Maternal regulation of the hypothalamic-pituitary-adrenal axis in the infant rat: the roles of feeding and stroking.

Twenty-four hours of maternal separation results in increased secretion of ACTH and corticosterone (CORT), suggesting the hypothalamic-pituitary-adrenal (HPA) axis is regulated by some aspect of maternal behavior. Previous results indicate that feeding plays a role in maintaining low levels of CORT in 12-day-old pups. In Experiment 1 basal and stress levels of CORT and ACTH were measured in maternally-deprived pups either provided or not with milk to determine whether: (1) feeding maintains ACTH secretion at low levels, and/or (2) feeding maintains the adrenal insensitive to ACTH. The results showed that, although ACTH levels were markedly low (compared to previous values reported by this laboratory) for both groups, only non-fed pups showed a robust increase in basal and stress CORT levels. During the deprivation period in Experiment 1, all pups were manually stroked to induce urination and defecation, suggesting an effect of stroking on ACTH secretion. Experiment 2 examined this hypothesis. Stroking suppressed stress-induced elevations of ACTH secretion due to maternal deprivation. CORT levels, however, were elevated in all deprived pups. The results indicate that maternal regulation of the infant's HPA axis occurs at multiple levels. Feeding appears to regulate adrenal sensitivity, whereas anogenital stroking inhibits the activation of centrally-controlled components of the axis.