Initial anesthesia was induced with an intramuscular (IM) injection of ketamine (75 mg/kg, Ketaset; Fort Dodge Animal Health, Fort Dodge, IA, USA) and xylazine (10 mg/kg, AnaSed; Lloyd, Inc., Shenandoah, IA, USA). A pad with circulating heated water (T/pump; Gaymar, Orchard Park, NY, USA) was used to maintain body temperature. A femoral vein and an artery were cannulated with polyethylene tubes for drug administration and blood pressure monitoring, respectively. Once the femoral vessels were cannulated, deep anesthesia was maintained for the remainder of the experiment via an intravenous infusion of sodium pentobarbital (∼10 mg/kg/h, Nembutal; Oak Pharmaceuticals, Lake Forest, IL, USA) using an infusion pump (Genie; Kent Scientific, Torrington, CT, USA). Prior to imaging, pupillary dilation was induced topically with tropicamide (0.5%; Alcon Laboratories, Inc., Fort Worth, TX, USA).