Effect of catecholamine precursors on stress-induced changes in motor activity, exploration, and brain monoamines in...

Male CF-1 mice aged 24 months showed the same level of motor activity and exploratory behavior as 3-month-old mice under control (no stress) conditions or 45 min following cold-swim stress. Within 90 min after stress exposure, motor activity level in both age-groups returned to control values. In contrast, 30-month-old mice showed lower motor activity under control conditions and marked reductions in activity and exploration following stress which recovered to only half of the control value 90 min after stress. These deficits in the 30-month-old animals were not reversed by L-tyrosine or L-dopa treatment prior to stress. L-Tyrosine supplement did not affect any of the behavioral measures despite having pronounced effects on brain tyrosine levels. Compared with 3-month-old mice, the 30-month-old animals had lower brain tyrosine following dietary L-tyrosine treatment, lower norepinephrine (NE), dopamine, and dihydroxyphenylacetic acid, but higher homovanillic acid levels, and after stress evidenced only slight increases in levels of blood corticosterone. Stress-induced decreases in brain NE and increases in serotonin and 5-hydroxyindoleacetic acid levels were observed in both age-groups. Data are discussed in terms of age-related alterations in brain monoamine functions and adrenocortical control mechanisms.