Randa E. Yee

Novel observations with FDOPA‐PET imaging after early nigrostriatal damage

Striatal 6‐[18F]fluoro‐L‐DOPA (FDOPA) kinetic rate constants were measured by positron emission tomography (PET) in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐treated squirrel monkeys. After scanning, stereological counts of dopaminergic neurons were done in substantia nigra, and dopamine (DA) and metabolite concentrations were determined in the caudate, putamen, and substantia nigra. Graded doses of MPTP produced animals with mild to moderate reductions (10–35%) in dopaminergic neurons, where the percent of cell loss was proportional to the amount of MPTP given. Striatal DA and metabolite concentrations were relatively unchanged in animals given 1.0 and 1.5 mg/kg of MPTP, but were significantly reduced after 2.0 mg/kg of MPTP. All animals injected with a single dose of MPTP showed no overt signs of parkinsonism. In contrast, DA and metabolite concentrations in the substantia nigra were significantly reduced for all MPTP‐treated animals. Reduction of dopaminergic indices in the substantia nigra did not parallel reductions in the striatum, indicating differential sensitivity of the nigrostriatal pathway to the neurotoxic effects of MPTP. The percent change in FDOPA uptake (Ki) and decarboyxlation (k3) after MPTP showed significant positive correlations to striatal DA levels, but not to the number of dopaminergic neurons. This suggests that FDOPA is a good index of striatal DA levels.

Nigrostriatal reduction of aromatic L-amino acid decarboxylase activity in MPTP-treated squirrel monkeys : In vivo and in vitro investigations

Aromatic L‐amino acid decarboxylase (AAAD) activity was examined in vivo with positron emission tomography (PET) using 6‐[18F]fluoro‐L‐DOPA (FDOPA) in squirrel monkeys lesioned with graded doses of the neurotoxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP). In vitro biochemical determinations of AAAD activity in caudate, putamen, substantia nigra, and nucleus accumbens were performed in the same animals to establish a direct comparison of in vivo and in vitro measurements. In vivo and in vitro AAAD activities in caudate/putamen were substantially reduced in animals treated with the highest dose of MPTP (2.0 mg/kg). The percent change in the striatal FDOPA uptake (Ki) and decarboxylation rate constant (k3) values resulting from MPTP treatment showed highly significant correlations with in vitro‐determined AAAD activities. However, decarboxylase rates within individual animals presented as ~ 10‐fold difference between in vivo and in vitro values. Lower in vivo k3 measurements may be attributed to several possibilities, including transport restrictions limiting substrate availability to AAAD within the neuron. In addition, reductions in AAAD activity in the substantia nigra did not parallel reductions in AAAD activity within the striatum, supporting the notion of a nonlinear relationship between nigrostriatal cell degeneration and terminal losses. This work further explores the role of AAAD in Parkinsons disease, a more important factor than previously thought.

Blood-brain barrier and neuronal membrane transport of 6-[18F]fluoro-L-DOPA.

The transport of 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]FDOPA) across the blood-brain barrier (BBB) and neuronal membranes was compared with that of L-3,4-dihydroxyphenylalanine (L-DOPA) in rats. The carotid injection method was used as a direct measurement of [18F]FDOPA, 1-[14C]-L-DOPA, and 3-[14C]-L-DOPA transport across the BBB, while isolated nerve terminals were used to examine neuronal membrane transport of [3H]-L-DOPA. [18F]FDOPA appeared to use the same large neutral amino acid carrier for BBB transport as L-DOPA and L-phenylalanine. In addition, carbidopa [L-alpha-hydrazino-alpha-methyl-beta-(3,4-dihydroxyphenyl)propionic acid] was found not to have direct interference with the transport carrier on the BBB, but indirectly inhibited aromatic L-amino acid decarboxylase (AAAD) activity in brain endothelium by depletion of pyridoxal phosphate, a necessary cofactor of the enzyme. In striatal and cortical synaptosomes, [3H]-L-DOPA uptake was inhibited by non-radioactive L-DOPA, FDOPA, and 6-fluoro-L-meta-tyrosine (6-FMT). The inhibition was significantly greater in terminals isolated from the striatum than in those from the cerebral cortex. FDOPA, 6-FMT, and L-DOPA equally inhibited the neuronal transport of [3H]-L-DOPA. This suggests that FDOPA and 6-FMT compete with L-DOPA at similar transport sites at the neuronal membrane.

Distribution Volume of Radiolabeled Large Neutral Amino Acids in Brain Tissue

Variations in the cerebellum to plasma ratio at late times in 6-[18F]fluoro-l-DOPA studies are shown to be consistent with competitive binding of large neutral amino acids for a common transporter in the blood—brain barrier and the stability of brain tissue large neutral amino acid level in the presence of plasma level changes. The distribution volume of an inert large neutral amino acid can be estimated from plasma and tissue large neutral amino acid levels and apparent half-saturation concentrations (Km) of the transporter in the blood—brain barrier. Stability of brain large neutral amino acid levels is supported by literature findings and can be explained by high saturation of the large neutral amino acid transporter at physiologic conditions.

Striatal Kinetic Modeling of FDOPA with a Cerebellar-Derived Constraint on the Distribution Volume of 3OMFD: A PET Investigation using Non-Human Primates

The peripherally born metabolite of FDOPA, 3-O-Methyl-FDOPA (3OMFD), crosses the blood-brain barrier, thus complicating positron emission tomography-FDOPA (PET-FDOPA) data analysis. In previous reports the distribution volume (DV) of 3OMFD was constrained to unity. We have recently shown that the forward transport rate-constant of FDOPA (KS1) and the cerebellum-to-plasma ratio (Cb/Cp), a measure for the DV of 3OMFD, are functions of plasma large neutral amino acid (LNAA) concentration. Given large interstudy and intersubject differences in plasma LNAA levels, variations in the DV of 3OMFD are significant. In this report, the authors propose a constraint on the DV of 3OMFD that accounts for these variations. Dynamic PET-FDOPA scans were performed on 12 squirrel monkeys and 12 vervet monkeys. Two sets of constraints were employed on the compartmental model—M1 or M2. In M1, the striatal DV of 3OMFD was constrained to unity; in M2, the striatal DV of 3OMFD was constrained to an estimate derived from the cerebellum. Striatal and cerebellar time-activity curves were fitted using FDOPA and 3OMFD plasma input functions. The estimate of KS1 and that of the compartmental FDOPA uptake-constant (Ki), both obtained using M2, were adjusted to values corresponding to average LNAA levels. Finally, Ki was compared with the graphical uptake-constant (PKi). With the use of constraint M2, intersubject variability of squirrel monkey kS3 and Ki was reduced by 45% and 53%, respectively; and for vervet monkeys, by 54% and 44%, respectively. Intersubject variability of Kl and Ki was further reduced after correction for variations in intersubject plasma LNAA levels (for squirrel monkeys, by 67% and 41%; for vervet monkeys, by 40% and 36%, respectively). Ki correlation to PKi was enhanced to identity. Finally, average cerebellar kC2 estimates were more than 2.5-fold higher than striatal kS2 estimates (P < 0.0001). In modeling of PET-FDOPA data, it cannot be assumed that the DV of 3OMFD is unity. The cerebellar-derived constraint furnishes a reliable estimate for the DV of 3OMFD. Invoking the constraint and correcting for variations in plasma LNAA significantly reduced interstudy and intersubject variations in parameter estimates.

Imaging and Therapeutics: The Role of Neuronal Transport in the Regional Specificity of L-DOPA Accumulation in Brain

PURPOSE To investigate the in vitro regional accumulation of L-3,4-dihydroxyphenylalanine (L-DOPA) in brain tissue. PROCEDURE Neuronal membrane transport of L-DOPA was investigated in rat and squirrel monkey brain tissue. The kinetics of L-DOPA regional transport was characterized, and the effect of amino acids on transport was evaluated using isolated nerve terminals from striatum and cerebral cortex. RESULTS When L-DOPA uptake was measured in modified Krebs-Ringer medium, transport occurred in both synaptosome preparations. In the presence of dilute protein-free plasma, uptake of L-DOPA was significantly present in striatal nerve terminals (P < 0.005), but was completely inhibited in terminals isolated from the cortex. L-DOPA transport in striatal synaptosomes was primarily inhibited by large neutral aromatic L-amino acids, in contrast to that in cortical synaptosomes that was mainly affected by large neutral aliphatic L-amino acids. A saturable component of influx was detected in both synaptosome preparations, where kinetic analysis revealed that the relative affinity of L-DOPA was greater for the carrier in the striatum than in the cortex. Based on the distribution of neuronal cell types within the two regions and the effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesioning in squirrel monkeys, the striatal specific accumulation of L-DOPA likely occurs within dopaminergic terminals. CONCLUSIONS These results demonstrate that the in vivo regional specificity of L-DOPA localization in brain tissue is primarily controlled by neuronal transport.

Distribution volume of 3-O-methyl-6.

The distribution volume (DY) of 6-[F-18]fluoro-L-DOPA (FDOPA) in the cerebellum recently has been linked using positron emission tomography (PET) to plasma large neutral amino acid (LNAA) concentrations in monkeys. In this article the authors provide additional experimental support for this relation by directly measuring the DY as the steady-state tissue to plasma radioactivity ratio in rats using a labeled LNAA analog 3-O-methyl-6-[F-18]FDOPA (OMFD), a compound that has no known specific enzyme or receptor interactions in brain tissue. The measured DY for OMFD (tissue OMFD concentration/plasma OMFD concentration) was found to be inversely related to plasma LNAA concentrations. The relation (DY = 1.5−0.00094* [LNAA], R^2 = 0.79) resulted in an 8% DY decrease per 100 nmol/mL plasma LNAA increase within the observed range of 330 to 510 nmol/mL. This was similar to recent noninvasive observations with FDOPA PET in vervet monkeys and with 6-[F-18]Fluoro-m-tyrosine PET in squirrel monkeys. The OMFD striatum to cerebellum (Str/Cb) ratio was greater than 1.0 for all measurements, averaging 1.09 ± 0.04, and was approximately equal to the Str/Cb LNAA ratio of 1.12 ± 0.05. This current study verifies the variation of DV of OMFD or FDOPA as a function of plasma LNAA concentrations and suggests the possibility of using OMFD for measuring cerebral LNAA noninvasively with PET.

Distribution volume of 3-O-methyl-6-[F-18]FDOPA in rat brain

The distribution volume (DY) of 6-[F-18]fluoro-L-DOPA (FDOPA) in the cerebellum recently has been linked using positron emission tomography (PET) to plasma large neutral amino acid (LNAA) concentrations in monkeys. In this article the authors provide additional experimental support for this relation by directly measuring the DY as the steady-state tissue to plasma radioactivity ratio in rats using a labeled LNAA analog 3-O-methyl-6-[F-18]FDOPA (OMFD), a compound that has no known specific enzyme or receptor interactions in brain tissue. The measured DY for OMFD (tissue OMFD concentration/plasma OMFD concentration) was found to be inversely related to plasma LNAA concentrations. The relation (DY = 1.5−0.00094* [LNAA], R^2 = 0.79) resulted in an 8% DY decrease per 100 nmol/mL plasma LNAA increase within the observed range of 330 to 510 nmol/mL. This was similar to recent noninvasive observations with FDOPA PET in vervet monkeys and with 6-[F-18]Fluoro-m-tyrosine PET in squirrel monkeys. The OMFD striatum to cerebellum (Str/Cb) ratio was greater than 1.0 for all measurements, averaging 1.09 ± 0.04, and was approximately equal to the Str/Cb LNAA ratio of 1.12 ± 0.05. This current study verifies the variation of DV of OMFD or FDOPA as a function of plasma LNAA concentrations and suggests the possibility of using OMFD for measuring cerebral LNAA noninvasively with PET.

Distribution Volume of 3-O-methyl-6-|[lsqb]|F-18|[rsqb]|FDOPA in Rat Brain

The distribution volume (DY) of 6-[F-18]fluoro-L-DOPA (FDOPA) in the cerebellum recently has been linked using positron emission tomography (PET) to plasma large neutral amino acid (LNAA) concentrations in monkeys. In this article the authors provide additional experimental support for this relation by directly measuring the DY as the steady-state tissue to plasma radioactivity ratio in rats using a labeled LNAA analog 3-O-methyl-6-[F-18]FDOPA (OMFD), a compound that has no known specific enzyme or receptor interactions in brain tissue. The measured DY for OMFD (tissue OMFD concentration/plasma OMFD concentration) was found to be inversely related to plasma LNAA concentrations. The relation (DY = 1.5−0.00094* [LNAA], R^2 = 0.79) resulted in an 8% DY decrease per 100 nmol/mL plasma LNAA increase within the observed range of 330 to 510 nmol/mL. This was similar to recent noninvasive observations with FDOPA PET in vervet monkeys and with 6-[F-18]Fluoro-m-tyrosine PET in squirrel monkeys. The OMFD striatum to cerebellum (Str/Cb) ratio was greater than 1.0 for all measurements, averaging 1.09 ± 0.04, and was approximately equal to the Str/Cb LNAA ratio of 1.12 ± 0.05. This current study verifies the variation of DV of OMFD or FDOPA as a function of plasma LNAA concentrations and suggests the possibility of using OMFD for measuring cerebral LNAA noninvasively with PET.