Effect of hypophysectomy and growth hormone on tryptophan and serotonin metabolism

The presented study reports data on brain serotonin (5 HT) metabolism in 2 experimental conditions which have in common a lack of pituitary growth hormone (GH) and discusses 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.