Genetic variation in serotonin transporter function affects human fear expression indexed by fear-potentiated startle

Abstract

The serotonin transporter (SERT) plays a crucial role in anxiety. Accordingly, variance in SERT functioning appears to constitute an important pathway to individual differences in anxiety. The current study tested the hypothesis that genetic variation in SERT function is associated with variability in the basic reflex physiology of defense. Healthy subjects (N = 82) were presented with clearly instructed cues of shock threat and safety to induce robust anxiety reactions. Subjects carrying at least one short allele for the 5-HTTLPR polymorphism showed stronger fear-potentiated startle compared to long allele homozygotes. However, short allele carriers showed no deficit in the downregulation of fear after the offset of threat. These results suggest that natural variation in SERT function affects the magnitude of defensive reactions while not affecting the capacity for fear regulation.

Introduction

The serotonin transporter plays a crucial role in anxiety. Accordingly, pharmacological adjustment of serotonin transporter (SERT) functioning by selective serotonin reuptake inhibitors (SSRIs) is among the most established clinical pharmacological strategies to control human anxiety disorders (Baldwin et al., 2005, Bandelow et al., 2002). It is therefore to be expected that genetic variation in SERT function underlies innate differences in individuals’ fear reactivity. In the current study we investigated how genetic variance in the SERT gene affects the up and down regulation of basic fear responses.

To this end we studied the impact of a well-known polymorphism in the promoter region of the SERT gene (also called 5-HTTLPR), which influences anxiety-related personality (Lesch et al., 1996). This common 5-HTTLPR polymorphism consists of an insertion/deletion of 43 bp in the 5′ regulatory region of the gene, resulting in either a long or short allele. The short allele is associated with reduced SERT transcriptional activity in vitro (Heils et al., 1996). Meta-analyses have more recently confirmed that carriers of the 5-HTTLPR short allele (S-carriers) report more anxiety related personality traits (Schinka et al., 2004, Sen et al., 2004). However effect sizes are small (Munafo et al., 2009) and the relation between this polymorphism and anxiety- and stress related psychopathology remains a matter of debate (Karg et al., 2011, Lonsdorf et al., 2009a, Wankerl et al., 2010).

A potentially fruitful approach to further investigate sequelae of genetic variability in serotonin transporter function is by establishing intermediate phenotypes associated with the 5-HTTLPR polymorphism (Canli, 2008, Domschke and Dannlowski, 2010). For example, imaging studies showed exacerbated amygdala activation towards fear cues in S-carriers (Hariri et al., 2002, Munafo et al., 2008). Another approach has been to compare 5-HTTLPR genotypes on peripheral psychophysiological measures such as skin conductance and startle during fear conditioning. In a classical conditioning study, S-carriers were more likely to show strong conditioned SCR responses than LL homozygotes who were found more often in a group of subjects that showed weak conditioning (Garpenstrand et al., 2001). More recently, evidence was presented for stronger SCRs in S-carriers during vicarious conditioning (Crisan et al., 2009). Finally, Lonsdorf et al. (2009b) reported that across the acquisition phase of a fear conditioning session, S-carriers showed stronger potentiation of the startle response. Potentiation of the startle reflex is a reliable index of the activation of the defensive system (Bradley et al., 2005) and is widely used as an objective and rather specific measure of fearful responding (Grillon and Baas, 2003, Hamm and Weike, 2005). Taken together these results suggest that increased neural threat processing in S-carriers as witnessed by stronger amygdala activations, may be reflected in autonomic measures of fear and defensive reflexes. Moreover, S-carriers may show altered amygdala regulation by the prefrontal cortex (PFC) (Heinz et al., 2005, Pacheco et al., 2009, Pezawas et al., 2005). Since the interaction between these regions mediates fear regulation (Hartley and Phelps, 2010, Quirk and Beer, 2006), this implies that increased anxiety in S-carriers could stem from a genetic deficit in the capacity to suppress fear.

Here we investigated further how the 5-HTTLPR polymorphism affects fear reactions and the downregulation of fear responses. An instructed fear paradigm was employed, in which fear reactions were elicited in healthy volunteers by presenting cues that are identified before the experiment as signaling threat of electric shock (Bocker et al., 2004, Grillon et al., 1991, Grillon et al., 1993). Moreover, startle was also measured after the termination of threat cues, during the transition from threat to a period of relative safety. In this way we could index how well subjects of each genotype were able to reduce fear when direct danger subsided. Previous work indicated that the capacity to return to a resting state after threat offset depends on prefrontal-limbic interactions (Klumpers et al., 2010a). Based on the evidence that S-carriers display increased fear reactivity which perhaps relates to altered prefrontal control, we hypothesized that S-carriers would (a) demonstrate greater reactivity to threat cues by showing stronger startle potentiation and (b) would show a reduced capacity for fear regulation as reflected in a slower decline of startle potentiation after the offset of threat cues.