Contextual fear conditioning in maternal separated rats: the amygdala as a site for alterations

The first 2 weeks of life are a critical period for neural development in rats. Repeated long-term separation from the dam is considered to be one of the most potent stressors to which rat pups can be exposed, and permanently modifies neurobiological and behavioral parameters. Prolonged periods of maternal separation (MS) usually increase stress reactivity during adulthood, and enhance anxiety-like behavior. The aim of this study was to verify the effects of maternal separation during the neonatal period on memory as well as on biochemical parameters (Na(+), K(+)-ATPase and antioxidant enzymes activities) in the amygdala of adult rats. Females and male Wistar rats were subjected to repeated maternal separation (incubator at 32 °C, 3 h/day) during postnatal days 1-10. At 60 days of age, the subjects were exposed to a Contextual fear conditioning task. One week after the behavioral task, animals were sacrificed and the amygdala was dissected for evaluation of Na(+), K(+)-ATPase and antioxidant enzymes activities. Student-t test showed significant MS effect, causing an increase of freezing time in the three exposures to the aversive context in both sexes. Considering biochemical parameters Student-t test showed significant MS effect causing an increase of Na(+), K(+)-ATPase activity in both sexes. On the other hand, no differences were found among the groups on the antioxidant enzymes activities [superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT)] in male rats, but in females, we found a significant MS effect, causing an increase of CAT activity and no differences were found among the groups on SOD and GPx activities. Our results suggest a role of early rearing environment in programming fear learning and memory in adulthood. An early stress experience such as maternal separation may increase activity in the amygdala (as pointed by the increased activity of Na(+), K(+)-ATPase), affecting behaviors related to fear in adulthood, and this effect could be task-specific.