PURPOSE. Docosahexaenoic acid (DHA), an n-3 fatty acid, is the major polyunsaturate in rod outer segments. The effect of long-term n-3 fatty acid deficiency on rod and cone phototransduction was investigated in the rhesus monkey. METHODS. From birth to Х9 years rhesus monkeys were fed an n-3-deficient diet (n ϭ 9) known to reduce retinal DHA by 80%. Monkeys in the control group (n ϭ 12) received either 8% ␣-linolenic acid (ALA) or 0.6% DHA, both of which support normal retinal DHA levels. None of the diets contained carotenoids. Photoactivation kinetics were assessed from the rate of increase and a P3 model fit of the ERG a-wave. Maximal cone amplitude and sensitivity were measured from the cone a-wave at 4 ms. The rod photoresponse and rod recovery were derived by using a paired flash method. RESULTS. Rod sensitivity was reduced by 40% in the n-3-deficient monkeys at 9 but not 4.5 years. The onset of the rising phase of the photoresponse was significantly delayed (P Ͻ 0.004) at 9 years. Rod recovery was delayed by 20% in n-3-deficient monkeys at both ages, but only for bright saturating flashes. Cone phototransduction was not altered by n-3 deficiency. CONCLUSIONS. Long-term dietary n-3 deficiency in the rhesus monkey was associated with two changes in retinal function. First, there was a delay in rod recovery that has remained relatively constant throughout life. Second, there was an agedependent loss in rod phototransduction sensitivity; the lack of dietary carotenoids may have contributed to this decline. (Invest Ophthalmol Vis Sci. 2009;50:4360 -4367) DOI:10.1167/ iovs.09-3640 D ocosahexaenoic acid (DHA or 22:6n-3), an n-3 fatty acid with six double bonds, is the major polyunsaturated fatty acid in rod outer segments (ROS). The highest concentration of DHA is found in the retina, particularly within the ROS where DHA accounts for up to 60% of phosphatidylethanolamine (PE) acyl chains. 1 PE is the major phospholipid class of the outer bi-lipid layer of ROS discs. The cone-dominated retinas of the goldfish and chick are also highly enriched in DHA. 1,2 The effects of dietary n-3 deficiency on retinal lipid composition have been well described. The most consistent change in retinal function in n-3-deficient rodents, as measured with the ERG, is a reduction in ERG amplitude. 7 described a nonlinear function that related the reduction in rod sensitivity to retinal DHA levels, but only at 16 weeks of age, by which time guinea pigs are adults. In contrast, there were no significant losses of rod sensitivity from n-3-deficient guinea pigs at 6 or 11 weeks of age, despite 35% to 40% reductions in retinal DHA at these time points. At 16 weeks, similar reductions in retinal DHA levels were associated with significantly reduced rod sensitivity. The results indicate lower retinal DHA in combination with aging alters rod photoreceptor function in the guinea pig. In our earlier studies of the rhesus monkey, the most consistent change in retinal function of n-3-deficient animals was a delay in rod recovery as measured from either ERG a-or b-waves, a change seen starting in infancy. 5,8 However, we found no alteration in rod phototransduction sensitivity in n-3-deficient rhesus monkeys at 4.5 years of age when these animals were juveniles or subadults. 8 Given the interaction between age and n-3 deficiency in guinea pigs, the purpose of the present study was to investigate the effect of long-term n-3 deficiency on phototransduction mechanisms in our rhesus monkeys, now mature adults aged 9.2 Ϯ 1.2 years. The monkeys examined in the present study are of particular interest, as their diets were also free of carotenoids, including the xanthophylls lutein and zeaxanthin, which form the macular pigment. In a previous study of monkeys fed the same diets as we used, no lutein or zeaxanthin was present in the serum, and there was no detectable macular pigment. 10 Although the effects of varying dietary carotenoid levels were not examined in the monkeys described herein, the effect of long-term deficiency of both carotenoids and n-3 fatty acids on retinal function and aging is of particular interest. These two nutrients have been identified as risk factors for age-related macular degeneration (AMD) and are being evaluated in a large-scale trial of AMD progression 11 (www.areds2.org/ Age-Related Eye Disease Study)
