Locomotor activity and the expression of orexin A and orexin B in aged female rhesus macaques.

Reduced activity has been linked to age-associated physiological changes but the underlying root cause is unclear. The goal of the present study was to compare the orexin neuronal system of old (23–29 years) female rhesus macaques with either active or sedentary 24-hour locomotor activity patterns. Using immunohistochemistry, we counted the number of orexin A and orexin B neurons in the lateral hypothalamic area of each animal. Overall, we observed no difference in the distribution pattern or number of either orexin A or orexin B immune-positive neurons between animals in the 2 groups. Thus, reduced activity in the elderly is unlikely to stem from a loss of orexin neuronal perikarya in the lateral hypothalamic area. This, however, does not rule out the possibility that the reduced activity stems from reduced orexin neuronal projections to arousal centers of the brain, such as the locus coeruleus, or from attenuated release of orexin.