Serum GH levels increased in 2- or 8-day-old rat pups when sucking mammary glands whose main milk ducts were ligated. Although intragastric administration of rat milk has been shown to increase serum GH levels in neonatal rats, ingestion of milk during suckling did not increase serum GH values further. In another experiment, 2-day-old pups obtained no milk when they were suckled by anaesthetized mothers, and in this instance the serum GH concentration of the pups decreased. This decrease was prevented if the mothers were injected with oxytocin to counteract the depressant effect of the anaesthesia on milk ejection; nevertheless, GH levels in neonatal animals failed to increase following suckling. Thus some aspect of maternal activity appears to be involved in the suckling-induced increase of serum GH in rat pups. To elucidate which components of maternal activity might be involved, the effects of manipulations of ambient temperature as well as stimulation of the oral or anogenital regions were examined. Exposing rat pups to 37 °C (nest temperature) during the 6-h separation period before suckling prevented the separation-induced decrease in serum GH levels of 2-day-old pups. Moreover, exposure to 37 °C for 30 min following a 6-h separation at room temperature (22 °C) mimicked the effect of suckling in increasing serum GH levels in the pups. Suckling following separation at 37 °C was unable to increase serum GH values further. In other studies, stimulation of the oral zone (feeding from a soft cannula) or anogenital zone (inducing urination and defecation with a wet brush) of the pups significantly decreased neonatal serum GH values. The painful stimulus of administration of s.c. hyperosmotic saline was without effect on serum GH levels.
In summary: (1) suckling, even without milk removal, increases serum GH levels in neonatal rats; (2) some aspect of maternal activity is involved in the suckling-induced increase of serum GH concentration in the neonate; (3) provision of a warm ambient temperature by the mother is a critical component of this maternal activity; (4) the effects of milk and the maternal activity associated with suckling are not additive in increasing serum values of GH in neonatal rats; and (5) oral feeding is not a suitable approach for studying the effects of milk on serum GH in neonatal rats.