Positron emission tomographic analysis of central D1 and D2 dopamine receptor occupancy in patients treated with classical neuroleptics and clozapine. Relation to extrapyramidal side effects

Positron emission tomography and selective radioligands were used to determine D1 and D2 dopamine receptor occupancy induced by neuroleptics in the basal ganglia of drug-treated schizophrenic patients. In 22 patients treated with conventional dosages of classical neuroleptics, the D2 occupancy was 70% to 89%. Patients with acute extrapyramidal syndromes had a higher D2 occupancy than those without side effects. This finding indicates that neuroleptic-induced extrapyramidal syndromes are related to the degree of central D2 occupancy induced in the basal ganglia. In five patients treated with clozapine, the prototype atypical antipsychotic drug, a lower D2 occupancy of 38% to 63% was found. This finding demonstrates that clozapine is also "atypical" with respect to the central D2 occupancy in patients. During treatment with clozapine, there is a low frequency of extrapyramidal syndromes, which accordingly may reflect the comparatively low D2 occupancy induced by clinical doses of clozapine. Classical neuroleptics, like haloperidol or sulpiride, did not cause any evident D1 occupancy, but the thioxanthene flupentixol induced a 36% to 44% occupancy. In four patients treated with clozapine, the D1 occupancy was 38% to 52%. The D1 occupancy induced by clozapine and flupentixol may contribute to the antipsychotic effect of these drugs.

Quantitative analysis of D2 dopamine receptor binding in the living human brain by PET

D2 dopamine receptors in the putamen of living human subjects were characterized by using the selective, high-affinity D2 dopamine receptor antagonist carbon-11-labeled raclopride and positron emission tomography. Experiments in four healthy men demonstrated saturability of [11C]raclopride binding to an apparently homogeneous population of sites with Hill coefficients close to unity. In the normal putamen, maximum binding ranged from 12 to 17 picomoles per cubic centimeter and dissociation constants from 3.4 to 4.7 nanomolar. Maximum binding for human putamen at autopsy was 15 picomoles per cubic centimeter. Studies of [11C]raclopride binding indicate that clinically effective doses of chemically distinct neuroleptic drugs result in 85 to 90 percent occupancy of D2 dopamine receptors in the putamen of schizophrenic patients.

Polymorphisms in the dopamine D2 receptor gene and their relationships to striatal dopamine receptor density of healthy volunteers

The density of striatal dopamine D2 receptors has been shown to vary considerably among healthy subjects. This variability might be due to genetic or environmental factors. In the present analysis we searched for relationships between dopamine D2 receptor gene (DRD2) polymorphisms and striatal dopamine D2 receptor density in vivo, as measured by positron emission tomography and [11C]raclopride in 56 healthy subjects. There was a significant association between presence of a putative functional DRD2 promoter allele (-141C Del) and high striatal dopamine receptor density (t= 2.32, P= 0.02). In agreement with some previous studies the presence of the DRD2 TaqIA1 allele was associated with measures of low dopamine receptor density (t=2.58, P=0.01). Also the DRD2 TaqIB1 allele was associated with low dopamine receptor density (t= 2.58, P= 0.01) wheras there was no significant relationship between another common silent intronic DRD2 short tandem repeat polymorphism (STRP) and striatal dopamine D2 receptor density. The results suggest that DRD2 genotypes may participate differentially in the regulation of striatal dopamine D2 receptor density in healthy human subjects. The results should be interpreted with caution because of the limited sample size.

The correlative triad among aging, dopamine, and cognition: Current status and future prospects

The brain neuronal systems defined by the neurotransmitter dopamine (DA) have since long a recognized role in the regulation of motor functions. More recently, converging evidence from patient studies, animal research, pharmacological intervention, and molecular genetics indicates that DA is critically implicated also in higher-order cognitive functioning. Many cognitive functions and multiple markers of striatal and extrastriatal DA systems decline across adulthood and aging. Research examining the correlative triad among adult age, DA, and cognition has found strong support for the view that age-related DA losses are associated with age-related cognitive deficits. Future research strategies for examining the DA-cognitive aging link include assessing (a) the generality/specificity of the effects; (b) the relationship between neuromodulation and functional brain activation; and (c) the release of DA during actual task performance.

D2 dopamine receptors in neuroleptic-naive schizophrenic patients. A positron emission tomography study with [11C]raclopride

Several groups have reported increased densities of D2 dopamine receptors in the basal ganglia of schizophrenic brains postmortem. The significance of this finding has been questioned, since an upregulation of receptor number may be a neuronal response to neuroleptic drug treatment. We have used positron emission tomography and [11C]raclopride to examine central D2 dopamine receptor binding in 20 healthy subjects and 18 newly admitted, young, neuroleptic-naive patients with schizophrenia. An in vivo saturation procedure was applied for quantitative determination of D2 dopamine receptor density (Bmax) and affinity (Kd). When the two groups were compared, no significant difference in Bmax or Kd values was found in the putamen or the caudate nucleus. The hypothesis of generally elevated central D2 dopamine receptor densities in schizophrenia was thus not supported by the present findings. In the patients but not in the healthy controls, significantly higher densities were found in the left than in the right putamen but not in the caudate nucleus.

Central D2-dopamine receptor occupancy in schizophrenic patients treated with antipsychotic drugs

Using positron emission tomography and the carbon 11-labeled ligand raclopride, central D2-dopamine receptor occupancy in the putamen was determined in psychiatric patients treated with clinical doses of psychoactive drugs. Receptor occupancy in drug-treated patients was defined as the percent reduction of specific carbon 11-raclopride binding in relation to the expected binding in the absence of drug treatment. Clinical treatment of schizophrenic patients with 11 chemically distinct antipsychotic drugs (including both classic and atypical neuroleptics such as clozapine) resulted in a 65% to 85% occupancy of D2-dopamine receptors. In a depressed patient treated with the tricyclic antidepressant nortriptyline, no occupancy was found. The time course for receptor occupancy and drug levels was followed after withdrawal of sulpiride or haloperidol. D2-dopamine receptor occupancy remained above 65% for many hours despite a substantial reduction of serum drug concentrations. In a sulpiride-treated patient, the dosage was reduced in four steps over a nine-week period and a curvilinear relationship was demonstrated between central D2-dopamine receptor occupancy and serum drug concentrations. The results demonstrate that clinical doses of all the currently used classes of antipsychotic drugs cause a substantial blockade of central D2-dopamine receptors in humans. This effect appears to be selective for the antipsychotics, since it was not induced by the antidepressant nortriptyline.

Kinetic analysis of central [11C]raclopride binding to D2-dopamine receptors studied by PET--a comparison to the equilibrium analysis

[11C]Raclopride binding to central D2-dopamine receptors in humans has previously been examined by positron emission tomography (PET). Based on the rapid occurrence of binding equilibrium, a saturation analysis has been developed for the determination of receptor density (Bmax) and affinity (Kd). For analysis of PET measurements obtained with other ligands, a kinetic three-compartment model has been used. In the present study, the brain uptake of [11C]raclopride was analyzed further by applying both a kinetic and an equilibrium analysis to data obtained from four PET experiments in each of three healthy subjects. First regional CBV was determined. In the second and third experiment, [11C]-raclopride with high and low specific activity was used. In a fourth experiment, the [11C]raclopride enantiomer [11C]FLB472 was used to examine the concentration of free radioligand and nonspecific binding in brain. Radio-activity in arterial blood was measured using an automated blood sampling system. Bmax and Kd values for [11C]raclopride binding could be determined also with the kinetic analysis. As expected theoretically, those values were similar to those obtained with the equilibrium analysis. In addition, the kinetic analysis allowed separate determination of the association and dissociation rate constants, kon and koff, respectively. Examination of [11C]raclopride and [11C]FLB472 uptake in brain regions devoid of specific D2-dopamine receptor binding indicated a fourth compartment in which uptake was reversible, nonstereoselective, and nonsaturable in the dose range studied.

Changes in cortical dopamine D1 receptor binding associated with cognitive training

Working memory is a key function for human cognition, dependent on adequate dopamine neurotransmission. Here we show that the training of working memory, which improves working memory capacity, is associated with changes in the density of cortical dopamine D1 receptors. Fourteen hours of training over 5 weeks was associated with changes in both prefrontal and parietal D1 binding potential. This plasticity of the dopamine D1 receptor system demonstrates a reciprocal interplay between mental activity and brain biochemistry in vivo.

Central D2-dopamine receptor occupancy in relation to antipsychotic drug effects: a double-blind PET study of schizophrenic patients

The relationship between central D2-dopamine receptor occupancy and antipsychotic drug effects was examined in a double-blind study. Raclopride was the compound used to induce a selective occupancy of the D2-dopamine receptors. In addition, 11C-labeled raclopride was the radioligand used to measure occupancy by positron emission tomography (PET). Seventeen schizophrenic patients were randomly assigned to one of three parallel groups treated for 4 weeks with daily doses of 2, 6, or 12 mg of raclopride. D2-receptor occupancy was determined by PET at steady-state conditions in 13 patients who completed the study. A statistically significant relationship was demonstrated between antipsychotic effect and degree of D2-receptor occupancy (p < 0.05). Patients with extrapyramidal side effects had significantly higher D2-receptor occupancy than those without (p = 0.02). The finding of a relationship between selective occupancy of the D2-dopamine receptors and clinical effects in schizophrenic patients principally provides new support for the dopamine hypothesis of antipsychotic drug action.

Distribution of D1- and D2-dopamine receptors, and dopamine and its metabolites in the human brain

Densities and distribution of D1-dopamine and D2-dopamine receptors were investigated in vitro using [3H]SCH 23390 and [3H]raclopride in receptor binding assays and autoradiography on human post mortem whole hemisphere slices to serve as anatomical correlates to PET studies using [11C]SCH 23390 and [11C]raclopride. In addition, the levels of dopamine and its metabolites were determined by HPLC in various brain regions. Both dopamine receptor subtypes, as well as dopamine, HVA and DOPAC, were primarily found in the basal ganglia. Very high densities of D1-dopamine receptors were found particularly in the medial caudate nucleus, whereas D2-dopamine receptors were evenly distributed throughout the caudate. The densities of D1- and D2-dopamine receptors were similar in the caudate nucleus and the putamen, whereas there were 4 to 7 times higher densities of the D1- than of the D2-dopamine receptors in several limbic and neocortical regions. The receptor distribution in the autoradiographic study was consistent with that demonstrated in the living human brain using [11C]SCH 23390 and [11C]raclopride.

Age-related dopamine D2/D3 receptor loss in extrastriatal regions of the human brain

Loss of dopamine D2-like receptors in the striatum has been associated with both normal human aging and impairment of cognitive and motor functions in the elderly. To investigate whether there are age-associated changes in dopamine D2 and D3 receptor subtypes (D2/3Rs) outside the striatum, a D2/3R selective high-affinity radioligand [11C]FLB 457 was used in positron emission tomography (PET) examinations for 24 normal healthy male subjects (age range 19-74 years). Significant age-related declines of D2/3Rs were detected in all the brain regions studied: the anterior cingulate cortex (decline of 13% per increase of a decade in age, P < 0.001). the frontal cortex (11%, P < 0.001), the lateral temporal cortex (10%, P < 0.001), the hippocampus (10%, P < 0.01), the medial temporal cortex (9%, P < 0.001), the amygdala (7%, P < 0.01), the medial thalamus (6%, P < 0.001) and the lateral thalamus (5%, P < 0.01). The rate of D2/3R decline was significantly faster in the frontal cortex as compared to the medial temporal cortex (P < 0.05, Bonferroni corrected) and as compared to the medial thalamus (P < 0.05, Bonferroni corrected). These results indicate that the previously demonstrated age-related decline in striatal dopamine D2 receptors extends to several extrastriatal regions in normal human males. Further, the rate of D2/3R decline may be faster in the frontal cortex as compared to the temporal and thalamic regions.

Substituted benzamides as ligands for visualization of dopamine receptor binding in the human brain by positron emission tomography

Two substituted benzamides, FLB 524 and raclopride, were labeled with 11C and examined for their possible use as ligands for positron emission tomography (PET)-scan studies on dopamine-2 (D-2) receptors in the brains of monkeys and healthy human subjects. Both ligands allowed the in vivo visualization of D-2 receptor binding in the corpus striatum caudate nucleus/putamen complex in PET-scan images. [11C]Raclopride showed a high ratio of specific striatal to nonspecific cerebellar binding, and the kinetics of binding of this ligand made it optimal for PET studies. The in vivo binding of [11C]raclopride in the striatum of cynomolgus monkeys was markedly reduced by displacement with haloperidol. This and previous in vitro data indicate that [11C]raclopride binds selectively to striatal D-2 dopamine receptors. In healthy human subjects, [11C]raclopride binding in the caudate nucleus/putamen was 4- to 5-fold greater than nonspecific binding in the cerebellum. In comparison with previously available ligands for PET-scan studies on central dopamine receptors in man, [11C]raclopride appears to be advantageous with regard to (i) specificity of binding to D-2 receptors, (ii) the high ratio between binding in dopamine-rich (caudate, putamen) and dopamine-poor (cerebellum) human brain regions, and (iii) rapid association and reversibility of specific binding. [11C]Raclopride should be a valuable tool for characterizing D-2 receptors in the brains of patients with neuropsychiatric disorders.

D1- and D2-dopamine receptor occupancy during treatment with conventional and atypical neuroleptics

Using positron emission tomography and the selective ligands 11C-SCH23390 and 11C-raclopride, central D1- and D2-dopamine receptor occupancy was determined in schizophrenic patients treated with clinical doses of classical and atypical neuroleptics. Treatment with ten chemically distinct classical neuroleptics resulted in a 65-89% occupancy of D2-dopamine receptors. This finding represents strong support for the hypothesis that the mechanism of action of antipsychotic drugs is indeed related to a substantial degree of D2-dopamine receptor occupancy. In two patients treated with the atypical neuroleptic clozapine, 300 mg b.i.d. and 150 mg b.i.d., the D2-dopamine receptor occupancy was 65 and 40%, respectively. D1-dopamine receptor occupancy was determined in six antipsychotic drug-treated patients. No D1-dopamine receptor occupancy was found in patients treated with sulpiride and perphenazine, compounds known to be selective D2-dopamine receptor antagonists. The highest D1-dopamine receptor occupancy, 42%, was found in the patient treated with clozapine 150 mg b.i.d. The effects of the atypical neuroleptic clozapine may be related to a combined effect on both D1- and D2-dopamine receptors.

PET analysis of human dopamine receptor subtypes using 11C-SCH 23390 and 11C-raclopride

Tracer doses of 11C-SCH 23390 and 11C-raclopride, selective D1-dopamine and D2-dopamine receptor antagonists, respectively, were injected intravenously into three healthy male volunteers and two drug-treated schizophrenic patients. Regional radioactivity in brain and plasma was followed during 1 h by positron emission tomography (PET). After injection of both ligands a high accumulation of radioactivity was observed in the dopamine-rich caudate putamen. Experiments with 11C-SCH 23390, but not 11C-raclopride, showed a conspicuous accumulation of radioactivity also in the neocortex. None of the ligands accumulated in the dopamine-poor cerebellum. Specific binding of 11C-raclopride in the putamen was reduced by more than 80% in schizophrenic patients treated with antipsychotic doses of sulpiride or cis(Z)-flupentixol decanoate. 11C-SCH 23390 binding was slightly reduced in both the cortex and the putamen after treatment with cis(Z)-flupentixol decanoate but not after sulpiride. The results indicate that clinical antipsychotic drug treatment with sulpiride and cis(Z)-flupentixol decanoate causes a substantial blockade of D2-dopamine receptors in the basal ganglia but has only a minor effect on D1-dopamine receptors.

Autoradiographic localization of 5-HT1A receptors in the post-mortem human brain using [3H]WAY-100635 and [11C]way-100635

The distribution of 5-HT1A receptors was examined in the post-mortem human brain using whole hemisphere autoradiography and the selective 5-HT1A receptor antagonist [3H]WAY-100635 ([O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridinyl)cyclohexanecarboxamide trihydrochloride). The autoradiograms showed very dense binding to hippocampus, raphe nuclei and neocortex. The labeling in neocortex was slightly lower than in the hippocampus and was mainly at superficial layers, although a faintly labeled band could be seen in deeper neocortical layers. Other regions, such as the amygdala, septum and claustrum, showed low densities caudatus and putamen, in cerebellum or in structures of the brain stem except in the raphe nuclei. The labeling of human 5-HT1A receptors with [3H]WAY-100635 was antagonised by the addition of 5-HT1A receptor ligands, 5-HT, buspirone, pindolol or 8-OH-DPAT (10 microM), leaving a very low background of non-specific binding. Saturation analysis of semiquantitative data from several human regions indicated that [3H]WAY-100635 has a Kd of approximately 2.5 nM. The selective labeling of 5-HT1A receptors with [3H]WAY-100635 clearly show that this compound is useful for further studies of the human 5-HT1a receptor subtype in vitro [11C]WAY-100635 is used for the characterization of 5-HT1A receptors with positron emission tomography (PET). WAY-100635 was also radiolabeled with the short-lived positron-emitting radionuclide carbon-11 (t1/2 = 20 min) and used for in vitro autoradiography on human whole hemisphere cryosections. [11C]WAY-100635 gave images qualitatively similar to those of [3H]WAY-100635, although with a lower resolution. Thus, the hippocampal formation was densely labeled, with lower density in the neocortex. Buspirone, pindolol or 8-OH-DPAT (10 microM), blocked all binding of [11C]WAY-100635. The in vitro autoradiography of the distribution of 5-HT1A receptors obtained with radiolabeled WAY-100635 provide detailed qualitative and quantitative information on the distribution of 5-HT1A-receptors in the human brain. Moreover, the studies give reference information for the interpretation of previous initial results at much lower resolution in humans with PET and [11C]Way-100635. These data provide a strong basis for expecting [11C]WAY-100635 to behave as a highly selective radioligand in vivo.

Age-related cognitive deficits mediated by changes in the striatal dopamine system

OBJECTIVE

The study examined the influence of losses in dopaminergic function on age-related cognitive deficits.

METHOD

Eleven healthy subjects (21-68 years of age) completed a set of cognitive tasks used to assess perceptual speed and episodic memory. D(2) receptor binding was measured in the caudate and the putamen by using positron emission tomography.

RESULTS

A gradual age-related deterioration was found for all cognitive tasks and for D(2) binding in both striatal structures. Statistical control of D(2) binding eliminated the age-related cognitive variation, whereas residual effects of D(2) binding were seen after the analysis controlled for age.

CONCLUSIONS

D(2) receptor binding is a more important factor than chronological age in accounting for variation in cognitive performance across the adult lifespan. Changes in dopaminergic neurotransmission play an important role in aging-related cognitive decline.

QTc interval prolongation and antipsychotic drug treatments: focus on sertindole

Since the 1960s, physicians have been aware of electrocardiographic (ECG) abnormalities and cases of sudden death associated with the use of antipsychotic drugs in patients with schizophrenia. Explanations for such deaths have traditionally focused on drug-induced prolongation of the QT interval leading to the development of life-threatening ventricular arrhythmias such as torsade de pointes (TdP). It is now apparent that most conventional and atypical antipsychotics can cause dose-related prolongation of the corrected QT interval (QTc), although there are important differences in the potency of individual agents. This review discusses potential mechanisms underlying QTc prolongation and arrhythmogenesis and examines the evidence for a relationship between antipsychotic drugs and prolongation of the QTc interval. New electrophysiological and epidemiological data are presented which suggest there may not be a clear-cut cause-effect relationship between QTc prolongation and the development of ventricular tachyarrhythmias for all atypical antipsychotics. For at least one of these agents (sertindole), counterbalancing mechanisms may act to reduce the risk of proarrhythmic activity arising as a result of QTc prolongation.

D1, D2, and 5-HT2 receptor occupancy in relation to clozapine serum concentration: a PET study of schizophrenic patients

OBJECTIVE

Central D1, D2, and 5-HT2 receptor occupancy in schizophrenic patients treated with clozapine was determined and related to clozapine serum concentrations.

METHOD

Seventeen patients treated with clozapine (125-600 mg/day) were examined with positron emission tomography (PET) and one to three of the following selective radioligands: [11C]SCH23390 (N = 11), [11C]raclopride (N = 16), and [11C]N-methylspiperone (N = 5). Clozapine concentration in serum was determined by gas chromatography/mass spectrometry.

RESULTS

D2 receptor occupancy (20%-67%) was lower than that previously determined in patients treated with classical neuroleptics (70%-90%). D1 receptor occupancy (36%-59%) was higher than that induced by classical neuroleptics (0%-44%). 5-HT2 receptor occupancy was very high (84%-94%), even at low clozapine doses. Despite a 20-fold range in clozapine serum concentration (105-2121 ng/ml) at the time of PET examination, D2 receptor occupancy was low in all patients and was not described by the curvilinear relationship between serum drug concentration and receptor occupancy that has been demonstrated for classical antipsychotics.

CONCLUSIONS

The results confirm in an extended series of patients that clozapine is atypical with regard to degree of D2 receptor occupancy, a finding that may explain the lack of extrapyramidal side effects. The combination of relatively high D1, low D2, and very high 5-HT2 receptor occupancy values is unique to clozapine. Clozapine serum concentrations have not been unequivocally shown to predict clinical effects. In this study, concentration did not predict degree of occupancy in brain. Thus, careful clinical titration cannot be replaced by monitoring of drug concentrations for optimization of clozapine treatment in individual patients.

PET study of the pre- and post-synaptic dopaminergic markers for the neurodegenerative process in Huntington's disease

PET and: markers for the pre- and postsynaptic neurons were used to study the dopamine system in vivo in Huntington's disease. The radioligands used were [11C]SCH 23390 for D1-receptors, [11C]raclopride for D2-receptors and [11C]beta-CIT for dopamine transporters. Five patients with Huntington's disease and five matched controls were recruited. Brain anatomy was examined by MRI. The findings in patients were as follows. Postsynaptic D1- and D2-receptor densities were similarly reduced in the striatum. A reduction in D1-receptor density was shown in the temporal cortex; it draws attention to the cortical degeneration in relation to the cognitive deficits observed in Huntington's disease. The reduction of D1- and D2-receptor binding potentials in the striatum correlated significantly with increasing duration of illness. The correlation between the duration of illness and decline of D1- and D2-receptors make these receptors valuable as quantitative markers for the Huntington's disease degenerative process. Besides postsynaptic changes, a significant 50% decrease of [11C]beta-CIT binding to the dopamine transporter was found in the striatum. A reduced striatal blood flow in Huntington's disease cannot be excluded and could account for a small part of the decrease in [11C]beta-CIT binding. We suggest that the finding reflects a loss of presynaptic terminals or a reduced expression of dopamine transporter in the nigrostriatal dopaminergic system in Huntington's disease.

Meta-analysis of cognitive performance in drug-naïve patients with schizophrenia

Cognitive deficits represent a significant characteristic of schizophrenia. However, a majority of the clinical studies have been conducted in antipsychotic drug treated patients. Thus, it remains unclear if significant cognitive impairments exist in the absence of medication. This is the first meta-analysis of cognitive findings in drug-naïve patients with schizophrenia. Cognitive data from 23 studies encompassing 1106 patients and 1385 controls published from 1992 to 2013 were included. Tests were to a large extent ordered in cognitive domains according to the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) battery. Analysis was performed with STATA using the random-effects model and heterogeneity as well as Egger's publication bias was assessed. Overall the results show that patients performed worse than healthy controls in all cognitive domains with medium to large effect sizes. Verbal memory, speed of processing and working memory were three of the domains with the greatest impairments. The pattern of results is in line with previous meta-analytic findings in antipsychotic treated patients. The present meta-analysis confirms the existence of significant cognitive impairments at the early stage of the illness in the absence of antipsychotic medication.

Transplantation in Parkinson's disease: two cases of adrenal medullary grafts to the putamen

Autologous adrenal medullary tissue was transplanted unilaterally to the putamen in two patients with severe Parkinson's disease. The first patient exhibited a transient, two-day improvement of motor performance in the limbs contralateral to the implantation site. He also had significantly longer episodes of normal function for about two months. The second patient reported a minor improvement of balance and gait, again lasting for two months. Electrophysiological studies of the motor readiness and auditory evoked potentials were consistent with increased catecholaminergic activity in the basal ganglia after transplantation in both patients. Positron emission tomography showed no postoperative alteration of receptor density in the putamen. No significant adverse effects of the transplantation were observed in the patients' performance on neurological and psychological tests or in their quantitative electroencephalogram and cerebral blood flow recordings. Immediately after the grafting, one patient showed transient signs of sympathetic hyperactivity, probably caused by release of catecholamines from the implanted tissue into the peripheral circulation. We conclude that catecholamine-rich cellular implants in the basal ganglia have transient beneficial effects in patients with severe Parkinson's disease.

Support for dopaminergic hypoactivity in restless legs syndrome: a PET study on D2-receptor binding

Clinical observations support a central role of the dopamine system in restless legs syndrome (RLS) but previous imaging studies of striatal dopamine D2-receptors have yielded inconclusive results. Extrastriatal dopaminergic function has hitherto not been investigated. Sixteen RLS patients naïve to dopaminergic drugs and sixteen matched control subjects were examined with PET. [11C]Raclopride and [11C]FLB 457 were used to estimate D2-receptor availability in striatum and extrastriatal regions, respectively. Examinations were performed both in the morning (starting between 10:00 and 12:00 h) and evening (starting at 18:00 h). Measures were taken to monitor and control for head movement during data acquisition. In the striatum, patients had significantly higher [11C]raclopride binding potential (BP) values than controls. In extrastriatal regions, [11C]FLB 457 BP was higher in patients than controls, and in the regional analysis the difference was statistically significant in subregions of thalamus and the anterior cingulate cortex. The diurnal variability in BP with [11C]FLB 457 and [11C]raclopride was within the previously reported test-retest reproducibility for both radioligands. The study supports involvement of the dopamine system in both striatal and extrastriatal brain regions in the pathophysiology of RLS. The brain regions where differences in D2-receptor binding were shown are implicated in the regulation of affective and motivational aspects of sensory processing, suggesting a possible pathway for sensory symptoms in RLS. Increased D2-receptor availability in RLS may correspond to higher receptor densities or lower levels of endogenous dopamine. Both interpretations are consistent with the hypothesis of hypoactive dopaminergic neurotransmission in RLS, as increased receptor levels can be owing to receptor upregulation in response to low levels of endogenous dopamine. The results do not support variations in dopamine D2-receptor availability as a correlate to the diurnal rhythm of RLS symptoms.

The role of the striatal dopamine transporter in cognitive aging

We examined the relationship of age-related losses of striatal dopamine transporter (DAT) density to age-related deficits in episodic memory and executive functioning in a group of subjects (n = 12) ranging from 34 to 81 years of age. The radioligand [(11)C]beta-CIT-FE was used to determine DAT binding in caudate and putamen. Results showed clear age-related losses of striatal DAT binding from early to late adulthood, and a marked deterioration in episodic memory (word and figure recall, face recognition) and executive functioning (visual working memory, verbal fluency) with advancing age. Most importantly, the age-related cognitive deficits were mediated by reductions in DAT binding, whereas DAT binding added systematic cognitive variance after controlling for age. Further, interindividual differences in DAT binding were related to performance in a test of crystallized intelligence (the Information subtest from the Wechsler Adult Intelligence Scale-Revised) that showed no reliable age variation. These results suggest that DAT binding is a powerful mediator of age-related cognitive changes as well as of cognitive functioning in general. The findings were discussed relative to the view that the frontostriatal network is critically involved in multiple cognitive functions.