Predicting cortisol stress responses in older individuals: Influence of serotonin receptor 1A gene (HTR1A) and stressful life events

Abstract

Considerable variability in the activity of the hypothalamus–pituitary–adrenal (HPA) axis in response to stress has been found in quantitative genetic studies investigating healthy individuals suggesting that at least part of this variance is due to genetic factors. Since the HPA axis is regulated by a neuronal network including amygdala, hippocampus, prefrontal cortex as well as brainstem circuits, the investigation of candidate genes that impact neurotransmitter systems related to these brain regions might further elucidate the genetic underpinnings of the stress response. However, aside from genetic risk factors, past stressful life events might also result in long-term adjustments of HPA axis reactivity. Here, we investigated the effects of the − 1019 G/C polymorphism in the HTR1A gene encoding the serotonin (5-HT) receptor 1A (5-HT_1A) and stressful life events experienced during childhood and adolescence on changes in cortisol levels in response to the Trier Social Stress Test (TSST) in a sample of healthy older adults (N = 97). Regression analyses revealed a significant effect of HTR1A genotype with the G allele being associated with a less pronounced stress response. In addition, an inverse relationship between past stressful life events and cortisol release but no gene × environment interaction was detected. The results further underscore the crucial role of functional serotonergic genetic variation as well as stressful events during critical stages of development on the acute stress response later in life.

Introduction

Encountering stressful situations leads to a wide variety of psychological and physiological changes including activation of the hypothalamic–pituitary–adrenal (HPA) axis. In short, stress results in the release of corticotropin releasing hormone (CRH) from the paraventricular nucleus (PVN) of the hypothalamus with a subsequent stimulation of adrenocorticotropin (ACTH) and cortisol secretion in humans. Cortisol as the major stress hormone influences numerous physiological systems, including CNS function, metabolism, cardiovascular function, immune system, muscle tissue, and bones (Kino and Chrousos, 2005). The degree of change in cortisol levels in response to stress varies widely even in healthy individuals with genetic as well as environmental factors as likely contributors (Kirschbaum and Hellhammer, 1999). Since dysregulation of HPA axis activity has been related to impairing conditions like depression, anxiety and several other disorders (for a review see e.g., Chrousos, 2009, Handwerger, 2009, Pariante and Lightman, 2008) which are associated with considerable suffering as well as elevated health care costs, it is vital to investigate potential genetic and environmental factors in greater detail. HPA axis activity is further modulated by additional neural circuits, including the brainstem, the amygdala, the hippocampus and the medial prefrontal cortex (PFC; Dedovic et al., 2009a, Jankord and Herman, 2008), and thus, one promising research strategy to further elucidate its genetic underpinnings might be the investigation of candidate genes that impact neurotransmitter systems related to these brain regions.

Serotonin (5-HT) has been implicated in more behavioral, physiological and pathological mechanisms than any other brain neurotransmitter (Azmitia, 2007) and differences in serotonergic function based on genetic variation and their association with behavioral outcomes have been investigated in numerous studies in healthy volunteers and clinical samples. Regarding the cortisol stress response, 5-HT has been found to modulate HPA axis reactivity (Holmes, 2008, Lanfumey et al., 2008) with serotonergic neurons from the raphe nuclei projecting to the PVN of the hypothalamus (Herman et al., 2005). In addition, serotonergic neurons innervate the amygdala (Jacobs and Azmitia, 1992) which in turn also has connections to the PVN via the bed nucleus of the stria terminalis (BNST) (Davis and Whalen, 2001, Jankord and Herman, 2008). The relationship between 5-HT and the HPA axis is not one-directional: administration of glucocorticoids or activation of the HPA axis through stress exposure has been found to affect the serotonergic system as well (Leonard, 2005), with CRH-immunoreactive cell fibers in the rostral and caudal raphe nuclei being part of the neuroanatomical basis for HPA axis influence on 5-HT neurotransmission (Linthorst, 2005). Thus, HPA axis and the serotonergic system form a multifaceted network (Porter et al., 2004). Here, we focus on the effects of variation in the gene encoding for the 5-HT_1A receptor (HTR1A) as one of the essential regulators of 5-HT function on the cortisol stress response.

5-HT_1A is a G-protein coupled receptor which in the raphe nuclei functions as an autoreceptor providing negative feedback to the afferent neuron. 5-HT_1A is also expressed postsynaptically in the hippocampus, entorhinal cortex, septum, periaqueductal gray, frontal cortex, and amygdala where they inhibit neuronal activity by hyperpolarization (Albert and Lemonde, 2004, Pineyro and Blier, 1999). Altered 5-HT_1A function has been suggested to contribute to the pathogenesis of anxiety and depressive disorders (Nash et al., 2008, Neumeister et al., 2004) and human as well as animal studies have linked 5-HT_1A to differences in emotional regulation: mice with higher aggression scores showed a heightened expression of 5-HT_1A receptors in the dorsal hippocampus compared to less aggressive mice (Korte et al., 1996) while 5-HT_1A knock-out mice display increased anxiety-related behaviors compared to normal wild-type controls although they also show decreased behavioral despair in response to stress (Gross and Hen, 2004, Heisler et al., 1998, Parks et al., 1998, Ramboz et al., 1998) which has been referred to as “antidepressed” phenotype (Richardson-Jones et al., 2010). Recently, mice strains in which only 5-HT_1A autoreceptor (but not postsynaptic receptor) function was altered were found to show changes of physiological stress responses, behavioral despair, and response to antidepressants but no differences in anxiety-like behavior (Richardson-Jones et al., 2010). Regarding the link between 5-HT_1A and the HPA axis, administration of a 5-HT_1A antagonist before exposing rats to single-prolonged stress resulted in decreased CRH and glucocorticoid receptor mRNA and protein levels (Wang et al., 2009). In turn, adrenalectomy in rats leads to anatomically specific decreases in serotonergic metabolic indices and to a significant increase in 5-HT_1A receptor binding and mRNA levels in the hippocampus (Grino et al., 1987). Consistently, 5-HT_1A receptor mRNA levels and 5-HT_1A-binding were reduced in the rat hippocampus after 2 weeks of chronic stress (Lopez et al., 1998).

In humans, HTR1A is located on chromosome 5q12.3 and contains in its transcriptional control region a functional C/G single nucleotide polymorphism (SNP) at position − 1019 (rs6295) with the G variant preventing binding of regulatory proteins resulting in an increased HTR1A gene expression and 5-HT_1A-mediated serotonergic neurotransmission (Albert and Lemonde, 2004, Lemonde et al., 2003). Consistently, 5-HT_1A autoreceptor density was reported to be increased in G allele carriers in a positron emission tomography (PET) study (Parsey et al., 2006b) although not in an earlier PET study (David et al., 2005). In vitro findings point to increased expression of 5-HT_1A autoreceptors but decreased 5-HT_1A expression in postsynaptic receptors in G allele carriers (Czesak et al., 2006). The HTR1A G allele has been associated with major depression (Anttila et al., 2007, Kraus et al., 2007, Lemonde et al., 2003, Neff et al., 2009, Parsey et al., 2006a), panic attacks (Huang et al., 2004) and panic disorder with agoraphobia (Rothe et al., 2004) and anxiety- and depression-related personality traits (i.e., neuroticism and harm avoidance; Strobel et al., 2003) although there are also inconsistent findings (Koller et al., 2006). Recently, neuroimaging studies reported a link between decreased amygdala reactivity and increased 5-HT_1A autoreceptor expression and the presence of the HTR1A G allele, respectively (Fakra et al., 2009, Fisher et al., 2006). However, the resulting consequences on downstream effector systems of the amygdala such as the HPA axis need to be investigated as well since such intermediate phenotypes can further narrow the gap between brain activation and behavioral variables.

Similarly, there is evidence on the potential modulation of the cortisol stress response by stressful life events (SLEs), which have been found to contribute substantially to less favorable neuropsychiatric outcomes (Paykel, 2003) like depression or anxiety. Particularly, SLEs occurring in critical early stages of life have been found to result in persisting negative effects over the course of life in human as well as in animal studies: in rats, an early adverse social environment (i.e., low maternal care) has been reported to increase stress responses in adult rats via epigenetic processes (overview in Weaver, 2007). In humans, the crucial role of past events particularly during early development has been reported in several studies (e.g., Fumagalli et al., 2007, Kaufman et al., 2000) as have been gene × environment interactions (e.g., Canli and Lesch, 2007, Caspi et al., 2010, Kaufman et al., 2004, Lesch, 2004), the latter, however, mainly focused on the role of genetic variation in 5-HT transporter function. Thus, in the present study we investigated the independent and joint effects of HTR1A − 1019 G/C genotype and SLEs (experienced during childhood and adolescence) on the cortisol stress response in an acute social stress paradigm in a sample of older adults which enabled us to address an additional question: whether the potentially negative effects of early SLEs persist throughout the life span into late adulthood or whether they only exert a transient influence that will eventually fade out.