Abstract
Some studies have demonstrated that aging is associated with impaired vestibular reflexes, especially otolithic reflexes, resulting in postural instability. However, the neurochemical basis of these age-related changes is still poorly understood. The L-arginine metabolic system has been implicated in changes in the brain associated with aging. In the current study, we examined the levels of L-arginine and its metabolizing enzymes and downstream metabolites in the vestibular nucleus complex (VNC) and cerebellum (CE) of rats with and without behavioral testing which were young (4 months old), middle-aged (12 months old) or aged (24 months old). We found that aging was associated with lower nitric oxide synthase activity in the CE of animals with testing and increased arginase in the VNC and CE of animals with testing. L-citrulline and L-ornithine were lower in the VNC of aged animals irrespective of testing, while L-arginine and L-citrulline were lower in the CE with and without testing, respectively. In the VNC and CE, aging was associated with lower levels of glutamate in the VNC, irrespective of testing. In the VNC it was associated with higher levels of agmatine and putrescine, irrespective of testing. In the CE, aging was associated with higher levels of putrescine in animals without testing and with higher levels of spermine in animals with testing, and spermidine, irrespective of testing. Multivariate analyses indicated significant predictive relationships between the different variables, and there were correlations between some of the neurochemical variables and behavioral measurements. Cluster analyses revealed that aging altered the relationships between L-arginine and its metabolites. The results of this study demonstrate that there are major changes occurring in L-arginine metabolism in the VNC and CE as a result of age, as well as behavioral activity. (C) 2017 IBRO. Published by Elsevier Ltd. All rights reserved.