Secondly, after surgery, changes in digestion or intestinal absorption may change biochemical signals in the brain; for example, changes in intestinal microflora, intestinal peptides (including ghrelin, glucagon-like peptide 1, peptide YY and cholecystokinin) are related to depression and anxiety, and bile acids can have a central impact on the emotional and behavioural responses caused by cocaine (48). Here, GCG is linked to major depressive disorder.