Time course of changes in striatal dopamine transporters and D2 receptors with specific iodinated markers in a rat model of Parkinson's disease

Alteration to Dopaminergic Synapses Following Exposure to Perfluorooctane Sulfonate (PFOS), in Vitro and in Vivo

Our understanding of the contribution exposure to environmental toxicants has on neurological disease continues to evolve. Of these, Parkinson’s disease (PD) has been shown to have a strong environmental component to its etiopathogenesis. However, work is still needed to identify and characterize environmental chemicals that could alter the expression and function of the nigrostriatal dopamine system. Of particular interest is the neurotoxicological effect of perfluorinated compounds, such as perfluorooctane sulfonate (PFOS), which has been demonstrated to alter aspects of dopamine signaling. Using in vitro approaches, we have elaborated these initial findings to demonstrate the neurotoxicity of PFOS to the SH-SY5Y neuroblastoma cell line and dopaminergic primary cultured neurons. Using an in vivo model, we did not observe a deficit to dopaminergic terminals in the striatum of mice exposed to 10 mg/kg PFOS for 14 days. However, subsequent exposure to the selective dopaminergic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) significantly reduced the expression of dopamine transporter (DAT) and tyrosine hydroxylase (TH), and resulted in an even greater reduction in DAT expression in animals previously exposed to PFOS. These findings suggest that PFOS is neurotoxic to the nigrostriatal dopamine circuit and this neurotoxicity could prime the dopamine terminal to more extensive damage following additional toxicological insults.

Relation between microPET imaging and rotational behavior in a parkinsonian rat model induced by medial forebrain bundle axotomy

The purpose of the current study was to examine the relation between apomorphine (APO) induced rotational behavior and the pre- and post-synaptic dopaminergic function in a parkinsonian rat model induced by medial forebrain bundle (MFB) axotomy. The brains of these rats were unilaterally lesioned by mechanical transection of the nigrostriatal dopamine pathway at the MFB. Behavioral studies were carried out by APO challenge prior to and 1, 3, and 5 weeks after MFB axotomy. MicroPET scans with [(11)C]CFT and [(11)C]raclopride were performed 2 days after the behavioral test. The two PET scans were separated by an interval of 24-48 h. Immunohistochemistry was conducted 4 days after the last PET scan. Our data showed that [(11)C]CFT binding decreased progressively 1, 3, and 5 weeks postlesion, and there was a significant nonlinear correlation between [(11)C]CFT uptake ratio (right/left) and APO induced rotations. In contrast, [(11)C]raclopride binding only increased significantly 3 weeks postlesion, and there was a positive linear correlation between [(11)C]raclopride uptake ratio (right/left) and APO induced rotations. Postmortem immunohistochemical studies confirmed the loss of both striatal dopamine fibers and nigral neurons on the lesioned side. These findings not only demonstrate the relation between APO induced rotational behavior and the pre- and post-synaptic dopamine function but also indicate the utility and validity of in vivo PET imaging in understanding disease mechanisms and progression, which should in turn lead to development of new therapies.

Study of 99mTc-TRODAT-1 imaging on human brain with children autism by single photon emission computed tomography

To evaluate the application values of^99mTc-2 β [N, N′ , - bis (2-mercaptoethy1) ethylenediamino] methyl, 3 β -(4-chlorophenyl) tropane (TRODAT-1) dopamine transporter (DAT) SPECT imaging in children autism, and offer the academic foundation to etiology, mechanism and clinical therapy of autism. Ten autistic children and ten healthy controls were examined with 99mTc-TRODAT-1 DAT SPECT imaging. Striatal specific uptake of 99mTc-TRODAT-1 was calculated with region of interest analysis according to the ratios between striatum and cerebellum [(STR-BKG)/BKG]. There was no difference in semiquantitative dopamine transporter between bilateral striatum in autistic children (p=0.562) and in normal controls (p=0.573); Dopamine transporter in brain of patients with autism increased significantly than that in normal controls (p=0.017). Dopaminergic nervous system is dysfunction in human brain with children autism, and DAT 99mTc-TRODAT-1 SPECT imaging on human brain will help the imaging diagnosis of children autism.

Evolution of the dynamic properties of the cortex-basal ganglia network after dopaminergic depletion in rats

It is well established that parkinsonian syndrome is associated with alterations of neuronal activity temporal pattern basal ganglia (BG). An increase in synchronized oscillations has been observed in different BG nuclei in Parkinson's disease patients as well as animal models such as 6-hydroxydopamine treated rats. We recently demonstrated that this increase in oscillatory synchronization is present during high-voltage spindles (HVS) probably underpinned by the disorganization of cortex-BG interactions. Here we investigated the time course of both oscillatory and motor alterations. For that purpose we performed daily simultaneous recordings of neuronal activity in motor cortex, striatum and substantia nigra pars reticulata (SNr), before and after 6-hydroxydopamine lesion in awake rats. After a brief non-dopamine-specific desynchronization, oscillatory activity first increased during HVS followed by progressive motor impairment and the shortening of SNr activation delay. While the oscillatory firing increase reflects dopaminergic depletion, response alteration in SNr neurons is closely related to motor symptom.

Striatal adenosine A(2A) receptor-mediated positron emission tomographic imaging in 6-hydroxydopamine-lesioned rats using [(18)F]-MRS5425

INTRODUCTION

A(2A) receptors are expressed in the basal ganglia, specifically in striatopallidal GABAergic neurons in the striatum (caudate-putamen). This brain region undergoes degeneration of presynaptic dopamine projections and depletion of dopamine in Parkinson's disease. We developed an (18)F-labeled A(2A) analog radiotracer ([(18)F]-MRS5425) for A(2A) receptor imaging using positron emission tomography (PET). We hypothesized that this tracer could image A(2A) receptor changes in the rat model for Parkinson's disease, which is created following unilateral injection of the monoaminergic toxin 6-hydroxydopamine (6-OHDA) into the substantia nigra.

METHODS

[(18)F]-MRS5425 was injected intravenously in anesthetized rats, and PET imaging data were collected. Image-derived percentage injected doses per gram (%ID/g) in regions of interest was measured in the striatum of normal rats and in rats unilaterally lesioned with 6-OHDA after intravenous administration of saline (baseline), D(2) agonist quinpirole (1.0 mg/kg) or D(2) antagonist raclopride (6.0 mg/kg).

RESULTS

Baseline %ID/g reached a maximum at 90 s and maintained plateau for 3.5 min, and then declined slowly thereafter. In 6-OHDA-lesioned rats, %ID/g was significantly higher in the lesioned side compared to the intact side, and the baseline total %ID/g (data from both hemispheres were combined) was significantly higher compared to quinpirole stimulation starting from 4.5 min until the end of acquisition at 30 min. Raclopride did not produce any change in uptake compared to baseline or between the hemispheres.

CONCLUSION

Thus, increase of A(2A) receptor-mediated uptake of radioactive MRS5425 could be a superior molecular target for Parkinson's imaging.

Dopamine agonists increase perseverative instrumental responses but do not restore habit formation in a rat model of Parkinsonism

Dopamine (DA) deafferentation of the dorsolateral striatum has been shown to prevent habit development, leaving instrumental behavior under action-outcome control that is persistently sensitive to modification of the motivational value of the reward. The present experiment further explored the basis of this dysfunction by examining the ability of intrastriatal DA agonist injections (D1 SKF 38393 or D2/D3 Quinpirole) during overtraining of a signaled instrumental task to restore habit formation in rats subjected to bilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway. Overtraining was followed by a test of goal sensitivity by satiety-specific devaluation of the reward. The results confirmed the impaired shift in performance from action to habit in control lesioned rats. However, lesioned rats repeatedly injected with quinpirole D2/D3 agonist showed an increase in non-rewarded instrumental responses (intertrials periods) during overtraining, suggesting the development of perseverative behavior. Following the procedure of devaluation, quinpirole D2/D3 agonist treatment, and to a lesser extent SKF 38393 D1 agonist, caused the persistence of sensitivity to reward devaluation, indicating clear goal-directed behavior despite extended training. This absence of restoration of habit formation by DA agonist treatment is discussed in the light of DA agonist effects in Parkinson patients.

Brain arachidonic acid cascade enzymes are upregulated in a rat model of unilateral Parkinson disease

Arachidonic acid (AA) signaling is upregulated in the caudate-putamen and frontal cortex of unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, a model for asymmetrical Parkinson disease. AA signaling can be coupled to D(2)-like receptor initiated AA hydrolysis from phospholipids by cytosolic phospholipase A(2) (cPLA(2)) and subsequent metabolism by cyclooxygenase (COX)-2. In unilaterally 6-OHDA- and sham-lesioned rats, we measured brain expression of cPLA(2), other PLA(2) enzymes, and COX-2. Activity and protein levels of cPLA(2) were significantly higher as was COX-2-protein in caudate-putamen, frontal cortex and remaining brain on the lesioned compared to intact side of the 6-OHDA lesioned rats, and compared to sham brain. Secretory sPLA(2) and Ca(2+)-independent iPLA(2) expression did not differ between sides or groups. Thus, the tonically increased ipsilateral AA signal in the lesioned rat corresponds to upregulated cPLA(2) and COX-2 expression within the AA metabolic cascade, which may contribute to symptoms and pathology in Parkinson disease.

Review. Positron emission tomography imaging studies of dopamine receptors in primate models of addiction

Animal models have provided valuable information related to trait and state variables associated with vulnerability to drug addiction. Our brain imaging studies in monkeys have implicated D2 receptors in cocaine addiction. For example, an inverse relationship between D2 receptor availability and rates of cocaine self-administration has been documented. Moreover, environmental variables, such as those associated with formation of the social hierarchy, can impact receptor availability and sensitivity to the abuse-related effects of cocaine. Similarly, both D2 receptor availability and cocaine self-administration can be altered by chronic drug administration and fluctuations in hormone levels. In addition, cocaine self-administration can be altered in an orderly fashion by presentation of an acute stressor, such as acting as an intruder into an unfamiliar social group, which can shift the cocaine dose-response curve to the left in subordinate monkeys and to the right in dominant animals, suggesting an interaction between social variables and acute stressors. Conversely, irrespective of social rank, acute environmental enrichment, such as increasing the size of the living space, shifts the cocaine dose-response curve to the right. These findings highlight a pervasive influence of the environment in modifying the reinforcing effects of cocaine and strongly implicate brain D2 receptors.

PE2I: a radiopharmaceutical for in vivo exploration of the dopamine transporter

The membrane dopamine transporter (DAT) has a pivotal role in the regulation of dopamine (DA) neurotransmission involved in a number of physiological functions and brain disorders. Molecular imaging techniques, such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT), are relevant tools to explore the DAT, and we developed the cocaine derivative N-(3-iodopro-2E-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl) nortropane (PE2I) that has proved to be a very potent radiopharmaceutical to image the DAT by these techniques. Several methods are available to obtain PE2I labeled with iodine-123 or -125, carbon-11 and tritium. The pharmacological properties of PE2I have demonstrated that it has good affinity for the DAT (4 nM) and is one of the most selective DAT ligands. [(125)I]PE2I characterized postmortem in human brains has revealed very intense and selective binding in the basal ganglia. Ex vivo autoradiography in rats has shown that high level of [(125)I]PE2I accumulates in the striatum and also in the substantia nigra and ventral tegmental area. [(125)I]PE2I accumulation in the rat striatum is rapid, high, and selective, providing a maximum striatum/cerebellum ratio of 10 during the first 30 min post injection. Using SPECT or PET, rapid, high, and selective accumulation of PE2I was found in the caudate nucleus and putamen in monkeys, whereas rapid wash out from the cerebellum was observed. In vivo investigations in healthy humans have demonstrated that PE2I has high striatal uptake, low nonspecific binding, low radiation exposure, and a fairly short scanning time. A number of findings in various animal models of Parkinson's disease in rats and monkeys have demonstrated the high efficacy of PE2I for detection of reduction in the density of DAT, thus showing the potential value of PE2I for early diagnosis and evaluation of treatment of this disease. The excellent properties of PE2I are basis for the development of new DAT tracers for use in future PET explorations using fluor-18.

In vivo imaging of disturbed pre- and post-synaptic dopaminergic signaling via arachidonic acid in a rat model of Parkinson's disease

BACKGROUND

Parkinson's disease involves loss of dopamine (DA)-producing neurons in the substantia nigra, associated with fewer pre-synaptic DA transporters (DATs) but more post-synaptic dopaminergic D2 receptors in terminal areas of these neurons.

HYPOTHESIS

Arachidonic acid (AA) signaling via post-synaptic D2 receptors coupled to cytosolic phospholipase A2 (cPLA2) will be reduced in terminal areas ipsilateral to a chronic unilateral substantia nigra lesion in rats given D-amphetamine, which reverses the direction of the DAT, but will be increased in rats given quinpirole, a D2-receptor agonist.

METHODS

D-amphetamine (5.0 mg/kg i.p.), quinpirole (1.0 mg/kg i.v.), or saline was administered to unanesthetized rats having a chronic unilateral lesion of the substantia nigra. AA incorporation coefficients, k* (radioactivity/integrated plasma radioactivity), markers of AA signaling, were measured using quantitative autoradiography in 62 bilateral brain regions following intravenous [1-(14)C]AA.

RESULTS

In rats given saline (baseline), k* was elevated in 13 regions in the lesioned compared with intact hemisphere. Quinpirole increased k* in frontal cortical and basal ganglia regions bilaterally, more so in the lesioned than intact hemisphere. D-amphetamine increased k* bilaterally but less so in the lesioned hemisphere.

CONCLUSIONS

Increased baseline elevations of k* and increased responsiveness to quinpirole in the lesioned hemisphere are consistent with their higher D2-receptor and cPLA2 activity levels, whereas reduced responsiveness to D-amphetamine is consistent with dropout of pre-synaptic elements containing the DAT. In vivo imaging of AA signaling using dopaminergic drugs can identify pre- and post-synaptic DA changes in animal models of Parkinson's disease.

Metabolic-dopaminergic mapping of the 6-hydroxydopamine rat model for Parkinson's disease

PURPOSE

The unilateral 6-hydroxydopamine (6-OHDA) lesion rat model is a well-known acute model for Parkinson's disease (PD). Its validity has been supported by invasive histology, behavioral studies and electrophysiology. Here, we have characterized this model in vivo by multitracer imaging [glucose metabolism and dopamine transporter (DAT)] in relation to behavioral and histological parameters.

METHODS

Eighteen female adult Wistar rats (eight 6-OHDA-lesioned, ten controls) were investigated using multitracer [(18)F]-fluoro-2-deoxy-D: -glucose (FDG) and [(18)F]-FECT {2'-[(18)F]-fluoroethyl-(1R-2-exo-3-exe)-8-methyl-3-(4-chlorophenyl)-8-azabicyclo(3.2.1)-octane-2-carboxylate} small animal positron emission tomography (PET). Relative glucose metabolism and parametric DAT binding images were anatomically standardized to Paxinos space and analyzed on a voxel-basis using SPM2: , supplemented by a template-based predefined volumes-of-interest approach. Behavior was characterized by the limb-use asymmetry test; dopaminergic innervation was validated by in vitro tyrosine hydroxylase staining.

RESULTS

In the 6-OHDA model, significant glucose hypometabolism is present in the ipsilateral sensory-motor cortex (-6.3%; p = 4 x 10(-6)). DAT binding was severely decreased in the ipsilateral caudate-putamen, nucleus accumbens and substantia nigra (all p < 5 x 10(-9)), as confirmed by the behavioral and histological outcomes. Correlation analysis revealed a positive relationship between the degree of DAT impairment and the change in glucose metabolism in the ipsilateral hippocampus (p = 3 x 10(-5)), while cerebellar glucose metabolism was inversely correlated to the level of DAT impairment (p < 3 x 10(-4)).

CONCLUSIONS

In vivo cerebral mapping of 6-OHDA-lesioned rats using [(18)F]-FDG and [(18)F]-FECT small animal PET shows molecular-functional correspondence to the cortico-subcortical network impairments observed in PD patients. This provides a further molecular validation supporting the validity of the 6-OHDA lesion model to mimic multiple aspects of human PD.

Reduced vesicular storage of dopamine causes progressive nigrostriatal neurodegeneration

The vesicular monoamine transporter 2 (VMAT2; SLC18A2) is responsible for packaging dopamine into vesicles for subsequent release and has been suggested to serve a neuroprotective role in the dopamine system. Here, we show that mice that express approximately 5% of normal VMAT2 (VMAT2 LO) display age-associated nigrostriatal dopamine dysfunction that ultimately results in neurodegeneration. Elevated cysteinyl adducts to L-DOPA and DOPAC are seen early and are followed by increased striatal protein carbonyl and 3-nitrotyrosine formation. These changes were associated with decreased striatal dopamine and decreased expression of the dopamine transporter and tyrosine hydroxylase. Furthermore, we observed an increase in alpha-synuclein immunoreactivity and accumulation and neurodegeneration in the substantia nigra pars compacta in aged VMAT2 LO mice. Thus, VMAT2 LO animals display nigrostriatal degeneration that begins in the terminal fields and progresses to eventual loss of the cell bodies, alpha-synuclein accumulation, and an L-DOPA responsive behavioral deficit, replicating many of the key aspects of Parkinson's disease. These data suggest that mishandling of dopamine via reduced VMAT2 expression is, in and of itself, sufficient to cause dopamine-mediated toxicity and neurodegeneration in the nigrostriatal dopamine system. In addition, the altered dopamine homeostasis resulting from reduced VMAT2 function may be conducive to pathogenic mechanisms induced by genetic or environmental factors thought to be involved in Parkinson's disease.

Whole-body distribution and radiation dosimetry of the dopamine transporter radioligand [(11)C]PE2I in healthy volunteers

INTRODUCTION

This study reports on the biodistribution and radiation dosimetry of a cocaine analog, the (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-tolyl)nortropane (PE2I), labeled with carbon 11 ([(11)C]PE2I). [(11)C]PE2I is used in positron emission tomography (PET) for examination of the dopamine neuronal transporter (DAT). DAT radioligands are often used to evaluate the progression of Parkinson's disease or the efficiency of neuroprotective therapeutics, and, typically, these studies required several successive PET scans.

METHODS

In three healthy male volunteers, whole-body scans were performed up to 2 h following intravenous injection of 321+/-6 MBq of [(11)C]PE2I. For each subject, regions of interest were defined over all visible organs to generate time-activity curves and calculate the percentage of injected activity. Time-activity data were fitted to a monoexponential model, as an uptake phase followed by a mono-exponential washout, or bi-exponential model to obtain residence times. With the use of the MIRD method, several source organs were considered in estimating residence time and mean effective radiation absorbed doses.

RESULTS

Blood pressure and ECG findings remained unchanged after radioligand injection. The primary route of clearance was renal. Ten minutes after injection, high activities were observed in the kidneys, urinary-bladder, stomach, liver, salivary glands and brain. The urine bladder wall, stomach and liver received the highest absorbed doses. The average effective dose of [(11)C]PE2I was estimated to be 6.4+/-0.6 microSv/MBq.

CONCLUSION

The amount of [(11)C]PE2I required for adequate DAT PET imaging results in an acceptable effective dose equivalent permitting two or three repeated cerebral PET studies, with the injection of 222 MBq for each study.

Overexpression of D2/D3 receptors increases efficacy of ropinirole in chronically 6-OHDA-lesioned Parkinsonian rats

Ropinirole, which is a non-ergot dopamine agonist derivative, exerts therapeutic benefits in Parkinson's disease (PD). Based on recent studies implicating dopamine receptors 2 and 3 (D2R and D3R) as possible targets of ropinirole, we over-expressed these dopamine receptor genes in the dopamine-denervated striatum of rodents to reveal whether their over-expression modulated ropinirole activity. Adult Sprague-Dawley rats initially received unilateral 6-hydroxydopamine lesion of the medial forebrain bundle. At 1 month after surgery, successfully lesioned animals (3 or less forelimb akinesia score, and 8 or more apomorphine-induced rotations/min over 1 h) were randomly assigned to intrastriatal injection (ipsilateral to the lesion) of blank lentiviral vector, D2R, D3R or both genes. At about 5 months post-lesion, ropinirole (0.2 mg/kg, i.p.) was administered daily for 9 consecutive days. The subtherapeutic dose of ropinirole improved the use of previously akinetic forelimb and produced robust circling behavior in lesioned animals with striatal over-expression of both D2R and D3R compared to lesioned animals that received blank vector. In contrast, the subtherapeutic dose of ropinirole generated only modest motor effects in lesioned animals with sole over-expression of D2R or D3R. Western immunoblot and autoradiographic assays showed enhanced D2R and D3R protein levels coupled with normalized D2R and D3R binding in the ventral striatum of lesioned animals with lentiviral over-expression of both D2R and D3R relative to vehicle-treated lesioned animals. Immunohistochemical analyses showed that D2R and D3R GFP fluorescent cells colocalized with enkephalin and substance P immunoreactive medium spiny neurons. These data support the use of the subtherapeutic dose of ropinirole in a chronic model of PD.

In vivo tomographic imaging studies of neurodegeneration and neuroprotection: a review

Noninvasive tomographic imaging methods including positron emission tomography (PET) and single photon emission computed tomography (SPECT) are extremely sensitive and are capable of measuring biochemical processes that occur at concentrations in the nanomolar range. Inherent to neurodegenerative processes is neuronal loss. Thus, PET or SPECT monitoring of biochemical processes altered by neuronal loss (changes in neurotransmitter turnover, alterations in receptor, transporter or enzyme concentrations) can provide unique information not attainable by other methods. Such imaging techniques can also be used to longtitudinally monitor the effects of neuroprotective treatments. This review highlights current imaging probes used to evaluate patients with specific neurodegenerative disorders (e.g., Alzheimer's Disease, Parkinson's Disease, Huntington's Chorea), including those that image receptors of the dopaminergic, cholinergic and glutamatergic systems. Areas of future research focus are also defined. It is clear that monitoring the progression of neurodegenerative disorders and the impact of neuroprotective treatments are two different but related goals for which noninvasive imaging via PET and SPECT methods plays a powerful and unique role.

D-Amphetamine stimulates D2 dopamine receptor-mediated brain signaling involving arachidonic acid in unanesthetized rats

In rat brain, dopaminergic D(2)-like but not D(1)-like receptors can be coupled to phospholipase A(2) (PLA(2)) activation, to release the second messenger, arachidonic acid (AA, 20:4n-6), from membrane phospholipids. In this study, we hypothesized that D-amphetamine, a dopamine-releasing agent, could initiate such AA signaling. The incorporation coefficient, k* (brain radioactivity/integrated plasma radioactivity) for AA, a marker of the signal, was determined in 62 brain regions of unanesthetized rats that were administered i.p. saline, D-amphetamine (2.5 or 0.5 mg/kg i.p.), or the D(2)-like receptor antagonist raclopride (6 mg/kg, i.v.) before saline or 2.5 mg/kg D-amphetamine. After injecting [1-(14)C]AA intravenously, k* was measured by quantitative autoradiography. Compared to saline-treated controls, D-amphetamine 2.5 mg/kg i.p. increased k* significantly in 27 brain areas rich in D(2)-like receptors. Significant increases were evident in neocortical, extrapyramidal, and limbic regions. Pretreatment with raclopride blocked the increments, but raclopride alone did not alter baseline values of k*. In independent experiments, D-amphetamine 0.5 mg/kg i.p. increased k* significantly in only seven regions, including the nucleus accumbens and layer IV neocortical regions. These results indicate that D-amphetamine can indirectly activate brain PLA(2) in the unanesthetized rat, and that activation is initiated entirely at D(2)-like receptors. D-Amphetamine's low-dose effects are consistent with other evidence that the nucleus accumbens, considered a reward center, is particularly sensitive to the drug.

Polychlorinated biphenyl-induced reduction of dopamine transporter expression as a precursor to Parkinson's disease-associated dopamine toxicity

Epidemiological and laboratory studies have suggested that exposure to polychlorinated biphenyls (PCBs) may be a risk factor for Parkinson's disease. The purpose of this study was to examine the potential mechanisms by which PCBs may disrupt normal functioning of the nigrostriatal dopamine (DA) system. We utilized an environmentally relevant exposure of PCBs (7.5 or 15 mg/kg/day Aroclor 1,254:1,260 for 30 days by oral gavage) to identify early signs of damage to the DA system. This dosing regimen, which resulted in PCB levels similar to those found in human brain samples, did not cause overt degeneration to the DA system as shown by a lack of change in striatal DA levels or tyrosine hydroxylase levels. However, we did observe a dramatic dose-dependent decrease in striatal dopamine transporter (DAT) levels. The observed reductions appear to be specific to the DAT populations located in the striatum, as no change was observed in other dopaminergic brain regions or to other neurotransmitter transporters present in the striatum. These data demonstrate that PCB tissue concentrations similar to those found in postmortem human brain specifically disrupt DA transport, which acts as a precursor to subsequent damage to the DA system. Furthermore, DAT imaging may be useful in evaluating alterations in brain function in human populations exposed to PCBs.

Synthesis, radiosynthesis and in vivo preliminary evaluation of [11C]LBT-999, a selective radioligand for the visualisation of the dopamine transporter with PET

LBT-999 (8-((E)-4-fluoro-but-2-enyl)-3beta-p-tolyl-8-aza-bicyclo[3.2.1]octane-2beta-carboxylic acid methyl ester), a cocaine derivative belonging to a new generation of highly selective dopamine transporter (DAT) ligands, and its corresponding carboxylic acid derivative, the latter used as precursor for labelling both with tritium and the positron-emitter carbon-11 (half-life: 20.38 min), were synthesized from (R)-cocaine. [(3)H]LBT-999 (>99% radiochemically pure, specific radioactivity of 3.1 TBq/mmol) was prepared from [(3)H]methyl iodide, allowing its in vitro pharmacological evaluation (K(D): 9 nM for DAT and IC(50) > 1000 nM for SERT and NET). Routine production batches of 4.5-9.0 GBq of iv injectable solutions of [(11)C]LBT-999 (with specific radioactivities ranging from 30 to 45 GBq/mumol) were prepared in 25-30 min (HPLC purification and formulation included) using the efficient methylation reagent [(11)C]methyl triflate. The preliminary in vivo pharmacological evaluation of [(11)C]LBT-999, using both biodistributions in rats and brain imaging in monkeys with positron emission tomography (PET), clearly illustrates that this ligand is an excellent candidate for quantification with PET of DAT in humans.

Correlation between quantitative imaging and behavior in unilaterally 6-OHDA-lesioned rats

We evaluated correlation between neurochemical and functional alterations of the nigrostriatal dopaminergic system in rat brains lesioned with 6-hydroxydopamine (6-OHDA), that model hemi-Parkinson's disease (PD), by using three different quantitative in vivo and in vitro methods. Rats unilaterally lesioned with different doses of 6-OHDA underwent two types of in vivo experiments: (1) a rotational behavioral study with methamphetamine (MAP) or apomorphine (APO); and (2) a positron emission tomography (PET) study with [11C]PE2I (radioligand for dopamine transporters) or [11C]raclopride (radioligand for dopamine D2 receptors). An in vitro autoradiographic study with the same radioligands was also conducted. The number of rotations after the MAP or APO injection increased with increased doses of 6-OHDA. The in vitro and in vivo binding of [11C]PE2I dose-dependently decreased in response to the 6-OHDA injections, while that of [11C]raclopride dose-dependently increased. There was a significant negative hyperbolic correlation between the number of rotations after MAP injection and the binding of [11C]PE2I. In contrast, there was a significant positive linear correlation between the number of rotations after APO injections and the binding of [11C]raclopride. These results robustly reveal a molecular pharmacological basis of parkinsonian symptoms in animal models of PD, and indicate the utility and validity of in vivo PET measurements in assessing pre- and post-synaptic dopaminergic functions.

Deleterious effects of minocycline in animal models of Parkinson's disease and Huntington's disease

Minocycline has been shown to exert anti-inflammatory effects underlying its putative neuroprotective properties in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease and in the R6/2 mouse model of Huntington's disease (HD). However, contradictory results have recently been reported. We report deleterious effects of minocycline in two phenotypic (toxic) models of Parkinson's disease and HD in monkey and mouse. Of seven MPTP-intoxicated female cynomolgus monkeys (0.2 mg/kg, i.v. until day 15), three received minocycline (200 mg b.i.d.). While placebo-MPTP-treated animals displayed mild parkinsonism at day 15, the minocycline/MPTP-treated animals tended to be more affected (P = 0.057) and showed a greater loss of putaminal dopaminergic nerve endings (P < 0.0001). In the 3-nitropropionic acid (3-NP) mouse model of HD, minocycline (45 mg/kg i.p.) was administered 30 min before each i.p. injection of 3-NP (b.i.d., cumulated dose, 360 mg/kg in 5 days). Mice receiving minocycline exhibited a worsening of the mean motor score with a slower recovery slope, more impaired general activity and significantly deteriorated performances on the rotarod, pole test and beam-traversing tasks. The histopathological outcome demonstrated that minocycline-treated mice presented significantly more severe neuronal cell loss in the dorsal striatum. The effect of minocycline vs. 3-NP was also investigated on hippocampal and cortical cell cultures. minocycline blocked 3-NP-induced neurotoxicity at certain doses (1 mm cortical neurons) but not at higher doses (10 mm). Thus, minocycline may have variable and even deleterious effects in different species and models according to the mode of administration and dose.

Therapeutic doses of L-dopa reverse hypersensitivity of corticostriatal D2-dopamine receptors and glutamatergic overactivity in experimental parkinsonism

Levodopa (l-dopa) therapy is still considered the gold-standard in the treatment of Parkinson's disease. However, the synaptic and cellular mechanisms involved in the amelioration of motor symptoms during this treatment are still unclear. To address this issue, we analysed the physiological and pharmacological properties of striatal glutamatergic and GABAergic synaptic transmission in an experimental model of Parkinson's disease. Single-cell recordings were performed in sham-operated rats, in 6-hydroxydopamine-lesioned animals and in rats receiving chronic l-dopa treatment following dopamine (DA) denervation. We utilized a dose of l-dopa (10 mg/kg, twice daily for 21 days) able to reverse motor deficits in about half of parkinsonian animals. In the striatum of parkinsonian animals showing therapeutic benefits following l-dopa treatment, we observed a reversal of glutamatergic overactivity and of the hypersensitivity of presynaptic D2 DA receptors controlling glutamate release from corticostriatal terminals. Conversely, no change was detected in the sensitivity of presynaptic D2 DA receptors modulating striatal GABA transmission in both parkinsonian and l-dopa-treated rats. We suggest that the reversal of striatal glutamatergic overactivity and the normalization of hypersensitive D2 DA receptors modulating excitatory transmission might underlie some of the therapeutic actions of l-dopa in Parkinson's disease.

Functional brain imaging in the differential diagnosis of Parkinson's disease

The accurate diagnosis of idiopathic Parkinson's disease (IPD) is not only important for deciding on treatment strategies and providing a prognosis, but also crucial for studies designed to investigate the aetiology and pathogenesis of parkinsonian disorders. Over recent decades, improvements in the characterisation of the parkinsonian syndromes have led to improvements in clinical diagnostic accuracy; however, clinical criteria alone are not always sufficient to distinguish between IPD and other parkinsonian syndromes, particularly in the early stages of disease and in atypical presentations. Therefore, in addition to the development and implementation of diagnostic clinical assessments, there is a need for available objective markers to aid the physician in the differential diagnosis of IPD. Functional neuroimaging holds the promise of improved diagnosis and allows assessment in early disease. In this review, the use of PET and single photon emission CT in the differential diagnosis of IPD are discussed.

Dopamine transporter: basic aspects and neuroimaging

The plasma membrane dopamine transporter (DAT) is found exclusively in dopamine neurones and seems to be the defining molecule of the dopamine neurone. It provides effective control over the intensity of dopamine-mediated signalling by recapturing the neurotransmitter released by presynaptic neurones. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) represent unique techniques for assessing in vivo DAT distribution in humans and offer reliable methods for studying nigrostriatal dopaminergic function in health and disease. The characteristics of different DAT radiotracers, the modifying influences of factors such as age, gender, smoking habit, and dopaminergic drugs on DAT transporters as well as their implication in evaluation of neuroimaging studies are discussed.

Ropinirole versus l-DOPA effects on striatal opioid peptide precursors in a rodent model of Parkinson's disease: implications for dyskinesia

The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-DOPA), remains the most common treatment for Parkinson's disease. However, following long-term treatment, disabling side effects, particularly L-DOPA-induced dyskinesias, are encountered. Conversely, D2/D3 dopamine receptor agonists, such as ropinirole, exert an anti-parkinsonian effect while eliciting less dyskinesia when administered de novo in Parkinson's disease patients. Parkinson's disease and L-DOPA-induced dyskinesia are both associated with changes in mRNA and peptide levels of the opioid peptide precursors preproenkephalin-A (PPE-A) and preproenkephalin-B (PPE-B). Furthermore, a potential role of abnormal opioid peptide transmission in dyskinesia is suggested due to the ability of opioid receptor antagonists to reduce the L-DOPA-induced dyskinesia in animal models of Parkinson's disease. In this study, the behavioural response, striatal topography and levels of expression of the opioid peptide precursors PPE-A and PPE-B were assessed, following repeated vehicle, ropinirole, or L-DOPA administration in the 6-OHDA-lesioned rat model of Parkinson's disease. While repeated administration of L-DOPA significantly elevated PPE-B mRNA levels (313% cf. vehicle, 6-OHDA-lesioned rostral striatum; 189% cf. vehicle, 6-OHDA-lesioned caudal striatum) in the unilaterally 6-OHDA-lesioned rat model of Parkinson's disease, ropinirole did not. These data and previous studies suggest the involvement of enhanced opioid transmission in L-DOPA-induced dyskinesia and that part of the reason why D2/D3 dopamine receptor agonists have a reduced propensity to elicit dyskinesia may reside in their reduced ability to elevate opioid transmission.

Uptake of 6-[18F]fluoro-L-dopa and [18F]CFT reflect nigral neuronal loss in a rat model of Parkinson's disease

In order to characterize the sensitivity of an analog of levodopa and a dopamine transporter ligand to detect defects in nigrostriatal function, the uptake of [(18)F]FDOPA and [(18)F]CFT was studied ex vivo in a rat model of Parkinson's disease. The brains of these rats were unilaterally lesioned with an intranigral injection of 6-hydroxydopamine. The lesioned animals were divided into three groups subject to their behavior after pharmacological challenges. Circling behavior was recorded after amphetamine, apomorphine, and L-DOPA challenge in order to predict lesion size. The spatial distribution of radioactivity after [(18)F]FDOPA or [(18)F]CFT injection in brain sections was determined with digital autoradiography. Regions of interest were left/right striatum, left/right substantia nigra, and cerebellum. The degree of unilateral lesion for each animal was confirmed by counting of nigral tyrosine hydroxylase-positive cell bodies. With both tracers the uptake in the lesioned side was lower than in the intact side in the striatum and in the substantia nigra. In conclusion, both tracers clearly demonstrated nigrostriatal dopaminergic hypofunction and correlated with the number of nigral dopaminergic neurons. However, [(18)F]FDOPA showed a much higher unspecific uptake of radioactivity, due to extensive metabolism; therefore, this tracer was less sensitive than the transporter tracer [(18)F]CFT to detect these defects.

Bilateral increase in striatal dopamine D2 receptor density in the 6-hydroxydopamine-lesioned rat: a serial in vivo investigation with small animal PET

Unilateral destruction of the substantia nigra by local application of 6-hydroxydopamine (6-OHDA) serves as an animal model for Parkinson's disease. In this study, the changes in neostriatal dopamine D(2) receptor density were investigated with a small animal positron emission tomograph (PET) before and after 6-OHDA lesion. PET scans were performed in 14 rats after injection of the D(2) receptor radioligand [(18)F] N-methylbenperidol. After the first scan (day 0), nigrostriatal pathways were lesioned by unilateral injections of 6-OHDA. Further PET scans were performed on days 2 and 14 post-lesion. For both striata, B(max) values were determined from saturation binding curves with non-linear regression analysis. In the striatum ipsilateral to the lesion, B(max) initially amounted to 19.3+/-1. 9 fmol/mg (mean+/-SD) and increased to 19.7+/-2.2 and 29.9+/-5.7 fmol/mg on days 2 and 14 post-lesion, respectively. Contralateral B(max) values increased from 19.2+/-2 fmol/mg prior to the lesion to 21.2+/-2.9 and 28.6+/-5.7 fmol/mg on days 2 and 14, respectively. On day 14, the ipsilateral saturation binding curve differed from the ipsilateral pre-lesion curve (P=0.04; F test). When the contralateral pre-lesion saturation binding curve was compared with the contralateral post-lesion curve on day 14, a P value of 0.08 was obtained. This first serial in vivo imaging study of 6-OHDA-lesioned rats showed a time-dependent increase in striatal D(2) receptor density on both sides, the increase being more pronounced ipsilateral to the lesion. This result implies that compensatory mechanisms in the intact hemisphere contribute to regenerative processes following nigrostriatal dopaminergic denervation. Overall, our findings show the feasibility of repetitive in vivo studies of striatal receptor density with a small animal tomograph. Moreover, the applied in vivo saturation binding technique provides a versatile method for the quantification of time-dependent changes in the concentration of receptor binding sites.

Subacute systemic 3-nitropropionic acid intoxication induces a distinct motor disorder in adult C57Bl/6 mice: behavioural and histopathological characterisation

Data on motor behavioural disorders induced by systemic 3-nitropropionic acid, an irreversible inhibitor of mitochondrial succinate dehydrogenase and their histopathological correlates in mice, are sparse. We thus further characterised the subacute 3-nitropropionic-acid-induced motor disorder and its time course in C57Bl/6 mice using standard behavioural tests, histopathological correlates and in vivo magnetic resonance imaging. Firstly, we studied two intoxication paradigms (340 and 560 mg 3-nitropropionic acid/kg, 7 days) compared to controls. The low-dose regimen induced only slight motor changes (reduced hindlimb stride length and rearing). The high-dose regimen induced significant (P<0.05) behavioural and sensorimotor integration deficits (pole test, rotarod, stride length, open-field spontaneous activity) but with 37.5% lethality at week one. The clinical motor disorder consisted of hindlimb clasping and dystonia, truncal dystonia, bradykinesia and impaired postural control. Histopathologically, there were discrete lesions of the dorsolateral striatum in 62.5% of mice together with a 32% reduction (P<0.0001) of the striatal volume, reduced caldbindin-D28K immunoreactivity in the lateral striatum, and met-enkephalin and substance P in the striatal output pathways. There was also a significant (P<0.05) 30-40% dopaminergic cell loss within the substantia nigra pars compacta. Secondly, we validated a semi-quantitative behavioural scale to describe the time course of the motor deficits and to predict the occurrence of striatal damage. We sought to determine whether it could also be disclosed in vivo by magnetic resonance imaging. The scale correlated with the striatal volume reduction (r(2)=0.57) and striatal cell loss (r(2)=0.87) but not with the loss of striatal dopaminergic terminals (dopamine transporter binding). Increased T2-signal intensity within the striatal lesion correlated with the cell loss (r(2)=0.66). We conclude that systemic administration of 3-nitropropionic acid in C57Bl/6 mice induces a distinct motor disorder and dose-dependent striatonigral damage, which are potentially useful to model human diseases of the basal ganglia.

Increased dopamine receptor signaling and dopamine receptor-G protein coupling in denervated striatum

Chronic interruption of the nigrostriatal dopaminergic pathway leads to sensitized dopaminergic responses in striatum. We attempted to explore the mechanism(s) underlying this dopaminergic supersensitivity by assessing dopamine receptor signaling and receptor-G protein coupling in unilateral 6-hydroxydopamine-lesioned rats. Dopamine-stimulated adenylyl cyclase activity as well as dopamine-activated guanosine 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding and [(3)H]palmitate incorporation by Galpha proteins were enhanced in tissues obtained from denervated striata without apparent changes in Galpha protein levels. Moreover, high-affinity binding sites of the D(1) dopamine receptor increased in lesioned compared with control striata without altering the expression level of the receptor. These denervation-mediated changes appear to correlate with the increase in D(1) dopamine receptor binding sites that co-immunoprecipitated with Galphas(olf)/q(11) proteins. In contrast, the total number of D(2) receptor binding sites was increased, yielding an increase in absolute number of high-affinity sites without significant changes in the proportion of high-affinity sites. Stimulation of the D(2) dopamine receptor enhanced coupling to Galphai protein; this was increased in the striata lesioned. The results provide an important molecular mechanism by which dopamine receptor-regulated signaling is enhanced following denervation of dopaminergic input to striatum. Although D(1) dopamine receptor supersensitivity appears to be mediated by enhanced coupling of the receptor to its G proteins, sensitization in the D(2) dopamine receptor system is mediated by increased D(2) receptor density and enhanced D(2) receptor-Gi protein coupling.

Protection of dopaminergic nigrostriatal afferents by GDNF delivered by microspheres in a rodent model of Parkinson's disease

The use of glial cell line-derived neurotrophic factor (GDNF) appears to be a promising strategy to promote survival and function of the nigrostriatal dopaminergic pathway damaged in Parkinson's disease (PD). However, effective intracerebral administration is required for optimal therapeutic benefit and tools to evaluate such therapies must be developed. A rodent model of PD was therefore developed using striatal injection of 6-hydroxydopamine (6-OHDA) with simultaneous implantation of GDNF-delivering microspheres. The effects of GDNF released from microspheres were assessed by classical methods such as amphetamine-induced rotating behavior and tyrosine hydroxylase (TH) immunoreactivity, as well as by quantitative autoradiography using PE2I, a dopamine transporter (DAT) radiotracer, which is also suitable for SPET imaging in humans. 6-OHDA-lesioned animals that received microspheres without GDNF were used as controls. During the first 3 weeks after simultaneous lesion and implantation, the amphetamine-induced rotating behavior of GDNF-treated rats was improved compared to controls and an increase in TH expression (+26%) was measured in the striatum 6 weeks after lesion. In accordance with these results, an increase in striatal PE2I-labeled DAT density was obtained (+17%) after 3 and 6 weeks of treatment. In conclusion, this study demonstrates the neuroprotective action of GDNF delivered by microspheres and suggests that PE2I may be an appropriate radiotracer for use in SPET scintigraphy to follow up treatment of PD in humans.

Neither the density nor function of striatal dopamine transporters were influenced by chronic n-3 polyunsaturated fatty acid deficiency in rodents

We hypothesized that the chronic dietary deficiency of n-3 polyunsaturated fatty acids (n-3 PUFAs) might affect the density and/or function of dopamine transporters (DAT), which have a major role in regulating the synaptic level of dopamine. This hypothesis was tested by investigating DAT in the striatum using three complementary methods in control and deficient rats. The density of DAT was determined by quantitative autoradiography using [(125)I]PE2I, a specific ligand of this transporter. Functional investigations were performed (i) in vitro by measuring [(3)H]dopamine uptake on synaptosomes, and (ii) in vivo using intracerebral microdialysis. The results demonstrated that neither the density nor the function of DAT were influenced by n-3 PUFA deficiency in the striatum. This suggests lower sensitivity to n-3 PUFA deficiency in the striatum than that previously observed in the frontal cortex.

Pharmacological characterisation of (E)-N-(3-iodoprop-2-enyl)-2beta-carbomethoxy-3beta-(4'-methylphenyl)nortropane (PE2I) binding to the rat neuronal dopamine transporter expressed in COS cells

The interaction of (E)-N-(3-iodoprop-2-enyl)-2beta-Carbomethoxy-3beta-(4'-methylphenyl) nortropane (PE2I) with the rat neuronal dopamine transporter (DAT) was studied in transfected COS cells by measuring its ability to inhibit DA uptake and by measuring its affinity in radioligand binding experiments. Saturable [3H]DA uptake was measured in COS cells transiently transfected with the cDNA sequence encoding the rat DAT. Pharmacological characterisation of this uptake revealed functional properties with a V(max) value of 45.05+/-2.62 pmol/mg protein per min and a K(m) value of 2.86+/-0.28 microM. The specific [3H]DA uptake was fully inhibited by 1 microM PE2I. Concentration response curves revealed the high potency of PE2I in inhibiting DA uptake (pEC(50) value of 8.70+/-0.33), 25 times higher than that observed for the reference DAT inhibitor, GBR 12935. On crude homogenates from transfected COS cells, PE2I displaced the specific binding of [3H]GBR 12935 with a pK(i) value of 7.73+/-0.13. Accordingly, [125I]PE2I was found to specifically recognise a single binding site population which is almost completely displaced by GBR 12935 and nomifensine. Saturation experiments revealed the high affinity of [125I]PE2I (K(D) value of 3.8+/-0.63 nM) that correlates with the high potency of PE2I in inhibiting the [3H]DA uptake. This contrasts with the results obtained with GBR 12935 for which a discrepancy was found between its high affinity in binding assays (K(D) value of 0.43+/-0.04 nM) and its rather low potency in functional assays (pEC(50) value of 7.30+/-0.05). A relatively high level of [3H]GBR 12935 binding was detected in non transfected COS cells. Such nomifensine resistant binding is attributed to the interaction of GBR 12935 with cytochrome P-450 as it was displaced by cis-(Z)-flupentixol (an inhibitor of cytochrome P-450). Such interaction was not observed using PE2I. Taken together, these data demonstrate that PE2I was a highly potent inhibitor of cloned DAT compared with GBR 12935 and provided a useful tool for further investigations in cells transfected with cDNA encoding the DAT.

Evaluation of neurotoxicity of TIQ and MPTP and of parkinsonism-preventing effect of 1-MeTIQ by in vivo measurement of pre-synaptic dopamine transporters and post-synaptic dopamine D(2) receptors in the mouse striatum

Parkinsonism-inducing neurotoxicity of 1,2,3,4-tetrahydroisoquinoline (TIQ), as contrasted to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and parkinsonism-preventing effect of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ) have been investigated in mice by measuring their effects on the in vivo binding of radioligand to pre-synaptic dopamine transporters (DATs) or to dopamine D(2) receptors (D2R) in the striatum. A significant reduction of the ligand-DATs binding was found in the mice treated with MPTP, but not with TIQ, under the dosage inducing behavioral abnormality and loss of tyrosine hydroxylase-positive cells in the substantia nigra. A slight decrease in the ligand-DATs binding was observed in the mice given a larger dose of TIQ. Compensatory up-regulation in the post-synaptic D2Rs was found in the MPTP-treated mice. Pre-treatment with (S)-enantiomer, but not (R)-enantiomer, of 1-MeTIQ prevented the degeneration of DATs to some extent. We concluded that the TIQ-induced parkinsonism model is different from the MPTP-induced model as evaluated by the radioligand-DATs binding and that (S)-1-MeTIQ has a preventing effect for the degeneration of the DATs to a certain extent.

Relationship between the appearance of symptoms and the level of nigrostriatal degeneration in a progressive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned macaque model of Parkinson's disease

The concept of a threshold of dopamine (DA) depletion for onset of Parkinson's disease symptoms, although widely accepted, has, to date, not been determined experimentally in nonhuman primates in which a more rigorous definition of the mechanisms responsible for the threshold effect might be obtained. The present study was thus designed to determine (1) the relationship between Parkinsonian symptom appearance and level of degeneration of the nigrostriatal pathway and (2) the concomitant presynaptic and postsynaptic striatal response to the denervation, in monkeys treated chronically with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine according to a regimen that produces a progressive Parkinsonian state. The kinetics of the nigrostriatal degeneration described allow the determination of the critical thresholds associated to symptom appearance, these were a loss of 43.2% of tyrosine hydroxylase-immunopositive neurons at the nigral level and losses of 80.3 and 81.6% DA transporter binding and DA content, respectively, at the striatal level. Our data argue against the concept that an increase in DA metabolism could act as an efficient adaptive mechanism early in the disease progress. Surprisingly, the D(2)-like DA receptor binding showed a biphasic regulation in relation to the level of striatal dopaminergic denervation, i.e., an initial decrease in the presymptomatic period was followed by an upregulation of postsynaptic receptors commencing when striatal dopaminergic homeostasis is broken. Further in vivo follow-up of the kinetics of striatal denervation in this, and similar, experimental models is now needed with a view to developing early diagnosis tools and symptomatic therapies that might enhance endogenous compensatory mechanisms.

Anatomic and biochemical correlates of the dopamine transporter ligand 11C-PE2I in normal and parkinsonian primates: comparison with 6-[18F]fluoro-L-dopa

Positron emission tomography (PET) coupled to 6-[18F]Fluoro-L-Dopa (18F-Dopa) remains the gold standard for assessing dysfunctionality concerning the dopaminergic nigrostriatal pathway in Parkinson's disease and related disorders. The use of ligands of the dopamine transporters (DAT) is an attractive alternative target; consequently, the current aim was to validate one of them, 11C-PE2I, using a multiinjection modeling approach allowing accurate quantitation of DAT densities in the striatum. Experiments were performed in three controls, three MPTP-treated (parkinsonian) baboons, and one reserpine-treated baboon. 11C-PE2I B'max values obtained with this approach were compared with 18F-Dopa input rate constant values (Ki), in vitro Bmax binding of 125I-PE2I, and the number of dopaminergic neurons in the substantia nigra estimated postmortem by stereology. In the caudate nucleus and putamen, control values for 11C-PE2I B'max were 673 and 658 pmol/mL, respectively, whereas it was strongly reduced in the MPTP-treated (B'max = 26 and 36 pmol/mL) and reserpine-treated animals (B'max = 338 and 483 pmol/mL). In vivo 11C-PE2I B'max values correlated with 18F-Dopa Ki values and in vitro 125I-PE2I Bmax values in the striatum and with the number of nigral dopaminergic neurons. Altogether, these data support the use of 11C-PE2I for monitoring striatal dopaminergic disorders and the effect of potential neuroprotective strategies.

Pallidal border cells: an anatomical and electrophysiological study in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkey

A dopamine transporter-radioligand binding study demonstrated a dopaminergic innervation around the pallidal complex in the normal monkey (n=5), i.e. where a subpopulation of pallidal neurons known as "border cells" is classically identified. Surprisingly, this peripallidal binding persists in monkeys rendered parkinsonian (n=5) with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. The border cell electrophysiological activity was then analysed in normal and parkinsonian monkeys (n=2), either in the untreated state or following administration of levodopa. Pallidal border cell firing frequency was significantly decreased after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment (8.9+/-0.7 vs 31.4+/-1.6Hz, P<0.05). This decrease was partly corrected by levodopa administration (19.2+/-1.0Hz, P<0.05 vs both normal and parkinsonian situations). The peripallidal dopaminergic innervation suggests that pallidal border cells are under a direct dopaminergic control, arising from the ventral tegmental area and/or the basal forebrain magnocellular complex, the role of which remains unknown. Moreover, the relative sparing of these dopaminergic fibers in parkinsonian monkeys suggests that they would exhibit specific adaptive properties totally different from those described in the nigrostriatal pathway.

Microdialysis as a tool for in vivo study of dopamine transporter function in rat brains

The role of dopamine as a major modulator of CNS function is well-known, and the homeostasis of dopamine is considered to be of major importance in the pathogenesis of several psychiatric and neurological diseases. Few methods are currently available for in vivo study of dopamine transporter function, which regulates extracellular levels of dopamine. Adapting the 'indicator diffusion' method applied to the microdialysis technique, we present here a suitable method for this functional investigation. We measured the cellular extraction of [3H]-MPP+, which is known to accumulate in the dopaminergic neurones through the DAT in the rat striatum, using [14C]-mannitol as reference substance characterized by absence of cellular accumulation. The cellular extraction was 0.41 and was almost abolished in the presence of the dopamine-uptake inhibitor cocaine, reaching 0.07. This suggested that extraction of [3H]-MPP+ was due to cellular uptake by dopamine transporters. Tissue analysis confirmed that [3H]-MPP+ was internalized in cells and that such transport was stopped by cocaine. Moreover, [3H]-MPP+ extraction was dramatically decreased after lesioning the nigro-striatal pathway with 6-hydroxydopamine, whereas [14C]-mannitol extraction was unchanged. It is concluded that the presented method can be used to study the functioning of the dopamine transporter in live animals.