Abstract
The effect of combinations of nicotinic acid, insulin and tryptophan on serotonin metabolism in different parts of the brain of adult female rats have been investigated. Tryptophan, 5-HT, 5-HIAA and tryptophan-hydroxylase were measured in the forebrain, cerebellum and brain stem. The effect of a full combined treatment on behaviour was also investigated. The administration of nicotinic acid and insulin did not change the concentrations of total and free tryptophan in the plasma. The treatment increased the tryptophan concentration in the brain stem and in the cerebellum, but caused no change in the forebrain. The concentration of 5-HT was reduced in the brain stem, but not in the forebrain or in the cerebellum and the treatment had no effect on 5-HIAA in any parts of the brain. Combining nicotinic acid or insulin with tryptophan reduced the tryptophan concentration in the forebrain, whereas it raised the concentration of tryptophan in the cerebellum. Full combined treatment caused a fall in total and free tryptophan concentration in the plasma on week 4. The treble-treatment raised the tryptophan concentration in the cerebellum and in the brain stem on the week 3. The concentration of 5-HT was reduced in the cerebellum on the week 2 onwards, whereas that in the forebrain was on week 5. In the cerebellum, the concentration of 5-HIAA was lower on the week 4 and in the forebrain it was lower on week 5. The activity of tryptophan-hydroxylase was lower in the brain stem and cerebellum after three weeks of treble-treatment, despite the increase in activity of the enzyme with increasing age. Plasma concentration of methionine and isoleucine was reduced in the animals treated with nicotinic acid and giving insulin prevented this change. The treble-treatment also reduced the concentrations of tyrosine and phenylalanine in the plasma The concentrations of tryptophan in the brain stem and cerebellum were positively correlated with total plasma tryptophan and were negatively correlated with the ratio of free plasma tryptophan to the sum of other neutral amino acids. The concentration of NEFA was positively correlated with free tryptophan in the plasma. The locomotor activity of the animals was inhibited after two weeks treble-treatment, and the condition was accompanied by an increase in defensive behaviour. The results have indicated that serotonin metabolism in different parts of the brain responds in different way's to nutritional factors and that these changes may be associated with modifications of the functioning of the particular regions.