Evidence for depressed catecholamine and enhanced serotonin metabolism in aging male rats: posssible relation to gondotropin secretion.

The steady state concentration of dopamine (DA), norepinephrine (NE), and serotonin (5HT) was determined and turnover estimated in several brain regions of young (3-4 months) and old (21 months) male Wistar rats. An estimate of DA and NE turnover was obtained by determining their depletion rates after treatment with alpha-methylpara-tyrosine, a tyrosine hydroxylase inhibitor. Serotonin turnover was estimated by determining its rate of increase after monoamine oxidase inhibition with pargyline. In old males, medial basal hypothalamic (MBH) DA concentration and depletion rate were significantly lower than in young males. DA concentration of the remaining hypothalamus also was lower in old than in young males, but depletion rates were not different. DA concentration and depletion rate in the olfactory tubercle were the same in both age groups. The steady state concentration of NE in the MBH and remaining hypothalamus, and the hypothalamic NE depletion rate, were significantly lower in old than in young animals. In both brain and hypothalamus, steady state concentrations of 5HT were the same in young and old rats, but by 30 min after monoamine oxidase inhibition with pargyline, hypothalamic, but not brain, 5HT increased more in old than in young males. This may indicate a greater turnover of 5HT in the hypothalamus of old than of young males. In non-drug treated (control) old male rats, serum LH and FSH were lower, serum prolactin was higher and serum TSH was the same in young male rats. These data suggest that a decrease in catecholamine and an increase in serotonin metabolism occur in the hypothalamus of old male rats. These changes may be related to the decrease in release of gonadotropins and increase in release of prolactin observed in these old male rats.