Purpose  To characterize the effects of circadian rhythm, feeding time, age, general anesthesia, and ocular hypotensive compounds on intraocular pressure (IOP) of the Tibetan monkey (Macaca thibetana).

Methods  Tibetan monkeys were trained for IOP measurement with the TonoVet® rebound tonometer without sedation or anesthesia.  Their circadian IOP fluctuation was monitored every 3 h.  Effects of changing the feeding time, general anesthesia, age (2-3 year-old versus 8-15 year-old animals), and various pharmacological agents, such as travoprost, timolol, naphazoline and spiradoline, on IOP were also evaluated.

Results  After behavioral training, conscious Tibetan monkeys were receptive to IOP measurement.  The lowest and highest IOP values in a circadian cycle were recorded at 3:00 AM (19.8 ± 0.4 mmHg, mean ± SEM, n = 12) and noon (29.3 ± 0.9 mmHg), respectively. Changing the feeding time from 11:30 AM to 12:30 PM lowered the noon IOP to 25.1 ± 1.2 mmHg.  General anesthesia lowered IOP in these monkeys, while IOP of young and matured animals were similar. Three hours after topical ocular administration, travoprost reduced IOP by 5.2 ± 0.6 mmHg (n = 6, p < 0.001), and timolol reduced IOP by 2.8 ± 0.7 mmHg (p < 0.05).  Naphazoline and spiradoline lowered IOP by 4.8 mmHg and 2.5 mmHg (both p < 0.001), respectively, 2 h after drug administration.

Conclusion  The circadian IOP fluctuation in conscious Tibetan monkeys and their responses to travoprost, timolol, and other experimental conditions are similar to other primates. These monkeys appear to be a suitable model for glaucoma research.