Effect of hypophysectomy and growth hormone on tryptophan and serotonin metabolism

The present study reports data on brain serotonin (5-HT) metabolism in two experimental conditions which have in common a lack of pituitary growth hormone (GH) and refers also the effect of a GH replacement therapy. In hypophysectomized female rats or genetically dwarf mice, which have a more selective deficiency of GH, brain tryptophan (TP) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were significantly higher than in age-matched controls. Brain levels of 5-HT were practically unchanged. A GH replacement therapy resulted in both hypophysectomized rats and dwarf mice in a significant reduction of brain TP and 5-HIAA concentrations. These results suggest that GH may exert an inhibitory action on 5-HT metabolism. Consonant with this view are the preliminary findings that in 7 individuals with elevated plasma GH levels for acromegaly, plasma TP levels were lower than in control subjects.