Stressor predictability influences open field behavior, pain sensitivity and brain MAO inhibitory activity (tribulin) in the rat

An unexpected increase in restraint duration alters the expression of stress response habituation

While habituation develops to a repeated psychological stressor, manipulating certain parameters of the stress challenge experience may lead to dishabituation of the stress response. In this experiment, we investigated whether the behavioral, endocrine, and neural responses (c-fos mRNA immediate early gene expression) to a psychological stressor (restraint) differ when the duration of the stressor given on the test day violates expectations based on prior stress experience. Rats experienced 10 min of daily restraint on Days 1-4 followed by a challenge with either the same duration (10 min) or a longer duration (30 min) of restraint on Day 5. Rats' behavior was video recorded during the Day 5 restraint episode, and trunk blood and brain tissue were collected 30 min following restraint onset. Struggling behavior was manually scored as active attempts to escape the restraint device. Rats who experienced the same duration of repeated restraint showed a significant decrease of plasma corticosterone (CORT) compared to the 10 min acute restraint group (habituation). In addition, these rats showed decreased active struggling over repeated restraint trials. Conversely, the rats showed an increased CORT response (dishabituation) when they experienced a longer duration of restraint on Day 5 than they had previously. These rats showed a habituated behavioral response during the first 10 min of restraint, however struggling behavior increased once the duration of restraint exceeded the expected duration (with a peak at 12 min). This peak in struggling behavior did not occur during 30 min acute restraint, indicating that the effect was related to the memory of previous restraint experience and not due to a longer duration of restraint. In contrast, these animals showed habituated c-fos mRNA expression in the paraventricular nucleus (PVN), lateral septum (LS), and medial prefrontal cortex (mPFC) in response to the increased stressor duration. Thus, there was a dissociation between c-fos mRNA expression in key stress responsive brain regions and the behavioral and endocrine response to increased stressor duration. This dissociation may have been due to a greater lag time for c-fos mRNA responses to reflect the impact of a dishabituation response. In conclusion, habituation of the endocrine and behavioral stress response occurred when the duration of the stressor matches the previous experience, while dishabituation of the stress response was triggered (with remarkable temporal precision) by an unexpected increase in stress duration.

Timing in the absence of supraspinal input III: regularly spaced cutaneous stimulation prevents and reverses the spinal learning deficit produced by peripheral inflammation

In the absence of brain input, spinal systems can adapt to new environmental relations. For example, spinally transected rats given a legshock each time the leg is extended exhibit a progressive increase in flexion duration that minimizes net shock exposure, a simple form of instrumental learning. This capacity for learning is modulated by prior stimulation; both variable shock and inflammation produce a lasting inhibition of learning. An extended exposure to fixed spaced shock has no adverse effect on learning and opposes the consequences of variable shock. The present studies expand on these findings and demonstrate that fixed stimulation ameliorates the impact of peripheral inflammation. Spinally transected rats were administered 900 fixed spaced legshocks before (Experiment 1) or 1,800 legshocks after (Experiment 2) a subcutaneous hindpaw injection of capsaicin. Learning was assessed 24 hr later. Treatment with fixed shock attenuated the capsaicin-induced inhibition of learning. These findings suggest that fixed stimulation promotes adaptive plasticity and may foster recovery after injury.

Cueing unavoidable physical but not emotional stress increases long-term behavioural effects in rats

Cueing a stressor can influence the perception of and the direct reaction to a stressor. Until now, only the direct reaction of an animal to the cue or to the subsequent stressor has been examined. This experiment assesses the long-term behavioural effects of cueing two different stressors. Rats were exposed to physical stress (PS) or emotional stress (ES) in a two-compartment box for 5 consecutive days. PS rats daily received mild foot shocks during 10 min. ES rats were forced to witness the PS rats being foot shocked. A control group was placed in the same box without receiving shocks. Every foot shock was either cued or not cued with a 3-s light signal. In an additional experiment the effect of longer and stronger foot shocks was also investigated. Effects of stress treatments were measured 5 days after the last stress session in a small open field. Data showed that PS and ES had opposite long-term effects on open field activity (activity PS decreased and ES increased). Cueing stress resulted in a more pronounced effect in the PS, but not in the ES group. Presenting the light signal during emotional stress and control treatment resulted in an overall behavioural activation in the open field. All animals were exposed to the light stimulus in an open field after habituation. Both PS groups showed less active exploration during the test, while all cue-naïve animals showed more ambulations. No emotional stress effect was present. The direct reaction of the animals to the cue also differed with treatment: cued PS animals became inactive, while animals of all other groups primarily reacted with active exploration. Increasing the intensity of the foot shocks resulted in an increased physical stress effect, similar to cued physical stress; no effect was found on emotional stress effects. The results indicated that stress associated cues can increase the long-term behavioural effects of physical but not of emotional stress. Exposure to the cue induces a conditioned response in the cued PS group. Witnessing cued or more severe stressors did not influence the emotional stress effect. The findings again demonstrate that emotional and physical stressors are fundamentally different.