Aging diurnal rhythms and chronic stress: Distinct alteration of diurnal rhythmicity of salivary α-amylase and cortisol

Abstract

The present study assessed diurnal profiles of salivary α-amylase (sAA), proposed as a marker of autonomic activity, and salivary cortisol in competitive ballroom dancers as well as age- and sex-matched controls to investigate age-related changes of basal activity and potential chronic psychosocial stress-related alterations. According to the Allostatic Load (AL) hypothesis of a cumulative wear and tear of the body we expected to see physiological accumulation of the effects of stress and age especially pronounced in older dancers. Dancers and controls collected five saliva samples throughout the day. Daily overall output of sAA was elevated in older adults while there was no effect of age on mean cortisol levels. Alterations of diurnal rhythms were only seen in younger male dancers showing a flattened diurnal profile of sAA and younger dancers and female older dancers showing a blunted diurnal rhythmicity of cortisol. Furthermore, we found a negative correlation between summary indices of basal sAA and the amount of physical activity. In conclusion, higher overall output of sAA in older adults is in line with the phenomenon of a sympathetic “drive” with increasing age. Furthermore, a lower output of sAA in people who are more physical active is in line with the hypothesis of an exercise-induced decrease of sympathetic activity. Overall, our study does not support the AL hypothesis, but rather highlights the importance of regular physical activity and social environment in promoting health.

Introduction

Organisms not only regulate their physiological functioning in response to acute environmental demands, but also rely on biological rhythms to continuously adapt to the environment. Thus, disruption of biological rhythms can impair the health and well-being. In humans, disturbed rhythmicity has been associated with a variety of mental and physical disorders (Koch et al., 2009, Tan et al., 2007, Wessa et al., 2006). The suprachiasmatic nucleus (SCN) in the hypothalamus is discussed as the main circadian pacemaker, integrating endogenous and exogenous information (Gillette and Tischkau, 1999). Almost every physiological system has some degree of circadian rhythm that can be influenced not only by light exposure and the typical sleep–wake cycle but also by factors like age, sex, and stress. Of particular interest in the research of normal and abnormal physiology is the contribution of age. During the process of aging, cumulative exposure to psychological and biological stressors can, over time, result in a distinct and stable pattern of dysregulations, particularly in older individuals who experienced more stressors throughout their life spans. Most research on dysregulations of daily rhythms has focused on the hypothalamus–pituitary–adrenal (HPA) axis. Whether similar dysregulations also occur in the Autonomic Nervous System (ANS) is less well known. Therefore, this study was designed to examine the effects of age and stress on a non-invasive marker of autonomic-functioning salivary α-amylase (sAA).

The ANS is one of the main stress-sensitive systems in humans, and shows a distinct circadian rhythm, with sympathetic activity increasing significantly during the day and decreasing during the night, while parasympathetic activity decreases during the day and increases during the night (Yamasaki et al., 1996). Seals and Dinenno (2004) conclude in their review that the primary influence of aging on the human sympathetic nervous system (SNS) is an elevation of tonic activity. Another age-associated change of peripheral sympathetic function is an impairment of the baroreflex, and reduced neuronal reuptake of noradrenaline (NA) leading to higher NA as well as elevated sympathetic muscle nerve activity (Lakatta, 1993, Lavi et al., 2006). In contrast, adrenaline (A), an adrenomedullary sympathetic marker, remains unchanged (Pfeiffer et al., 1983, Leenen et al., 2005). Overall, there seems to be an age-related increase in systemic adrenergic “drive”. In addition to age, other factors that need to be taken into account are those that activate the SNS, such as mental and physical stresses. So far, studies investigating chronic stress effects on various autonomic markers usually focus on topics such as job stress, racial discrimination, or burden of caregiving. The letter chronic stress experience has been associated, for example, with reduced HRV (Lampert et al., 2005, Schubert et al., 2009). In a recent study conducted by Mausbach et al. (2008), caregiving stress was associated with decreased β₂-adrenergic receptor sensitivity, which may contribute to the development of cardiovascular illness among caregivers. This desensitization may occur by way of a chronic stress-induced greater release of adrenaline and noradrenaline from the adrenal medulla (Mausbach et al., 2008). In summary, studies investigating chronic stress effects on autonomic functions showed an increase of sympathetic activity and a concomitant decrease of parasympathetic function (Pickering, 2006).

Recently, salivary α-amylase (sAA) has gained interest as an indicator of autonomic activity. Since pharmacological studies were able to show a direct link between sAA and sympathetic nervous system (SNS) activity (van Stegeren et al., 2006, Ehlert et al., 2006), determination of sAA activity under both physically and psychologically stressful conditions is used more and more in biobehavioral research. Summarizing the existing literature, early studies suggested sAA as non-invasive measure of plasma noradrenaline (NA) concentration in humans (Chatterton et al., 1996, 1997; Skosnik et al., 2000). Recently, a distinct diurnal rhythm of sAA was described in younger adults and children (Rohleder et al., 2004, Nater et al., 2007, Wolf et al., 2008). However, only little is known about possible confounders of basal sAA levels (for a review Rohleder and Nater, 2009). Nearly all our previous knowledge concerning diurnal changes of basal sAA activity stems from studies with adult participants. Earlier studies showed lower morning and higher afternoon values in young adults (Jenzano et al., 1987, Rantonen and Meurman, 2000, Li and Gleeson, 2004). Yamaguchi et al. (2006) on the contrary, found no diurnal rhythm of sAA in students using a newly developed handheld electronic device that allows immediate amylase measurement (Yamaguchi et al., 2006). More recent studies revealed a pronounced diurnal rhythm characterized by a strong decrease after awakening and steadily increasing values during the day with peaks in the late afternoon in adults (Rohleder et al., 2004, Nater et al., 2007). Just recently, the same diurnal profile was shown in children and adolescents (Wolf et al., 2008). Previous studies investigating basal sAA activity in healthy elderly subjects have led to the conclusion that sAA activity does not change with age (Ben-Aryeh et al., 1986, Aguirre et al., 1987, Salvolini et al., 1999). However, in all of the latter studies sAA activity was measured only once per participant, but to the best of our knowledge, no studies have assessed full diurnal profiles in elderly individuals. Furthermore, studies investigating the cumulative effects of repeated or chronic stress on sAA rhythmicity are rare. One study reported an association between chronic stress and increases in daytime sAA levels (Nater et al., 2007). In a more recent study, Rohleder et al. (2009) reported a flattening of sAA diurnal profiles in cancer caregivers, which normalized at the end of the observation period. To address these gaps, our study was designed to investigate the association of aging and chronic stress experiences with diurnal rhythms of sAA.

The concept of Allostatic Load (AL) has served as a framework for a large body of research on the understanding of stress–health relationships. It is based on the hypothesis that there is a physiological accumulation of the effects of chronic stress—most notably seen during the process of aging. This concept refers to the cumulative biological wear and tear that can result from repeated stress system activation, leading to a chronic dysregulation of these systems, which finally causes damage to dependent tissues and organ systems (McEwen, 1998). For instance, age-associated elevations in sympathetic neuronal outflow result in higher concentrations of catecholamines and thus increase cardiac work. The resulting imbalance between oxygen supply and demand might lead to, e.g. cardiovascular hypertrophy, angina pectoris, or hypertension (Schulkin, 2004). Other hypothesized consequences are HPA axis dysregulations, i.e. impaired negative feedback and altered diurnal rhythmicity (for a review McEwen and Seeman, 1999), with its own set of hypothesized consequential dysregulations in for example the cardiovascular, immune, and the metabolic systems. In addition to the investigation of HPA axis dysregulations as a possible early indicator of AL (Gunnar and Vazquez, 2001), determination of basal diurnal sAA activity seems to be a unique possibility to extend our knowledge of age and chronic stress effects on autonomic/sympathetic nervous system.

In this study, competitive ballroom dancing was chosen as a model for chronic, non-habituating psychosocial stress. We have previously shown that competitive ballroom dancing induced substantial increases in cortisol, which did not habituate across competitions, was independent of individual experience, and was not a result of the physiological exercise component (Rohleder et al., 2007). Theoretically, repeated, non-habituating responses to social-evaluative conditions, which are characteristic for the lives of competitive ballroom dancers, should be associated with stress system dysregulations. Thus, we hypothesized that ballroom dancers would show altered diurnal patterns of salivary α-amylase and cortisol secretion in comparison with a control group of non-dancers. The second purpose of our study was to investigate age-related changes of basal amylase and cortisol diurnal rhythms, and potential stress-related alterations of these differences. Because of the potentially longer accumulation of wear and tear, we expected to see more pronounced alterations in older dancers in comparison with younger dancers and controls. In addition, we expected to see a sympathetic drive associated higher overall α-amylase activity in older adults. For cortisol profiles it was shown, that chronic stress situations characterized by uncontrollability, emotions like shame, and threats to the social self are associated with flatter rhythms and higher daily output (Miller et al., 2007). Salivary cortisol profiles were obtained from the same samples as amylase for a comparison of the effects of chronic stress and age on diurnal profiles and the interaction of both stress-sensitive systems.