Imaging and Quantification of Brain Serotonergic Activity using PET

Abstract: This thesis investigates the potential of using positron emission tomography (PET) to study the biosynthesis and release of serotonin (5HT) at the brain serotonergic neuron. As PET requires probe compounds with specific attributes to enable imaging and quantification of biological processes, emphasis was placed on the evaluation of these attributes.The experiments established that the 5HT transporter radioligand [11C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile, [11C]DASB, is suitable for imaging and quantification of transporters in rats and rhesus monkeys. In addition, the binding of [11C]DASB in brain tissue is decreased when 5HT concentrations are increased by tranylcypromine administration. The sensitivity of [11C]DASB binding, under these experimental conditions, to increased endogenous 5HT concentrations demonstrates the potential of in vivo monitoring of 5HT release in rat and monkey models.The irreversible binding of 5-hydroxy-L-[?-11C]tryptophan, [11C]HTP, in the monkey brain was lower in the presence of NSD1015, which was used to inhibit the decarboxylase step in 5HT synthesis. [11C]HTP seems thus to have potential for tracking changes in the activity of this biosynthesis enzyme. In contrast, the accumulation of [11C]HTP was unaffected by clorgyline, which was used to inhibit metabolism of the probe in the brain. This appears to indicate that elimination of the main metabolite from the brain could be negligible and thus will not alter [11C]HTP quantification. The extent and distribution of the irreversible binding of a substrate for the first enzyme in 5HT formation, ?-[11C]methyl-L-tryptophan, [11C]AMT, was different from those for [11C]HTP. This suggests that the two studied probe compounds provide estimates related to the enzyme activity of different steps in the 5HT biosynthesis pathway.A reference tissue version of the Patlak method for the analysis of data obtained by PET was also developed. This approach takes into account irreversible binding in the reference region and appears, therefore, to yield more reliable parameter estimates than the conventional reference Patlak analysis. The method is recommended for parameter estimation of [11C]HTP data when no metabolite-corrected plasma curve is available.Knowledge of altered 5HT synthesis and release in disease states and the consequences for effective pharmacotherapy can improve our knowledge of the aetiology of certain psychiatric and neurological diseases and enhance our ability to design more effective drugs.