Abstract
Fear acquisition and extinction are crucial mechanisms in the etiology and maintenance of anxiety disorders. Moreover, they might play a pivotal role in conveying the influence of genetic and environmental factors on the development of a (more or less) stronger proneness for, or resilience against psychopathology. There are only few insights in the neurobiology of genetically and environmentally based individual differences in fear learning and extinction. In this functional magnetic resonance imaging study, 74 healthy subjects were investigated. These were invited according to 5-HTTLPR/rs25531 (S+ vs. LALA; triallelic classification) and TPH2 (G(-703)T) (T+ vs. T-) genotype. The aim was to investigate the influence of genetic factors and traumatic life events on skin conductance responses (SCRs) and neural responses (amygdala, insula, dorsal anterior cingulate cortex (dACC) and ventromedial prefrontal cortex (vmPFC)) during acquisition and extinction learning in a differential fear conditioning paradigm. Fear acquisition was characterized by stronger late conditioned and unconditioned responses in the right insula in 5-HTTLPR S-allele carriers. During extinction traumatic life events were associated with reduced amygdala activation in S-allele carriers vs. non-carriers. Beyond that, T-allele carriers of the TPH2 (G(−703)T) polymorphism with a higher number of traumatic life events showed enhanced responsiveness in the amygdala during acquisition and in the vmPFC during extinction learning compared with non-carriers. Finally, a combined effect of the two polymorphisms with higher responses in S- and T-allele carriers was found in the dACC during extinction. The results indicate an increased expression of conditioned, but also unconditioned fear responses in the insula in 5-HTTLPR S-allele carriers. A combined effect of the two polymorphisms on dACC activation during extinction might be associated with prolonged fear expression. Gene-by-environment interactions in amygdala and vmPFC activation may reflect a neural endophenotype translating genetic and adverse environmental influences into vulnerability for or resilience against developing affective psychopathology.
Highlights
More than 25% of the western population develops an anxiety or mood disorder at least once in a lifetime [1]
Enhanced startle potentiation but not skin conductance responding to the conditioned stimulus (CS+) has been observed in S-allele carriers compared with noncarriers during fear conditioning [17]
General Unconditioned and Conditioned Responses General unconditioned and conditioned neural responses during the first and second half of acquisition have been found in all Region of interest (ROI) in the whole group
Summary
More than 25% of the western population develops an anxiety or mood disorder at least once in a lifetime [1]. Not every individual encountering a highly stressful situation develops an anxiety or mood disorder [2,4] These individual differences may in part be genetically driven and/or based on prior learning experiences including traumatic life events [10]. Garpenstrand and colleagues [16] found an overrepresentation of the 5-HTTLPR S-allele in 20 individuals that showed good acquisition compared with 20 individuals that showed bad acquisition of conditioned SCRs (preselected from a group of 346 fear conditioned individuals). These Sallele carriers were characterized by stronger fear conditioned SCRs compared with non-carriers. There is one study that utilized an instructed fear paradigm showed that S-allele carriers compared with LL-homozygotes exhibit stronger fear potentiated startle responses [21]
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