METABOLISM OF A TRYPTOPHAN LOAD IN THE HYPOTHALAMUS AND OTHER BRAIN REGIONS

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Abstract
Abstract— Results confirm previous findings that after injecting rats with 50mg/kg tryptophan the percentage increase of 5‐hydroxytryptamine metabolism (as shown by 5‐hydroxyindolylacetic acid changes) is particularly small in the hypothalamus. However, 15–30 min after tryptophan injection (when brain 5‐hydroxytryptamine changes were maximal) percentage 5‐hydroxytryptamine increases in the hypothalamus and in the rest of the brain were comparable. The small 5‐hydroxyindolylacetic acid changes in the hypothalamus are consistent with a long 5‐hydroxytryptamine turnover time therein as indicated by experiments using pargyline or probenecid and by the relatively small increases of 5‐hydroxytryptamine after injecting tryptophan into tranylcypromine treated rats. When 5‐hydroxytryptamine synthesis was partially inhibited by p‐chlorophenylalanine and tryptophan was injected, there was a large percentage rise of hypothalamic 5‐hydroxytryptamine but the concentration found in rats given neither drug was not attained and 5‐hydroxyindolylacetic acid showed little change. Elsewhere in the brain 5‐hydroxytryptamine attained concentrations comparable to those in rats given neither drug and 5‐hydroxyindolylacetic acid rose considerably. Results are discussed in relation to the contributions made to brain 5‐hydroxytryptamine turnover by functional and non‐functional metabolism.