Heightened arousal in the 2-week-old rat: the importance of starvation

The 2-week-old rat has frequently been reported to be hyperactive; however, the causes of this are disputed. In 3 experiments, with split-litter designs, the following possibilities were eliminated as causative factors: (a) body temperature; (b) nest seeking; and (c) maturation of peripheral sensory, motor, and endocrine systems. In contrast, the isolated 2-week-old rat, in a novel environment, was shown to be very reactive to hunger, however mild. Further, this was true whether the hunger was self-generated during a forced switch in diet or external origin due to food deprivation. These data are consistent with and supportive of the hypothesis that the 2-week-old rat does not have functionally-mature forebrain inhibitory structures with which to modulate arousal. Thus, even a mild arousal stimulus results in extreme hyperactivity. Further, these results, together with previously published results, suggest that the concurrence of isolation, starvation, and novelty of environment are necessary, but none of these alone is sufficient, for the 2-week-old rat to become hyperactive.