Endocrine adaptations in the foal over the perinatal period

In adapting to life ex utero, the foal encounters a number of physiological challenges. It has to assume the nutritional, respiratory and excretory functions of the placenta and activate full regulatory control over its own internal environment for the first time. To achieve this, there must be structural and functional changes to a wide range of tissues including several endocrine glands. In most species, including the horse, these maturational changes begin in late gestation and continue into the first few days of neonatal life. Consequently, during this perinatal period, there are major changes in the sensitivity and/or set point of key endocrine axes, which alter the circulating hormone concentrations in the foal. In turn, these endocrine changes are responsible for many of the other physiological adaptations essential for neonatal survival. The perinatal alterations in the hypothalamic-pituitary-adrenal (HPA) axis are particularly important in these processes, although the sympatho-adrenal medullary axis and endocrine pancreas also have key roles in ensuring homeostasis during the multiple novel stimuli experienced at birth. Abnormalities in the perinatal endocrine profile caused by adverse conditions before or after birth may, therefore, lead to maladaptation or aid survival of the newborn foal depending on the specific circumstances. This review examines the perinatal changes in endocrinology in normal and compromised foals and the role of these endocrine changes in the physiological adaptations to extrauterine life with particular emphasis on the HPA axis, adreno-medullary catecholamines and the endocrine pancreas.