A further analysis of behavioral and endocrine effects of unpredictable chronic stress

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Toler nee can develop when rats are repeatedly exposed to some predictable stressors. This does not occur, however, when rats are exposed to unpredictable chronic stress. In this study we have analyzed some behavioral and endocrine effects in male and female rats treated daily with unpredictable emotional stressor (ES-groups) or unpredictable physical stressors (PS-groups) over a 14-day period. Animals were then submitted to three behavioral tests at 24 hr intervals. Experiment 1 shows that when rats were tested in an enriched environment both total motor activity and exploration of the novel object were impaired by the Ps treatment. This suggests the occurrence of motivational deficit. The fact that the PS-groups also showed increased intratest defecation suggests increased emotionality. When animals were submitted to an emergence test the PS-groups showed longer emergence latency, lower frequency of emergences and lower time spent exploring the emergence compartment than the Es- and the C-group. This strongly supports that the PS treatment increased emotionality in rats. When Es- and PS-groups were exposed to a forced swim test they showed longer immobility duration (despair reaction) but only the PS-group displayed lower frequency of jumps (escape reaction). Results of all tests performed revealed that females were more resistent than males to the behavioral effects of the PS treatment. The day after the behavioral testing was completed, basal levels of corticosterone and prolactin were investigated in male subjects. The PS-group showed higher baseline levels of these “stress labile” hormones than the ES and the C-group. When animals were submitted to restraint stress before sampling for hormone assay, corticosterone and prolactin responses were similar in all groups. In Experiment 2 it was found that the PS treatment caused constant diestrus in female rats whereas the Es treatment caused minor estrous cycle distrubances. The results provide further evidence supporting that the chronic administration of unpredictable chronic stress is a useful animal model of human depression.