DHA Restriction in Early Development Leads to Distortions in Visual Pathways
If you pause to think about it, the ability of the eye to coordinate its visual responses to different types of inputs is quite remarkable. Besides responding to light with visual images, the eye responds to sound signals by turning the eye and head toward the direction of the sound. It responds to touch to determine the nature of the object it feels and processes three-dimensional vision as well. These coordinated functions occur as a result of neurons from the retina converging with other sensory neurons in two specialized brain structures. These are the superior colliculus and the lateral geniculate nucleus. The pathways between the retina, the superior colliculus and lateral geniculate nucleus and the visual cortex are illustrated in the Figure. For retinal neurons to work properly together, they must be arranged in specific patterns in these brain structures. Anything that interferes with the formation of these patterns may affect the complex function of these structures and the communication between the retinal neurons and the visual cortex in the brain. As it happens, nutrient deprivation in early development and interference with the metabolism of particular fatty acids can do just that. This article describes studies conducted in Brazil by researchers interested in the effects of nutritional deprivation in early life on the development of the visual system. Their work was carried out in laboratory animals, but is relevant to human nutrition because intakes of some fatty acids are very low and visual function is similar to human vision. The investigators examined the effects of omega-3 fatty acid restriction in animals prior to mating, with the restriction continued through pregnancy and lactation, up to 6 weeks after delivery. Restricted animals were fed small but sufficient amounts of linoleic acid, an essential fatty acid that must be supplied in the diet. They had no source of long-chain omega-3s, the fatty acids found mainly in seafood. Control animals received a diet sufficient in all the fatty acids and nutrients needed for growth and development. One group of omega-3-restricted animals received omega-3s one week after delivery to see if the effects of prenatal restriction could be corrected after birth. When the infant animals were evaluated at one month of age, the omega-3-restricted animals had about half the amount of DHA in the superior colliculus as found in the control animals. The investigators also observed that the arrangement and distribution of the retinal neurons in the superior colliculus was disordered compared with the controls. Instead of their orderly pattern, the neurons were fused, spread over a wider area and extended beyond their usual zone. This disarray was observed as early as 13 days after birth. Similar disarray was observed in the lateral geniculate nucleus as well. In contrast to the disorder in the omega-3 restricted animals, those replenished with fish oil after birth had neuronal patterns similar to the controls. The provision of long-chain omega-3s restored the normal pattern of neurons in the superior colliculus. Using a different approach, the researchers also showed that the omega-3 restricted animals had a longer period to complete the formation of neuronal patterns in these structures. This might be an adaptation to the omega-3 restriction. These complicated, but elegant, studies extend our understanding of how long-chain omega-3s, particularly DHA, contribute to optimum visual development and its integration with other sensory pathways. The authors suggested that DHA is likely involved in establishing the correct pattern and arrangement of retinal neurons in the superior colliculus and lateral geniculate nucleus. They showed that critical nutrients and timing are necessary for optimum visual development. Next time you turn your head to see where that songbird is sitting, think of how nutrition affects such a complex process.