Effects of post-training beta-endorphin and Met-enkephalin administration on two-way active avoidance task in well-nourished and protein malnourished rats.

Protein malnutrition during suckling period or throughout the life affects the hypothalamic beta-endorphinergic system of adult rats. In the present study, rats were undernourished during suckling by feeding their dams an 8% casein diet whereas well-nourished dams received a 25% casein diet from birth until weaning (21 day of postnatal life). After weaning, the offsprings were maintained with the same diet as their dams. When rats were 3 month-old, they were subjected to two-way active avoidance task. Protein malnutrition did not affect the performance in the two-way active avoidance task. Post-training beta-endorphin or Met-enkephalin administration impaired the retention of shuttle avoidance task in both well-nourished and undernourished rats. However, the amnesic effect of the peptides was only achieved in undernourished rats with higher doses of opioids when compared to the well-nourished rats. These data suggest that undernourished rats present alterations in opioid sensitivity which may be related to changes in the levels of beta-endorphin previously observed both in brain and hypothalamus of early undernourished adult rats.