Propionate and butyrate increased AANAT and melatonin levels mediated by p-CREB. (A) Representative images (left) and quantification of intestinal p-CREB and CREB (right) levels detected using Western blotting after administration of sodium propionate or sodium butyrate to germfree rats (n = 6). (B) Representative images (left) and quantification of AANAT and p-CREB (right) levels in BON-1 cells detected using Western blotting after propionate/butyrate (10 mM) treatment for 24 h with or without 666-15 pretreatment (1 μM) for 2 h (n = 6). (C) Melatonin concentration in BON-1 cell supernatant detected using ELISA after propionate/butyrate (10 mM) treatment for 24 h with or without 666-15 pretreatment (1 μM) for 2 h (n = 6). GF, germfree rats receiving PBS gavage; GP, germfree rats receiving propionate gavage; GB, germfree rats receiving butyrate gavage; Pro, propionate; But, butyrate. * p < 0.05, ** p < 0.01, *** p < 0.001, according to one-way ANOVA with Tukey’s post-hoc test.