Table 1D.

Humoral communication pathway.

Microbiota-gut-brain communication routes		GI and Brain effects	References
A. SNS	A1. Catecholamine (Noradrenaline)	A1.1 Alteration of growth, motility and virulence of pathogenic and commensal bacteria	Moreira et al., 2016
B. HPA axis	B1. Corticosteroids → anti-inflammatory and immunosuppressive actions	B1.1 Regulation of brain-immune pathway	Dantzer, 2018
C. Microbial-derived metabolites	C1. SCFA	C1.1 Stimulation of SNS, release of mucosal 5-HT, influence of memory and learning processes, ↓ Aβ aggregates and neuroprotective, cognitive, and anti-depressive effects	Carabotti et al., 2015; Ho et al., 2018; Dalile et al., 2019; Fu et al., 2019
	C2. Neuropeptides (PYY, NPY, CCK, GLP-1, GLP-2, and substance P)	C2.1 Influence on the ENS and effect on brain (patho)physiology	Cani et al., 2013
	C3. Folate	C3.1 Precursor in the brain biosynthesis of monoamine neurotransmitters (5-HT, epinephrine, and dopamine), implication in cognitive performances by reducing inflammation and homocysteine levels, support to the structure and functions of juvenile brain cells	Miller, 2008; Rossi et al., 2011; Oikonomidi et al., 2016; Ma et al., 2019; Naninck et al., 2019
	C4. 5-HT	C4.1 Immunomodulation, regulation of GI motility, secretion and sensation	Desbonnet et al., 2008; Kashyap et al., 2013; Oh et al., 2015; Reigstad et al., 2015
	C5. Dopamine	C5.1 Neurotransmitter involved in reward, motivation, memory, attention, and regulation of body movements, correlated with neurodegenerative disorders	Tsavkelova et al., 2000; Asano et al., 2012; Evrensel and Ceylan, 2015; Maiti et al., 2017
	C6. γ-aminobutyric acid (GABA)	C6.1 Neurotransmitter implicated in anxiety and depression	Schousboe and Waagepetersen, 2007; Bravo et al., 2011; Asano et al., 2012; Barrett et al., 2012; Ko et al., 2013; Selhub et al., 2014