Rapoport et al1 have concluded that brain delivery of the major dopamine metabolite, homovanillic acid (HVA), to cerebrospinal fluid (CSF) is reduced during human aging. This conclusion is contrary to other studies reporting that HVA concentration is increased2- 4 or its levels maintained5 in elderly individuals compared with young adults. The authors’ conclusion is based on the fact that the CSF flow rate, measured in milliliters per day, was found to be reduced by about 50% in elderly subjects compared with young subjects. Multiplying similar HVA concentrations obtained in the CSF samples from young and elderly subjects by the corresponding CSF flow rate, a 50% decline in the HVA delivery from the brain to CSF in elderly individuals was obtained. However, the authors have not considered that the CSF volume increases with normal aging owing to age-related cerebral atrophy.6- 8 Considering the same age ranges used by Rapoport et al of 21 to 26 years and 67 to 84 years, the CSF volume would be doubled for the elderly group. This volume increase gives rise to a dilution of the metabolites delivered by the brain, resulting in a decreased HVA concentration in CSF. Taking into account the greatly increased volume of CSF, it would compensate for the decreased flow rate of HVA concentrations in the CSF of the elderly group as presented by Rapoport et al, and, therefore, from their results, the brain delivery of HVA to CSF did not appear to change significantly during human aging.
Martí MB, Cano AH, Villareal EML, Martínez SG, Bernet IM. Brain Delivery of Homovanillic Acid to Cerebrospinal Fluid During Human Aging. Arch Neurol. 2005;62(4):690. doi:10.1001/archneur.62.4.690-a