The role of serotonergic and dopaminergic mechanisms and their interaction in Levodopa-induced dyskinesias

Long–term levodopa treatment in Parkinson’s disease (PD) is commonly associated with troublesome levodopa–induced dyskinesias (LIDs). Striatal serotonergic terminals amid the degenerating dopaminergic ones are proposed to play an important role in LIDs by taking up exogenous levodopa and releasing dopamine in an unregulated fashion. However, to date, the underlying mechanisms of LIDs are not fully understood. By using single photon emission computed tomography (SPECT) with 123I–Ioflupane and positron emission tomography (PET) with 11C–DASB and 11C–PE2I, the clinical studies conducted for this thesis aimed (a) to estimate the role of striatal dopamine transporter (DAT) availability in early PD as a prognostic marker for LIDs, (b) to explore whether striatal DAT availability changes over time are related to the appearance of LIDs, (c) to estimate the role of striatal serotonin-to-dopamine transporter (SERT–to–DAT) binding ratios to LIDs, and (d) to look for a relation between the changes in striatal SERT, DAT and SERT–to–DAT binding ratios over time and the appearance of LIDs. The main findings are as follows: (a) in early PD, striatal DAT availability alone does not predict the appearance of future LIDs, (b) at later stages, the occurrence of LIDs may be dependent on the magnitude of DAT decline in the putamen, (c) the SERT–to–DAT binding ratio in the putamen is increased in PD patients as compared to controls, and within PD, it is higher in patients with LIDs as compared to nondyskinetic patients, (d) as PD continues to progress, putaminal serotonergic terminals remain relatively unchanged in comparison to the dopaminergic ones and the aforementioned imbalance (as reflected by the binding ratio) increases over time. These findings provide fundamental insight in the pathophysiology of LIDs and have direct implications for further research towards novel therapeutics in PD dyskinesia.Open Acces

PET-imaging in depression and antidepressant therapies : focus on the serotonin system and the cerebral glucose metabolism

The main scope of the research summarised in this dissertation comprises the use of positron emission tomography to investigate the role of the serotonin transporter in depression and antidepressant therapies. Hereby, several studies were performed using the PET radiotracer [11C]DASB, which specifically targets the serotonin transporter. To allow qualitative and safe research with this radiotracer, the first research topic focussed on the optimization of the radiotracer’s purification procedure and its quality control. Using this radiotracer, a first-in-dog study was carried out to investigate the radiotracer’s distribution and to define the appropriate image quantification methods. Subsequently, this radiotracer was used to perform a dose-occupancy in the dog to estimate the optimal dosing regimen to treat dogs with behavioural disorders with escitalopram. A second part of the dissertation focuses on rats and the current position of repetitive transcranial magnetic stimulation (rTMS) in the rat. Hereby, several additional objectives were put forward. The first objective comprised the evaluation of the accuracy of a for rodents adapted human neuronavigation system to perform rTMS in the rat. A second objective was the investigation of the construct validity of two depression models in terms of altered regional glucose metabolism. This was investigated via a PET study using the radiotracer [18F]FDG. Finally, for the preferred depression model, which was the one based on chronic corticosterone injections, the scope was extended from the serotonin transporter to the serotonin 5-HT1A and 5-HT2A receptors to explore the role of the serotonin system in the pathophysiology of this depression model in the rat. For this purpose, three radiotracers were applied: [11C]DASB, [18F]MPPF, and [18F]altanserin. This allowed to image the serotonin transporters, the 5-HT1A receptors, and the 5-HT2A receptors, respectively

Computational analysis of the response of the monoaminergic neurotransmitter system and stress- and sex-steroid hormone systems to administration of antidepressant drugs

Fewer than half of depressed patients who take antidepressants experience complete remission of symptoms after 4-6 weeks of daily administration. The reason for this clinically observed heterogeneity in antidepressant response is still unclear. The majority of antidepressant drugs today are designed to enhance the brain’s production of one or more of the monoaminergic neurotransmitters (serotonin [5HT], norepinephrine [NE], and dopamine [DA]). In order to better understand how the brain adapts to chronic antidepressant administration, we developed a computational model that represents the known interactions of the neurobiology of depression. This model was expanded with further knowledge extraction, and also re-structured as computational tools were improved. Our model is based on the neuroadaptation hypothesis, whereby the brain homeostatically adapts to chronic antidepressant administration by adjusting the strengths of transmitter-system components (TSCs) in order to return brain-region activity levels back toward their pre-drug baselines. The main finding was that the model can adapt through many different pathways, arriving at many different TSC-strength configurations but not all of the adapted configurations are also associated with therapeutic elevations in the monoamines. These results provide insight into the heterogeneity among individuals in response to chronic antidepressants. We expanded this model to incorporate the stress-hormone response and the male sex-steroid system, and postulated that if only a subset of adapted configurations to chronic antidepressant are therapeutic, then it is possible that individual, pairs, or subsets of TSCs are responsible for mediating the therapeutic state. Through several modes of analyses, including sensitivity, correlation, and linear temporal-logic, we found that therapeutic neuroadaptation to chronic antidepressant is an overdetermined process that depends on multiple TSCs, providing a potential explanation for the clinical finding that no single antidepressant alleviates depressive symptoms in all patients. Our models can be used to systematically facilitate the clinical practice of antidepressant augmentation by providing the means to computationally screen for antidepressant drug/hormone combinations that could potentially be more therapeutic than single drugs by themselves

Effects of Smoking and Gender on Tetrahydroisoquinolines and Beta-Carbolines in a Healthy Population and During Alcohol Detoxification

The purpose of this investigation was to evaluate the effects of smoking and gender on 1) tetrahydroisoquinolines (TIQs) and beta-carbolines (BCs) in a population of healthy subjects and 2) TIQs in an alcohol-dependent population undergoing in-patient detoxification. Comparison in plasma TIQs between the populations was additionally conducted. To support the clinical investigations, a HPLC-FD method was developed and validated to assess plasma concentrations of BCs, harman and norharman, while a HPLC-ESI-MS/MS method was validated to quantify the TIQs, R/S-salsolinol along with dopamine. Forty-one young volunteers were recruited including 19 nonsmokers (NS), 11 light smokers (LS) and 11 heavy smokers (HS), stratified by their smoking history. Each group had, at least, 5 males and females. Plasma samples were obtained for analyte measurement within 30 minutes of smoking for LS and HS groups. Twoway ANCOVA was performed on the log-transformed concentrations. Significant differences were found between HS-NS and LS-NS in analyte concentrations. A comparison to eighteen subjects (6 NS, LS and HS) abstaining from smoking for 15 hours resulted in a difference only between NS and HS, suggesting that acute tobacco smoking has a major influence on circulating TIQs and BCs between smoking status groups. In a study involving thirty-five alcohol dependent subjects (12 NS, 11 LS, and 12 HS, balanced with gender), TIQ measurements were taken on day 1, 2, 3, 8 and 15 of inpatient detoxification. A significant effect of time was observed, with TIQ concentrations slightly increasing from admission to day 15. Both factors of smoking status and gender did not have a significant effect on plasma TIQ\u27s at any of the time points evaluated. Although, measures of acute and chronic alcohol intake had no effect on TIQ levels, liver function showed moderate correlation with plasma TIQs. Comparison of both populations showed that alcoholics had a lower average TIQ concentration than healthy subjects. The results indicate that smoking status 1) has an effect on plasma TIQs and BCs in healthy individuals and 2) does not have an effect in alcoholics during detoxification. The alcoholics possessed lower TIQ concentrations than the healthy subjects. No gender effect was observed in either study

[¹⁸F]fluorination of biorelevant arylboronic acid pinacol ester scaffolds synthesized by convergence techniques

Aim: The development of small molecules through convergent multicomponent reactions (MCR) has been boosted during the last decade due to the ability to synthesize, virtually without any side-products, numerous small drug-like molecules with several degrees of structural diversity.(1) The association of positron emission tomography (PET) labeling techniques in line with the “one-pot” development of biologically active compounds has the potential to become relevant not only for the evaluation and characterization of those MCR products through molecular imaging, but also to increase the library of radiotracers available. Therefore, since the [18F]fluorination of arylboronic acid pinacol ester derivatives tolerates electron-poor and electro-rich arenes and various functional groups,(2) the main goal of this research work was to achieve the 18F-radiolabeling of several different molecules synthesized through MCR. Materials and Methods: [18F]Fluorination of boronic acid pinacol esters was first extensively optimized using a benzaldehyde derivative in relation to the ideal amount of Cu(II) catalyst and precursor to be used, as well as the reaction solvent. Radiochemical conversion (RCC) yields were assessed by TLC-SG. The optimized radiolabeling conditions were subsequently applied to several structurally different MCR scaffolds comprising biologically relevant pharmacophores (e.g. β-lactam, morpholine, tetrazole, oxazole) that were synthesized to specifically contain a boronic acid pinacol ester group. Results: Radiolabeling with fluorine-18 was achieved with volumes (800 μl) and activities (≤ 2 GBq) compatible with most radiochemistry techniques and modules. In summary, an increase in the quantities of precursor or Cu(II) catalyst lead to higher conversion yields. An optimal amount of precursor (0.06 mmol) and Cu(OTf)2(py)4 (0.04 mmol) was defined for further reactions, with DMA being a preferential solvent over DMF. RCC yields from 15% to 76%, depending on the scaffold, were reproducibly achieved. Interestingly, it was noticed that the structure of the scaffolds, beyond the arylboronic acid, exerts some influence in the final RCC, with electron-withdrawing groups in the para position apparently enhancing the radiolabeling yield. Conclusion: The developed method with high RCC and reproducibility has the potential to be applied in line with MCR and also has a possibility to be incorporated in a later stage of this convergent “one-pot” synthesis strategy. Further studies are currently ongoing to apply this radiolabeling concept to fluorine-containing approved drugs whose boronic acid pinacol ester precursors can be synthesized through MCR (e.g. atorvastatin)