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2 Mental Health Research Institute, 1 Department of Pediatrics, and 3 Department of Psychiatry, University of Michigan, Ann Arbor, Michigan 48109-0720
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Dexamethasone is commonly used to lessen the morbidity of chronic lung disease in premature infants, but little is known regarding neurological consequences of its prolonged use. To study neurological effects of dexamethasone, we have developed a rat model in which newborn pups are exposed to tapering doses of dexamethasone at a time corresponding neurodevelopmentally to human exposure in the neonatal intensive care unit. On postnatal day (PD) 2, litters were divided into three groups: 1) handled controls, 2) saline-injected animals, and 3) animals injected with tapering doses of intramuscular dexamethasone between PD 3 and 6. Somatic growth and brain weight were decreased in dexamethasone-treated animals. Dexamethasone-treated animals demonstrated delays in gross neurological development on PD 7 and 14 but not PD 20. In late adolescence (PD 33), dexamethasone-treated animals were less active in light and dark environments, while demonstrating a blunted serum corticosterone response to a novel stress. The dissociation between behavioral and hormonal stress responsiveness suggests that neonatal dexamethasone exposure permanently alters central nervous system function, particularly within the neuroendocrine stress axis. This may lead to increased risk for learning impairment and maladaptive responses to the environment.
steroids; prematurity; brain; corticosterone; limbic-hypothalamic-pituitary-adrenal axis
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INTRODUCTION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IN EXTREMELY LOW BIRTH WEIGHT (ELBW) infants with severe respiratory distress syndrome, postnatal dexamethasone (Dex) is often used to lessen the morbidity of chronic lung disease. Even though clinical trials fail to consistently demonstrate significant improvement in mortality or length of stay, prolonged courses of up to 42 days are still used in many centers (3, 13, 25, 26). The time of treatment for premature infants corresponds to the viable perinatal period from 24 to 40 wk postconception, when the human central nervous system (CNS) undergoes profound structural and functional transformations, making it particularly vulnerable to a variety of external influences. Common side effects of Dex use in ELBW infants include hypertension, bowel perforation, infection, ventricular hypertrophy, catabolic changes, and alteration of the limbic-hypothalamic-pituitary-adrenal (LHPA) axis (1, 7, 11, 19, 27, 33, 34). Despite these observations, very little data have been generated with regard to possible glucocorticoid effects on the developing CNS.
Clinical studies have demonstrated that premature infants receiving long-term Dex therapy have reduced linear growth, decreased weight gain, and smaller head circumferences (8, 21, 42). During the acute phase of Dex exposure, changes in gross neuromotor function have also been noted (9, 56). Not surprisingly, recent clinical studies have linked Dex therapy to long-term neurological effects, including an increased incidence of cerebral palsy at 1 yr corrected for age and decreased cerebral volume as measured by neuroimaging (30, 37).
Animal models are beginning to emerge investigating the neurological effects and possible mechanisms of perinatal glucocorticoids (10, 17, 18). Although caution is necessary when extrapolating from animal models to the clinical setting, there is very little variation in the general sequence of brain growth between laboratory animals and humans (14); the main differences being the timing of events that lead to spurts in brain growth. In humans, neuronal proliferation is completed before the 24th week of gestation, exceptions being the cerebellum and dentate gyrus (14). After this gestational age, glia continue to proliferate and oligodendroglial maintain ongoing myelination, with a peak in brain growth occurring near term. In contrast to humans, rodents have their brain growth spurt after birth. It is estimated that in terms of brain growth rate, periventricular germinal matrix composition, neurochemical expression, electroencephalographic patterns, and synapse formation, at approximately postnatal day (PD) 7, the rat brain is roughly equivalent in development to that of a full-term human infant (38 to 40 wk postconception; Refs. 14, 15, 23). With the use of this model of cross-species neurodevelopment, the brain of a rat pup at birth (PD 1) corresponds to that of a human fetal brain at or near 22-24 wk gestation (15, 55). Extrapolating this brain growth velocity model further, the PD 2 brain approximates that of a 26- to 27-wk-old human brain in terms of gross neurodevelopment, the PD 3 brain corresponds with that of a 28- to 29-wk-old human brain, and the PD 6 brain corresponds with that of a 35- to 36-wk-old human brain. Given these developmental correlations, the neonatal rat pup provides a model in which to investigate effects of a prolonged, tapering course of neonatal Dex on the developing CNS.
The present study was designed to develop a rat model system in which to study long-term effects of a prolonged, tapering course of neonatal Dex administration. Unlike other studies, where single doses of Dex were given at particular postnatal ages (6, 16), our objectives were twofold: 1) to provide Dex during a postnatal age in the rat that corresponds to the neurodevelopmental time point at which human premature infants receive prolonged Dex therapy and 2) to provide Dex in tapering doses (between PD 3 and PD 6). The specific goal of this model is to more closely mimic glucocorticoid protocols provided in the neonatal intensive care setting where 42-day (6 wk) courses are still administered.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals. Adult Sprague-Dawley rats (Charles Rivers, Wilmington, MA) were housed in our animal unit and maintained in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals. All animals were kept under constant temperature (25 ± 2°C) and photoperiodicity (14:10-h light-dark cycle), and they were provided with food and water ad libitum.
With the use of the trio mating system (2 females:1 male), 20 female rats were mated. Females were then housed in pairs until estimated gestational day 18, at which point they were housed individually. The day of birth was designated PD 1. On PD 2, each litter was sexed and culled to 12 pups (6 males:6 females) to ensure equality in nutrition and maternal care within litters. Pups were separated into three treatment groups on PD 3, with each group represented within one litter, to control for variations in maternal behavior. On PD 8, a male and a female pup from each treatment group were killed, reducing the litter size to six animals for the remainder of the study.Drug treatment. As stated above, each litter was assigned to three treatment groups: handled controls (Han), saline-injected animals (Veh), and Dex-treated animals (Dex). All pups within each litter were removed from their mother and treated (or handled) between the hours of 1100 and 1300 for a period of 5 min. Animals in the Dex group received an intramuscular injection of Dex in tapering doses on PD 3 through 6 (0.5 mg/kg on PD 3, 0.25 mg/kg on PD 4, 0.125 mg/kg on PD 5, and 0.05 mg/kg on PD 6; American Pharmaceutical Partners, Los Angeles, CA). Animals in the Veh group received equivalent volumes of intramuscular sterile saline as the Dex animals, and animals in the Han group received no injection but were handled during the same time period on PD 3 through 6.
Animal measurements.
Weight and length were recorded before handling or injection on
PD 3-6, 8, 14, and
20 for each treatment group. Length was measured from the
nose to the base of the tail (rump length) in each pup. Procedures for
testing neurological responses were derived and adapted from those
reported by Altman and McCrady, Fox, and Wahlsten (2, 20,
50). Each pup was submitted to testing between the hours of 1100 and 1300 on PD 7, 14, and 20. Two
investigators performed the evaluation of the responses of all animals
using a 0-5 rating scale with a score of 5 corresponding to the
mature response (Table 1). Measures used
to examine neonatal neurodevelopment included posture, righting reflex,
postural flexion and extension, vibrissa placing, forelimb and hindlimb
placing, geotaxis, and bar hold. Physical maturity was measured by
observing eye opening, ear opening, ear folding, fur development, and
tooth eruption.
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Behavioral testing. All behavioral testing occurred between the hours of 0700 and 1000. On PD 21, open-field behavior was assessed. Rats were placed in the center of a brightly illuminated 70 × 70 × 31-cm Plexiglas box with the floor divided into 16 equal squares. Motor activity and behavioral measures were recorded during a 2-min test period. An animal was considered to have "crossed squares" when its entire body entered a new square. The amount of time spent rearing and grooming was counted sequentially throughout the test. Defecation and urination were also recorded.
Light-dark preference was tested on PD 33. The testing apparatus consisted of a covered 30 × 60 × 30-cm Plexiglas shuttle-box with a stainless steel grid floor suspended above corncob bedding. Boxes were divided into two equal-sized compartments with a 12-cm-wide opening. The light compartment was constructed of white Plexiglas and was brightly illuminated; the dark compartment was constructed of black Plexiglas and was minimally lit. Each animal was placed in the dark compartment, and the amount of time elapsed before the animal entered the lit side was recorded. Locomotor activity as well as time spent in each compartment were monitored by photocells located on the wall of each box, with the number of photocell beams interrupted per unit time recorded with a microprocessor. Total testing time was 5 min.Adrenocortical response to novelty stress.
On PD 33, blood sampling was performed via the tail nick
method at 15, 30, 60, and 120 min following exposure to the preference box, with a basal time point obtained before the procedure. Blood was
collected in tubes containing EDTA and then spun at 2,000 rpm for 7 min. Serum was collected from the spun sample and stored at 
20°C.
Brain weights.
Brain weights were obtained during necropsy on PD 8,
23, and 35. Brains were then quick-frozen in
liquid isopentane (40°C) and stored at 80°C for future
processing and analysis.
Statistical analysis.
Body weight, length, rate of growth, brain weight, hormonal values, and
behavioral data were averaged across treatment groups and ages. Results
were subject to ANOVA considering age, sex, and treatment
simultaneously. Total and individual neurological scores were also
averaged across treatment groups and ages, and these results were
subject to analysis using the Kruskal-Wallis test of nonparametric
values. For all tests, when sex differences were determined to be
nonsignificant, the data were collapsed across the respective variable.
The level of significance was set at P 
0.05, and
post hoc comparisons were done using Fisher's protected
least-significant differences tests.
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RESULTS |
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METHODS
RESULTS
DISCUSSION
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Growth measurements.
Females weighed significantly less than males in each of the treatment
groups at all ages (PD 4-8, n = 40, P < 0.001; PD 14 and 20,
n = 20, P < 0.05), and Dex-treated
animals weighed significantly less than the comparison groups on
PD 4-8 (n = 80), PD
14 (n = 40), and PD 20 (n = 40, P < 0.0001; Fig.
1A). Rump length was significantly less in Dex-treated animals on PD 7 (n = 80), PD 14 (n = 40), and PD 20 (n = 40, P < 0.0001; Fig.
1B). Dex-treated pups gained weight at a slower rate than
comparison animals for up to 2 wk after treatment was discontinued
(n = 40, P < 0.0001). A significant
difference was found in the rate of length gain only during the first
week of life (n = 80, P < 0.0002).
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Brain weights.
Brain weights in Dex-treated animals were reduced on PD 8 and 23 compared with both Veh and Han controls. However,
this effect only reached significance between Dex and Veh animals
(n = 10, P < 0.05; Fig.
2A). In contrast to the
differences observed in total brain weights between groups, Dex-treated
animals maintain a higher ratio of brain weight to body weight compared
with both Veh and Han groups at PD 8 and 23 (n = 10, P < 0.05; Fig.
2B). Interestingly, on PD 35, the Veh group had a
significantly lower brain weight-to-body weight ratio than either the
Dex or Han groups (n = 10, P < 0.05;
Fig. 2B).
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Neurological examination. For global neurological assessment, a cumulative neurological test score for each treatment group was assessed (Table 1). Although Dex-treated animals exhibited a reduced total neurological score compared with the Veh and Han groups on PD 7 (mean total neurological scores ± SE: Dex 44.53 ± 0.572; Han 47.79 ± 0.392; Veh 47.03 ± 0.530; n = 80, P < 0.001), no significant difference on the total neurological score was noted on PD 14 and 20.
Differences observed on the neurological exam on PD 7 included delays in physical maturation (ear unfolding) and neurodevelopment (immature posture, forelimb placing, and postural extension reflexes) in Dex-treated animals compared with controls (n = 80, Han P < 0.05; Veh P < 0.01; Fig. 3). On PD 14, Dex-treated animals also exhibited delays in physical maturation (ear folding and fur development) and neurodevelopment (posture and vibrissa reflex; n = 40; Han P < 0.05; Veh P < 0.05; Fig. 4). Interestingly, Dex-treated animals exhibited a marked acceleration in eye opening at this postnatal age (n = 40, P 0.0001; Fig. 4). By PD
20, minor delays of physical maturation and posture persisted in
Dex-treated animals, with only delayed fur development in Dex-treated
animals reaching significance (n = 40, P 0.05; Fig. 5).
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Behavioral testing. In open-field testing, there were no significant sex differences observed. No significant differences between Dex-treated animals and comparison groups on PD 21 were observed in the number of squares crossed, time spent rearing, or the amount of defecation and urination. There was, however, a trend effect for the number of squares crossed, with Dex-treated animals crossing fewer squares than the control groups (mean number of squares crossed ± SE: Dex 9.98 ± 1.60; Veh 13.95 ± 1.86; Han 10.43 ± 1.2).
On PD 28, Dex-treated animals were less active in both dark and light environments compared with the control groups (n = 25, P < 0.05; Fig. 6A). There were no significant differences in emergence time to the light compartment or total time spent in either the light or dark compartments among the three groups. No sex differences were observed.
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Adrenocortical response to novelty stress. On PD 8, basal corticosterone levels in Dex-treated animals were suppressed when compared with Veh and untreated controls (mean serum corticosterone concentration in µg/dl ± SE: Dex 0.175 ± 0.05; Veh 0.660 ± 0.16; Han 0.552 ± 0.17; n = 19, P < 0.05). No differences in basal corticosterone levels were noted between groups on PD 23 (mean serum corticosterone concentration in µg/dl ± SE: Dex 1.43 ± 0.39; Veh 2.16 ± 0.79; Han 1.78 ± 0.52; n = 8) or on PD 33 (mean serum corticosterone concentration in µg/dl ± SE: Dex 0.69 ± 0.45; Veh 0.79 ± 0.42; Han 1.02 ± 0.37; n = 10). On PD 33, in response to novelty stress, Dex-treated animals demonstrated an early blunted corticosterone peak, followed by an early termination to baseline (Fig. 6B). There were no sex differences noted in basal corticosterone levels at all ages examined or in response to novelty stress.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In the present study, we have investigated early and long-term effects of a Dex treatment regimen given during the first week of life in the infant rat. Unlike previous studies that have investigated the effects of neonatal exposure to glucocorticoids in rats (4, 22, 49), we administered tapering doses of Dex between PD 3 and 6 in an attempt to mimic a prolonged 42-day treatment regimen commonly used in the neonatal intensive care setting. The timing of drug administration is critical in our rat model, and it corresponds to the third trimester of human pregnancy, a period of growth and development that renders a brain highly vulnerable to insult in the human neonate (15). We found that Dex treatment induced significant decreases in length and weight gain in the rat pup that persisted for up to 2 wk after treatment. Dex-treated animals also demonstrated differences in neurological development at PD 7 and 14 but not at PD 20 upon weaning. Interestingly, although absolute brain weights were found to be significantly less in Dex-treated animals compared with Han controls on PD 8, 23, and 35, this relationship disappears when corrected for differences in somatic weight. Beyond these somatic and CNS observations, we found that in early adolescence, Dex-treated animals demonstrate altered behavioral responses to a novel environment. A blunted corticosterone response to novelty stress was also observed in adolescent animals exposed to Dex during the early neonatal period. These findings point to the long-lasting effects of Dex administration during a time of peak brain development, even when a tapering dose regimen is implemented to minimize adverse effects from glucocorticoid exposure.
Somatic growth measurements.
Our study suggests that a regimen of decreasing Dex exposure early in
life has a lasting impact on somatic growth. One possible explanation
for the decreased somatic growth observed in the Dex-treated pups is
inadequate nutritional intake during the postnatal period. Although we
cannot exclude this possibility, we presume that the somatic and brain
weight deficits observed in Dex-treated animals are primarily due to
direct Dex effects on catabolism and tissue accretion. Other
investigators have clearly shown that the dam spends more time giving
nutrition, stimulation, and warmth to a litter that is perceived to
have poor health (29, 53). Dams readily recognize
disparities in their young. Some factors that appear to heighten
maternal sensitivity to their young are decreased weight, reduction in
motor movement, and enhanced vocalizations (12, 43). In
our study, Dex-treated animals were easily identified visually, and
abnormal results found in the neurological examination are within the
repertoire of pup behaviors that would provoke nurturing from the dam
(43). In support of Dex-induced catabolic effects on
weight gain, previous reports have demonstrated that Dex alters
metabolic pathways. In a setting of sufficient caloric intake, Dex has
been shown to increase protein catabolism and prevent adequate growth
(8, 27, 42, 52). For example, in a piglet model, Weiler
and colleagues (52), using an oral dose of Dex at 0.5 mg · kg
1 · day1 for 15 consecutive days, demonstrated that Dex rapidly induces protein
catabolism beyond the capacity for an anabolic state, resulting in an
elevation of blood urea nitrogen and a reduction of circulating total
protein. These findings correlated with functional measures of reduced
growth and lean body mass (52). Leucine kinetic studies in
Dex-treated human infants (46) and piglets (24) further support this argument. These studies have
established that, rather than an inadequate nutritional intake that
would preclude anabolism during and after Dex exposure, the major
contributing factor to the failure to thrive observed in infants given
Dex is the elevation in protein catabolism. In energy expenditure studies of human infants with chronic lung disease, reduced growth in
Dex-treated infants could not be explained by increased energy expenditure or decreased caloric intake (21, 27, 42). It should be noted that in the present study, a significant weight difference is observed between Dex-treated and control animals on
PD 4, only 24 h after the first Dex dose. This
observation lends support to the notion that decreased nutritional
intake, particularly fluid and free water, may play a more acute role in weight disparity immediately after Dex exposure. However, given the
body of evidence supporting the scenario of increased catabolism and
decreased accretion of tissue, nutritional intake seems unlikely to be
the sole explanation for decreased somatic growth (27).
Neurological development. Our data also reveal that Dex-treated animals experience transient variations in neurological development compared with control animals. On PD 7, Dex-treated animals exhibited mild differences in the gross neurological exam, which included immature posture at rest and delayed forelimb placing and postural extension reflexes. Differences in neurodevelopment were also observed on PD 14, with an abnormal response to vibrissa placing and a persistence of the forelimb and hindlimb grasp reflexes. As a functional unit, vibrissa maintain an extensive representation in the sensory motor cortex, with considerable modulatory input from cerebellar and vestibular systems (51, 54). It is important to note that gross neurological deficits observed early on in our Dex-treated pups were no longer evident by PD 20, suggesting that the pathways relevant to organization of reflexes and behavior assessed on our exam reached acceptable criteria of normality by the time of weaning.
Primitive reflexes appear and disappear in defined sequences during specific periods of development. Absence of an expected primitive reflex, or persistence beyond an expected time of extinction, typically indicates severe dysfunction within the CNS (32). Little data exist pertaining to specific neurological processes that might impinge on the organization of these reflexes in the rodent. However, it is known that an "immature posture" reflex consists of a predominance of flexion that is characteristic in the first 2 days of life. Postural flexion inherently inhibits gross motor actions, such as crawling and rooting activities. Thus postural flexion beyond 3 days of life signifies generalized CNS dysfunction, which may or may not be permanent (20). In contrast, strong stereotyped reflexes and limb placement reactions are expected to be evident after PD 3, persisting to PD 9. These reflexes provide the organism with important information concerning body orientation. Delays in development of such complicated behavioral reflexes in Dex-treated pups would indicate perturbations or alterations in multiple integrated circuitry, including contributions from sensory regions, major developing motor regions, and the cerebellum (20). A growing body of evidence is emerging to support the notion that early neurodevelopmental delays observed in animals treated with a prolonged course of Dex during early neonatal life may be due primarily to an inhibition of the normal myelination processes, including in vitro and in vivo animal studies and evaluations of very low birth weight human infants treated with Dex (18, 30, 35, 41, 45). Morphological evaluations to ascertain changes in myelination or other gross neuroanatomic parameters were not performed in the present study, and it remains unclear at present whether permanent CNS changes have occurred that are not evident on our neurological exam.Brain weights. Dex treatment correlated with low brain weights in our young animals, persisting into early adolescence (PD 35). Interestingly, when corrected for variations in somatic weight, brain weights did not differ between groups. Although such a discrepancy provides an argument that catabolism and nutrition play a major role in brain size reduction, the fact that Dex-treated animals have smaller brains than Veh and Han animals still remains. Ferguson and Holson (18), in fact, report reduced brain weight in 28-day-old animals treated with Dex on PD 7. Similar findings have also been reported in human clinical studies (30). Murphy and colleagues (30) have recently reported a 30% reduction in cerebral tissue volume on neuroimaging in premature infants treated with Dex compared with untreated, age-matched controls.
From a mechanisms perspective, decreased brain weight in Dex-treated animals is not unexpected in light of previous studies of prenatal Dex exposure, where neuronal maturation, replication, differentiation, programmed cell death, and organization of synaptic connections were clearly affected in brain regions that are at the peak of neuronal mitosis (31, 36, 55). Although the formation of new neuronal cells in the brain is limited, other brain cell types are actively dividing and differentiating postnatally at all levels of the CNS in many species, including primates (5). During the time frame of postnatal myelination and differentiation, Dex may be exerting damaging effects on the developing CNS, with reduction in brain weight as one outcome. Interestingly, in a randomized study of premature infants treated with Dex, an increased incidence of cerebral palsy was observed at 1 yr of age (37), an outcome that has been attributed primarily to white matter damage (32). Although in our study low brain weights also corresponded with somatic weight loss, one cannot rule out the possibility that Dex treatment interferes with neurogenesis within specific vulnerable brain structures.Adrenocortical response to novelty stress. Our evaluation of the LHPA axis revealed that basal corticosterone levels were suppressed by Dex, but only in the PD 8 Dex-treated animals. This finding is of interest given that the developing rat already exhibits a reduced basal corticosteroid level and a minimal corticosteroid response to stressful stimuli between PD 4 and 14 [stress hyporesponsive period (SHRP)]. During the SHRP, under circumstances of stress, the ACTH and corticosterone responses are limited in magnitude and sustained, indicating both an immature activation of the stress axis and an immature termination of the stress response (44). The termination of the stress response is mediated through corticoid receptors in the developing CNS [type I or mineralocorticoid receptor (MR) and type II or glucocorticoid receptor (GR); Refs. 38-40]. The abundance and pattern of distribution of MR and GR mRNA are individually distinct during development in the hippocampus, with highest signal intensity found on PD 10. Beyond this age, as GR mRNA expression diminishes, MR mRNA expression continues to evolve, acquiring its adult-like distribution after PD 28 (48). Thus, during the early SHRP and up to ~10 days of age, the hippocampus is acquiring the greatest number of GRs that would become important for the reactive negative feedback action of the LHPA axis, as has been demonstrated by van Oers and colleagues (47). Because Dex has a prolonged half-life and higher affinity for GR compared with corticosterone, one can conclude that the low basal corticosterone levels observed in our Dex-treated animals on PD 8 are a result of prolonged pituitary and brain GR occupation that consequentially enhances negative feedback at this age.
In the present study, we observed a blunted corticosterone response to novelty stress, with a prompt return to basal levels in the Dex-treated animals at PD 33. This is in agreement with Felszeghy and colleagues (16) who reported a suppressed elevation of both ACTH and corticosterone concentrations in response to restraint stress in adult rats that were exposed to a fixed dose of Dex on PD 1, 3, and 5. Our findings also suggest that there is an immediate and long-lasting effect of Dex on the developing LHPA axis, even though Dex was administered in tapering doses. Additionally, Dex-treated animals were less active in both dark and light environments compared with the control groups on PD 33, suggesting a possible anxious state in these young animals while displaying a blunted corticosterone response. In conclusion, early exposure to glucocorticoids has long-lasting effects on neurodevelopment and neuroendocrine function even when drug doses are deliberately reduced to decrease adverse events. In particular, effects on stress responsiveness and behavior are dissociated and not evident until adolescence. These findings raise concerns about maladaptive behavioral strategies that may be subtle and not recognizable until later in development. Such effects may have important implications on learning, mood, and, ultimately, quality of life in an organism.Perspectives
Nationwide, neonatal intensive units provide medical care to infants born as early as 24 wk gestation. Dex treatment is readily used in these newborns with the purpose of reducing time of hospitalization. Our study mimics the equivalent of a 42-day Dex taper commonly used for this purpose. Our findings, while underscoring the somatic effects of steroids that have been reported in other studies, also contribute new information regarding long-term effects of Dex on stress response and behavior later in life. In early adolescence, Dex-treated animals are less active in both dark and light environments compared with controls, a behavioral response suggestive of an anxious state in these animals. Despite this state of increased anxiety, the adrenocortical response to stress is blunted, creating a dissociation between the behavioral and physiological responses that may be viewed as maladaptive. Given that adequate glucocorticoid activation is vital for learning [memory acquisition is dependent on optimal exposure to elevated glucocorticoid levels (28)], the results of this present study should raise concern with regards to the neurodevelopmental repercussions a prolonged steroid course may have on the premature human infant. The exposed infant's ability to mobilize energy resources and to promote anabolic processes during the period of Dex exposure is an acute matter. Perhaps of greater concern are the long-term consequences on behavior, particularly on anxiety and attention, which may interfere with later learning and adjustment in the early school years. Our findings point to the need to study mechanisms of cognitive and behavioral effects of treating premature infants with Dex during their early postnatal life.| |
ACKNOWLEDGEMENTS |
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We thank S. Burke, L. Bain, and J. Stewart for superb technical assistance.
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FOOTNOTES |
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This work was supported by a Robert Wood Johnson Foundation Fellowship to C. R. Neal, Jr. (RWJ-030811), a National Institute of Child Health and Human Development (NICHD) Junior Investigator Award to C. R. Neal, Jr. (P30-HD28820), a National Institute of Mental Health Program Project to S. J. Watson, Jr. and D. M. Vásquez (PO1-MH42251), and an NICHD Award to D. M. Vásquez (HD/DK-37431).
Address for reprint requests and other correspondence: C. R. Neal, Jr., Mental Health Research Institute, 205 Zina Pitcher Place, Ann Arbor, MI 48109-0720 (E-mail: crnj{at}umich.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 18 April 2001; accepted in final form 6 September 2001.
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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