Elsevier

NeuroImage

Volume 62, Issue 1, 1 August 2012, Pages 130-136
NeuroImage

5-HTTLPR status predictive of neocortical 5-HT4 binding assessed with [11C]SB207145 PET in humans

https://doi.org/10.1016/j.neuroimage.2012.05.013Get rights and content

Abstract

Serotonin (5-HT) is a neuromodulator affecting myriad aspects of personality and behavior and has been implicated in the pathophysiology of affective disorders including depression and anxiety. The 5-HTTLPR is a common genetic polymorphism within the promoter region of the gene coding for the serotonin transporter such that the S allele is associated with reduced transcriptional efficacy compared to the L allele, potentially contributing to increased serotonin levels. In humans, this genetic variant has been linked to inter-individual variability in risk for affective disorders, related aspects of personality and brain function including response to threat. However, its effects on aspects of serotonin signaling in humans are not fully understood.

Studies in animals suggest that the 5-HT 4 receptor (5-HT4) shows a monotonic inverse association with long-term changes in serotonin levels indicating that it may be a useful measure for identifying differences in serotonergic neurotransmission. In 47 healthy adults we evaluated the association between 5-HTTLPR status and in vivo 5-HT4 receptor binding assessed with [11C]SB207145 positron emission tomography (PET).

We observed a significant association within the neocortex where [11C]SB207145 binding was 9% lower in S carriers compared to LL homozygotes. We did not find evidence for an effect of season or a season-by-5-HTTLPR interaction effect on regional [11C]SB207145 binding.

Our findings are consistent with a model wherein the 5-HTTLPR S allele is associated with relatively increased serotonin levels. These findings provide novel evidence supporting an effect of 5-HTTLPR status on serotonergic neurotransmission in adult humans. There were no indications of seasonal effects on serotonergic neurotransmission.

Highlights

► We evaluate if the 5-HTTLPR variant predicts serotonin 4 receptor (5-HT4) binding. ► We assess 5-HT4 binding with [11C]SB207145 PET in 47 healthy human volunteers. ► 5-HTTLPR status significantly predicts neocortical [11C]SB207145 binding. ► S allele carriers show 9% reduced binding across neocortex versus LL homozygotes. ► This further suggests that 5-HTTLPR affects 5-HT neurotransmission in adult humans.

Introduction

Serotonin (5-hydroxytryptamine, 5-HT) is a neuromodulator with significant effects on emotional behavior, including anxiety and sensitivity to threat (Blier and de Montigny, 1998, Lucki, 1998). Genetic polymorphisms that putatively affect serotonin signaling have been associated with affective disorders (e.g., depression and anxiety disorders) and aspects of personality that confer risk for these disorders (Caspi et al., 2003, Fakra et al., 2009, Lemonde et al., 2003, Wankerl et al., 2010). A clearer understanding of how these genetic variants modulate aspects of serotonin signaling is critical for leveraging that genetic information to model underlying brain chemistry and brain function. In turn, this knowledge can be used to identify how underlying serotonin signaling and its effects on brain function may contribute to variability in personality and risk for affective disorders (Hariri, 2009). Imaging genetics using neuroreceptor binding positron emission tomography (PET) has emerged as an approach for evaluating the impact of genetic variants on molecular mechanisms in humans, in vivo. The 5-HTTLPR is a common genetic variant within the gene (SLC6A4) coding for the serotonin transporter (5-HTT) where the ‘long’ (L) allele exhibits increased serotonin transporter (5-HTT) transcription in vitro relative to the ‘short’ (S) allele, putatively affecting 5-HT signaling (Lesch et al., 1996). As discussed within a recent review of genetic sources of variability in serotonin signaling as measured with PET, there is evidence suggesting that 5-HTTLPR status is associated with individual differences in aspects of serotonin signaling (Willeit and Praschak-Rieder, 2010).

Previous studies have reported reduced 5-HTT binding in S carriers with the PET radioligand [11C]DASB(Kalbitzer et al., 2009, Praschak-Rieder et al., 2007, Reimold et al., 2007), though other studies, including one using the PET radioligand [11C]McN5652, have reported no relation (Murthy et al., 2010, Parsey et al., 2006a). Additionally, the 5-HTTLPR S allele has been associated with decreased 5-HT type 1A receptor (5-HT1A) binding (David et al., 2005), though other studies have reported no relation (Borg et al., 2009). Recent studies also suggest that genetic variation putatively modulating 5-HT signaling may moderate the effect of environmental factors on 5-HT signaling, such as seasonal variation (Kalbitzer et al., 2010). This is consistent with a growing corpus of research suggesting that consideration of environmental triggers may be integral for identifying the impact of serotonin-related genetic variants on brain chemistry and related effects on behavior (Caspi et al., 2010). Though numerous studies have applied imaging genetics to identify serotonergic mechanisms modulated by common genetic polymorphisms, their effect on serotonin signaling is not fully understood (Willeit and Praschak-Rieder, 2010). Notably, identifying genetic sources of variability in serotonin signaling is limited by the capacity to effectively quantify it in vivo with PET (Paterson et al., 2010).

Recently, our lab has validated the quantitative use of a novel radioligand, [11C]SB207145, for measuring 5-HT type 4 receptor (5-HT4) binding with PET in humans (Marner et al., 2009, Marner et al., 2010). Recent animal studies have reported that cerebral 5-HT4 binding shows a monotonic response to pharmacologically induced changes in cerebral 5-HT levels, at least when exposure had lasted 2–3 weeks (Licht et al., 2009, Vidal et al., 2009). Furthermore, genetically modified expression of the gene coding for the 5-HTT induced significant alterations in 5-HT4 density in mice (Jennings et al., 2011). Specifically, mice over-expressing 5-HTT (i.e., resulting in reduced 5-HT signaling) showed increased 5-HT4 density while knock-out of the gene coding for 5-HTT (i.e., resulting in increased 5-HT signaling) resulted in reduced 5-HT4 density. From studies in humans we also know that acute blockade of 5-HTT with citalopram, putatively resulting in increased 5-HT signaling, did not modify [11C]SB207145 binding (Marner et al., 2010). Taken together, these findings suggest that [11C]SB207145 PET may be a useful proxy for chronically, but not acutely, altered cerebral 5-HT levels. Thus, evaluating the impact of 5-HTTLPR status on [11C]SB207145 binding would benefit our understanding of how this polymorphism may bias cerebral 5-HT neurotransmission in humans, in vivo.

In the current study, we sought to determine whether 5-HTTLPR status predicted [11C]SB207145 binding in 47 healthy adult volunteers. Based on the anticipated effects of the 5-HTTLPR on synaptic 5-HT signaling and the monotonic association between 5-HT4 levels and 5-HT signaling, we hypothesized that S carriers would exhibit reduced [11C]SB207145 binding relative to L/L individuals. Additionally, we evaluated whether seasonal fluctuation or daylight minutes moderated the effect of 5-HTTLPR status on [11C]SB207145 binding, hypothesizing that the S allele would be associated with greater season-associated fluctuations in [11C]SB207145 binding reflecting a diminished capacity for regulating 5-HT signaling in response to an environmental stressor (Kalbitzer et al., 2010).

Section snippets

Participants

Forty-seven healthy participants (35 males; age: mean ± s.d. = 34.9 ± 17.7, median = 26.9, range = 20–86) were recruited by advertisement for a research protocol approved by the Ethics Committee of Copenhagen and Frederiksberg, Denmark [KF-01-274821, KF-11-282837, H-KF-274821]. After complete description of the study to the participants, written informed consent was obtained from all participants. Exclusion criteria for all participants included 1) primary psychiatric disease, 2) substance or drug abuse

[11C]207145 binding and demographic variables

A summary of demographic variables and PET parameters can be found in Table 1. Genotype groups based on observed allele frequency were in Hardy–Weinberg equilibrium (X2 = 0.09, p = 0.77). Differences between genotype groups for demographic variables and PET parameters can be found in Table 1. Age was significantly different between 5-HTTLPR genotype groups such that LL individuals were older than S carriers (p = 0.007). Additionally, we observed differences in BMI and weight-adjusted injected mass

Discussion

We used an imaging genetics approach to evaluate the association between 5-HTTLPR status and [11C]SB207145 binding, assessed using in vivo PET, within a population of healthy adults with no history of psychopathology. Consistent with our hypothesis, we identified an association between 5-HTTLPR status and [11C]SB207145 binding such that S carriers exhibited reduced binding relative to L/L homozygotes. Specifically, we observed this effect within the neocortex and a trend for a similar effect

Conclusions

In summary, we find evidence for an association between 5-HTTLPR status and 5-HT4 binding, assessed with [11C]SB207145 PET, such that S carriers display reduced neocortical binding relative to LL homozygotes. We do not observe evidence for an effect of season on [11C]SB207145 binding, which may reflect regulation of 5-HT signaling in response to seasonal variation via alternative mechanisms (e.g., regulation of 5-HTT levels). Considering animal studies that indicate the 5-HT4 receptor shows a

Financial disclosures

The authors declare no conflict of interest.

Acknowledgments

We would like to thank G. Thomsen, S. Larsen, A. Dyssegaard, L. Bech, K. Christiansen and L. Freyr for their assistance in scheduling and data collection at both the MR and PET centers. We would like to gratefully acknowledge The John and Birthe Meyer Foundation for the donation of the Cyclotron and PET-scanner. This study was funded by a center grant to Cimbi from the Lundbeck Foundation. Data presented here were also presented by Dr. Fisher as a poster at the 2011 European Congress of

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