A Proof-of-concept Study Evaluating the Microbiota-gut-brain Axis

The key role of gut microbiota in keeping local and systemic homeostasis is termed the "Microbiota-gut-brain axis", which is a complex bidirectional communication system between the gastrointestinal tract and the brain. The hypothalamic-pituitary adrenal (HPA) axis takes part to this bidirectional communication by releasing corticotrophin-releasing factor (CRF), which facilitates the release of adenocorticotrophin hormone (ACTH) from the pituitary, which enters systemic circulation to lead to the release of cortisol from the adrenal glands. Many reports indicating that this hormonal cascade has a significant role in the adjustment of several functions like gastrointestinal transit, visceral sensation and permeability of the intestinal wall.

The etiology of intestinal FDG uptake without pathologic lesions is not fully understood. Tohihara et al. reported physiologic bowel FDG activity at the delayed phase was more than that at the early phase in dual-time images, and postulated FDG secretion was the major cause of physiologic uptake. Franquet et al. reported that physiologic bowel FDG uptake was inhibited by antibiotics, such as rifaximin. Some studies proposed that a specific type of bacteria in the lumen plays a role in gathering FDG, and it explain individual differences in physiologic bowel FDG activity. Previous studies debated about if FDG transfer from the blood to the bowel lumen through a transcellular or paracellular pathways. The GLUT transporters are known to export glucose from mucosal cells to the blood, but it is doubtful they can also transport in the opposite direction. If bowel FDG uptake is associated to intestinal permeability, FDG is likely to migrate through a paracellular pathway because intestinal permeability is adjusted by paracellular tight junction.

There is strong evidence that microbial strains may generate neuroactive molecules such as neurotransmitters, which may interfere with gut and brain functions. Furthermore, gut microbiota compositional changes may affect pathogenesis in patients with Parkinson's disease (PD). A previous hypothesis of PD pointed disease originates in the enteric nervous system and spreads via autonomic neurons to the brain, eventually causing PD. Besides, several studies support the clinical use of Tc-99m TRODAT-1 SPECT in assessing the neurodegenerative status of PD.

To date, no radionuclide imaging studies for correlation between physiologic bowel FDG uptake and dopamine transporter degeneration have been elucidated. The investigators hope to have insight into pathophysiology of PD by investigating the association between the pattern of intestinal FDG activity and Tc-99m TRODAT-1 SPECT images. In addition, research in this field opens the possibility to use neuroactive molecule-producing probiotics as new potential therapeutic tools for patients with PD.