(A) Relative colonic mRNA expression (fold-difference) of solute carrier family 6, member 4 (Slc6a4, encoding an ortholog of SERT, 5-HT transporter) versus 5-HT4 receptor, in germ free and microbiota-colonized mice (n = 16 germ-free, GF; n = 13 humanized, HM; n = 16 conventionally raised, CR). (B) Similar to panel (A), Slc6a4 versus Htr4 mRNA expression in both males (n = 26) and females (n = 19). (C) Slc6a4 versus Htr4 mRNA expression in colonic segments from all mice showing coordinate expression (n = 45 mice). (D) Relative mRNA expression of tryptophan hydroxylase 1 (Tph1) versus chromogranin A (Chga) in mouse colonic segments from all three colonization states and both sexes (n = 46 mice).