Age-dependent correlation of motor performance with neurotransmitter synthetic enzyme activities in mice.

The performance of male C57BL/6J mice (Mus musculus) was assessed in a battery of tests designed to detect age-related losses in motor abilities and also to examine individual differences within age groups. Studied were 4-, 18-, and 24-month-old mice. Within 5 days following the completion of the behavioral battery, the mice were killed, and brain tissue was taken for determinations of choline acetyltransferase (EC 2.3.1.6), L-glutamic acid decarboxylase (EC 4.1.1.15) and tyrosine hydroxylase (EC 1.14.16.2) activities in the frontoparietal + temporal + occipital cortex, hippocampus, striatum, and cerebellum. Two composite test scores were derived for each animal. The first composite score comprised measurements of locomotor activity, and the second reflected strength and coordination abilities. Test scores included in each composite correlated highly with that composite score, but not with the other composite. This observation suggests that there is a high degree of internal consistency for the composite scores and that the composite scores represent independent behavioral factors. Significant correlations between the composite scores and regional enzyme activities were detected; these correlations varied with age. In most cases, the sign (+, -) of the correlation was reversed at 24 months as compared with 4 months of age. These results suggest that the relations between performance and enzyme activities vary with age, and may reflect altered neurotransmitter function during senescence. This work serves as an example of how direct correlational analyses may help to elucidate behavioral-neurochemical interactions in aging.