We studied the effects of treatment with (-)-deprenyl, a monoamine oxidase B inhibitor, on plasma levels of insulin-like growth factor-I (IGF-I) (as indicator of growth hormone (GH) secretion), levels of monoamines and their metabolites, and the activity and content of tyrosine hydroxylase - the rate-limiting enzyme in the biosynthesis of catecholamines - in the hypothalamus and hypophysis of old male rats.  Male Wistar rats (22 months old) were treated with 2 mg deprenyl/kg body weight s.c. three times a week for 2 months.  At the end of the treatment period, blood was collected for measurement of plasma IGF-I levels by radioimmunoassay (RIA).  The concentrations of dopamine, serotonin (5-HT) and their main metabolites were determined by high performance liquid chromatography (HPLC) with electrochemical detection, and the tyrosine hydroxylase content in hypothalamus and hypophysis was determined by enzyme-linked immunoabsorbent assay (ELISA). (-)-Deprenyl treatment produced a pronounced increase in dopamine and 5-HT in both the hypothalamus and hypophysis (P < 0.01).  The main dopaminergic metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), decreased in hypothalamus but not in hypophysis, and treatment had no effect on the concentration of 5-hydroxyindole-3-acetic acid (5-HIAA).  The tyrosine hydroxylase activity and tyrosine hydroxylase content increased in hypothalamus and hypophysis (P < 0.05). In the hypophysis the increase in tyrosine hydroxylase activity was consistent with the increase in tyrosine hydroxylase amount.  Moreover, (-)-deprenyl treatment restored the IGF-I plasma levels in old rats to a concentration similar to those found in young animals.  Postulated anti-aging effects of (-)-deprenyl could hence be due to restoration of hypothalamic hormones such as GH.

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