazepine receptor binding was also increased in the cerebral cortex of the HD mice. Changes in opioid and dopamine D1 receptor densities were more subtle and influenced by the genetic background of the mice. Our findings are consistent with the hypothesis that alterations in neurotransmitter receptor density precede cell loss and may be an active cellular response to the initial stages of HD pathogenesis.     

No support for regional selectivity in clozapine-treated patients: a PET study with [(11)C]raclopride and [(11)C]FLB 457

OBJECTIVE: The authors' goal was to test the hypothesis of extrastriatal D(2) receptor selectivity as the mechanism of action of clozapine. METHOD: Positron emission tomography (PET) was used to examine extrastriatal as well as striatal dopamine D(2) receptor occupancy in four patients treated with clozapine and three patients treated with haloperidol. The reference radioligand [(11)C]raclopride was used for determination of D(2) receptor occupancy in the striatum. The radioligand [(11)C]FLB 457 was chosen for determination of D(2) receptor occupancy in the thalamus, the temporal cortex, and the frontal cortex. RESULTS: In patients treated with haloperidol the D(2) receptor occupancy was high in all examined brain regions. In clozapine-treated patients the D(2) receptor occupancy was relatively low in both the striatum and the extrastriatal regions. CONCLUSIONS: The results from the present study give no support for the hypothesis of regional selectivity as the mechanism of action for clozapine.     

Spinocerebellar ataxia type 1, 2, and 3 and restless legs syndrome: striatal dopamine D2 receptor status investigated by [11C]raclopride positron emission tomography.

In spinocerebellar ataxias (SCAs), up to 30% of patients complain of restless legs syndrome (RLS). In primary RLS, a putative role of the dopaminergic system has been postulated. To assess dopaminergic function in SCA1, 2, and 3, dopamine D(2) receptor binding potential (BP) was assessed by [(11)C]raclopride positron emission tomography in 10 SCA patients, 4 of whom suffered from RLS as demonstrated by polysomnography. BP was compared to 9 age-matched control subjects. In 2 SCA patients, striatal BP was clearly reduced (<2 SD below the mean of controls). However, there were no significant group differences between SCA and controls, largely owing to a significantly higher variance of striatal BP in SCA. BP was negatively correlated with disease duration. The fit suggests an increased BP in early stages, followed by a moderate decline in all quantified regions (caudate, dorsal putamen, ventral striatum) presumably reflecting a progressive loss of D(2) receptors. RLS in SCA was not accompanied by a significant reduction of D(2) receptor availability in the striatum. This missing correlation may point to an extrastriatal origin of RLS.     

Asymmetrical changes of dopamine receptors in the striatum after unilateral dopamine depletion

Dopamine plays an important role in modulating synaptic transmission in the striatum and has great influence on the function of the basal ganglia. Degeneration of dopamine neurons in the substantia nigra (SN) is the major cause of many neurological disorders, and the reduction of dopamine innervation results in alterations of dopamine receptors in the striatum. It has been shown that the nigrostriatal dopamine system has functional and neurochemical asymmetry. To investigate the lateralization of dopamine receptors in the striatum after dopamine denervation, the present study used quantitative autoradiography to compare the changes in dopamine receptor binding in the left and right striatum in rats after unilateral dopamine depletion. In comparison to control levels, dopamine D1)-like receptor binding, labeled with [3H]-SCH23390, in the dorsal striatum was reduced 2 weeks after unilateral lesions of the SN with 6-hydroxydopamine. D1-like receptor binding was decreased in the ipsilateral striatum following unilateral lesions of either the left or right SN. The left and right striatum responded similarly to unilateral SN lesions, as there were no significant differences in the percent decrease in D1-like binding in the two striata. In contrast, D2-like receptor binding, labeled with [3H]-spiroperidol, was significantly increased in the dorsal striatum following an ipsilateral SN lesion. Furthermore, the up-regulation of D2-like receptors in the right striatum was significantly greater than that in the left striatum after an ipsilateral lesion. The asymmetrical up-regulation of striatal D2 receptors after extensive dopamine depletion might contribute to the lateralization of the nigrostriatal system observed in some pathological conditions.     

D2 dopamine receptors in the rat prefrontal cortex: characterization and alteration by stress

D2 dopamine (DA) receptors were characterized in the medial prefrontal cortex (MPFC) of the rat by employing radioligand binding techniques which greatly reduced [3H]spiperone binding to filters, S2 serotonin receptors and spirodecanone sites. Competition studies suggested that the MPFC contains a higher proportion of D2 [high] receptors than does the striatum. The IC50 values of DA receptor antagonists in assays of MPFC tissue were correlated with their antipsychotic potencies. Stress (footshock or tailshock) increased the density of MPFC D2 dopamine receptors and decreased their affinity for [3H]spiperone. These changes were delayed in onset, being present at 27 h but not at 3 h after exposure to stress. The binding assay detected the changes when it was performed at pH 7.9 but not when the pH was reduced to 6.2. D2 dopamine receptors in the striatum and nucleus accumbens were not affected by stress.     

Is regionally selective D2/D3 dopamine occupancy sufficient for atypical antipsychotic effect? an in vivo quantitative [123I]epidepride SPET study of amisulpride-treated patients

OBJECTIVE: Atypical antipsychotic drug treatment is clinically effective with a low risk of extrapyramidal symptoms. Explanations for the mechanism underlying this beneficial therapeutic profile of atypical over typical antipsychotic agents include 1) simultaneous antagonism of dopamine D(2) and serotonin 5-HT(2A) receptors or 2) selective action at limbic cortical dopamine D(2)-like receptors with modest striatal D(2) receptor occupancy. Amisulpride is an atypical antipsychotic drug with selective affinity for D(2)/D(3) dopamine receptors and provides a useful pharmacological model for examining these hypotheses. The authors' goal was to evaluate whether treatment with amisulpride results in "limbic selective" D(2)/D(3) receptor blockade in vivo. METHOD: Five hours of dynamic single photon emission tomography data were acquired after injection of [(123)I]epidepride (approximately 150 MBq). Kinetic modeling was performed by using the simplified reference region model to obtain binding potential values. Estimates of receptor occupancy were made relative to a healthy volunteer comparison group (N=6). RESULTS: Eight amisulpride-treated patients (mean dose=406 mg/day) showed moderate levels of D(2)/D(3) receptor occupancy in the striatum (56%), and significantly higher levels were seen in the thalamus (78%) and temporal cortex (82%). CONCLUSIONS: Treatment with amisulpride results in a similar pattern of limbic cortical over striatal D(2)/D(3) receptor blockade to that of other atypical antipsychotic drugs. This finding suggests that modest striatal D(2) receptor occupancy and preferential occupancy of limbic cortical dopamine D(2)/D(3) receptors may be sufficient to explain the therapeutic efficacy and low extrapyramidal symptom profile of atypical antipsychotic drugs, without the need for 5-HT(2A) receptor antagonism.     

Dopamine D2 receptor SPECT in corticobasal syndrome and autopsy-confirmed corticobasal degeneration

BackgroundCorticobasal degeneration (CBD) is a rare neurodegenerative disorder characterized by tau-positive neuronal and glial lesions in the cortex and striatum with neuronal loss in cortical regions and in the substantia nigra. Striatal dopamine D2 receptor binding in autopsy-confirmed CBD has not been studied before.MethodsWe performed D2 receptor single photon emission computerized tomography using ¹²³I-IBZM in nine patients with a clinically diagnosed corticobasal syndrome (CBS) and on ten healthy controls. Two of the patients subsequently came to autopsy and were diagnosed with CBD.ResultsOverall striatal D2 receptor binding was preserved in 8/9 patients, but more asymmetric than in controls. Overall striatal binding in pathologically confirmed CBD was reduced in one case and normal in the other, and was lower contralateral to the clinically more affected side in both.ConclusionThis first study on D2 receptor imaging in autopsy-confirmed CBD demonstrates that loss of postsynaptic striatal neurons in CBD is a variable finding. Given the heterogeneity of our findings in pathology-confirmed cases, D2 receptor imaging seems to be of little practical value in the diagnostic workup of patients with CBS.