One hundred and eight female Wistar rats were placed on tryptophan sufficient or tryptophan deficient diets. Six animals from each group were sacrificed after 2 days, 4 days and multiples of 4 days. Plasma tryptophan level was found to reflect changes in brain serotonin level. Furthermore, plasma tryptophan and brain serotonin were significantly decreased after as little as 4 days on the tryptophan deficient diet. The levels of plasma tryptophan and brain serotonin were relatively constant from 4 to 28 days, at which time both showed further decreases.
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Culley et al. (1963) studied this question.