Neuroreceptor Bindings and Synaptic Activity in Visual System of Monocularly Enucleated Rat

Development of PET radiopharmaceuticals and their clinical applications at the Positron Medical Center

The Positron Medical Center has developed a large number of radiopharmaceuticals and 36 radiopharmaceuticals have been approved for clinical use for studying aging and geriatric diseases, especially brain functions. Positron emission tomography (PET) has been used to provide a highly advanced PET-based diagnosis. The current status of the development of radiopharmaceuticals, and representative clinical and methodological results are reviewed.

Adenosine receptor ligands and PET imaging of the CNS

Advances in radiotracer chemistry have resulted in the development of novel molecular imaging probes for adenosine receptors (ARs). With the availability of these molecules, the function of ARs in human pathophysiology as well as the safety and efficacy of approaches to the different AR targets can now be determined. Molecular imaging is a rapidly growing field of research that allows the identification of molecular targets and functional processes in vivo. It is therefore gaining increasing interest as a tool in drug development because it permits the process of evaluating promising therapeutic targets to be stratified. Further, molecular imaging has the potential to evolve into a useful diagnostic tool, particularly for neurological and psychiatric disorders. This chapter focuses on currently available AR ligands that are suitable for molecular neuroimaging and describes first applications in healthy subjects and patients using positron emission tomography (PET).

Increased regional cerebral blood flow but normal distribution of GABAA receptor in the visual cortex of subjects with early-onset blindness

Before the completion of visual development, visual deprivation impairs synaptic elimination in the visual cortex. The purpose of this study was to determine whether the distribution of central benzodiazepine receptor (BZR) is also altered in the visual cortex in subjects with early-onset blindness. Positron emission tomography was carried out with [(15)O]water and [(11)C]flumazenil on six blind subjects and seven sighted controls at rest. We found that the CBF was significantly higher in the visual cortex for the early-onset blind subjects than for the sighted control subjects. However, there was no significant difference in the BZR distribution in the visual cortex for the subject with early-onset blindness than for the sighted control subjects. These results demonstrated that early visual deprivation does not affect the distribution of GABA(A) receptors in the visual cortex with the sensitivity of our measurements. Synaptic elimination may be independent of visual experience in the GABAergic system of the human visual cortex during visual development.

Visualization of sigma1 receptors in eyes by ex vivo autoradiography and in vivo positron emission tomography

Sigma receptors are present on the neurons of the central nervous system and in peripheral organs. They have also been demonstrated in ocular tissues by in vitro membrane binding assays. We have investigated whether sigma(1) receptors can be demonstrated in rat eyes by ex vivo autoradiography using [(11)C]SA4503, a selective radioligand. We also tested whether in vivo positron emission tomography (PET) can be used to show sigma(1) receptors in rabbit eyes. In rats, a high accumulation of [(11)C]SA4503 was found in the iris-ciliary body and retina. A carrier-loading experiment showed that the receptor-specific binding of [(11)C]SA4503 was approximately 75% of the total binding in the brain. Sigma(1) receptors were also detected in the projecting terminals of the retina to the superior colliculus. PET showed radioactivity in the anterior segment including the iris-ciliary body and retina, and pretreatment or displacement by a sigma receptor ligand (haloperidol), suggested that the PET signal reflects radioligand-receptor binding. The high density of sigma(1) receptors in the iris-ciliary body and retina was confirmed by ex vivo autoradiography. In conclusion, the iris-ciliary body and retina are rich in sigma(1) receptors, and PET may be used to investigate the in vivo distribution of these neuroreceptors in the eye.

Preclinical studies on [11C]MPDX for mapping adenosine A1 receptors by positron emission tomography

In previous in vivo studies with mice, rats and cats, we have demonstrated that [11C]MPDX ([1-methyl-11C]8-dicyclopropylmethyl-1-methyl-3-propylxanthine) is a potential radioligand for mapping adenosine A1 receptors of the brain by positron emission tomography (PET). In the present study, we performed a preclinical study. The radiation absorbed-dose by [11C]MPDX in humans estimated from the tissue distribution in mice was low enough for clinical use, and the acute toxicity and mutagenicity of MPDX were not found. The monkey brain was clearly visualized by PET with [11C]MPDX. We have concluded that [11C]MPDX is suitable for mapping adenosine A1 receptors in the human brain by PET.