Resumo

INTRODUCTION: Mammals exposed to early-life adversity such as maternal separation or social isolation from conspecifics adversely affects brain development and adult behavior. The nature and severity of these effects depend on the age at isolation which can be associated to different periods of neuronal plasticity. The effects of isolation on exploratory activity have been shown to be different depending on the stage of development of animals in isolation. AIM: This study aimed at evaluating the effect of chronic affective isolation in different periods of brain development on the exploratory behavior. METHODS: In the isolation studies four groups of rats (n=12/each) were used. In two groups the pups remained with their mothers (6/mother) until weaning (21 days, 40g) when they were allocated randomly to one of two conditions: 1) grouped, housed 4/cage and handled 3 times a week; 2) isolated, housed individually and handled once a week (isolation rearing). In the other two groups, the young adult rats (140g) were allocated in the same conditions (social isolation). Behavioral tests began after ten weeks. In the maternal separation (MS) the pups (n=10) underwent a daily-3h separation from their mothers from PND1 to weaning (PND21) and the controls (n=11) left undisturbed. Following weaning animals were housed in groups of 4 for 5 weeks before testing. All groups were tested in a circular open arena. The animals were put into the middle of the arena and the behavioral responses scored every minute for 5 minutes: Number of crossings (horizontal exploration) and number of rearings (vertical exploration). Only the groups from the MS study had the number of fecal pellets counted when the animals were submitted to a recognition memory test for 3 days (4 expositions to arena). Controls and isolated groups were compared by Student t-test and the level of significance was p<0.05. RESULTS: All conditions of isolation induced a significant reduction on the vertical exploration while no change was induced on the horizontal exploration. Isolation rearing showed a reduction of 42% ( p=0,001) in the number of rearings only in the first minute in the arena while social isolation showed a reduction of 40 - 43% (p=0,01) on the first two minutes in the arena. MS induced a reduction of 41% (p=0,01) in this score only in the second minute in the arena. When these animals were exposed 4 times to arena on the memory test, they showed a significant decrease in fecal pellets when compared to controls (90%, p=0,02; 79%, p=0,01and 77%, p=0,01). CONCLUSION: Chronic affective stress in any period of development did not induce locomotor hyperactivity as previously reported. However, a significant reduction was found in exploratory activity. In addition, the reductions in fecal pellets observed on MS suggest an anxiogenic effect as the rats responded more intensely to stressor stimulus such as a novel environment.