Aged Fischer 344 rats exhibit altered locomotion in the absence of decreased locomotor activity: exacerbation by nomifensine.

A novel force plate actometer was used to measure locomotor activity and gait in young (6 months) versus aged (24 months) Fischer 344 rats. The actometer revealed altered gait in the aged rats in the absence of decreased locomotor activity. The catecholamine uptake inhibitor, nomifensine increased locomotor activity in both groups and exacerbated the gait alteration in the aged group. Analyses of whole brain tissue levels of dopamine (DA), 3,-4 dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in the substantia nigra and dorsal striatum revealed no significant differences between the two age groups. In the young (but not aged) rats, distance traveled was negatively correlated with striatal DOPAC + HVA/DA tissue ratios (a measure of DA turnover). In the aged (but not the young) rats, positive correlations were observed between distance traveled and DOPAC + HVA/DA ratios in the substantia nigra. Neither striatal nor nigral DA content was significantly correlated with distance traveled in either age group. These findings demonstrate that aged rats may exhibit functional changes in locomotor activity in the absence of quantitative changes in nigrostriatal DA content.