Human brain serotonin synthesis capacity measured in vivo with alpha-[C-11]methyl-L-tryptophan

Local cerebral serotonin synthesis capacity was measured with alpha-[C-11]methyl-L-tryptophan ([C-11]AMT) in normal adult human brain (n = 10; five males, five females; age range, 18-38 years, mean 28.3 years) by using positron emission tomography (PET). [C-11]AMT is an analog of tryptophan, the precursor for serotonin synthesis, and is converted to alpha-[C-11]methyl-serotonin ([C-11]AM-5HT), which is trapped in serotonergic neurons because [C-11]AM-5HT is not degraded by monoamine oxidase. Kinetic analysis of [C-11] activity in brain after injection of [C-11]AMT confirmed the presence of a compartment with unidirectional uptake that represented approximately 40% of the activity in the brain at 50 min after tracer administration. The undirectional rate constant K, which represents the uptake of [C-11]AMT from the plasma to brain tissue followed by the synthesis and physiologic trapping of [C-11]AM-5HT, was calculated using the Patlak graphic approach on a pixel-by-pixel basis, thus creating parametric images. The rank order of K values for different brain regions corresponded well to the regional concentrations of serotonin in human brain (P < .0001). High serotonin synthesis capacity values were measured in putamen, caudate, thalamus, and hippocampus. Among cortical regions, the highest values were measured in the rectal gyrus of the inferior frontal lobe, followed by transverse temporal gyrus; anterior and posterior cingulate gyrus; middle, superior, and inferior temporal gyri; parietal cortex; occipital cortex, in descending order. Values in women were 10-20% higher (P < .05, MANOVA) throughout the brain than those measured in men. Differences in the serotonin synthesis capacity between men and women measured in this study may reflect gender differences of importance to both normal and pathologic behavior. This study demonstrates the suitability of [C-11]AMT as a tracer for PET scanning of serotonin synthesis capacity in human brain and provides normal adult values for future comparison with patient groups.

Altered serotonin synthesis in the dentatothalamocortical pathway in autistic boys

Based on reports of increased platelet serotonin in 30 to 50% of autistic subjects, abnormal serotonergic neurotransmission may be important in the pathogenesis of autism. However, serotonin metabolite measurements in cerebrospinal fluid of autistic subjects have failed to demonstrate consistent abnormalities. Using alpha-[11C]methyl-L-tryptophan as a tracer for serotonin synthesis with positron emission tomography, we now report unilateral alterations of serotonin synthesis in the dentatothalamocortical pathway in autistic boys. Asymmetries of serotonin synthesis were found in frontal cortex, thalamus, and dentate nucleus of the cerebellum in all 7 boys, but not in the 1 autistic girl studied. Decreased serotonin synthesis was found in the left frontal cortex and thalamus in 5 of the 7 boys and in the right frontal cortex and thalamus in the 2 remaining autistic boys. In all 7 cases, elevated serotonin synthesis in the contralateral dentate nucleus was observed. Statistically significant differences between autistic boys and their nonautistic siblings (n = 5) were obtained when comparing asymmetry indices for frontal cortex, thalamus, and dentate nucleus combined as well as individually for frontal cortex and thalamus. These serotonergic abnormalities in a brain pathway, important for language production and sensory integration, may represent one mechanism underlying the pathophysiology of autism.

Analysis of [C-11]alpha-methyl-tryptophan kinetics for the estimation of serotonin synthesis rate in vivo

We describe the tracer kinetic analysis of [C-11]-labeled alpha-methyl-tryptophan (AMT), an analogue of tryptophan, which has been developed as a tracer for serotonin synthesis using positron emission tomography (PET) in human brain. Dynamic PET data were acquired from young healthy volunteers (n = 10) as a series of 22 scans covering a total of 60 minutes and analyzed by means of a three-compartment, four-parameter model. In addition, functional images of the K-complex were created using the Patlak-plot approach. The application of a three-compartment model resulted in low identifiability of individual k-values, especially that of k3. Model identifiability analysis using a singular value decomposition of the final sensitivity matrix showed parameter identifiability to increase by 50% when the Patlak-plot approach was used. K-complex values derived by the Patlak-plot approach overestimated the compartmental values by 10 to 20%, because of the violation of the dynamic equilibrium assumption. However, this bias was fairly constant in all structures of the brain. The rank order of K-complex values from different brain regions corresponded well to the regional concentrations of serotonin in human brain (P < 0.0001). These results indicate that the Patlak-plot method can be readily applied to [C-11]AMT data in order to create functional images of the K-complex, reflecting serotonin synthesis rate, within an acceptable error margin.

Effects of cardiac sympathetic innervation on coronary blood flow

BACKGROUND

The role of cardiac sympathetic nerves in regulating coronary blood flow is controversial. We sought to determine the degree to which cardiac efferent sympathetic signals modulate coronary blood flow. The heterogeneous sympathetic reinnervation in transplanted hearts provides a model for studying the vasomotor responses to adrenergic stimulation in reinnervated and denervated coronary territories of the same heart.

METHODS

We studied 14 cardiac-transplant recipients who had normal coronary arteries and no evidence of rejection and 8 normal subjects. We used positron-emission tomography with [(11)C]hydroxyephedrine, an analogue of norepinephrine, to delineate sympathetic innervation. Using [(13)N]ammonia, we measured myocardial blood flow at rest, during adenosine-induced hyperemia, and in response to sympathetic stimulation induced by cold pressor testing.

RESULTS

In the transplant recipients, the uptake of [(11)C]hydroxyephedrine was greater in the territory served by the left anterior descending artery (0.15+/-0.01) than in those served by the right coronary artery (0.07+/-0.01, P<0.001) or the circumflex artery (0.09+/-0.01, P<0.001). The basal flow was similar in all three regions, as was the percent increase in flow during hyperemia. However, the increase in flow in response to cold pressor testing was higher in the territory of the left anterior descending artery (46+/-10 percent) than in those of the right coronary artery (16+/-5 percent, P=0.01) or the circumflex artery (23+/-6 percent, P=0.06), although the changes in hemodynamics and levels of circulating catecholamines were similar. No such regional differences were observed in the normal subjects.

CONCLUSIONS

Increases in coronary blood flow in response to sympathetic stimulation correlated with the regional norepinephrine content in the cardiac sympathetic-nerve terminals. These findings suggest that cardiac adrenergic signals play an important part in regulating myocardial blood flow.

Successful and unsuccessful approaches to imaging carcinoids: comparison of a radiolabelled tryptophan hydroxylase inhibitor with a tracer of biogenic amine uptake and storage, and a somatostatin analogue

A mouse mastocytoma model was used to determine the biodistribution and tumour uptake of four radiopharmaceuticals developed to target the serotonin synthetic pathway in carcinoid tumours. Three of the compounds were competitive inhibitors of the rate-limiting enzyme of serotonin synthesis, tryptophan hydroxylase. Radiolabelled iodo-dL-phenylalanine (iodine-131 PIPA) was found to have the highest uptake and tumour-to-liver ratio. Four patients with known carcinoid tumours were then injected with 0.5 mCi 131I-PIPA and imaged at 1, 4, 24 and 48 h post-injection. The radiopharmaceutical, however, failed to localize in the known tumour sites. This result was in contrast to the authors experience of 131I- and 123I-MIBG imaging of carcinoid tumours. Seven patients with known metastatic carcinoid tumours, two patients with symptoms of recurrence following tumour resection, one patient with completely resected disease, and two patients with a flushing syndrome of uncertain aetiology were studied with 131I-MIBG. Three of the seven patients with known metastatic disease had positive 131I-MIBG scans. Both patients with clinical evidence of recurrent disease had negative scans, as did the patient who was considered to have had complete resection of her primary tumour. The two patients with idiopathic flushing syndrome also had negative scans. Among seven patients imaged with 123I-MIBG there were four true-negative scans and one false-negative, the latter in a patient with biochemical and CT evidence of recurrence. In a seventh patient with distant metastases there was variable uptake in some of the lesions. Four patients were studied with indium-111 pentetreotide . Two patients with metastatic carcinoid disease had positive scans, although hepatic metastases were not seen in one. Another two with idiopathic flushing syndrome had normal studies. The literature suggests that up 50% of carcinoid tumour cases are detected with 131I-MIBG, compared to a sensitivity of 87% reported with somatostatin receptor imaging using 111In-pentetreotide. The experience with 123I-MIBG is much less extensive. The mechanisms of carcinoid tumour localization for each of the three classes of radiotracers are discussed and contrasted to their varying sensitivities. The relative success of 131I-MIBG and 111In-pentetreotide relative to 131I-PIPA may be related to the fact that 131I-MIBG is actively taken up and stored by the enterochromaffin cells of the tumours and 111In-pentetreotide binds to cell surface receptors, whereas 131I-PIPA binds to tryptophan hydroxylase, which may be present in quantities too small to permit tumours to be imaged.

The kinetics of copper-62-PTSM in the normal human heart

Copper-62-labeled pyruvaldehyde bis(N-4-methylthiosemicarbazone) copper(II) (PTSM) is a generator-produced myocardial perfusion tracer. Animal studies have shown high myocardial tissue extraction and prolonged retention. The aim of this study was to define myocardial kinetics of 62Cu-PTSM and to determine its suitability for evaluating myocardial perfusion at rest and during pharmacological vasodilation in human subjects. In six healthy volunteers, 62Cu-PTSM was administered at baseline and during a 6-min adenosine infusion (140 micrograms/kg/min). Dynamic PET imaging with high temporal resolution was performed over 20 min. Good image quality was observed at rest and following adenosine. Myocardial kinetics demonstrated prolonged tissue retention with a clearance half-life of 105 +/- 49 min at rest and 101 +/- 65 min following adenosine (p = ns). Copper-62-PTSM tissue retention was quantified and showed only a 1.97-fold increase from rest to adenosine studies. This suggests attenuation of tracer retention at high flow rates. Copper-62-PTSM represents a promising new radiopharmaceutical for the evaluation of myocardial perfusion in the human heart.

Effect of aspartame-derived phenylalanine on neutral amino acid uptake in human brain: a positron emission tomography study

The possible effects of elevation of the plasma phenylalanine level secondary to the ingestion of aspartame on brain amino acid uptake in human subjects have been investigated by means of positron emission tomography (PET). 1-[11C]Aminocyclohexanecarboxylate [( 11C]ACHC) is a poorly metabolized synthetic amino acid that crosses the blood-brain barrier by the same carrier that transports naturally occurring large neutral amino acids. Quantitative test-retest PET studies were performed on 15 individuals. Seven received two identical baseline scans, whereas eight received a baseline scan followed by a scan performed approximately 40-45 min following ingestion of an orange-flavored beverage containing 34 mg/kg of body weight of the low-calorie sweetener aspartame, a dose equivalent to the amount in 5 L of diet soft drink consumed all at once by the study subjects, weighing an average of 76 kg. The 40-45-min interval was selected to maximize the detection of possible decreases in ACHC uptake resulting from increased competition for the carrier, because the plasma phenylalanine level is known to peak at this time. We observed an 11.5% decrease in the amino acid transport rate constant K1 and a smaller decrease in the tissue distribution volume of ACHC (6%). Under conditions of normal dietary use, aspartame is thus unlikely to cause changes in brain amino acid uptake that are measurable by PET.

Use of [11C]aminocyclohexanecarboxylate for the measurement of amino acid uptake and distribution volume in human brain

A quantitative positron emission tomographic (PET) method to measure amino acid blood-brain barrier (BBB) transport rate and tissue distribution volume (DV) has been developed using 11C-labeled aminocyclohexanecarboxylate (ACHC), a nonmetabolized amino acid analogue. Dynamic PET data were acquired as a series of 15 scans covering a total of 60 min and analyzed by means of a two-compartment, two-parameter model. Functional images were calculated for the amino acid transport rate constants across the BBB and the amino acid DV in the brain. Results show [11C]ACHC to have an influx rate constant in gray matter of approximately 0.03-0.04 ml g-1 min-1, indicating a single-pass extraction fraction of approximately 5-7%. The intersubject coefficient of variation was approximately 15% while intrasubject variability of repeat scans was only slightly greater than 5%. Studies were performed in 15 young normal volunteer control subjects, 5 elderly controls, 7 patients with probable Alzheimer's disease, and one patient with phenylketonuria. Results indicate that [11C]-ACHC will serve as the basis of a method for measuring amino acid transport rate and DV in the normal and pathological human brain.

omega-123I-hexadecanoic acid metabolic probe of cardiomyopathy

The utility of omega-123I-hexadecanoic acid myocardial scintigraphy as a metabolic probe of cardiomyopathies was investigated. Sixteen patients with a variety of cardiomyopathies and myopathies that involve cardiac muscle and ten volunteers were imaged in the postabsorptive state in a 40 degrees LAO projection after a standard dose of omega-123I-hexadecanoic acid. An elimination T1/2 was calculated from the left ventricular myocardial time-activity curve. An uptake index, corrected for chest wall attenuation, was also computed in 7 of 10 volunteers and 8 of 16 patients. Of the 16 patients, only 2 had distinctly abnormal omega-123I-hexadecanoic acid myocardial tracer kinetics. The first patient had a metabolic disorder of which carnitine deficiency was one component. The second patient had endocardial fibroelastosis, a process which has been linked to disorders which deprive the myocardium of oxygen and energy. Therefore, the cardiomyopathy may have been caused by some abnormality of cardiac metabolism other than carnitine deficiency. Although of limited utility in the overall cardiomyopathic population, omega-123I-hexadecanoic acid myocardial scintigraphy should be further investigated as a screening test for carnitine deficiency and related metabolic abnormalities in patients at risk.