Polymorphism of the tryptophan hydroxylase 2 (TPH2) gene is associated with chimpanzee neuroticism

The A328 V/E (rs2887147) polymorphisms in human tryptophan hydroxylase 2 compromise enzyme activity

Human tryptophan hydroxylase 2 (hTPH2) is the rate-limiting enzyme for serotonin biosynthesis in the brain. A number of naturally-occurring single nucleotide polymorphisms (SNPs) have been reported for hTPH2. We investigated the activity and kinetic characteristics of the most common missense polymorphism rs2887147 (A328 V/E; 0.92% allelic frequency for the two different reported SNPs at the same site) using bacterially expressed hTPH2. The recombinant full-length enzyme A328E had no measurable enzyme activity, but A328V displayed decreased enzyme activity (Vmax). A328V also displayed substrate inhibition and decreased stability compared to the wild-type enzyme. By contrast, in constructs lacking the N-terminal 150 amino acid regulatory domain, the A328V substitution had no effect; that is, there was no substrate inhibition, enzyme stabilities (for wild-type and A328V) were dramatically increased, and Vmax values were not different (while the A328E variant remained inactive). These findings, in combination with molecular modeling, suggest that substitutions at A328 affect catalytic activity by altering the conformational freedom of the regulatory domain. The reduced activity and substrate inhibition resulting from these polymorphisms may ultimately reduce serotonin synthesis and contribute to behavioral perturbations, emotional stress, and eating disorders.

The neurobiology of human aggressive behavior: Neuroimaging, genetic, and neurochemical aspects

In modern societies, there is a strive to improve the quality of life related to risk of crimes which inevitably requires a better understanding of brain determinants and mediators of aggression. Neurobiology provides powerful tools to achieve this end. Pre-clinical and clinical studies show that changes in regional volumes, metabolism-function and connectivity within specific neural networks are related to aggression. Subregions of prefrontal cortex, insula, amygdala, basal ganglia and hippocampus play a major role within these circuits and have been consistently implicated in biology of aggression. Genetic variations in proteins regulating the synthesis, degradation, and transport of serotonin and dopamine as well as their signal transduction have been found to mediate behavioral variability observed in aggression. Gene-gene and gene-environment interactions represent additional important risk factors for aggressiveness. Considering the social burden of pathological forms of aggression, more basic and translational studies should be conducted to accelerate applications to clinical practice, justice courts, and policy making.

Comparative analyses of the Pan lineage reveal selection on gene pathways associated with diet and sociality in bonobos

Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) diverged into distinct species approximately 1.7 million years ago when the ancestors of modern-day bonobo populations were separated by the Congo River. This geographic boundary separates the two species today and the associated ecological factors, including resource distribution and feeding competition, have likely shaped the divergent social behavior of both species. The most striking behavioral differences pertain to between group interactions in which chimpanzees behave aggressively towards unfamiliar conspecifics, while bonobos display remarkable tolerance. Several hypotheses attempt to explain how different patterns of social behavior have come to exist in the two species, some with specific genetic predictions, likening the evolution of bonobos to a process of domestication. Here, we utilize 73 ape genomes and apply linkage haplotype homozygosity and structure informed allele frequency differentiation methods to identify positively selected regions in bonobos since their split from a common pan ancestor to better understand the environment and processes that resulted in the behavioral differences observed today. We find novel evidence of selection in genetic regions that aid in starch digestion (AMY2) along with support for two genetic predictions related to self-domestication processes hypothesized to have occurred in the bonobo. We also find evidence for selection on neuroendocrine pathways associated with social behavior including the oxytocin, serotonin, and gonadotropin releasing hormone pathways.

Association between temperament related traits and single nucleotide polymorphisms in the serotonin and oxytocin systems in Merino sheep

Animal temperament is defined as the consistent behavioral and physiological differences that are seen between individuals in response to the same stressor. Neurotransmitter systems, like serotonin and oxytocin in the central nervous system, underlie variation in behavioral traits in humans and other animals. Variations like single nucleotide polymorphisms (SNPs) in the genes for tryptophan 5-hydroxylase (TPH2), the serotonin transporter (SLC6A4), the serotonin receptor (HTR2A), and the oxytocin receptor (OXTR) are associated with behavioral phenotype in humans. Thus, the objective of this study was to identify SNPs in those genes and to test if those variations are associated with the temperament in Merino sheep. Using ewes from the University of Western Australia temperament flock, which has been selected on emotional reactivity for more than 20 generations, eight SNPs (rs107856757, rs107856818, rs107856856 and rs107857156 in TPH2, rs20917091 in SLC6A4, rs17196799 and rs17193181 in HTR2A, and rs17664565 in OXTR) were found to be distributed differently between calm and nervous sheep. These eight SNPs were then genotyped in 260 sheep from a flock that has never been selected on emotional reactivity, followed by the estimation of the behavioral traits of those 260 sheep using an arena test and an isolation box test. We found that several SNPs in TPH2 (rs107856757, rs107856818, rs107856856 and rs107857156) were in strong linkage disequilibrium, and all were associated with behavioral phenotype in the nonselected sheep. Similarly, rs17196799 in HTR2A was also associated with the behavioral phenotype.

A review of research in primate sanctuaries

While non-human primate studies have long been conducted in laboratories, and more recently at zoological parks, sanctuaries are increasingly considered a viable setting for research. Accredited sanctuaries in non-range countries house thousands of primates formerly used as subjects of medical research, trained performers or personal pets. In range countries, however, sanctuaries typically house orphaned primates confiscated from illegal poaching and the bushmeat and pet trafficking trades. Although the primary mission of these sanctuaries is to rescue and rehabilitate residents, many of these organizations are increasingly willing to participate in non-invasive research. Notably, from a scientific standpoint, most sanctuaries provide potential advantages over traditional settings, such as large, naturalistic physical and social environments which may result in more relevant models of primates' free-ranging wild counterparts than other captive settings. As a result, an impressive scope of research in the fields of primate behaviour, cognition, veterinary science, genetics and physiology have been studied in sanctuaries. In this review, we examine the range and form of research that has been conducted at accredited sanctuaries around the world. We also describe the potential challenges of sanctuary-based work and the considerations that external researchers may face when deciding to collaborate with primate sanctuaries on their research projects.

Personality links with lifespan in chimpanzees

Life history strategies for optimizing individual fitness fall on a spectrum between maximizing reproductive efforts and maintaining physical health over time. Strategies across this spectrum are viable and different suites of personality traits evolved to support these strategies. Using data from 538 captive chimpanzees (Pan troglodytes) we tested whether any of the dimensions of chimpanzee personality - agreeableness, conscientiousness, dominance, extraversion, neuroticism, and openness - were associated with longevity, an attribute of slow life history strategies that is especially important in primates given their relatively long lives. We found that higher agreeableness was related to longevity in males, with weaker evidence suggesting that higher openness is related to longer life in females. Our results link the literature on human and nonhuman primate survival and suggest that, for males, evolution has favored the protective effects of low aggression and high quality social bonds.

The association between glutamine repeats in the androgen receptor gene and personality traits in dromedary camel (Camelus dromedarius)

Temperament traits such as fearfulness are important as they define an animal’s responses to its environment and handling. The increasing automation of daily tasks and growing population limits contact between camels and humans. Such limitations contribute to fear of humans and changes in physical environment. Monoamine oxidase A (MAOA) and androgen receptor (AR) genes are important candidates associated with mammal personality. In our analysis, MAOA exon 15 showed no polymorphism but a novel polymorphism was seen in the camel AR exon 1; 16, 17, 18, and 19 glutamine repeats were detected. We genotyped 138 camels belonging to four Egyptian breeds: Maghrabi (n = 90), Sudani (n = 15), Somali (n = 23), and Baladi (n = 10) for AR. Out of the 90 genotyped Maghrabi camels, we evaluated responses of 33 and 32 mature females to a novel object and exposure to an unfamiliar person, respectively. AR gene showed a significant association based on the principal component (PC) score, which indicated the fear of human touch, and the PC score indicates fear during interaction with novel objects. Individuals carrying a shorter genotype in homozygote (S/S) were found to be more fearful. Furthermore, we found that Sudani and Somali breeds had a higher frequency of shorter genotype (S/S), which was associated with increased fearfulness. These findings reflect the behavioral tendency and consequently, affect the use of this breed. This is the first report showing the role of AR glutamine repeats influencing a behavioral trait in dromedary camels and leading to inter-breed differences. Fear-related traits reported here are important because camels cope with various types of stresses and fear, resulting from the demands of intensive production systems and racing events. However, further studies, employing functional genomics and linkage analysis are necessary for confirming the relationship between fearfulness and genetic variation.

Bonobo personality traits are heritable and associated with vasopressin receptor gene 1a variation

Despite being closely related, bonobos and chimpanzees show remarkable behavioral differences, the proximate origins of which remain unknown. This study examined the link between behavioral variation and variation in the vasopressin 1a receptor gene (Avpr1a) in bonobos. Chimpanzees are polymorphic for a ~360 bp deletion (DupB), which includes a microsatellite (RS3) in the 5′ promoter region of Avpr1a. In chimpanzees, the DupB deletion has been linked to lower sociability, lower social sensitivity, and higher anxiety. Chimpanzees and bonobos differ on these traits, leading some to believe that the absence of the DupB deletion in bonobos may be partly responsible for these differences, and to the prediction that similar associations between Avpr1a genotypes and personality traits should be present in bonobos. We identified bonobo personality dimensions using behavioral measures (Sociability_B, Boldness_B, Openness_B, Activity_B) and trait ratings (Assertiveness_R, Conscientiousness_R, Openness_R, Agreeableness_R, Attentiveness_R, Extraversion_R). In the present study we found that all 10 dimensions have nonzero heritabilities, indicating there is a genetic basis to personality, and that bonobos homozygous for shorter RS3 alleles were lower in Attentiveness_R and higher in Openness_B. These results suggest that variations in Avpr1a genotypes explain both within and between species differences in personality traits of bonobos and chimpanzees.

Chimpanzee sociability is associated with vasopressin (Avpr1a) but not oxytocin receptor gene (OXTR) variation

The importance of genes in regulating phenotypic variation of personality traits in humans and animals is becoming increasingly apparent in recent studies. Here we focus on variation in the vasopressin receptor gene 1a (Avpr1a) and oxytocin receptor gene (OXTR) and their effects on social personality traits in chimpanzees. We combine newly available genetic data on Avpr1a and OXTR allelic variation of 62 captive chimpanzees with individual variation in personality, based on behavioral assessments. Our study provides support for the positive association of the Avpr1a promoter region, in particular the presence of DupB, and sociability in chimpanzees. This complements findings of previous studies on adolescent chimpanzees and studies that assessed personality using questionnaire data. In contrast, no significant associations were found for the single nucleotide polymorphism (SNP) ss1388116472 of the OXTR and any of the personality components. Most importantly, our study provides additional evidence for the regulatory function of the 5' promoter region of Avpr1a on social behavior and its evolutionary stable effect across species, including rodents, chimpanzees and humans. Although it is generally accepted that complex social behavior is regulated by a combination of genes, the environment and their interaction, our findings highlight the importance of candidate genes with large effects on behavioral variation.

Neuroanatomical correlates of personality in chimpanzees (Pan troglodytes): Associations between personality and frontal cortex

Converging empirical data suggests that a set of largely consistent personality traits exist in both human and nonhuman primates; despite these similarities, almost nothing is known concerning the neurobiological basis of these traits in nonhuman primates. The current study examined associations between chimpanzee personality traits and the grey matter volume and asymmetry of various frontal cortex regions in 107 captive chimpanzees. Chimpanzees rated as higher on Openness and Extraversion had greater bilateral grey matter volumes in the anterior cingulate cortex. Further, chimpanzee rated as higher on Dominance had larger grey volumes in the left anterior cingulate cortex and right Prefrontal Cortex (PFC). Finally, apes rated higher on Reactivity/Unpredictability had higher grey matter volumes in the right mesial PFC. All associations survived after applying False Discovery Rate (FDR) thresholds. Results are discussed in terms of current neuroscientific models of personality which suggest that the frontal cortex, and asymmetries in this region, play an important role in the neurobiological foundation of broad dispositional traits.

Genetic influences on response to novel objects and dimensions of personality in Papio baboons

Behavioral variation within and between populations and species of the genus Papio has been studied extensively, but little is known about the genetic causes of individual- or population-level differences. This study investigates the influence of genetic variation on personality (sometimes referred to as temperament) in baboons and identifies a candidate gene partially responsible for the variation in that phenotype. To accomplish these goals, we examined individual variation in response to both novel objects and an apparent novel social partner (using a mirror test) among pedigreed baboons (n = 578) from the Southwest National Primate Research Center. We investigated the frequency and duration of individual behaviors in response to novel objects and used multivariate factor analysis to identify trait-like dimensions of personality. Exploratory factor analysis identified two distinct dimensions of personality within this population. Factor 1 accounts for 46.8 % of the variance within the behavioral matrix, and consists primarily of behaviors related to the "boldness" of the subject. Factor 2 accounts for 18.8 % of the variation, and contains several "anxiety" like behaviors. Several specific behaviors, and the two personality factors, were significantly heritable, with the factors showing higher heritability than most individual behaviors. Subsequent analyses show that the behavioral reactions observed in the test protocol are associated with animals' social behavior observed later in their home social groups. Finally we used linkage analysis to map quantitative trait loci for the measured phenotypes. Single nucleotide polymorphisms in a positional candidate gene (SNAP25) are associated with variation in one of the personality factors, and CSF levels of homovanillic acid and 3-methoxy-4-hydroxyphenylglycol. This study documents heritable variation in personality among baboons and suggests that sequence variation in SNAP25 may influence differences in behavior and neurochemistry in these nonhuman primates.

Association study of TPH2 polymorphisms and bipolar disorder in the Han Chinese population

OBJECTIVE

Bipolar disorder (BPD) is a serious and common mental disorder with high heritability. The serotonergic system is known to be implicated in the etiology of the disorder. Tryptophan hydroxylase isoform-2 (TPH2), which controls the synthesis of serotonin in the brain, has been suggested as a candidate gene for BDP. The aim of this study was to examine the association between the polymorphisms in TPH2 and BPD.

METHODS

We conducted a case-control study by genotyping six SNPs (rs10784941, rs1386494, rs2171363, rs4760816, rs1386486, and rs1872824) in 506 bipolar patients and 507 controls of Chinese Han origin.

RESULTS

rs10784941 was not in the Hardy-Weinberg equilibrium and therefore excluded from further analysis. rs1386486 and rs1872824 showed statistically significant differences between cases and controls in genotype frequencies (rs1386486: p=0.043351; rs1872824: p=0.016563), but no association in allele frequencies. Strong LD was found among rs1386494, rs2171363 and rs4760816, but no positive association with BPD was found for haplotypes.

CONCLUSION

Our results indicate that in the Han Chinese population TPH2 may be a potential susceptibility gene for bipolar disorder. Further studies are needed to validate this association.

Androgen receptor and monoamine oxidase polymorphism in wild bonobos

Androgen receptor gene (AR), monoamine oxidase A gene (MAOA) and monoamine oxidase B gene (MAOB) have been found to have associations with behavioral traits, such as aggressiveness, and disorders in humans. However, the extent to which similar genetic effects might influence the behavior of wild apes is unclear. We examined the loci AR glutamine repeat (ARQ), AR glycine repeat (ARG), MAOA intron 2 dinucleotide repeat (MAin2) and MAOB intron 2 dinucleotide repeat (MBin2) in 32 wild bonobos, Pan paniscus, and compared them with those of chimpanzees, Pan troglodytes, and humans. We found that bonobos were polymorphic on the four loci examined. Both loci MAin2 and MBin2 in bonobos showed a higher diversity than in chimpanzees. Because monoamine oxidase influences aggressiveness, the differences between the polymorphisms of MAin2 and MBin2 in bonobos and chimpanzees may be associated with the differences in aggression between the two species. In order to understand the evolution of these loci and AR, MAOA and MAOB in humans and non-human primates, it would be useful to conduct future studies focusing on the potential association between aggressiveness, and other personality traits, and polymorphisms documented in bonobos.

Personality in chimpanzees (Pan troglodytes): exploring the hierarchical structure and associations with the vasopressin V1A receptor gene

One of the major contributions of recent personality psychology is the finding that traits are related to each other in an organized hierarchy. To date, however, researchers have yet to investigate this hierarchy in nonhuman primates. Such investigations are critical in confirming the cross-species nature of trait personality helping to illuminate personality as neurobiologically-based and evolutionarily-derived dimensions of primate disposition. Investigations of potential genetic polymorphisms associated with hierarchical models of personality among nonhuman primates represent a critical first step. The current study examined the hierarchical structure of chimpanzee personality as well as sex-specific associations with a polymorphism in the promoter region of the vasopressin V1a receptor gene (AVPR1A), a gene associated with dispositional traits, among 174 chimpanzees. Results confirmed a hierarchical structure of personality across species and, despite differences in early rearing experiences, suggest a sexually dimorphic role of AVPR1A polymorphisms on hierarchical personality profiles at a higher-order level.

Genetic influences on receptive joint attention in chimpanzees (Pan troglodytes)

Despite their genetic similarity to humans, our understanding of the role of genes on cognitive traits in chimpanzees remains virtually unexplored. Here, we examined the relationship between genetic variation in the arginine vasopressin V1a receptor gene (AVPR1A) and social cognition in chimpanzees. Studies have shown that chimpanzees are polymorphic for a deletion in a sequence in the 5′ flanking region of the AVPR1A, DupB, which contains the variable RS3 repetitive element, which has been associated with variation in social behavior in humans. Results revealed that performance on the social cognition task was significantly heritable. Furthermore, males with one DupB⁺ allele performed significantly better and were more responsive to socio-communicative cues than males homozygous for the DupB- deletion. Performance on a non-social cognition task was not associated with the AVPR1A genotype. The collective findings show that AVPR1A polymorphisms are associated with individual differences in performance on a receptive joint attention task in chimpanzees.

Genetic origins of social networks in rhesus macaques

Sociality is believed to have evolved as a strategy for animals to cope with their environments. Yet the genetic basis of sociality remains unclear. Here we provide evidence that social network tendencies are heritable in a gregarious primate. The tendency for rhesus macaques, Macaca mulatta, to be tied affiliatively to others via connections mediated by their social partners - analogous to friends of friends in people - demonstrated additive genetic variance. Affiliative tendencies were predicted by genetic variation at two loci involved in serotonergic signalling, although this result did not withstand correction for multiple tests. Aggressive tendencies were also heritable and were related to reproductive output, a fitness proxy. Our findings suggest that, like humans, the skills and temperaments that shape the formation of multi-agent relationships have a genetic basis in nonhuman primates, and, as such, begin to fill the gaps in our understanding of the genetic basis of sociality.

A common TPH2 haplotype regulates the neural processing of a cognitive control demand

The monoamine neurotransmitter, serotonin, critically regulates the function of the cerebral cortex and is involved in psychiatric disorders. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of serotonin with the neuron-specific TPH2 isoform present exclusively in the brain and encoded by the TPH2 gene on chromosome 12q21. The haplotype structure of TPH2 was defined for 16 single-nucleotide polymorphisms (SNPs) in a healthy subject population and a haplotype block analysis confirmed the presence of a six SNP haplotype in a yin configuration that has previously been associated with risk for suicidality, depression, and anxiety disorders. Functional magnetic resonance imaging (fMRI) was used to assess the influence of TPH2 variation on brain function related to cognitive control using the Multi-Source Interference Task (MSIT). The MSIT-related blood oxygen level-dependent (BOLD) response was increased with increasing copies of the TPH2 yin haplotype for the dorsal anterior cingulate cortex (dACC), right inferior frontal cortex (IFC), and anterior striatum. A functional connectivity analysis further revealed that increasing numbers of the TPH2 yin haplotype was associated with diminished functional coupling between the dACC and the right IFC, precentral gyrus, parietal cortex and dlPFC. A moderation analysis indicated that the relationship between neural processing networks and cognitive control was significantly modulated by allelic variation for the TPH2 yin haplotype. These findings suggest that the association of risk for psychiatric disorders with a common TPH2 yin haplotype is related to the inefficient functional engagement of cortical areas involved in cognitive control and alterations in the mode of functional connectivity of dACC pathways.

Human-specific SNP in obesity genes, adrenergic receptor beta2 (ADRB2), Beta3 (ADRB3), and PPAR γ2 (PPARG), during primate evolution

Adrenergic-receptor beta2 (ADRB2) and beta3 (ADRB3) are obesity genes that play a key role in the regulation of energy balance by increasing lipolysis and thermogenesis. The Glu27 allele in ADRB2 and the Arg64 allele in ADRB3 are associated with abdominal obesity and early onset of non-insulin-dependent diabetes mellitus (NIDDM) in many ethnic groups. Peroxisome proliferator-activated receptor γ (PPARG) is required for adipocyte differentiation. Pro12Ala mutation decreases PPARG activity and resistance to NIDDM. In humans, energy-expense alleles, Gln27 in ADRB2 and Trp64 in ADRB3, are at higher frequencies than Glu27 and Arg64, respectively, but Ala12 in PPARG is at lower frequency than Pro12. Adaptation of humans for lipolysis, thermogenesis, and reduction of fat accumulation could be considered by examining which alleles in these genes are dominant in non-human primates (NHP). All NHP (P. troglodytes, G. gorilla, P. pygmaeus, H. agilis and macaques) had energy-thrifty alleles, Gly16 and Glu27 in ADRB2, and Arg64 in ADRB3, but did not have energy-expense alleles, Arg16, Gln27 and Trp64 alleles. In PPARG gene, all NHP had large adipocyte accumulating type, the Pro12 allele.

CONCLUSIONS

These results indicate that a tendency to produce much more heat through the energy-expense alleles developed only in humans, who left tropical rainforests for savanna and developed new features in their heat-regulation systems, such as reduction of body hair and increased evaporation of water, and might have helped the protection of entrails from cold at night, especially in glacial periods.

A polymorphic indel containing the RS3 microsatellite in the 5' flanking region of the vasopressin V1a receptor gene is associated with chimpanzee (Pan troglodytes) personality

Vasopressin is a neuropeptide that has been strongly implicated in the development and evolution of complex social relations and cognition in mammals. Recent studies in voles have shown that polymorphic variation in the promoter region of the arginine vasopressin V1a receptor gene (avpr1a) is associated with different dimensions of sociality. In humans, variation in a repetitive sequence element in the 5' flanking region of the AVPR1A, known as RS3, have also been associated with variation in AVPR1a gene expression, brain activity and social behavior. Here, we examined the association of polymorphic variation in this same 5' flanking region of the AVPR1A on subjective ratings of personality in a sample of 83 chimpanzees (Pan troglodytes). Initial analyses indicated that 34 females and 19 males were homozygous for the short allele, which lacks RS3 (DupB(-/-)), while 18 females and 12 males were heterozygous and thus had one copy of the long allele containing RS3 (DupB(+/-)), yielding overall allelic frequencies of 0.82 for the DupB(-) allele and 0.18 for the DupB(+) allele. DupB(+/+) chimpanzees were excluded from the analysis because of the limited number of individuals. Results indicated no significant sex difference in personality between chimpanzees homozygous for the deletion of the RS3-containing DupB region (DupB(-/-)); however, among chimpanzees carrying one allele with the DupB present (DupB(+/-)), males had significantly higher dominance and lower conscientiousness scores than females. These findings are the first evidence showing that the AVPR1A gene plays a role in different aspects of personality in male and female chimpanzees.

Advances in tryptophan hydroxylase-2 gene expression regulation: new insights into serotonin-stress interaction and clinical implications

Serotonin (5-HT) modulates the stress response by interacting with the hormonal hypothalamic-pituitary-adrenal (HPA) axis and neuronal sympathetic nervous system (SNS). Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in 5-HT biosynthesis, and the recent identification of a second, neuron-specific TPH isoform (TPH2) opened up a new area of research. While TPH2 genetic variance has been linked to numerous behavioral traits and disorders, findings on TPH2 gene expression have not only reinforced, but also provided new insights into, the long-recognized but not yet fully understood 5-HT-stress interaction. In this review, we summarize advances in TPH2 expression regulation and its relevance to the stress response and clinical implications. Particularly, based on findings on rhesus monkey TPH2 genetics and other relevant literature, we propose that: (i) upon activation of adrenal cortisol secretion, the cortisol surge induces TPH2 expression and de novo 5-HT synthesis; (ii) the induced 5-HT in turn inhibits cortisol secretion by modulating the adrenal sensitivity to ACTH via the suprachiasmatic nuclei (SCN)-SNS-adrenal system, such that it contributes to the feedback inhibition of cortisol production; (iii) basal TPH2 expression or 5-HT synthesis, as well as early-life experience, influence basal cortisol primarily via the hormonal HPA axis; and (iv) 5'- and 3'-regulatory polymorphisms of TPH2 may differentially influence the stress response, presumably due to their differential roles in gene expression regulation. Our increasing knowledge of TPH2 expression regulation not only helps us better understand the 5-HT-stress interaction and the pathophysiology of neuropsychiatric disorders, but also provides new strategies for the treatment of stress-associated diseases.