Recent Progress on the Effects of Weaning and Nutrition on the Action of Physiologically Active Peptides in the Ruminant

In the present mini-review, we will introduce our recent new findings accumulated at the Animal Physiology Laboratory over the last several years, in particular with reference to studies on leptin and ghrelin, which are involved in both the control of the somatotropic axis (GHRH/GHS-GH-IGF-I system) and in the regulation of the energy balance of domestic animals. Our interest is also focused on weaning because it is the most drastic event that neonates have to experience through their lives. 1) We compared the effect of feeding on basal plasma GH levels in 3-(milk-fed) and 12-week old (concentrates and roughage-fed) Holstein bull calves, and found that feeding caused a rise in GH levels at 3 weeks but a reduction at 12 weeks of age. 2) Plasma leptin levels, which were increased with aging in Japanese Black cattle, were not changed during the 5 hours after feeding. However, the expression of leptin, and CCK_A and CCK_B receptors in the stomachs of calves was abolished after weaning. This finding was confirmed even when calves were maintained on a milk replacer diet with an intragastric injection of VFA until 13 weeks of age. In addition, we demonstrated that VFA increased the expression of leptin, but suppressed that of the leptin receptor (OB-Ra) in calf anterior pituitary cells. 3) Although the basal ghrelin level in the plasma of goats decreases as the animal ages, parallel with basal GH levels as well as the responses of GH to GH secretagogues, milk feeding appears not to reduce the basal ghrelin levels in calves and goats. Weaning significantly decreased the ghrelin content to 30% of that seen pre-weaning in the stomachs. These findings will substantially contribute to the studies on the control of the somatotropic axis and the regulation of the energy balance of ruminant animals