Multidimensional structure of anxiety-related behavior in early-weaned rats

Early environmental stimuli affect various aspects of physical and behavioral development. Weaning is one of the most important events in the early stage of life, and recently we have found that precocious weaning augments anxiety and aggressiveness in mice. Here, we report the presence of virtually identical phenomena in rats. To understand the multidimensional structure of anxiety-related behavior, the influence of early weaning upon behavior in adulthood was investigated using three behavioral tests: the elevated plus-maze test, the hole-board test and the open-field test. Two groups of rats were prepared. One was weaned from the dam at 16 days of age (early-weaned group) and the other at 30 days (normally weaned group) as a control. Both groups were subjected to the three tests at 8-10 weeks of age. The elevated plus-maze test revealed lower frequency of entry to and shorter duration of stay in the open arms in the early-weaned animals. In the hole-board test, the early-weaned rats showed lower frequency and shorter duration of head dipping into the holes. And in the open-field test, the early-weaned rats tended to stay at the central square for a shorter period and to defecate more frequently. The behavioral parameters of the three tests were combined and subjected to principal component analysis (PCA). The factorial scores for six extracted factors were compared between the early-weaned and normally weaned groups, and it was revealed that the early-weaned rats had a lower score in Factor 1 (non-anxious exploration) and Factor 5 (risk assessment behavior). Taken together, these results suggest that the time of weaning had a considerable impact on behavioral development, particularly with respect to anxiety-related behaviors.