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Bioscience Reports, Vol. 25 Nos. 3/4, June/August 2005 ( 2005) DOI: 10.1007/s10540-005-2888-3 Neonatal Programming of Body Weight Regulation and Energetic Metabolism Egberto Gaspar de Moura1,3 and Magna Cottini F Passos2 Programming is an epigenetic phenomena by which nutritional, hormonal, physical psychological and other stressful events acting in a critical period of life, such as gestation and lactation, modifies in a prolonged way certain physiological functions. This process was preserved by natural selection as an important adaptive tool for survival of organisms living in nutritional impaired areas. So, malnutrition during gestation and lactation turns on different genes that provide the organism with a thrifty phenotype. In the case of an abundant supply of nutrients after this period, those organisms that were adapted to a low metabolic waste and higher energy utilization will be in a higher risk of developing metabolic diseases, such as obesity, hyperlipidemia, diabetes mellitus and hypertension. The kind of malnutrition, duration and intensity are important for the type of programming obtained. We discuss some of the hormonal and metabolic changes that occur in gestation or lactation, when malnutrition is applied to the mothers and their offspring. Some of these changes, such as an increase of maternal triiodothyronine (T3), leptin and glucocorticoids (GC) and decrease in prolactin are by itself potential programming factors. Most of these hormones can be transfer through the milk that has other important macronutrients composition changes in malnourished dams. We discuss the programming effects of some of these hormones upon body weight and composition, leptin, thyroid and adrenal functions, and their effects on liver, muscle and adipose tissue metabolism and the consequences on thermogenesis. KEY WORDS: Programming; neonatal; body weight; malnutrition; hormones; leptin; thyroid. INTRODUCTION Some studies have shown that gestation and/or lactation could be a critical period for the future nutritional and hormonal status of the progeny, a relationship that has been termed programming. This term is defined as the basic biological phenomena that putatively underlie relations among nutritional experiences of early life and early diseases (Lucas et al., 1994). Among the programming factors that act in a 1 Dept. Ciências Fisiológicas, Instituto de Biologia Roberto Alcantara Gomes, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brasil. 2 Dept. Nutrição Aplicada, Instituto de Nutrição, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brasil. 3 To whom should be addressed. E-mail: [email protected] 251 0144-8463/05/0800-0251/0 2005 Springer Science+Business Media, Inc. 252 Moura and Passos critical period in the initial phases of life are: nutrition, hormones, physical stimuli, like temperature, light/dark cycle, and other stressful events. Most of the authors studied malnutrition over gestation or both gestation and lactation periods (Coleoni et al., 1983; Anguita et al., 1993; Burns et al, 1997; Bertin et al., 1999). However, few studies were restricted to evaluate malnutrition only during the lactation period (Passos et al., 2000; Ramos et al, 2000; Moura et al., 2002), that seems to be the most crucial period in mammals to the establishment of programming. Hormones and their receptors exert marked tissue-specific and age specific effects on development (Dauncey et al., 2001). During fetal development, the coordinated actions of IGF-1, IGF-2, insulin, thyroid hormones (TH) and glucocorticoids (GC) play central roles in the control of differentiation, growth and maturation (Fowden, 1995). The present paper reviews the programming of endocrine and intermediary metabolism, with emphasis on the energy balance and thermogenesis. PROGRAMMING ENDOCRINE CHANGES Endocrine dysfunctions were shown in adult animals whose mothers were submitted to malnutrition, stressful events and hormonal dysfunctions or treatment during lactation and/or pregnancy. Here, we discuss the programming of glucose metabolism regulation, with emphasis in the mechanism of insulin resistance, and the programming of hormonal systems as TH, GC, catecholamine and leptin that have profound effects on metabolic rate. The combination of insulin resistance and changes in the action of lipolytic hormones can result in the higher prevalence of the metabolic syndrome in adult animals that were malnourished or submitted to hormonal treatment during the prenatal or neonatal periods. PROGRAMMING BODY WEIGHT Malnutrition and Body Weight Programming Obesity and related metabolic disorders have become a major health issue in modern society and it is widely accepted that obesity in humans is commonly due to lifestyle and dietary factors. The mechanisms by which diet and environment modulate the physiological systems that control appetite, weight regulation and the etiology of metabolic disorders are poorly understood. Increasing evidence suggests that the origin of some metabolic disorders, which manifest in adult life, may have their roots before birth. Early support for the ‘‘fetal origins’’ hypothesis was based on epidemiological studies of adverse health outcomes in humans in which size at birth was related to the occurrence of metabolic or cardiovascular disorders at an adult age (Osmond et al., 1993). Many studies have shown that a small size or thinness at birth are associated with an increased propensity to adverse health outcomes in adulthood including obesity, type 2 diabetes, abnormal lipid and carbohydrate metabolism, coronary heart disease and elevated blood pressure (Godfrey and Barker, 2000). Prospective Neonatal Programming of Body Weight Regulation 253 animal studies to test the ‘‘fetal origins’’ or ‘‘programming’’ hypothesis have imposed perturbations such as moderate to severe maternal undernutrition, feeding of lowprotein diets throughout pregnancy, or GC exposure during fetal development. The large majority of studies confirm that perturbations of fetal growth and development lead to obesity, reduced insulin sensitivity (Holemans et al., 1996), elevated blood pressure (Langley-Evans et al., 1996; Tonkiss et al., 1998) and growth hormone dysfunctions (Woodall et al., 1996). There is increasing evidence that obesity may also be associated with impaired fetal development since epidemiological studies suggests that children born with intrauterine growth retardation (IUGR) has an increased risk of developing obesity later in life (Desai and Hales, 1997). This was clearly shown in the Dutch Famine Study where poor maternal nutrition resulted in increased rates of obesity in adult males (Ravelli et al., 1976). However, obesity seems to develop only if the nutritional conditions improve thereafter, since in Russia, Stanner et al. (1997) failed to observe programming of obesity in adults, whose mothers suffered famine during gestation at the German siege of Leningrad in World War II. The uterine milieu seems to be more important than genetic factors to the programming of growth and it has been estimated that 62% of the variation of human birthweight results from the intrauterine environment, compared with 20% and 18% from maternal and paternal genes, respectively (Holt, 2002). We have studied an animal model for programming of body weight, that concomitantly affects leptin and TH function at adult age (Passos et al., 2000; Passos et al., 2002; Teixeira et al., 2002; Dutra et al., 2003; Passos et al., 2004). In this model we submitted the mothers to a low-protein (8%) or energy restricted (40%) diet throughout lactation. At weaning the pups received a normal diet and the body weight and food intake were monitored until 180 days of age. We demonstrated with this experimental model that the kind of mother’s nutrition during lactation may influence the body weight of their offspring in the adult life, and this can be mainly associated with protein and lipid milk concentration (Passos et al., 2000). Protein-restricted (PR) mothers had lower protein concentration in milk, while energy-restricted (ER) mothers had higher lipid milk concentration. Both malnourished offspring had lower body weight until weaning. After weaning, though, ER offspring were heavier than the controls, while PR presented lower body weight from birth to 180 days old (Passos et al., 2000). In addition, those animals had several abnormalities in their leptin action and thyroid function as will be discussed in the next section. A schematic view of body weight programming can be seen in Fig. 1. Numerous investigations have shown that nutrition markedly influences the synthesis and metabolism of many hormones involved in development, growth and metabolism (Dauncey et al., 2001). Effects are exerted both by specific nutrients and by changes in overall food intake, as occurs during undernutrition, A further mechanism by which nutrition modulates hormone action is by regulation of hormone receptors. Particularly important is the finding that the response to nutrition can be tissue-specific, because this factor enables highly specific and diverse functional responses to a given circulating hormone level. Postnatal undernutrition down regulates GH receptor gene expression in liver, whereas it is up regulated in muscle (Dauncey et al., 1994; Weller et al., 1994). This situation will affect the GH-IGF-I 254 Moura and Passos Protein restricted dams during lactation Adult offspring Body weight Leptin resistance Pituitary leptin receptors Serum leptin at weaning Energy restricted dams during lactation Adult offspring Body weight Leptin resistance Pituitary leptin receptors Serum leptin at weaning and Fat milk Fig. 1. Model of body weight programming and leptin secretion and action at adult age. axis and limit growth by reducing hepatic IGF-I synthesis (Katsumata et al., 2000). The probable mechanism by which early malnutrition may exert permanent or longterm effects on development is that the actions of hormone receptors on cellular development persist long after subsequent optimization of nutrition has restored hormone receptor expression to its appropriate level. Despite some reports in IUGR born children, of changes in the GHRH-GHIGF1 axis (Albertsson-Wikland et al., 1998) and GH resistance (Cutfield et al., 2002), those changes are transitory since in a cohort study in UK, they failed to relate urinary GH and serum IGF-I in adults to birthweight, other measurements at birth, or weight at 1 year (Holt et al., 2004). So it seems, that the long term changes in body composition and intermediary metabolism may be attributed to the programming effects on other hormones, such as leptin, epinephrine, TH and GC. Leptin and Body Weight Programming Leptin is a protein produced by the obesity gene (ob) and is secreted mainly by the adipocyte and regulates food ingestion and energetic balance (Zhang et al., 1994). This hormone regulates energy disposal in the peripheral adipose tissue by specific hypothalamic signals, and affects many functions such as body weight, food intake, body temperature, and metabolic rate (Friedman and Halaas, 1998). These effects are mediated in part by leptin’s ability to modulate hypothalamic function in the arcuate nucleus (Arc) of the hypothalamus. The signaling form of the leptin receptor (ObRB) is coexpressed with neuropeptide Y (NPY) and agouti-related peptide (AgRP) in a group of orexigenic neurons and with proopiomelanocortin (POMC) that contain the anorectic peptide a-melanocyte-stimulating hormone Neonatal Programming of Body Weight Regulation 255 (a-MSH) and cocaine- and amphetamine-regulated transcript (CART) in a group of anorexigenic neurons (Erickson et al., 1996). Increased NPY activity and reduced POMC activity appear to increase feeding and fat deposition, whereas reduced NPY activity and increased POMC activity decrease feeding and body mass (Huszar et al., 1997). Leptin increases the firing rate of POMC neurons and decreases NPY firing rate in the Arc (Cowley et al., 2001). Several mice strains develop abnormalities in leptin or in its receptor expression. Obesity in ob/ob mice results from a mutation in the leptin gene that leads to hyperphagia, excess body weight, and decreased thermogenesis; all these effects can be reversed by exogenous leptin administration (Hwa et al., 1997). Other genetic rodent models of obesity (db/db) result from the mutation in the functional leptin receptor (Chua et al., 1996). Unfortunately, human studies have shown the scarcity of such mutations as causes of obesity (Considine et al., 1996), and leptin deficiency is a rare occurrence in human obesity. Instead, most obese humans exhibit elevated levels of circulating leptin correlated to body mass index (Considine et al., 1996). Leptin is transported into the brain through a saturable system of short form of leptin receptors (ObRa and ObRc) where its actions are to modulate energy functions through the long-form of leptin receptor (ObRb) and activation of a JAKSTAT and PI3P signaling pathway (Banks et al., 1996; Friedman and Halaas, 1998). Thus, several steps in the leptin signaling pathway can be affected, leading to the development of obesity. In the ob/ob hypothalamus, the amounts of NPY RNAs are increased, whereas the RNAs for POMC are decreased and leptin treatment of these animals normalizes the amount of these RNAs (Stephens et al., 1995). The number and activity of excitatory or inhibitory synapses to these neurons also is regulated by the presence of leptin (Pinto et al., 2004). Bouret et al. (2004a) showed the specificity of these leptin morphogenetic effects, since other hypothalamic nuclei or nerval projections to the limbic system were not affected, by the absence of leptin in the ob/ob model or the treatment of those animals with leptin. Leptin treatment of neonatal ob-mice on days 4–12 of life reversed the reduction in nerve fibres to POMC neurons (Bouret et al., 2004b). However, if the treatment was postponed to the adult age it did not result in the formation of anorexigenic and orexigenic nerve fibres. This confirms that there is a critical period for the formation of leptin-dependent neurons in mice. Although, this neural plasticity is not completely limited to this initial phase of life (Pinto et al., 2004). These data give a morphological support for the programming hypothesis, since different levels of leptin during hypothalamic morphogenesis could permanently changes the distribution of excitatory or inhibitory synapses to NPY or POMC neurons, modifying permanently the response of these neurons to orexigenic or anorexigenic stimuli. Some authors have suggested that the higher leptin serum concentrations could downregulate hypothalamic leptin receptors or their blood-brain barrier carriers, which could result in leptin resistance and start a vicious cycle of higher food ingestion and even higher leptin serum concentration (Considine et al., 1996). Mice made obese by feeding on a high-fat diet were shown to lose their response to peripherally administered leptin, but not to centrally administered leptin (Van Heek et al., 1997); these studies suggest that impairment of leptin transport might contribute to the development of obesity. However, Martin et al. (2000) also showed an 256 Moura and Passos association between leptin resistance in obese rats and leptin receptor downregulation. Therefore, the failure of the hypothalamus to detect leptin or leptin resistance may occur for one of several reasons leptin may fail to cross the blood-brain barrier, the hypothalamic receptors may be downregulated, or downstream signaling may be inhibited. Leptin decreases food intake 2 h after injection, in adult rats kept in a 24 h fasting, and then allowed to eat ad libitum a standard diet (Martin et al., 2000). In contrast animals whose mothers were submitted to PR or ER diets during lactation showed no decrease in the feeding response to this acute dose of peripheral leptin (Passos et al., 2004), characterizing a hypothalamic leptin resistance. The leptin resistance, which is observed in programmed offspring, may be a mechanism induced by the experience of a nutrient-deprived intrauterine environment to store large quantities of triglycerides when food is plentiful. Leptin resistance would therefore create a competitive advantage in preparation for a nutrientdeprived environment to store as much fat as possible when food becomes available. Thus, leptin resistance may be a mechanism for the thrifty phenotype proposed by Hales and Barker (1992). However, when hypercaloric nutrition persists for long periods of time, leptin resistance may lead to adipogenic diabetes mellitus (Breier et al., 2001). Both PR and ER offspring presented at weaning higher serum leptin concentrations (Teixeira et al., 2002). As leptin is present in milk it is possible that this hormone is transfer to the pups through the milk (Casabiell et al., 1997; Houseknecht et al., 1997). The higher fat milk concentration could increase leptin production in the pups. This relation was evidenced in humans (Mantzoros et al., 1997) and animals (Rousseau et al., 1997; Trottier et al., 1998). So, we hypothesized that this higher serum leptin at weaning could be one of the factors that programs the endocrine disorders observed at adult age. Based on these findings we developed another experimental model where the pups were injected with murine leptin (8 lg/100 g BW, sc) during the first 10 or last 10 days of lactation (Cravo et al., 2002). In this study we observed that hiperleptineamia in both the first and the last 10 days of lactation was associated with higher serum leptin concentration at 150 days of age and , paradoxically, with higher food intake and body weight, suggesting also a leptin resistance. PROGRAMMING THYROID FUNCTION Neonatal TH Dysfunctions and Thyroid Function Programming The widespread actions of hormones during development are exemplified by the striking effects of TH on differentiation, growth and metabolism of many tissues and cell types (Fisher et al., 1977). Particularly well recognized are the effects of TH on myelination and development of the central nervous system (Barradas et al., 2000), and TH deficiency during the perinatal period results in severe mental and physical retardation (Oppenheimer and Schwartz, 1997; Chan and Kilby, 2000). There are a considerable bulk of data about the effects of maternal hyperthyroidism and programming of the thyroid function in the offspring. Neonatal Neonatal Programming of Body Weight Regulation 257 hyperthyroidism is associated to secondary hypothyroidism in the adulthood (Varma and Crawford, 1979; Dussault e cols., 1982). Walker and Courtain (1985) showed that this TSH insensitivity to the lower T4 and T3 concentration in adult rats that were T4 treated in the neonatal period, were in part due to a higher pituitary deiodinase activity. The effects of maternal or neonatal hypothyroidism in programming thyroid function were less marked than those observed for hyperthyroidism (Pracyk et al., 1992), consisting mainly in TSH changes. Neonatal Malnutrition and Thyroid Function Programming Several changes in thyroid economy are detected in food-restricted animals. TH secretion (Moura et al., 1987) and metabolism (Harris et al., 1978), as well as thyroid regulation by hypothalamic and pituitary hormones (Jolin and Lamas, 1984) are impaired in this situation. Similar changes were reported for PR adult humans (Rastogi et al., 1974) and animals (Rostom de Mello et al., 1989). However, few studies could correlate thyroid dysfunction in adulthood with malnutrition in the first days of life. During gestation, protein malnutrition causes a delay in thyroid follicle formation and a reduction in gland area, follicle number, colloid space and cell size in the thyroid tissue of fetal and neonatal pups from PR dams (Shrader et al., 1977), A decrease in T3 level with normal serum T4 and thyroid stimulating hormone (TSH) levels was shown in the fetuses or newborns from PR lactating dams (Friedman and Zeman, 1979). Oberkotter and Rasmussen (1992) reported that the offspring of chronically ER dams showed lower serum T3 and higher serum reverse T3 (rT3, an inactive metabolite of T4) during lactation. We showed that protein malnutrition only during lactation was associated with thyroid dysfunction in 60-days-old (Ramos et al., 1997) or 180 days old offspring (Passos et al., 2002). Those changes may be related to maternal thyroid dysfunction, like the higher T3 serum concentration in PR dams (Ramos et al., 2000). Those mothers, probably, had an increase in T3 thyroidal production, plus an increase in T4 to T3 conversion, due to higher thyroid, brown adipose tissue (BAT) and skeletal muscle deiodinase activities (Lisboa et al., 2003). Only in the mammary gland, the D1 activity was lower in the 4th day of lactation. Considering that this enzyme also converts T3 to T2, that is inactive, it is possible that this effect contributes to a higher transfer of T3 from the maternal serum to the milk. In fact, protein malnourished mothers transfer more T3 to their offspring (Passos et al., 2001 a). A schematic view of thyroid hormone metabolism in malnourished mothers can be seen in Fig. 2. We also showed that protein malnutrition during lactation programs a higher T4 and T3 serum concentration in the adult offspring (Passos et al., 2002), in part due to a higher liver deiodinase activity in those animals (Dutra et al., 2003). The hyperthyroidism in animals from protein-restricted mothers could explain the low body weight observed in these animals. Similar results were found by Coleoni et al. (1983), who showed that adult rats whose mothers had been fed a low protein diet during gestation and lactation and the offspring were kept up to 50 days after weaning on this diet presented higher T3 serum concentrations and higher T3-inducible enzymes, as hepatic glycerophosphate dehydrogenase and malic enzyme, even after a long period of nutritional recovery. In that study, malnutrition 258 Moura and Passos Protein restricted dams during lactation malnourished pups malnourished dams T3 T4 D1 mammary gland Serum T3 Milk T3 Serum T3 T3 T4 Thyroid D1 Skelectal muscle Brown adipose tissue Fig. 2. Transfer to T3 through the milk in protein restricted lactating rats. D1-deiodinase type 1. was imposed during perinatal period until puberty. In contrast, deprivation in our experimental model was applied only during the lactation period, and we already observed the same permanent changes at the thyroid function. So, the critical phase for the metabolic imprinting seems coincide with the lactation period. Leptin and Thyroid Function Programming Although leptin secretion was initially reported exclusively from adipose tissue, other studies have identified leptin production in a few others tissues including the brain and pituitary (Morash et al., 1999), skeletal muscle. (Wang et al., 1998), placenta (Senaris et al., 1997), stomach (Bado et al., 1998), and epithelial cells of the mammary gland (Smith-Kirwin et al., 1998). Jin et al. (2000) have reported leptin in the rat anterior pituitary cells that produces the b subunit for thyroid–stimulating hormone (TSH) and follicle-stimulating hormone (FSH)/luteinizing hormone (LH). It becomes subsequently clear that leptin functions not only in the regulation of food intake and energy metabolism, but Neonatal Programming of Body Weight Regulation 259 also in different areas, i.e., secretory regulation of pituitary hormones (Ahima et al., 2000). Studies in normal rodents have demonstrated the expression of leptin receptor in the pituitary by reverse transcriptase polymerase chain reaction (Jin et al., 2000) and also by immunoblotting and immunohistochemical analysis (Sone et al., 2001; Vicente et al., 2004). It has been shown that leptin regulates the function of GH cells (Sone et al., 2001) and also has acute stimulatory effect on TSH release, in vivo, acting probably at the hypothalamus (Legradi, 1997; Seoane et al., 2000). In a recent study (Vicente et al., 2004), we demonstrated that the protein or energy restriction during lactation is associated with a higher expression of leptin receptor in the pituitary glands of adult offspring (Fig. 3). These animals had higher T4 and T3 serum concentrations and lower TSH. The lower serum TSH is in agreement with thyroid hormones negative feedback on the hypothalamic-pituitary axis. However, the higher expression of leptin receptor in the pituitary gland could also indicate an inhibitory effect of leptin on TSH secretion, since it has been demonstrated that the in vitro effect of leptin on TSH release is inhibitory (OrtigaCarvalho et al., 2002). These animals showed hypothalamic leptin resistance, as discussed above (Passos et al., 2004), and the increase in SOCS-3 expression was associated with both leptin resistance and higher ObRb expression (Bjorbaek et al., 1998). Conversely, if this higher expression of pituitary leptin receptor is associated with a resistance to leptin action, it could also contribute to the lower TSH. These data provide new insights regarding the physiological importance of nutritional status and consequent changes in leptin levels during neonatal period, Fig. 3. Immunohistochemical staining for leptin receptor in pituitary of adult rats whose mothers were fed a control (2A), protein-restricted (2B), and energy-restricted (2C) diet during. 260 Moura and Passos and suggest that leptin may play an important role on the thyroid-pituitary axis regulation in the adult. Nutrition has a major influence both on the hypothalamic-pituitary-thyroid axis and on nuclear TH binding in peripheral tissues. A low energy intake reduces thyroid gland activity, plasma TH levels and total TR numbers, and the decrease in nuclear TH binding is even greater when metabolic demand is increased by lowering the environmental temperature (Swoap et al., 1994). IUGR also down regulates TR (Dauncey et al., 2001). Thus, when energy is restricted, because of either a low intake or a high expenditure, the reduction in TR will limit responsiveness to TH and result in reduced growth and metabolism. Besides programming, another factor by which environmental changes may interfere with fetal and neonatal thyroid function, even before conception, is the genome imprinting. This phenomenon is characterized by the fact that some genes are grouped and can express in a different way if the allele is paternal or maternal (Fergunson-Smith and Surani, 2001). So, genomic imprinting is a form of nonmendelian epigenetic process in which the two parental alleles of a gene are differentially expressed according to parental origin and depending on environmental factors express only one parental allele. Around 50 genes are known to be genomic imprinted, including the DIO3 gene that express deiodinase III in placenta, affecting the transfer of thyroid hormones from the mother to the foetus (Tsai et al., 2002). PROGRAMMING DIABETES MELLITUS (DM) There are strong inverse relationships between birthweight and the risk of developing type 2 DM, evidenced by epidemiological studies (Ozanne and Hales, 2002). In a 64-year-old men population of UK, Hales et al. (1991) showed an association between low birthweight and glucose intolerance. It was confirmed later by Phillips et al. (1994) that carried out insulin tolerance tests on 103 men and women in Preston and found that in people with similar current body weight, the insulin resistance was greater in those who had a lower birthweight. Conversely, at each birthweight, the insulin resistance was greater in those with heavier body weights at adulthood, more pronounced in those who had developed obesity as adults (Hales et al., 1991). Short pre pubertal IUGR children also have impaired insulin sensitivity compared to their normal birthweight peers (Hofman et al., 1997). In a regional cohort study, in France, Leger et al. (1997), showed that IUGR was associated to reduce final height, raised insulin and proinsulin concentrations in young adults born small for gestational age. In Sweden, another study of 1333 men aged 50–60 confirmed that reduced fetal growth is associated with insulin resistance and non-insulin dependent diabetes mellitus (Lithell et al., 1996). The molecular basis for these programming are under intense investigation, but the complete understanding of this process is still elusive. Fetal and neonatal life is a crucial period for pancreatic b-cell development (Rahier et al., 1981). In rats, the mother’s protein malnutrition during the first days of lactation was associated with a decrease of insulin secretion in the 60- and 90-dayold progeny (Moura et al., 1997). This deficiency can be accelerated by a high-fat diet (Holness, 1996). The adult offspring, whose mothers were PR during pregnancy, exhibited an increase in gluconeogenesis metabolism (Burns et al., 1997) and a Neonatal Programming of Body Weight Regulation 261 significant decrease in pancreatic insulin content and beta-cell mass (Bertin et al., 1999). It seems that an amino acid specific deficiency can be responsible for this programming effect, since the mother’s diet supplementation with taurine prevents the impairment of insulin secretion in the offspring of PR mothers (Cherif et al., 1998). In the other side, the ‘‘insulin resistance syndrome’’ is composed of a wide range of metabolic disorders including glucose intolerance, hyperinsulinemia, increased very-low-density lipoprotein (VLDL), decreased high-density lipoprotein (HDL), and hypertension (Howard, 1999). Insulin resistance is thought to contribute towards the development of hypertension, hyperinsulinism, dyslipidemia, obesity, and cardiovascular disease, known as syndrome X or metabolic syndrome (Reaven, 1993). This syndrome was also related to low birthweight (Barker et al., 1993). Hales and Barker (1992) proposed the ‘‘thrifty phenotype’’ hypothesis which suggests that intrauterine malnutrition leads to adaptive responses that changes the nutrient disposal to the organs, affecting their normal growth. The brain is spared in detrimental of other organs, such as liver, pancreas and muscle. One adaptation of this hypothesis, is the ’fetal salvage’ hypothesis (Hofman et al., 1997) that adds to the ‘‘thrifty phenotype’’ hypothesis the permanent reduction in the skeletal muscle glucose transporter number or function, and in the antilipolytic action of insulin (Ozanne et al., 2001), that leads a stimulation of b-cell to produce larger amount of insulin to achieve glycaemic control that eventually leads to b-cell exhaustion. More importance is given to the insulin resistance rather than through b cell dysfunction (Lithell et al., 1996), Other alternative hypothesis for the association of low birthweight and type2 DM in adulthood were proposed, including a genetic link, but it fail to be confirmed, because these mutations affecting both fetal growth and insulin secretion or action are very rare (Ozanne and Hales, 2002). Reinforcing the hypothesis of programming in detrimental of a genetic link are the studies with twins that were discordant for type 2 DM (Poulsen et al., 1997; Bo et al., 2000). In both studies, the twin that was born with the lower birthweight was the one that presented the higher risk of developing type 2 DM. Other forms of programming can be associated to insulin resistance as the higher serum cortisol in adults that had low birthweight (Phillips et al., 1998) and GC treatment during gestation, that will be discussed in the next section. PROGRAMMING OF THE HYPOTHALAMUS–PITUITARY–ADRENAL (HPA) AXIS Malnutrition and hormonal treatment during gestation or lactation are stressful events that can activate the HPA axis. Despite the use of appropriated controls, models of HPA disruption during this critical period of life can be useful to individualize the programming effects of malnutrition and isolated hormonal treatments from those caused by pure stress. Several different manipulations in early development can program HPA function in adult primates, guinea pigs, sheep and rats. Examples of prenatal manipulation are maternal stress (Welberg and Seckl, 2001; Weinstock, 2001), exposure to synthetic GC (Matthews, 2000; Liu et al., 2001) and nutrient restriction (Lingas and Matthews, 2001; Lesage et al., 2002). Postnatal manipulations include neonatal 262 Moura and Passos handling (Meaney et al., 2000), modified maternal behavior (Meaney, 2001) and exposure to GC (Bakker et al., 2001). Maternal treatment with synthetic GC (e.g. dexamethasone) has provided a useful tool to investigate the impact of GC on the development and subsequent function of the HPA axis. In the rat, daily treatment with synthetic GC in the final week of gestation results in elevated basal plasma corticosterone levels in the adult offspring (Welberg et al., 2001]. This was associated with increased blood pressure (Levitt et a1., 1996) and obesity in adulthood (Bjorntorp and Rosmond, 2000). In another study, maternal synthetic GC treatment on gestational days 17 to 19 resulted in adult offspring that mounted a greater corticosterone response to stress (Muneoka et al., 1997). Prenatal dexamethasone (dex) exposure in the rat lowers birthweight and programs for lower adult body weight, smaller fat pads, and relatively preserved muscle mass, fasting and postprandial hyperglycemia, associated with increased hepatic gluconeogenesis driven by elevated liver and adipose tissue GC receptor (GR) expression (Cleasby et al., 2003). Increased hepatic gluconeogenesis is probably explained by permanent elevation in expression of hepatic phosphoenolpyruvate carboxykinase in dex-treated offspring (Nyirenda et al., 1998). Paradoxically, plasma leptin was increased in 1-yr-old rats given dex continuously during their third week of gestation (Sugden et al., 2001). Permanent GR up-regulation has been reported in the offspring of PR mothers, which also exhibit insulin resistance and hypertension (Bertram et al., 2001). However, in the model of dex prenatal exposure, GR is not widespread up-regulated, because its expression is lower at the hippocampus (Welberg et al., 2001). So, programming can affect hormonal regulation in a tissue-specific way. Regardless the causing factors, time and duration of the stimulus, that can cause differences on HPA programming, prenatal or neonatal GC increase programs for a higher vigilance status in adulthood, increasing the ability to survive to a menacing environment. PROGRAMMING OF SYMPATHOADRENAL FUNCTION Environmental exposures at critical period of development permanently alter sympathoadrenal function in mammals. The sympathetic innervations of peripheral tissues and the responsiveness of sympathetic nerves and adrenal medulla to standard stimuli are susceptible to modification by exposures in early life, such as environmental temperature, nutrition and stress. Effects of temperature exposure in early life on thermoregulation appear to be qualitatively different from those of cold and heat acclimatization in adult animals (Ferguson et al., 1981). Although less extensively studied than the effects of environmental temperature, both maternal and neonatal nutrition contribute to development of SNS anatomy and function. It was shown, recently, that young adult humans that were born with low birthweight showed increase in their sympathetic activity, that could contribute to hypertension (Boguszewski et al., 2004). One model often employed to manipulate neonatal nutrition is variation in litter size (Young, 2002). Animals reared in small litters are considered to be ‘‘overnourished’’, whereas those reared in large litters are ‘‘undernourished’’ The Neonatal Programming of Body Weight Regulation 263 stimulatory effect of dietary sucrose on cardiac SNS activity was completely absent in rats reared from one day of age in small litters (Young, 2002). These findings are potentially important, because the impairment in activation of cardiac sympathetic nerves by dietary sucrose seen in rats reared in small litters is similar to reported effects of the neurotoxin, gold thioglucose, in adult mice (Young, 1980). Gold thioglucose destroys neurons within the ventromedial hypothalamus (among other sites), a lesion that is dependent upon the presence of insulin (Debons et al., 1968). Because circulating insulin levels are increased in neonatal rats reared in small litters (Plagemann et al., 1992) and because administration of insulin to neonatal rats alters hypothalamic morphology (Plagemann et al., 1992; Plagemann et al., 1999), neonatal hyperinsulinemia might induce permanent changes in insulin-dependent SNS responses, such as those related to dietary carbohydrate. These findings are analogous to the impairments observed in ventilatory regulation in adult rats exposed to hyperoxia during the first month of life (Ling et al., 1996), and suggest that regulatory pathways governing vegetative function could be susceptible to desensitization and/or damage by excessive stimulation at an early age. SUMMARY AND CONCLUSIONS Programming is based on the observation that environmental changes can reset the developmental path during a critical period of life, when the tissues still have some plasticity and are in a higher proliferating and differentiating phase. Leptin seems to be a hormone that reunites all the predicates to be a signaling factor of nutritional environmental conditioning the animal to different levels of nutrient supply during life. 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