Rats were anesthetized with a combination of 75 mg/kg ketamine and 5 mg/kg xylazine (i.p.). Jugular catheters were surgically implanted, and the distal end of the catheter was attached to a back mount that could be connected to a fluid swivel (Instech Laboratories, Plymouth Meeting, PA, USA), as previously described (Schindler et. al., 2016 (link)). Rats received ketoprofen (2.5 mg/kg, s.c., Butler Schein, Owings Mills, MD) for pain management immediately following surgery. Rats were given at least 7 days to recover before starting experimental sessions. Catheters were flushed 5 days a week with gentamicin saline solution (0.2 mL, 5 mg/mL, Butler Schein, Owings Mills, MD) prepared from stock gentamicin.