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The etiology of anxiety disorders is multifactorial with contributions from both
genetic and environmental factors. Several susceptibility genes of anxiety disorders or
anxiety-related intermediate phenotypes have been identified, including the
serotonin transporter gene (5-HTT) and the neuropeptide S receptor gene (NPSR1),
which have been shown to modulate responses to distal and acute stress experiences.
For instance, gene-environment interaction (GxE) studies have provided evidence
that both 5-HTT and NPSR1 interact with environmental stress, particularly
traumatic experiences during childhood, in the moderation of anxiety traits, and
both 5-HTT and NPSR1 have been implicated in hypothalamic-pituitary-adrenal
(HPA) axis reactivity – an intermediate phenotype of mental disorders – in response
to acute stress exposure. The first part of this thesis aimed to address the interplay of
variations in both 5-HTT and NPSR1 genes and distal stress experiences, i.e.
childhood trauma, in the moderation of anxiety-related traits, extended by
investigation of the potentially protective effect of positive influences, i.e. elements of
successful coping such as general self-efficacy (GSE), on a GxE risk constellation by
introducing GSE as an indicator of coping ability (“C”) as an additional dimension in
a GxExC approach conferring – or buffering – vulnerability to anxiety. Increased
anxiety was observed in 5-HTTLPR/rs25531 LALA genotype and NSPR1 rs324981 AA
genotype carriers, respectively, with a history of childhood maltreatment but only in
the absence of a person’s ability to cope with adversity, whereas a dose-dependent
effect on anxiety traits as a function of maltreatment experiences irrespective of
coping characteristics was observed in the presence of at least one 5-HTT S/LG or
NSPR1 T allele, respectively. The second part of this thesis addressed the respective
impact of 5-HTT and NPSR1 variants on the neuroendocrine, i.e. salivary cortisol
response to acute psychosocial stress by applying the Maastricht Acute Stress Test
(MAST). A direct effect of NPSR1 – but not 5-HTT – on the modulation of acute
stress reactivity could be discerned, with carriers of the more active NPSR1 T allele
Summary
III
displaying significantly higher overall salivary cortisol levels in response to the MAST
compared to AA genotype carriers.
In summary, study 1 observed a moderating effect of GSE in interaction with
childhood maltreatment and 5-HTT and NPSR1, respectively, in an extended GxExC
model of anxiety risk, which may serve to inform targeted preventive interventions
mitigating GxE risk constellations and to improve therapeutic interventions by
strengthening coping ability as a protective mechanism to promote resilient
functioning. In study 2, a modulation of HPA axis function, considered to be an
endophenotype of stress-related mental disorders, by NPSR1 gene variation could be
discerned, suggesting neuroendocrine stress reactivity as an important potential
intermediate phenotype of anxiety given findings linking NPSR1 to dimensional and
categorical anxiety. Results from both studies may converge within the framework of
a multi-level model of anxiety risk, integrating neurobiological, neuroendocrine,
environmental, and psychological factors that act together in a highly complex
manner towards increasing or decreasing anxiety risk.
Sustained anxiety is considered as a chronic and future-oriented state of apprehension that does not belong to a specific object. It is discussed as an important characteristic of anxiety disorders including panic disorder, generalized anxiety disorder (GAD) and posttraumatic stress disorder (PTSD). Experimentally, sustained anxiety can be induced by contextual fear conditioning in which aversive events are unpredictably presented and therefore the whole context becomes associated with the threat. This thesis aimed at investigating important mechanisms in the development and maintenance of sustained anxiety: (1) facilitated acquisition and resistant extinction of contextual anxiety due to genetic risk factors (Study 1), and (2) the return of contextual anxiety after successful extinction using a new reinstatement paradigm (Study 2). To this end, two contextual fear conditioning studies were conducted in virtual reality (VR). During acquisition one virtual office was paired with unpredictable mildly painful electric stimuli (unconditioned stimulus, US), thus becoming the anxiety context (CXT+). Another virtual office was never paired with any US, thus becoming the safety context (CXT-). Extinction was conducted 24 h later, i.e. no US was presented, and extinction recall was tested another 24 h later on Day 3. In both studies context-evoked anxiety was measured on three different response levels: behavioral (anxiety-potentiated startle reflex), physiological (skin conductance level), and verbal (explicit ratings). In Study 1, participants were stratified for 5-HTTLPR (S+ risk allele vs. LL no risk allele) and NPSR1 rs324981 (T+ risk allele vs. AA no risk allele) polymorphisms, resulting in four combined genotype groups with 20 participants each: S+/T+, S+/LL, LL/T+, and LL/AA. Results showed that acquisition of anxiety-potentiated startle was influenced by a gene × gene interaction: only carriers of both risk alleles (S+ carriers of the 5-HTTLPR and T+ carriers of the NPSR1 polymorphism) exhibited significantly higher startle magnitudes in CXT+ compared to CXT-. However, extinction recall as measured with anxiety-potentiated startle was not affected by any genotype. Interestingly, the explicit anxiety level, i.e. valence and anxiety ratings, was only influenced by the NPSR1 genotype, in a way that no risk allele carriers (AA) reported higher anxiety and more negative valence in response to CXT+ compared to CXT-, whereas risk allele carriers (T+) did not. Study 2 adopted nearly the same paradigm with the modification that one group (reinstatement group) received one unsignaled US at the beginning of the experimental session on Day 3 before seeing CXT+ and CXT-. The second group served as a control group and received no US, but was immediately exposed to CXT+ and CXT-. Results showed a return of anxiety on the implicit and explicit level (higher startle responses and anxiety ratings in response to CXT+ compared to CXT-) in the reinstatement group only. Most important, the return of contextual anxiety in the reinstatement group was associated with a change of state anxiety and mood from extinction to test, that is the more anxiety and negative mood participants experienced before the reinstatement procedure, the higher their return of anxiety was. In sum, results of Study 1 showed that facilitated contextual fear conditioning on an implicit behavioral level (startle response) could be regarded as an endophenotype for anxiety disorders, which can contribute to our understanding of the etiology of anxiety disorders. Results of Study 2 imply that anxiety and negative mood after extinction could be an important facilitator for the return of anxiety. Furthermore, the present VR-based contextual fear conditioning paradigm seems to be an ideal tool to experimentally study mechanisms underlying the acquisition and the return of anxiety. Future studies could investigate clinical samples and extend the VR paradigm to evolutionary-relevant contexts (e.g., heights, darkness, open spaces).
The present work investigated the neural mechanisms underlying cognitive inhibition/thought suppression in Anderson’s and Green’s Think/No-Think paradigm (TNT), as well as different variables influencing these mechanisms at the cognitive, the neurophysiological, the electrophysiological and the molecular level. Neurophysiological data collected with fNIRS and fMRI have added up to the existing evidence of a fronto-hippocampal network interacting during the inhibition of unwanted thoughts. Some evidence has been presented suggesting that by means of external stimulation of the right dlPFC through iTBS thought suppression might be improved, providing further evidence for an implication of this region in the TNT. A combination of fNIRS with ERP has delivered evidence of a dissociation of early condition-independent attentional and later suppression-specific processes within the dlPFC, both contributing to suppression performance. Due to inconsistencies in the previous literature it was considered how stimulus valence would influence thought suppression by manipulating the emotional content of the to-be-suppressed stimuli. Findings of the current work regarding the ability to suppress negative word or picture stimuli have, however, been inconclusive as well. It has been hypothesized that performance in the TNT might depend on the combination of valence conditions included in the paradigm. Alternatively, it has been suggested that inconsistent findings regarding the suppression of negative stimuli or suppression at all might be due to certain personality traits and/or genetic variables, found in the present work to contribute to thought inhibition in the TNT. Rumination has been shown to be a valid predictor of thought suppression performance. Increased ruminative tendencies led to worse suppression performance which, in the present work, has been linked to less effective recruitment of the dlPFC and in turn less effective down-regulation of hippocampal activity during suppression trials. Trait anxiety has also been shown to interrupt thought suppression despite higher, however, inefficient recruitment of the dlPFC. Complementing the findings regarding ruminative tendencies and decreased thought inhibition a functional polymorphism in the KCNJ6 gene, encompassing a G-to-A transition, has been shown to disrupt thought suppression despite increased activation of the dlPFC. Through the investigation of thought suppression at different levels, the current work adds further evidence to the idea that the TNT reflects an executive control mechanism, which is sensitive to alterations in stimulus valence to some extent, neurophysiological functioning as indicated by its sensitivity to iTBS, functional modulations at the molecular level and personality traits, such as rumination and trait anxiety.