Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2015 Jan 9.
Published in final edited form as: J Neuropathol Exp Neurol. 2014 Feb;73(2):115–122. doi: 10.1097/NEN.0000000000000034

Serotonin Metabolites in the Cerebrospinal Fluid in the Sudden Infant Death Syndrome: In Search of a Biomarker of Risk

Ingvar J Rognum 1,2, Hoa Tran 1, Elisabeth A Haas 3, Keith Hyland 4, David S Paterson 1, Robin L Haynes 1, Kevin G Broadbelt 1, Brian J Harty 5, Othon Mena 6, Henry F Krous 3, Hannah C Kinney 1
PMCID: PMC4287961  NIHMSID: NIHMS652341  PMID: 24423636

Abstract

Clinical biomarkers are urgently needed in the sudden infant death syndrome (SIDS) to identify living infants at risk because it because it occurs without occurs without clinical warning. Previously, we reported multiple serotonergic (5-HT) abnormalities in nuclei of the medulla oblongata that help mediate protective responses to homeostatic stressors. Here we test the hypothesis that 5-HT-related measures are abnormal in the cerebrospinal fluid (CSF) of SIDS infants compared to autopsy controls, as a first step towards their assessment as diagnostic biomarkers of medullary pathology. Levels of CSF 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), the degradative products of 5-HT and dopamine, respectively, were measured by high performance liquid chromatography in 57 SIDS and 29 non-SIDS autopsy cases. Tryptophan (Trp) and tyrosine (Tyr), the substrates of 5-HT and dopamine, respectively, were also measured. There were no significant differences in 5-HIAA, Trp, HVA, or Tyr levels between the SIDS and non-SIDS groups. These data preclude use of 5-HIAA, HVA, Trp or Tyr measurements as CSF biomarkers of 5-HT medullary pathology in infants at risk. They provide, however, important information about monoaminergic measurements in human CSF at autopsy and their developmental profile in infancy that is applicable to multiple pediatric disorders beyond SIDS.

Keywords: 5-hydroxyindoleacetic acid, brainstem, dopamine, high performance liquid chromatography, postmortem interval, tryptophan, tyrosine

Introduction

The sudden infant death syndrome (SIDS) is defined as the sudden death of an infant less than 12 months of age that is related to a sleep period, and remains unexplained after a complete autopsy and death scene investigation (1). It is the leading cause of postneonatal infant mortality in westernized countries today (2). Currently, there are no biomarkers for SIDS risk in living infants, a major problem given that the death occurs suddenly, without warning, in seemingly health infants, and is almost always unwitnessed. Yet, these infant deaths can be preceded by subclinical dysfunction in respiratory and autonomic control and sleep-wake patterns (3-12); moreover, certain features at autopsy indicate chronic, albeit subtle, hypoxia (13-16). Thus, all SIDS deaths are not likely to be acute, but rather, to be preceded by a subclinical prodrome. Therefore, we reason that the risk for at least some SIDS cases prior to death can potentially be detected by clinical biomarkers, thereby allowing for the identification of affected living infants and the possibility for pharmacologic or other specific interventions to prevent death.

Over the last two decades, our group has provided mounting evidence for serotonergic (5-HT) pathology in the homeostatic network of the medulla oblongata in the majority of SIDS infants (17-19). In four independent studies, we have reported altered 5-HT receptor binding (17, 19, 20) in autonomic- and respiratory- related nuclei of the medulla that are comprised of 5-HT neurons and/or receive 5-HT projections, i.e., the so-called medullary 5-HT homeostatic network (21), in SIDS cases. Moreover, independent investigators have confirmed medullary abnormalities in 5-HT receptors in SIDS infants by immunocytochemical methods (22, 23). Further, we demonstrated a significant reduction in SIDS of 5-HT itself and tryptophan hydroxylase 2 (TPH2), the major biosynthetic enzyme for 5-HT synthesis, in two 5-HT source nuclei with altered 5-HT1A binding in SIDS, the raphe obscurus (RO) and paragigantocellularis lateralis (PGCL) (17). These results point to a deficiency of 5-HT in the pathogenesis of the medullary 5-HT pathology in affected SIDS cases. The medullary 5-HT network plays a key role in mediating protective responses to homeostatic stressors, e.g., hypercarbia, hypoxia, and asphyxia (21, 24, 25). Thus, defects in this system may impair the SIDS infant's responses to such stressors, for example, asphyxia in the prone (face-down) sleep position, thereby leading to sudden death in a critical developmental period (26, 27). While 5-HT abnormalities are not the only reported neurochemical abnormalities in SIDS brainstems, they are among the most robust and reproducible (25), and these findings have suggested the formulation by us that at least a SIDS subset is part of a spectrum of “developmental serotonopathies” which include known inborn errors of 5-HT metabolism, autism, and prenatal exposure to selective serotonergic reuptake inhibitors (24).

The objective of the following study was to determine if levels of 5-HT-related parameters are decreased in the cerebrospinal fluid (CSF) of SIDS infants, as a first step towards discovering a potential tissue biomarker of abnormalities of the 5-HT medullary network in infants at risk for SIDS in a clinically accessible tissue. We further reasoned that new information accrued in this study about the methodology of 5-HT-related measurements in human CSF and their developmental profile would be directly relevant to multiple neurological disorders involving 5-HT beyond the SIDS problem (e.g., 24). The major 5-HT-related parameters that can be readily measured in the human CSF are 5-HIAA and tryptophan (Trp), the essential amino acid involved in 5-HT synthesis, both utilizing clinically available high performance liquid chromatography (HPLC) techniques. In this study, we tested the hypothesis that 5-HIAA and/or Trp levels in the CSF are decreased in SIDS infants compared to controls dying of known causes. We reasoned that the decreased tissue levels of 5-HT in the medullary (raphe and extra-raphe) nuclei reported by us previously in SIDS infants result in a secondary decrease of tissue 5-HIAA (17), and thus its decreased release into the CSF with decreased levels in that compartment. Because we did not previously find any differences in dopamine metabolite levels or turnover directly in the medullary 5-HT system in SIDS cases compared to controls (17), we hypothesized here that there are no differences in dopamine metabolites in the CSF in SIDS versus non-SIDS groups. In addition, we tested the hypothesis that 5-HIAA and Trp levels in the CSF in SIDS infants are significantly different from those of infants dying with unequivocal asphyxia, to determine if the putative 5-HT-related alterations in SIDS are potentially secondary to terminal asphyxia. To explore further the potential effects of hypoxia/anoxia upon 5-HIAA and Trp levels, we also included a group of SIDS infants who suffered a cardiopulmonary arrest and anoxic encephalopathy, and were ventilated mechanically for 6-31 hours prior to death. Information about the potential effects of terminal hypoxia-ischemia upon monoaminergic levels in postmortem CSF is of importance in its own right in assessing such levels in human autopsy populations across the age spectrum.

We further tested the hypotheses that there are no significant differences between 5-HIAA and/or Trp levels in the CSF between infants who are classified as SIDS and those classified as “undetermined”, and that both groups have levels significantly decreased from those of infants dying of known causes of death. Our hypothesis is based on the premise that this “undetermined” group of sudden and unexplained deaths share common underlying abnormalities with those of the SIDS group, that the distinction between the two groups is largely a semantic one, and that the two groups share a common biomarker, e.g., altered 5-HIAA and/or Trp levels in the CSF. Finally, we were interested in determining if any SIDS cases met the diagnostic screening criteria for a known inborn error of 5-HT metabolism, thereby potentially unmasking a subgroup of infants with genetic bioaminergic disorders that present as sudden death in seemingly well infants (30). The identification of such a SIDS subset would carry tremendous implications for future newborn screening programs in order to identify this rare, but important, group at risk, given that several 5-HT inborn errors of metabolism are treatable (30).

Materials and Methods

Clinical Database

All CSF specimens were obtained between 2004 and 2011 from the San Diego County Medical Examiner's Office, San Diego, CA. Samples were accrued , in accordance to California law Chapter 955, Statutes of 1989 (SB1069). This law permits the use of autopsy tissues from infants with sudden death for research without parental consent. SIDS cases were defined as above (1). “Acute” controls (AC) were infants less than 12 months of age who died unexpectedly (in some instances with a minor or acute illness within 48 hours of death), and in whom an autopsy and death scene investigation established a known cause of death (31). “Chronic” controls (CC) were infants less than 12 months of age who had clinical chronic illnesses, but who nevertheless died suddenly and unexpectedly; a complete autopsy confirmed the chronic illness (31). The acute asphyxia (AA) group was defined as infants less than 12 months of age in whom the death scene investigation determined unequivocally an acute lethal asphyxial event, e.g., overlaying, crib accident, and the autopsy itself did not demonstrate a chronic or lethal intrinsic disease process. The resuscitated SIDS (RSIDS) group was defined as infants less than 12 months of age who were discovered unresponsive, underwent successful cardiopulmonary resuscitation, and were subsequently ventilated; these infants were withdrawn from the ventilator due to severe post-arrest anoxic encephalopathy and were classified as SIDS at autopsy. The undetermined (UND) group was defined as infants less than 12 months of age in whom the findings at autopsy or death scene may have contributed to death, but their causative role in death was uncertain. Clinicopathologic variables were recorded upon review of autopsy and death scene investigation reports, including gestational age, race, gender, and circumstances of death (e.g., position found upon discovery). The diagnosis in each case and control was made without knowledge of the CSF data. The Committee on Clinical Investigation at Boston Children's Hospital, MA, approved this study.

Neurotransmitter-Related Measurements in CSF

The site of CSF collection for the remaining cases was either posterior lumbar subarachnoid space or cisterna magna. Upon collection, all CSF samples were stored at −80°C until analysis. Obviously bloody specimens were excluded from biochemical analysis due to red cell contamination. Samples were thawed on ice, and centrifuged at 3500g/4°C for 10 minutes. Reverse phase HPLC with electrochemical detection was performed on the CSF supernates for 5-HIAA and HVA analyses at Medical Neurogenetics, Atlanta, GA, with internal standards, according to previously published protocols (32, 33). Measurements of the CSF levels of Trp and tyrosine (Tyr) were performed in the same set of SIDS cases and non-SIDS controls. The analysis of these amino acids was performed at Medical Neurogenetics, Atlanta, GA, by reversed-phase HPLC with fluorescence detection (34). All bioaminergic CSF measurements were made without knowledge of diagnosis, age, or other clinical factors.

Statistical Analysis

Descriptive statistics were performed with regards to demographic information and study measures for each of the study groups (SIDS and non-SIDS). A one-way ANOVA was used for the comparison of age and PMI between the different groups, and Fisher's exact test was used for the comparison of the percentage of gender and prematurity between groups. Models of analysis of covariance (ANCOVA) were used to compare the levels of 5-HIAA, HVA, Trp, and Tyr between the SIDS group and various non-SIDS groups, separately and combined, controlling for postconceptional age (PCA) and PMI. A Tukey-Kramer adjustment was used to account for the multiple comparisons being made within each model.

Results

Clinical Database

The total dataset consisted of 86 cases, with the following classification: SIDS, n=52; AC, n=9; CC, n=2; AA, n=8; RSIDS, n=5, and UND, n=10 (Table 1). There was no significant difference in postconceptional age (PCA), gestational age (GA), or postnatal age (PNA), percentage of preterm birth or gender, or postmortem interval (PMI) among the study groups (Table 2). The median PMI for each of the groups was ≤24 hours (data not shown), with a range of 5.5 to 37 hours (only one case >30 hours). There was no significant effect of increasing postmortem interval upon CSF levels of 5-HIAA (Fig. 1), in contrast to adverse effects upon Trp and Tyr levels within certain groups (see below). All 86 cases were included in the analysis of 5-HIAA, HVA, Trp and Tyr (Tables 3 to 6, respectively). The causes of death in the non-SIDS groups and the circumstances of death in the AA group are provided in Table 1. There was no effect of increasing age for any of the four measures (5-HIAA, HVA, Trp, Tyr), in any group, or all groups combined (Fig. 2).

Table 1.

Demographic information and cause of death in acute controls, chronic controls, and acute asphyxial group.

Acute Controls Sex GA (w) PNA (w) PCA (w) Cause of Death
1 M 40 21.0 61.0 Hyperthermia (excessive bedding)
2 F 40 3.5 43.5 Histiocytoid cardiomyopathy
3 M 35 2.0 37.0 Bronchopulmonary dysplasia secondary to lung disease of prematurity
4 M 36 0.3 36.3 Fatty acid oxidation disorder
5 M 38 15.5 53.5 Acute aspiration pneumonia
6 M 40 11.0 51.0 Acute bacterial meningitis
7 M 40 24.5 64.5 Drowning
8 M 39 1.0 40.0 Acute gastrointestinal illness of unknown etiology with dehydration.
9 M 36 2.5 38.5 Congenital heart defect (ventricular septal defect and bicuspid aortic valve with stenosis) with minimal acute symptoms preceding death
Chronic Controls Sex GA (w) PNA (w) PCA (w) Cause of Death
1 M 41 2.0 43.0 Disseminated herpes simplex Type II viral infection
2 F 40 28.5 68.5 Hemolytic anemia
Acute Asphyxia Sex GA (w) PNA (w) PCA (w) Findings upon death scene investigation
1 M 40 9 49 Found face down in mattress with lower body on “boppy” pillow (feet in air).
2 F 40 13 53 Found with head inside pillow case between two pillows
3 F 36 32 68 Found with body wedged between bed springs and air mattress
4 F 40 23 63 Found with head covered by adult pillow
5 M 39 6 45 Found between arm and torso of intoxicated mother in adult bed
6 F 31 12 43 Face down in unsafe sleeping environment
7 M 40 33 73 Found with head wrapped in blanket
8 F 40 4 44 Found with face and head wedged between co-sleeper and sofa back
Open in a new tab

Legend: Abbreviations: F, female; M, male; w, weeks

Table 2.

Demographics of the entire study cohort.

N Male Premature (GA<37wks) Mean (Standard Deviation)
N (%) N (%) PCA (weeks) GA (weeks) PNA (weeks) PMI (hours)
SIDS 52 25 (48%) 6 (12%) 52.7 (8.6) 39.0 (3.1) 13.9 (8.1) 22.2 (5.4)
Acute Controls (AC) 9 7 (78%) 2 (22%) 50.6 (12.2) 38.7 (1.9) 11.9 (10.9) 21.1 (8.1)
Chronic Controls (CC) 2 2 (100%) 1 (50%) 40.8 (3.2) 38.5 (3.5) 2.3 (0.4) 23.5 (4.9)
Acute Asphyxia (AA) 8 3 (38%) 2 (25%) 54.8 (11.7) 38.3 (3.2) 16.5 (11.4) 16.5 (8.8)
Resuscitated SIDS (RSIDS) 5 2 (40%) 1 (20%) 57.5 (8.0) 38.2 (2.7) 19.3 (8.0) 16.7 (2.2)
Undetermined cases(UND) 10 7 (70%) 2 (20%) 53.0 (14.0) 37.8 (4.1) 15.1 (12.5) 20.1 (8.3)
Non-SIDS controls (AC+CC+AA) 19 12 (63%) 5 (26%) 51.3 (11.8) 38.5 (2.5) 12.8 (11.1) 19.4 (8.2)
P-value comparing demographic by diagnosis 0.27 0.37 0.45 0.91 0.31 0.14
Open in a new tab

Legend: A one-way ANOVA was used for the comparison of age and PMI between the different groups, and Fisher's exact test was used for the comparison of the percentage of gender and prematurity between groups. These comparisons do not include the non-SIDS control group, which is composed of the acute and chronic controls and the cases of acute asphyxia. Abbreviations: N, sample size; PCA, postconceptional weeks; GA, gestational age; PNA, postnatal age; PMI, post-mortem interval.

Fig. 1.

Fig. 1

Open in a new tab

There is no significant effect of increasing postmortem interval (PMI) upon CSF levels of either 5-HIAA levels (left) or HVA levels (right) in the SIDS or non-SIDS groups (the latter comprised of acute controls, chronic controls, and acute asphyxial cases combined). Solid circles, SIDS; open circles, autopsy controls.

Table 3.

5-Hydroxyindoleacetic Acid (5-HIAA) comparisons by diagnosis, adjusting for post-conceptional age (PCA) and postmortem interval (PMI)

5-HIAA
Adjusted Mean (Standard Error) in nmol/L P-value of Pair-wise Comparisons between Groups of Interest
SIDS cases, n=52 1191.8 (67.6) SIDS vs. AC (0.839)
SIDS vs. CC (0.915)
SIDS vs. AA (0.999)
SIDS vs. RSIDS (1.000)
SIDS vs. UND (0.526)
AC vs. CC (0.999)
AC vs. AA (0.861)
Acute Controls (AC), n=9 1399.9 (161.0)
Chronic Controls (CC), n=2 1546.8 (347.8)
Acute Asphyxia (AA), n=8 1126.7 (175.1)
Resuscitated SIDS (RSIDS), n=5 1150.3 (220.5)
Undetermined cases(UND), n=10 904.7 (152.6)
Open in a new tab

Legend: A Tukey-Kramer adjustment was used to account for the multiple comparisons. The p-values are presented in the brackets. There was no difference in 5-HIAA levels between the SIDS and non-SIDS (AC, CC, and AA groups combined) (p-value=0.824). Abbreviation: vs., versus.

Table 6.

Tyrosine (Tyr) comparisons by diagnosis, adjusting for post-conceptional age (PCA) and postmortem interval (PMI).

TYR
Adjusted Mean (Standard Error) P-value of Pair-wise Comparisons between Groups of Interest1
SIDS cases 224.0 (11.2) SIDS vs. AC (0.902)
SIDS vs. CC (1.000)
SIDS vs. AA (0.997)
SIDS vs. RSIDS (1.000)
SIDS vs. UND (0.943)
AC vs. CC (1.000)
AC vs. AA (0.859)
Acute Controls (AC) 254.2 (26.8)
Chronic Controls (CC) 248.0 (81.7)
Acute Asphyxia (AA) 208.6 (29.1)
Resuscitated SIDS (RSIDS) 211.3 (36.6)
Undetermined cases(UND) 198.7 (25.4)
Open in a new tab

Legend: A Tukey-Kramer adjustment was used to account for the multiple comparisons. The p-values are presented in the brackets. There was no difference in Tyr levels between SIDS and non-SIDS (AC, CC, and AA groups combined) (p-value=0.966). Abbreviation: vs., versus.

Fig. 2.

Fig. 2

Open in a new tab

The levels of 5-HIAA and HVA across all groups are relatively constant over infancy, the age range studied. Solid circles, SIDS; open circles, non-SIDS group; resuscitated SIDS (RSIDS), grey shaded circles.

Measurements of 5-HIAA in the CSF of SIDS versus non-SIDS Groups

There were no significant differences in mean 5-HIAA levels between the SIDS group, and AC, CC, AA, RSIDS, and UND groups (p-value>0.05) for all five comparisons (Table 3). In a separate ANCOVA model, there was no significant difference in 5-HIAA levels between the SIDS group, non-SIDS group (defined as combined AC, CC, and AA groups), RSIDS group, and UND group, adjusting for PCA and PMI (data not shown). Although there was a minimal reduction (9%) in 5-HIAA levels on average in the SIDS group compared to non-SIDS groups, it was not statistically significant (p-value=0.824).

Measurements of Trp in the CSF of SIDS versus non-SIDS Groups

There was a significant positive correlation in the non-SIDS group between Trp and PMI (correlation coefficient [r]=0.48, p-value=0.037), as previously suggested in human CSF autopsy analysis (35), and non-significant positive correlations in the SIDS group (r=0.14, p-value=0.313), RSIDS group (r=0.03, p-value=0.965), and the UND group (r=0.53, p-value=0.113). Using an ANCOVA model to compare the groups while adjusting for PCA and PMI, we found no significant differences in mean Trp levels between the SIDS cases and the AC, CC, AA, RSIDS, and UND groups (p-value>0.05 for all five comparisons) (Table 5).

Table 5.

Tryptophan (Trp) comparisons by diagnosis, adjusting for post-conceptional age (PCA) and postmortem interval (PMI)

TRP
Adjusted Mean (Standard Error) P-value of Pair-wise Comparisons between Groups of Interest1
SIDS cases 52.1 (2.8) SIDS vs. AC (0.999)
SIDS vs. CC (1.000)
SIDS vs. AA (1.000)
SIDS vs. RSIDS (0.985)
SIDS vs. UND (0.975)
AC vs. CC (1.000)
AC vs. AA (0.999)
Acute Controls (AC) 55.0 (6.6)
Chronic Controls (CC) 50.1 (14.2)
Acute Asphyxia (AA) 51.2 (7.2)
Resuscitated SIDS (RSIDS) 45.8 (9.0)
Undetermined cases(UND) 47.0 (6.2)
Open in a new tab

Legend: A Tukey-Kramer adjustment was used to account for the multiple comparisons. The p-values are presented in the brackets. There was no difference in TRP levels between SIDS and non-SIDS (AC, CC, and AA groups combined) (p-value=0.999). Abbreviation: vs., versus.

Measurements of HVA in the CSF of SIDS versus non-SIDS Groups

In fitting an ANCOVA model for the HVA data, a significant PCA-diagnosis interaction (p-value<0.0001) was observed. The difference between HVA and diagnoses, adjusting for PMI, varied with PCA, with an increase in HVA levels with increasing age (data not shown). In exploring this interaction further, we found that it was due to two outliers, one case from the RSIDS group (HVA=15,420 nmol/l, PCA=67 weeks), the other case from the UND group (HVA=15, 520 nmol/l, PCA=82 weeks). Exclusion of these two outliers eliminated the significant PCA-diagnosis interaction (Table 2). There were significant differences in HVA level among the pair-wise comparisons between the SIDS and AC groups, and between the AA and AC groups (Table 4). Upon excluding the single outlier (HVA=11070 nmol/L) in the AC group from the analyses, the significant differences disappeared. There was no significant difference in HVA levels between the SIDS and AA groups (p=0.989) (Table 4). A separate ANCOVA model was performed to compare HVA levels between the SIDS, non-SIDS (AC, CC, and AA combined), RSIDS, and UND groups adjusting for PCA and PMI.

Table 4.

Homovanillic Acid (HVA) comparisons by diagnosis, adjusting for post-conceptional age (PCA) and postmortem interval (PMI)

HVA
Adjusted Mean (Standard Error) in pmol/L P-value of Pair-wise Comparisons between Groups of Interest
SIDS cases, n=52 1557.7 (177.4) SIDS vs. AC (0.004)
SIDS vs. CC (0.963)
SIDS vs. AA (0.989)
SIDS vs. RSIDS (0.485)
SIDS vs. UND (0.729)
AC vs. CC (0.923)
AC vs. AA (0.019)
Acute Controls (AC), n=9 3305.2 (423.0)
Chronic Controls (CC), n=2 2321.9 (915.6)
Acute Asphyxia (AA), n=8 1245.9 (462.4)
Resuscitated SIDS (RSIDS), n=5 2755.1 (645.3)
Undetermined cases(UND), n=10 914.2 (424.0)
Open in a new tab

Legend: A Tukey-Kramer adjustment was used to account for the multiple comparisons. The p-values are presented in the brackets. There was no difference in HVA levels between the SIDS and non-SIDS (AC, CC, and AA groups combined) (p-value=0.130). Abbreviation: vs., versus.

Measurements of Tyr in the CSF of SIDS versus non-SIDS Groups

There was a significant positive correlation of Tyr with PMI in the non-SIDS group (r=0.50, p-value=0.034), as previously suggested in human CSF autopsy analysis (35). There was a non-significant positive correlations in the SIDS group (r=0.16, p-value=0.250), RSIDS group (r=0.38, p-value=0.522), and the UND group (r=0.48, p-value=0.161). Using an ANCOVA model to compare the groups while adjusting for PCA and PMI, we found no significant differences in mean Tyr levels between the SIDS cases and the AC, CC, AA, RSIDS, and UND groups (p-value≥0.05 for all five comparisons) (Table 6).

Analysis for Potential Known Inborn Errors of 5-HT Metabolism

In order to determine if the entire SIDS cohort (n=52) contained cases with low 5-HIAA and HVA levels in the range of those found in known inborn errors of monoamine metabolism, we compared the 5-HIAA and HVA levels published in inborn errors (36) as a reference. There were no SIDS cases that demonstrated combined low levels of 5-HIAA and HVA within this disease reference range (Fig. 3).

Fig. 3.

Fig. 3

Open in a new tab

There are no SIDS cases in the entire dataset (n=52) that demonstrate combined low levels of 5-HIAA and HVA within the reference levels of known inborn errors of 5-HT metabolism, indicated in the shaded rectangle in the lower left portion of the graph (36).

Discussion

The measurement of 5-HIAA and Trp in CSF provide an important tool already in clinical practice for detecting and monitoring disorders of brain 5-HT levels and metabolism (turnover) (19, 20). Of interest is the relevance of these tools to SIDS, as well as to multiple other neurological disorders known to involve 5-HT in children (24, 30, 34, 36-40) and adults (41-45). In this study, we did not find a significant difference in the two standard measures of 5-HT metabolism in human CSF, i.e., 5-HIAA and Trp, between SIDS cases and infants dying of a range of acute and chronic disorders. Thus, our data suggest that 5-HIAA or Trp is not a reasonable biomarker for SIDS, and that the need to correlate CSF levels with medullary tissue 5-HT markers in the same SIDS cases is negated. We also did not find a difference in dopaminergic markers between the SIDS and non-SIDS cases, as anticipated since we did not find differences in dopaminergic parameters in medullary tissues between SIDS and control (17). We also did not find significant differences in 5-HIAA, Trp, HVA, or Tyr measures between the UND and the SIDS group, indicating that these metabolites in the CSF do not provide a distinguishing biomarker. Of note, there were no changes in the levels of any bioaminergic CSF parameter analyzed in the SIDS cases or controls within the first year of life, suggesting that this time frame is not a critical developmental period for these CSF parameters across infancy in general. Irrespective of the SIDS problem, this study also provides information directly relevant to assessment of monoaminergic metabolites in postmortem human CSF, including potential effects of terminal hypoxia-ischemia upon them, as highlighted below.

Measurements of Serotonergic Parameters in the CSF of SIDS versus non-SIDS Groups

Synthesis of 5-HT in the brain occurs in raphe and extra-raphe neurons of the entire brainstem through the conversion of the precursor molecule, Trp, to 5-hydroxytryptophan (5-HTP) by TPH2, and then to 5-HT via the enzymatic action of aromatic l-amino acid decarboxylase (AADC) which has 5-HTP and L-DOPA as substrates (37). Tryptophan is also metabolized in the kynurenine pathway and used for protein synthesis (46). Serotonin is degraded to 5-HIAA by monoamine oxidase (MAO) (47). Terminals from the raphe 5-HT source neurons in the brainstem form a sub- and supra-plexus around the ependyma lining the fourth ventricle, and release extra-synaptic 5-HT, including directly into the CSF (Fig. 4) (48, 49). In addition, 5-HIAA in the brain extracellular space diffuses into the CSF, such that this compartment serves as a so-called 5-HIAA “sink” (50). Serotonin itself cannot reliably be measured in the CSF because its levels are constitutively negligible (less than 10 pg/ml) (51). Nevertheless, the CSF concentration of 5-HIAA correlates with 5-HT levels and turnover in brain parenchyma (29, 52). Tryptophan crosses the blood brain barrier (in competition with other large neutral amino acids) for incorporation into brain 5-HT(53,54). It is perhaps not surprising that we did not find reduced 5-HIAA levels in the CSF in the SIDS cases as we did not previously observe significantly reduced 5-HIAA or 5-HT/5-HIAAA levels in the medullary tissue in SIDS infants. Our inability to detect a significant reduction in Trp in the CSF in the SIDS cases suggests that the reduction in medullary 5-HT is due to TPH2 deficiency, and not to the dietary availability of Trp, or the capability to transport Trp across the blood brain barrier.

Fig. 4.

Fig. 4

Open in a new tab

Diagram of the CSF “sink” of tissue 5-HIAA. The 5-HIAA in the CSF diffuses freely from the brainstem parenchyma, or alternatively, represents degradation of 5-HT released directly into the CSF at the ependymal plexuses of the 5-HT terminals of 5-HT source neurons in the raphe and extra-raphe regions of the entire brainstem. See text.

Two previous studies reported significantly increased levels of 5-HIAA, HVA, Trp, and Tyr in the CSF of SIDS infants compared to controls) (55, 56). Our lack of confirmation of these studies may reflect a discrepancy in the control populations, as the previous studies mainly involved CSF measurements in living control infants for comparison, and our study was based upon autopsy controls only. In one previous report, the metabolite levels were similar between autopsy and living controls, and the SIDS values were elevated relative to these two control populations combined (55). We found, however, that the autopsy controls in our study were >3-fold higher than published values of living controls (57), suggesting that autopsy and living control values cannot be combined. These data need to be considered in selecting controls for CSF assessments in other pediatric (24, 30, 34, 36-40) and adult (41-45) disorders affecting central monoaminergic metabolism.

Potential Relationship of CSF Bioaminergic Biomarkers to Asphyxia to SIDS

The potential effect of asphyxia/hypoxia on CSF levels of 5-HIAA and HVA is of concern in SIDS research due to experimental reports of abnormal brainstem measures of 5-HT (and dopaminergic) parameters secondary to hypoxia-ischemia (17, 38), and elevated CSF levels in adults who have undergone successful cardiopulmonary resuscitation after cardiac arrest (45). Thus, interpretation of potentially altered levels in SIDS infants must account for (possible) asphyxia engendered in an unsafe sleeping environment (21, 24, 25). We found, however, no significant differences in 5-HIAA or Trp (and Tyr) levels in the CSF between the SIDS and AA groups. There were also no differences in these metabolite concentrations between the SIDS and RSIDS group, the latter group with potential secondary chronic (6-31 hours) hypoxic-ischemic injury complicating the putative intrinsic defect.

Analysis of Potential Inborn Errors of 5-HT Metabolism in the SIDS Group

Measures of 5-HIAA and HVA are used to screen clinically for inborn errors of 5-HT metabolism, including a genetic deficiency of AADC (30). In this study, however, we found no SIDS outliers with significant decreases in combined 5-HIAA and HVA levels compared to controls,. Thus, these data suggest that infants with known inborn errors of 5-HT metabolism are not embedded within the SIDS population. Of note, infants with known inborn bioaminergic errors are recognized by a clinical phenotype of mild to severe developmental delay, dystonia, and seizures (30, 52); they may be associated with autonomic dysfunction, such as temperature instability and hypotension, with rare reports of cardiorespiratory arrest (30). Nevertheless, sudden death in a clinically healthy infant, as in SIDS, has not been described.

Potential Limitations of the Study

Of concern is the potential compromise of the validity of postmortem measures of 5-HT parameters by lengthy postmortem intervals, with the definition of “lengthy” may be within minutes or even seconds after death. Importantly, our data indicate no change in 5-HIAA and HVA levels with an increasing postmortem interval between 5.5 and 37 hours. This observation is supported by the report of stable levels up to a PMI of 30 hours in serial samples (29). Still, our finding that the 5-HIAA and HVA levels in the entire autopsy dataset (SIDS and non-SIDS controls) were >3-fold higher than that of published values for living infants and children (irrespective of neurological disease) (57) suggests that rapid changes in these parameters occur immediately after death. Thus, comparison of an autopsy study group must be made to an appropriate autopsy control group, not living control standards. A second potential limitation of this study is that CSF was obtained at autopsy by two approaches, i.e., lumbar puncture and cisternal tap, that was not always specifically documented, because an uneven concentration gradient in CSF metabolites has been reported in older children and adults, including at autopsy (28), and values may vary by rostral or caudal site. Nevertheless, this putative gradient may not be present in infants, as a 5-HIAA and HVA gradient was not observed in six living infants who underwent neurosurgery for hydrocephalus (40).

Conclusion

In conclusion, we report no alterations in 5-HT-related parameters in the CSF of SIDS infants compared to that in infants dying of various disorders, including asphyxia. The finding of unaltered levels of 5-HIAA in SIDS is consistent with our previous report of likewise unaltered levels of this metabolite in medullary tissues of SIDS infants with altered 5-HT levels (17). Thus, this study does not negate the 5-HT brainstem hypothesis in SIDS. This study focuses the ongoing search for 5-HT biomarkers of 5-HT medullary pathology in SIDS infants in other readily accessible tissue compartments, notably in serum and platelets, or with different 5-HT-related markers (e.g.mTPH2, 5-HT1A receptor protein by proteomics) in future studies. It also provides important information about the methodology of monoaminergic measurements in human CSF at autopsy, including considerations related to postmortem interval and terminal hypoxia-ischemia at autopsy, as well as their developmental profile in infancy that is applicable to multiple disorders of the pediatric and adult brain beyond SIDS.

Acknowledgements

The authors are grateful for the dedicated assistance in this study of the medical examiners of the San Diego County Medical Examiner's Office, San Diego, CA. We also appreciate the helpful input of Dr. Siri Opdal and Professor Åshild Vege, University of Oslo, Norway, in the course of the study. We thank Drs. Eugene E. Nattie and Richard D. Goldstein for critical reading of the manuscript in preparation.

Sources of Funding: This study was funded by the Translational Research Program, Boston Children's Hospital, MA; Norwegian ExtraFoundation for Health and Rehabilitation (IJR); Norwegian SIDS and Stillbirth Society (IJR); First Candle/SIDS Alliance; CJ Foundation for SIDS; Jacob Neil Boger Foundation for SIDS; Marley Jaye Cerella Foundation for SIDS; Eunice Kennedy Shriver National Institute of Child Health and Development (P01-042774441) (HCK), and Intellectual and Developmental Disabilities Research Center, Boston Children's Hospital, MA (P30-HD18655).

Role of Each Author (over seven authors):

This study represents a team of investigators in the diverse and integrated aspects of this NICHD-funded research program in the sudden infant death syndrome. Ingvar J. Rognum, MD: Study design, preparation of specimens, data preparation, analysis, and interpretation, chart review, manuscript preparation, corresponding author (work in preparation for PhD thesis)

Hoa Tran, PhD: Technical assistance in preparation and shipping of specimens, manuscript input and review

Elisabeth A. Haas, MPH: Study coordinator in the accrual, tracking, and shipping of specimens from the San Diego County Medical Examiner's Office; supervisor of linkage of biochemical data to autopsy and death scene investigation reports; manuscript input and review

Keith Hyland, PhD: Scientist and supervisor of high performance liquid chromatography analysis of bioamine metabolites in SIDS and control cerebrospinal David S. Paterson, PhD: Study design, data analysis and interpretation, and manuscript input and review

Robin L. Haynes, PhD: Study design, data analysis and interpretation, and manuscript input and review

Kevin G. Broadbelt, PhD: Study design, data analysis and interpretation, and manuscript input and review

Brian J. Harty, MA: Statistical analysis and interpretation, manuscript input and review

Othon Mena, MD: Deputy medical examiner at and study liaison to the San Diego County Office, manuscript input and review

Henry F. Krous, MD: Study design, external standardization of phenotyping of autopsy cases, manuscript input and review

Hannah C. Kinney, MD: Principal Investigator of the research program, study design and overall supervision, chart review, data analysis and interpretation, manuscript preparation, funding provision

References

  • 1.Krous HF, Beckwith JB, Byard RW, Rognum TO, Bajanowski T, Corey T, Cutz E, Hanzlick R, Keens TG, Mitchell EA. Sudden infant death syndrome and unclassified sudden infant deaths: a definitional and diagnostic approach. Pediatrics. 2004e;114:234–8. doi: 10.1542/peds.114.1.234. [DOI] [PubMed] [Google Scholar]
  • 2.Heron M. Deaths: leading causes for 2008. Natl Vital Stat Rep. 60:1–94. [PubMed] [Google Scholar]
  • 3.Sridhar R, Thach BT, Kelly DH, Henslee JA. Characterization of successful and failed autoresuscitation in human infants, including those dying of SIDS. Pediatr Pulmonol. 2003e;36:113–22. doi: 10.1002/ppul.10287. [DOI] [PubMed] [Google Scholar]
  • 4.Meny RG, Carroll JL, Carbone MT, Kelly DH. Cardiorespiratory recordings from infants dying suddenly and unexpectedly at home. Pediatrics. 1994e;93:44–9. [PubMed] [Google Scholar]
  • 5.Poets CF. Apparent life-threatening events and sudden infant death on a monitor. Paediatr Respir Rev. 2004e;5(Suppl A):S383–6. doi: 10.1016/s1526-0542(04)90068-1. [DOI] [PubMed] [Google Scholar]
  • 6.Kinney HC, Myers MM, Belliveau RA, Randall LL, Trachtenberg FL, Fingers ST, Youngman M, Habbe D, Fifer WP. Subtle autonomic and respiratory dysfunction in sudden infant death syndrome associated with serotonergic brainstem abnormalities: a case report. J Neuropathol Exp Neurol. 2005e;64:689–94. doi: 10.1097/01.jnen.0000174334.27708.43. [DOI] [PubMed] [Google Scholar]
  • 7.Franco P, Szliwowski H, Dramaix M, Kahn A. Decreased autonomic responses to obstructive sleep events in future victims of sudden infant death syndrome. Pediatr Res. 1999e;46:33–9. doi: 10.1203/00006450-199907000-00006. [DOI] [PubMed] [Google Scholar]
  • 8.Schechtman VL, Lee MY, Wilson AJ, Harper RM. Dynamics of respiratory patterning in normal infants and infants who subsequently died of the sudden infant death syndrome. Pediatr Res. 1996e;40:571–7. doi: 10.1203/00006450-199610000-00010. [DOI] [PubMed] [Google Scholar]
  • 9.Kahn A, Groswasser J, Rebuffat E, Sottiaux M, Blum D, Foerster M, Franco P, Bochner A, Alexander M, Bachy A, et al. Sleep and cardiorespiratory characteristics of infant victims of sudden death: a prospective case-control study. Sleep. 1992e;15:287–92. doi: 10.1093/sleep/15.4.287. [DOI] [PubMed] [Google Scholar]
  • 10.Thach BT. Tragic and sudden death. Potential and proven mechanisms causing sudden infant death syndrome. EMBO Rep. 2008e;9:114–8. doi: 10.1038/sj.embor.7401163. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Shannon DC, Kelly DH, O'Connell K. Abnormal regulation of ventilation in infants at risk for sudden-infant-death syndrome. N Engl J Med. 1977e;297:747–50. doi: 10.1056/NEJM197710062971403. [DOI] [PubMed] [Google Scholar]
  • 12.Southall DP, Stevens V, Franks CI, Newcombe RG, Shinebourne EA, Wilson AJ. Sinus tachycardia in term infants preceding sudden infant death. Eur J Pediatr. 1988e;147:74–8. doi: 10.1007/BF00442617. [DOI] [PubMed] [Google Scholar]
  • 13.Vege A, Chen Y, Opdal SH, Saugstad OD, Rognum TO. Vitreous humor hypoxanthine levels in SIDS and infectious death. Acta Paediatr. 1994e;83:634–9. doi: 10.1111/j.1651-2227.1994.tb13096.x. [DOI] [PubMed] [Google Scholar]
  • 14.Jones KL, Krous HF, Nadeau J, Blackbourne B, Zielke HR, Gozal D. Vascular endothelial growth factor in the cerebrospinal fluid of infants who died of sudden infant death syndrome: evidence for antecedent hypoxia. Pediatrics. 2003e;111:358–63. doi: 10.1542/peds.111.2.358. [DOI] [PubMed] [Google Scholar]
  • 15.Takashima S, Armstrong D, Becker LE, Huber J. Cerebral white matter lesions in sudden infant death syndrome. Pediatrics. 1978e;62:155–9. [PubMed] [Google Scholar]
  • 16.Del Bigio MR, Becker LE. Microglial aggregation in the dentate gyrus: a marker of mild hypoxic-ischaemic brain insult in human infants. Neuropathol Appl Neurobiol. 1994e;20:144–51. doi: 10.1111/j.1365-2990.1994.tb01173.x. [DOI] [PubMed] [Google Scholar]
  • 17.Duncan JR, Paterson DS, Hoffman JM, Mokler DJ, Borenstein NS, Belliveau RA, Krous HF, Haas EA, Stanley C, Nattie EE, Trachtenberg FL, Kinney HC. Brainstem serotonergic deficiency in sudden infant death syndrome. JAMA. 303:430–7. doi: 10.1001/jama.2010.45. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 18.Paterson DS, Belliveau RA, Trachtenberg F, Kinney HC. Differential development of 5-HT receptor and the serotonin transporter binding in the human infant medulla. J Comp Neurol. 2004e;472:221–31. doi: 10.1002/cne.20105. [DOI] [PubMed] [Google Scholar]
  • 19.Paterson DS, Trachtenberg FL, Thompson EG, Belliveau RA, Beggs AH, Darnall R, Chadwick AE, Krous HF, Kinney HC. Multiple serotonergic brainstem abnormalities in sudden infant death syndrome. JAMA. 2006e;296:2124–32. doi: 10.1001/jama.296.17.2124. [DOI] [PubMed] [Google Scholar]
  • 20.Panigrahy A, Filiano J, Sleeper LA, Mandell F, Valdes-Dapena M, Krous HF, Rava LA, Foley E, White WF, Kinney HC. Decreased serotonergic receptor binding in rhombic lip-derived regions of the medulla oblongata in the sudden infant death syndrome. J Neuropathol Exp Neurol. 2000e;59:377–84. doi: 10.1093/jnen/59.5.377. [DOI] [PubMed] [Google Scholar]
  • 21.Kinney HC, Thach BT. The sudden infant death syndrome. N Engl J Med. 2009e;361:795–805. doi: 10.1056/NEJMra0803836. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.Machaalani R, Say M, Waters KA. Serotoninergic receptor 1A in the sudden infant death syndrome brainstem medulla and associations with clinical risk factors. Acta Neuropathol. 2009e;117:257–65. doi: 10.1007/s00401-008-0468-x. [DOI] [PubMed] [Google Scholar]
  • 23.Ozawa Y, Okado N. Alteration of serotonergic receptors in the brain stems of human patients with respiratory disorders. Neuropediatrics. 2002e;33:142–9. doi: 10.1055/s-2002-33678. [DOI] [PubMed] [Google Scholar]
  • 24.Kinney HC, Broadbelt KG, Haynes RL, Rognum IJ, Paterson DS. The serotonergic anatomy of the developing human medulla oblongata: implications for pediatric disorders of homeostasis. J Chem Neuroanat. 41:182–99. doi: 10.1016/j.jchemneu.2011.05.004. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25.Kinney HC, Richerson GB, Dymecki SM, Darnall RA, Nattie EE. The brainstem and serotonin in the sudden infant death syndrome. Annu Rev Pathol. 2009e;4:517–50. doi: 10.1146/annurev.pathol.4.110807.092322. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Filiano JJ, Kinney HC. A perspective on neuropathologic findings in victims of the sudden infant death syndrome: the triple-risk model. Biol Neonate. 1994e;65:194–7. doi: 10.1159/000244052. [DOI] [PubMed] [Google Scholar]
  • 27.Rognum TO, Saugstad OD. Biochemical and immunological studies in SIDS victims. Clues to understanding the death mechanism. Acta Paediatr Suppl. 1993e;82(Suppl 389):82–5. doi: 10.1111/j.1651-2227.1993.tb12886.x. [DOI] [PubMed] [Google Scholar]
  • 28.Tagliamonte A, Biggio G, Vargiu L, Gessa GL. Free tryptophan in serum controls brain tryptophan level and serotonin synthesis. Life Sci II. 1973e;12:277–87. doi: 10.1016/0024-3205(73)90361-5. [DOI] [PubMed] [Google Scholar]
  • 29.Stanley M, Traskman-Bendz L, Dorovini-Zis K. Correlations between aminergic metabolites simultaneously obtained from human CSF and brain. Life Sci. 1985e;37:1279–86. doi: 10.1016/0024-3205(85)90242-5. [DOI] [PubMed] [Google Scholar]
  • 30.Pearl PL, Taylor JL, Trzcinski S, Sokohl A. The pediatric neurotransmitter disorders. J Child Neurol. 2007e;22:606–16. doi: 10.1177/0883073807302619. [DOI] [PubMed] [Google Scholar]
  • 31.Broadbelt KG, Paterson DS, Rivera KD, Trachtenberg FL, Kinney HC. Neuroanatomic relationships between the GABAergic and serotonergic systems in the developing human medulla. Auton Neurosci. 154:30–41. doi: 10.1016/j.autneu.2009.10.002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.Howells DW, Smith I, Hyland K. Estimation of tetrahydrobiopterin and other pterins in cerebrospinal fluid using reversed-phase high-performance liquid chromatography with electrochemical and fluorescence detection. J Chromatogr. 1986e;381:285–94. doi: 10.1016/s0378-4347(00)83594-x. [DOI] [PubMed] [Google Scholar]
  • 33.Hyland K. Estimation of tetrahydro, dihydro and fully oxidised pterins by high-performance liquid chromatography using sequential electrochemical and fluorometric detection. J Chromatogr. 1985e;343:35–41. doi: 10.1016/s0378-4347(00)84565-x. [DOI] [PubMed] [Google Scholar]
  • 34.Hyland KS, I. Howells DW, Clayton PT, Leonard JV. The determination of pterins, biogenic amine metabolites and aromatic amino acids in cerebrospinal fluid using isocratic reverse phase liquid chromatography with in series dual cell coulometric electrochemical and fluorescence detection: use in the study of inborn errors of dihydropteridine reductase and 5,10 methylenetetrahydrofolatye reductase. In: Wachter HC, H.Ch. Pfleiderer W, editors. Biochemical and Clinical Aspects of Pteridines. Vol. 4. Walter de Gruyter; NY: 1985. pp. 85–99. [Google Scholar]
  • 35.Karkela J, Scheinin M. Tryptophan and biogenic amine metabolites in post mortem human cisternal fluid: effects of post-mortem interval and agonal time. J Neurol Sci. 1992e;107:239–45. doi: 10.1016/0022-510x(92)90295-v. [DOI] [PubMed] [Google Scholar]
  • 36.Pons R, Ford B, Chiriboga CA, Clayton PT, Hinton V, Hyland K, Sharma R, De Vivo DC. Aromatic L-amino acid decarboxylase deficiency: clinical features, treatment, and prognosis. Neurology. 2004e;62:1058–65. doi: 10.1212/wnl.62.7.1058. [DOI] [PubMed] [Google Scholar]
  • 37.Hyland K. Inherited disorders affecting dopamine and serotonin: critical neurotransmitters derived from aromatic amino acids. J Nutr. 2007e;137:1568S–72S. doi: 10.1093/jn/137.6.1568S. discussion 73S-75S. [DOI] [PubMed] [Google Scholar]
  • 38.Blennow M, Zeman J, Dahlin I, Lagercrantz H. Monoamine neurotransmitters and metabolites in the cerebrospinal fluid following perinatal asphyxia. Biol Neonate. 1995e;67:407–13. doi: 10.1159/000244193. [DOI] [PubMed] [Google Scholar]
  • 39.Tay SK, Poh KS, Hyland K, Pang YW, Ong HT, Low PS, Goh DL. Unusually mild phenotype of AADC deficiency in 2 siblings. Mol Genet Metab. 2007e;91:374–8. doi: 10.1016/j.ymgme.2007.04.006. [DOI] [PubMed] [Google Scholar]
  • 40.Gopal SC, Pandey A, Das I, Gangopadhyay AN, Upadhyaya VD, Chansuria JP, Singh TB. Comparative evaluation of 5-HIAA (5-hydroxy indoleacetic acid) and HVA (homovanillic acid) in infantile hydrocephalus. Childs Nerv Syst. 2008e;24:713–6. doi: 10.1007/s00381-007-0546-8. [DOI] [PubMed] [Google Scholar]
  • 41.Fabbri C, Marsano A, Serretti A. Genetics of serotonin receptors and depression: state of the art. Curr Drug Targets. 2003;14:531–48. doi: 10.2174/1389450111314050004. [DOI] [PubMed] [Google Scholar]
  • 42.Loane C, Wu K, Bain P, Brooks DJ, Piccini P, Politis M. Serotonergic loss in motor circuitries correlates with severity of action-postural tremor in PD. Neruology. 2013;80:1850–5. doi: 10.1212/WNL.0b013e318292a31d. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.Martinez A, Finegersh A, Cannon DM, Dustin I, Nugent A, Herscovitch P, Theodore WH. The 5-HT1A receptor and 5-HT transporter in temporal lobe epilepsy. Neurology. 2013;80:1465–71. doi: 10.1212/WNL.0b013e31828cf809. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 44.De Grandis E, Serrano M, Perez-Duenas B, Ormazabal A, et al. Cerebrospinal fluid alterations fo the serotonin product, 5-hydrozyindolacetic acid, in neurological disorders. J Interit Metab Dis. 2010;33:803–9. doi: 10.1007/s10545-010-9200-9. [DOI] [PubMed] [Google Scholar]
  • 45.Odink J, Karkela J, Thissen JT, Marnela KM. Biogenic amine metabolites in human CSF after hypoxia due to cardiac arrest. Acta Neurol Scand. 1989e;80:6–11. doi: 10.1111/j.1600-0404.1989.tb03834.x. [DOI] [PubMed] [Google Scholar]
  • 46.Heidelberger C, Gullberg ME, et al. Tryptophan metabolism; concerning the mechanism of the mammalian conversion of tryptophan into kynurenine, kynurenic acid, and nicotinic acid. J Biol Chem. 1949e;179:143–50. [PubMed] [Google Scholar]
  • 47.Weissbach H, Redfield BG, Udenfriend S. Soluble monoamine oxidase; its properties and actions on serotonin. J Biol Chem. 1957e;229:953–63. [PubMed] [Google Scholar]
  • 48.Chan-Palay V. Serotonin axons in the supra- and subependymal plexuses and in the leptomeninges; their roles in local alterations of cerebrospinal fluid and vasomotor activity. Brain Res. 1976e;102:103–30. doi: 10.1016/0006-8993(76)90578-3. [DOI] [PubMed] [Google Scholar]
  • 49.Simpson KL, Fisher TM, Waterhouse BD, Lin RC. Projection patterns from the raphe nuclear complex to the ependymal wall of the ventricular system in the rat. J Comp Neurol. 1998e;399:61–72. doi: 10.1002/(sici)1096-9861(19980914)399:1<61::aid-cne5>3.0.co;2-8. [DOI] [PubMed] [Google Scholar]
  • 50.Verleysdonk S, Hamprecht B, Rapp M, Wellard J. Uptake and metabolism of serotonin by ependymal primary cultures. Neurochem Res. 2004e;29:1739–47. doi: 10.1023/b:nere.0000035810.08543.97. [DOI] [PubMed] [Google Scholar]
  • 51.Visser AK, van Waarde A, Willemsen AT, Bosker FJ, Luiten PG, den Boer JA, Kema IP, Dierckx RA. Measuring serotonin synthesis: from conventional methods to PET tracers and their (pre)clinical implications. Eur J Nucl Med Mol Imaging. 38:576–91. doi: 10.1007/s00259-010-1663-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 52.Wester P, Bergstrom U, Eriksson A, Gezelius C, Hardy J, Winblad B. Ventricular cerebrospinal fluid monoamine transmitter and metabolite concentrations reflect human brain neurochemistry in autopsy cases. J Neurochem. 1990e;54:1148–56. doi: 10.1111/j.1471-4159.1990.tb01942.x. [DOI] [PubMed] [Google Scholar]
  • 53.Gessa GL, Biggio G, Fadda F, Corsini GU, Tagliamonte A. Tryptophan-free diet: a new means for rapidly decreasing brain tryptophan content and serotonin synthesis. Acta Vitaminol Enzymol. 1975e;29:72–8. [PubMed] [Google Scholar]
  • 54.Williams WA, Shoaf SE, Hommer D, Rawlings R, Linnoila M. Effects of acute tryptophan depletion on plasma and cerebrospinal fluid tryptophan and 5-hydroxyindoleacetic acid in normal volunteers. J Neurochem. 1999e;72:1641–7. doi: 10.1046/j.1471-4159.1999.721641.x. [DOI] [PubMed] [Google Scholar]
  • 55.Caroff J, Girin E, Alix D, Cann-Moisan C, Sizun J, Barthelemy L. [Neurotransmission and sudden infant death. Study of cerebrospinal fluid]. C R Acad Sci III. 1992e;314:451–4. [PubMed] [Google Scholar]
  • 56.Cann-Moisan C, Girin E, Giroux JD, Le Bras P, Caroff J. Changes in cerebrospinal fluid monoamine metabolites, tryptophan, and gamma-aminobutyric acid during the 1st year of life in normal infants. comparison with victims of sudden infant death syndrome. Biol Neonate. 1999e;75:152–9. doi: 10.1159/000014091. [DOI] [PubMed] [Google Scholar]
  • 57.Hyland K, Surtees RA, Heales SJ, Bowron A, Howells DW, Smith I. Cerebrospinal fluid concentrations of pterins and metabolites of serotonin and dopamine in a pediatric reference population. Pediatr Res. 1993e;34:10–4. doi: 10.1203/00006450-199307000-00003. [DOI] [PubMed] [Google Scholar]

RESOURCES