BIOLOGY OF THE NEONATE, Volume 77, Number 2: Pages 69-82,
February 2000.

Can Adverse Neonatal Experiences
Alter Brain Development and Subsequent

K. J. S. Anand     Frank M. Scalzo

Department of Pediatrics, University of Arkansas for Medical Sciences, and Pain Neurobiology Laboratory,
Arkansas Children's Hospital Research Institute, Little Rock, Ark., USA.

Pain · N-Methyl-D-aspartate · Excitotoxicity · Apoptosis

Self-destructive behavior in current society promotes a search for psychobiological factors underlying this epidemic. Perinatal brain plasticity increases the vulnerability to early adverse experiences, thus leading to abnormal development and behavior. Although several epidemiological investigations have correlated perinatal and neonatal complications with abnormal adult behavior, our understanding of the underlying mechanisms remains rudimentary. Models of early experience, such as repetitive pain, sepsis, or maternal separation in rodents and other species have noted multiple alterations in the adult brain, correlated with specific behavioral phenotypes depending on the timing and nature of the insult. The mechanisms mediating such changes in the neonatal brain have remained largely unexplored. We propose that lack of N-methyl-D-aspartate (NMDA) receptor activity from maternal separation and sensory isolation leads to increased apoptosis in multiple areas of the immature brain. On the other hand, exposure to repetitive pain may cause excessive NMDA/excitatory amino acid activation resulting in excitotoxic damage to developing neurons. These changes promote two distinct behavioral phenotypes characterized by increased anxiety, altered pain sensitivity, stress disorders, hyperactivity/attention deficit disorder, leading to impaired social skills and patterns of self-destructive behavior. The clinical important of these mechanisms lies in the prevention of early insults, effective treatment of neonatal pain and stress, and perhaps the discovery of novel therapeutic approaches that limit neuronal excitotoxicity or apoptosis. Copyright 2000 S. Karger AG, Basel

[CIRP note: Apoptosis is defined by Stedman's Medical Dictionary (1995) as "simple deletion of scattered cells by fragmentation into membrane bound particles which are phagocytosed by other cells; believed to be due to programmed cell death."]

Aberrant behavior, mental illness, and drug abuse have reached epidemic proportions amongst adolescents and adults today. The high prevalence of such psychopathologies imposes a signficant burden of direct and indirect health care costs in developed countries [1, 2]. Self-destructive behaviors (e.g., suicide, drug abuse) are also associated with social disruption and the propagation of sexually transmitted diseases, leading to extensive societal and emotional consequences [3].

While the causes of such behavior are no doubt complex, adverse experiences around birth (many of which are preventable) may change the course of brain development and predispose individuals to abnormal behavior. Indeed several investigations have correlated perinatal and neonatal complications with these adult psychopathologies Abnormal conditions around birth have been associated with behavioral and emotional problems during childhood [4-8], major psychoses such as schizophrenia [9], anxiety or depression, suicides in adolescence [13] and adulthood [14, 15], altered responses to pain [16-18], or intractable pain states [19].

Much progress has been made in understanding the neurobiological basis for altered adult behavior resulting from experimental manipulations in the neonatal or perinatal period [20, 21]. These elegant studies have mainly focused on neuroanatomical, hormonal, and neurochemical alterations in the adult brain casued by models of early experience, such as repetitive pain, sepsis, or maternal separation. However, changes occurring in the neonatal brain as a result of these experimental manipulations have been largely unexplored. As a result, little is known about the pathogenesis or underlying mechanisms that lead to the neurobiological or behavioral changes noted in adult animals. We propose novel hypotheses that may help to investigate the mechanisms leading to long-term changes in adult rat behavior, resulting from modification in brain structure and function during the neonatal period.

Long-Term Behavioral Sequelae Related to Perinatal Complications

Based on a cross-sectional case-control study comparing 412 suicide victims with 2,901 matched controls, Jacobson et al. [14] related adult self-destructive behavior with the traumatic experiences around the birth of these individuals. They suggested that imprinting at birth may predispose individuals to certain patterns of behavior that remain masked throughout most of adult life, but may be triggered during conditions of extreme stress [14]. In subsequent studies, these authors controlled for genetic, socioeconomic and demographic factors by using biological siblings as controls. For suicides committed by violent means (e.g., by using a firearm, jumping from a height or in front of a train, hanging and strangulation), the significant risk factors were those perinatal events that were likely to cause pain in the newborn [15]. A composite score derived from factors such as forceps or vacuum delivery, presentation other than vertex, resuscitation and other neonatal complications was significantly correlated with adult suicide in male infants. [15]. Multiple risk factors markedly increased the likelihood of the male off-spring subsequently committing violent suicide (relative risk [RR] confidence intervals [CI] 1.8-13.0). Maternal opioid therapy during delivery reduced the risk for subsequent suicides (RR 0.26, 95% CI 0.09-0.69), perhaps by making the infants less sensitive to the painful interventions at birth [15]. Another study [13] comparing 52 adolescents who committed suicide and 104 matched controls found that lack of antenatal care, chronic maternal disease during pregnancy, and neonatal respiratory compromise increased the risk for adolescent suicide (RR 3.42, 95% CI 1.87-5.30).

Interestingly, sedatives given in the absence of painful interventions were noted to increase the risk for subsequent drug abuse. Administration of multiple doses of opiates, barbiturates and nitrous oxide to mothers during delivery were found to increase the occurrence of subsequent opiate (RR 4.7, 95% CI 1.8-12.0, p = 0.002) or amphetamine (RR 5.6, 95% CI 1.6-16.9, p = 0.005) addiction in the offspring as compared to when no drug was given [22, 23]. Obstetric factors had greater influence on future drug addiction than socioeconomic factors [24,25] Neonatal hypoxia was not a significant risk factor in any of these studies [13-15, 22-25] suggesting that such long-term effects were not associated with extensive brain damage. Conversely, maternal smoking during pregnancy showed a dose-response relationship with persistent criminal behavior in male offspring, after controlling for demographic, parental, and perinatal risk factors [26].

Full-term neonates who experienced stressful deliveries also showed increased behavioral responses to heel lancing and increased cortisol responses to vaccination at 4-6 months age [27]. Gunnar et al. [28, 29] reported that infants born following obstetric complications showed altered responses to handling and an elevated adrenocortical response to a second heelstick performed 24 h later. Together these data suggest that prenatal or perinatal stress may enhance subsequent responses to stressful or painful events, and perhaps alter adult behavior.

Long-Term Neurobehavioral Effects Related to Neonatal Interventions

Early clinical interventions in human neonates have multiple long-term effects [30,31] that can be briefly summarized here. Ex-premature infants often have more educational, behavioral and emotional difficulties during school age [32, 33] and adolescence [7,8,34] as compared to their ex-full term peers. There is not evidence relating these long-term outcomes to neonatal pain or stress, although such an association may be supported by preliminary findings from the clinical studies reviewed below.

Figure one
Fig. 1. Schematic diagram of the effects of neonatal brain and maternal separation on brain plasticity in the neonatal and long-term effects on subsequent brain development and behavior. We propose that maternal separation and sensory isolation in the neonatal period may lead to decreased NMDA activity and increased apoptosis in multiple areas of the immature brain. These early changes will predispose individuals to develop a behavioral phenotype associated with anxiety and stress disorders during adult life. On the other hand, exposure to repetitive pain may cause execesive NMDA/EAA activation resulting in excitotoxic damage to developing neurons. These changes may promote a behavioral phenotype characterized by decreased pain sensitivity and hyperactivity during adult life. These two distinct behavioral phenotypes will resulted in impaired social/cognitive skills and specific patterns of self-destructive behavior.

Multiple invasive procedures in premature infants cause marked fluctuations in intracranial pressure leading to early intraventricular hemorrhage or periventricular leukomalacia [30]. In preterm infants of 24-32 weeks gestation, the incidence of poor neurological outcomes (severe intraventricular hemorrhage, PVL, or neonatal death) were reduced by continuous infusions of low-dose morphine in a randomized clinical trial [35]. Nursing interventions that provide comfort/analgesia were correlated with a decreased incidence of severe IVH and improved neurological and cognitive function during later infancy [36]. Preterm neonates exposed to care in a neonatal intensive care unit for 4 weeks had increased physiological responses and diminished behavioral responses to heel lancing as compared to age-matched controls, and this pattern of responses was most strongly predicted by the number of painful procedures that the former group had received [37].

Table 1. Mechanisms supporting increased pain sensitivity in neonates.
Table one
     Reproduced with permission from Porter et al.[31].

Grunau et al. [38] reported that 18-month-old ex-preterm neonates were less sensitive to pain as compared with ex-full term infants. The greater the number of procedures they had experienced as neonates, the less responsive they were to pain during infancy [38]. Altered behavior continued at older ages (4-5 years), with increased somatization in the ex-preterm infants [16], and greater affective responses to depicted medical pain at the age of 8-10 years [17]. Thus exposure to painful stimuli may alter the responses to subsequent stimuli, implying that the clinical benefits of optimal pain management may persist well beyond the duration of therapeutic effects.

Little is known about the effects of full-term pain in neonates. Circumcision seemed to disrupt their post-natal adaptation [39] and treatment of circumcision pain with a topical anesthetic decreased their responses to vaccination pain at 4-6 months of age [18]. From other experimental studies, it is now evident that very young infants can readily learn tasks and retain their responses over a period of several days [40]. Together, these studies provide evidence for classical associative learning in both newborn animals and humans, and promote the possibility that memories for pain may be recorded biologically [41], but are not accessible to conscious recall [40]

Some aspects of Normal Neonatal Development

By themselves, the clinical correlations reviewed above are not sufficient to postulate the long-term effects of early experience. Corroborative data from animal experiments and mechanistic explanations of how abnormal sensory inputs in the neonatal period may lead to long-term changes in brain structure and behavior are necessary to substantiate these clinical observations. The following sections describe the impact of abnormal simulation in the newborn period within the context of maternal-infant interactions from human and animal experiments. We propose mechanistic hypotheses based on overstimulation or understimulation of newborns that may provide a plausible rationale for some of the clinical observations noted above.

Figure 1 outlines some normal processes that regulate early brain development and abnormal sensory experiences that can alter the cellular mechanisms, the organization of stress-responsive systems, and subsequent behavior. The mechanisms for these phenomena can only be examined within the framework of what now constitutes normal stimulation for the newborn infant (associated with factors such as maternal-infant interaction [42], neonatal painful stimulation [43], brain growth and plasticity).

Maternal-Infant Interaction
        Studies of newborn behavior have demonstrated a dynamic interactive process of the neonatal with its mother and environment [44-46]. In this setting, both maternal and infants behaviors play pivotal roles in producing mother-infant bonding and providing optimal stimulation for development of the neonate's brain. It was proposed that the mother-infant relationship represents the prototype for relationship with the self and all other social relationships [47], thus highlighting its importance in subsequent behavior.

Maternal-infant interactions were examined by Bowlby [48-50], who then proposed the general concepts of attachment theory. He described the response to maternal separation in two phases, a 'protest' phase representing highly activated attachment behavior, and a 'despair' phase to prepare the infant for passive survival through a combination of energy conservation and withdrawal from danger [48]. Hofer [42] discovered the maternal regulators of infant physiology and behavior, and classified them into three categories: (a) thermal-metabolic, (b) nutrient-interoceptive, and (c) behavioral-sensorimotor. These regulated the infant's activity level, sucking behavior, oxygen consumption, sleep-wake states, circadian rhythms, as well as hormonal, cardiovascular, immune and neuroendocrine responses. From this perspective, infantile responses to maternal separation were viewed as resulting from a single mechanisms, that is, a release from maternal regulation. [42].

Kuhn and Schanberg [51] noted that sensorimotor stimulation of the infant rat altered the regulation of growth hormone secretion, and Field [52] reported the beneficial effects of tactile-kinesthetic stimulation on weight gain and development of preterm neonates. Recently these principles have helped to formulate 'kangaroo care' (placement of preterm neonates between their mother's breasts), which may provide additional physiological and neurodevelopmental benefits in critically ill neonates [53, 54]. Several lines of evidence underscore the importance of maternal care-giving practices on neonatal physiology and brain development, with similar results reported in a variety of species [42]. The neonatal brain appears to be well equipped for the transmission and integration of all sensory stimuli (auditory, visual, and tactile) required for the developmental tasks in this time period.

The long-term effects of physical handling or other enrichment paradigms in neonatal animals further substantiate the importance of early childhood experiences. For example, Pieretti et al. [55] found that newborn mice removed from the dam for 10 min each day developed significantly higher latencies for the tail flick and hot plate tests at P25 and P45, and these effects were inhibited by naloxone pretreatment. The vast literature on the long-term effects of postnatal handling [21, 56-58] suggests highly specific effects on adult behavior and cognitive ability.

Behavioral and Cognitive Activity of Human Newborns
        Within hours after birth, human infants show significantly increased preferences for their mothers voice [59], smell [60], and facial features [61] over those of a stranger. Term and preterm neonates actively regulate their behavioral states to maintain an optimum level of stimulation, and unstressed infants can smoothly transition from one behavioral state to another [62, 63]. Even very premature neonates actively perceive, learn, and organize information [64], while constantly striving to control their sensory input [62, 65]. Preterm infants will mount a coordinated response to a painful stimulus [66, 67] associated with changes in their arousal, behavior, and physiology [68, 69]. Als et al. [70] have noted that preterm infants actively approach and favor experiences that are developmentally supportive and actively avoid experiences that are developmentally disruptive. These behaviors support conservation of energy, sleep-wake cycles and the achievement of developmental milestones. Improved clinical and neuromaturational outcomes have resulted from developmentally supportive nursing care [36, 71] and 'kangaroo care' in preterm infants [54].

Increased Sensitivity to Pain in Neonates
       The traditional view that neonates were relatively insensitive to pain was refuted more than a decade ago. [66, 72]. A comparative study of human infants and rat pups reported that withdrawal reflex thresholds increased during gestation and with postnatal age, suggesting lower pain thresholds at earlier developmental stages [73]. Multiple other studies have demonstrated that the intensity and duration of pain responses were developmentally regulated [74-77]. Teng and Abbott [75] found that thresholds for formalin-induced pain increased 2.4-fold from 3 to 15 days of age, and 11-fold from the newborn to the adult rat. Thermal stimulation using withdrawal latencies to a hot plate [76] or tail flick [77] also reported lower pain thresholds in the younger rats. Mechanisms underlying increased pain sensitivity in neonatal rat pups are summarized in table 1 and described elsewhere [78-81]. The 2 weeks after birth in new born rat pups and the last trimester of human gestation represent critical windows for the organization of spinal cord mechanisms [78, 79]. During these critical periods, the supraspinal transmission of pain impulses at is facilitated, whereas modification of these impulses is rudimentary. [82-84].

For many years, cortical activity was not thought to be necessary for pain perception [85]. Studies using positron emission tomography challenged these concepts by showing activation of the anterior cingulate cortex, primary somatosensory cortex, and the prefrontal cortex [86, 87]. Multiple lines of structural and functional evidence now support the role of cortical mechanisms for pain perception in adult humans and animals [88]. Neonatal positron emission tomography scans also indentified somatosensory areas as the most active sites in the developing brain [89]. By 24-26 weeks of gestation, putative nociceptive fibers from the ventroposterior thalamus have fully penetrated the primary somatosensory cortex, providing the final anatomical link for the developing pain/tactile system [90]. Cellular organization of the somatosensory cortex and other supraspinal areas during early development is largely determined by the initiation of incoming afferent activity and activity-dependent changes in gene expression.

Mechanisms of Neonatal Brain Plasticity
        Parallel experiments in the somatosensor and visual cortex suggest that activity-dependent effects on gene expression underlie the establishment of cortical maps during development [91]. Current data suggest complex interactions between early neonatal experience and the gene products controlling cellular and neurotransmitter development, which may lead to apoptosis in development in developing cortical neurons. For example, insulin-like growth factors and their receptors were most abundant in the sensory cortex, hippocampus, and cerebellum [92]. Trophic factors such as IGF-1 promote neuronal survival and differentiation, whereas other trophic factors (such as BDNF, NT4/5, NT-3, via TrkB receptors) chemotactically stimulated the migration of embryonic neurons to their cortical targets [93].

The neonatal period is characterized by peak rates of brain growth [94, 95] and cortical neurons are endowed with an exhuberant production of synaptic vesicles [96]. During the processes of migration and differentiation, large numbers of neurons undergo apoptosis or programmed cell death from diverse areas of the developing cortex. Using semiquantitative methods, Rabinowitz et al. [97] calculated that the number of neurons in the human cortex reaches a maximumat 28 weeks of gestation, then then declines by about 70% to achieve a stable number of neurons at birth. The lack of afferent inputs produced by the blockade of N-methyl-D-aspartate (NMDA) receptors in developing neurons markedly accentuated the occurrence of apoptosis in the neonatal rat brain with the maximun effects occurring in 7-day-old (P7) rates [98]. Thus NMDA activity in the neonatal rat brain may be required for cell survival and the factors that regulate apoptosis in the neonatal brain would play important roles in the final development of the somatosensory cortex.

The density of NMDA receptors increases postnatally reaching peak levels of expression in the neonatal brain [99, 100]. Whole-cell patch-clamp analysis showed NMDA-induced Ca2+ currents developing at P7 whereas AMPA/KA-induced currents were larger and developed earlier [101]. Depending on the cellular context, Ca2+ influx through voltage-sensitive Ca2+ channels increased cell survival, whereas Ca2+ influx via the MNDA receptors in postnatal neurons mediated excitotoxic cell death [102-104]. The peak susceptibility to MNDA-mediated excitotoxicity occurred between P7 and P15, from accentuation of its metabolic effects [105, 106] or reduction of the voltage-dependent Mg2+ block [107], potentially correlated with the expression of NMDA subunits in immature neurons [108]. Permanent alterations in the structure of NMDA receptors occurred following exposure to neonatal hypoxic stress, resulting in increased susceptibility to NMDA-induced toxicity thereafter [109, 110]. NMDA receptors play a central role in thte activity-dependent changes in dendritic length or spine density, synaptic stability, long-term potentiation (LTP) or depression, and other processes that mediate the heightened plasticicy of the neonatal brain. [82, 90, 111]

NMDA-dependent mechanisms are not only implicated in the spinal transmission of pain but also in the long-term effects of pain (such as hyperalgesia, allodynia, windup, and central sensitization [112]) leading to the pathogenesis of chronic pain states [113, 114]. Accumulating evidence suggests that exposure to repetitive neonatal pain may promote an increased susceptibility to chronic pain states mediated by NMDA-dependent neuroplasticity [19, 115]. Although intense research activity has focused on peripheral and spinal cord mechanisms that support these phenonema [114, 116], we believe that the significant role of NMDA-mediated neuroplasticity in supraspinal areas remains unexplored. The following findings suggest such a role.

Craig and Malenka [41] demonstrated that projections from the ventrobasal thalamus to layer IV neurons in the somatosensory (S1) cortex are organized by NMDA-dependent LTP. Induction of LTP occurred in P3-P7 rats but was absent in P8-P14 rates [41]. Further experiments showed that NMDA-mediated LTP is responsible for the learning and memory associated with classical fear conditioning [117, 118], and that early experiences can modify the capacity for learning in the adult [119]. It is particularly fascinating that the capacity for adult learning can be enhanced [119] or inhibited [120, 121] by neonatal experiences. Thus it is plausible that the underlying templates for subsequent cognitive ability and vulverability to psychiatric disorders may be formed as a result of abnormal neonatal stimulation [9, 14, 15].

Critical Periods of Vulnerability
       Critical periods of vulnerability occur just before and after birth, when the development of the underlying neoronal circuitry is more susceptible to pertubation than at any other time of life. Disruptive experiences at this time may have a greater impact on subsequent neurobiological and behavioral development [42, 47]. There is increasing clinical appreciation for the long-term, often permanent effects of early sensory experiences on the patterns of subsequent behavior [20].

A search for critical periods may be aided by the ontogenetic changes in pain threshold during early development. Falcon et al. [77] found that the nadir for thermal pain thresholds in infant rats occurred at P6 and similar age-related patterns were noted in thresholds for the cutaneous withdrawal reflex [73] and foot-shock responses [122]. When rat pups of different age groups (P4, P7, P14) were tested in different graded stimuli in our laboratory, the number of ultrasonic cries at P7 were significantly increased (ANOVA, F = 57.94, p < 0.0001). Pain behaviors occurred in response to saline injections (0.01%) at all ages tested (P4, P7, P14). Thus, sensitivity to painful stimulation may be maximal in rat pups at 6-9 days of age, a period that corresponds with the neurological maturity of full term infants at birth.

We postulate that a critical window of neonatal brain development occurs around the time of birth in human neonates and at 6-9 days in rat pups. This period also coincides with the peak rates of brain growth [95] and an exuberant production of synaptic sites [96]. At this time, the dorsal horn of the spinal cord and other supraspinal regions show a peak density of NMDA receptors [99, 100], decreased thresholds for NMDA- or kainate-induced seizures [123], an increased magnitude of Ca2+ currents from NMDA activity [107], and a greater susceptibility of brain cells to NMDO induced toxicity [102, 103, 124]. Neonatal brain cells have altered molecular mechanisms for Ca2+ signalling [102] which appear to determine neuronal survival, whereas blockade of NMDA receptors in this period causes widespread apoptosis [98].

Data related to normal brain development in the neonate reviewed above support the assertion that adverse sensory experience during the neonatal period may have widespread and far-ranging effects on subsequent developmental events. In the sections below, we examine the effects of overstimulation or understimulation in neonatal rat pups leading to permanent changes in adult behavior and cognitive ability.

Abnormal stimulation of the Newborn

Numerous animal experiments over the past 30 years have demonstrated the long-term effects of early experience on subsequent behavior [21, 125]. These data suggest that both excessive sensory stimulation and the lack of appropriate stimulation in neonatal rats may lead to permanent changes in adult rat behavior, altered hormonal or immune responses to subsequent stress, associated with altered expression of neurotransmitters, their receptors, and/or other cellular processes associated with repetitive neonatal pain and maternal separation, used repetively as representative models of over-or understimulation.

Longterm effects of pain in neonatal rats
       The long-term effects of acute or repetive pain in neonatal rat pups have remained largely unexplored. Reynolds and Fitzgerald [126] reported that skin wounds placed in newborn rates (at P0, P7) caused a marked hyper-innervation and decreased thresholds in the injured area lasting longer than 3 months after the injury. Nerve sprouting from A and C fibers was markedly increased around skin wounds place on the day of birth (P0) as compared with older ages, and was stimulated by nerve growth factor in older rats and unknown neurotrophic factors in the neonate [127]. Neonatal rats subjected to carrageenan-induced inflammation on P1 developed a persistent expansion (by approximately 30%) in the receptive field of the dorsal horn neurons [128], although the mechanisms mediating these changes remain unknown.

Behavioral changes in rats following exposure to repetitive pain as neonates were first reported from our laboratory [129]. Rat pups subjected to four needle pricks each day from P0 to P7 showed greater preference for alcohol as adults, increased defensive withdrawal and hyper-vigilance behavior. Interestingly, these animals had diminished neuronal activity in the somatosensory cortex following hot-plate exposure as compared to the adult rats receiving tactile stimulation as neonates. These results were correlated with greater exploratory and escape behavior seen in the tactile group, whereas the experimental animal's behavior was more reflective of fear conditioning [129]. Further experiments showed that exposure to prolonged inflammatory pain (repetitive formalin injections in the neonatal period) was associated with decreased body weight, decreased pain sensitivity (hot plate tests), and decreased locomotor activity in the adult rat. Amongst the rats exposed to neonatal pain, adult females had increased alcohol preference and consumption as compared to males, consistent with our previous findings from repetitive acute pain [129]. These preliminary results suggest that neonatal pain leads to an increased expression (and/or coupling) of opiod receptors in adult rats [128], which is increased preference for sucrose [130, 131], decreased pain sensitivy, and the normalization of locomotor activity by concomitant morphine treatment in neonatal rats [132]. When morphine preceded the neonatal formalin injections, it was noted to reverse or ameliorate the long-term effects from exposure to neonatal inflammatory pain. Thus, the persistent behavioral changes from repetitive neonatal pain depend on the type of stimulation (needle prick vs. inflammation), age at the time of stimulation, concurrent treatment with opioids, and perhaps other unexplored factors. The neurobiological mechanisms underlying these changes are the focus of intense reseach activity.

Longterm Effects of Maternal Separation
       Prolonged maternal separation leads to the development of exaggerated hormonal responses and altered neurotransmitter release, which is qualitatively different from the effects of postnatal handling [133, 134]. Persistent effects of maternal separation occurred only if the daily separation paradigm exceeded 180 min [56], leading to elevated baseline and stress-induced plasma corticosterone levels, and reduced efficiency of glucocorticoid negative feedback in adult rats [56, 135]. Repetive maternal separation in neonatal rats was associated with accentuated stress responses, increased vulnerabity to stress and anxiety disorders, neophobia, early development of diabetes, hypertension, cognitive defects, and shorter life spans in adult rats [136, 137]. Adult rats isolated as neonates for 1 h/day from P2 to P9 had reduced susceptibility to an amphetamine challenge [138]. Enhanced amphetamine effects were associated with elevated dopamine levels in the ventral stratum, possibly linked to pertubations of the dopamine receptor systems as a result of isolation [139]. Early isolation may contribute to the sensitization of reward mechanisms in adult animals, from changes in the dopaminergic mesolimbocortical pathways [140, 141].

Any links between the animal experimental data reviewed above and recent epidemiological data on the long-term effects of perinatal or neonatal complications are speculative at best. How do adverse neonatal experiences lead to long-term changes in behavior and brain function? For these relationships to be investigated rigorously, the potential mechanisms by which the experimental findings can be applied to explain such preliminary data must be understood. Answers to such questions will not only help to understand the mechanisms by which long-term effects are produced, but will also define the clinical importance of interventions that treat or reduce the impact of abnormal stimulation.

Proposed Cellular Mechanisms

We propose two novel hypotheses that can be tested in the laboratory to explain the link between perinatal sensory experiences and adult behavior: (a) excessive neonatal stimulation resulting from perinatal trauma or other insults causes NMDA-mediated excitotoxicity in multiple areas of the developing brain, and (b) lack of appropriate sensory stimulation in the neonatal period serves to enhance the normal occurrence of developmental apoptosis in the neonatal brain.

Excitotoxicity Hypothesis
        Prolonged pain causes excessive activation of central afferent pain pathways. This activation leads to release of glutamate and other excitatory neurotransmitters from spinal and supraspinal neurons. Binding of glutamate to metabatrophic and NMDA receptors causes removal of the MG2+ block and allows intracellular Ca2+ influx, ultimately leading to the phosphorylation of second messengers and altered gene regulation. NMDA-induced neuroplasticity is known to mediate the wind-up and central sensitization of pain pathways [112-115]. It is evident that these phenomena are accentuated and prolonged in the developing nervous system [19, 101-107, 112-115], although the supraspinal excitotoxic effects of pain-related NMDA activity have not been investigated in immature animals.

Figure two
Fig. 2. Schematic diagram for the mechanisms that mediate pain-induced excitotoxity in the neonatal brain. PKC = Protein kinase C; PKA = Protein kinase A; µ-OR = µ-opioid receptor; Gi/Go = inhibitory G-proteins; IEG = immediate early genes; APD = action potential duration; HSP-70 = heat shock protein-70.

We hypothesize that repetitive neonatal pain causes excessive NMDA activation leading to excitotoxic damage in the brain regions noted above. Figure 2 illustrates the well-known phenomenon of NMDA-mediated excitotoxity [142]. Repetitive pain exacerbates excitotoxic damage, which may extend to adjacent neurons via nitric oxide and products of cell necrosis [112-115]. Structural and functional changes in the NMDA receptor may increase the neonates sensitivity to future excitotoxic insults. Furthermore, NMDA receptor-mediated intracellular changes [101-103] may lead to the development of hyperalgesic states by increasing the efficacy of NMDA receptor-activated Ca2+ channels. We predict that cumulative neuronal damage from repetitive pain during infancy results in abnormal behavior regulation during adolescence and adult life.

Apoptosis Hypothesis
        The current mechanisms of apoptosis and their interaction with NMDA and opioid receptor systems is illustrated in figure 3. Apoptosis or programmed cell death is mediated by the stimulation of multiple cell surface receptors (e.g. death receptors DR1, DR2, DR3, DR4, DR5) or by the internal activation of reactive oxygen species, mitochondrial damage, or Ca2+-calmodulin-dependent mechanisms. The most prominent initiators of apoptosis include the binding of Fas ligand to CD95 (Fas/Apo 1/DR1), release of cytochrome C from mitochondria, or binding of tumor necrosis factor to its receptor (TNFR1), which activate the initator caspase enzymes (e.g., caspase-3, caspase-6, caspase-7) which, in essence, rapidly dismantle the organelles and genetic machinery of the cell. Caspase enzymes are differentially expressed in different tissues, and caspase-3 serves as the main effector enzyme in neurons. In situ hybridization histochemistry revealed a profound developmental regulation of the caspase-3 transcript in rat brain, with relatively high levels of caspase-3 mRNA observed in neurons of the fetal and neonatal brain and low levels of mRNA in neurons of the adult brain [143]. Spatial and temporal distributions of apoptotic cells in developing rodents showed an inverse correlation with Bcl-2 oncoprotein expression in visual, sensory, and motor cortices but not in the frontal cortex or hippocampus [144]. Expression of the protein kinase C substrate, neurograni, also seems regulated by afferent activity in areas associated with the pain/tactile system [145].

Figure three
Fig. 3. Schematic diagram for the apoptopic mechanisms mediated via lack of stimulation of NMDA receptors in the neonatal brain. See text for explanation. FasL = Fas ligand; FADD = Fas associated death domain; TRADD = TNF receptor-associated death domain; TNFα = tumor necrosis factor-α TNFR1 = TNF receptor-1; JNK = Jun NH2-kinase; TRAF2 = TNFR-associated death domain; RIP = receptor-interacting protein; NFkB = nuclear factor k-B; NIK = NFk B-inducing kinase; IKK = inhibitor of k-B; + = stimulates; - = inhibits.

Ikonomidou et al. [98] reported recently that NMDA receptor blockade results in widespread activation of neuronal apoptosis in rat pups, and the peak effect occurred on P7. These data are consistent with previous reports suggesting that NMDA activation promotes the survival of developing neurons [104]. These recent findings led us to hypothesize that the adverse behavioral effects of isolation may be mediated by aberrant regulation of apoptosis in the neonatal brain. Lack of stimulation from the environment will reduce afferent activity and may interfere with the ontogenetic regulation of apoptosis, leading to long-term behavior changes. The anatomical distribution of apoptosis resulting from isolation may be different from that determined by NMDA blockade and has not been investigated.

Clinical significance

The mechanisms of excitotoxicity and apoptosis described above must be explored to define whether abnormal stimulation around birth may predispose individuals to an number of neuropsychiatric and behavioral disorders. If the findings from epidemiological studies that have correlated early experiences and adult self-destructive behavior can be explained by patterns of increased excitotoxicity or apoptosis, novel therapeutic strategies can be developed [146]. The clinical importance of these mechanisms lies in the prevention of early insults, developing effective treatments for neonatal pain and stress, and the discovery of novel therapeutic approaches that limit neuronal excitotoxicity or apoptosis. It would also be important to identify and provide such early intervention for children or adolescents who have been exposed to such perinatal or neonatal conditions. [4-11]. For example, recent data suggest that environmental enrichment during development reduces spontaneous apoptotic cell death and protects against kainate-induced excitotoxicity [147].

The public health importance of abnormal stimulation during the neonatal period cannot be overemphasized. While programs for formulating appropriate health policies and public education campaigns must disseminate this message, it is also important for these effects to be investigated, particularly with a view to developing effective therapeutic strategies for the growing childen and adolescents who were exposed to abnormal conditions during the neonatal period.


Supported by grants from the National Institute of Child Health and Human Development (HD 36484), National Institute on Drug Abuse (DA 08240), and the Arkansas Children's Hospital Foundation.


  1. Booth BM, Zhang M, Rost KM, Clardy JA, Smith LG, Smith GR: Measuring outcomes and costs for major depression. Psychopharmacol Bull 1997;33:653-658. [Abstract]
  2. Garnick DW, Hendricks AM, Comstock C, Horgan C: Do individuals with substance abuse diagnoses incur higher charges than individuals with other chronic conditions? J Subst Abuse Treat 1997;14:457-465. [Abstract]
  3. Cuffel B: Disruptive behavior and the determinants of costs in the public mental health system. Psychiatr Serv 1997;48:1562-1566. [Abstract]
  4. Hack M, Taylor HG, Klein N, Eiben R, Schatschneider C, Mercuri-Minich N: School-age outcomes in children with birth weights under 750 g. N Engl J Med 1994;331:753-803. [Full Text]
  5. Saigal S, Szatmari P, Rosenbaum P, et al: Cognitive abilities and school performance of extremely low birth weight children and matched term control children at age 8 years: A regional study. J Pediatr 1991;118:751. [PubMed]
  6. Whitfield MF, Grunau RVE, Holsti L: Extreme prematurity (<800 g) at school age: Multiple areas of hidden disability. Arch Dis Child 1997;77:F85-F90. [Full Text]
  7. Levy-Shiff R, Einat G, Har-Even D, et al: Emotional and behavioral adjustment in children born prematurely. J Clin Child Psychiatry 1994;23:323-333. [Abstract]
  8. Botting N, Powls A, Cooke RW: Attention deficit hyperactivity disorders and other psychiatric outcomes in very low birthweight children at 12 years. J Child Psychol Psychiatry 1997;38:931-941. [PubMed]
  9. Cannon M, Murray RM: Neonatal origins of schizophrenia. Arch Dis Child 1998;78:1-8. [Full Text]
  10. Mullen PE, Martin JI, Anderson JC, Herbison GP: Childhood sexual abuse and mental health of adult life. Br J Psychiatry 1993;163:721-732.
  11. Goodyear IM: Development psychopathology: The impact of recent life events in anxious and depressed school-age children. J R Soc Med 1994;87:327-329. [PubMed Central]
  12. Anand KJS, Nemeroff CB: Developmental aspects of psychoneuroendocrinology; in Lewis M (ed): Textbook of Child and Adolescent Psychiatry. Baltimore, Williams & Wilkins, 1996, pp 64-86.
  13. Salk L, Lipsitt LP, Sturner WQ, Reilly BM, Levat RH: Relationship of maternal and perinatal conditions to eventual adolescent suicide. Lancet 1985;i:624-627.
  14. Jacobson B, Eklund G, Hamberger L, Linnarsson D, Sedvall G, Valverius M: Perinatal origin of adult self-destructive behavior. Acta Psychiatr Scand 1987;76:364-371.
  15. Jacobson B, Bygdeman M: Obstetric care and proneness of offspring to suicides as adults: Case-control stuedy. BMJ 1998;317:1346-1349. [Full Text]
  16. Grunau RV, Whitfield MF, Petrie JH, Fryer EL: Early pain experience, child and family factors, as precursors of somatization: A prospective study of extremely premature and fullterm children. Pain 1994;56:353-359. [PubMed]
  17. Grunau RVE, Whitfield MF, Petrie J: Children's judgments about pain at age 8-10 years: Do extremely low birthweight (<1000 g) children differ from full birthweight peers. J Child Psychol Psychiatry 1998;39:587-594.
  18. Taddio A, Katz J, Ilersich AL, Koren G: Effect of neonatal circumcision in pain response during subsequent routine vaccination. Lancet 1997;49:599-603
  19. McCormack K, Prather P, Chapleo C: Some new insights into the effects of opioids in phasic and tonic nociceptive tests. Pain 1998;78:79-98. [PubMed]
  20. Winberg J: Do neonatal pain and stress program the brain's response to future stimuli? Acta Paediatr 1998;87:723-725.
  21. Hall FS: Social deprivation of neonatal, adolescent, and adult rats has distinct neurochemical and behavioral consequences. Crit Rev Neurobiol 1998;12:129-162. [PubMed]
  22. Jacobson B, Nyberg K, Gronbladh L, Eklund G, Bygdeman M, Rydberg U: Opiate addiction in adult offspring through possible imprinting after obstetric treatment. BMJ 1990;301:1067-1070. [PubMed Central]
  23. Jacobson B, Nyberg K, Eklund G, Bygdeman M, Rydberg U: Obstetric pain medication and eventual adult amphetamine addiction in offspring. Acta Obstet Gynecol Scand 1988;67:677-682. [Abstract]
  24. Nyberg K, Allebeck P, Eklund G, Jacobson B: Obstetric medication versus residental area as perinatal risk factors for subsequent adult drug addiction in offspring. Paediatr Perinat Epidemiol 1993;7:23-32.
  25. Nyberg K, Allebeck P, Eklund G, Jacobson B: Socio-economic versus obstetric risk factors for drug addiction in offspring. Br J Addict 1992;87:1669-1676. [PubMed]
  26. Brennan PA, Grekin ER, Mednick SA: Maternal smoking during pregnancy and adult male criminal outcomes. Arch Gen Psychiatry 1999;56:215-219. [Abstract]
  27. Ramsay DS, Lewis M: The effects of birth condition on infants' cortisol response to stress. Pediatrics 1995;95:546-549. [Abstract]
  28. Gunnar MR: Reactivity of the hypothalamic-pituitary-adrenocortical system to stressors in normal infants and children. Pediatrics 1992;90:491-497. [Abstract]
  29. Gunnar M, Hertsgaard L, Larson M, Rigatuso J: Cortisol and behavioral responses to repeated stressors in the human newborn. Dev Psychobiol 1991;24:487-506. [Abstract]
  30. Anand KJS: Clinical importance of pain and stress in preterm neonates. Biol Neonate 1998;73:1-9. [Abstract]
  31. Anand KJS, Grunau RVE, Oberlander TF: Developmental character and long-term consequences of pain in infants and children. Child Adolesc Psychiatr Clin N Am 1997;6:703-724.
  32. Hack M, Taylor HG, Klein N, et al: 1994 School-age outcomes in children with birth weights under 750 g. N Engl J Med 1999;331:753. [Abstract]
  33. Whitfield MF, Grunau RVE, Holsti L: Extreme prematurity (<800 g) at school age: Multiple areas of hidden disability. Arch Dis Child 1997;77:F85-F90. [Full Text]
  34. Saigal S, Feeny D, Rosenbaum P, Furlong W, Burrows E, Stoskopf B: Self-perceived health status and health-related quality of life of extremely low-birth-weight infants at adolescence. JAMA 1996;276:453-459.[Abstract]
  35. Anand KJS, McIntosh N, Lagercrantz H, Young TE, Vasa RK, Barton BA: Analgesia and sedation in ventilated preterm neonates: Results from the pilot NOPAIN trial. Arch Pediatr Adolesc Med 1999;153:331-338. [Abstract]
  36. Als H, Lawhon G, Duffy FH, McAnulty GB, Gibes-Grossman R, Blickman JG: Individualized developmental care for the very low-birth-weight preterm infant. Medical and neurofunctional effects. JAMA 1994;272:853-858. [Abstract]
  37. Johnston CC, Stevens BJ: Experience in a neonatal intensive care unit affects pain response. Pediatrics 1996;98:925-930. [Abstract]
  38. Grunau RV, Whitfield MF, Petrie JH: Pain sensitivity and temperament in extremely low-birth-weight premature toddlers and preterm and full-term controls. Pain 1994;58:341-346. [PubMed]
  39. Dixon S, Snyder J, Holve R, et al: Behavioral effects of circumcision with and without anesthesia. J Dev Behav Pediatr 1984;5:246-250. [PubMed]
  40. Anand KJS, Rovnaghi C, Walden M, Churchill J: Consciousness, behavior, and clinical impact of the definition of pain. Pain Forum 1999;8:64-73.
  41. Crair MC, Malenka RC: A critical period for long-term potentiation at thalamocortical synapses. Nature 1995;375:325-328. [Abstract]
  42. Hofer MA: Early relationships as regulators of infant physiology and behavior. Acta Paediatr Suppl 1994;397:9-18. [PubMed]
  43. Johnston CC, Collinge JM, Henderson SJ, Anand KJS: A cross-sectional survey of pain and pharmacological analgesia in Canadian neonatal intensive care units. Clin J Pain 1997;13:308-312. [Abstract]
  44. Simion F, Butterworth G: The Development of Sensory, Motor and Cognitive Capacities in Early Infancy: From Perception to Cognition. Hove, Psychology Press/Erlbaum (UK)/Taylor & Francis, 1998, vol 16, p 352.
  45. Oates J, Sheldon S: Cognitive Development in Infancy. Hove, Erlbaum, 1987, vol 9, p 305.
  46. Beebe B, Lachmann F, Jaffe J: Mother-infant interaction structures and presymbolic self- and object representations. Psychoanal Dialog 1997;7:133-182. [Abstract]
  47. Rosenblum LA, Andrews MW: Influences of environmental demand on maternal behavior and infant development. Acta Paediatr Suppl 1994;397:57-63. [Abstract]
  48. Bowlby J: Attachment and Loss. Vol 1: Attachment. New York, Basic Books, 1969.
  49. Bowlby J: Attachment and Loss. Vol 2: Separation: Anxiety and Anger. New York, Basic Books, 1973.
  50. Bowlby J: Attachment and Loss. Vol 3: Loss. New York, Basic Books, 1980.
  51. Kuhn CM, Schanberg SM: Stimulation in infancy and brain development; in Carroll BJ, Barrett JE (eds): Psychopathology and the Brain. New York, Raven Press, 1991, pp 97-111.
  52. Field T: Alleviating stress in newborn infants in the intensive care unit; in Lester BM, Tronick E (eds): Clinics in Perinatology. Stimulation and the Preterm Infant. Philadelphia, Saunders, 1990, vol 17, part 5, pp 1-9.
  53. Mooncey S, Giannakoulopoulos X, Glover V, Acolet D, Modi N: The effect of mother-infant skin-to-skin contact on plasma cortisol and endorphin concentrations in preterm newborns. Infant Behav Dev 1997;20:553-557.
  54. Ludington-Hoe SM, Swinth JY: Developmental aspects of kangaroo care. J Obstet Gynecol Neonatol Nurs 1996;25:691-703. [PubMed]
  55. Pieretti S, d'Amore A, Loizzo A: Long-term changes induced by developmental handling on pain threshold: Effects of morphine and naloxone. Behav Neurosci 1991;105:215-218. [PubMed]
  56. Plotsky PM, Meaney MJ: Early postnatal experience alters hypothalamic corticotropin-releasing factor (CRF) mRNA, median eminence CRF content and stress-induced release in adult rats. Brain Res Mol Brain Res 1993;18:195-200. [PubMed]
  57. Caldji C, Tannenbaum B, Sharma S, Francis D, Plotsky PM, Meaney MJ: Maternal care during infancy regulates the development of neural systems mediating the expression of fearfulness in the rat. Proc Natl Acad Sci USA 1998;95:5335-5340 [Full Text]
  58. Kempermann G, Kuhn HG, Gage FH: More hippocampal neurons in adult mice living in an enriched environment. Nature 1997;386:493-495. [Abstract]
  59. DeCasper A, Fifer W: Of human bonding: Newborns prefer their mothers' voices. Science 1980;208:1174-1176. [Abstract]
  60. MacFarlane A: Olfaction in the development of social preferences in the human neonate. Ciba Found Symp 1975;33:103-117. [PubMed]
  61. Field T, Woodson R, Greenberg R, Cohen D: Discrimination and imitation of facial expressions by neonates. Science 1982;218:179-181. [Abstract]
  62. Als H, Brazelton TB: A new model of assessing the behavioral organization in preterm and fullterm infants: Two case studies. J Am Acad Child Psychiatry 1981;20:239-263. [PubMed]
  63. Brazelton TB: Behavioral competence of the newborn infant. Semin Perinatol 1979;3:35-44. [PubMed]
  64. Stone L: Introduction to the capabilities of the newborn; in Stone L, Smith H, Murphy L (eds): The Competent Infant. New York, Basic Books, 1973, pp 239-253.
  65. Connolly K, Bruner J: The Growth of Competence. New York, Academic Press, 1973.
  66. Anand KJS, Hickey PR: Pain and its effects in the human neonate and fetus. N Engl J Med 1987;317:1321-1329.
  67. Craig KD, Whitfield MF, Grunau RV, Linton J, Hadjistavropoulos HD: Pain in the preterm neonate: Behavioural and physiological indices. Pain 1993;52:287-299. [PubMed]
  68. Gladman G, Chiswick ML: Skin conductance and arousal in the newborn. Arch Dis Child 1990;65:1063-1066. [Abstract]
  69. Anand KJS, Sippell WG, Aynsley-Green A: Randomised trial of fentanyl anaesthesia in preterm babies undergoing surgery: effects on the stress response. Lancet 1987;i:62-66.
  70. Als H, Lawhon G, Brown E, et al: Individualized behavioral and environmental care for the very low birth weight preterm infant at high risk for bronchopulmonary dysplasia: Neonatal intensive care unit and developmental outcome. Pediatrics 1986;78:1123-1132. [Abstract]
  71. Fleisher BE, VandenBerg K, Constantinou J, et al: Individualized developmental care for very-low-birth-weight premature infants. Clin Pediatr 1995;34:523-529. [Abstract]
  72. Porter FL, Miller RH, Marshall RE: Neonatal pain cries: Effect of circumcision on acoustic features and perceived urgency. Child Dev 1986;57:790-802.
  73. Fitzgerald M, Shaw A, MacIntosh N: The postnatal development of the cutaneous flexor reflex: A comparative study in premature infants and newborn rat pups. Dev Med Child Neurol 1988;30:520-526.
  74. Guy ER, Abbott FV: The behavioral response to formalin in preweanling rats. Pain 1992;51:81-90. [PubMed]
  75. Teng CJ, Abbott FV: The formalin test: A dose-response analysis at three development stages. Pain 1998;76:337-347.
  76. Hu D, Hu R, Berde CB: Neurologic evaluation of infant and adult rats before and after sciatic nerve blockade. Anesthesiology 1997;86:957-965. [Abstract]
  77. Falcon M, Guendellman D, Stolberg A, Frenk H, Urca G: Development of thermal nociception in rats. Pain 1996;67:203-208. [PubMed]
  78. Fitzgerald M, Anand KJS: The developmental neuroanatomy and neurophysiology of pain; in Schechter N, Berde C, Yaster M (eds): Pain Management in Infants, Children and Adolescents. Baltimore, Williams & Wilkins, 1993, pp 11-32.
  79. Marti E, Gibson SJ, Polak JM, et al: Ontogeny of peptide and amino-containing neurons in motor, sensory, and autonomic regions of rat and human spinal cord. J Comp Neurol 1987;266:332-359.
  80. Bicknell HR, Beal JA: Axonal and dendritic development of substantia gelatinosa neurons in the lumbosacral spinal cord of the rat. J Comp Neurol 1984;226:508-522. [PubMed]
  81. Pignatelli D, Silva DA, Coimbra A: Postnatal maturation of primary afferent termintions in the substantia gelatinosa of the rat spinal cord. An electron microscope study, brain. Brain Res 1989;491:33-44.
  82. Vicario-Abejon C, Collin C, McKay RD, Segal M: Neurotrophins induce formation of functional excitatory and inhibitory synapses between cultured hippocampal neurons. J Neurosci 1998;18:7256-7271. [Abstract]
  83. Flint AC, Liu X, Kriegstein AR: Nonsynaptic glycine receptor activation during early neocortical development. Neuron 1998;20:43-53. [Full Text]
  84. Ben-Ari Y, Khazipov R, Leinekugel X, Caillard O, Gaiarsa JL: GABAA, NMDA and AMPA receptors: A developmentally regulated 'ménage à trois'. Trends Neurosci 1997;20:523-529.
  85. Kenshalo DR, Willis WD: The role of the cerebral cortex in pain sensation; in Peters A (ed): Cerebral Cortex. New York, Plenum Press, 1991, vol 9, pp 153-212.
  86. Jones AK, Brown WD, Friston KJ, Qi LY, Frackowiak RS: Cortical and subcortical localization of response to pain in man using positron emission tomography. Proc R Soc Lond 1991;244:39-44. [PubMed]
  87. Derbyshire SW, Jones AK: Cerebral responses to a continual tonic pain stimulus measured using positron emission tomography. Pain 1998;76:127-135. [PubMed]
  88. Treede R-D, Kenshalo DR, Gracely RH, Jones AKP: The cortical representation of pain. Pain 1999;79:105-111.
  89. Chugani HT: A critical period of brain development: Studies of cerebral glucose utilization with PET. Prevent Med 1998;27:184-188. [Abstract]
  90. Kostovic I, Rakic P: Developmental history of the transient subplate zone in the visual and somatosensory cortex of the macaque monkey and human brain. J Comp Neurol 1990;297:441-470.
  91. Jones EG: The role of afferent activity in the maintenance of primate neocortical function. J Exp Biol 1990;153:155-176. [Abstract]
  92. Bondy C, Werner H, Roberts CJ Jr, LeRoith D: Cellular pattern of type-I insulin-like growth factor receptor gene expression during maturation of the rat brain: Comparison with insulin-like growth factors I and II. Neuroscience 1992;46:909-923. [PubMed]
  93. Behar TN, Dugich-Djordjevic MM, Li YX, et al: Neurotrophins stimulate chemotaxis of embryonic cortical neurons. Eur J Neurosci 1997;9:2561-2570. [PubMed]
  94. Rakic P: DNA synthesis and cell division in the adult primate brain. Ann NY Acad Sci 1985;457:193-211. [Abstract]
  95. Rakic P: Images in neuroscience. Brain development, VI: Radial migration and cortical evolution. Am J Psychiatry 1998;155:1150-1151. [Abstract]
  96. Rakic P, Bourgeois J-P, Eckenhoff MF, Zecevic N, Goldman-Rakic PS: Concurrent overproduction of synapses in diverse regions of the primate cerebral cortex. Science 1986;232:232-235. [Abstract]
  97. Rabinowicz T, de Courten-Myers GM, Petetot JM, Xi G, de los Reyes E: Human cortex development: Estimates of neuronal numbers indicate major loss late during gestation. J Neuropathol Exp Neurol 1996;55:320-328. [PubMed]
  98. Ikonomidou C, Bosch F, Miksa M, Olney JW, et al: Blockade of NMDA receptor and apoptotic neurodegeneration in the developing brain. Science 1999;283:70-74.
  99. Rao H, Jean A, Kessler JP: Postnatal ontogeny of glutamate receptors in the rat nucleus tractus solitarii and ventrolateral medulla. J Auton Nerv Syst 1997;65:25-32. [Abstract]
  100. Chahal H, D'Souza SW, Barson AJ, Slater P: Modulation by magnesium of N-methyl-D-aspartate receptors in developing human brain. Arch Dis Child Fetal Neonatal Ed 1998;78:F116-F120. [Abstract]
  101. Colwell CS, Cepeda C, Crawford C, Levine MS: Postnatal development of glutamate receptor-mediated responses in the neostriatum. Dev Neurosci 1998;20:154-163. [Abstract]
  102. Ghosh A, Greenberg ME: Calcium signaling in neurons: molecular mechanisms and cellular consequences. Science 1995;268:239-244. [Abstract]
  103. McDonald JW, Johnston MV: Physiological and pathophysiological role of excitatory amino acids during central nervous system development. Brain Res Rev 1990;15:41-70.
  104. Lipton SA, Nakanishi N: Shakespeare in love - With NMDA receptors? Nature Med 1999;5:270-271. [Abstract]
  105. Lipartiti M, Lazzaro A, Zanoni R, Mazzari S, Toffano G, Leon A: Monosialoganglioside GM1 reduces NMDA neurotoxicity in neonatal rat brain. Exp Neurol 1991;113:301-305. [PubMed]
  106. Jacquin T, Gillet B, Fortin G, Pasquier C, Beloeil JC, Champagnat J: Metabolic action of N-methyl-D-aspartate in newborn rat brain ex vivo: 31p magnetic resonance spectroscopy. Brain Res 1989;497:296-304. [PubMed]
  107. Mitani A, Watanabe M, Kataoka K: Functional change of NMDA receptors related to enhancement of susceptibility to neurotoxicity in the developing pontine nucleus. J Neurosci 1998;18:7941-7952. [Abstract]
  108. Kim WT, Kuo MF, Mishra OP, Delivoria-Papadopoulos M: Distribution and expression of the subunits of N-methyl-D-aspartate (NMDA) receptors; NR1, NR2A and NR2B in hypoxic newborn piglet brains. Brain Res 1998;799:49-54. [Abstract]
  109. Otoya RE, Seltzer AM, Donoso AO: Acute and long-lasting effects of neonatal hypoxia on (+)-3-[125I]MK-801 binding to NMDA brain receptors. Exp Neurol 1997;148:92-99. [Abstract]
  110. Delivoria-Papadopoulos M, Mishra OP: Mechanisms of cerebral injury in perinatal asphyxia and strategies for prevention. J Pediatr 1998;132(suppl):30-34. [PubMed]
  111. Kim JJ, Foy MR, Thompson RF: Behavioral stress modifies hippocampal plasticity through N-methyl-D-aspartate receptor activation. Proc Natl Acad Sci USA 1996;93:4750-4753. [Abstract]
  112. Kim YI, Na HS, Yoon YW, Han HC, Ko KH, Hong SK: NMDA receptors are important for both mechanical and thermal allodynia from peripheral nerve injury in rats. Neuroreport 1997;8:2149-2153. [Abstract]
  113. Zhuo M: NMDA receptors-dependent long term hyperalgesia after tail amputation in mice. Eur J Pharmacol 1998;349:211-220. [Abstract]
  114. Baranauskas G, Nistri A: Sensitization of pain pathways in the spinal cord: cellular mechanisms. Prog Neurobiol 1998;54:349-365. [Abstract]
  115. Chiang CY, Hu JW, Sessle BJ: NMDA receptor involvement in neuroplastic changes induced by neonatal capsaicin treatment in trigeminal nociceptive neurons. J Neurophysiol 1997;78:2799-2803. [Abstract]
  116. De Felipe C, Pinnock RD, Hunt Sp: Modulation of chemotropism in the developing spinal cord by substance P. Science 1995;267:899-905. [Abstract]
  117. Rogan MT, Staubil U, LeDoux JE: Fear conditioning induces associative long-term potentiation in the anygdala. Nature 1997;390:604-607. [Full Text]
  118. McKernan MG, Shinnick-Gallagher P: Fear conditioning induces a lasting potentiation of synaptic currents in vitro. Nature 1997;390:607-610. [Abstract]
  119. Knudsen EI: Capacity for plasticity in the adult owl auditory system expanded by juvenile experience. Science 1998;279:1531-1533. [Abstract]
  120. McEwen BS: The plasticity of the hippocampus is the reason for its vulnerability. Neurosciences 1994;6:239-246.
  121. Liu D, Diorio J, Tannenbaum B, et al: Maternal care, hippocampal glucocorticoid receptors, and hypothalamic-pituitary-adrenal responses to stress. Science 1997;277:1659-1662. [Abstract]
  122. Collier AC, Bolles RC: The ontogenesis of defensive reactions to shock in preweanling rats. Dev Psychobiol 1980;13:141-150. [PubMed]
  123. Wasterlain CG, Shirasaka Y: Seizures, brain damage and brain development. Brain Dev 1994;16:279-295. [PubMed]
  124. Meldrum B, Garthwaite J: Excitatory amino acid neurotoxicity and neurodegenerative disease. Trends Pharmacol Sci 1990;11:379-387. [PubMed]
  125. Fillion TJ, Blass EM: Infantile experience with suckling odors determines sexual behavior in male rats. Science 1986;231:729-731.
  126. Reynolds ML, Fitzgerald M: Long-term sensory hyperinnervation following neonatal skin wounds. J Comp Neurol 1995;358:487-498. [PubMed]
  127. Reynolds M, Alvares D, Middleton J, Fitzgerald M: Neonatally wounded skin induces NGF-independent sensory neurite outgrowth in vitro. Dev Brain Res 1997;102:275-283. [PubMed]
  128. Rahman W, Fitzgerald M, Aynsley-Green A, Dickenson AH: The effects of neonatal exposure to inflammation and/or morphine on neuronal responses and morphine analgesia in adult rats; in Jensen TS, Turner JA, Wiesenfeld-Hallin Z (eds): Proc 8th World Congree on Pain. Seattle, IASP Press, 1997, vol 8, pp 783-794.
  129. Anand KJS, Coskun V, Thrivikraman KV, Nemeroff CB, Plotsky PM: Long-term behavioral effects of repetitive pain in neonatal rat pups. Physiol Behav 1999;66:627-637. [PubMed]
  130. Shide DJ, Blass EM: Opioid mediation of odor preferences induced by sugar and fat in 6-day-old rats. Physiol Behav 1991;50:961-966. [Abstract]
  131. Stromberg MF, Meister S, Volpicelli JR, Ulm RR: Morphine enhances selection of both sucrose and ethanol in a two-bottle test. Alcohol 1997;14:55-62. [Abstract]
  132. Medina Jimenez M, Lujan Estrada M, Rodriguez R: Influence of chronic prenatal and postnatal administration of naltrexone in locomotor activity induced by morphine in mice. Arch Med Res 1997;28:61-65. [PubMed]
  133. Kuhn CM, Pauk J, Schanberg SM: Endocrine responses to mother-infant separation in developing rats. Dev Psychobiol 1990;23:395-410. [PubMed]
  134. Meaney MJ, Diorio J, Francis D, et al: Early environmental regulation of forebrain glucocorticoid receptor gene expression: Implications for adrenocortical responses to stress. Dev Neurosci 1996;18:49-72. [PubMed]
  135. Viau V, Sharma S, Plotsky P, Meaney M: Increased plasma ACTH responses to stress in nonhandled compared with handled rats require basal levels of corticosterone and are associated with increased levels of ACTH secretagogues in the median eminence. J Neurosci 1993;13:1097-1105. [Abstract]
  136. Seckl JR, Meaney MJ: Early life events and later development of ischaemic heart disease. Lancet 1993;342:1236. [PubMed]
  137. Meaney MJ, Aitken DH, van Berkel C, Bhatnagar S, Sapolsky RM: Effect of neonatal handling on age-related impairments associated with the hippocampus. Science 1988;239:766-768. [Abstract]
  138. Kehoe P, Shoemaker WJ, Triano L, Hoffman J, Arons C: Repeated isolation in the neonatal rat produces alterations in behavior and ventral striatal dopamine release in the juvenile following amphetamine challenge. Behav Neurosci 1996;110:1435-1444. [PubMed]
  139. Kehoe P, Clash K, Skipsey K, Shoemaker WJ: Brain dopamine response in isolated 10-day-old rats: Assessment using D2 binding and dopamine turnover. Pharmacol Biochem Behav 1996;53:41-49. [PubMed]
  140. Cahib S, Puglisi-Allegra S, D'Amato FR: Effects of postnatal stress on dopamine mesolimbic system responses to aversive experiences in adult life. Brain Res 1993;604:232-239. [PubMed]
  141. Zimmerberg B, Shartrand AM: Temperature-dependent effects of maternal separation on growth, activity, and amphetamine sensitivity in the rat. Dev Psychobiol 1992;25:213-226. [PubMed]
  142. Tsumoto T, Kimura F, Nishigori A: A role of NMDA receptors and Ca2+ influx in synaptic plasticity in the developing visual cortex. Adv Exp Med Biol 1990;268:173-180. [PubMed]
  143. Ni B, Wu X, Su Y, et al: Transient global forebrain ischemia induces a prolonged expression of the caspase-3 mRNA in rat hippocampal CA1 pyramidal neurons. J Cereb Blood Flow Metab 1998;18:248-256. [Abstract]
  144. Chan WY, Yew DT: Apoptosis and Bcl-2 oncoprotein expression in the human fetal central nervous system. Anat Rec 1998;252:165-175. [Abstract]
  145. Alvarez-Bolado G, Rodriguez-Sanchez P, Tejero-Diez P, Fairen A, Diez-Guerra FJ: Neurogranin in the development of the rat telencephalon. Neuroscience 1996;73:565-580. [Abstract]
  146. Schierle GS, Hansson O, Leist M, Nicotera P, Widner H, Brundlin P: Caspase inhibition reduces apoptosis and increases survival of nigral transplants. Nature Med 1999;5:97-100. [Abstract]
  147. Young D, Lawlor PA, Leone P, Dragunow M, During MJ: Environmental enrichment inhibits spontaneous apoptosis, prevents seizures, and is neuroprotective. Nature Med 1999;5:448-453. [Abstract]

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