Dietary tryptophan and the selection of ethyl alcohol in different strains of rats

Rats of three different strains were tested systematically for their selection of water or ethanol offered in a two-choice situation. In order that preference-aversion curves could be derived, the concentrations of ethanol were increased in strength from 3 to 30% over an 11 or 12 day test sequence. L-tryptophan was added to the rat's diet in 1% or 3% mixtures for a predetermined interval beginning before or during a test for ethanol preference. Given to female hooded rats of the Royal Victoria strain, excess tryptophan elevated the ethanol preference threshold and significantly increased ethanol intake according to the dose administered. However, in male hooded rats of the Long-Evans strain, tryptophan augmented only slightly the selection of ethanol. Male Sprague-Dawley rats, an albino strain, were unaffected by the addition of this essential amino acid to their diet. Although tryptophan may exert its effect on ethanol intake through an alteration in the metabolism of cerebral serotonin, such an effect seems to be dependent upon the strain of rat tested.