Effects of maternal and sibling deprivation on basal and stress induced hypothalamic-pituitary-adrenal components in the infant rat

Abstract

Prolonged maternal deprivation during early infancy increases basal- and stress-induced corticosterone (CORT) levels, but the underlying mechanism is not clear. In general, stressors activate the hypothalamic-pituitary-adrenal (HPA) axis, with secretion and compensatory synthesis of hypothalamic cortcotropin-releasing hormone (CRH). In the infant rat, we have demonstrated that maximally tolerated acute cold stress induced a robust elevation of plasma CORT throughout the first 2 postnatal weeks. However CRH messenger RNA (CRH-mRNA) abundance 4 h subsequent to cold stress was enhanced only in rats aged 9 days or older. This suggests a developmental regulation of the CRH component of the HPA-response to this stressor. The present study examined whether increased basal and cold stress-induced CORT levels after 24 h of maternal deprivation were due to enhanced CRH-mRNA abundance in the hypothalamic paraventricular nucleus (PVN). CRH-mRNA abundance, and basal- and cold-induced plasma CORT levels were measured in maternally deprived 6 and 9-day-old pups compared to non-deprived controls. Maternal deprivation increased basal and cold-induced CORT levels on both 6 and 9-day-old rats. CRH-mRNA abundance in the PVN of deprived rats did not differ from that in non-deprived rats. Our results indicate that the enhanced basal and stress-induced plasma CORT observed after 24 h maternal deprivation is not due to increased CRH-mRNA abundance in the PVN.

Fig. 1

Time course of basal and cold stress-induced plasma corticosterone in 24 h maternally deprived 6-day-old rats. *Basal CORT was significantly elevated in individually deprived (I-DEP) and group-deprived rats (G-DEP) compared with non-deprived controls (N-DEP), P<0.05. **Basal CORT was significantly elevated in G-DEP compared with 1-DEP rats. Cold stress-induced peak plasma corticosterone was significantly higher in I-DEP or G-DEP versus N-DEP rats, (P < 0.05). Values represent the mean ± SEM of 6–8 rats per group.

Fig. 2

Time course of basal and cold stress-induced plasma corticosterone in 24 h maternally deprived 9-day-old rats. Basal CORT was significantly elevated in individually deprived rats (I-DEP) and group-deprived rats (G-DEP), compared with non-deprived controls (N-DEP). Evaluated separately, both G-DEP and I-DEP plasma corticosterone levels were significantly higher than those of the N-DEP group. Cold stress-induced peak plasma corticosterone was significantly higher and more sustained in G-DEP or I-DEP versus N-DEP rats and G-DEP versus I-DEP rats. Values are the mean ± SEM of 6–8 rats per group. *Significantly different from N-DEP (P < 0.05); ⁺Significantly different from I-DEP (P < 0.05).

Fig. 3

Steady-state CRH messenger RNA abundance in the paraventricular nucleus of 9-day-old rats. Groups were: individually deprived (I-DEP); group-deprived (G-DEP); and controls (N-DEP). Differences among groups were not statistically significant (P > 0.05). Values are expressed in arbitrary units and presented as mean ± SEM of 6–12 sections from a minimum 4 rats.