In-vivo measurement of LDOPA uptake,dopamine reserve and turnover in the rat brain using [18F]FDOPA PET

Longitudinal measurements of dopamine (DA) uptake and turnover in transgenic rodents may be critical when developing disease-modifying therapies for Parkinson''s disease (PD). We demonstrate methodology for such measurements using [¹⁸F]fluoro-3,4-dihydroxyphenyl-L-alanine ([¹⁸F]FDOPA) positron emission tomography (PET). The method was applied to 6-hydroxydopamine lesioned rats, providing the first PET-derived estimates of DA turnover for this species. Control (n=4) and unilaterally lesioned (n=11) rats were imaged multiple times. Kinetic modeling was performed using extended Patlak, incorporating a k_loss term for metabolite washout, and modified Logan methods. Dopaminergic terminal loss was measured via [¹¹C]-(+)-dihydrotetrabenazine (DTBZ) PET. Clear striatal [¹⁸F]FDOPA uptake was observed. In the lesioned striatum the effective DA turnover increased, shown by a reduced effective distribution volume ratio (EDVR) for [¹⁸F]FDOPA. Effective distribution volume ratio correlated (r>0.9) with the [¹¹C]DTBZ binding potential (BP_ND). The uptake and trapping rate (k_ref) decreased after lesioning, but relatively less so than [¹¹C]DTBZ BP_ND. For normal controls, striatal estimates were k_ref=0.037±0.005 per minute, EDVR=1.07±0.22 and k_loss=0.024±0.003 per minute (30 minutes turnover half-time), with repeatability (coefficient of variation) ≤11%. [¹⁸F]fluoro-3,4-dihydroxyphenyl-L-alanine PET enables measurements of DA turnover in the rat, which is useful for developing novel therapies for PD.     

The hypothalamus in MPTP-induced parkinsonism

1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) has been shown to produce a parkinsonian syndrome in humans and other primates. Recent studies have demonstrated that in humans the hypothalamus has the highest binding density for (³H) MPTP, which corresponds to monoamine oxidase type B (MAO-B). There is evidence that the conversion of MPTP to the toxic compound MPP⁺ takes place in the hypothalamus; subsequently, MPP⁺ is transported to the striatal system, where destruction of nigrostriatal dopamine neurons occurs. Thus, the hypothalamus appears to be a primary target organ of MPTP toxicity. This assumption is supported by the observation that monkeys exposed to MPTP exhibit extensive pathological lesions in the hypothalamus which are manifested clinically by the development of life-threatening anorexia requiring forced feeding to overcome. We discuss the clinical implications of MPTP-induced hypothalamic damage to the pathophysiology of MPTP-induced parkinsonism and to Parkinson disease. It is suggested that consideration of hypothalamic involvement in MPTP-induced parkinsonism may provide a broader understanding of the pathophysiology of parkinsonism and may, in addition, account for the preliminary observations that MAO-B inhibitors retard the progression of Parkinson disease and possibly prolong life expectancy.

---

Sommario La l-metil-4-fenil-1,2,5,6 tetraidropiridina (MPTP) produce una sindrome parkinsoniana nell'uomo e in alcuni primati; studi recenti hanno dimostrato che nell'uomo l'ipotalamo ha la più alta capacità di legare il (3H) MPTP. è dimostrato che la conversione dell'MPTP nel composto tossico MPP+ si svolge nell'ipotalamo; successivamente lo MPP+ è trasportato al sistema striato e quindi si determina una distribuzione dei neuroni della dopamina nigrostriatale. Risulta così che l'ipotalamo è l'organo bersaglio primario della tossicità dell'MPTP e ciò è dimostrato dalla osservazione che le scimmie esposte al-l'azione dell'MPTP presentano estese lesioni patologiche nell'ipotalamo e manifestano clinicamente una anoressia di tale gravità da richiedere l'alimentazione forzata. Noi discutiamo le implicazioni cliniche del danno ipotalamico da MPTP con la patofisiologia del parkinsonismo indotto da MPTP e col morbo di Parkinson. Riteniamo che l'interessamento ipotalamico nel Parkinson indotto da MPTP può dare una più ampia interpretazione della fisiopatofisiologia del parkinsonismo e può dare spiegazione di osservazioni preliminari che dimostrano che i MAO-B inibitori ritardano la progressione del morbo di Parkinson e prolungano probabilmente l'aspettativa di vita.

     

6-[18F]fluoro-L-dopa probes dopamine turnover rates in central dopaminergic structures

6-[18F]Fluoro-L-DOPA (FDOPA) cerebral kinetics and metabolism were correlated in normal primates (Macaca nemestrina) and primates with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced unilateral Parkinsonism. Application of a tracer kinetic model to positron emission tomography (PET) data indicated that the model allows reliable estimation of FDOPA blood brain barrier transport, decarboxylation and release of stored 6-[18F]fluorodopamine (FDA) radioactivity in normal striatum (k4 = 0.005/min, turnover half-time greater than or equal to 2 hr), in agreement with biochemical data. PET scans of MPTP treated monkeys revealed 40-50% reduction in total striatal activity in comparison with pre-MPTP scans. Monkey brain biochemical analysis revealed that the reduction in activity was mainly due to a decrease in FDA and its metabolites, 6[18F]fluorohomovanillic (FHVA) and 6-[18F]fluoro-3, 4-dihydroxyphenylacetic acid (FDOPAC). The remaining activity in tissue was 3-0-methyl-6-[18F]fluoro-L-DOPA (3-OMFD) of peripheral origin. The (FHVA + FDOPAC)/FDA ratio was 1:2 in normal putamen and greater than or equal to 6:1 in the lesioned putamen, indicative of a dramatic increase in turnover of FDA. Both kinetic and biochemical data indicate that FDOPA labels a slow turnover rate pool of dopamine in rat and primate brain. This turnover rate for stored dopamine (DA) is accelerated with dopaminergic cell losses (e.g., MPTP-induced Parkinsonism).     

Striatal and extrastriatal dopamine release measured with PET and [18F] fallypride

The amphetamine challenge, in which positron emission tomography (PET) or single photon emission computed tomography radioligand binding following administration of amphetamine is compared to baseline values, has been successfully used in a number of brain imaging studies as an indicator of dopaminergic function, particularly in the striatum. [¹⁸F] fallypride is the first PET radioligand that allows measurement of the effects of amphetamine on D2/D3 ligand binding in striatum and extra‐striatal brain regions in a single scanning session following amphetamine. We scanned 15 healthy volunteer subjects with [¹⁸F] fallypride at baseline and following amphetamine (0.3 mg/kg) using arterial plasma input‐based modeling as well as reference region methods. We found that amphetamine effect was robustly detected in ventral striatum, globus pallidus, and posterior putamen, and with slightly higher variability in other striatal subregions. However, the observed effect sizes in striatum were less than those observed in previous studies in our laboratory using [¹¹C] raclopride. Robust effect was also detected in limbic extra‐striatal regions (hippocampus, amygdala) and substantia nigra, but the signal‐to‐noise ratio was too low to allow accurate measurement in cortical regions. We conclude that [¹⁸F] fallypride is a suitable ligand for measuring amphetamine effect in striatum and limbic regions, but it is not suitable for measuring the effect in cortical regions and may not provide the most powerful way to measure the effect in striatum. Synapse 64:350–362, 2010. © 2009 Wiley‐Liss, Inc.     

NMSP binding to dopamine and serotonin receptors in MPTP-induced parkinsonism: relation to dopa therapy

We tested the hypothesis that N-methylspiperone binding to dopamine D2 receptors must be reduced when L-dopa therapy of parkinsonism augments the binding of dopamine to the receptors and improves the clinical state expressed by the Hoehn & Yahr stage. A patient with MPTP-induced parkinsonism underwent two positron emission tomographic studies of the D2-like dopamine receptors with N-[11C]methylspiperone (NMSP). The first study took place 3 days after cessation of the L-dopa medication, the second 5 days after its resumption. Noticeable clinical deterioration occurred during both studies, consistent with significant dopamine receptor blockade by NMSP and elevated NMSP binding in both scans. The dopa treatment did not reduce the NMSP binding. On the contrary, the rate of binding of NMSP (k3) was increased on-dopa, compared to off-dopa. The increase was consistent with the slightly greater dopamine receptor density estimated after resumption of the dopa therapy. The NMSP binding to serotonin receptors suggested lower synaptic serotonin on-dopa than off-dopa. The results are consistent with negative correlation between the Hoehn & Yahr stage and the amount of dopamine bound to dopamine D2 receptors. Low synaptic serotonin may explain the depression seen in some patients on dopa for Parkinson's disease.     

Tremor is associated with PET measures of nigrostriatal dopamine function in MPTP-lesioned monkeys

Unilateral intracarotid artery (ICA) MPTP infusion, along with sequential systemic doses of MPTP, produces near complete degeneration of the nigrostriatal pathway on the side of infusion (ipsilateral) and variable levels of damage in the contralateral hemisphere accompanied by varying levels of parkinsonism (overlesioned hemiparkinsonian model). Positron emission tomography and the dopamine (DA) metabolism tracer [(18)F]6-fluoro-l-m-tyrosine (FMT) were used to evaluate the relationship between DA metabolism and clinical features of parkinsonism in 14 overlesioned hemiparkinsonian monkeys. Monkeys were rated on a parkinsonian scale that included ratings of bradykinesia, fine motor skills (FMS), and rest tremor. Because the monkeys tended to show more severe clinical signs on the side of the body contralateral to ICA MPTP infusion, we calculated asymmetry scores for each of the clinical features as well as for FMT uptake (K(i)) in the caudate and putamen. Tremor asymmetry was associated with FMT uptake asymmetry in the putamen. No such relationship was observed for FMS or bradykinesia. The overall severity of tremor (mild, moderate/severe) was associated with FMT uptake in the caudate and putamen. Postmortem biochemical analysis for a subset of monkeys showed that the monkeys with moderate/severe tremor had significantly lower DA levels in both caudate and putamen than those with mild tremor. In addition, K(i) values were significantly correlated with DA levels in both caudate and putamen. These findings support the idea that nigrostriatal degeneration contributes to rest tremor.     

Net influx of plasma 6-[18F]fluoro-L-DOPA (FDOPA) to the ventral striatum correlates with prefrontal processing of affective stimuli

Dopaminergic neurotransmission in the ventral and dorsal striatum interact with central processing of rewarding and reward-indicating stimuli, and may affect frontocortical-striatal-thalamic circuits regulating goal-directed behaviour. Thirteen healthy male volunteers were investigated with multimodal imaging, using the radioligand 6-[(18)F]fluoro-l-DOPA (FDOPA) for positron emission tomography (PET) measurements of dopamine synthesis capacity, and also functional magnetic resonance imaging (fMRI) in a cognitive activation paradigm. We calculated the correlation between FDOPA net blood-brain influx (; ml/g/min) in the ventral and associative dorsal striatum and BOLD signal changes elicited by standardized affectively positive, negative and neutral visual stimuli. The magnitude of in the ventral striatum was positively correlated with BOLD signal increases in the left anterior cingulate cortex and right insular operculum elicited by positive vs. neutral stimuli, but not negative vs. neutral stimuli. In the dorsal striatum, the magnitude of was positively correlated with processing of positive and negative stimuli in the left dorsolateral prefrontal cortex. These findings suggest that dopamine synthesis capacity in the ventral striatum correlates with the attentional processing of rewarding positive stimuli in the anterior cingulate cortex of healthy subjects. Dopaminergic neurotransmission in the associative dorsal striatum has been associated previously with habit learning. The observed correlation between dopamine synthesis capacity in the dorsal striatum and BOLD signal changes in the dorsolateral prefrontal cortex suggests dopaminergic modulation of processing of emotional stimuli in brain areas associated with motor planning and executive behaviour control.     

Effect of hypoxia/hypercapnia on metabolism of 6-[(18)F]fluoro-L-DOPA in newborn piglets

There is evidence that the dopaminergic system is sensitive to altered p(O(2)) in the immature brain. However, the respective enzyme activities have not been measured in the living neonatal brain together with brain oxidative metabolism. Therefore 18F-labelled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography (PET) was used to estimate the activity of the aromatic amino acid decarboxylase (AADC) in the brain of fifteen newborn piglets (2-5 days old). Two PET scans were performed in each piglet. Eleven animals underwent a period of normoxia and moderate hypoxia/hypercapnia (H/H). The remaining four animals were used as an untreated control group. Simultaneously, the brain tissue p(O(2)) was recorded, the regional cerebral blood flow (CBF) was measured with colored microspheres and the cerebral metabolic rate of oxygen (CMRO(2)) was determined. In addition, in four untreated and six H/H treated piglets the relative amounts of fluorodopamine and the respective metabolites were determined in brain tissue samples using HPLC analysis. H/H conditions were induced by lowering the inspired fraction of oxygen from 0.35 to 0.10 and adding CO(2) to the inspired gas resulting in an arterial p(CO(2)) between 74 and 79 mmHg. H/H elicited a more than 3-fold increase of the CBF (P<0.05) so that the CMRO(2) remained unchanged throughout the H/H period. Despite this, the brain tissue p(O(2)) was reduced from 19+/-4 to 6+/-3 mmHg (P<0.05). The permeability-surface area product of FDOPA (PS(FDOPA)) was unchanged. However, the transfer rate of FDOPA (k(3)(FDOPA)) of the nigrostriatal dopaminergic system and the relative amounts of fluorodopamine and the respective metabolites were significantly increased (P<0.05). It is suggested that H/H induces an increase of AADC activity. However, an H/H-induced CBF increase maintains bulk O(2) delivery and preserves CMRO(2).     

In vivo PET studies of the dopamine D2 receptors in rhesus monkeys with long-term MPTP-induced parkinsonism

Studies of dopamine (DA) receptor binding in early parkinsonian patients, or in models of Parkinson's disease, have revealed a supersensitivity of the D2-like receptor subtype as compared to age-matched controls. The lack of upregulation in advanced patients is often attributed to the effects of prolonged antiparkinsonian therapy, but the impact of therapy vs. intrinsic mechanisms in untreated patients or animals with long-term lesions of the DA nigrostriatal pathway has been difficult to address. We studied, in vivo, by PET using the DA D2 receptor ligand raclopride, the status of the DA receptors in normal rhesus monkeys and those with acute (3 months) or long-term (10 years) MPTP-induced nigrostriatal lesions. Compared to age-matched controls, there was no change in raclopride binding in MPTP-treated animals without parkinsonian symptoms. There was a significant increase in raclopride binding in the putamen (but not caudate nucleus) of all the animals displaying rigidity, hypo- and bradykinesia. This increase was greater in the animals with acute lesions (32%) than with established, long-term lesions (18%). There was no correlation between the postmortem striatal DA concentrations and in vivo raclopride binding but there was a correlation between PET raclopride binding and [(3)H]raclopride binding in vitro. Complex changes in D2 receptor binding occur in various stages of parkinsonism. Antiparkinsonian therapy is unlikely to be solely responsible for the lack of upregulation found in advanced parkinsonian patients but may be a contributing factor.     

Broad neuroprotective profile of nicotinamide in different mouse models of MPTP-induced parkinsonism

The factors contributing to substantia nigra pars compacta (SNc) dopamine (DA) neuron death and striatal DA depletion in Parkinson's disease (PD) are still poorly understood. However, mitochondrial dysfunction, cellular energy depletion and oxidative stress appear to play important roles in the pathogenesis of PD. In view of this, the current study examined the potential of nicotinamide, a form of the B-complex vitamin niacin, to protect against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced SNc cell loss and striatal DA depletion in two mouse MPTP models that respond differently to putative neuroprotective agents. Adult male C57Bl/6 mice received nicotinamide (125, 250 or 500 mg/kg i.p.) prior to either acute (four injections in 1 day at 2-h intervals) or sub-acute (two injections per day at 4-h intervals for 5 days) MPTP administration. Striatal DA levels, changes in numbers of tyrosine hydroxylase (TH)- and cresyl violet-stained cells in the SNc at 2 and 6 weeks following the last MPTP exposure were analyzed. Nicotinamide administration resulted in a dose-dependent sparing of striatal DA levels and SNc neurons in acute MPTP-treated animals. Only the highest dose of nicotinamide had similar effects in sub-acute MPTP-treated animals. At 6 weeks after MPTP exposure, there was some spontaneous recovery of striatal DA levels in both models: neuroprotective effects were still apparent in acute but not sub-acute MPTP-treated animals. These results show neuroprotective effects of nicotinamide in different mouse Parkinson models associated with different forms of cell death and suggest that nicotinamide may have broad neuroprotective potential in PD.     

6-[18F]fluoro-L-DOPA-PETstudies show partial reversibility of long-term effects of chronic amphetamine in monkeys

The acute and long-term effects of chronic amphetamine administration on the striatal dopamine system in monkeys were assessed with 6-[¹⁸F]fluoro-L-DOPA (FDOPA) and positron emission tomography (PET). Vervet monkeys (Cerecopithecus aethiops) were administered amphetamine doses, i.m., that increased from 4 mg/kg/d to 18 mg/kg/d over a 10 day period. Post-amphetamine FDOPA-PET scans at 1–2, 3–4, and 6 week time points in individual subjects showed persistent decrements in dopamine synthesis capacity as reflected by FDOPA influx rate constant (K_i) values being ∼30% that of pre-drug assessment. In other animals that were administered the same drug regimen, biochemical analysis of striatal regions at 1–2 weeks post-drug indicated that dopamine concentrations were decreased by ∼95% throughout caudate and putamen regions, while the homovanillic acid/dopamine level ratio was increased 3–10-fold. Post-drug FDOPA-PET K_i values remained consistently low up to 6 weeks; however, at the 5–6 month time point, relative increases in FDOPA-K_i values (∼53% of pre-drug values) were observed for all subjects, indicative of partial recovery of striatal dopamine synthesis capacity. These results demonstrate that FDOPA-PET can reveal temporal activity changes within the striatal dopamine system of individual subjects. The apparent, partial reversibility of amphetamine's neurotoxic effects suggests a plasticity of dopaminergic function that may include regeneration of dopaminergic terminals and compensatory increases in residual dopamine synthesis rates. The persistence of the partial decrement in dopamine synthesis capacity, however, may indicate a long term component of amphetamine's toxic effects. © 1996 Wiley-Liss, Inc.     

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.     

[123I]FP-CIT binds to the dopamine transporter as assessed by biodistribution studies in rats and SPECT studies in MPTP-lesioned monkeys

[¹²³I]FP-CIT (N-ω-fluoropropyl-2β-carbomethoxy-3β-{4-iodophenyl}tropane), a radioiodinated cocaine analogue, was evaluated as an agent for the in vivo labeling of dopamine (DA) transporters by biodistribution studies in rats and by single photon emission computed tomography (SPECT) studies in unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)–lesioned monkeys. In rats, intravenous injection of [¹²³I]FP-CIT resulted in high accumulation of radioactivity in the striatum. Less pronounced uptake was seen in brain areas with high densities of serotonergic uptake sites. While striatal uptake of radioactivity after injection of [¹²³I]FP-CIT was displaced significantly by GBR12,909 but not by fluvoxamine, the opposite was observed in brain areas known to be rich of serotonin transporters. Monkeys which were unilaterally treated with neurotoxic doses of MPTP showed severe loss of striatal [¹²³I]FP-CIT uptake at the side of treatment. The results of this study indicate that [¹²³I]FP-CIT, although not being a selective radioligand, binds specifically to the striatal DA transporter in vivo and thus suggest that [¹²³I]FP-CIT promises to be a suitable radioligand for SPECT imaging of DA transporters in humans. Synapse 27:183–190, 1997. © 1997 Wiley-Liss, Inc.     

Attenuation of MPTP-induced dopaminergic neurotoxicity by a serotonin uptake blocker

Summary Monoamine oxidase-B (MAO-B) has been determined to be the enzyme responsible for the conversion of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) into its toxic metabolite 1-methyl-4-phenylpyridine ion (MPP+). Since this enzyme has been localized primarily in astrocytes and serotonergic neurons, it would appear that MPP+ is being produced outside the dopaminergic neurons. To investigate this possibility, the administration of MPTP was preceded by systemically administered fluoxetine. In keeping with its demonstrated ability to inhibit uptake into serotonergic neurons and serotonin uptake into astrocytes, fluoxetine pretreatment resulted in a significant attenuation of MPTP-induced depletions of striatal dopamine and serotonin concentration. These results support the extra-dopaminergic production of MPP+.     

18F]FDOPA PET and clinical features in parkinsonism due to manganism.

Manganese exposure reportedly causes a clinically and pathophysiologically distinct syndrome from idiopathic Parkinson's disease (PD). We describe the clinical features and results of positron emission tomography with 6-[18F]fluorodopa ([18F]FDOPA PET) of a patient with parkinsonism occurring in the setting of elevated blood manganese. The patient developed parkinsonism associated with elevated serum manganese from hepatic dysfunction. [18F]FDOPA PET demonstrated relatively symmetric and severely reduced [18F]FDOPA levels in the posterior putamen compared to controls. The globus pallidum interna had increased signal on T1-weighted magnetic resonance imaging (MRI) images. We conclude that elevated manganese exposure may be associated with reduced striatal [18F]FDOPA uptake, and MRI may reveal selective abnormality within the internal segment of the pallidum. This case suggests that the clinical and pathophysiological features of manganese-associated parkinsonism may overlap with that of PD.     

[18F]CFT ([18F]WIN 35,428), a radioligand to study the dopamine transporter with PET: Characterization in human subjects

We have characterized the usage of [¹⁸F]CFT (also known as [¹⁸F]WIN 35,428) as a radioligand for in vivo studies of human dopamine transporter by PET. CFT was labeled with ¹⁸F to a high specific activity, and dynamic PET scans were conducted in healthy volunteers at various time points up to 5 h from [¹⁸F]CFT injection. The regional distribution of [¹⁸F]CFT uptake correlated well with the known distribution of dopaminergic nerve terminals in the human brain and also with that of other dopamine transporter radioligands. Striatal binding peaked at 225 min after injection and declined thereafter, demonstrating the reversible nature of the binding to the dopamine transporter. Therefore, due to the relatively long half-life of ¹⁸F (109.8 min), PET scans with [¹⁸F]CFT could easily be conducted during the binding equilibrium, allowing estimation of B_max/K_d values (i.e., binding potential). Binding potentials for putamen and caudate measured at equilibrium were 4.79 ± 0.11 and 4.50 ± 0.23, respectively. We were able to also visualize midbrain dopaminergic neurons (substantia nigra) with [¹⁸F]CFT in some subjects. In conclusion, the labeling of CFT with ¹⁸F allows PET scans to be conducted at binding equilibrium, and therefore a high signal-to-noise ratio and reliable quantification of binding potential can be achieved. With a high resolution 3D PET scanner, the quantification of extrastriatal dopamine transporters should become possible. Synapse 28:244–250, 1998. © 1998 Wiley-Liss, Inc.     

[18F]CFT ([18F]WIN 35,428), a radioligand to study the dopamine transporter with PET: Biodistribution in rats

We describe the ¹⁸F-radiolabelling synthesis (¹⁸F; T1/2 = 109.8 min) of 2-β-carbomethoxy-3β-(4-fluorophenyl)tropane (also known as CFT or WIN 35,428) and the biodistribution of this compound in rats. ¹⁸F-labelled CFT has high chemical and radiochemical purity and relatively high specific radioactivity [specific radioactivity up to 14.8 GBq/μmol (400 mCi/μmol) at end of synthesis]. Striatum to cerebellum radioactivity uptake ratios were calculated from digitised images of rat brain slices recorded with a phosphoimaging device, the maximum ratio of about 10 was obtained at 2 h postinjection. Pretreatment of the rats with a specific dopamine transport inhibitor, GBR 12909, showed that CFT binding is specific in striatum. The highest accumulation of ¹⁸F-radioactivity was found in the liver, urine, striatum, and kidney of the rat. Clearance from blood was rapid. The uptake in bone was low, indicating that [¹⁸F]CFT is not defluorinated. The relatively long half-life of ¹⁸F makes it possible to study the uptake of [¹⁸F]CFT in the brain, as equilibrium between specific and non-specific binding is reached. This will improve the signal to noise ratio as compared to positron emission tomography (PET) studies with [¹¹C]CFT (¹¹C; T_1/2 = 20.4 min). CFT labelled with ¹⁸F is clearly a promising radioligand for PET studies of the dopamine transporter system in humans. © 1996 Wiley-Liss, Inc.     

MPTP-induced parkinsonism: acceleration of biochemical and behavioral recovery by GM1 ganglioside treatment.

The effects of GM1 ganglioside administration on functional recovery and recovery of caudate nucleus dopamine levels have been assessed in cats made parkinsonian by administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Cats made severely parkinsonian by MPTP administration began to show spontaneous functional recovery by the third week after MPTP, as had been observed in previous studies with this model. In contrast, cats with similar initial impairment but which received 3 weeks of GM1 ganglioside treatment (30 mg/kg, i.p. daily) showed an accelerated behavioral recovery, showing significant functional improvement after the first week of GM1 treatment and almost normal function by the end of the third week of treatment. The GM1-treated cats had caudate nucleus dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and HVA levels significantly increased above levels measured in saline-treated MPTP control cats. A second group of cats received MPTP only until the first signs of parkinsonism were observed and thus overall had a less severe initial syndrome than the cats described previously. Again, while all cats showed functional recovery over time, the recovery process was accelerated in GM1-treated cats. GM1 treatment also caused a significant increase in caudate dopamine levels in these cats. These results suggest that GM1 ganglioside administration can result in increased dopamine levels even in the heavily denervated striatum and accelerate functional recovery after an MPTP-induced lesion of the nigrostriatal dopamine system in the cat. This suggests that GM1 or other trophic factor therapies may be fruitful treatment strategies for a disorder of nigrostriatal function such as Parkinson's disease.     

Progressive loss of striatal dopamine terminals in MPTP-induced acute parkinsonism in cynomolgus monkeys using vesicular monoamine transporter type 2 PET imaging ([18F]AV-133)

The 1-methyl-4-phenyl-1,2,3,4-tetrahydropyridine (MPTP)-induced parkinsonism model, particularly in non-human primates, remains the gold-standard for studying the pathogenesis and assessing novel therapies for Parkinson’s disease. However, whether the loss of dopaminergic neurons in this model is progressive remains controversial, mostly due to the lack of objective in vivo assessment of changes in the integrity of these neurons. In the present study, parkinsonism was induced in cynomolgus monkeys by intravenous administration of MPTP (0.2 mg/kg) for 15 days; stable parkinsonism developed over 90 days, when the symptoms were stable. Noninvasive positron emission tomographic neuroimaging of vesicular monoamine transporter 2 with 9-[¹⁸F] fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]AV-133) was used before, and 15 and 90 days after the beginning of acute MPTP treatment. The imaging showed evident progressive loss of striatal uptake of [¹⁸F]AV-133. The dopaminergic denervation severity had a significant linear correlation with the clinical rating scores and the bradykinesia subscores. These findings demonstrated that [¹⁸F]AV-133 PET imaging is a useful tool to noninvasively evaluate the evolution of monoaminergic terminal loss in a monkey model of MPTP-induced parkinsonism.