Unexpected stress termination triggers a relief-like response: A simple behavioral paradigm for studying positive...

This study aimed to develop a behavioral paradigm in rats to examine the neurobehavioral mechanisms underlying the relief response after the termination of acute stress. The cage test (CT)-a reliable method for behavioral assessment-was used for all procedures. Experiment 1 evaluated which stressor produced the strongest effects on exploratory activity and 50-kHz ultrasonic vocalizations. Stressors included a single 24-hour housing in empty (EC), wet bedding (WB), and flooded (FC) cages. Afterwards, animals were quickly transferred to the CT with dry bedding (DB) for a 5-minute test. Control groups consisted of animals housed and tested either on DB or WB. This experiment was repeated three times, with animals receiving different treatments each week. Experiment 2 assessed whether the relief response varied after 6, 12, or 24 hours of stress exposure, while Experiment 3 examined the effects of stress and relief on target genes and brain regions involved in stress and reward processing. We found that, after stress, DB increased 50-kHz calls, with FC producing the strongest effect even after repeated testing. WB also elicited strong responses, though these responses diminished with repetition. Both stressors increased frequency-modulated calls, with longer exposures inducing the greatest responses. Stress and relief altered gene expression in the medial prefrontal cortex, nucleus accumbens, and hippocampus, with the relief experience counteracting many stress-induced changes. Our paradigm proved useful in inducing a positive contrast effect on stress removal, characterized by higher rates of 50-kHz calls-especially the frequency-modulated subtypes-a phenomenon we interpret as a relief-like response.