were injected intraperitoneally with the triple antibiotic cocktail every day – thus bypassing the issue of intestinal absorption. These animals did not show the increased caecal size associated with large-scale knockdown of gut bacteria (data not shown). Since our most robust behavioral effect of oral antibiotics was in the CPP test, we examined effects of parenteral antibiotics in this assay and found no effect of such treatment on cocaine CPP, either at the 10 mg/kg dose (Fig. 4a – two-tailed t test: p = 0.57, t = 0.58) or 5 mg/kg dose (Fig. 4b – p = 0.97, t = 0.04). These findings support our interpretation that the behavioral effects observed with oral antibiotic treatment are due to depletion of gut microbiota and not due to systemic effects of the antibiotic cocktail.
