Age effects on monoamine turnover of the rat substantia nigra.

Measurement of turnover of dopamine (DA), noradrenaline (NA) and serotonin (5-hydroxytryptamine (5-HT), and their metabolites has been performed in 6- and 24-month-old rats. Dopamine synthesis in 24-month-old rats did not show any change with respect to 6-month-old rats. However, our results seem to indicate decreased DA release in 24-month-old rats. This hypothesis could be supported by the changes found in the 3-methoxytyramine (3-MT) accumulation rate after monoamine oxidase inhibition with pargyline. The turnover of NA and its main metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG) decreased in 24-month-old rats compared with 6-month-old rats. Serotonin synthesis did not change in 24-month-old rats with respect to 6-month-old rats. However, the metabolism of 5-HT quantified as turnover of 5-hydroxy-3-indoleacetic acid (5-HIAA) increased in 24-month-old rats with respect to 6-month-old rats. The monoamine oxidase B:monoamine oxidase-A ratio increased in 24-month-old rats. The significance of these changes is discussed.