Paul Cumming

MR-based statistical atlas of the Göttingen minipig brain

Thedomestic pig is increasingly being used as an experimental model for brain imaging studies with positron emission tomography (PET). The recording of radiotracer uptake by PET gives functional and physiological information, but with poor spatial resolution. To date, anatomical regions of interest in pig brain have been defined in MR images obtained for each individual animal, because of the lack of a standard stereotaxic coordinate system for the pig brain. In order to define a stereotaxic coordinate system, we coregistered T1-weighted MR images from 22 male Göttingen minipigs and obtained a statistically defined surface rendering of the average minipig brain in which stereotaxic zero is defined by the position of the pineal gland. The average brain is now used as a target for registration of dynamic PET data, so that time-activity curves can be extracted from standard volumes of interest. In order to define these volumes, MR images from each individual pig were manually segmented into a total of 34 brain structures, including cortical regions, white matter, caudate and putamen, ventricular system, and cerebellum. The mean volumes of these structures had variances in the range of 10-20%. The 34 brain volumes were transformed into the common coordinate system and then used to generate surface renderings with probabilistic threshold greater than 50%. This probabilistic threshold gave nearly quantitative recovery of the mean volumes in native space. The probabilistic volumes in stereotaxic space are now being used to extract time-radioactivity curves from dynamic PET recordings.

Methylphenidate-evoked changes in striatal dopamine correlate with inattention and impulsivity in adolescents with attention deficit hyperactivity disorder.

Abnormal central dopamine (DA) neurotransmission has been implicated in the impulsivity, inattention, and hyperactivity of attention deficit hyperactivity disorder (ADHD). We hypothesized that a pharmacological challenge with methylphenidate (MP) at a therapeutic dose increases extracellular DA concentrations in proportion to the severity of these specific ADHD symptoms. To test this hypothesis, we measured by PET the effect of acute challenge with MP on the availability of striatal binding sites for [11C]raclopride (pB), an index of altered interstitial DA concentration, in nine unmedicated adolescents (1 female, 8 males; age 13.7 +/- 1.8 years) with a current diagnosis of ADHD. We estimated the pB of [11C]raclopride for brain dopamine D2/3 receptors first in a baseline resting condition, and again after an acute challenge with MP (0.3 mg/kg, p.o.), and calculated the percentage change in (%DeltapB) in left and right striatum. On another day, measurements of impulsivity and inattention were performed using a computerized continuous performance test. There was a significant correlation between the magnitude of %DeltapB in the right striatum and the severity of inattention and impulsivity. MP-evoked %DeltapB correlated with standard scores for impulse control (r = 0.68; P = 0.02), attention (r = 0.81; P = 0.005), information processing (r = 0.66; P = 0.02), and consistency of attention, or variability (r = 0.60; P = 0.04). In conclusion, the results link inattention and impulsivity with sensitivity of brain DA receptor availability to an MP challenge, corroborating the hypothesis that MP serves to potentiate decreased DA neurotransmission in ADHD.

6-[18F]fluoro-l-DOPA Metabolism in Living Human Brain: A Comparison of Six Analytical Methods

In 11 normal volunteers and six patients with Parkinsons disease, we compared six different analyses of dopaminergic function with l-3,4-dihydroxy-6-[18F]fluorophenylalanine (FDOPA) and positron emission tomography (PET). The caudate nucleus, putamen, and several reference regions were identified in PET images, using magnetic resonance imaging (MRI). The six analyses included two direct determinations of DOPA decarboxylase activity (kD3, k*3), the slope–intercept plot based on plasma concentration (K), two slope–intercept plots based on tissue content (kr3, ks3), and the striato–occipital ratio [R(T)]. For all analyses, the difference between two groups of subjects (normal volunteers and patients with Parkinsons disease) was larger in the putamen than in the caudate. For the caudate nucleus, the DOPA decarboxylase activity (kD3, k*3), tissue slope–intercept plots (kr3, ks3), and striato–occipital ratio [R(T)] analyses significantly discriminated between the normal volunteers and the patients with Parkinsons disease (p < 0.005) [with least significance for k*3 (p < 0.05)], while the plasma slope–intercept plot (K) failed to do so. For the putamen, the values for kD3, k*3, K, kr3, ks3 and R(T) of normal volunteers were significantly higher than those of patients (p < 0.005) [with least significance for K (p < 0.025)]. Linear correlations were significant between kD3 and ks3; kD3 and kr3; kD3 and R(T); and kD3 and k*3, in this order of significance. We found no correlation between kD3 and K values in the caudate nucleus.

Human Striatal l-DOPA Decarboxylase Activity Estimated in vivo Using 6-[18F]fluoro-DOPA and Positron Emission Tomography: Error Analysis and Application to Normal Subjects

DOPA decarboxylase is the enzyme directly responsible for the synthesis of the neurotransmitters dopamine and serotonin, and indirectly of noradrenaline, in brain. We used the decarboxylation coefficient (kD3) of 6-[18F]fluoro-DOPA (FDOPA) to denote the relative activity of l-DOPA decarboxylase in vivo in the human brain. To determine the relative enzyme activity with positron emission tomography (PET), we evaluated the model that separates the metabolism into compartments of nondiffusible and diffusible (i.e., transient) tracer metabolites. Error analysis indicated that the least-squares optimization alone was not sufficient to yield accurate estimates of kD3 in the presence of the inherent error of PET. To improve the accuracy of the kD3 estimates by optimizing the number of parameters, we introduced biological constraints which included a tracer partition volume (Ve) common to frontal cortex and striatum, and a fixed ratio (q) between the blood–brain barrier transport coefficients of O-methyl-[18F]fluoro-DOPA and FDOPA, the two sources of radioactivity in plasma. We found that a two-step analysis yielded sufficiently accurate estimates of kD3. The two steps include the initial estimation of the partition volume in frontal cortex and the subsequent use of this value to determine kD3 in striatum and other structures. We studied twelve healthy controls (age 45 ± 15 years). The average kD3 value was 0.081 ± 0.024 min−1 (coefficient of variation (COV) 30%) for caudate nucleus, 0.074 ± 0.013 min“1 (COV 18%) for putamen, and 0.010 ± 0.005 min−1 (COV 50%) for cerebral cortex.

Compartmental analysis of dopa decarboxylation in living brain from dynamic positron emission tomograms.

The trapping of decarboxylation products of radiolabelled dopa analogs in living human brain occurs as a function of the activity of dopa decarboxylase. This enzyme is now understood to regulate, with tyrosine hydroxylase, cerebral dopamine synthesis. Influx into brain of dopa decarboxylase substrates such as 6‐[18F]fluorodopa and β‐[11C]dopa measured by positron emission tomography can be analyzed by solution of linear differential equations, assuming irreversible trapping of the decarboxylated products in brain. The isolation of specific physiological steps in the pathway for catecholamine synthesis requires compartmental modelling of the observed dynamic time‐activity curves in plasma and in brain. The several approaches to the compartmental modelling of the kinetics of labelled substrates of dopa decarboxylase are now systematically and critically reviewed. Labelled catechols are extensively metabolized by hepatic catechol‐O‐methyltransferase yielding brain‐penetrating metabolites. The assumption of a fixed blood–brain permeability ratio for O‐methyl‐6‐[18F]fluorodopa or O‐methyl‐β‐[11C]dopa to the parent compounds eliminates several parameters from compartmental models. However, catechol‐O‐methyltransferase activity within brain remains a possible factor in underestimation of cerebral dopa decarboxylase activity. The O‐methylation of labelled catechols is blocked with specific enzyme inhibitors, but dopa decarboxylase substrates derived from m‐tyrosine may supplant the catechol tracers. The elimination from brain of decarboxylated tracer metabolites can be neglected without great prejudice to the estimation of dopa decarboxylase activity when tracer circulation is less than 60 minutes. However, elimination of dopamine metabolites from brain occurs at a rate close to that observed previously for metabolites of glucose labelled in the 6‐position. This phenomenon can cause systematic underestimation of the rate of dopa decarboxylation in brain. The spillover of radioactivity due to the limited spatial resolution of tomographs also results in underestimation of dopa decarboxylase activity, but correction for partial volume effects is now possible. Estimates of dopa decarboxylase activity in human brain are increased several‐fold by this correction. Abnormally low influx of dopa decarboxylase tracers in the basal ganglia is characteristic of Parkinsons disease and other movement disorders. Consistent with postmortem results, the impaired retention of labelled dopa is more pronounced in the putamen than in the caudate nucleus of patients with Parkinsons disease; this heterogeneity persists after correction for spillover. Current in vivo assays of dopa decarboxylase activity fail to discriminate clinically distinct stages in the progression of Parkinsons disease and are, by themselves, insufficient for differential diagnosis of Parkinsons disease and other subcortical movement disorders. However, potential new avenues for therapeutics can be tested by quantifying the rate of metabolism of exogenous dopa in living human brain. Synapse 29:37–61, 1998.

Formation and clearance of interstitial metabolites of dopamine and serotonin in the rat striatum: an in vivo microdialysis study.

Abstract: In vivo microdialysis was employed in order to characterize the steady‐state kinetics of the turnover of specific dopamine and serotonin metabolites in the rat striaturn 48 h after surgery. Inhibitors of monoamine oxidase (MAO; pargyline) and catechol‐O‐methyltransferase (COMT; Ro 40‐7592) were administered, either separately or in conjunction, at doses sufficient to block these enzymes in the CNS. In some experiments, the acid metabolite carrier was blocked with probenecid. Temporal changes were then observed in the efflux of interstitial dopamine, 3‐methoxytyramine (3‐MT), 3,4‐dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5‐hydroxyindoleacetic acid (5‐HIAA). The fractional rate constants for the accumulation or disappearance of the metabolites could be determined after pharmacological blockade of catabolic enzymes or the acid metabolite carrier. Interstitial 5‐HIAA was found to be cleared with a half‐life of approximately 2 h. After blockade of either MAO or COMT, HVA disappeared with a half‐life of 17 min. Experiments employing probenecid suggested that some of the interstitial HVA was cleared by the acid metabolite carrier, the remainder being cleared by a probenecid‐insensitive process, possibly conjugation. After MAO inhibition, DOPAC disappeared with an apparent half‐life of 11.3 min. The rate of 3‐MT accumulation after pargyline indicated that the majority of interstitial HVA (>95%) is formed from DOPAC rather than 3‐MT. The formation of 3‐MT from interstitial dopamine, calculated from the accumulation rate of 3‐MT after pargyline, appeared to follow first‐order kinetics (k = 0.1 min−1).

Inverted-U-shaped correlation between dopamine receptor availability in striatum and sensation seeking

Sensation seeking is a core personality trait that declines with age in both men and women, as do also both density and availability of the dopamine D2/3 receptors in striatum and cortical regions. In contrast, novelty seeking at a given age relates inversely to dopamine receptor availability. The simplest explanation of these findings is an inverted-U-shaped correlation between ratings of sensation seeking on the Zuckerman scale and dopamine D2/3 receptor availability. To test the claim of an inverted-U-shaped relation between ratings of the sensation-seeking personality and measures of dopamine receptor availability, we used PET to record [11C]raclopride binding in striatum of 18 healthy men. Here we report that an inverted-U shape significantly matched the receptor availability as a function of the Zuckerman score, with maximum binding potentials observed in the midrange of the scale. The inverted-U shape is consistent with a negative correlation between sensation seeking and the reactivity (“gain”) of dopaminergic neurotransmission to dopamine. The correlation reflects Zuckerman scores that are linearly linked to dopamine receptor densities in the striatum but nonlinearly linked to dopamine concentrations. Higher dopamine occupancy and dopamine concentrations explain the motivation that drives afflicted individuals to seek sensations, in agreement with reduced protection against addictive behavior that is characteristic of individuals with low binding potentials.

Subchronic haloperidol downregulates dopamine synthesis capacity in the brain of schizophrenic patients in vivo.

The antipsychotic effect of neuroleptics cannot be attributed entirely to acute blockade of postsynaptic D2-like dopamine (DA) receptors, but may arise in conjunction with the delayed depolarization block of the presynaptic neurons and reduced DA synthesis capacity. Whereas the phenomenon of depolarization block is well established in animals, it is unknown if a similar phenomenon occurs in humans treated with neuroleptics. We hypothesized that haloperidol treatment should result in decreased DA synthesis capacity. We used 6-[18F]fluoro-L-dopa (FDOPA) and positron emission tomography (PET) in conjunction with compartmental modeling to measure the relative activity of DOPA decarboxylase (DDC) (kD3, min−1) in the brain of nine unmedicated patients with schizophrenia, first in the untreated condition and again after treatment with haloperidol. Patients were administered psychometric rating scales at baseline and after treatment. Consistent with our hypothesis, there was a 25% decrease in the magnitude of kD3 in both caudate and putamen following 5 weeks of haloperidol therapy. In addition, the magnitudes of kD3 in cerebral cortex and thalamus were also decreased. Psychopathology as measured with standard rating scales improved significantly in all patients. The decrease of kD3 in the thalamus was highly significantly correlated with the improvement of negative symptoms. Subchronic treatment with haloperidol decreased the activity of DDC in the brain of patients with schizophrenia. This observation is consistent with the hypothesis that the antipsychotic effect of chronic neuroleptic treatment is associated with a decrease in DA synthesis, reflecting a depolarization block of presynaptic DA neurons. We link an alteration in cerebral catecholamine metabolism in human brain with the therapeutic action of neuroleptic medication.

Regulation of DOPA Decarboxylase Activity in Brain of Living Rat

Abstract: To test the hypothesis that l‐DOPA decarboxylase (DDC) is a regulated enzyme in the synthesis of dopamine (DA), we developed a model of the cerebral uptake and metabolism of [3H]DOPA. The unidirectional blood‐brain clearance of [3H]DOPA (KD1) was 0.049 ml g−1 min−1. The relative DDC activity (kD3) was 0.26 min−1 in striatum, 0.04 min−1 in hypothalamus, and 0.02 min−1 in hippocampus. In striatum, 3,4‐[3H]dihydroxyphenylacetic acid ([3H]DOPAC) was formed from [3H]DA with a rate constant of 0.013 min−1, [3H]homovanillic acid ([3H]HVA) was formed from [3H]DOPAC at a rate constant of 0.020 min−1, and [3H]HVA was eliminated from brain at a rate constant of 0.037 min−1. Together, these rate constants predicted the ratios of endogenous DOPAC and HVA to DA in rat striatum. Pargyline, an inhibitor of DA catabolism, substantially reduced the contrast between striatum and cortex, in comparison with the contrast seen in autoradiograms of control rats. At 30 min and at 4 h after pargyline, kD3 was reduced by 50% in striatum and olfactory tubercle but was unaffected in hypothalamus, indicating that DDC activity is reduced in specific brain regions after monoamine oxidase inhibition. Thus, DDC activity may be a regulated step in the synthesis of DA.

Pharmacokinetics of Plasma 6-[18F]Fluoro-l-3,4-Dihydroxyphenylalanine ([18F]FDOPA) in Humans

Like native DOPA, [18F]-6-fluoro-l-3,4-dihydroxyphenylalanine ([18F]FDOPA) is subject to methylation and decarboxylation. To determine the rates of formation and elimination of [18F]FDOPA metabolites, plasma from human subjects undergoing positron emission tomographic (PET) studies was analyzed by high-performance liquid chromatography (HPLC). In addition to the principal metabolite O-methyl-[18F]FDOPA (OMe-[18F]FDOPA), two decarboxylated metabolites were detected in plasma from carbidopa pretreated subjects. The concentrations of each metabolite during 90 min following tracer injection could be described as a function of the concentration of [18F]FDOPA, and two rate constants; k0, the rate of formation, and k–1, the rate of clearance. Plasma metabolite time series generated from total plasma activity curves and measured rate constants were in close agreement with the actual concentrations determined by HPLC fractionation. Population means for k0 (0.011 ± 0.002 min−1) and k–1, (0.010 ± 0.003 min−1) were used to generate “simulated” plasma curves. The measured and generated plasma curves were used as inputs for estimation of partition and decarboxylation coefficients of [18F]FDOPA in brain. The use of generated input functions from normal population means of transfer coefficients did not introduce a systematic error into the estimate of the enzyme activity. However, the high variability of these estimates in patients precludes the use of this technique as an alterative to individual HPLC measurements.