Ontogeny of Home-orienting Behavior in Rat Pups During the First Half of the Lactation Period: Effects of Early Postnatal Protein Malnutrition on Repeated but not Single Testing

The ontogeny of home-orienting behavior was investigated during the first half of the lactation period in postnatally protein malnourished pups using repeated or single testing at 7, 9 and 11 days of life. During the lactation period (21 days) each litter (6 male and 2 female pups) was provided with 16% (control) or 6% (low protein) casein diets. To study home-orienting ability the well-nourished and malnourished pups were divided into two groups: the independent group (single testing), in which the pup was tested only once at 7, 9 or 11 days of age, and the dependent group (repeated testing), in which the same animal was tested once a day at 7, 9 and 11 days of age. Latency to reach the nest bedding, latency to reach the fresh bedding, time spent in the half of the box that contained nest bedding, time spent in the half of the box that contained fresh bedding, and total number of entered sections were recorded in 3 min sessions. Postnatal protein malnutrition significantly impaired the ability (higher latencies) to locate nest bedding in the group of pups repeatedly tested at 7, 9 and 11 days of age. This difference was not found in the group of pups tested just once at 7, 9 or 11 days of age. Both groups showed increased ability to find nest bedding across age. Locomotor activity was similar for well-nourished and malnourished pups in both groups, suggesting that impaired performance to find nest bedding in malnourished pups repeatedly tested at 7, 9 and 11 days of age cannot be attributed to differences in locomotion ability. The results suggest a possible role of spatial learning in the home-orienting behavior when the same pup is repeatedly tested at 7, 9 and 11 days of age as compared with pups tested just once on each of those days. This impairment of spatial learning in response to nest odor in malnourished pups is discussed in terms of its relevance for social behavior in early infancy and adulthood.