GO5MJa

BdmeMmmeR.ECbsc

Kb

Dna1atSrcaseio(

D

A

R

0d

enetics of Monoamine Metabolites in Baboons:verlapping Sets of Genes Influence Levels of-Hydroxyindolacetic Acid, 3-Hydroxy-4-ethoxyphenylglycol, and Homovanillic Acid

effrey Rogers, Lisa J. Martin, Anthony G. Comuzzie, J. John Mann, Stephen B. Manuck, Michelle Leland,nd Jay R. Kaplan

ackground: Monoamine neurotransmitters (serotonin, dopamine, and norepinephrine) are associated with several psychiatricisorders. Limited evidence suggests that monoamine levels are heritable, but no information concerning genetic relationships amongonoamines is available. Further genetic analysis can help explain phenotypic correlations among monoamine levels and might

ventually help identify genes involved in response to therapy or risk of psychopathology.ethods: Levels of the monoamine metabolites homovanillic acid (HVA), 5-hydroxyindolacetic acid (5-HIAA), and 3-hydroxy-4-ethoxyphenylglycol (MHPG) were measured in cerebrospinal fluid from 271 baboons (Papio hamadryas). Variance componentsethods were used to estimate heritabilities, and multivariate analyses were used to estimate genetic correlations (pleiotropy) and

nvironmental correlations between metabolites.esults: Each metabolite exhibited significant heritability in baboons (5-HIAA: h2 � .30 � .17; MHPG: h2 � .36 � .16; HVA: h2 �

50 � .19). Multivariate analyses revealed genetic correlations between 5-HIAA and HVA and between HVA and MHPG.nvironmental correlations were found between 5-HIAA and HVA and between 5-HIAA and MHPG.onclusions: Overlapping, nonidentical sets of genes influence individual variation in 5-HIAA, MHPG, and HVA levels amongaboons. The phenotypic correlation between 5-HIAA and HVA observed in nonhuman primates and humans is likely due to bothhared genetic and environmental factors. Genetic analyses of monoamine levels in primates can provide novel informationoncerning the genetics of variation among humans.

ey Words: Serotonin, norepinephrine, dopamine, primate, herita-ility, pleiotropy

ysregulation of the monoamine neurotransmitter sys-tems is thought to contribute to numerous forms ofpsychopathology. Abnormalities of serotonergic and

oradrenergic neurotransmission are implicated in mood andnxiety disorders (Barchas and Altemus 1999; Heninger et al996; Krystal et al 2001; Manji et al 2001; Roy et al 1986), and bothntidepressant and anxiolytic medications commonly targethese systems (Blanco et al 2003; Nemeroff and Owens 2002;heehan and Mao 2003). Risk of suicide has been shownepeatedly to be associated with low cerebrospinal fluid (CSF)oncentrations of the serotonin metabolite 5-hydroxyindolaceticcid (5-HIAA), as well as variation in the expression of certainerotonin receptors (Mann 2003). Finally, dopamine is consid-red central to reward mechanisms underlying addictive behav-or (Maldonado 2003; Uhl 1999), and dopamine receptor antag-nists figure prominently in the treatment of schizophreniaFreedman 2003).

epartments of Genetics (JR, LJM, AGC) and Physiology and Medicine (ML),Southwest Foundation for Biomedical Research, and Southwest Na-tional Primate Research Center (JR), San Antonio, Texas; Department ofPsychiatry (JJM), Columbia University College of Physicians and Sur-geons, New York, New York; Behavioral Physiology Laboratory (SBM),University of Pittsburgh, Pittsburgh, Pennsylvania; and Departments ofPathology (JRK) and Comparative Medicine (JRK), Wake Forest Univer-sity School of Medicine, Winston-Salem, North Carolina.

ddress reprint requests to Dr. Jeffrey Rogers, Southwest Foundation forBiomedical Research, Department of Genetics, 7620 N.W. Loop 410, SanAntonio, TX 78227.

eceived June 16, 2003; revised December 17, 2003; accepted December 21,2003.

006-3223/04/$30.00oi:10.1016/j.biopsych.2003.12.017

Susceptibility to the psychiatric illnesses cited above is nowrecognized to be influenced in part by genetic variation amongindividuals (Hettema et al 2001; Kendler et al 1995; Lewis andLevitt 2002; Sullivan et al 2000). This raises the question ofgenetic heritability of monoamine neurotransmitter levels, mono-amine receptor expression or sensitivity, and other aspects ofthese systems. The exact role of monoamine function in psychi-atric illness is not clear; however, a more complete understand-ing of the genetic basis of individual variation in monoaminefunction will be valuable for a number of reasons. For example,such information might suggest new targets for therapy andcould direct attention to genetic pathways that might contributeto individual variation in response to pharmacotherapy or pos-sibly disease risk itself.

Previous studies have shown that levels of monoamine me-tabolites in CSF are heritable among people (Oxenstierna et al1976), but the available data are quite limited, and further studyis warranted. Many questions remain, including whether thereare possible genetic explanations for correlations observedamong monoamine metabolite levels. Serotonin turnover, asindexed by 5-HIAA levels in CSF, is correlated with dopamineturnover, measured as CSF levels of homovanillic acid (HVA)(Agren et al 1986). Serotonin function can also influence norad-renergic function, as demonstrated by the finding that serotoner-gic neurons project to noradrenergic neurons in the locusceruleus (Cooper et al 2003). The significance of these correla-tions for psychopathology is not known; however, severalpsychiatric illnesses (e.g., anxiety disorders and depression) havebeen associated with more than one monoamine neurotransmit-ter. It might be valuable to determine whether the phenotypiccorrelations among these monoamines result from shared ge-netic factors underlying their functional variation, or whether theobserved correlations are entirely the result of parallel responsesto environmental factors or developmental experience.

BIOL PSYCHIATRY 2004;55:739–744© 2004 Society of Biological Psychiatry

maltmfsmCrMstthmbaodhiCgaselnt

olpetppatsbglmi

M

B

aTm(oiowb

m

740 BIOL PSYCHIATRY 2004;55:739–744 J. Rogers et al

w

The previous approaches used to investigate the genetics ofonoamine metabolites in humans have been inadequate to

ddress questions of genetic relationships among the metabo-ites. One approach to this problem is the measurement of thehree metabolites in CSF of multiple individuals from largeultigenerational families. This might be impractical in human

amilies, but nonhuman primates are a valuable model for thetudy of monoamine metabolism and the influence of variation inonoamine function on expressed behavior. For example, lowSF 5-HIAA concentrations in macaques are predictive of higherates of impulsive or aggressive behavior (Higley et al 1996c;ehlman et al 1994; Westergaard et al 1999), reduced amounts of

ocial interaction (Higley et al 1996d), and shorter latency toouch a novel object (A.J. Bennett, Ph.D., personal communica-ion, February 9, 2004). Two studies indicate that, as amongumans, individual variation among rhesus macaques (Macacaulatta) in CSF levels of monoamine metabolites is influencedy genetic variation. Higley et al (1993) found that both maternalnd paternal genetic contributions influence monoamine metab-lite levels (5-HIAA for serotonin, HVA for dopamine, 3-hy-roxy-4-methoxyphenylglycol [MHPG] for norepinephrine);owever, the rhesus monkeys in that study could not be linkednto extended pedigrees, and the sample size was fairly small.larke et al (1995) found significant differences across sireroups of rhesus monkeys for 5-HIAA and HVA, but again nodditional pedigree information was used, and sample size wasmall. Consequently, although these results provide importantvidence that genetic variation influences monoamine metabo-ite levels in this species, these analyses could not address aumber of specific issues, including genetic relationships amonghe monoamines.

Taken as a whole, previous research suggests that the activityf monoamine neurotransmitters and the levels of their metabo-ites found in CSF are heritable in both humans and nonhumanrimates. But a detailed understanding of the genetic andnvironmental determinants of the levels of monoamine neuro-ransmitters or their metabolites is not yet available. Using a largeopulation of pedigreed baboons (Papio hamadryas), theresent study investigated the quantitative genetics of mono-mine neurotransmitter activity, as represented by CSF levels ofhe monoamine metabolites 5-HIAA, HVA, and MHPG. Thistudy used variance component methods to estimate the herita-ilities of individual metabolite levels, as well as to estimate theenetic correlations (i.e., pleiotropy) between pairs of metabo-ites. A more complete understanding of genetic influences ononoamine levels in nonhuman primates will provide insight

nto possible genetic effects on monoamine function in humans.

ethods and Materials

aboon SubjectsSubjects were 271 baboons (olive baboons, Papio hamadryas

nubis, yellow baboons, P. h. cynocephalus, and their hybrids).he animals were drawn from the multigenerational colonyaintained at the Southwest National Primate Research Center

SNPRC) in San Antonio, Texas. Molecular genetic studies dem-nstrate that these animals exhibit high levels of genetic variabil-ty (Rogers et al 2000). The animals are housed in social groupsf 15–40 animals. The sire and dam of each infant is known,hich allows reconstruction of the ancestry of any given animalack to wild-caught founders three to five generations ago.

Study subjects were not inbred, and no assortative mating foronoamine or behavioral phenotypes has ever been performed.

ww.elsevier.com/locate/biopsych

The 271 animals in this study are members of 10 families, rangingin size from 4 to 59 baboons. Breeding groups are formed with8–15 adult females and one adult male, so pedigrees consist oflarge paternal half-sibships. Forty-five animals were exposed forvarying lengths of time to nursery rearing during the first year oflife, and this environmental factor is included in genetic analyses(see below). All procedures were approved by the InstitutionalAnimal Care and Use Committee of the Southwest Foundation forBiomedical Research.

Monoamine AssaysCerebrospinal fluid was drawn once from the cisterna magna

of each study baboon. Cerebrospinal fluid was collected onMonday mornings, after the removal of the animals from theirhome social cages the previous Friday. Animals were housedindoors in single cages from Friday to Monday. A series ofnoninvasive behavioral tests were performed on Saturday andSunday. These tests will be described in detail elsewhere butinvolved only brief exposure of study animals to novel objects(toys). Cerebrospinal fluid samples were obtained 30 min or lessafter sedation with RAAK (rompun, acepromazine, atropine, andketamine). The CSF samples were immediately placed on wetice, and within 60 min centrifuged to pellet any contaminants,including blood cells. Aliquots of the supernatant were split intomultiple cryogenic tubes and frozen at �80°C less than 90 minafter collection. After storage at �80°C, samples were shipped tothe laboratory of one author (JJM) at Columbia University forassay of monoamine metabolites. The time between collectionand assay varied but was generally less than 5 months.

Homovanillic acid, 5-HIAA, and MHPG were assayed follow-ing standard methods (Scheinin et al 1983). A measured aliquotof each sample was mixed with an equal volume of cold mobilephase. The mixture was then filtered at centrifugation (6000g for40 min at 4°C) and part of the filtrate transferred to a 300-FLmicroinjection insert. This material was then analyzed by high-performance liquid chromatography with electrical detection,which allows simultaneous measurement of HVA, 5-HIAA, andMHPG.

Quantitative Genetic AnalysisVariance Components Analysis. Univariate quantitative ge-

netic analyses were conducted with data for each neurotransmit-ter metabolite to estimate the influence of specific variables(additive genetic variation, covariates [including sex, age, andbody weight], identified environmental factors, and randomunidentified environmental effects) on levels of CSF metabolitesin this population. These analyses used maximum likelihoodvariance decomposition methods, implemented in the computerpackage SOLAR (Almasy and Blangero 1998). The variancecomponents approach is an extension of the strategy developedby Amos (1994). Briefly, the covariance matrix for a pedigree isgiven by:

� � 2��g2���e

2

where �g2 is the genetic variance due to residual additive genetic

factors, � is the kinship matrix representing the pairwise kinshipcoefficients among all animals, �e

2 is the variance due toindividual-specific environmental effects, and I is an identitymatrix. Significance of heritability was tested by comparing thelikelihood of the model in which �g

2 is constrained to zero withthat of a model in which �g

2 is estimated. Twice the differencebetween the two loge likelihoods of these models yields a teststatistic, which is asymptotically distributed as a 1/2:1/2 mixture

o

vTinca1tmpihsof

h1itcmt

we

acblccaegsmtiap

R

ppamrTbfa(o

J. Rogers et al BIOL PSYCHIATRY 2004;55:739–744 741

f a 2(1) variable and a point mass at zero (Self and Liang 1987).Before testing for the significance of heritability, phenotype

alues for each individual are adjusted for a series of covariates.he covariates used in this model included age, sex, age sex

nteraction, body weight, and nursery rearing. Exposure toursery rearing (affecting 17% of the study subjects) was in-luded as a covariate because previous analyses have suggestedpotential impact on monoamine metabolite levels (Higley et al992; Sanchez et al 2001). Furthermore, the data sets wererimmed before genetic analysis by deletion of any monoamineetabolite value that was 4 standard deviations or more from theopulation mean. This data-cleaning procedure reduces the

mpact of problematic or invalid data on the final estimate oferitability and is important because extreme outliers can have aubstantial impact on the estimated heritability of a trait. A totalf four outlier values were removed for 5-HIAA, one outlier valueor HVA, and none for MHPG.

Bivariate Analysis. The basic variance components approachas been extended to a multivariate framework (Comuzzie et al994, 1996). In the multivariate model, the phenotype covariances further decomposed to include the genetic correlation betweenraits due to shared additive genetic effects (pleiotropy) and theorrelation between traits due to shared unmeasured environ-ental effects. Thus, the covariation between two individuals for

wo traits is given by

� � � �11 �12

�12 �12� (2)

here � is a covariance matrix of 2 2 covariance matrices, thelements of which are defined by

�ab � 2��g�ga�gb � ��e�ea�eb (3)

nd where a and b are trait 1 or trait 2, �g is the additive geneticorrelation between the two traits, and �e is the correlationetween unmeasured environmental effects. The genetic corre-ation estimates the proportion of genetic variance shared inommon between the two traits. If a � b, then �g is 1.0 and theovariance of a pair of relatives simplifies to Equation 1. Thispproach has been implemented in SOLAR version 2.0. Inssence, this analysis uses all pairwise comparisons among theenealogically related animals to determine whether the mea-urement of one phenotype in one animal is correlated witheasurement of a second phenotype in closely related animals

hat share genes in common. If the correlation is significant, thiss evidence that a gene or set of genes shared among thosenimals by virtue of common ancestry are influencing both thosehenotypes.

esults

The mean values and ranges for HVA, 5-HIAA, and MHPG areresented in Table 1. These results indicate that substantialhenotypic variation exists among the study baboons. Values ofll 271 animals for HVA and MHPG were distributed approxi-ately normally (kurtosis of 1.78 and 1.00, respectively). After

emoval of one outlier value for HVA, kurtosis decreased to .45.he distribution of raw 5-HIAA values was more kurtotic (66.78),ut after removal of four outliers this was reduced to 1.79. It hasrequently been observed within human populations that there isphenotypic correlation between CSF levels of 5-HIAA and HVA

e.g., Agren et al 1986). This phenotypic correlation (�P) was alsobserved in the SNPRC baboons (Table 2), with � estimated to

P

be .64 (p � .05). The other two pairs of metabolites exhibitweaker but statistically significant phenotypic correlations.

The identified covariates account for only a small proportionof variation in these traits. Levels of HVA were influenced by sex,age, age squared, and body weight, but together these variablesaccount for only 13% of the variance in HVA. Levels of MHPGwere associated with age and sex, with a combined effect ofcovariates of 6%, and 5-HIAA was influenced only by sex (6%).The simultaneous analyses of genetic and environmental effectsshow no statistically significant contribution of rearing status(scored as a dichotomous variable: mother-reared vs. nursery-reared) to the variance of any of the CSF monoamine levels.

Results of the quantitative genetic analyses are presented inTable 3. All three monoamine metabolites exhibit significantadditive genetic variation (heritability). The estimates of herita-bility range from .30 to .50, but given the standard errors of theestimates, the differences between values are not statisticallysignificant. In addition, the residual values of 5-HIAA afteradjusting for HVA also show significant additive genetic effects(h2 � .46).

There are significant genetic and/or environmental correla-tions among all pairs of monoamine metabolites (Table 4).Homovanillic acid and 5-HIAA exhibit substantial environmentalcorrelation (�E � .71) and genetic correlation (�G � .50). Thereis a strong genetic correlation between MHPG and HVA (�G �.91) but no significant environmental correlation between them.In contrast, MHPG and 5-HIAA provide evidence for a sharedenvironmental effect (�E � .36) with no significant geneticcorrelation. These analyses indicate that, among the baboonsstudied here, individual variation in the three monoamine neu-rotransmitters is influenced by overlapping sets of both geneticand environmental factors. No genetic or environmental corre-lation is estimated at 1.00, which suggests that each neurotrans-mitter has its own unique set of influences. At the same time,each neurotransmitter shares at least some components of vari-ance with each of the others.

Discussion

Variation in monoamine neurotransmission is associated witha number of psychiatric disorders in humans, as part of theiretiology, treatment, or both. The level of monoamine metabolitesin CSF is one index of monoamine turnover that has been useful

Table 2. Phenotypic Correlations (�P)

5-HIAA MHPG

HVA .64a .38a

5-HIAA — .27a

HVA, homovanillic acid; 5-HIAA, 5-hydroxyindolacetic acid; MHPG,3-hydroxy-4-methoxyphenylglycol.

aSignificant at p � .05

Table 1. Monoamine Metabolite Levels in SNPRC Baboons

Neurotransmitter Metabolite nMean (SD)pmol/mL Range

Dopamine HVA 270 573 (177) 230 –1131Serotonin 5-HIAA 267 247 (119) 76 – 688Norepinephrine MHPG 271 86 (29) 21–197

SNPRC, Southwest National Primate Research Center; HVA, homo-vanillic acid; 5-HIAA, 5-hydroxyindolacetic acid; MHPG, 3-hydroxy-4-methoxyphenylglycol.

www.elsevier.com/locate/biopsych

fsosmtsiisv

hmdmsflsiewt[miomspo

mha1tm

TB

P

HM5HR

ac

TM

T

MHM

5

742 BIOL PSYCHIATRY 2004;55:739–744 J. Rogers et al

w

or assessment of monoamine functionality. The exact relation-hip between monoamine neurotransmission and psychopathol-gy is not understood. Nevertheless, a more complete under-tanding of the genetic basis of individual variation inonoamine turnover and metabolite levels will be valuable for

he investigation and interpretation of these biological relation-hips. For example, the identification of specific genes thatnfluence serotonin or norepinephrine levels might providemportant clues regarding the genetic pathways involved inusceptibility to anxiety disorders or depression, or individualariation in response to pharmacotherapy.

Previous studies of both humans and nonhuman primatesave found evidence that CSF levels of these monoamineetabolites are heritable, but limitations of sample size and studyesign have made investigation of genetic relationships amongonoamine levels impossible. The present study overcomes

uch limitations by using quantitative genetic methods to per-orm a combined, multivariate analysis of the genetic control ofevels of monoamine metabolites found in the CSF of randomlyelected pedigreed baboons. By examining large extended fam-lies, we were able to estimate the magnitude of genetic influ-nces on variation in specific monoamines (i.e., heritability), asell as to assess the degree of pleiotropy among these traits (i.e.,

o determine the degree to which the same or different genescurrently unidentified] influence variation in specific pairs ofonoamines). The results indicate that interindividual variation

n each of the metabolites is partially heritable and that thebserved phenotypic correlations between specific pairs ofonoamine metabolites are due to different combinations of

hared environmental and shared genetic influences. In thisopulation of baboons, pleiotropy is an important determinantf monoamine metabolite levels.

Among the CSF monoamine metabolites, serotonin has beenost frequently associated with behavioral phenomena in non-uman primates. Low CSF 5-HIAA concentrations in macaquesre predictive of higher rates of impulsive behavior (Higley et al996c; Mehlman et al 1994), reduced amounts of social interac-ion (Higley et al 1996d), higher rates of severe aggression andortality (Higley et al 1996a, 1996b; Westergaard et al 1999), and

able 3. Estimates of Heritability of Monoamine Metabolites in SNPRCaboons

henotype h2 p n

VA .50 � .19 .0016 270HPG .36 � .16 .0022 271

-HIAA .30 � .17 .016 267VA: 5-HIAA .36 � .14 .0012 270esidual 5-HIAA (HVA-free) .46 � .16 .00026 266

SNPRC, Southwest National Primate Research Center; HVA, homovanilliccid; MHPG, 3-hydroxy-4-methoxyphenylglycol; 5-HIAA, 5-hydroxyindola-etic acid.

able 4. Genetic and Environmental Correlations among Monoamineetabolites

rait 1 Trait 2Environmental

Correlation pGenetic

Correlation p

HPG HVA .14 � .15 .41 .91 � .22 .01VA 5-HIAA .71 � .10 .0007 .50 � .29 .025HPG 5-HIAA .36 � .14 .02 .07 � .36 .84

MHPG, 3-hydroxy-4-methoxyphenylglycol; HVA, homovanillic acid;-HIAA, 5-hydroxyindolacetic acid.

ww.elsevier.com/locate/biopsych

shorter latency to touch a novel object (A.J. Bennett, Ph.D.,personal communication, February 9, 2004). Low CSF serotoninlevels are associated with greater impulsivity in vervet monkeys(Fairbanks et al 2001). From the perspective of natural primatebehavior, it is intriguing that low serotonin levels predict earlydispersal of male macaques from their natal social groups(Kaplan et al 1995; Mehlman et al 1995). Similarly, anubisbaboons, which disperse from their natal groups, have lower CSF5-HIAA levels than do sympatric hamadryas baboons, whichtypically do not disperse (Kaplan et al 1999). Environmentaleffects on 5-HIAA are also known. High dietary cholesterolinduced greater serotonergic activity and less aggressive/impul-sive behavior in a population of macaques (Kaplan et al 1994; seealso Kaplan et al 1997).

Among the other neurotransmitters, high levels of MHPGhave been associated with greater reactivity to novelty in rhesusmacaques (Clarke et al 1996; Higley et al 1991), whereas higherlevels of dopaminergic activity (assessed through CSF HVA) areassociated with the expression of social dominance amongcynomolgus macaques (Kaplan et al 2002). Within a wild popu-lation from Ethiopia, individual vervet monkeys that expressedimpulsive, risk-taking behavior had lower levels of MHPG thanother individuals that did not show those behaviors (Fairbanks etal 1999). These studies of vervet monkeys and two species ofmacaques all support the conclusion that variation in levels ofmonoamine metabolites is indicative of differential CNS functionand is associated with behavioral variation.

If all three monoamine metabolites are heritable, are theyinfluenced by the same or different genes? The present dataindicate that MHPG and HVA share significant genetic variance(�G � .91). There are two alternative, although not mutuallyexclusive, hypotheses to explain this high genetic correlation.First, because norepinephrine is derived directly from dopaminethrough enzymatic addition of a single hydroxyl group, it is quiteplausible that genetic differences among individuals that affectlevels of dopamine (and hence HVA) might also influencenorepinephrine (and MHPG). If the rate-limiting step in dopa-mine synthesis is influenced by genetic differences amongbaboons, then the same genetic variance might explain variationin norepinephrine levels (Cooper et al 2003, p. 186); however,these two neurotransmitters are secreted by two different sets ofCNS neurons. The alternative hypothesis is that the geneticcorrelation between CSF levels of HVA and MHPG is the result ofvariation in one or more genes that influence the activity of boththe dopaminergic and noradrenergic systems but is not simplythe consequence of rate-limiting steps in the synthetic pathway.This question can be resolved by identification of the specificgene or genes that are exerting these effects.

Agren et al (1986) examined turnover in the serotonin anddopamine systems by measuring CSF levels of 5-HIAA and HVAin two populations of patients and also compared serotonin anddopamine levels in specific brain regions in dogs. Agren et alconfirmed the phenotypic correlation between serotonin anddopamine and their metabolites. In addition, their statisticalmodeling suggests that dopamine turnover is controlled byserotonin turnover. Our results for baboons are entirely consis-tent with these findings, though Agren et al were not able todetermine whether the correlation between these two mono-amines is due to genetic or environmental factors.

The present results indicate that the phenotypic correlationobserved between 5-HIAA and HVA across the baboons reflectsboth genetic and environmental covariance. That is, some of thesame genes that influence individual variation in CSF concentra-

ttisfr5bglbod

esme(ilaicpclptl

vtcstrcni

ailnamorraefsdsrerh

ta

J. Rogers et al BIOL PSYCHIATRY 2004;55:739–744 743

ions of 5-HIAA also influence variation in HVA. In addition,here are environmental factors (currently unidentified) thatnfluence both 5-HIAA and HVA. Although 5-HIAA and HVAhow both types of correlation, the pairwise analyses with MHPGound only genetic or only environmental correlation, and as aesult the overall phenotypic correlation is greater between-HIAA and HVA than for either of these with MHPG. Theaboon data suggest the possibility of shared genetic effects (i.e.,enetic correlation) between HVA and 5-HIAA in humans. Theack of genetic correlation between MHPG and 5-HIAA in theaboons indicates that whatever gene or genes are acting onbserved variation in both 5-HIAA and HVA do not have aetectable influence on levels of MHPG.

The simultaneous analyses of genetic and environmentalffects showed no statistically significant contribution of rearingtatus (mother-reared vs. nursery-reared) to the variance in CSFonoamine levels. Previous research has indicated that rearing

xperience can affect monoamine levels in nonhuman primatesHigley et al 1992; Sanchez et al 2001). The present study was notntended to assess the effect of rearing on neurotransmitterevels. Length of time spent in the nursery varied across subjects,nd only 17% had any exposure at all. These circumstances maket difficult to detect small or modest effects of nursery rearing. Weonclude that in this population of baboons, assayed with thisrotocol, the rearing of animals following the system of nurseryare used at the SNPRC did not exert a substantial influence onevels of monoamine metabolites. We cannot rule out theossibility of a small effect of this rearing experience on theseraits, or that a different nursery-rearing protocol might exert aarger effect.

It is not possible at this time to identify the environmentalariables that account for the environmental correlations be-ween 5-HIAA and HVA and between 5-HIAA and MHPG. It isonceivable that social experience during development (e.g., thetyle of maternal behavior received as an infant) might affecthese neurotransmitter systems. The nature of current socialelationships among the adult animal subjects in their homeages immediately before CSF collection, or the social domi-ance rank of a subject baboon’s mother during that animal’snfancy, are also possible explanations.

The present analysis has three notable limitations. First,ctivity of the three neurotransmitter systems was measuredndirectly through quantitative analysis of monoamine metaboliteevels in cisternal CSF. This method has been valuable in a largeumber of previous studies of nonhuman primates (see above)nd has also been useful in studies of humans. Nevertheless, CSFetabolite levels are influenced by factors other than the activityf the neuronal systems that synthesize and release these neu-otransmitters. Second, this approach provides no informationegarding the specific brain regions that contribute to variationmong individual baboons. Finally, the heritability values andstimated genetic and environmental correlations are valid onlyor the specific research protocol used in this study. The animalubjects were removed from their home social groups on oneay, and CSF samples were collected after 2 more full days iningle caging, with other baboons similarly caged in the sameoom. If monoamine metabolite levels are affected by thisxperience, then the results reported here will not necessarilyeflect monoamine levels measured when animals are in theirome social groups.

Although our conclusions must be limited to the conditions ofhe testing, it is clear that metabolites of serotonin, dopamine,nd norepinephrine exhibit heritable variation in baboons, that

these three monoamines show phenotypic pairwise correlationsacross subjects, and that those phenotypic correlations resultfrom shared genetic effects (MHPG–HVA), shared environmentaleffects (5-HIAA–MHPG), or both (5-HIAA–HVA).

If a similar genetic architecture for these neurotransmittersexists in humans, then people who are genetically predisposedto low levels of dopamine turnover might also be geneticallypredisposed to lower-than-average levels of turnover in seroto-nin and norepinephrine. The results for baboons suggest that thegenetic correlations among HVA, MHPG, and 5-HIAA are not dueto the action of one set of genes on all three phenotypes.Dopamine (HVA) and serotonin (5-HIAA) share some geneticeffects in common, whereas dopamine and norepinephrine(MHPG) share others. Knowledge of the genetic architecture ofthese traits among humans would be valuable in interpretingpopulation-based studies of these neurotransmitters. It wouldalso facilitate genetic studies using linkage analysis to map andidentify the functional genes that influence CSF monoaminelevels in humans or nonhuman primates. When two phenotypesare genetically correlated, linkage analyses that include bothphenotypes in a bivariate analysis will have more power thanunivariate analyses of one phenotype or the other (Almasy et al1997). The present analysis illustrates that studies of nonhumanprimate models, such as baboons and macaques, can makeimportant contributions to understanding the genetic basis ofindividual variation in these parameters.

This work was supported by National Institute of Health GrantNo. R01-MH65462 (JR), Conte Neuroscience Center Grant Nos.MH62185 (JJM), HL5666 (JRK), and HL40962 (SBM), andthrough the base grant from the Southwest National PrimateResearch Center: Grant No. NIH P51-RR013986.

We thank Dr. Yung-Yu Huang for expert assistance with themonoamine assays, and four anonymous reviewers for theirhelpful comments.

Agren H, Mefford IN, Rudorfer MV, Linnoila M, Potter WZ (1986): Interactingneurotransmitter systems. A non-experimental approach to the 5-HIAA-HVA correlation in human CSF. J Psychiatr Res 20:175–193.

Almasy L, Blangero J (1998): Multipoint quantitative trait linkage analysis ingeneral pedigrees. Am J Hum Genet 62:1198 –1221.

Almasy L, Dyer TD, Blangero J (1997): Bivariate quantitative trait linkageanalysis: Pleiotropy versus co-incident linkage. Genet Epidemiol 14:953–958.

Amos CI (1994): Robust variance components approach for assessing ge-netic linkage in pedigrees. Am J Hum Genet 54:535–543.

Barchas JD, Altemus M (1999): Biochemical hypotheses of mood and anxietydisorders. In: Siegel GJ, Agranoff BW, Albers RW, Fisher SK, Uhler MD,editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6thed. New York: Lippincott Williams and Wilkins, 1073–1094.

Blanco C, Schneier FR, Schmidt A, Blanco-Jerez CR, Marshall RD, Sanchez-Lacay A, Liebowitz MR (2003): Pharmacological treatment of social anx-iety disorder: A meta-analysis. Depress Anxiety 18:29 –40.

Clarke AS, Hedeker DR, Ebert MH, Schmidt DE, McKinney WT, Kraemer GW(1996): Rearing experience and biogenic amine activity in infant rhesusmonkeys. Biol Psychiatry 40:338 –352.

Clarke AS, Kammerer CM, George KP, Kupfer DJ, McKinney WT, Spence MA,Kraemer GW (1995): Evidence for heritability of biogenic amine levels inthe cerebrospinal fluid of rhesus monkeys. Biol Psychiatry 38:572–577.

Comuzzie AG, Blangero J, Mahaney MC, Haffner SM, Mitchell BD, Stern MP,MacCluer JW (1996): Genetic and environmental correlations amonghormone levels and measures of body fat accumulation and topogra-phy. J Clin Endocrinol Metab 81:597–600.

Comuzzie AG, Blangero J, Mahaney MC, Mitchell BD, Stern MP, MacCluer JW(1994): Genetic and environmental correlations among skinfolds. Int JObesity 18:413–418.

www.elsevier.com/locate/biopsych

C

F

F

FH

H

H

H

H

H

H

H

H

K

K

K

K

K

744 BIOL PSYCHIATRY 2004;55:739–744 J. Rogers et al

w

ooper JR, Bloom FE, Roth RH (2003): The Biochemical Basis of Neuropharma-cology, 8th ed. Oxford: Oxford University Press.

airbanks LA, Fontenot MB, Phillips-Conroy JE, Jolly CJ, Kaplan JR, Mann JJ(1999): CSF monoamines, age and impulsivity in wild grivet monkeys(Cercopithecus aethiops aethiops). Brain Behav Evol 53:305–312.

airbanks LA, Melega WP, Jorgensen MJ, Kaplan JR, McGuire MT (2001):Social impulsivity inversely associated with CSF 5-HIAA and fluoxetineexposure in vervet monkeys. Neuropsychopharmacology 24:370 –378.

reedman R (2003): Schizophrenia. N Engl J Med 349:1738 –1749.eninger GR, Delgado PL, Charney DS (1996): The revised monoamine the-

ory of depression: A modulatory role for monoamines, based on newfindings for monoamine depletion experiments in humans. Pharma-copsychiatry 29:2–11.

ettema JM, Neale MC, Kendler KS (2001): A review and meta-analysis of thegenetic epidemiology of anxiety disorders. Am J Psychiatry 158:1568–1578.

igley JD, King ST Jr, Hasert MF, Champoux M, Suomi SJ, Linnoila M (1996a):Stability of interindividual differences in serotonin function and its rela-tionship to severe aggression and competent social behavior in rhesusmacaque females. Neuropsychopharamocology 14:67–76.

igley JD, Mehlman PT, Higley SB, Fernald B, Vickers SG, Lindell SG, et al(1996b): Excessive mortality in young free-ranging male nonhuman pri-mates with low cerebrospinal fluid 5-hydroxyindoleacetic acid concen-trations. Arch Gen Psychiatry 53:537–543.

igley JD, Mehlman PT, Poland RE, Taub DM, Vickers J, Suomi SJ, Linnoila M(1996c): CSF testosterone and 5-HIAA correlate with different types ofaggressive behaviors. Biol Psychiatry 40:1067–1082.

igley JD, Suomi SJ, Linnoila M (1991): CSF monamine metabolite concen-trations vary according to age, rearing, and sex, and are influenced bythe stressor of social separation in rhesus monkeys. Psychopharmacol-ogy 103:551–556.

igley JD, Suomi SJ, Linniola M (1992): A longitudinal assessment of CSFmonoamine metabolite and plasma cortisol concentrations in youngrhesus monkeys. Biol Psychiatry 32:127–145.

igley JD, Suomi SJ, Linnoila M (1996d): A nonhuman primate model of typeII alcoholism? Part 2. Diminished social competence and excessive ag-gression correlates with low cerebrospinal fluid 5-hydroxyindoleaceticacid concentrations. Alcohol Clin Exp Res 20:643–650.

igley JD, Thompson WW, Champoux M, Goldman D, Hasert MF, KraemerGW, et al (1993): Paternal and maternal genetic and environmentalcontributions to cerebrospinal fluid monoamine metabolites in rhesusmonkeys (Macaca mulatta). Arch Gen Psychiatry 50:615–623.

aplan JR, Fontenot MB, Berard J, Manuck SB, Mann JJ (1995): Delayeddispersal and elevated monoaminergic activity in free-ranging rhesusmonkeys. Am J Primatol 35:229 –234.

aplan JR, Manuck SB, Fontenot B, Mann JJ (2002): Central nervous systemmonoamine correlates of social dominance in cynomolgus monkeys.Neuropsychopharmacology 26:431–443.

aplan JR, Muldoon MF, Manuck SB, Mann JJ (1997): Assessing the observedrelationship between low cholesterol and violence-related mortality:Implications for suicide risk. Ann N Y Acad Sci 836:57–80.

aplan J, Phillips-Conroy J, Fontenot MB, Jolly CJ, Fairbanks LA, Mann JJ(1999): Cerebrospinal fluid monoaminergic metabolites differ in wildanubis and hybrid (anubis x hamadryas) baboons: Possible relationshipsto life history and behavior. Neuropsychopharmacology 20:517–524.

aplan JR, Shively CA, Fontenot MB, Morgan TM, Howell SM, Manuck SB, et al

(1994): Demonstration of an association among dietary cholesterol, cen-

ww.elsevier.com/locate/biopsych

tral serotonergic activity, and social behavior in monkeys. PsychosomMed 56:479 –484.

Kendler KS, Walters EE, Neale MC, Kessler RC, Heath AC, Eaves LJ (1995): Thestructure of the genetic and environmental risk factors for six majorpsychiatric disorders in women: Phobia, generalized anxiety disorder,panic disorder, bulimia, major depression and alcoholism. Arch Gen Psy-chiatry 52:374 –383.

Krystal JH, D’Souza DC, Sanacora G, Goddard AW, Charney DS (2001): Cur-rent perspectives on the pathophysiology of schizophrenia, depressionand anxiety disorders. Med Clin North Am 85:559 –577.

Lewis DA, Levitt P (2002): Schizophrenia as a disorder of development. AnnRev Neurosci 25:409 –432.

Maldonado R (2003): The neurobiology of addiction. J Neural Transm66(suppl):1–14.

Manji HK, Drevets WC, Charney DS (2001): The cellular neurobiology ofdepression. Nat Med 7:541–547.

Mann JJ (2003): Neurobiology of suicidal behaviour. Nat Rev Neurosci 4:819 –828.

Mehlman PT, Higley JD, Faucher I, Lilly AA, Taub DM, Vickers J, et al(1994): Low CSF 5-HIAA concentrations and severe aggression andimpaired impulse control in nonhuman primates. Am J Psychiatry151:1485–1491.

Mehlman PT, Higley JD, Faucher I, Lilly AA, Taub DM, Vickers J, et al (1995):Correlation of CSF 5-HIAA concentration with sociality and the timing ofemigration in free-ranging primates. Am J Psychiatry 152:907–913.

Nemeroff CB, Owens MJ (2002): Treatment of mood disorders. Nat Neurosci5(suppl):1068 –1070.

Oxenstierna G, Edman G, Iselius L, Oreland L, Ross SB, Sedvall G (1976):Concentrations of monoamine metabolites in the cerebrospinal fluid oftwins and unrelated individuals: A genetic study. J Psychiatr Res 20:19 –29.

Rogers J, Mahaney MC, Witte SM, Nair S, Newman D, Wedel S, et al (2000): Agenetic linkage map of the baboon (Papio hamadryas) genome basedon human microsatellite polymorphisms. Genomics 67:237–247.

Roy A, Jimerson DC, Pickar D (1986): Plasma MHPG in depressive disordersand relationship to the dexamethasone suppression test. Am J Psychiatry143:846 –851.

Sanchez MM, Ladd CO, Plotsky PM (2001): Early adverse experience as adevelopmental risk factor for later psychopathology: Evidence from ro-dent and primate models. Dev Psychopathol 13:419 –449.

Scheinin M, Chang WH, Kirk KL, Linnoila M (1983): Simultaneous determina-tion of 3-methoxy-4-hydroxyphenylglycol, 5-hydroxyindoleacetic acid,and homovanillic acid in cerebrospinal fluid with high-performance liq-uid chromatography using electrochemical detection. Anal Biochem131:246 –253.

Self SG, Liang K-Y (1987): Asymptotic properties of maximum likelihood ratiotests under nonstandard conditions. J Am Stat Assoc 82:605–610.

Sheehan DV, Mao CG (2003): Paroxetine treatment of generalized anxietydisorder. Psychopharmacol Bull 37(suppl 1):64 –75.

Sullivan PF, Neale MC, Kendler KS (2000): Genetic epidemiology of majordepression: Review and meta-analysis. Am J Psychiatry 157:1552–1562.

Uhl GR (1999): Neurochemical bases of drug abuse. In: Siegel GJ, et al,editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6thed. New York: Lippincott Williams and Wilkins, 1095–1107.

Westergaard GC, Suomi SJ, Higley JD, Mehlman PT (1999): CSF 5-HIAA andaggression in female macaque monkeys: Species and inter-individual

differences. Psychopharmacology 146:440 –446.