Loss of neurons or neuronal functions over time has been hypothesized to contribute to the dysregulation of autonomic functions observed in aging. In this study, we evaluated the total number of the hypothalamic hypocretin (orexin) immunopositive neurons in 100, 400, 800 and 1000-day-old male and female C57Bl/6 mice that are commonly used in aging studies in vertebrates. Males had 15-20% more hypocretin immunopositive neurons (HIN) than females at all ages examined. Neuronal number for both sexes was stable in the first 400 days of life, but started declining between 400 and 800 days with rates of approximately 1 neuron/day. The rate of loss doubled in males between 800 and 1000 days of age. The total average number of HIN for males was 2251 ± 139 at 100 days, 2235 ± 112 at 400 days, 1914 ± 81 at 800 days, and 1596 ± 301 at 1000 days. The total average number of HIN for females was 1805 ± 76 at 100 days, 1887 ± 118 at 400 days, and 1521 ± 181 at 800 days. Evaluation of the time-dependent decline in the number of hypocretin immunopositive neurons may help to explain the physiological changes in sleep or energy homeostasis regulation during aging.
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