Age-associated changes in the circadian rhythm of retinal N-acetylserotonin and melatonin in rats with pigmented eyes.

Using a semiquantitative immunohistochemical procedure, we have evaluated the 24 hour circadian rhythm of N-acetylserotonin- and melatonin-like immunoreactivity in the retina of male hooded rats that have pigmented eyes. Three and twenty months old Long Evans rats were used. Animals were kept under a 14 light (L): 10 dark (D) lighting cycle and were sacrificed during the month of June. Four time points were assessed in the 24 hour light/dark cycle. Retinal N-acetylserotonin and melatonin were assessed by immunohistochemistry and microphotometric procedures as previously used by us. In young animals, the intensity of retinal N-acetylserotonin and melatonin immunofluorescence was significantly different (P less than 0.005, DF = 17) in animals killed during the light vs the dark period with peak values during the dark period. In contrast, retinal N-acetylserotonin and melatonin-immunofluorescence in old animals showed a flattened 24 hour rhythm, with light values as high as those observed during the dark period. The age-associated bluntness of the 24 hour rhythm was more noticeable for N-acetylserotonin than for melatonin immunofluorescence.