Injected tryptophan increases brain but not plasma tryptophan levels more in ethanol treated rats.

In previous studies, long term treatment with ethanol has been shown to enhance brain 5-hydroxytryptamine 5-(HT) metabolism by increasing the activity of the regulatory enzyme tryptophan hydroxylase and or availability of circulating tryptophan secondarily to an inhibition of hepatic tryptophan pyrrolase. In the present study ethanol treatment given for two weeks decreased hepatic apo-tryptophan pyrrolase but not total tryptophan pyrrolase activity in rats. Tryptophan levels in plasma and brain did not increase significantly. But there was a marked increase of 5-HT but not 5-hydroxyindoleacetic acid (5-HIAA) concentration in brain, suggesting a possible increase in the activity of tryptophan hydroxylase. The effect of a tryptophan load on brain 5-HT metabolism was therefore compared in controls and ethanol treated rats. One hour after tryptophan injection (50 mg/kg i.p.) plasma concentrations of total and free tryptophan were identical in controls and ethanol treated rats, but the increases of brain tryptophan 5-HT and 5-HIAA were considerably greater in the latter group. The results are consistent with long term ethanol treatment enhancing brain serotonin metabolism and show that brain uptake/utilization of exogenous tryptophan is increased in ethanol treated rats and may be useful to understand the role and possible mechanism of tryptophan/serotonin involvement in mood regulation.