Endocrine and behavioural plasticity in response to juvenile stress in the semi-precocial rodent Octodon degus

The present study in the South American rodent Octodon degus shows for the first time that the postnatal development of hypothalamic–pituitary–adrenal axis function in this semi-precocial species differs from that of altricial rodents, i.e. rats or mice, in several aspects. Our experiments revealed a particular pattern of hypothalamic–pituitary–adrenal axis activity during the first 3 weeks of life characterized by (i) a period of low plasma glucocorticoid concentrations, during which (ii) brief stress exposure (1 h parental separation) is able to elevate glucocorticoids significantly. In addition, (iii) repeated stress exposure (1 h parental separation daily) during the first 3 weeks of life resulted in females, but not in males, in an attenuated separation-induced increase of glucocorticoids, and a higher behavioural activity in both sexes at postnatal day 21. These data indicate that parental separation early in life acts as a ‘strong’ stressor in this species, which on the long run can alter endocrine stress response at the time of weaning in a sex-specific manner. These findings support the role of the hypothalamic–pituitary–adrenal axis as one of the key factors mediating the effects of early life stress on the neuronal network and behaviour in O. degus.