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.