The 5HTT and Behavior
Primates exhibit polymorphisms in the gene for 5HTT (SLC6A4), based on the number of variable repeat sequences appearing in the promoter region of the gene.1,9,15-18
In humans, these polymorphisms include short (S) and long alleles (L), recognized more than a decade ago, as well as more recently discovered long-allele variants, based on single-nucleotide A→G substitution yielding two variants (LG
). One variant (LG
) behaves physiologically like the S allele, whereas the other (LA
) behaves differently from the other two alleles (S and LG
). Relative to the LA
allele, the S and LG
alleles are associated with a decreased capacity for serotonin reuptake, the main function of the 5HTT protein.
Many scientists find the 5HTT alleles to be of considerable relevance for translational research on developmental psychopathology. Research reliably demonstrates in vitro functional differences among 5HTT alleles that parallel the effects of selective-serotonin reuptake inhibitors. This research in turn reliably extends considerable work linking 5HT to behavior. For example, manipulations of 5HTT function in rodents and nonhuman primates affect behaviors relevant to psychopathology, such as an organism's response to threats or rewards.9
Specifically, at least in rodents, reducing 5HTT activity early in life increases measurements of fear assessed much later in life; this effect does not occur in mature rodents, thereby linking 5HTT, behavior, and development. However, despite agreement on the importance of studying 5HTT, disagreement surrounds some aspects of the research.
One area of disagreement concerns the expected magnitude of associations between SLC6A4 and psychopathology. Initial findings did relate the 5HTT gene to marked variations in neuroticism or harm avoidance, and subsequent studies extended the findings to other phenotypes, including anxiety and major depressive disorders.9
However, recent meta-analyses suggested that associations are weaker than originally thought.15
This supports conclusions in other work noting that genetically complex phenotypes, such as mood and anxiety disorders, show small associations with individual genes.19-21
Subsequent work then suggested that large effects manifest in high-stress environments, due to powerful gene-by-environment interactions.9
However, here, too, recent meta-analyses suggested that this may not be the case.18
Imaging Genetics, Child Psychiatry, and the 5HTT
Most scientists agree that major advances can accrue from research in humans that directly links brain function to genes. Without measurements of brain function, work attempting to link genetically mediated variation in 5HTT function to behavior is not easily integrated with work in rodents and nonhuman primates, given inadequacies in animal models of psychopathology.9,19-21
Basic-to-clinical translation is easier when it is based on research in multiple species, each relating the 5HTT to amygdala–prefrontal cortex (PFC) function.1,2,5,21
The advantage of work in this area reflects the fact that highly similar phenotypes can be studied in different species. This approach is easier to apply for particular phenotypes, such as fear conditioning or attention orienting,5
where data demonstrate similar brain–behavior associations in rodents, nonhuman primates, and humans. Other phenotypes, such as those emphasizing data from verbal reports, are less easily applied across species. Moreover, when comparable phenotypes are studied, invasive experiments in animal models can probe the manner in which specific polymorphisms shape brain functions studied in imaging. Using such invasive techniques, neuroscientists can elucidate the manner in which functionality of polymorphisms maps onto behaviors and brain functions with precision that is not achievable in less invasive studies performed with humans.
Controversy has arisen about some aspects of 5HTT imaging–genetics.19,20,22
Data in healthy adults link the SLC6A4 genotype to an amygdala–PFC circuit.1,16,17
The most consistent finding17
is that healthy adults with low-activity variants of the 5HTT gene manifest greater amygdala threat-related reactivity than do those with high-activity variants. This led to suggestions that relations between the SLC6A4 genotype and diagnosis are weak because they are indirect and attenuated by diverse sources of noise. It was further suggested that the association between the SLC6A4 genotype and amygdala–PFC function is more direct and, hence, stronger. However, there is controversy about the reasonableness of these suggestions, specifically with regard to the expected magnitude of associations between genes and imaging.19-22
Initial work suggested the presence of large effects,1
but contrary views suggest that imaging–genetics is vulnerable to similar weaknesses plaguing other psychiatric–genetics studies.19-23
These weaknesses are attributed to incorrect assumptions about the simplicity of gene effects on neuroimaging and other neuroscience-based phenotypes, which may indeed exhibit genetic architectures that are as complex as clinical phenotypes, yielding small genotype–phenotype associations.
Clearly, imaging–genetics has promise and complexity, and important work focuses on structural and functional correlates of genetic variability in amygdala–PFC circuitry. In research on 5HTT, one set of complexities concerns interpretations of amygdala-activation data in adults. Initial interpretations emphasized the role of decreased 5HTT function in threat hypersensitivity, consistent with work in rodents and nonhuman primates showing that decreased 5HTT function produces threat hypersensitivity through effects on the amygdala.1,9,16
However, initial fMRI findings in humans were open to other interpretations, in part because fMRI lacks absolute quantification. Thus, for example, between-group differences in amygdala activity in response to neutral and threatening faces can reflect increased responses to one stimulus (e.g., threatening faces) or decreased responses to the other (e.g., neutral faces). Although the initial studies suggested that low-activity 5HTT alleles predict enhanced threat
responsiveness, more recent work used additional fMRI conditions. These more recent studies showed that low-activity 5HTT alleles predict normal
amygdala response to threats, in tandem with reduced
response to neutral
stimuli, which could explain the previous findings.24,25
These new findings generate novel research opportunities and raise questions about the comparability of imaging–genomic and basic 5HTT data on threat responding. Development further complicates interpretations: fMRI data suggest that the amygdala responds more strongly to threats in adolescents than adults, at least under some circumstances.26
Therefore, although there may be associations between the 5HTT genotype and responses to neutral stimuli in adults, in youth there may be associations between the 5HTT genotype and responses to threat stimuli. Work in rodents finds interrelated developmental differences in amygdala function, response to 5HTT manipulations, and threat responding.9
As a result, it is important to establish in humans the degree to which the SLC6A4 genotype specifically affects responding to neutral or emotional stimuli at various ages. Adding yet further complexity to this emerging set of findings, studies using positron emission tomography find no relation between the SLC6A4 genotype and in vivo measurements of 5HTT binding potential in adults.27
Although many explanations could produce such unexpected, negative findings, some suggest that they result from development: effects of genetic variation in 5HTT may shape behavior early in life in ways that are not reflected in adult measurements of 5HTT binding potential.
Other complexities concern the contexts under which amygdala hypersensitivity manifests. Early interpretations of imaging–genetics 5-HTT data treated amygdala hyperactivity as a relatively static phenomenon, correlated with genotype. However, amygdala activity is plastic, changing with the context of experimental tasks.28
Between-group differences in amygdala function related to anxiety or age only manifest in specific experimental contexts.26,29
One would expect a similar context dependency on genetic effects.
Ideally, fMRI experiments designed to elucidate associations between amygdala activity and genetic predictors of psychopathology should use particular tasks. These tasks should be selected based on their ability to elicit, in studies in patients and healthy subjects, between-group differences in task performance or autonomic physiology. This approach generates research that directly links many of the processes depicted in . Unfortunately, most 5HTT imaging–genetics work uses tasks that do elicit strong amygdala responses in subjects, considered as a group, but do not possess such prior knowledge linking neural system function, behavior in the laboratory, and clinical features. Thus, clinical applicability is limited in most published work, because it is does not use tasks sensitive to psychopathology-related perturbations that manifest on the employed cognitive tasks. Consideration of experimental context appears particularly important in child psychiatry, given the powerful effects of context on children's behavior and brain function. As such, the experimental context ideally suited for eliciting between-group differences in children may contrast with that ideally suited for adults.
Only one pediatric fMRI study has examined the relation between the SLC6A4 genotype and amygdala function.30
Unlike prior studies in adults, this study used an experimental task where prior independent data demonstrated between-group differences in task performance relevant to mood and anxiety disorders.31
The study then mapped 5HTT-related associations with amygdala function under task conditions previously shown to elicit behavioral differences between controls and adolescents with familial or personal risk for mood and anxiety disorders.
shows data from two fMRI studies of amygdala function, i.e., the pediatric imaging–genetics study of the SLC6A4 genotype discussed above,30
and a study on adolescent anxiety and major depressive disorders.32
Three findings emerged from these studies. First, as expected, there was an association between enhanced amygdala response and the low-activity 5HTT S or LG
alleles in 33 healthy adolescents (). This occurred specifically when adolescents rated levels of fear experienced while viewing fear faces. Second, like healthy adolescent carriers of the low-activity 5HTT alleles, adolescents with anxiety or major depressive disorders showed a greater amygdala response than did healthy adolescents while rating fear levels to fear faces (). Third, genetics and diagnosis interacted: in the 31 psychiatrically impaired adolescents, genetic associations with amygdala function exhibited an opposite
trend in these patients as had manifested in healthy adolescents (). Thus, individual differences related to genes and to diagnosis may shape amygdala response to threats in youth.
FIGURE 2 5-HTT imaging–genetics in adolescents. Note: Illustrated are findings from two studies on relations among amygdala function, 5-HTT genotype, and developmental psychopathology. Amygdala topography is displayed on the left-hand side, in an axial (more ...)
These two studies may provide important clues on the modulating effects from psychopathology in imaging–genetics. Few prior fMRI studies in any age group assessed such modulation by simultaneously studying healthy and impaired subjects. However, given the small samples and existing complexities, these data should encourage more definitive studies in larger samples, studied under experimental conditions where links to psychopathology have been established through independent studies of information processing.