Accommodative function in rhesus monkeys: effects of aging and calorie restriction

Abstract

Numerous degenerative changes in the visual system occur with age, including a loss of accommodative function possibly related to hardening of the lens or loss of ciliary muscle mobility. The rhesus monkey is a reliable animal model for studying age-related changes in ocular function, including loss of accommodation. Calorie restriction (CR) is the only consistent intervention to slow aging and extend lifespan in rodents, and more recently the beneficial effects of CR have been reported in nonhuman primates. The goal of the present study was to evaluate age-related changes in ocular accommodation and the potential effect of long-term (>8 years) CR on accommodation in male and female rhesus monkeys. Refraction, accommodation (Hartinger coincidence refractometer), and lens thickness (A-scan ultrasound) were measured in 97 male and female rhesus monkeys age 8-36 years under Telazol/acepromazine anesthesia. Refraction and accommodation measurements were taken before and after 40% carbachol corneal iontophoresis to induce maximum accommodation. Half the animals were in the control (CON) group and were fed ad libitum. The CR group received 30% fewer calories than age- and weight-matched controls. Males were on CR for 12 years and females for eight years. With increasing age, accommodative ability declined in both CON and CR monkeys by 1.03 ± 0.12 (P = 0.001) and 1.18 ± 0.12 (P = 0.001) diopters/year, respectively. The age-related decline did not differ significantly between the groups (P = 0.374). Baseline lens thickness increased with age in both groups by 0.03 ± 0.005 mm/year (P = 0.001) and 0.02 ± 0.005 mm/year (P = 0.001) for the CON and CR groups, respectively. The tendency for the for the lens to thicken with age occurred at a slower rate in the CR group vs. the CON group but the difference was not statistically significant (P = 0.086). Baseline refraction was -2.8 ± 0.55 and -3.0 ± 0.62 diopters for CON and CR, respectively. Baseline refraction tended to become slightly more negative with age (P = 0.070), but this trend did not differ significantly between the groups (P = 0.587). In summary, there was no difference in the slope of the age-related changes in accommodation, lens thickness, or refraction in the carbachol-treated eyes due to diet. These data are consistent with previous findings of decreased accommodative ability in aging rhesus monkeys, comparable to the age-dependent decrease in accommodative ability in humans. This study is the first to indicate that the accommodative system may not benefit from calorie restriction.

Keywords: Macaca mulatta; accommodation; diet; refraction; vision.