Stressful life events play an important role in the aetiology of human mood disorders and are frequently modelled by chronic social defeat (SD) in rodents. Exploratory phenotype in rats is a stable trait that is likely related to inter-individual differences in reactivity to stress. The aim of the study was to confirm that low levels of exploratory activity (LE) are, in rodents, a risk factor for passive stress coping, and to clarify the role of medium (ME) and high (HE) exploratory disposition in the sensitivity to SD.

We examined the effect of SD on male Wistar rats with LE, ME, and HE activity levels as measured in the exploration box. After SD, the rats were evaluated in social preference, elevated zero maze, and open-field tests. Brain tissue levels of monoamines were measured by high-performance liquid chromatography.

Rats submitted to SD exhibited lower weight gain, higher sucrose consumption, showed larger stress-induced hyperthermia, lower levels of homovanillic acid in the frontal cortex, and higher levels of noradrenaline in the amygdala and hippocampus. Open-field, elevated zero maze, and social preference tests revealed the interaction between stress and phenotype, as only LE-rats were further inhibited by SD. ME-rats exhibited the least reactivity to stress in terms of changes in body weight, stress-induced hyperthermia, and sucrose intake.

Both low and high novelty-related activity, especially the former, are associated with elevated sensitivity to social stress. This study shows that both tails of a behavioural dimension can produce stress-related vulnerability.

Objective: The trait of experiencing positive affect could make a unique contribution to the pathogenesis of affective disorders. Animal models of positive emotionality are scarce but 50-kHz ultrasonic vocalizations (USVs) in rats have been associated with rewarding experience. We have previously reported that persistent inter-individual differences in expression of 50-kHz USVs (chirps) exist, and that male rats producing fewer 50-kHz USVs are more sensitive to chronic variable stress (CVS). In this study we examined the effect of CVS on extracellular serotonin (5-HT) levels in hippocampus, comparing high-chirping (HC) and low-chirping (LC) rats.

Methods: Male rats were classified as HC- and LC-rats on the basis of stable levels of USV response using sessions of tickling-like stimulation. CVS procedure lasted 4 weeks. The administration of citalopram (1 μM) and measurements of levels of 5-HT were done by microdialysis. Corticosterone levels were also measured from trunk blood.

Results: Male LC-rats were more sensitive to CVS: the effect of stress on body weight gain was larger and corticosterone levels from full blood were higher in the stressed LC animals as compared to both the unstressed groups and the stressed HC animals. While no baseline differences in extracellular 5-HT levels in hippocampus were found between groups, the increase in extracellular 5-HT levels induced by citalopram was much higher in LC-rats.

Conclusion: Chronic stress appears to modify hippocampal 5-HT overflow in rats with low positive affectivity. This finding supports the notion of greater vulnerability to CVS in male rats with low positive affectivity.