Richard Torstenson

Amphetamine effects on dopamine release and synthesis rate studied in the Rhesus monkey brain by positron emission tomography

SummaryPositron emission tomography (PET) was used in a multitracer protocol to evaluate D-amphetamine induced effects on dopamine biosynthesis rate and release in propofol anesthetized Rhesus monkeys.l-[β-11C]DOPA was used as biochemical probe to study the brain dopamine biosynthesis rate whilst dopamine release was followed by the binding displacement of the [11C]-radiolabelled dopamine receptor antagonists, raclopride and N-methylspiperone. Studies were performed with either a constant rate intravenous infusion of D-amphetamine aiming at plasma concentrations of 0.2 to 25 ng/ml or with intravenous bolus doses of 0.1 and 0.4 mg/ kg. Decreased binding of the dopamine receptor antagonists was measured in both modes of D-amphetamine administration but notably [11C]N-methylspiperone was less able to sense D-amphetamine induced release of dopamine. At plasma concentrations aimed above 1 ng/ml a levelling off of the binding of [11C]raclopride at 68 ± 8.1% of the baseline value indicated that displacement was only possible from a fraction of the binding sites. Amphetamine was observed to increase the rate constant forl-[β-11C]DOPA utilization in the brain. This was most likely due to an acutely induced subsensitivity of presynaptic dopamine receptors.l-[β-11C]DOPA and [11C]raclopride were found suitable to indicate changes in dopamine synthesis rate and release respectively using PET and can be used to mirror drug-induced changes of brain dopaminergic function.

Kinetic compartment modeling of [11C]-5-hydroxy-L-tryptophan for positron emission tomography assessment of serotonin synthesis in human brain.

The substrate for the second enzymatic step in serotonin synthesis, 5-hydroxy-L-tryptophan, labeled in the β-position ([11C]-HTP), was used for positron emission tomography (PET) measurements in six healthy human participants, examined on two occasions. One- and two-tissue kinetic compartment modeling of time-radioactivity curves was performed, using arterial, metabolite-corrected [11C]-HTP values as input function. The availability of unchanged tracer in arterial blood plasma was ⩽ 80% up to 60 minutes after injection, while [11C]-hydroxyindole acetic acid and [11C]-serotonin accounted for the remaining radioactivity, amounting to ⩽16% and ⩽4%, respectively. Compartment modeling was performed for brain stem, putamen, caudate nucleus, anterior cingulate, white matter, and superior occipital, occipitotemporal, and temporal cortices. The average biologic half-life for plasma-to-tissue equilibrium was 7 to 12 minutes, and the volume of distribution was 0.2 to 0.5 μL·mL−1. In all regions except white matter, the kinetic compartment model that included irreversible [11C]-HTP trapping showed significantly improved model fits with respect to a one-tissue compartment model. The [11C]-HTP trapping rate constant depended on the estimated tissue availability of the serotonin precursor tryptophan, known to reflect serotonin synthesis in healthy individuals, and correlated with serotonin tissue concentration and synthesis rates reported previously in literature. These findings suggest the use of [11C]-HTP PET measurements to investigate serotonin synthesis.

Effects of the substituted (S)-3-phenylpiperidine (−)-OSU6162 on PET measurements in subhuman primates: Evidence for tone-dependent normalization of striatal dopaminergic activity

(−)‐OSU6162 is a substituted (S)‐3‐phenylpiperidine derivative which exhibits some affinity to the dopamine D2 receptor family. In vivo, the compound displays a unique normalizing profile on psychomotor activity by an intriguing mixture of stimulatory and inhibitory properties. In the present investigation, some of the effects of (−)‐OSU6162 on central dopaminergic function were studied by positron emission tomography (PET) and L‐[11C]DOPA in anaesthetized female rhesus monkeys. (−)‐OSU6162 displayed a dopaminergic tone‐dependent effect with a reduction in the striatal L‐[11C]DOPA influx rate in monkeys with high baseline values and an increased striatal L‐[11C]DOPA influx rate in animals with low baseline values. Infusion of (−)‐OSU6162 for a whole day resulted in a stable effect with no evidence of tolerance. (−)‐OSU6162 also stabilized dopaminergic function by attenuating the upregulation of the striatal L‐[11C]DOPA influx rate which has previously been shown to occur following 6R‐BH4 or 6R‐BH4 + L‐tyrosine infusions. This “Protean” effect of (−)‐OSU6162 on the striatal dopaminergic function corresponds to previous behavioral observations in intact animals and demonstrates a true functional correlation to the measures obtained with L‐[11C]DOPA and PET. The normalizing and stabilizing profile of (−)‐OSU6162 should be of value in treating a variety of disorders where an underlying dysregulation or disruption of dopaminergic function can be assumed. Synapse 28:280–287, 1998.

Tetrahydrobiopterin in the treatment of children with autistic disorder: a double-blind placebo-controlled crossover study.

Abstract: Twelve children, all boys, aged 4 to 7 years, with a diagnosis of autistic disorder and low concentrations of spinal 6R-l-erythro-5,6,7,8-tetrahydrobiopterin (tetrahydrobiopterin) were selected to participate in a double-blind, randomized, placebo-controlled, crossover study. The children received a daily dose of 3 mg tetrahydrobiopterin per kilogram during 6 months alternating with placebo. Treatment-induced effects were assessed with the Childhood Autism Rating Scale every third month. The results showed small nonsignificant changes in the total scores of Childhood Autism Rating Scale after 3- and 6-month treatment. Post hoc analysis looking at the 3 core symptoms of autism, that is, social interaction, communication, and stereotyped behaviors, revealed a significant improvement of the social interaction score after 6 months of active treatment. In addition, a high positive correlation was found between response of the social interaction score and IQ. The results indicate a possible effect of tetrahydrobiopterin treatment.

Effect of apomorphine infusion on dopamine synthesis rate relates to dopaminergic tone

The effects of apomorphine on the striatal L-[11C]DOPA influx rate was examined in anaesthetized Rhesus monkeys using positron emission tomography (PET). In comparison with baseline conditions, the addition of a continuous infusion of apomorphine produced decreases in the striatal L-[11C]DOPA influx rate in all the monkeys examined. The effect of apomorphine infusion also showed a dose-dependent trend. In individual monkeys, the magnitude of the effect showed a baseline dopaminergic tone-dependency; that is, the effect of apomorphine was most pronounced in monkeys with high baseline influx rates, and in monkeys with lower baseline values apomorphine induced a weaker effect. Studies of radiolabeled tracer and radiolabeled metabolites formed in plasma confirmed that apomorphine infusion did not induce any change in the peripheral elimination or metabolite formation of L-[11C]DOPA. The decreased striatal L-[11C]DOPA influx rate induced by apomorphine was interpreted as an agonist effect on dopamine autoreceptors regulating the dopamine synthesis rate. The observation of a baseline dopaminergic tone-dependent effect is in agreement with earlier results showing this influence on the striatal influx rate as measured with the tracer L-[11C]DOPA. A priori, it can be established that L-[11C]DOPA and PET provide a method not only to study the structural integrity of the presynaptic dopaminergic system but also to study the homeostasis-regulating mechanisms of this neurotransmitter system in vivo. The ability to measure condition-dependent effects in individuals should be of great importance in determining specific pathophysiological mechanisms underlying degenerative and functional disorders affecting the dopaminergic system.

L‐DOPA modulates striatal dopaminergic function in vivo: Evidence from PET investigations in nonhuman primates

Significant increases in striatal L‐[11C]DOPA retention were observed in adult female rhesus monkeys with positron emission tomography (PET) following administration of drugs that increase cerebral L‐DOPA concentrations. The monkeys were scanned twice: at baseline (using 10–50 μg of tracer substance) and during continuous administration of L‐DOPA (3 or 15 mg/kg/h) and 6‐R‐Erythro‐4,5,6,7‐tetrahydrobiopterin (6R‐BH4) (5 mg/kg/h) and during combined administration of both drugs. PET scans of L‐[11C]DOPA distribution were obtained in GE2048‐15B or GE4096‐15WB Plus positron tomographs. In all studies the specific striatal L‐[11C]DOPA influx rate increased by an average of 17–20%. These increases were significantly higher than the retest variability obtained with saline infusions under identical experimental conditions. In individual monkeys the magnitude of increase in the striatal L‐[11C]DOPA influx rate varied from no effect of the drug infusion to a 45% increase. Taken together, the results of this study demonstrate that L‐DOPA in itself can affect dopaminergic neurotransmission in vivo and also adds further evidence that the neuromodulatory effects of the amino acid are predominantly autoreceptor antagonist‐like. The findings most likely have importance for the further understanding of the dopaminergic system in neurodegenerative and psychiatric disorders. Synapse 25:56–61, 1997.

Effects of the substituted (S)-3-phenylpiperidine (−)-OSU6162 on PET measurements of [11C]SCH23390 and [11C]raclopride binding in primate brains

The substituted (S)-3-phenylpiperidine (-)-OSU6162 belongs to a novel class of functional modulators of dopaminergic systems. In vivo, (-)-OSU6162 has a unique stabilising profile on dopaminergic functions. In vitro this compound exhibits low affinity for the dopamine D2 receptor, but due to its similarity to neuroleptics on brain dopaminergic neurochemistry and different postsynaptic effects it has been characterised as a preferential dopamine autoreceptor antagonist. To further clarify the effects of (-)-OSU6162 on the postjunctional nigrostriatal dopaminergic system, dopamine receptor binding was measured in rhesus monkeys (Macaca mulatta) by positron emission tomography (PET) using the D1 and D2 dopamine receptor radioligands [11C]SCH23390 and [11C]raclopride respectively, before and during continuous intravenous infusions of(-)-OSU6162. Additionally, the test-retest variability of sequential [11C]SCH23390 scans was estimated. Following the administration of (-)-OSU6162, [11C]raclopride binding in striatum was dose-dependently decreased with a 76% reduction occurring after 3.0 mg/kg per h continuous infusion. Whereas (-)-OSU6162 in the lower doses had no effect on [11C]SCH23390 binding, the highest dose, 3.0 mg/kg per h, increased [11C]SCH23390 binding, which may indicate a potentiating effect on D1 dopamine receptor mediated functions. Thus, in contrast to the conditions in vitro, (-)-OSU6162 produces a high displacement of raclopride from D2 receptors in vivo.

A Comparison of 11C-Labeled l-DOPA and l-Fluorodopa as Positron Emission Tomography Tracers for the Presynaptic Dopaminergic System

11C-labeled 3,4-Dihydroxy-phenyl-l-alanine (L-DOPA) and l-fluorodopa were used as tracers for the functional state of the presynaptic dopamine system in anesthetized monkeys with positron emission tomography, The radiotracer disposition in brain tissue and plasma were studied and effects induced by pharmacologic challenges were evaluated, 6R-l-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4) increased the striatal influx rate constant, e.g., striatal Ki for l-[β-11C]DOPA, but it induced no effect on the Ki-value using l-[β-11C]-6-fluorodopa. Studies of radiolabeled tracer and metabolites in plasma showed substantial differences between the two tracers. At baseline conditions, 60% unchanged l-[β-11C]DOPA was detected in plasma 50 minutes after tracer injection and the 3-O-methylated fraction accounted for 25% of total radioactivity. For l-[β-11C]-6-fluorodopa, the relation was inverse; about 25% unchanged tracer and 60% 3-O-methyl metabolite were present in plasma after 50 minutes. A site-specific 11C-labeling in the carboxylic position in the molecules revealed a significant specific retention of radioactivity in striatum with l-[carboxy-11C]-6-fluorodopa but not with l-[carboxy-11C]DOPA. The 3-O-methyl metabolite of l-DOPA is known to pass the blood-brain barrier and may interfere with the calculation of the Ki value using a brain reference region. Thus, extensive 3-O-methylation in circulation of the fluorinated analog could obscure the detectability of potential functional change in striatal Ki of the tracer when using a reference tissue model for calculation.

Synthesis of [11C-methyl]-(−)-OSU6162, its regional brain distribution and some pharmacological effects of (−)-OSU6162 on the dopaminergic system studied in the rhesus monkey by positron emission tomography

The labelling of the presynaptic dopamine receptor antagonist (-)-OSU6162, ((S)-(-)-3-(3-(methylsulfonyl)phenyl)-1-propylpiperidine) was performed by an alkylation with [11C]methyl iodide of the thio anion (-)-OSU1281, followed by a selective oxidation to the corresponding methyl sulfone, [11C-methyl]-(-)-OSU6162. The total radiochemical yield calculated from the produced [11C]carbon dioxide to final product was about 25% and the time of synthesis was in the range of 40 min from end of bombardment. The synthesis of the precursor, (-)-OSU1281, was performed from (-)-3PPP in a three-step synthesis. The regional brain distribution of (-)-OSU6162 radiolabelled with 11C was studied in rhesus monkeys by means of positron emission tomography, PET. [11C-Methyl]-(-)-OSU6162 was rapidly and uniformly distributed to gray matters of the brain, and no decrease of radioactivity uptake in the brain was seen after pretreatment with 1 to 3 mg/kg/h of (-)-OSU6162. The effect of doses of 1 to 3 mg/kg/h of (-)-OSU6162 on the dopamine binding was studied by PET using [11C-methyl]raclopride. Radioactivity in the striatum was significantly and dose-dependently decreased by (-)-OSU6162 (r = 0.88), supporting competition with dopamine for selective binding to dopamine receptors.

Regional Brain Distribution and Binding of the Muscarinic Receptor Agonist CI-979 Studied by Positron Emission Tomography in the Monkey

The regional brain distribution and selective binding of the cholinergic muscarinic receptor agonist CI-979 labelled with 11C was studied in rhesus monkeys by means of positron emission tomography. The selective binding was measured as displacement of [11C]CI-979-derived radioactivity following constant-rate infusion of CI-979 at doses of 0.5-10 micrograms/kg/h. An extensive and rapid distribution of [11C]CI-979 was observed to the basal ganglia and temporal occipital cortices, i.e., regions of the brain with a high density of muscarinic receptors. The radioactivity was dose-dependently decreased in cortical regions following infusions of unlabelled CI-979 (0.5-10 micrograms/kg/h), indicating selective receptor binding of [11C]CI-979 in these brain regions. The binding of [11C]CI-979 was unaltered or even higher in the striatum following unlabelled CI-979 infusion. The high densities of autoreceptors in the striatum may explain the inhibitory feedback mechanism on endogenous acetylcholine induced by the muscarinic receptor agonist. Since muscarinic receptor agonists release acetylcholine, the higher releasable pool of acetylcholine in the stratum will induce feedback inhibition of acetylcholine release and less competition between acetylcholine and CI-979 at the muscarinic receptors. This may explain the differences between cortex and striatum in [11C]CI-979 binding. The cerebral blood flow was not changed by the infusion of unlabelled CI-979, supporting the assumption that effects of CI-979 in brain may be due to the interaction with brain muscarinic receptors. Despite a relative rapid clearance of [11C]CI-979 radioactivity from binding in the brain characteristic of a receptor agonist, the radioligand might be useful in positron emission tomography studies to reveal changes in cholinergic receptors and functional activity in Alzheimer patients treated with muscarinic agonists.