Reduced Brain Delivery of Homovanillic Acid to Cerebrospinal Fluid During Human Aging

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Abstract
Background Markers of human brain dopamine metabolism are reported to decline with age. However, the cerebrospinal fluid (CSF) concentration of homovanillic acid (HVA), a major dopamine metabolite, is reported to not change or to increase in elderly individuals. Objective To estimate the rate of delivery of HVA from the brain to CSF, taking into account the HVA concentration gradient in the spinal subarachnoid space and CSF flow. Methods Homovanillic acid concentrations were measured in 5 serial 6-mL aliquots of CSF removed from the L3-4 or L4-5 interspaces of 7 healthy young (mean ± SD age, 28.7 ± 4.6 years) subjects and 7 healthy elderly (mean ± SD age, 77.1 ± 6.3 years) subjects. Cisterna magna HVA concentrations were estimated from the slopes of the HVA concentrations along the spinal subarachnoid space. The products of cisternal HVA concentrations and published values for CSF flow were used to estimate lower limits for brain delivery of HVA to CSF, according to the Fick principle. Results The mean ± SD HVA concentration in the initial lumbar CSF sample in the young subjects, 116 ± 66 pmol/mL, did not differ significantly from 140 ± 86 pmol/mL in the elderly subjects. Estimated cisternal HVA concentrations equaled 704 and 640 pmol/mL, respectively, in the young and elderly subjects. Multiplying these concentrations by CSF flow rates of 591 and 294 mL/d, respectively, gave lower limits for rates of delivery of HVA from the brain to CSF. These rates equaled 416 and 175 nmol/d, respectively. Conclusion A 50% decline in the lower limit for the rate of HVA delivery from the brain to CSF in elderly individuals is consistent with other evidence that brain dopaminergic neurotransmission declines with age.