Using rodent models to simulate stress of physiologically relevant severity : when, why and how
Date
2012
Authors
Journal Title
Journal ISSN
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Publisher
InTech, Rijeka, Croatia
Abstract
Given the demands of modern life, it is no wonder that the concept of stress has become a
household topic for discussion. Also in the academic realm the phenomenon which is stress,
is topping the charts in terms of research interest. The short term costs as well as the long
term maladaptive effects of stress have been a popular topic of research in especially
physiology and psychology for the past few decades, ever since Hans Selye defined the term
“stress” in 1956 (Selye, 1956). Stress-related chronic disease, such as cardiovascular disease,
diabetes and depression, places an ever-increasing burden on society – medically, socially
and financially. Therefore, if we are to limit the spread and impact of this “pandemic”, it is
imperative to properly manage the effects of stress on our bodies. This of course, is only
possible if we have a complete understanding of the body’s response to stress.
The response to stress is almost never localised and contained. Rather, a stress response is
initiated in response to a local physical (e.g. contusion to skeletal muscle) or mental (e.g. the
loss of a loved one) stressor, but always culminates in a wide-spread, systemic response
process that affects many organs and systems. Consider for a moment a less complex
research model in a different discipline. Metabolic pathways (e.g. the Krebs cycle or
glycolysis) can easily be manipulated in cell culture assays using one single cell type at a
time, since these pathways (including substrate supply and waste removal systems) are
contained in its entirety within each cell. In contrast, with the stress response pathways this
is clearly not the case.
The stress response is a complex network of events, which is directed via two interlinked
pathways, one endocrine (the hypothalamic pituitary adrenal (HPA)-axis) and one neural (the locus coeruleus norepinephrine (LC-NE) or sympatho-adrenal medullary (SAM)-
system). While the neural pathway is mainly activated neurally in response to stress
perception, leading to the well-known “fight-or-flight” response, the endocrine pathway has
many more triggers. Apart from neural activation, the HPA-axis is also activated by a large
number of hormones and even chemical messengers, such as interleukin-6, a cytokine and
mediator of inflammation, which is known to increase cortisol secretion. A contributing
factor to the complexity of the HPA-axis is the fact that cortisol, the main end product of this
stress response, has both endocrine and metabolic functions. Although cortisol is commonly
known as the “stress hormone” in the context of psychological stress, its main function is
actually metabolic – to maintain glucose supply to the brain. Therefore, the HPA-axis is
structured not only for activation in response to perceived stress, but also to react to
metabolic stimuli. Furthermore, while the stress response should be powerful and fast in an
acute stress situation, the response should be controlled and relatively more limited in a
situation of chronic stress, to prevent detrimental effects to the organism in the long term.
One can appreciate therefore the need for relatively complex signalling networks in this
regard, which serves to activate, limit or inhibit the stress response. To achieve this,
numerous molecular mechanisms are in place, and react and interact in response to various
stress signals. To give just one example, the glucocorticoid receptor, which is present on
most cells to enable cortisol’s effect on these cells, is up-regulated in response to acute stress,
but down-regulated after a period of chronic stress.
Such complexities make the choice of a suitable stress research model both a difficult, and
vital one. While some mechanisms, e.g. activation agents of specific adrenal or pituitary cell
types, may be elucidated in cell culture, a whole-system model is required in order to assess
the net effect of any stressor to these systems. This does not imply that there is no place for
ex vivo or in vitro studies in the discipline of stress research, far from it! A large number of
cell-based – and more recently organotypic culture-based – studies have contributed
substantially to our understanding of specific mechanisms and/or partial pathways relevant
to stress. The important point here is that ideally, in vitro work should at some point be
followed up by in vivo investigations, in order to test the applicability of results obtained in
vitro, to a whole system.
The importance of in vivo assessments, and the need for conducting them in a model
specifically suitable to answer the question at hand, is clear when one considers the huge
number of described animal models in the scientific literature. Apart from more
conventional models using genetically “intact” rodents, recent advances in biotechnology
have made possible research using non-physiological models such as gene-knock out
animals. These animals may be genetically modified to erase the gene coding for a particular
protein, so that the researcher may elect to produce animals completely lacking a particular
protein of interest (e.g. IL-6 knockout mice), or in some cases lacking it in only one organ or
system (e.g. STAT-3 knocked out or “switched off” in skeletal muscle only). These models
may be used to shed light on various in vivo mechanisms which could previously not be
properly elucidated using the conventional methods. However, these models have their limitations. For example, when doing research on inflammation, an animal in which a proinflammatory
cytokine was knocked out, may display increased or decreased basal levels of
other pro-inflammatory cytokines, or an altered anti-inflammatory cytokine profile, or even
up- or down-regulated cytokine responses on activation, as a spontaneous compensatory
mechanism. The resultant net effect of the genetic manipulation therefore may result in a
model that is not physiologically accurate, and responses measured may not accurately
reflect normal in vivo responses. Furthermore, these compensatory mechanisms and/or the
mere absence of an important protein may also result in other – sometimes unanticipated –
side-effects (such as severe constipation in IL-6 knockout mice). Apart from being a
confounding factor in the intended study, in some cases these undesired outcomes may
result in poor health or even shortened life expectancy of the experimental animal, so that it
limits the application of such a model even further. Of course, chain-reaction compensatory
responses will also limit the extent to which results obtained in such models, may be
extrapolated to a (at least genetically) normal situation.
Relatively “old-fashioned”, or more conventional methods, when applied optimally,
therefore still have an important place in research, both in applied areas such as
pharmacology and in areas of basic research. Only when a situation that is physiologically
relevant is recreated or simulated, can one realistically assess either the response to a
challenge, or the outcome of a remedial intervention.
Therefore, in this chapter, I would like to reflect on methods used to simulate a variety of
stressors to the body, starting with a variety of models used to simulate psychological stress,
ranging in severity from non-extreme (mild) psychological stress to extreme mental trauma.
I will also discuss general considerations in picking the appropriate animal model to use,
which may determine the difference between success and failure in your research. Details
on the various models will be provided, including issues such as repeatability and
standardisation. Models will also be discussed in terms of their suitability for different
research approaches or objectives, as well as in terms of their limitations. Arguments for and
against the use of any particular model will also be illustrated using actual research data.
Description
Keywords
Stress-related chronic diseases -- Research, Stress (Physiology)
Citation
Smith C. Using rodent models to simulate stress of physiologically relevant severity: when, why and how (Chapter 10, pp 211-230). IN: Glucocorticoids: new recognition of our familiar friend. Editor X. Qian, InTech, Rijeka, Croatia, 2012 (ISBN 978-953-51-0872-6)