Mean ± SD homovanillic acid (HVA) concentrations in young and elderly healthy subjects (n = 7 in each group) in five 6-mL cerebrospinal fluid (CSF) aliquots removed serially from the L3-4 or L4-5 interspaces. There was no significant difference in the mean concentration in any 6-mL aliquot between the 2 groups. The mean ± SD slopes of the HVA concentrations, 39.2 ± 2.4 pmol/mL per aliquot and 33.3 ± 3.4 pmol/mL per aliquot for young and elderly subjects, respectively, do not differ significantly (P >.05).
Rapoport SI, Schapiro MB, May C. Reduced Brain Delivery of Homovanillic Acid to Cerebrospinal Fluid During Human Aging. Arch Neurol. 2004;61(11):1721-1724. doi:10.1001/archneur.61.11.1721
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.
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.
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.
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.
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.
Certain biomarkers of brain dopamine neurotransmission are reported to decline with human aging. These include D2 receptor densities,1 the number of dopamine transporter immunoreactive neurons,2 and striatal dopamine content.3 Nevertheless, cerebrospinal fluid (CSF) concentrations of homovanillic acid (HVA), the major dopamine metabolite, are reported to be elevated4- 6 or to not differ significantly7 in elderly individuals compared with young adults. Homovanillic acid within CSF is derived exclusively from brain tissue, as systemically administered HVA does not enter the CSF space.8 Thus, the CSF concentration of HVA can be a measure of brain dopamine metabolism. Its sources are dopamine synapses in the basal ganglia and frontal cortex, from which dopamine diffuses into the lateral ventricles, whereas the spinal cord makes no contribution.9,10
One reason for the discrepancy between evidence of declining brain dopaminergic function but not of CSF concentration of HVA with age is that washout of HVA within CSF is reduced in elderly individuals. Indeed, when measuring the recovery of CSF pressure after removing 10 mL of CSF from the lumbar space11,12 or when infusing artificial CSF into the lateral cerebral ventricle in patients with normal pressure hydrocephalus,13 CSF flow F was found to be reduced by about 50% in elderly subjects compared with young subjects. (A 50% age reduction in F also has been reported in rats.14) Another possible cause for the discrepancy is that HVA transport out of CSF by a short-chain fatty acid transporter8,15 decreases with age.
In this article, we determined the rate of HVA delivery from the brain to CSF in young and elderly subjects, when taking into account published values for F12 and new values for lumbar CSF concentration of HVA.
Under an institutional review board–approved protocol, we studied 7 healthy young adults (mean ± SD age, 28.7 ± 4.6 years; range, 21-26 years; 3 men and 4 women) and 7 healthy elderly adults (mean ± SD age, 77.1 ± 6.3 years; range, 67-84 years; 5 men and 2 women), who had not taken off any medication for at least 2 weeks. Each subject was medically screened with a physical examination and routine blood tests and was in excellent health. Cerebrospinal fluid flow rates have been reported for these subjects.12
Subjects consumed a low-monoamine diet for 72 hours. After 9 hours of overnight bed rest and fasting, lumbar punctures were performed between 8:30 and 10:30 AM, when the subject was recumbent. Cerebrospinal fluid was collected in five 6-mL aliquots (total 30 mL) from the L3-4 or L4-5 interspace and frozen at −80°C for later analysis. Only clear CSF with normal cell counts and protein and glucose concentrations was used.
Homovanillic acid was assayed by high-performance liquid chromatography with electrochemical detection,16 using 5-fluoro-homovanillic acid as an internal standard. Group differences between means were analyzed by a repeated analysis of variance, and differences between slopes and intercepts of plots of CSF concentration of HVA against aliquot number were determined by linear regression. Means ± SDs are given. Statistical significance was taken as P ≤ .05.
The Figure presents mean values for HVA concentration in the five 6-mL CSF aliquots that were removed serially from the 7 young and 7 elderly subjects. A repeated analysis of variance demonstrated a statistically significant relation between HVA concentration and aliquot number (P = .001) but an insignificant difference in mean HVA concentration in any aliquot between the 2 groups. In the young and elderly subjects, mean±SD HVA concentration in the initial (caudal) 6-mL aliquot equaled 116 ± 56 and 140 ± 84 pmol/mL, respectively (Table). The mean±SD slopes of plots of HVA concentration against aliquot number, 39.2 ± 2.4 and 33.3 ± 3.4 pmol/mL per aliquot, did not differ significantly between groups. We extrapolated these slopes to estimate HVA concentration along the spinal subarachnoid space, taking its volume as 96 mL.17 Assuming that HVA concentration in the most cephalad aliquot of this space equaled the HVA concentration in the cisterna magna, we calculated that cisternal HVA concentration equaled 704 pmol/mL and 640 pmol/mL in the young and elderly subjects, respectively (Table).
The rate of HVA transfer by bulk CSF flow from the cisterna magna to the spinal subarachnoid space was calculated as the product of cisternal HVA concentration and CSF flow, F. Flows have been reported to fall by about 50% in these elderly subjects compared with the young subjects, from 591 to 274 mL/d (Table).12 Flows measured by intraventricular infusion also have been reported to decline, from 720 mL/d at 29 years of age to 403 mL/d at 79 years of age.13
We multiplied the values for F in the Table12 by the corresponding cisternal HVA concentration to calculate HVA transfer rates by CSF flow from the cisterna magna to the spinal subarachnoid space. These rates equaled 416 and 175 nmol/d, respectively (Table). We also calculated HVA outflow rates from the L3-4 or L4-5 lumbar spaces as the product of F and HVA concentration in the most caudal lumbar aliquot. These rates equaled 69 and 38 nmol/d, respectively, in young and elderly subjects (Table). The differences between the outflow and inflow rates equaled 347 and 137 nmol/d, respectively. They represent, according to the Fick principle,18 rates of removal of HVA from the spinal subarachnoid CSF.
We calculated that the rate of transfer of HVA via CSF flow, from the cisterna magna to the spinal subarachnoid space, falls by about 50% with age, from 416 to 175 nmol/d. According to the Fick principle,18 these latter rates are “lower limits” for HVA delivery from the brain to CSF. They ignore any HVA loss from CSF at the choroid plexus and subarachnoid membranes by an outward short-chain fatty acid transporter and by bulk flow into the sagittal sinus through cranial subarachnoid villi.15,19 Additionally, we estimated that HVA is removed from the spinal subarachnoid space before CSF reaches the L3-4 or L4-5 interspaces at rates of 347 and 137 nmol/d in young and elderly subjects, respectively (Table). This removal can involve outward transport by the fatty acid transporter at the spinal cord8,20 or outflow of CSF via spinal subarachnoid villi.19
Our analysis, which suggests that HVA diffusion from the brain (mainly the basal ganglia and frontal cortex9,10) into CSF falls by half with age, can explain the discrepancy between reports of reduced brain dopaminergic function1- 3 but not of lumbar CSF concentrations of HVA in elderly individuals.4- 7 In this regard, our measured HVA concentrations in the initial lumbar CSF samples from the young and elderly groups, 116 ± 56 pmol/mL and 140 ± 84 pmol/mL, respectively, are in the range of published concentrations.4- 7
Our results remain to be validated, because they are based on a number of assumptions and on data from a small number of subjects. We extrapolated a linear concentration gradient along a 96-mL spinal subarachnoid space from the concentrations in the 5 aliquots removed but our cisternal HVA concentration calculated in this way agrees with published values.21,22 It is unlikely that our results were markedly affected by non-HVA dopamine metabolites in CSF, which represent only 12% of the HVA concentration,23 or by diurnal changes in CSF flow, which are only plus or minus 20%.24 Future studies should involve larger numbers of subjects and correlated measurements, if possible, of F and HVA concentrations in lumbar and/or lateral ventricle CSF.12,13,25 Multiplying a known lateral ventricle HVA concentration by F would give a more realistic lower bound for HVA delivery by the brain than multiplying cisternal HVA concentration by F.
Measured urinary HVA excretion rates and carotid-jugular HVA concentration differences suggest that the net rate of HVA production by the brain in young adults is 5.4 to 7.0 μmol/d.26,27 This rate is 13 to 17 times higher than our estimated lower limit of 416 nmol/d (Table). Thus, although cisternal HVA concentration × F reflects brain dopamine metabolism, this product represents at most 5% to 10% of the actual rate of HVA production by the brain. The remaining HVA produced, 90% to 95% of the net, likely is transported by a short-chain fatty acid transporter directly from the brain to the blood at the cerebral capillaries or from CSF at the choroid plexus or transferred by bulk flow via cranial arachnoid villi.15,19,28
In summary, the product of cisternal HVA concentration or, for that matter, of lumbar HVA concentration, and of CSF flow F is more useful for evaluating brain dopamine metabolism than is lumbar HVA concentration alone. This conclusion applies to aging as well as to a brain in which CSF flow is altered. For example, lumbar HVA concentrations in patients withAlzheimer disease are reported to equal or exceed control concentrations4,29 but CSF flow appears reduced in this condition.25 Similar considerations may apply to Parkinson disease.23 More generally, our results suggest that neglecting CSF flow differences in aging or disease can lead to an incorrect interpretation of the physiological or biochemical significance of measured CSF concentrations of substances derived from the brain.
Correspondence: Stanley I. Rapoport, MD, Brain Physiology and Metabolism Section, Bldg 10, Room 6N-202, National Institute on Aging, National Institutes of Health, Bethesda, MD 20892 (firstname.lastname@example.org).
Accepted for Publication: April 2, 2004.
Author Contributions:Study concept and design: Rapoport and May. Acquisition of data: Schapiro and May. Analysis and interpretation of data: Rapoport. Drafting of the manuscript: Rapoport. Critical revision of the manuscript for important intellectual content: Rapoport, Schapiro, and May. Statistical analysis: Rapoport. Obtained funding: Rapoport. Administrative, technical, and material support: Schapiro. Study supervision: Rapoport.
Funding/Support: This study was supported by the Intramural Research Program of the National Institute on Aging, Bethesda, Md.
Acknowledgment: We thank Irwin Kopin, MD, for his relevant criticisms and important suggestions; Eileen Daly, BSc, for measuring cerebrospinal fluid homovanillic acid concentrations; and Kenneth Kirk, PhD, for providing the homovanillic acid analytical standard, 5-fluoro-homovanillic acid.