Exchange of 5-hydroxyindoleacetic acid between the spinal cord and lumbar cerebrospinal fluid.

1. After inhibition of 5-hydroxyindoleacetic acid (5-HIAA) transport by repeated administration of probenecid in cats, this acid accumulates linearly in the lumbosacral cord, lumbar cerebrospinal fluid (c.s.f.) and cisternal fluid during 5 hr of experiment. 2. A simple mathematical analysis of these results shows that (a) the rates of 5-HIAA accumulation in the lumbar c.s.f. and cisternal c.s.f. are faster than in the cord, (b) about 22% of 5-HIAA formed in the cord diffuses into lumbar c.s.f. and (c) by measuring 5-HIAA in the lumbar c.s.f. the concentration and turnover of 5-HIAA and turnover of serotonin in the lumbosacral cord can be calculated as well as the changes of 5-HIAA in the cisternal c.s.f. predicted. 3. These results suggest that it is possible to calculate the level and turnover of endogenous substances in the central nervous system on the basis of their measurement in the c.s.f.