ReviewA users guide to HPA axis research☆
Introduction
Glucocorticoid hormones are powerful regulators of all mammalian physiological systems including the central nervous system [1], [2], [3]. These regulatory effects depend on a dynamic and complex profile of ultradian, circadian and stress reactive glucocorticoid hormone circulating levels [4], [5]. Glucocorticoid hormone production and secretion is controlled by the hypothalamic-pituitary-adrenal (HPA) axis neuroendocrine system (Fig. 1A). A number of pathological biomedical conditions are associated with altered HPA axis activity profiles. Dysregulation of HPA axis activity is strongly associated with some mental health disorders (e.g. depression, PTSD, schizophrenia) [6], [7], [8] and other biomedical disorders (e.g. Type II diabetes, hypertension, chronic fatigue syndrome, fibromyalgia and chronic facial pain) [9], [10], [11], [12], [13]. Altered glucocorticoid hormone profiles also contribute to the adverse health effects of persistent psychological or physiological stress [14], [15], [16]. These altered profiles may be manifest by changes in basal glucocorticoid hormone secretion patterns and/or alterations in the response to acute stressor challenge [7], [17]. There is also considerable interest in the prospect that signs of HPA axis dysregulation serve as valuable biomarkers in the clinical setting. For example, HPA axis dysregulation is a significant covariate for various subgroups of certain disorders, and it has been associated with differential treatment responsiveness, recovery rate, and probability of disease relapse [18], [19], [20], [21], [22]. Consequently, study of HPA axis and glucocorticoid physiology continues to be an important scientific biomedical endeavor.
There are a number of excellent reviews of glucocorticoid physiology and its importance for health (e.g. [4], [23], [24], [25], [26], [27]). Most of these reviews, however, only touch on some of the conceptual and logistical aspects that one must consider when conducting and evaluating HPA axis-related research. In this paper, our primary objective is to provide a rudimentary Users Guide that may help assist others with the design and interpretation of research that includes HPA axis manipulations and measurements, especially within the realm of in vivo stress neurobiology. We will begin by reviewing the components and functional operation of the HPA axis. This knowledge is essential in order to manipulate and measure the HPA axis in a meaningful fashion. We will then examine strategies for manipulating the HPA axis, and discuss considerations for measuring aspects of HPA axis function. We acknowledge that this guide is rodent centric, as that reflects both our first hand expertise, and to some extent the species bias of biomedical research. However, the molecular, cellular and systems-level components and function of the HPA axis are highly conserved in mammals. Consequently, many of the principles and considerations outlined here can be readily applied to study of other species including humans.
Section snippets
Overview of the HPA axis
Glucocorticoid hormones are the systemic effector hormone of the HPA axis. The principal circulating glucocorticoid hormone in humans, many other mammals (e.g. dogs and hamsters) and most fish is the steroid molecule cortisol. In rats, mice, birds and most reptiles it is the closely related molecule, corticosterone (Fig. 1B) [28]. In this Users Guide we will use CORT as an abbreviation for both of these molecules except in cases where the distinction is important.
Although we do not have
Experimental manipulation of the HPA axis
Because CORT is the primary effector hormone of the HPA axis, the general research objective of HPA axis physiology research is to test whether a particular aspect of circulating CORT (ultradian, circadian or reactive) is necessary or sufficient for dependent measures of interest. This general objective is typically achieved by either directly or indirectly manipulating CORT levels, or by treating the subject with MR/GR agonists or antagonists.
HPA axis hormone blood measures
There are three important objectives necessary for accurate measurement of HPA axis hormonal activity: 1) to obtain blood samples that reflect the actual circulating hormone levels at a point in time of interest that are not confounded by the sampling method itself, 2) to collect and process blood samples in a manner that preserves the hormone molecule present within the plasma sample, and 3) to use a hormone analysis method that is reliable and sensitive. We discuss these objectives below and
Concluding perspectives
The vast scientific research literature examining glucocorticoid biological actions does not reflect a peculiar tunnel vision of scientists. Rather it reflects the remarkably widespread actions of glucocorticoid hormones and the diverse range of physiological processes affected by them. The information that is conveyed by this systemic hormonal signal – time of day and presence of environmental challenges — is critical for survival. So it is not surprising that virtually every cell and system
Acknowledgements
We are grateful to our many scientific partners—mentors, trainees, collaborators and professional colleagues—who have shared with us their invaluable insights, perspectives and tricks of the trade concerning HPA axis related research. We regret the fact that we have often forgotten the source of these gems of information and have not given appropriate credit to many of these individuals in this Users Guide.
Dr. Robert L. Spencer is currently supported by NSF grant IOS1456706, and Dr. Terrence
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This manuscript is submitted in fond memory of our colleague and friend, Dr. Randall Sakai, who continually strived to improve the quality of experiments being conducted, the impact of the work being published, and the training models we use to foster the next generation. One of us (RLS) worked closely with Randall as fellow postdoctoral trainees at Rockefeller University, and Randall was a significant force in shaping my scientific development. He enlisted me in his mission to establish rigorous conditions for each of our scientific measurements and to adopt thoughtful experimental procedures that would provide meaningful insights into the physiology of the research subject. Randall's impeccably high standards continue to drive us forward, and we hope this article will provide useful guidance for those who find themselves pursuing and interpreting studies in HPA axis regulation.