Feeding in infancy: short- and long-term effects on cardiovascular function

Psychobiological Roots of Early Attachment

New laboratory research has revealed a network of simple behavioral, physiological, and neural processes that underlie the psychological constructs of attachment theory. It has become apparent that the unique features of early infant attachment reflect certain unique features of early infant sensory and motor integration, learning, communication, and motivation, as well as the regulation of biobehavioral systems by the mother–infant interaction. In this article, I will use this new knowledge to answer three major questions that have remained unsettled in our understanding of early human attachment: What creates an attachment bond? Why is early maternal separation stressful? How can early relationships have lasting effects? I will discuss the implications of these new answers for human infants and for the development of mental processes. Attachment remains useful as a concept that, like hunger, describes the operation of subprocesses that work together within the frame of a vital biological function.

Parental potentiation of vocalization as a marker for filial bonds in infant animals

Maternal and paternal potentiation of vocalization are two parts of a promising model of early life social bonds that has been and can be a useful tool in research. Most mammalian infants vocalize when isolated. Interactions with adult females just before isolation have been found to increase vocalizations in several species. Interactions with littermates and other social stimuli do not. In guinea pigs and pigs, the response is specific to the dam. In rats and octagon degus, an unrelated adult female from the colony is sufficient. The presence of an intact adult male in the test chamber with dam-reared pups evokes behavioral inhibition, a fear response. Previous exposure to the male in the home cage, biparental rearing, dramatically transforms the response of the pup. The pup treats the adult male as it does its dam, including potentiation of vocalization during a subsequent isolation. This article outlines the methods, advantages, and disadvantages of parental potentiation as a research tool, as well as a brief review of the evidence supporting its use as a marker for filial attachment. Future research directions are outlined.

Changing mechanisms of opiate tolerance and withdrawal during early development: animal models of the human experience

Human infants may be exposed to opiates through placental transfer from an opiate-using mother or through the direct administration of such drugs to relieve pain (e.g., due to illness or neonatal surgery). Infants of many species show physical dependence and tolerance to opiates. The magnitude of tolerance and the nature of withdrawal differ from those of the adult. Moreover, the mechanisms that contribute to the chronic effects of opiates are not well understood in the infant but include biological processes that are both common to and distinct from those of the adult. We review the animal research literature on the effects of chronic and acute opiate exposure in infants and identify mechanisms of withdrawal and tolerance that are similar to and different from those understood in adults. These mechanisms include opioid pharmacology, underlying neural substrates, and the involvement of other neurotransmitter systems. It appears that brain circuitry and opioid receptor types are similar but that NMDA receptor function is immature in the infant. Intracellular signaling cascades may differ but data are complicated by differences between the effects of chronic versus acute morphine treatment. Given the limited treatment options for the dependent infant patient, further study of the biological functions that are altered by chronic opiate treatment is necessary to guide evidenced-based treatment modalities.

Duration of early maternal separation and prediction of schizotypal symptoms from early adolescence to midlife

Early childhood experiences influence the capacity for healthy social and emotional development. The present study uses longitudinal data to determine whether early maternal separation predicted the subsequent development of schizotypal personality disorder (SPD) symptoms assessed repeatedly from early adolescence over the following 20 years. Within this community sample (N=766), multilevel linear regression analyses revealed the duration of separation from mother in the first 2 years of life predicted elevated SPD symptoms. This relationship was specific to children with mother-reported early angry emotional behavior. These results provide support for the role of early childhood psychosocial risk factors in the development of subsequent schizophrenia spectrum symptoms in emotionally vulnerable children.

The measurement of blood pressure and the detection of hypertension in children and adolescents

Despite the publication of several expert committee guidelines for the measurement of blood pressure (BP) and the diagnosis of hypertension in children and adolescents, it was our perception and clinical experience that there still appeared to be a general lack of standardisation of BP measurement techniques and little consensus on the criteria for diagnosing hypertension. To investigate this further, we have conducted a postal survey of consultant-grade paediatricians who were members of the British Paediatric Association (BPA). A total of 1500 questionnaires were sent out and 708 analysable replies were received (47.1%). This showed that 68.6% of paediatricians routinely measured BP, at least on one occasion, in children or adolescents attending their outpatient clinics, 17.7% started at or soon after birth, 12.3% started at the age of 1 year, 20.0% at 3 years, 12.0% from 7 years of age and 3.5% from the age of 13. Only 60.5% reported that they had a choice of four or more different cuff sizes in their clinic. Forty-one percent of respondents reported that the BP was always or sometimes measured by nurses. Fifty-one percent of respondents measured diastolic BP at the phase of muffling of sound (Korotkoff phase IV), 31.9% used the disappearance of sound (phase V) whilst 15.9% claimed that they measured both end-points. The criteria for diagnosing a child as being hypertensive varied greatly; 17.9% reported that they responded to the systolic BP alone, 13.5% to the diastolic BP alone, 65.9% relied on both pressures, and 2.7% responded to either the systolic or diastolic pressure if it was raised. Furthermore, 12.9% diagnosed hypertension if the BP exceeded the 90th percentile in relation to age and 41.8% used the 95th percentile. However 45.3% of respondents employed a higher dividing line. In hospitalised children, leg blood pressures were measured routinely by 30.3%, although a further 44.0% would do so if aortic coarctation or other vascular diseases were suspected. Despite considerable variation in clinical practice, techniques and criteria, only 11.4% of clinicians would manage the patients themselves, with the remainder referring the child on to the appropriate specialist. The survey suggests a general lack of standardisation of BP measurement techniques and little consensus on the criteria for diagnosing hypertension amongst paediatricians. Simplified, shortened and updated guidelines on hypertension in paediatric practice and research are needed.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Thermoregulatory and cardiac responses of infant spontaneously hypertensive and Wistar-Kyoto rats to cold exposure

Cardiovascular function during cold exposure is dependent on effective thermoregulation. This dependence is particularly apparent in infants. For example, we have previously demonstrated that in infant rats during cold exposure, cardiac rate is directly related to their ability to produce heat endogenously. The primary source of endogenous heat production for infant rats is brown adipose tissue (BAT). Because of the dependence of cardiac rate on effective thermoregulation in the cold and because hypertension in spontaneously hypertensive rats (SHR) is influenced by the preweanling environment, in this study we examined the thermoregulatory and cardiac rate responses of infant SHR and Wistar-Kyoto rats (WKY) to varying levels of cold exposure. In experiment 1, 7- to 8-day-old SHR and WKY were acclimated at a thermoneutral air temperature (35 degrees C) and then exposed to successive decreases in ambient temperature (30.5 degrees C, 26.5 degrees C, 23 degrees C, and 17 degrees C) while thermal and metabolic measures were recorded. Although both strains increased BAT thermogenesis and oxygen consumption in response to cold exposure, SHR cooled more than WKY and exhibited lower levels of oxygen consumption at the lowest air temperatures. Experiment 2 was identical to experiment 1 except that cardiac rate was also measured. Again, SHR exhibited substantial thermoregulatory deficits compared with WKY; in addition, they were less able than WKY to maintain cardiac rate at the 2 lowest air temperatures tested. Finally, in experiment 3, infant SHR exhibited diminished BAT thermogenesis in response to a range of doses of a selective beta3-adrenoceptor agonist. We hypothesize that long-term thermoregulatory deficits during the early postnatal period influence cardiovascular function and contribute to the development of hypertension in SHR.

Cardiovascular responses to feeding in the neonate during the first four days of life

Prior work has demonstrated acute cardiovascular responses associated with breast and bottle feeding of newborn infants that consist of increases in both blood pressure and heart rate. This current study sought to determine if the amplitude of these responses is related to the age of the infant and/or the amount of nutrient ingested during the study period. Results show that the magnitude of the feeding responses does not change over the first four days of life. In a second study it was found that changes in systolic blood pressure during feeding are positively correlated with volume of nutrient intake during the initial phase of feeding (r = +0.75, p < 0.01). These results suggest that some of the individual variability in the increase in blood pressure during feeding is related to the amount of nutrient ingested. It is not known whether these differences are due to differences in sucking effort or other aspects of feeding efficiency.

Maternal care, hippocampal glucocorticoid receptors, and hypothalamic-pituitary-adrenal responses to stress

Variations in maternal care affect the development of individual differences in neuroendocrine responses to stress in rats. As adults, the offspring of mothers that exhibited more licking and grooming of pups during the first 10 days of life showed reduced plasma adrenocorticotropic hormone and corticosterone responses to acute stress, increased hippocampal glucocorticoid receptor messenger RNA expression, enhanced glucocorticoid feedback sensitivity, and decreased levels of hypothalamic corticotropin-releasing hormone messenger RNA. Each measure was significantly correlated with the frequency of maternal licking and grooming (all r's > -0.6). These findings suggest that maternal behavior serves to "program" hypothalamic-pituitary-adrenal responses to stress in the offspring.

Long-term effects of neonatal sodium restriction on blood pressure

In 1980, a randomized trial was conducted among 476 Dutch newborn infants to study the effect of a low or normal sodium diet on blood pressure during the first 6 months of life. At the end of the trial, systolic blood pressure in the low sodium group (n = 231) was 2.1 mm Hg lower than in the control group (n = 245). To investigate whether contrasting levels of sodium intake in infancy are associated with blood pressure differences in adolescence, we measured blood pressure in 167 children from the original cohort (35%) after 15 years of follow-up. We assessed the differences in systolic and diastolic blood pressure levels between the diet groups using a multivariate regression model with adjustment for potential confounders. The adjusted systolic blood pressure at follow-up was 3.6 mm Hg lower (95% confidence interval, -6.6 to -0.5) and the diastolic pressure was 2.2 mm Hg lower (95% confidence interval, -4.5 to 0.2) in children who had been assigned to the low sodium group (n = 71) compared with the control group (n = 96). These findings suggest that sodium intake in infancy may be important in relation to blood pressure later in life.

Enduring effects of infant feeding experiences on adult blood pressure

OBJECTIVE

Vulnerability to psychosomatic diseases is influenced by events early in life. The objective of this article is to discuss animal research that demonstrates relationships between feeding experiences and growth in infancy and risk of hypertension in adulthood.

METHOD

Subjects were spontaneously hypertensive rats (SHR) and their normotensive Wistar Kyoto progenitors. Initial experiments involved observations of the behaviors of rat mothers and their infants and follow-up measurements of blood pressures. Further studies focused on measurements of infant blood pressure during feeding, and recent investigations manipulated weight gain and sex hormones early in life.

RESULTS

Infant rats whose mothers were seen nursing more often had increased blood pressure as adults. Each time rat mothers delivered milk to their young, the nursing pups' blood pressures rose dramatically. These feeding-induced increases in blood pressure have been observed in the young of many species including humans. They are mediated by autonomic nervous system activity and are larger in SHR pups. Finally, animals that gain weight rapidly as infants as a consequence of being reared in small litters had higher adult blood pressure; but, this effect is seen only in intact males.

CONCLUSIONS

Adult physiologic traits can be influenced by the joint actions of genetic predisposition and essential psychosocial interactions during early development. Animal models can stimulate new ideas, provide important confirmations and elaborations of hypotheses from human investigations, and afford experimental approaches for identifying mechanisms underlying the transduction of behavioral experience to disease susceptibility.

Effects of neonatal growth on adult blood pressures of borderline hypertensive rats

We conducted this study to test the hypothesis that there are long-term effects of litter-size manipulations during the preweaning period on growth and adult blood pressure of rats. Litter size of genetically homogeneous borderline hypertensive rats, which were produced by cross-mating male Wistar-Kyoto rats with female spontaneously hypertensive rats, was manipulated from 10 to 16 days of age. In addition, a subset of males and females was castrated within the first 30 hours of life. Body weights were measured at several preweaning and postweaning ages, and tail-cuff blood pressures were recorded at 90 days of age. Intact and castrated pups of both sexes that were reared in small (n = 4) litters from 10 to 16 days of age gained nearly twice the weight of animals reared in large (n = 9 to 12) litters during this period. Intact males from small litters had significantly higher adult blood pressures than those from large litters. These long-term effects remained even in groups matched for adult weight and length. Neonatal castration of males completely blocked the consequences of litter-size manipulation on adult blood pressure, suggesting either an organizational or activational role for androgens. Neither intact nor neonatally castrated females exhibited differences in adult blood pressure as a function of litter-size manipulation.

Is blood pressure in later life affected by events in infancy?

This article reviews experimental and clinical evidence of whether primary hypertension (HTN) later in life is influenced by events early in life. The experimental evidence is drawn from studies in inbred strains of HTN-prone rats; the clinical evidence is drawn from studies in children and adults of the influence of genetics, nutrition, and stress on adult blood pressure (BP). Adult BP in HTN-prone rats is significantly influenced in the preweaning period by salt intake and genetic factors regulating extra-cellular fluid volume, and by maternal-infant interactions. BPs of children track with BPs of their parents. Children of parents with primary HTN are insulin resistant and have lower average cation flux values across cell membranes as do their parents; children and their parents with secondary HTN do not. Children with low birth weight have a higher prevalence of HTN as adults than better-nourished peers. Salt intake in children affects BP response to stress. Average salt consumption among different cultures correlates with the prevalence of HTN in those cultures. Varying salt intake of infants and children has little influence on BP later in childhood. The evidence suggests simple measures that might lower the risk for HTN in HTN-prone children in general. However, at present we lack reliable means for identifying children at risk for HTN specific means to lower that risk.

Early relationships as regulators of infant physiology and behavior

In recent years, animal research has revealed a network of simple behavioral and biological processes that underlie the psychological constructs we use to define early social relationships. Hidden within the observable interactions of parent and offspring are sensorimotor, thermal and nutrient-based events which have unexpected and widespread regulatory effects on infant behavior and physiology. The complex pattern of responses resulting from early separation in infant rats can be traced to the abrupt withdrawal of a number of discrete, independent regulatory processes which had been acting on individual components of the infant's physiology and behavior. These regulatory processes also appear to mediate long-term shaping effects exerted by early relationships, for example, on the vulnerability of the adult rat to hypertension and stress ulcer. In human development, early regulatory interactions may provide a bridge between biological and psychological processes in the development of our earliest mental representations.

Timing of preweanling maternal effects on development of hypertension in SHR rats

Cross-fostering of litters was used to determine the timing of preweanling maternal influences on the development of high blood pressure in spontaneously hypertensive (SHR) rats. The SHR litters were either reared by their natural mothers or reciprocally cross-fostered to normotensive Wistar-Kyoto (WKY) mothers for postnatal days 1-7, 1-14, 1-21, 8-21, or 15-21. All SHR litters were weaned at 21 days of age and males were housed in groups of two to three per cage until physiological measures were obtained at 100 days of age. At 100 days of age, all rats were surgically prepared with tail artery catheters and, on the day after surgery, direct measures of mean arterial pressure (MAP, mmHg) and heart rate (HR, bpm) were obtained while rats were resting and undisturbed in their individual home cages. Our findings indicate that cross-fostering SHR pups to WKY foster mothers was attended by significant effects on body weights at weaning and on adult MAPs. Compared to control SHRs, cross-fostered SHRs, with the exception of the 15-21-day group, were significantly heavier at weaning. By 100 days of age, body weights of SHRs were similar across treatment groups. Basal MAPs of SHRs cross-fostered for days 1-7, 1-14, 1-21, and 8-21, but not days 15-21, were reduced significantly compared to SHR controls reared by their natural mothers. In contrast, basal HRs were not affected in any of the cross-fostered SHR groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental sensitivity to high dietary sodium chloride in borderline hypertensive rats

The present study compared the postweaning blood pressures and body weights of borderline hypertensive rats exposed to a high (8%) sodium chloride maternal diet either from conception to weaning or only during the weaning period with borderline hypertensive rats consistently exposed to a normal (1%) sodium chloride maternal diet. Because the effects of early sodium chloride exposure may be most evident during a subsequent challenge, rats from each group were assigned to receive either an 8% sodium chloride or a 1% sodium chloride diet from 8 to 17 weeks of age. Exposure to an 8% sodium chloride diet from conception through weaning increased the adult blood pressure of borderline hypertensive rats compared with that of controls exposed to a 1% sodium chloride diet; exposure to an 8% sodium chloride diet only during weaning did not increase blood pressure. An 8% sodium chloride diet beginning at 8 weeks of age increased systolic blood pressure. The effects of perinatal and adult exposure to high dietary sodium chloride were additive. Behavioral observations and urinary electrolyte measures confirmed that pups exposed to an 8% sodium chloride diet during weaning ingested the high-sodium chloride diet. The blood pressure and heart rate response to autonomic nervous system ganglionic blockade were assessed at 17 weeks of age. Borderline hypertensive rats exposed to an 8% sodium chloride diet from conception through weaning showed an increased bradycardic response, but no difference in depressor response, to ganglionic blockade. These data suggest that the window of developmental sensitivity for modulation of blood pressure regulation by high dietary sodium chloride occurs during prenatal and early postnatal development.

Regulation of sodium and body fluid homeostasis during development: implications for the pathogenesis of hypertension

The spontaneously hypertensive rat (SHR) is an important animal model of human essential hypertension. During the first month of life, increased retention of sodium is present in the SHR which appears to be mediated by the renin-angiotensin system. The present review will discuss the role that increased activity of the renin-angiotensin system plays in sodium/body fluid regulation during early development. It is hypothesized that disordered regulation of sodium/body fluid homeostasis during this stage leads to pathological cardiovascular regulation in adulthood. Through an understanding of the relationship between sodium/body fluid balance in the young and cardiovascular function in the adult insights may be gained into both the pathological state of hypertension and the critical role played by early development in shaping homeostatic mechanisms in adulthood.

Nature/nurture and the nature of nurture in the etiology of hypertension

Four reviews on the the role of developmental factors in hypertension are introduced and set in historical context. Recent research in the laboratory rat has shown that the preweaning environment makes an important contribution to the level of blood-pressure reached in adult life in genetic models of hypertension. Both of the most commonly used models of hypertension, the SHR and SS/Jr rat strains, exhibit lower BP in adult life, if they are fostered shortly after birth to mothers from their normotensive control strains. It has been suggested that it is the idiosyncratic maternal behavior of the hypertensive mothers which contributes to the elevated BP of their offspring, and it has been amply demonstrated that there is an association between a constellation of behaviors emitted by rat mothers and the adult BP of their offspring in a wide variety of genetic groups (inbred hypertensive animals, F1's and F2's). In addition to the above, maternal environment has been demonstrated to have a significant impact on the pathophysiological response of hypertensive animals to a high salt diet. Being raised by an SHR mother, versus an SS/Jr mother, increases the magnitude of BP increases to a high salt diet, susceptibility to hemorrhagic stroke, body weight loss and the risk of mortality. A variety of physiological systems are undergoing rapid change during the preweaning period and may mediate the effects of differences in the maternal environment. These include the renin-angiotensin system and the peripheral sympathetic nervous system. Nutritional factors may be involved in all of the phenomena referred to above. Thus, any physiological mechanisms that are proposed to link maternal behavior to its effects on the physiology of adult animals should recognize the involvement of nutritional factors. Research on the role of developmental factors such as maternal behavior in genetic models of hypertension is at the interface of two growing disciplines: behavior genetics and developmental psychobiology. The methodological and conceptual contributions of these fields to advancing our understanding of these phenomena is emphasized.

Maternal influences on cardiovascular pathophysiology

Elevated blood pressure (BP) is of special clinical significance because of its association with pathophysiologies such as heart disease, renal failure, and stroke. We described the development of a protocol for use with hypertensive rats in which prepubertal exposure to a high salt (8% NaCl) diet results in a pathophysiological syndrome including rapid increase in BP, failure to maintain normal weight gain, renal damage, cerebrovascular lesions, and early mortality. These phenomena are described for the inbred spontaneously hypertensive rat (SHR), and for reciprocal F1 hybrids of a cross between SHR and the Dahl salt-sensitive (SS/Jr) inbred strain. The study with reciprocal F1s revealed striking effects of maternal environment on pathophysiological response to a high salt diet. F1s nurtured by SHR mothers weighed less at 35 days of age, and after exposure to the high salt diet suffered more rapid BP increases, greater incidence of stroke, body weight loss, and mortality, than F1s nurtured by SS/Jr dams. These results suggest that maternal mediation of the nutritional status of the animal may play an important role in determining susceptibility to elevated BP and subsequent pathophysiology associated with exposure to a high salt diet. The implication of these findings for human hypertension is briefly discussed.