Dopamine, a monoamine neurotransmitter, is implicated in diverse functions of the central nervous system (CNS), such as motivation, cognition, and motor control. In the CNS, dopamine is synthesized by dopaminergic neurons located only in specific brain regions. Outside of the nervous system, dopamine is also found in the blood plasma, but dopamine and its metabolites, except for levodopa, cannot cross the blood-brain barrier. Entacapone is a catechol-O-methyl transferase inhibitor, which inhibits the degradation of levodopa into 3-methoxy-4-hydroxy-L-phenylalanine.
The aim of this study was to investigate the effects of entacapone, a modulator of dopamine, in the mouse hippocampus. Male 7-week-old C57BL/6J mice were randomly divided into 4 groups: a vehicle-treated group and groups treated with 10, 50, and 200 mg/kg entacapone. Vehicle or entacapone were administered orally once a day for 21 days, and then the animals were euthanized for immunohistochemistry and western blot analysis to reveal the effects of entacapone on hippocampal neurogenesis. Administration of entacapone significantly increased the number of proliferating (Ki67-positive) cells and differentiating (doublecortin-positive) neuroblasts in the dentate gyrus. Furthermore, the administration of entacapone significantly increased the expression of brain-derived neurotrophic factor (BDNF) and tyrosine kinase receptor B (TrkB), and the expression of their nuclear effector, the phosphorylated cAMP response element-binding protein (pCREB).
In this study, we also investigated glucose utilization after entacapone treatment in the mouse brain using 18[F]-fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT). Our results confirmed that glucose uptake was significantly increased in the olfactory bulb, laterodorsal tegmentum, hippocampus, subiculum, and retrosplenial cortex after entacapone treatment.
To evaluate the effects of entacapone on the hippocampal expression of proteins, 2-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry were performed. Results of 2-DE and MALDI-TOF mass spectrometry, and subsequent proteomic analysis, revealed an altered protein expression profile in the hippocampus after entacapone treatment. Proteomic analysis indicated that treatment with entacapone induced expressional changes in the proteins that are involved in synaptic transmission, cellular processes, cellular signaling, the regulation of cytoskeletal structure, energy metabolism, and various subcellular enzymatic reactions.
In this thesis, we have shown the effects of entacapone on the mouse hippocampus using morphological, radiologic, and proteomic analyses. Results of the study suggest that treatment with entacapone influences hippocampal neurogenesis by changing the BDNF-TrkB-pCREB signaling, which might result from the activation of dopaminergic neurotransmission between the hippocampus and the ventral tegmental area. In addition, the expressional changes of proteins involved in synaptic transmission, cellular processes, and cellular signaling support that the activation of dopaminergic transmission might stimulate the hippocampus, which might explain the observed morphological and proteomic changes in the hippocampus.