Six male adult rhesus monkeys (Macaca mulatta; age, 6.7 ± 1.3 years) were included in this study. All animals initially were anesthetized for measurement procedures with intramuscular ketamine (15 mg/kg; Henry Schein Animal Health, Dublin, OH, USA) and xylazine (1.5 mg/kg; Akorn, Inc., Decatur, IL, USA), as well as a subcutaneous injection of atropine sulfate (0.05 mg/kg; Butler Schein Animal Health, Dublin, OH, USA). Animals were then intubated with an endotracheal tube to breathe air supplemented with pure oxygen at a ratio of 9:1 (air:oxygen). This was achieved by a gas mixer, which mixed the gas from two corresponding compressed gas tanks. The ending oxygen concentration that the animals breathed in was approximately 30% to ensure sufficient oxyhemoglobin saturation. During anesthesia animals' body temperature was kept at 37°C using a warming blanket. Heart rate, end tidal CO2, as well as arterial oxygen saturation were recorded continuously. Arterial blood pressure was recorded continuously after cannulating one superficial tibial artery with a 27-gauge needle coupled to a pressure transducer and an amplifier system (BLPR2 and Lab-Trax-4/24T; World Precision Instruments, Sarasota, FL, USA).
For Doppler OCT readings, pupils were dilated using 1% tropicamide (Alcon Laboratories, Inc., Fort Worth, TX, USA). During all procedures, anesthesia was maintained by intravenous pentobarbital (8–12 mg/kg/h) infusion by means of an infusion pump (Aladdin; World Science Instruments, Inc.).
Optic nerve transection was performed under general anesthesia with 1.5% to 3% isoflurane in oxygen. The optic nerve was approached via lateral orbitotomy, zygomatic bone excision, and the dissection of the lateral rectus muscle. The optic nerve was cut approximately 6 to 8 mm proximal of globe entry and care was taken not to severe the central retinal artery. After reattaching the lateral rectus muscle, facial muscles and skin incisions were sutured.
To ensure the integrity of the central retinal artery, fundus fluorescein angiography was performed 7 to 10 days after ONT.