specifically selected because they are well characterized as being non-absorbed from the intestine4272829. This allows for maximal debulking of gut bacteria and minimal off-target effects due to systemic absorption. The antibiotic mixture was changed every two days and animals were weighed during the first week of experimentation to ensure body weight was maintained. Animals remained on antibiotic treatments throughout all cocaine treatments and behavioral paradigms. To measure any behavioral effects of parenterally-administered antibiotics we injected animals with 1.67 mg/kg vancomycin, 20 mg/kg neomycin, and 293 U/kg bacitracin in PBS every day for seven days prior to behavioral testing. During days of behavioral testing injections were performed >2 hours prior to the start of testing to avoid any association of the injection with the chamber. For experiments with short-chain fatty acids (SCFA) animals were given three days of antibiotic or control treatment, followed by addition of the three principle SCFAs produced by bacteria at concentrations similar to those found endogenously in the gut (67.5 mM acetate, 40 mM butyrate, 25.9 mM propionate) as has been reported in multiple previous studies examining effects of SCFA in mice3132. All animal protocols were approved by the Institutional Animal Care and Use Committee (IACUC) at
