Functional gastrointestinal disorders and gut-brain axis: What does the future hold?

Table 2 Methods used to study brain - Gut interaction in functional gastrointestinal disorders

Link to be tested	Name of the test	Evidence	Comments
Microbiota-gut-brain axis
	Germ-free mice	Abrams et al[45]	This has been the most widely used technique to study the gut-brain axis. Germ-free mice are compared with healthy to look for changes in desired characteristics or behaviors
	Antibiotic-treated mice	Verdú EF et al[59]	Antibiotics are used to induce changes in the composition of microbiota and then these treated mice are compared with untreated mice to look for the desired characteristics. Antibiotics are useful for selectively eliminating certain bacteria from the gut, allowing the growth of other strains.
	Mice treated with probiotics	Mohle et al[64]	Once germ-free mice have been studied, they can be injected with probiotics to establish the reciprocity of the relationship that has been studied.
The interactions between visceral, peripheral and central pathways
	Functional MRI (fMRI)	Tillisch et al[72]; Aziz et al[73]; Mayer et al[75], and Labus et al[76]	fMRI measures the changes in oxygenated and deoxygenated hemoglobin where oxygenated hemoglobin denotes the group of neurons that have increased activity. They are useful in studying the complex relationship between visceral stimuli and brain response.
	PET imaging	Tillisch[72]	PET imaging has the advantage of probing a particular receptor by developing a radiolabeled ligand. This important feature can be used to assess specific receptor activities during pain and stress response in control and FGID patients.
	Structural MRI (sMRI)	Seminowicz et al[77]	Whole and regional brain images using sMRIs have been used to study differences between individuals with FGIDs and control groups

FGIDs: Functional gastrointestinal diseases; MRI: Magnetic resonance imaging; PET: Positron emission tomography.