Region-selective decline of in vivo lipid synthesis in the aged rat visual system

[14C]palmitate and [3H]choline were injected intravitreally and, at the same time, intraventricularly in Wistar male rats at 4, 10, and 24 mo of age. The precursor incorporation into lipids of the retina, optic nerve tract, superior colliculus, and lateral geniculate body was followed for 2 h. The specific radioactivity of precursors pool (choline, phosphorylcholine, and free fatty acids) showed a marked decrease in optic nerve tract and lateral geniculate body of aged rats, whereas in retinal tissue and superior colliculus no changes were observed as a function of age. In rats of the three age groups, whole retina and superior colliculus showed neither changes of choline incorporation into phosphatidylcholine and sphingomyelin nor alteration of palmitate incorporation into diacylglycerols, triacylglycerols, and major phospholipid classes as a function of age. In sharp contrast, the optic nerve tract and, to a lesser extent, the lateral geniculate body exhibited a significant age-related decline of either the incorporation of both precursors into all lipid classes or the specific radioactivities of endogenous precursor pools. We concluded that the visual pathway structures are metabolically affected in a different manner by aging. Particularly, the ability of the retina and superior colliculus to metabolize lipids appeared to be age invariant. The marked decline of lipid biosynthesis with age, for some visual structures, is consistent with the trend generally observed in metabolic turnover and function of other CNS regions.