Stress and Motivated Behavior Institute

Development of Anxiety and Anxious States

Anxiety and anxiety-related disorders are among the most prevalent mental disorders in the industrialized world 1,2. A subset of these disorders are directly related to stress and coping. Rat models can be particularly useful for the study of stress-induced impairments of physical and mental health. The ability to exert experimental control over stressor presentation and cue conditions has allowed for substantial advances in the neurobiology of stress and stress-induced emotional states.

A prime example is fear. Associations between an aversive event and signals of the aversive event, whether discrete or contextual, produce a set of physiological (autonomic, immune, and endocrine) and neurobehavioral (freezing, defecation, vocalization, potentiated startle) adjustments that have been well described 3-6. Converging evidence points to a fear circuit, which includes the CeA 7-10, prefrontal cortex 11-13, BNST 14, ventral tegmental area 15-17, and locus coeruleus (LC) 18-20 that mediate discrete and contextual cued fear.

Less is known concerning stress-induced anxious states. There are a number of reasons for this. For one, anxiety is an inherently difficult experimental problem. Unlike fear, anxiety is not observable as a cue-elicited response, but is considered free-floating. The maintenance of stress-induced anxiety is often attributable to mental representations of either the precipitating aversive event or associated internal or external representations. Animal models must rely on behavioral disruptions; the mentalistic aspects are beyond the purview of animal research. Moreover, anxiety is not a unitary construct. Anxiety has different forms and features, with a continuum from normal to pathological. A single animal model is not likely to capture such a multifaceted clinical disorder 21,22.

Inescapable stress provides an important model system for understanding aspects of anxious states. An anxious state is apparent as behavioral disruptions that persist in the post-stressor period in the absence of cues reminiscent of the precipitating stressor 23-29. This persistent stress-induced anxious state only develops in response to particularly intense stressors 30. While fear-related learning has been extensively elaborated, learning changes during anxious states has gone largely unnoticed. Typically, the end result of the associative process is elaborated, but not the associative process itself 31-33. In part, this is a natural result of the learning paradigms chosen. Acquisition occurs too rapidly in simple fear-related paradigms. In others, the nature of learning and role of the associative process in the completion of the task is unclear 34. Most of these have difficulty distinguishing learning from performance. Learning changes may be obscured by differences in appetite, activity or arousal.

To clearly expose the nature of the associative process after exposure to stress, we have assessed acquisition of the CCER in the freely-moving rat 25,35-37. Nonassociative changes in reactivity to the conditioned stimulus (CS) and unconditional stimulus (US) can be explicitly measured 38. Importantly, the neural substrates for acquisition of the CCER have been extensively elaborated 39,40. Acquisition depends on brain stem sensory and motor nuclei, and integration within the deep cerebellar nuclei 41; modulatory structures include the amygdala 42, prefrontal cortex 43,44, and hippocampus 45-47.

A body of work demonstrates that exposure to inescapable stress proactively facilitates acquisition of the CCER in male rats 48,35,37. Facilitated acquisition after exposure to stress is truly associative in nature. Faster learning cannot be explained by enhanced processing of the US apparent as an exaggerated unconditional response (UR) 38. Nor is it the product of pseudoconditioning; stressed rats given explicitly unpaired training do not exhibit heightened reactivity to the CS merely as a function of US exposure 35. Faster acquisition is apparent from 1 to 4 days after stressor cessation in the absence of cues associated with the precipitating stressor and regardless of the interstimulus interval 25. Stress-induced facilitated acquisition of the CCER by rats is consistent with the classic work of Spence and colleagues in the 1950’s showing faster acquisition of the CCER in anxious human subjects 49,50. Facilitated acquisition of the CCER serves not only as a probe of an anxious state, but provides an index of the associative potential during an anxious state. Enhanced associative learning provides the impetus for second order conditioning and generalization, ways in which stress-induced anxious states may be perpetuated. Further, it is presumed that the neurobiological basis for a stress-induced enhancement of learning is also the basis for developing and maintaining an anxious state.

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