18F-2β-Carbomethoxy-3β-(4-chlorophenyl)-8-(2-fluoroethyl)nortropane (18F-FECNT), a PET radioligand for the dopamine transporter (DAT), generates a radiometabolite that enters the rat brain. The aims of this study were to characterize this radiometabolite and to determine whether a similar phenomenon occurs in human and nonhuman primate brains by examining the stability of the apparent distribution volume in DAT-rich (striatum) and DAT-poor (cerebellum) regions of the brain. Methods: Two rats were infused with 18F-FECNT and sacrificed at 60 min. Extracts of brain and plasma were analyzed by high-performance liquid chromatography (HPLC) and liquid chromatography–mass spectrometric (LC-MS) techniques. Two human participants and 3 rhesus monkeys were injected with 18F-FECNT and scanned kinetically, with serial arterial blood analysis. Results: At 60 min after the injection of rats, 18F-FECNT accumulated to levels about 7 times higher in the striatum than in the cortex and cerebellum. The radiometabolite was distributed at equal concentrations in all brain regions. The LC-MS techniques identified N-dealkylated FECNT as a major metabolite in the rat brain, and reverse-phase HPLC detected an equivalent amount of radiometabolite eluting with the void volume. The radiometabolite likely was 18F-fluoroacetaldehyde, the product expected from the N-dealkylation of 18F-FECNT, or its oxidation product, 18F-fluoroacetic acid. The distribution volume in the cerebellum increased up to 1.7-fold in humans between 60 and 300 min after injection and 2.0 ± 0.1-fold (mean ± SD; n = 3) in nonhuman primates between 60 and 240 min after injection. Conclusion: An 18F-fluoroalkyl metabolite of 18F-FECNT originating in the periphery confounded the measurements of DAT in the rat brain with a reference tissue model. Its uniform distribution across brain regions suggests that it has negligible affinity for DAT (i.e., it is an inactive radiometabolite). Consistent with the rodent data, the apparent distribution volume in the cerebellum of both humans and nonhuman primates showed a continual increase at late times after injection, a result that may be attributed to entry of the radiometabolite into the brain. Thus, reference tissue modeling of 18F-FECNT will be prone to more errors than analysis with a measured arterial input function.

http://jnm.snmjournals.org/content/47/3/520.full.pdf

PET imaging of the dopamine transporter with 18F-FECNT: a polar radiometabolite confounds brain radioligand measurements.

[123I]N-ω-fluoropropyl-2β-carbomethoxy-3β-{4-iodophenyl}nortropane ([123I]FP-CIT) single photon emission computed tomography (SPECT) is a frequently and routinely used technique to detect or exclude dopaminergic degeneration by imaging the dopamine transporter (DAT) in parkinsonian and demented patients. This technique is also used in scientific studies in humans, as well as in preclinical studies to assess the availability of DAT binding in the striatum. In routine clinical studies, but also in scientific studies, patients are frequently on medication and sometimes even use drugs of abuse. Moreover, in preclinical studies, animals will be anesthetized. Prescribed drugs, drugs of abuse, and anesthetics may influence the visual interpretation and/or quantification of [123I]FP-CIT SPECT scans. Here, we discuss the basic principle of how drugs and anesthetics might influence the visual interpretation and/or quantification of [123I]FP-CIT SPECT scans. We also review drugs which are likely to have a significant influence on the visual interpretation and/or quantification of [123I]FP-CIT SPECT scans. Additionally, we discuss the evidence as to whether frequently prescribed drugs in parkinsonian and demented patients may have an influence on the visual interpretation and/or quantification of [123I]FP-CIT SPECT scans. Finally, we discuss our recommendations as to which drugs should be ideally withdrawn before performing a [123I]FP-CIT SPECT scan for routine clinical purposes. The decision to withdraw any medication must always be made by the specialist in charge of the patient’s care and taking into account the pros and cons of doing so.

Dopamine transporter imaging with [(123)I]FP-CIT SPECT: potential effects of drugs.

Small animal imaging with high resolution single photon emission tomography.

Positron emission tomography (PET) provides a means of studying physiological and pharmacological processes as they occur in the living brain. Mice, rats, dogs, cats, pigs and non-human primates are often used in studies using PET. They are commonly anaesthetized with ketamine, propofol or isoflurane in order to prevent them from moving during the imaging procedure. The use of anaesthesia in PET studies suffers, however, from the drawback of possibly altering central neuromolecular mechanisms. As a result, PET findings obtained in anaesthetized animals may fail to correctly represent normal properties of the awake brain. Here, we review findings of PET studies carried out either in both awake and anaesthetized animals or in animals given at least two different anaesthetics. Such studies provide a means of estimating the extent to which anaesthesia affects the outcome of PET neuroimaging in animals. While no final conclusion can be drawn concerning the ‘best’ general anaesthetic for PET neuroimaging in lab...

https://journals.sagepub.com/doi/pdf/10.1258/la.2012.011173

Anaesthesia for positron emission tomography scanning of animal brains.

Neuro-imaging studies have shown altered, yet often inconsistent, serotonergic and dopaminergic neurotransmission in patients with obsessive–compulsive disorder (OCD). We investigated both serotonergic and dopaminergic neurotransmission in 9 drug-naive dogs with compulsive behaviour, as a potential model for human OCD. Single photon emission computed tomography was used with 123 I-R91150 and 123 I-FP-CIT, in combination with 99m Tc-ECD brain perfusion co-registration, to measure the serotonin (5-HT) 2A receptor, dopamine transporter (DAT) and serotonin transporter (SERT) availability. Fifteen normally behaving dogs were used as reference group. Significantly lower 5-HT2A receptor radioligand availability in frontal and temporal cortices (bilateral) was observed. Further, in 78% of the compulsive dogs abnormal DAT ratios in left and right striatum were demonstrated. Interestingly, both increased and decreased DAT ratios were observed. Finally, significantly lower subcortical perfusion and (hypo)thalamic SERT availability were observed in the compulsive dogs. This study provides evidence for imbalanced serotonergic and dopaminergic pathways in the pathophysiology of compulsions in dogs. The similarities with the altered neurotransmission in human OCD provide construct validity for this non-induced, natural canine model, suggesting its usefulness for future investigations of the pathophysiology of human OCD as well as the effectiveness of psychopharmacological interventions.

Serotonin 2A receptor, serotonin transporter and dopamine transporter alterations in dogs with compulsive behaviour as a promising model for human obsessive-compulsive disorder

BackgroundIsoflurane administration is known to increase extracellular dopamine (DA) concentration Because the dopamine transporter (DAT) is a key regulator of DA, it is likely affected by isoflurane This study investigates the hypothesis that isoflurane inhibits DA reuptake by causing DAT to be t

Interaction of isoflurane with the dopamine transporter.

PURPOSE: 8-(2-[18F]fluoroethyl)-2β-carbomethoxy-3β-(4-chlorophenyl)nortropane ([18F]FECNT) is a radiotracer for positron emission tomography (PET) used to trace the location and binding potential (BP) of the dopamine transporter (DAT). We tested the hypothesis that adrenergic receptor agonists increase the BP of [18F]FECNT for the DAT. PROCEDURES: Anesthetized monkeys were scanned twice to determine the ligand BP at baseline and during administration of phenylephrine (3–6 μg/kg/min) or norepinephrine (0.5–1 μg/kg/min). Standard immunohistochemistry and immunoblot analyses were performed in rats to determine if the quantity of DAT changed with phenylephrine. [18F]FECNT uptake in human embryonic kidney (HEK) cells, stably transfected with DAT cDNA, was measured by gamma scintillation counting during phenylephrine. RESULTS: The PET measured BP of [18F]FECNT increased by 50% and 45% during the phenylephrine and norepinephrine infusion, respectively. The immunohistochemistry and immunoblot analyses did not show a difference in total DAT. CONCLUSION: Adrenergic agonists increase the BP of [18F]FECNT in monkey striatum.

Jian Hua

Papers

Interaction of isoflurane with the dopamine transporter.

Phenylephrine and norepinephrine increase dopamine transporter ligand binding in striatum.