The HPA system during the postnatal development of CD1 mice and the effects of maternal deprivation.

In this study we describe in 9- and 18-day-old CD1 mice (i) the basal and stress-induced activity of markers of the HPA system in blood, brain and pituitary, (ii) the effects of a 24-h maternal deprivation and (iii) the influence of anogenital stimulation (stroking) in deprived pups on these markers. We found low basal concentrations of ACTH and corticosterone at postnatal day (pnd) 9 and no or little response to a mild stressor at this age, confirming the existence of a SHRP in mice. At pnd 18 the mice displayed an adult-like ACTH and corticosterone response following a mild stressor. Maternal deprivation resulted in enhanced basal and stress levels of corticosterone at both ages. Interestingly, basal ACTH levels were elevated following maternal deprivation at pnd 9. At pnd 18 maternal deprivation resulted in a blunted ACTH response. Maternal deprivation resulted in a down-regulation of GR, MR, CRH and POMC transcript in the brain. However, maternally deprived 18-day-old pups displayed increased levels of CRH and POMC transcript, while GR and MR mRNA was also down-regulated. Anogenital stroking could reverse maternal deprivation effects on ACTH and MR mRNA, but not CRH mRNA. We conclude that (i) at the two measured time points the HPA axis develops similar in mice as in rats, (ii) maternal deprivation has pronounced effects in mice, which are similar to that found in the rat and (iii) there are a number of significant differences, which mainly concern the central CRH-ACTH components of the axis.