Early postnatal protein malnutrition affects learning and memory in the distal but not in the proximal cue version of the Morris water maze

Learning and memory of early postnatal protein malnourished rats were investigated in the Morris water maze. During the lactation period (21 days) each litter (mother plus six male and two female pups) was provided with 16% (well-nourished) or 6% (malnourished) protein diets. After weaning, rats remained on the same diet until 49 days of age. From day 50 on all animals were fed a commercial lab chow. Experiments started on day 70. In experiment I (proximal cue version) the animals were trained to escape from water to a visible platform (3 cm above the water level) in six trials daily for four consecutive days, completing 24 trials. In experiment II (distal cue version) the animals were trained to escape from water to a submerged platform using the same procedure as in experiment II. After the 24th trial, the platform was removed and the animals were submitted to a 60-s trial (probe trial). Seven and twenty-eight days after training, the retention test was conducted in one 180-s trial. The results showed no impairment of the learning or memory of malnourished animals tested in the proximal cue version but an increased latency and distance traveled to find the submerged platform in the distal cue version of the procedure. In the distal cue version the malnourished animals also showed increased latency to find the platform 7 and 28 days after the test training. No difference due to diet was found in the probe trial test indicating that, once the task is acquired, malnourished rats can manage extra-maze cues as easily as well-nourished rats. It is suggested that the present results can be due to alterations produced by protein malnutrition in the hippocampal formation or also to reflect the higher emotionality of rats following early malnutrition, specially considering the fact that postnatally malnourished animals are more reactive to unpleasant or aversive stimuli as cold water.