Behavioural Brain Research 156 (2005) 45–52 Research report Multidimensional structure of anxiety-related behavior in early-weaned rats Kahoru Kanari, Takefumi Kikusui ∗ , Yukari Takeuchi, YujiMori Laboratory of Veterinary Ethology, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan Received 19 December 2003; received in revised form 6 May 2004; accepted 10 May 2004 Available online 19 June 2004 Abstract 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. © 2004 Elsevier B.V. All rights reserved. Keywords: Maternal environment; Anxiety behavior; Principal component analysis 1. Introduction It is well known that events in the early stage of life have long-lasting influences on the behavior of individuals in adulthood. Maternal deprivation during the first 10 days of neonatal period, when the mother rat shows intensive nurs- ing behaviors, such as grooming and licking, augments the anxiety-related behaviors of the pups in their adulthood with- out influencing somatic growth [4,20,28]. In addition, it is known that the maternal deprivation deteriorates the resis- tance against cancer [15] and enhances the neuroendocrine response to stressors [10,16,25]. ∗ Corresponding author. Tel.: +81 3 5841 5475; fax: +81 3 5841 8190. E-mail address: akikus@mail.ecc.u-tokyo.ac.jp (T. Kikusui). In these previous studies, the manipulation of maternal care was conducted during the first week of the neonatal period, i.e., the stress hypo-responsive period [19]. In the normal developmental process of rats, the pups are able to eat, maintain body temperature and evacuate by themselves from 13 days of age [27]. The neuroendocrine system be- comes sensitive to stressors from this point throughout the weaning period [19]. Weaning is one of the most important events characterizing the early developmental stage. After weaning, the offspring must become nutritionally and behav- iorally independent from their dam, so it is highly conceiv- able that the weaning procedure influences their physiologi- cal and neurobehavioral development. In support of this idea, early-weaned rats have been reported to show a low rate of play–fighting behavior and low exploratory behavior in their adulthood [13], and early weaning has also been shown to 0166-4328/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bbr.2004.05.008