The intrauterine and early postnatal environments are critical in the development of offspring. Previous work has documented that maternal HF diet throughout gestation and suckling can have long-term metabolic consequences at weaning and in adulthood (14
). In this study, we used a cross-fostering procedure to determine whether prenatal or postnatal HF-diet exposure had a greater influence on offspring metabolic phenotype. Our data suggest that the maternal HF diet during the suckling period is more critical in determining metabolic consequences for offspring such as leptin resistance.
Other studies have used a HF-diet period before conception to induce maternal obesity, including 5 or 6 weeks before mating and throughout gestation and suckling (15
). Maternal obesity alone has significant effects on oocyte development, maturation, and embryo development (30
), any of which could have adverse effects on offspring independent of diet during gestation and suckling. Maternal obesity could also produce diabetic conditions, before or during pregnancy, which could predispose the offspring to metabolic side effects (32
). Consistent with previous studies using HF diet only during gestation, those dams fed the HF diet consumed more calories during gestation and lactation, but body weight did not differ significantly between the dietary groups throughout the experiment, perhaps suggesting that the HF-fed dams may have increased their energy expenditure during this time (33
). Plasma insulin and blood glucose levels on GD21 were similar in CHOW- and HF-diet-fed dams, suggesting HF dams had not developed gestational diabetes, although this remains to be tested directly.
Maternal HF diet, prenatally or postnatally, resulted in a significant attenuation of pSTAT3 activation in male offspring, whereas exposure only to the prenatal maternal HF diet resulted in decreased pSTAT3 activation in females. This sex difference was lost by PND21, when male and female pups cross-fostered to HF diet dams postnatally were less sensitive to leptin compared with the CHOW-CHOW group. It is unclear why there were sex differences in the response to leptin at PND10 but not at PND21. We and others have reported sex differences in offspring response to maternal HF diet in adulthood (15
). It is intriguing to postulate that those differences in metabolic programming in males and females have origins during the early postnatal period, even before puberty, and may be related to deficits in hypothalamic development secondary to impaired leptin signaling during the neonatal period.
An unexpected outcome was the finding that the prenatal HF diet alone (HF-CHOW) group had no effect, with the exception of PND10 leptin sensitivity, compared with the CHOW-CHOW control group, suggesting three possibilities as follows: 1) prenatal HF diet exposure alone was not sufficient to impair leptin sensitivity and that the exposure to maternal HF diet during the postnatal period is required for this phenomena, 2) the prenatal HF diet imparts increased susceptibility to metabolic abnormalities that is precipitated when provided with a HF diet after weaning, or 3) cross-fostering the prenatal HF pups to a CHOW-fed dam postnatally corrects for any impairment in leptin sensitivity that may have developed during the prenatal period. More detailed study of the time course of the effects of maternal diet on leptin signaling during the early postnatal period may distinguish among these possibilities.
Two of the downstream targets in the leptin-signaling cascade are AMPK and ACC. AMPK and ACC in the hypothalamus are dephosphorylated in response to leptin activation of STAT3 signaling, and the resulting decreases in pAMPK and pACC in the hypothalamus inhibit food intake in adult rats (34
). There was no change in total or pAMPK or pACC after the leptin challenge in all four groups compared with baseline levels. The pSTAT3 response to leptin is clear in rodent neonates as early as PND1 (38
). Behaviorally, mouse pups do not respond to peripheral leptin on PND17 but do display a decrease in food intake by PND28 (39
). Thus, although we observe a significant activation of the STAT3 pathway, it appears that the downstream mediators, such as AMPK and ACC, may not normally be functional until after weaning. This signaling pathway may mature later in development to allow neonates to maximize their food intake during a period of rapid growth (40
). This notion may be further supported by and is consistent with leptin’s primary role as a critical trophic factor for neurodevelopment rather than an adiposity signal that controls food intake during the early postnatal period.
The precise timing of when pathways that control food intake become functional and influence behavior remains to be determined. It is clear, however, that pups suckled by dams fed a HF diet have a deficit in their pSTAT3 response to peripheral leptin, at least up to PND21, and this may be the reason those offspring are hyperphagic and remain heavier through adulthood compared with CHOW offspring. In addition, we administered leptin peripherally, and it is possible that there is a deficit in the transport of leptin across the blood–brain barrier (BBB) as a result of obesity or maternal HF diet, pre- or postnatally. Diet-induced obese rats develop deficits in leptin transport across the BBB, although whether this is due to obesity, age, or a combination of both is unknown (41
The field of early-life metabolic programming has made significant progress in establishing the offspring phenotypes resulting from changes in perinatal diet. However, the question of what mechanisms are responsible for these outcomes remains. There were no differences in leptin content of maternal milk on PND10, but there was greater leptin and fat content of milk of dams fed a HF diet on PND21. Leptin can be transmitted to offspring via maternal milk; however, the obese and leptin-resistant phenotype was evident by PND10, indicating that milk leptin may have an influence during the later postnatal period but is not responsible for the early metabolic phenotype. Alternative possibilities include other hormones that were not measured, such as ghrelin, which has recently been implicated as a trophic factor in hypothalamic development (42
) and fatty acid composition of maternal milk (26
). In addition, we previously reported that neonatal offspring of HF-fed dams consume more milk in an independent ingestion test (25
). Greater caloric consumption could also have a significant influence on pups’ development of obesity, as demonstrated by studies using small litters to increase milk availability and consumption in rat and mouse pups (43
). These possibilities all represent future directions for our studies.
Postnatal HF pups had greater circulating leptin, independent of their prenatal dams’ diet, as early as PND10 in this study and may contribute to alterations in hypothalamic development and leptin resistance. Others have shown that elevating neonatal leptin levels with exogenous leptin administration to offspring of ad libitum, chow-fed dams results in an increase in the risk of obesity compared with saline treatment (45
). In the converse situation, Vickers et al. (48
) showed that neonatal leptin treatment to restore plasma leptin levels to normal reversed developmental programming in offspring of undernourished dams. Thus, manipulation of neonatal leptin levels has significant effects on offspring metabolic phenotype.
This then calls into question whether hypothalamic development proceeds normally in offspring from dams fed a HF diet, prenatally, postnatally, or both. In rodents, hypothalamic neuronal proliferation occurs primarily during midgestation, but the development of neural projections from these neurons to their downstream target sites is initiated during the early postnatal period (23
). Leptin has been identified as a critical trophic factor that influences the development of the hypothalamic projections, which continues during the early postnatal period in rats (22
). Alterations in the pattern of leptin secretion (premature peak, excess, or deficiency) during neonatal life have significant adverse effects on hypothalamic development and metabolic phenotype (21
). We previously found that pups born to HF-fed dams had higher plasma leptin levels than those from CHOW-fed dams beginning during the first postnatal week, and this persisted throughout the suckling period (24
), suggesting that hypothalamic development may be altered in offspring suckled by HF-fed dams.
In summary, our studies demonstrate that a perinatal HF diet influences offspring metabolic phenotype in a time- and sex-dependent manner. The next step is to elucidate the mechanisms responsible for adverse consequences of HF-diet exposure early in life. Additional important studies will be those directed at determining the mechanisms involved in correction of HF diet–related deficits by manipulation of postnatal diet (i.e., CHOW diet) or behavior (e.g., exercise) and in further refining the critical windows for development of systems regulating energy homeostasis and associated metabolic processes.