The Neurobiology Of Stress

The term stress typically refers to a behavioral state elicited by challenging or threatening events. This concept arises from nearly a century of research starting with the seminal work of Cannon19 and Selye86. In these early studies, various physiological systems were similarly affected by disparate environmental events, which had in common a potential to disrupt homeostasis or threaten animal well-being. Initially, emphasis was placed primarily on stressor-induced activation of peripheral endocrine systems. This

Craig W. Berridge, Psychology Department, University of Wisconsin, Madison, WI 53706. Rodrigo A. EspaƱa, Department of Neurology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, 02115.

work identified the activation of both peripheral catecholamine systems and the pituitary-adrenal axis as hallmark features of the state of stress. The activation of these systems results in enhanced ability of the animal to physically contend with a challenging situation.

More recently, investigation has focused on the central neural systems involved in the affective, cognitive, and behavioral components of stress. This raises the longstanding issue regarding the psychologically defining features of stress. In contrast to the well-delineated physiological indices of stress, the affective and cognitive features of stress remain less clear. The terms stress and anxiety are frequently used interchangeably. However, the precise definition of these terms and the relationship between stress and anxiety are poorly understood. Typically, anxiety is viewed as having both anticipatory and affective components. In contrast, as defined above, stress is most commonly viewed as a response to a present challenge. Moreover, the extent to which stress has an affective component, which can or cannot be dissociated from anxiety is unclear. Regardless of the exact configuration of cognitive and affective responses associated with stress, it appears a heightened level of readiness for action is paramount to a state of stress. A prominent component of this preparatory state is an elevated level of arousal defined, for the purposes of this review, by a heightened awareness of, and sensitivity to, environmental stimuli. In fact, it can be argued that a defining feature of the state of stress is sustained high levels of arousal and the associated sympathetic activation. In addition to alterations in arousal level, stress is also associated with alterations in a variety of state-dependent processes, including attention, memory, and sensory information processing.4'6'10

Candidate neural systems that participate in stress-related alterations in arousal state and state-dependent behavioral processes include catecholamine neurotransmitter systems. For example, it has long-been known that stress is associated with a robust activation of cerebral noradrenergic and dopaminergic neurons resulting in increased release of norepinephrine (NE) and dopamine (DA) in a variety of terminal fields, particularly those terminal fields associated with higher cognitive and affective processes (e.g. prefrontal cortex, amygdala).14'27'50'89'95 Relatively recent evidence suggests these neurotransmitter systems modulate behavioral and forebrain neuronal activity states as well as state-dependent cognitive (e.g. working memory, attention) and physiological processes.5'10'16'79 Combined, these observations suggest a prominent role of central catecholaminergic systems in both adaptive and possibly maladaptive stressor-induced alterations in cognition and affect.

The peptide neurotransmitter' CRF' plays a pivotal role in stress-related activation of the HPA axis, involving the release of CRF from neurons located within the paraventricular nucleus of the hypothalamus (PVN) into the median eminence. Subsequent to the characterization of this peptide in 1981,97 it was observed that CRF-containing neurons and fibers are found outside the hypothalamus' including within neocortex, limbic structures and autonomic nuclei.70'85 Additional work demonstrated a variety of stress-like behavioral and physiological actions of CRF when injected centrally into animals. Among these stress-like actions of CRF are the activation of dopaminergic28 and noradrenergic systems'28'98 arousal-enhancing actions'20'21 and elevation of behavioral indices of stress and/or anxiety.29'54 These observations suggested a pivotal role of CRF in the coordination of a variety of behavioral and physiological responses in stress.29 Consistent with this hypothesis, blockade of CRF neurotransmission via central administration of CRF antagonists reverses a variety of behavioral and physiological effects of stress.29

Combined, these observations suggest critical roles of catecholaminergic and CRF systems in regulating the constellation of behavioral and physiological processes associated with stress. For the purposes of this review, it is of interest that HCRT efferents target dopaminergic, noradrenergic and CRF systems, suggesting a possible role of HCRT in stress. Consistent with this hypothesis, recent evidence begins to suggest a role of HCRT in behavioral and physiological responding in stress and arousal.

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