Handling during suckling alters rat behavior but does not reverse the deleterious effects of undernutrition on naltrexone-induced inhibition of exploratory activity.

The effects of undernutrition during suckling, early environmental stimulation and naltrexone administration on the locomotor activity test were investigated in adult rats. Young rats were undernourished from delivery until weaning (23 days) by feeding their dams a diet containing 7% casein diet. Mothers of well-nourished rats were fed a 25% casein diet. After weaning, both groups of rats received standard lab chow. From day 2 to day 8 after delivery the animals were stimulated by being placed individually in plastic containers for 4 minutes. From day 12 to 19 the litters were removed from their home cages and placed for 10 min in plastic cages. Exploratory behavior was assessed using a biocompartmentalized open field. The number of crossings from one compartment to the other and the number of rearings were recorded. Undernourished animals crossed less frequently than control animals and presented a higher latency to the first crossing response. Early handling increased the total number of rearing responses regardless of nutritional treatment. Naltrexone administration caused a dose-dependent reduction in the number of rearing responses only in the well-nourished group.