The principal function of the serotonin transporter is to remove serotonin from the synapse, returning it to the presynaptic neuron where the neurotransmitter can be degraded or re-released at a later time. A polymorphism in the promoter region of the serotonin transporter gene (5-HTTLPR) has been found to affect the transcription rate of the gene, with the short (s) allele transcriptionally less efficient that the alternate long (l) allele. In 2003, Caspi and colleagues examined the relationship between 5-HTTLPR, stress and depression using a prospective, longitudinal birth cohort and found that subjects carrying the less functional 5-HTTLPR s allele reported greater sensitivity to stress1.

This study has been cited over 2000 times in the scientific literature and generated a great deal of excitement and controversy around the potential of gene × environment interaction studies2. To date, there have been 55 follow-up studies, exploring whether 5-HTTLPR moderates the relationship between stress and depression, with some studies supporting the association between the 5-HTTLPR s allele and greater stress sensitivity and others not. Two recent meta-analyses have assessed a subset of these studies and concluded that there is no evidence supporting the presence of genetic moderation3, 4.

Since their publication, these meta-analyses have been criticized for only including a subset of the studies investigating the relationship between 5-HTTLPR stress and depression5–9. In fact, while 56 primary data studies have assessed whether 5-HTTLPR moderates the relationship between stress and depression, the Munafo and Risch meta-analyses included only 5 and 14 of those studies respectively10–48. Further, Uher and McGuffin have demonstrated that the larger, Risch meta-analysis included a significantly greater proportion of negative replication studies than positive replication studies8.

There are multiple reasons that the studies included in the meta-analyses were limited. First, the primary study data needed for traditional meta-analysis was often not available, either in the original publications or in follow-up email inquiries to study authors. For instance, Munafo and colleagues reported that 15 studies met criteria for inclusion in their meta-analysis. However, they were only able to obtain the primary study data needed for inclusion for five of those studies. There is no evidence that the studies that were able to be included in the meta-analyses were of higher “quality” than those not included.

Another reason why many studies were not included in the Risch and Munafo meta-analysis is that both meta-analyses focused exclusively on studies that explored an interaction between 5-HTTLPR and stressful life events (SLEs) in the development of depression. The original Caspi article, however, not only reported an interaction between 5-HTTLPR and SLEs, but also an interaction between 5-HTTLPR and childhood maltreatment stress. Nine studies have attempted to replicate this interaction with childhood maltreatment, but these studies were not included in the meta-analyses.

Some observers have noted that the SLE study design may have limited power to detect genetic moderation effects because they are susceptible to biases introduced by impaired recall of stressors by subjects and highly variable stressors between subjects9, 45. A newer class of studies has attempted to bypass these potential problems by focusing on specific populations that have experienced a substantial, specific stressor. These studies test whether 5-HTTLPR moderates the relationship between a specific stressor and depression. Eighteen studies have employed such specific stressor designs, but like the childhood maltreatment studies, these studies were excluded from the previous meta-analysis.

In this study, rather than focus on a limited of studies, we sought to perform a meta-analysis on the entire body of work assessing the relationship between 5-HTTLPR, stress and depression. Unfortunately, different types of studies have generally used different study designs to explore this question, rendering it very difficult to combine the studies into a single traditional meta-analysis. An approach useful in situations where equivalent raw data are not available across all studies, is to combine the studies at the level of significance tests 49. The Liptak-Stouffer Z-score method is a well-validated method for combining p-values across studies that has been utilized widely across genomics and biostatistics 50–56. In this study, we utilize the Liptak-Stouffer Z-score method to combine the results from studies investigating whether the 5-HTTLPR variant moderates the relationship between stress and depression.

/pdf/the-serotonin-transporter-promoter-variant-5-httlpr-stress-1dalduwj08.pdf

The serotonin transporter promoter variant (5-HTTLPR), stress, and depression meta-analysis revisited: evidence of genetic moderation.

The ability to modify mice genetically has been one of the major breakthroughs in modern medical science affecting every discipline including psychiatry. It is hoped that the application of such technologies will result in the identification of novel targets for the treatment of diseases such as depression and to gain a better understanding of the molecular pathophysiological mechanisms that are regulated by current clinically effective antidepressant medications. The advent of these tools has resulted in the need to adopt, refine and develop mouse-specific models for analyses of depression-like behavior or behavioral patterns modulated by antidepressants. In this review, we will focus on the utility of current models (eg forced swim test, tail suspension test, olfactory bulbectomy, learned helplessness, chronic mild stress, drug-withdrawal-induced anhedonia) and research strategies aimed at investigating novel targets relevant to depression in the mouse. We will focus on key questions that are considered relevant for examining the utility of such models. Further, we describe other avenues of research that may give clues as to whether indeed a genetically modified animal has alterations relevant to clinical depression. We suggest that it is prudent and most appropriate to use convergent tests that draw on different antidepressant-related endophenotypes, and complimentary physiological analyses in order to provide a program of information concerning whether a given phenotype is functionally relevant to depression-related pathology.

In search of a depressed mouse: utility of models for studying depression-related behavior in genetically modified mice.

https://www.childpsych.theclinics.com/article/S1056-4993(14)00003-0/pdf

The Biological Effects of Childhood Trauma

HPA Axis Reactivity: A Mechanism Underlying the Associations Among 5-HTTLPR, Stress, and Depression

Stress, serotonin, and hippocampal neurogenesis in relation to depression and antidepressant effects

The aim of the present study was to investigate the mechanisms underlying the desensitization of 5-HT1A receptors in the dorsal raphe and hypothalamus of serotonin (5-HT) transporter knock-out mice (5-HTT −/−). The density of 5-HT1A receptors in the dorsal raphe was reduced in both male and female 5-HTT −/− mice. This reduction was more extensive in female than in male 5-HTT −/− mice. 8-OH-DPAT-induced hypothermia was absent in female 5-HTT −/− and markedly attenuated in 5-HTT +/− mice. The density of 5-HT1A receptors also was decreased significantly in several nuclei of the hypothalamus, amygdala, and septum of female 5-HTT −/− mice. 5-HT1A receptor mRNA was reduced significantly in the dorsal raphe region, but not in the hypothalamus or hippocampus, of female 5-HTT +/− and 5-HTT −/− mice. G-protein coupling to 5-HT1A receptors and G-protein levels in most brain regions were not reduced significantly, except that Go and Gi1 proteins were reduced modestly in the midbrain of 5-HTT −/− mice. These data suggest that the desensitization of 5-HT1A receptors in 5-HTT −/− mice may be attributable to a reduction in the density of 5-HT1A receptors. This reduction is brain region-specific and more extensive in the female mice. The reduction in the density of 5-HT1A receptors may be mediated partly by reduction in the gene expression of 5-HT1A receptors in the dorsal raphe, but also by other mechanisms in the hypothalamus of 5-HTT −/− female mice. Finally, alterations in G-protein coupling to 5-HT1Areceptors are unlikely to be involved in the desensitization of 5-HT1A receptors in 5-HTT −/− mice.

https://www.jneurosci.org/content/jneuro/20/21/7888.full.pdf

Reduction in the density and expression, but not G-protein coupling, of serotonin receptors (5-HT1A) in 5-HT transporter knock-out mice: gender and brain region differences.

The aim of the present study was to determine whether alterations in 5-hydroxytryptamine (5-HT)1A receptors would be found in knockout mice lacking the serotonin transporter (5HTT). Hypothermic and neuroendocrine responses to the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetraline (8-OHDPAT) were used to examine the function of 5-HT1A receptors. Initial studies evaluated the dose-response and time course of 8-OH-DPAT-induced hypothermia and hormone secretion in normal CD-1 mice (the background strain of the 5-HTT knockout mice). 8-OH-DPAT dose-dependently produced hypothermic responses that peaked at 20 min postinjection. 8-OHDPAT-induced hypothermia was blocked by the 5-HT1A antagonist WAY-100635. 8-OH-DPAT dose-dependently increased the concentrations of plasma oxytocin, corticotropin, and corticosterone. In the 5-HTT knockout (2/2) mice, the hypothermic response to 8-OH-DPAT (0.1 mg/kg s.c.) was completely abolished. Furthermore, 5-HTT2/2 mice had significantly attenuated plasma oxytocin and corticosterone responses to 8-OH-DPAT. No significant changes in the hypothermic or hormonal responses to 8-OH-DPAT were observed in heterozygous (5-HTT1/2) mice. [ 3 H]8-OH-DPAT- and [ 125 I]MPPI [4-(29-methoxyphenyl)-1-[29-[N-(20-pyridinyl)-iodobenzamido]ethyl]piperazine]-binding sites in the hypothalamus and [ 125 I]MPPI-binding sites in the dorsal raphe were significantly decreased in 5-HTT2/2 mice. The results indicate that lack of the 5-HTT is associated with a functional desensitization of 5-HT1A receptor responses to 8-OH-DPAT, which may be a consequence, at least in part, of the decrease in density of 5-HT1A receptors in the hypothalamus and dorsal raphe of 5-HTT2/2 mice. The 5-hydroxytryptamine (5-HT; serotonin) transporter (5HTT) is an essential component in serotonergic neurotransmission. The 5-HTT regulates 5-HT concentrations in the synaptic cleft and adjacent brain extracellular fluid locations via recycling released serotonin, thus terminating the action of 5-HT. The 5-HTT is the target of serotonin reuptake inhibitors (SRIs), a group of widely used antidepressant and antianxiety agents. The exact mechanism of therapeutic action of the SRIs remains unknown because the onset of clinical improvement usually takes 2 to 4 weeks or longer after the initiation of SRI administration. Adaptive changes in the serotonergic system that are initiated by blockade of 5-HT reuptake have been implicated in these delayed therapeutic effects (Briley and Moret, 1993; Blier and De Montigny, 1998). A number of studies have demonstrated that chronic

Reduction of 5-hydroxytryptamine (5-HT)(1A)-mediated temperature and neuroendocrine responses and 5-HT(1A) binding sites in 5-HT transporter knockout mice.

Armin Heils

Papers

Reduction in the density and expression, but not G-protein coupling, of serotonin receptors (5-HT1A) in 5-HT transporter knock-out mice: gender and brain region differences.

Reduction of 5-hydroxytryptamine (5-HT)(1A)-mediated temperature and neuroendocrine responses and 5-HT(1A) binding sites in 5-HT transporter knockout mice.