Effects of early weaning on anxiety and autonomic responses to stress in rats

Environmental stimuli affect various aspects of the early physical and behavioral development in rats. One of the most important events in the early stage of life is weaning, and we recently reported that precocious weaning augments anxiety and aggressiveness in rats and mice. In the present study, we investigated the autonomic responses to stress in two groups of rats: the early-weaned group (weaned at 16 days of age), and the normally weaned group (weaned at 30 days) as a control. First, the early and normally weaned rats were subjected to an elevated plus-maze test to assess their anxiety levels. It was confirmed that early-weaned male rats, but not the females, showed a lower frequency of entry into and shorter duration of stay in the open arms of the maze compared to the normally weaned rats. Subsequently, the two groups were either placed in a novel clean cage or exposed to an unfamiliar conspecific, and their heart rates and core body temperatures were monitored to evaluate their autonomic stress responses. There was an exacerbation of autonomic responses, such as stress-induced hyperthermia and tachycardia, and an alternation of behavioral responses, including increased sniffing, and decreased grooming and resting. These effects of early weaning were significant only in males. In contrast, when rats encountered an unfamiliar individual, no significant differences were observed between the two groups in either sex. This suggests that stimuli emanating from an unfamiliar intruder were too intense to detect the augmentation of stress responses in the early-weaned rat. The results of the present study demonstrate that precocious weaning augments, not only behavioral but also autonomic responses, to stressful conditions with sexually dimorphic patterns, i.e. more profoundly in males than in females.