Aged Fischer 344 rats exhibit altered orolingual motor function: relationships with nigrostriatal neurochemical measures

The present study utilized a novel behavioral preparation to measure differences in orolingual motor function between young (6 months) and aged (24 months) Fischer 344 (F344) rats. Rats were trained to lick an isometric force-sensing operandum for water reinforcement so that the number of licks per session, licking rhythm and lick force could be compared between the two groups. The aged rats exhibited a greater number of licks per session, but a slowed licking rhythm, compared to the young rats. Lick force did not differ significantly between the groups. The dopamine (DA) uptake inhibitor nomifensine decreased all three measures in both groups. Analyses of whole brain tissue content of 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. Differences were observed between the two groups with respect to relationships between behavioral and neurochemical tissue measures. Striatal DA content and the number of licks per session were positively correlated for the young rats but not for the aged rats. In the aged rats, but not the young rats, positive correlations were also observed between licking rhythm and the DOPAC+HVA/DA ratio in the substantia nigra. These findings suggest that age-related alterations in orolingual motor function may relate in part to functional changes in DA neuronal circuits.