S.c. challenge with whole heat-killed M. vaccae in M. vaccae–preimmunized mice increased 5-HT and 5-HT metabolite concentrations in the medial prefrontal cortex. Graphs illustrate 5-HIAA, 5-HT, and l-tryptophan concentrations in the left and right hemispheres of each brain region 12 h following s.c. injections of vehicle or whole heat-killed M. vaccae in M. vaccae–preimmunized mice (n=8–9). Abbreviations: Mv, s.c. challenge with heat-killed M. vaccae, Sal, s.c. challenge with saline vehicle; TRP, l-tryptophan. For additional abbreviations, see Fig. 1 legend. * P≤0.05, compared with M. vaccae–preimmunized, vehicle-injected controls based on multifactor ANOVA with repeated measures analysis within each brain region.