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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.
At birth, the foal faces a number of physiological challenges in adapting to life ex utero including assuming the nutritional, respiratory and excretory functions of the placenta along with full regulatory control over its own internal environment for the first time (Sangild et al. 2000). Many of the homeostatic functions, such as thermo- and gluco-regulation, are not active before birth but, like pulmonary respiration, are vital after delivery (Fowden et al. 1998). Specific tissues and organ systems of the foal must, therefore, be competent to carry out their new functions at or shortly after birth, if the neonate is to survive the passage from intra- to extrauterine life. Consequently, preparations for this transition begin during late gestation and involve maturational changes in the structure and functional capacity of key tissues essential for immediate neonatal survival (Silver 1990; Sangild et al. 2000). Longer-term adaptations in these and other tissues then continue over the next few days of neonatal life in response to the novel stimuli of high pO2, cold exposure, enteral nutrition, locomotion and behavioural interactions.
Endocrine glands are amongst the tissues adapting in function during the perinatal period. Changes in the sensitivity and/or set points of several major endocrine axes have been observed both before and/or shortly after birth in a number of species including the horse (Berg et al. 2007; Fowden and Forhead 2009). Indeed, these endocrine changes and the ensuing alterations in circulating hormone concentrations are often responsible for the maturational changes seen in other physiological systems during the perinatal period. The glucocorticoids, in particular, have a wide range of maturational effects but many other hormones, such as the catecholamines and pancreatic hormones, also have important roles in maintaining homeostasis during the stress of adapting to extrauterine life (Fowden et al. 1998; Fowden and Forhead 2009). In addition to the normal maturational processes, there are also changes in the intrauterine development and perinatal adaptation of the endocrine glands in response to adverse conditions that alter the environment in utero, such as maternal ill health, nutrient deprivation or placental dysfunction (Bertram and Hanson 2002; McMillen and Robinson 2005; Fowden et al. 2006). This review examines the perinatal changes in endocrinology in normal and compromised foals and the role of these changes in the physiological adaptations to extrauterine life with particular emphasis on the hypothalamic-pituitary-adrenal (HPA) axis, adreno-medullary catecholamines and the endocrine pancreas.
The hypothalmic-pituitary-adrenal axis
Activation of the hypothalmic-pituitary-adrenal (HPA) axis is important for many of the adaptive processes essential for life ex utero. In all species studied to date, including the horse, there is an increase in the cortisol concentration in the fetal circulation during the period immediately before birth as a result of developmental changes at all levels of the HPA axis (Fig 1) (Liggins 1994; Challis et al. 2000). This increase in fetal glucocorticoid availability induces structural and functional changes in a variety of different tissues, including the lungs, liver, kidneys and gastrointestinal tract (Silver 1990; Sangild et al. 2000). It also activates many of the physiological processes that are vital at birth but have little or no function in fetal life, such as pulmonary ventilation, glucogenesis and renal sodium conservation (Fowden et al. 1998). In many species, prepartum activation of the fetal HPA axis is also involved in the onset of labour. In man and other primates, parturition is facilitated by increased adrenal output of oestrogen precursors, whereas in ruminants, it is the cortisol surge itself that induces labour by actions on uteroplacental steroidogenesis and prostaglandin production (Wood and Cudd 1997; Challis et al. 2000). Consequently, in some species, adrenal glucocorticoids act to coordinate prepartum maturation of the fetus with the onset of uterine contractile activity, which maximises the chances of delivering viable young (Silver 1990). In horses, the signal for the onset of labour is not as clear as in other species but may involve increased adrenal secretion of steroids (Silver 1994).
The hypothalmic-pituitary-adrenal axis and cortisol production
In the horse, activation of the HPA axis occurs late in gestation compared with other species (Rossdale et al. 1973; Fowden and Silver 1995; Wood and Cudd 1997). Fetal cortisol concentrations only begin to rise about 5 days before birth and then escalate rapidly towards term in pony foals (Fig 1b). This is associated with a doubling of adrenal weight over the last 5% of gestation, primarily due to increased growth of the zona fasciculata in the adrenal cortex (Comline and Silver 1971). Similar increments in adrenal weight and cortisol concentrations are observed in fetal sheep and pigs during late gestation but these begin proportionately earlier in gestation and occur more slowly than in fetal horses (Wood and Cudd 1997). In general, plasma adrenocorticotropic hormone (ACTH) concentrations rise in parallel with the cortisol concentrations in the fetus (Silver and Fowden 1994; Cudd et al. 1995). Thus, in sheep, there is a progressive increase in plasma ACTH concentrations from about 15 days before birth, whereas, in horses, fetal plasma ACTH is not elevated until the day of delivery (Fig 1a). Fetal adreno-cortical activity, therefore, appears to rise over the last 15% of gestation in sheep and pigs but is confined to the final 1–2% of gestation in the horse (Fowden and Silver 1995). However, fetal plasma cortisol binding capacity declines over a slightly longer period before birth in the horse than in other species; thus, the period of increased prepartum cortisol exposure may be longer than appears from measurements of total cortisol concentrations alone (Cudd et al. 1995). Nevertheless, overall, there is a very narrow window just before delivery of the foal for glucocorticoids to stimulate fetal maturation in preparation for birth. This may explain the greater incidence of prematurity and maladaptation in newborn foals than seen in other precocious species (Rossdale and Silver 1982).
In contrast to other species (Silver 1990), cortisol concentrations continue to rise for 2 h after birth in full-term pony foals before falling to basal values 24 h after delivery (Fig 1b). Concentrations then remain stable for the next 10–14 days (Fig 1b). Plasma ACTH concentrations fall progressively from birth to reach basal level by 8 h after delivery (Fig 1a). Similar decreases in plasma ACTH and cortisol are observed between the first and subsequent days after birth in Thoroughbred (TB), Standardbred and Quarter Horse (QH) foals (Wong et al. 2009; Hart et al. 2009a). In pony foals delivered prematurely before 320 days, cortisol concentrations are lower than normal at birth and fail to increase significantly over the next 2 h (Silver et al. 1984). In contrast, plasma ACTH concentrations continue to rise after birth in premature foals and are 2–5-fold higher than normal during the first 2 h after birth (Silver et al. 1984). In term newborn foals showing mixed premature and full-term characteristics (Rossdale et al. 1984), cortisol concentrations are intermediate between those of premature and mature foals at birth and rise in the 2 h after birth but to a lesser extent than seen in mature neonates born spontaneously at full-term (Silver et al. 1984). If the premature foals born close to full-term survive beyond the first 48 h, their cortisol concentrations are raised compared with healthy mature foals (Panzani et al. 2009). Even when delivery is induced 24–48 h before full-term on the basis of milk parameters (Ousey et al. 1984), plasma cortisol concentrations are elevated 2–3 fold throughout the first 10 days of life in pony foals that otherwise appear normal and mature by clinical criteria (Holdstock et al. 2011). Overall, these observations show that the cortisol concentrations observed in the immediate neonatal period are determined by the maturity of the foal, and possibly, by its method of delivery.
High concentrations of cortisol and ACTH are also seen in foals aged 1–7 days hospitalised for a range of illnesses including septic and nonseptic conditions (Gold et al. 2007; Hurcombe et al. 2008; Panzani et al. 2009; Hart et al. 2009b). Nonsurvival of sick foals is associated with both higher and lower cortisol concentrations and with a high ACTH:cortisol ratio compared with healthy foals (Gold et al. 2007; Hurcombe et al. 2008; Panzani et al. 2009; Hart et al. 2009b), which suggests that some sick foals are capable of mounting a stress response including activation of adrenal cortisol secretion while others suffer from adrenal exhaustion or insufficiency. Foals born prematurely following maternal placentitis may have precocious adrenocortical maturation, despite displaying other clinical features of prematurity, and can survive with minimal intensive care (Rossdale et al. 1991; Ousey 2004). When mares with placentitis are treated with antimicrobials and immunomodulators, gestational length is close to normal and foals delivered alive have cortisol concentrations within the normal range in the majority of cases (Bailey et al. 2010). In contrast, in fescue toxicosis when fetal HPA development is suppressed by the dopaminergic action of the maternally ingested alkaloids, foals delivered after a full term have low concentrations of cortisol and ACTH at birth and usually do not survive (Brendemeuhl et al. 1995; Cross et al. 1995).
Adrenocortical sensitivity to ACTH in terms of cortisol secretion changes dramatically in the foal during the perinatal period. Before about 290 days of gestation, the fetal adrenal cortex appears to be unresponsive to exogenously administered ACTH1-24 but, thereafter, there is a small but significant rise in fetal plasma cortisol when ACTH is administered (Fig 2a) (Silver and Fowden 1994; Lyle et al. 2010). This response then increases with increasing proximity to delivery (Fig 2a). Adrenocortical sensitivity to exogenous ACTH1-24 is maximal at birth and then declines again in newborn pony foals, although the postnatal cortisol responses are greater than those observed prenatally (Fig 2a). Similar temporal changes in adrenocortical responsiveness to exogenous administration of ACTH have been observed in newborn TB and QH foals, in response to low and high doses of ACTH (Wong et al. 2009; Hart et al. 2009a). In pony foals delivered prematurely before 320 days, the cortisol response to exogenous ACTH1-24 is small at birth and resembles that of the fetus in late gestation (Rossdale et al. 1982; Silver et al. 1984; Silver and Fowden 1995). It remains suppressed for the next 2 days if the foal survives, despite high endogenous ACTH concentrations (Silver et al. 1984). When gestational length is shortened by maternal administration of dexamethasone beginning after 315 days, the foals are small at birth yet viable with normal neonatal cortisol concentrations (Ousey et al. 2011). However, there is a tendency for reduced cortisol responses to exogenous ACTH1-24 in the 15–24 h period after birth of these foals (Ousey et al. 2011). Blunted cortisol responses to exogenous ACTH are also observed in sick foals hospitalised for sepsis and other disorders, such as multiple organ dysfunction syndrome, which suggests that adrenal exhaustion can occur when stress is prolonged (Hart et al. 2009b). These observations show that the ability of the equine adrenal gland to secrete cortisol develops only late in gestation and is directly related to the viability of the foal. The cortisol response to ACTH administration, therefore, provides a useful clinical test of the maturity and health status of newborn foals (Rossdale et al. 1984; Wong et al. 2009).
The developmental profile of the adrenocortical response to stressful stimuli, such as hypoglycaemia and hypotension, differs from that seen in response to exogenous administration of ACTH1-24 (Fig 2b) (Silver et al. 1987; Silver and Fowden 1995; O'Connor et al. 2005). The increment in plasma cortisol in response to insulin-induced hypoglycaemia is smaller and delayed relative to that evoked by direct administration of ACTH1-24 in both near-term fetuses and neonates (Fig 2b). However, by 7–14 days, there is a rapid, sustained increase in plasma cortisol concentrations in response to both hypoglycaemia and hypotension (Silver et al. 1987; O'Connor et al. 2005). The cortisol response to hypoglycaemia, therefore, rises progressively with postnatal age in contrast to that seen in response to exogenous ACTH administration (Fig 2a). This suggests that, during the perinatal period, there may be developmental changes occurring at the hypothalamic-pituitary level that determine the secretion of endogenous, bioactive ACTH. Certainly, there is little, if any, ACTH release in response to insulin-induced hypoglycaemia in the fetus, even close to term, yet by 3–4 days after birth this response is brisk and significantly greater in magnitude than that seen prenatally (Fig 3a). However, despite the greater increment in ACTH, adrenocortical sensitivity to endogenous ACTH appears to be less 3–4 days after birth than in newborn foals (Fig 3b), consistent with the findings with exogenous ACTH1-24 administration (Fig 2a). Rapid rises in ACTH and cortisol concentrations are also seen in 7-day-old foals in response to acute hypotension (O'Connor et al. 2005). However, by 14 days, the ACTH response to this physiological challenge is significantly less than at 7 days, despite a similar cortisol increment (O'Connor et al. 2005). There may, therefore, be changes in the bioactivity of pituitary ACTH during the neonatal period or, alternatively, other factors, such as adrenal innervation, may act to increase adrenocortical sensitivity to endogenous ACTH during stressful conditions in foals by age 14 days (Edwards 1997; Wood and Cudd 1997).
The cellular and molecular mechanisms responsible for the perinatal changes in basal and stimulated HPA function in the foal remain largely unknown. At the pituitary level, there is little evidence for changes in the morphology or density of the corticotrophs observed in the fetal ovine pituitary during the prepartum period (Fowden and Silver 1995). However, the alterations in the pulsatile pattern of fetal ACTH concentrations with proximity to delivery suggest that there are prepartum changes in hypothalamic secretion of the ACTH releasing factors, corticotrophin releasing hormone (CRH) and arginine vasopressin, and/or in the abundance of the receptor for these neuropeptides on the corticotrophs of the foal (Cudd et al. 1995). At the adrenal level, production of cortisol depends on 3 key rate-limiting enzymes: cholesterol side chain cleavage (P450SCC), 3β-hydroxysteroid dehydrogenase (3βHSD) and 17α-hydroxylase (P450C17). Cytochrome P450SCC, which converts cholesterol to pregnenolone (P5) is present in the zona glomerulosa and putative zona reticularis of the equine adrenal gland from as early as 150 days of gestation and rises in abundance in these regions, and in the zona fasciculata, with increasing gestational age (Han et al. 1995). The enzyme 3βHSD synthesises progesterone from P5 and increases in abundance, predominantly in the zona fasciculata, from about 280 days of gestation onwards, in association with ultrastructural changes to this zone indicative of increased steroid synthesis (Webb and Stevens 1981; Han et al. 1995). In contrast, P450C17, required for cortisol production, is only detectable in the equine adrenal gland at low concentrations until very close to term when its expression increases in parallel with the prepartum rise in fetal cortisol concentrations (Fig 1b). By birth, all 3 enzymes are abundant throughout the zona fasciculata of the neonatal adrenal gland and show little further change in expression during the first few weeks of post natal life (Han et al. 1995). These perinatal changes in cytoarchitecture and enzyme content of the equine adrenal gland are, therefore, consistent with the increased responsiveness to exogenous ACTH1-24 between late gestation and birth (Fig 2a), and with the positive correlation observed between the cortisol response to ACTH1-24 of the newborn foal and its gestational age at delivery (Ousey et al. 2011). They also support the suggestion that other factors are involved in the subsequent post natal changes in adreno cortical ACTH responsiveness (Fig 2a). These factors include adrenal blood flow, ACTH receptor density, ACTH clearance and the release of adreno-medullary peptides with effects on the adrenal cortex (Fowden and Silver 1995; Wood and Cudd 1997).
In rat dams, environmental challenges during pregnancy influence HPA function in the newborn pups but little is known about the immediate neonatal consequences of suboptimal intrauterine conditions on this axis in more precocious species such as the horse (Lesage et al. 2001; Fowden et al. 2006). Dysphagia and chronic weight loss during TB pregnancy caused by infection with Streptococcus equi increases the basal cortisol concentrations of their growth restricted foals 15–24 h after birth but has little effect on the cortisol response to exogenous ACTH1-24 at this age, irrespective of the nutritional plane of the mother at the time of infection (Ousey et al. 2008). Neonatal HPA function is also altered when fetal growth is manipulated by embryo transfer between equine breeds of different sizes (Allen et al. 2002; Giussani et al. 2003; Ousey et al. 2004). Both intrauterine growth-restriction (IUGR) caused by transferring a TB embryo into a pony uterus and fetal macrosomia induced by the reciprocal transfer of a pony embryo into a TB mare alter cortisol responses to ACTH and physiological stimuli in the newborn foal. In TB foals growth restricted by embryo transfer into pony mares, basal cortisol concentrations are high 12 h after birth, despite normal ACTH concentrations, although the cortisol response to exogenous ACTH1-24 is small compared with that of normally grown TB foals during the first day post partum (Ousey et al. 2004). However, 4–5 days later, the cortisol response to ACTH1-24 and HPA sensitivity to acute hypotension are no different in IUGR and normally grown TB foals (Giussani et al. 2003; Ousey et al. 2004). In contrast, overgrown pony foals delivered by TB mares have normal basal concentrations of cortisol and ACTH for 10 days after birth and a normal cortisol response to exogenous ACTH1-24 at 1 and 5 days (Giussani et al. 2003; Ousey et al. 2004). However, these overgrown pony foals had a reduced ACTH response to acute hypotension on Day 6, which was coupled with a cortisol response similar to that seen in normal pony foals with greater ACTH responses (Giussani et al. 2003). Neonatal adrenocortical sensitivity to endogenous ACTH, therefore, appears to be enhanced during stressful conditions when intrauterine growth is enhanced above the genetic norm by increasing the surface area of the placenta (Allen et al. 2002; Giussani et al. 2003). Conditions during intrauterine development, therefore, appear to affect HPA function of the newborn foal in the 10-day period after birth, although the changes are subtle in some instances and may take several days to develop.
The hypothalmic-pituitary-adrenal (HPA) axis and progestagen production
The fetal HPA axis may have a role in controlling the length of gestation in the mare through adrenal production of pregnenolone, P5 (Silver 1994; Chavatte et al. 1997; Ousey 2004; Fowden et al. 2008). This steroid is believed to be the main precursor for uteroplacental production of the progestagens that control uterine quiescence during the second half of equine pregnancy (Silver 1994; Chavatte et al. 1997; Ousey et al. 2003). Although the specific progestagens involved in this process remain unknown (Ousey et al. 2000a; 2001), uteroplacental uptake of P5 from the fetal circulation and total maternal progestagen concentrations both rise during late gestation in parallel with the structural and functional developments in the fetal adrenal (Ousey et al. 2003). Indeed, when development of the fetal HPA axis is impaired by fescue toxicosis, maternal progestagen concentrations remain low throughout pregnancy and gestation is often prolonged (Brendemeuhl et al. 1995; Cross et al. 1995). Conversely, premature activation of the fetal HPA axis by intrauterine stress from placentitis, interbreed embryo transfer, or by fetal CRH or ACTH injection increases maternal progestagen concentrations and leads to early delivery in some instances (Rossdale et al. 1992; Ousey et al. 1998; Allen et al. 2002; Ousey 2004; Morris et al. 2007). Moreover, removal of the fetal gonad, another potential source of P5, has little effect on the normal profile of maternal progestagen concentrations during late gestation (Pashen and Allen 1979). Collectively, these findings suggest that the fetal HPA axis is involved in uteroplacental progestagen production and uterine quiescence in the mare.
Integrated hypothalmic-pituitary-adrenal (HPA) function during the prepartum period
By combining the cortisol and progestagen data, a possible sequence of maturational changes can be described for the integrated function of the fetal HPA axis during the prepartum period as follows. Before 290 days of gestation (>30 days from delivery), the fetal HPA axis has a basal level of activity and is relatively unresponsive to stimuli. Circulating concentrations of ACTH are low and the adrenal appears to produce P5 primarily, due to a lack of P450C17 (Fig 4a). Much of the cortisol circulating in the fetus at this time may be of maternal origin derived by transplacental passage down the concentration gradient (Fowden et al. 2008). After 300 days of gestation (25–5 days before delivery), activity of the HPA axis increases (Fig 4b). The adrenal cortex grows and develops morphologically, possibly due to increased ACTH exposure caused by changes in the pattern and bioactivity of the ACTH released by the pituitary (Comline and Silver 1971; Webb and Stevens 1981; Cudd et al. 1995). With low P450C17 expression, this leads to increased adrenal P5 production, which, in turn, enhances uteroplacental production of several progestagens (Holtan et al. 1991; Rossdale et al. 1992; Ousey et al. 2003). These steroids maintain uterine quiescence in the face of increasing uterine stretch and lead to rising total progestagen concentrations in the mare during the last 20–25 days of gestation (Fig 4b). Once the adrenal gland expresses sufficient P450C17 in the last 5 days or so of gestation, it appears to switch from P5 to cortisol production with the result that fetal cortisol concentrations rise and uteroplacental synthesis and maternal concentrations of total progestagens fall (Fig 4c). Withdrawal of the progestagenic block on uterine contractility is, therefore, coordinated with cortisol-stimulated maturation of the fetal tissues in the horse through this putative switch in adrenal steroid synthesis. Certainly, when delivery occurs prematurely either naturally or by induction with oxytocin before 320 days, plasma P5 and other progestagen concentrations are high in the newborn foal in conjunction with low cortisol concentrations, consistent with a switch in adrenal steroidogenesis very close to term (Holtan et al. 1991; Panzani et al. 2009). Thus, paradoxically, the fetal HPA axis appears to act sequentially first to prevent and then to facilitate the onset of labour in the mare (Fig 4).
The stimulus for the rise in adrenal P450C17 expression so close to term remains unknown but may depend on removal of an inhibitor or on sufficient exposure to bioactive ACTH or to cortisol itself. The rising fetal cortisol concentrations may also have direct effects on uteroplacental progestagen production as administration of synthetic glucocorticoids to either the mare or fetus increases total maternal progestagen concentrations and can induce early delivery depending on the gestational age at treatment (Alm et al. 1975; Rossdale et al. 1992; Ousey et al. 2011). However, the specific progestagen profile in the mare differs between maternal and fetal treatment with ACTH or synthetic glucocorticoids, despite transplacental passage of natural and exogenous glucocorticoids from the dam (Rossdale et al. 1992; Ousey et al. 1998, 2000b, 2011). Collectively, these findings suggest that maternal as well as fetal HPA function may influence the periparturient endocrine profiles, particularly during adverse conditions. The trigger for the final endocrine cascade that leads to parturition, therefore, appears to be multifactorial in the mare and not solely dependent on increased cortisol production by the fetal adrenal gland as occurs in ruminants (Silver 1994; Wood and Cudd 1997; Challis et al. 2000).
Sympatho-adrenal medullary axis
The sympatho-adrenal medullary system secretes catecholamines and several other neuropeptides with cardiovascular and metabolic effects into the circulation. It is, therefore, involved in regulating homeostasis and responds to a range of stresses common in newborn animals, such as hypoglycaemia and hypotension (Silver 1990; Spurlock and Furr 1990). Plasma concentrations of the catecholamines, adrenaline and noradrenaline, increase towards term in normal pony foals to peak at or shortly after birth before returning to basal values within 10–14 days of birth (Fig 1c). Noradrenaline concentrations were generally higher than adrenaline concentrations throughout the perinatal period with the exception of the period immediately after birth (Fig 1c). Neither concentration appears to be affected by the breed of horse, at least 6 days after birth (Giussani et al. 2003; Forhead et al. 2004). When delivery is induced prematurely with oxytocin, noradrenaline concentrations are higher and adrenaline lower in the 2 h after birth than that seen in full-term neonates (Silver et al. 1984). In stressed premature foals, noradrenaline concentrations may be as much as 5-fold higher than normal, although adrenaline concentrations remain low (Silver et al. 1984). However, induction of delivery within 24–48 h of full-term has little effect on the temporal profile of catecholamine concentrations over the first 10 days after birth (Holdstock et al. 2011). At birth, total catecholamine concentrations are inversely related to the pH of umbilical arterial blood in both premature and full-term foals (Silver et al. 1984). The sympatho-adrenal medullary axis, therefore, appears to be largely responsible for the marked perinatal surge in catecholamine concentrations in the foal and is sensitive to stressful stimuli, at least by late gestation.
Sympatho-adrenal medullary responses to asphyxia and insulin-induced hypoglycaemia have been studied in foals before and after birth (Comline and Silver 1971; Silver et al. 1987; Silver and Fowden 1995). These responses increase during late gestation and, again, between birth and 2 weeks before resembling mature responses by about age 12 weeks (Comline and Silver 1971; Silver et al. 1987; Silver and Fowden 1995). In contrast, there is no change in the catecholaminergic response to acute hypotension between 7 and 14 days of post natal life, despite the concomitant changes in HPA sensitivity (O'Connor et al. 2005). Studies on anaesthetised foals show a doubling of total catecholamine output by the adrenal gland between late gestation and 2 weeks after birth in response to direct stimulation of the peripheral end of the cut splanchnic nerve (Comline and Silver 1971). Since there is no change in adrenal catecholamine content over this period (Comline and Silver 1971), these observations suggest that splanchnic innervation to the equine adrenal medulla becomes progressively more effective at releasing catecholamines during the perinatal period. Indeed, increased adrenal innervation by age 14 days may be responsible for the enhanced adrenocortical sensitivity to ACTH seen in response to acute hypotension as splanchnic nerve stimulation and the release of adrenal neuropeptides, such as vasoactive intestinal peptide, have been shown to increase adrenal cortisol output in response to exogenous ACTH in young, suckling calves (Edwards 1997). Indeed, earlier innervation of the adrenal gland in pony foals overgrown by development in a TB mare may explain their enhanced adrenocortical response to acute hypotension relative to normally grown pony foals delivered by pony mares (Giussani et al. 2003). Stressful stimuli that activate both the HPA and sympatho-adrenal medullary axes may, therefore, cause greater cortisol secretion per unit of ACTH than seen in response to administration of ACTH alone. However, sectioning of the splanchnic nerve to the adrenal gland has little effect on the catecholaminergic response to asphyxia, or anoxia, in either the fetus or the neonate until about 2 weeks after birth of the foal (Comline and Silver 1971). This suggests that, unlike the lamb, adreno-medullary cells of the foal remain directly sensitive to pO2 for some time after birth, despite the increasing effectiveness of the innervation (Comline et al. 1965; Comline and Silver 1971).
In late gestation, the sympatho-adrenal medullary response of the foal to hypoglycaemia is solely noradrenergic, whereas, by 7–14 days after birth, it is primarily adrenergic (Fig 5). At the nadir of the glucose concentrations in the older foals, there are significant increases in both catecholamine concentrations but the increment in plasma adrenaline is 10-fold greater than the noradrenergic response (Silver et al. 1987; Silver and Fowden 1995). Similar developmental changes in the relative adrenal output of the 2 catecholamines are seen in response to asphyxia of the foal (Comline and Silver 1971). There is also a perinatal switch from noradrenaline to adrenaline output by the adrenal gland in response to direct stimulation of the splanchnic nerve with adrenaline accounting for 20–30% of the total adrenal catecholamine output just before term but over 70% of the total two weeks after birth of the foal (Comline and Silver 1971). In part, this may be due to perinatal activation of the HPA axis as cortisol is known to induce activity of phenyl-N-methyl-transferase, the enzyme responsible for adrenaline synthesis from noradrenaline, in the adrenal gland of fetal sheep near term (Comline et al. 1970; Coulter et al. 1991). This suggestion is consistent with the lower adrenaline concentrations seen in newborn foals delivered prematurely before the final prepartum cortisol surge (Silver et al. 1984). However, fetal adrenaline and cortisol concentrations are not correlated during late gestation and there is no change in the adrenaline content of the equine adrenal between late gestation and 2 weeks after birth (Comline and Silver 1971; Giussani et al. 2005). Consequently, factors other than cortisol, such as the improving innervation, may contribute to the increasing adrenaline output of the adrenal gland during this period. In neonates of other species, the pattern and frequency of discharge in the splanchnic nerves is known to cause differential release of the various adreno-medullary secretions (Edwards 1997).
Noradrenaline and adrenaline are known to affect cardiovascular function in fetal and newborn foals (Hollis et al. 2006; O'Connor et al. 2006). They raise blood pressure by increasing both systemic vascular resistance and heart rate at all ages studied from mid gestation to 14 days after birth, although they are more effective pressor agents after than before birth (O'Connor et al. 2005, 2006; Hollis et al. 2006). Neonatal sensitivity to the catecholamines and sympatho-adrenal medullary responses to homeostatic challenges are affected by the conditions experienced in utero. The overgrown pony foal produced by embryo transfer into a TB mare has diminished plasma catecholamine responses to acute nitroprusside-induced hypotension 6 days after birth but a raised basal arterial blood pressure, despite similar basal catecholamine concentrations to normally grown pony foals delivered by pony mares (Giussani et al. 2003). These changes are coupled to an increased baroreflex threshold and reduced baroreflex sensitivity in the pony foal delivered by a TB mare. Conversely, TB foals with IUGR resulting from development in a pony mare have augmented sympatho-adrenal responses to acute hypotension but no change in basal blood pressure, baroreflex threshold or basal catecholamine concentrations, although baroreflex sensitivity was increased relative to normal TB foals delivered by TB mares (Giussani et al. 2003). Changes in the secretion and action of the catecholamines during the perinatal period, therefore, have an essential role in the cardiovascular and other adaptations to life ex utero. Administration of noradrenaline or adrenoreceptor agonists may, therefore, be a useful clinical treatment for the neonatal hypotension frequently seen in premature and other compromised foals (Hollis et al. 2006).
The endocrine pancreas
Before birth, fetal glucose concentrations are controlled primarily by the transplacental passage of glucose from the mother and, hence, the main role of the endocrine pancreas in utero is not to regulate the glucose concentration but to match the rate of fetal glucose utilisation to the rate of placental glucose supply (Fowden and Hill 2001; Fowden and Forhead 2009). However, at birth, the endocrine pancreas must become directly involved in gluco-regulation and maintain normoglycaemia in the face of the increased metabolic demands of the neonate and switch from a continuous to an intermittent supply of nutrients. Episodes of hypoglycaemia are common in the foal during the neonatal period, which suggests that the mechanisms of glycaemic control are not always fully competent at birth in this species (Spurlock and Furr 1990).
Pancreatic β cell function
Although basal insulin concentrations in the foal tend to be higher at age 14 days than earlier in pre- or post natal life (Fig 1e), there is little variation in the plasma insulin concentration from 160 days of gestation to birth or from Day 1 to Day 10 post partum in normal foals, even though there is a marked post natal increase in the glucose concentration (Fowden et al. 1980, 1982; Holdstock et al. 2004). The main increase in plasma insulin concentrations, therefore, appears to occur over the first 8 h post partum in association with the onset of sucking (Fig 1e). The temporal profile of neonatal insulin concentrations appears to be unaffected when labour is induced 24–48 h before full term based on pre-colostral electrolyte concentrations (Fowden et al. 1984; Holdstock et al. 2012). There is also little evidence for differences in insulin concentrations between pony and TB foals during early neonatal life (Fowden et al. 1984; Forhead et al. 2004). Comparison of the relationships between the endogenous concentrations of insulin and glucose during the perinatal period shows that the set point for glucose-stimulated insulin secretion in the foal shifts progressively to higher glucose concentrations from late gestation through to 24–48 h after birth when the relationship resembles that in the mature horse (Fowden et al. 1980). In keeping with this, there are changes in the pancreatic β cell sensitivity to exogenous glucose during the perinatal period (Fowden et al. 1982, 1984, 2005; Holdstock et al. 2004).
Exogenous administration of glucose to fetal horses does not evoke insulin secretion before about 200 days of gestation (Fowden et al. 1980). Thereafter, there is a prompt increase in the fetal insulin concentration in response to exogenous glucose. This response increases between 260 and 290+ days of gestational age and then, in late gestation (>300 days), increases with proximity to delivery as indicated by the prepartum rise in circulating cortisol concentrations (Fowden et al. 1980, 2005). In late gestation, the glucose-stimulated insulin increment was 3-fold greater in fetuses with cortisol concentrations >15 ng/ml than in those of the same gestational age with lower cortisol concentrations (Fowden et al. 2005). At 2 h after birth, the pancreatic β cell response to glucose is low compared with that seen either in late gestation or at age 5–7 days (Fowden et al. 1982, 1984). Since the pancreatic β cell response to glucose varies little between 24 h and Day 9 post partum (Holdstock et al. 2004), the suppressed response seen immediately after birth may be due to the elevated catecholamine concentrations (Fig 1c), as both adrenaline and noradrenaline are known to inhibit insulin secretion in fetal and mature animals (Fowden and Hill 2001). In contrast, the pancreatic β cell response to the amino acid, arginine, shows little change in magnitude either during late gestation or between Days 2 and 10 post partum, although these responses are smaller than those evoked by glucose at all ages studied (Fowden et al. 1984, 2005; Holdstock et al. 2004). Since arginine and glucose act through different pathways to stimulate insulin secretion (Fowden and Hill 2001), these observations suggest that the major perinatal changes in β cell function may occur in the glucose signalling pathway, upstream of the mechanism of insulin vesicle release common to both pathways.
Proinsulin is detectable in plasma from newborn foals and increases in concentration over the first 24–48 h post partum, although absolute concentrations are only 5–10% of those of insulin (Holdstock et al. 2004). Proinsulin concentrations also increase after administration of exogenous glucose and arginine but not in response to the smaller increments in endogenous glucose seen after feeding (Holdstock et al. 2004). The proinsulin responses are also smaller and more prolonged than the corresponding insulin responses (Holdstock et al. 2004). These observations suggest that there may be changes to the rate of proinsulin cleavage within the insulin vesicles or in proinsulin clearance from the circulation as the glucoregulatory demands on the β cells rise with the intermittent provision of nutrients after birth. Overall, the perinatal changes in the sensitivity and set point of the pancreatic β cells to glycaemic changes, particularly between the immediate prepartum period and 48 h after birth suggest that these cells are responsive to the other endocrine changes occurring at delivery and/or to the release of gut peptides and hormones after feeding for the first time (Ousey et al. 1995).
In common with other species (Fowden and Forhead 2009), insulin acts to lower glucose concentrations in the foal both before and after birth (Silver et al. 1987; Silver and Fowden 1995; George et al. 2009). This hypoglycaemic action of insulin appears to be less effective shortly after birth, as glucose clearance is slower in response to exogenous glucose administration 12–24 h after birth than in older foals, despite a normal insulin response (Holdstock et al. 2004). This apparent tissue resistance to insulin immediately after birth may be due to the higher concentrations of catecholamines and cortisol, which cause glucogenesis and insulin antagonism, respectively. In other species, there are also changes in the abundance of insulin receptors and proteins in the intracellular insulin signalling pathways during late gestation, which may influence the action of insulin perinatally (Fowden and Hill 2001; Muhlhauser et al. 2009). Whether similar developmental changes occur in these signalling pathways in equine tissues remains unknown.
Insulin concentrations are low after birth in premature and ill foals (Fowden et al. 1984; Barsnick et al. 2011). Compared with the full-term foal, the pancreatic β cell response to exogenous glucose is also decreased 2 h after birth in foals delivered prematurely by induction before 320 days (Fowden et al. 1984). In part, these findings may reflect the inhibitory effect of the higher than normal noradrenaline concentrations in premature foals (Silver et al. 1984). Neonatal β cell responses to exogenous glucose are also affected by induction of delivery closer to term and by adverse conditions in utero. Induction of delivery 24–48 h before full term leads to an enhanced β cell sensitivity to glucose without any change in glucose clearance, which suggests that the increased insulin secretion may be a compensatory response to greater insulin resistance caused by the hypercortisolaemia of these foals (Holdstock et al. 2012). An increased β cell response to glucose is also observed in 2-day-old pony foals overgrown in utero by transfer as embryos into TB mares (Forhead et al. 2004). In these circumstances, the apparent increase in β cell sensitivity to glucose may reflect β cell proliferation before birth in response to an increased fetal supply of nutrients via the larger than normal placenta (Allen et al. 2002). However, as basal insulin concentrations were increased and glucose clearance was normal in these overgrown foals (Forhead et al. 2004), there may also be a degree of tissue insulin resistance, although this was not associated with any change in basal cortisol or catecholamines concentrations or in the hypoglycaemic response to acute administration of insulin (Giussani et al. 2003; Forhead et al. 2004; Ousey et al. 2004). Similarly, maternal undernutrition during mid to late gestation caused by maternal infection with Streptococcus equi increased glucose-stimulated insulin secretion in 5-day-old foals when the mares were on a moderate relative to a high plane of nutrition at the time of infection (Ousey et al. 2008). This exaggerated insulin response was unlikely to reflect insulin resistance as there was no change in basal insulin concentrations or in the hypoglycaemic response of acute insulin administration (Ousey et al. 2004). Similar improvements in β cell sensitivity to glucose have been observed in juvenile offspring of sheep and rats undernourished during pregnancy (Ozanne and Hales 1999; Clarke et al. 2000). Conditions during both pre- and immediate post natal development, therefore, appear to have an important role in determining pancreatic β cell function of the newborn foal but the extent to which this reflects direct changes in the β cells or indirect responses to tissue insulin resistance remains to be determined.
Pancreatic α cell function
Much less is known about pancreatic α cell function in the foal. In contrast to insulin, glucagon concentrations increase during late gestation to peak at birth and, then, decline progressively during the 10 days after birth of the foal (Fig 1d). Basal glucagon concentrations show a similar temporal pattern in newborn foals induced to deliver 24–48 h before full-term based on milk parameters (Holdstock et al. 2012). Compared with normal 1–2-day-old foals, glucagon concentrations are raised 10 fold in septic foals and 2 fold in those hospitalised for other illnesses, such as failure of passive immunity, limb deformities and encephalopathy (Barsnick et al. 2011). Equine pancreatic α cells respond to arginine from late gestation onwards but appear to be relatively insensitive to changes in glycaemia, even at 10 days after birth in both induced and spontaneously delivered foals (Fowden et al. 1999; Holdstock et al. 2012). The responses to arginine appear to be greater in utero than neonatally but this may reflect, in part, the slower clearance of arginine from the fetal circulation (Fowden et al. 1999; Holdstock et al. 2011). Glucagon secretion in response to arginine increased over the first 10 days post partum in normal term foals and was greater in foals induced to deliver 24–48 h before term than in those born spontaneously at term (Holdstock et al. 2012). Collectively, these observations suggest that glucagon functions as a stress hormone in the foal during the perinatal period. This is consistent with its known action in activating fetal glucogenesis in other species and with the immediate need for an endogenous source of glucose after the placenta is lost at birth (Fowden et al. 1998; Sangild et al. 2000). The pancreatic α cells, therefore, act to ensure a glucose supply to insulin insensitive tissues, such as the brain, during adverse conditions when glucose availability is limited, while the pancreatic β cells are involved in regulating glucose uptake by insulin sensitive tissues, such as skeletal muscle, during the normal variations in glucose availability associated with the intermittent patterns of feeding and exercise seen in newborn foals.
There are several other endocrine systems, such as the renin-angiotensin system, somatotrophic axis, adipokines and leptin and the thyroid hormones, that adapt perinatally and contribute to the physiological adjustments required to thrive ex utero (Fowden and Forhead 2009). In keeping with cold exposure for the first time, concentrations of thyroxine and tri-iodothyronine (T3) are high in the foal for the first 48 h after birth and then fall to stable values for the following 3 weeks (Murray and Luba 1993). Concentrations of T3, in particular, increase in the 2 h after birth of full-term pony foals in parallel with the rise in cortisol concentrations and are low in premature foals with hypocortisolaemia (Silver et al. 1991). Cortisol may, therefore, stimulate formation of T3 from thyroxine by activation of tissue deiodinases in the foal as occurs in the lamb during the prepartum period (Forhead et al. 2006). Cortisol may also be involved in inducing angiotensin converting enzyme (ACE) as pulmonary ACE activity increases during late gestation to peak at birth in line with the fetal cortisol concentrations in the foal (O'Connor et al. 2002). Angiotensin II is a known pressor agent in the fetal horse near term and concentrations of its precursor, angiotensinogen, decline immediately after birth in full-term foals but are high and remain elevated after premature delivery, consistent with the potential action of cortisol in activating pulmonary ACE (Broughton Pipkin et al. 1982; O'Connor et al. 2005). Concentrations of insulin-like growth factor (IGF)-I and leptin also increase in newborn foals in the first few days after birth and then stabilise at mature values by 2–3 weeks (Hess-Dudan et al. 1994; Berg et al. 2007). The initial neonatal increase in these concentrations may reflect, in part, gastrointestinal uptake of IGF-I and leptin directly from the milk before gut closure (Hess-Dudan et al. 1994; Sangild et al. 2000; Berg et al. 2007). Neonatal leptin concentrations appear to be unaffected by either sepsis or other illnesses but are lower in hospitalised foals that subsequently die than in those that survive (Barsnick et al. 2011).
In summary, there are major adaptations in several endocrine systems of the foal during the perinatal period (Fig 1), which are influenced by the conditions it experiences in utero and by its maturity at birth. These endocrine changes are closely inter-related and critical for the structural and functional adaptations in many of the other physiological systems essential for neonatal survival. The perinatal alterations in HPA function are particularly important in these processes, although the sympatho-adrenal medullary axis and endocrine pancreas also have key roles in ensuring homeostasis during the novel challenges associated with life ex utero. Changes in the functioning of the HPA axis begin before birth and lead to a switch in adrenal steroidogenesis, which results in prepartum maturation of the foal and, possibly, also in the onset of labour in the mare. Changes in the HPA axis continue after birth and are associated with alterations in functioning of the pituitary and adrenal glands. In turn, the 5-fold increase in perinatal cortisol availability is influential in adapting many physiological systems to their new postnatal roles, including several other endocrine axes. The sympatho-adrenal medullary axis becomes progressively more sensitive to stimuli during the perinatal period, in part, due to the increasing effectiveness of the splanchic innervation. Catecholamine concentrations increase 20-fold in the immediate neonatal period and have an important role in maintaining blood pressure and glycaemia during the transition from intra- to extrauterine environments. Similarly, there are changes in the function of the pancreatic α and β cells during the perinatal period, which are important in the more long-term metabolic adaptations to enteral nutrition. Although insulin and glucagon concentrations change only 2–3-fold during the perinatal period, these changes are essential for maintaining a glucose supply and in establishing good glycaemic control with the loss of parenteral nutrition via the placenta.
Abnormalities in the perinatal endocrine profile caused by adverse conditions before or after birth may, therefore, lead to neonatal maladaptation and a poor prognosis for the foal. Alternatively, they may be beneficial and aid its survival, despite early delivery and/or IUGR. In addition to the immediate neonatal effects, endocrine abnormalities during the perinatal period may have long-term consequences for the foal by programming the structure and function of its tissues more permanently with implications for its physiological phenotype later in life (Rossdale and Ousey 2002; McMillen and Robinson 2005; Fowden et al. 2006). Indeed, recent studies suggest that overexposure of the foal to cortisol in the immediate neonatal period can influence HPA function shortly thereafter and have metabolic effects many months later (Jellyman et al. 2012; Valenzuela et al. 2011). The endocrine profile of the newborn foal may, therefore, provide a good index of both the conditions experienced during intrauterine development and the likelihood of physiological abnormalities arising later in life. Thus, closer monitoring of neonatal endocrine status may have potential benefits to equine clinical practice and to the horse racing industry more generally.
Authors' declaration of interests
No conflicts of interest have been declared.
Source of funding
We are also indebted to the Horserace Betting Levy Board for their financial support over many years.
We would like to thank the many members of the Departments of Physiology, Development and Neuroscience who helped with the experimental and biochemical aspects of these studies. In particular, we would like to thank Peter Rossdale, without whose enthusiasm and expert training none of these studies would have begun or progressed to completion.