Effects of neonatal hypoxia on brain development in the rat: immediate and long-term biochemical alterations in discrete regions

Perinatal Hypoxia and Ischemia in Animal Models of Schizophrenia

Intrauterine or perinatal complications constitute a major risk for psychiatric diseases. Infants who suffered from hypoxia-ischemia (HI) are at twofold risk to develop schizophrenia in later life. Several animal models attempt to reproduce these complications to study the yet unknown steps between an insult in early life and outbreak of the disease decades later. However, it is very challenging to find the right type and severity of insult leading to a disease-like phenotype in the animal, but not causing necrosis and focal neurological deficits. By contrast, too mild, repetitive insults may even be protective via conditioning effects. Thus, it is not surprising that animal models of hypoxia lead to mixed results. To achieve clinically translatable findings, better protocols are urgently needed. Therefore, we compare widely used models of hypoxia and HI and propose future directions for the field.

Neonatal Hypoxia Ischaemia: Mechanisms, Models, and Therapeutic Challenges

Neonatal hypoxia-ischaemia (HI) is the most common cause of death and disability in human neonates, and is often associated with persistent motor, sensory, and cognitive impairment. Improved intensive care technology has increased survival without preventing neurological disorder, increasing morbidity throughout the adult population. Early preventative or neuroprotective interventions have the potential to rescue brain development in neonates, yet only one therapeutic intervention is currently licensed for use in developed countries. Recent investigations of the transient cortical layer known as subplate, especially regarding subplate's secretory role, opens up a novel set of potential molecular modulators of neonatal HI injury. This review examines the biological mechanisms of human neonatal HI, discusses evidence for the relevance of subplate-secreted molecules to this condition, and evaluates available animal models. Neuroserpin, a neuronally released neuroprotective factor, is discussed as a case study for developing new potential pharmacological interventions for use post-ischaemic injury.

Neurochemical and physiological correlates of a critical period of respiratory development in the rat

Despite its vital importance to life, respiration is not mature at birth in mammals, but rather, it undergoes a great deal of growth, refinement, and adjustments postnatally. Many adjustments do not follow smooth paths, but assume abrupt changes during certain postnatal periods that may render the animal less capable of responding to respiratory stressors. The present review focuses on neurochemical and physiological correlates of a critical period of respiratory development in the rat. In addition to an imbalanced expression of reduced excitatory and enhanced inhibitory neurotransmitters, a switch in the expressions of gamma-aminobutyric acid (GABA)A receptor subunits from alpha3 to alpha1 occurs around postnatal day (P)12 in the pre-Bötzinger nucleus and the ventrolateral subnucleus of the solitary tract nucleus. Possible subunit switches in a number of other neurotransmitter receptors are discussed. These neurochemical changes are paralleled by ventilatory adjustments at the end of the second postnatal week. At P13 and under normoxia, respiratory frequency reaches its peak before assuming a gradual fall, and both tidal volume and minute ventilation exhibit a significant rise prior to a plateau or a gradual decline until P21. The response to acute hypoxia is markedly reduced between P12 and P16, being lowest at P13. Thus, the end of the second postnatal week can be considered as a critical period of respiratory development, during which multiple neurochemical and physiological adjustments and switches are orchestrated at the same time, rendering the system extremely dynamic but, at the same time, vulnerable to externally imposed perturbations and insults. The critical period embodies a time of multi-system, multifaceted growth and adjustments. It is a plastic, transitional period that is also a part of the normal development of the respiratory system.

Ontogeny of altered dendritic morphology in the rat prefrontal cortex, hippocampus, and nucleus accumbens following Cesarean delivery and birth anoxia

We used a delayed Cesarean birth model and the Golgi-Cox staining method to investigate the effects of perinatal anoxia on prefrontal cortex (PFC) and hippocampal (CA1) pyramidal neurons as well as nucleus accumbens (NAcc) medium spiny neurons. Dendritic morphology in these regions was studied on postnatal days (P) 2, 7, 14, 21, 35, and 70 in male Sprague-Dawley rats born either vaginally (VAG) or by Cesarean section either with (C + anoxia) or without (C-only) anoxia. The most striking birth group differences seen were at the level of dendritic spine densities on P35. During this postnatal period the dendritic spine density of PFC neurons was significantly lower in C + anoxia and C-only animals than in VAG controls; however, by P70 PFC spine densities in all birth groups were comparable. In contrast, hippocampal spine densities on P35 were comparably greater in C + anoxia animals than in VAG controls, whereas in C-only animals spine densities were lower than controls; here again, by P70 all groups had comparable hippocampal spine densities. In NAcc greater spine densities were seen on medium spiny neurons of C + anoxia animals on P35. These findings provide evidence that perinatal insult in the form of Cesarean birth with or without anoxia alters the dendritic development of PFC and hippocampal pyramidal neurons and to some extent also of NAcc medium spiny neurons. They also suggest that perinatal anoxia can alter the neuronal development of key structures thought to be affected in such late-onset dopamine-related disorders as schizophrenia and Attention Deficit Hyperactivity Disorder (ADHD).

Developmental plasticity of the carotid chemoafferent pathway in rats that are hypoxic during the prenatal period

The chemoreflex pathway undergoes postnatal maturation, and the perinatal environment plays a critical role in shaping respiratory control system. We investigated the role of prenatal hypoxia on the maturation of the chemoreflex neural circuits regulating ventilation in rat. Effects of hypoxia (10% O2) from the 5th to the 20th day of gestation were studied on male offspring at birth and on postnatal days 3, 7, 21 and 68. Maturation of the respiratory control system was assessed by in vivo tyrosine hydroxylase (TH) activity measurement in peripheral chemoreceptors (carotid bodies, petrosal ganglia), and in brainstem catecholaminergic cell groups (A2C2c and A1C1 areas in the medulla, A5 and A6 areas in the pons). Resting ventilation and ventilatory response to hypoxia were evaluated as functional sequelae. In peripheral structures, prenatal hypoxia reduced TH activity within the first postnatal week and enhanced it later. In contrast, in central areas, prenatal hypoxia upregulated TH activity within the first postnatal week and downregulated it later. The in vivo TH activity impairment is therefore tissue specific, with an opposite effect on the peripheral and central neural circuits. A shift of the effect of prenatal hypoxia occurred between 1 and 3 weeks, indicating a postnatal temporal effect of prenatal hypoxia. An important period in the development of the chemoafferent pathway occurred between the first and the third postnatal week. Functionally, prenatal hypoxia impaired resting ventilation and ventilatory response to hypoxia. The alterations of the catecholaminergic components of the chemoafferent pathway resulting from prenatal hypoxia might contribute to impair postnatal respiratory behaviour.

Novel two-dimensional morphometric maps and quantitative analysis reveal marked growth and structural recovery of the rat hippocampal regions from early hypothyroid retardation

Effects of postnatal hypothyroidism and recovery from this condition on regional growth of the rat hippocampus (HC) were studied using two-dimensional (2D) foldout, morphometric maps of HC and its constituent CA1-CA4 regions. The maps were derived from unfolding serial coronal sections of the rat forebrain, consisting of the entire rostrocaudal extent of HC pyramidal cell layer in the normal control and hypothyroid weanling (P25, postnatal day 25) and young adult (P90) male rats, as well as animals allowed to recover from hypothyroid-induced growth retardation at weaning. The maps revealed novel views of HC regions for assessment of topological relationships and measurement of surface areas of the HC cortical sheet (pyramidal cell layer). In normal control P90 rats, the unfolded HC on each side extended 4 times more laterally than rostrocaudally; total HC surface area was about 40 mm(2), compared to 30 mm(2) in the weanling, indicating 35% growth from P25 to P90; CA1 took up 52% of the total HC surface area, followed by CA3 (31%) and CA2 and CA4, 8% each. Hypothyroidism resulted in significant (p<0.01) 11% and 20% reductions in the HC surface area in P25 and P90 rats, respectively; CA1 and CA4 regions suffered the most reductions while CA3 and CA2 regions the least. Recovering rats examined at P90 exhibited remarkable growth plasticity and recovery in HC regions, as evident by their near normal HC cortical surface area values, compared to age-matched controls. The 2D maps also revealed growth deficits in all HC regions of the hypothyroid rats; recovery in these parameters occurred across all dimensions, although the anterior-posterior growth was more severely affected than the mediolateral one. These results are confirmed and extended by volumetric analysis of laminar volumes of HC regions presented in a companion paper [Farahvar, A., Darwish, N., Sladek, S., Meisami, E., in press. Marked recovery of functional metabolic activity and laminar volumes in the rat hippocampus and dentate gyrus following postnatal hypothyroid growth retardation: a quantitative cytochrome oxidase study. Exp. Neurol.]. These results imply that HC regions, in contrast to whole brain, possess exceptional growth plasticity, as shown by ability to dramatically recover from early hypothyroid retardation; also 2D morphometric maps are useful tools to visualize complex and convoluted regional sheet of HC cortex and depict quantitative aspects of growth in normal and experimental conditions.

Maternal exposure of rats to nicotine via infusion during gestation produces neurobehavioral deficits and elevated expression of glial fibrillary acidic protein in the cerebellum and CA1 subfield in the offspring at puberty

Maternal smoking during pregnancy is known to be a significant contributor to developmental neurological health problems in the offspring. In animal studies, nicotine treatment via injection during gestation has been shown to produce episodic hypoxia in the developing fetus. Nicotine delivery via mini osmotic pump, while avoiding effects due to hypoxia-ischemia, it also provides a steady level of nicotine in the plasma. In the present study timed-pregnant Sprague-Dawley rats (300-350 g) were treated with nicotine (3.3 mg/kg, in bacteriostatic water via s.c. implantation of mini osmotic pump) from gestational days (GD) 4-20. Control animals were treated with bacteriostatic water via s.c. implantation of mini osmotic pump. Offspring on postnatal day (PND) 30 and 60, were evaluated for changes in the ligand binding for various types of nicotinic acetylcholine receptors and neuropathological alterations. Neurobehavioral evaluations for sensorimotor functions, beam-walk score, beam-walk time, incline plane and grip time response were carried out on PND 60 offspring. Beam-walk time and forepaw grip time showed significant impairments in both male and female offspring. Ligand binding densities for [3H]epibatidine, [3H]cytisine and [3H]alpha-bungarotoxin did not show any significant changes in nicotinic acetylcholine receptors subtypes in the cortex at PND 30 and 60. Histopathological evaluation using cresyl violet staining showed significant decrease in surviving Purkinje neurons in the cerebellum and a decrease in surviving neurons in the CA1 subfield of hippocampus on PND 30 and 60. An increase in glial fibrillary acidic protein (GFAP) immuno-staining was observed in cerebellum white matter as well as granular cell layer of cerebellum and the CA1 subfield of hippocampus on PND 30 and 60 of both male and female offspring. These results indicate that maternal exposure to nicotine produces significant neurobehavioral deficits, a decrease in the surviving neurons and an increased expression of GFAP in cerebellum and CA1 subfield of hippocampus of the offspring on PND 30 and 60. The results show that although 60-day-old male and female rat offspring of mothers exposed to nicotine during gestation did not differ from control in body weight gain or nicotinic acetylcholine receptors ligand binding, they exhibited significant sensorimotor deficits that were consistent with the neuropathological alterations seen in the brain. These neurobehavioral and pathological deficits indicate that maternal nicotine exposure may produce long-term adverse health effects in the offspring.

Cholinergic systems in brain development and disruption by neurotoxicants: nicotine, environmental tobacco smoke, organophosphates

Acetylcholine and other neurotransmitters play unique trophic roles in brain development. Accordingly, drugs and environmental toxicants that promote or interfere with neurotransmitter function evoke neurodevelopmental abnormalities by disrupting the timing or intensity of neurotrophic actions. The current review discusses three exposure scenarios involving acetylcholine systems: nicotine from maternal smoking during pregnancy, exposure to environmental tobacco smoke (ETS), and exposure to the organophosphate insecticide, chlorpyrifos (CPF). All three have long-term, adverse effects on specific processes involved in brain cell replication and differentiation, synaptic development and function, and ultimately behavioral performance. Many of these effects can be traced to the sequence of cellular events surrounding the trophic role of acetylcholine acting on its specific cellular receptors and associated signaling cascades. However, for chlorpyrifos, additional noncholinergic mechanisms appear to be critical in establishing the period of developmental vulnerability, the sites and type of neural damage, and the eventual outcome. New findings indicate that developmental neurotoxicity extends to late phases of brain maturation including adolescence. Novel in vitro and in vivo exposure models are being developed to uncover heretofore unsuspected mechanisms and targets for developmental neurotoxicants.

Maternal exposure to bacterial endotoxin during pregnancy enhances amphetamine-induced locomotion and startle responses in adult rat offspring

An increased incidence of schizophrenia has been associated with several perinatal insults, most notably maternal infection during pregnancy and perinatal hypoxia. This study used a rat model to directly test if maternal exposure to bacterial endotoxin (lipopolysaccharide, LPS) during pregnancy alters behaviors relevant to schizophrenia, in offspring at adulthood. The study also tested if postnatal anoxia interacted with gestational LPS exposure to affect behavior. At adulthood, offspring from dams administered LPS on days 18 and 19 of pregnancy showed significantly increased amphetamine-induced locomotion, compared to offspring from saline-treated dams. A period of anoxia on postnatal day 7 had no effect on amphetamine-induced locomotion and there was no interaction between effects of gestational LPS and postnatal anoxia on this behavior. Offspring from LPS-treated dams also showed enhanced acoustic startle responses as adults, compared to offspring from saline-treated dams. In offspring tested for pre-pulse inhibition (PPI) of acoustic startle and for apomorphine modulation of PPI, no effects of either gestational LPS or of postnatal anoxia and no interactions between LPS and anoxia were observed. It is concluded that maternal LPS exposure during pregnancy in the rat may be a useful model to study mechanisms responsible for effects of maternal infection on behaviors relevant to schizophrenia, in offspring.

Immunological detection of 4-hydroxynonenal protein adducts in developing pontine and Purkinje neurons and in karyorrhexis in pontosubicular neuronal necrosis

Four-hydroxynonenal (HNE) has been proposed as an important marker of radical-induced lipid peroxidation. The principal objective of this study was to assess the occurrence of lipid peroxidation in normal perinatal brain and brains with one form of pontosubicular neuronal necrosis (PSN). Immunochemical studies using an antibody against HNE-modified protein were performed in controls aged from 20 weeks of gestation to 64 years, and patients with PSN. Immunohistochemical study showed developmental and aging changes of positive staining in Purkinje cells and pontine neurons (27 weeks-7 months, 50 and 64 years). In addition, karyorrhectic cells in pontine nuclei with PSN were positively stained. Immunoblotting revealed that a 75-kDa protein, which is speculated to be mitochondrial complex-1 protein, was the most intensely expressed among multiple immunoreactive proteins. Our results identified the presence of oxidative stress in the perinatal neuron, and this oxidative stress may contribute to some forms of karyorrhectic death.

Perinatal expression of heat shock proteins HSC 70 and HSP 70 in neural and non neural tissues of the piglet

Stress of different kinds during early perinatal life can result in severe consequences for further development. To determine possible involvement of heat shock proteins in brain development, the expression of HSC 70 and HSP 70 was determined in brain regions (cerebellum, cortex, hippocampus, hypothalamus and striatum) and non neural tissues (liver, lungs and kidneys) at birth and during early development of the piglet. In brain regions, HSC 70 expression was decreased during the few hours following birth. With the exception of cortex, hippocampus and kidney where a decrease of expression was observed, HSP 70 did not show significant changes during early development. These results are discussed in terms of using the piglet model of development to study the effect of different kinds of stress like hypoxia or temperature changes on brain development.

Perinatal expression of heat-shock protein 90 in different regions of the brain and in non-neural tissues of the piglet

Important stressful events occur at birth or within the few hours that follow. To establish a possible involvement of stress proteins, expression of heat-shock protein 90 was determined by Western blotting in several regions of the brain and in non-neural tissues of the developing piglet (fetal to 10 days and adult). Expression was found in all the tissues studied. While comparable values were found in the whole brain during development, decreased expressions were observed from 4 to 8 h to 2 days after birth in cerebellum, cortex, hypothalamus and striatum. In hippocampus, low expression was observed from 4 h postnatally onward. In non-neural tissues, low expression was observed after birth and in the adult for heart, liver and lungs. In kidney, low values were found from birth to 1 day of age. Changes in environmental parameters like temperature and/or hypoxia can be related to differential expressions of heat-shock proteins and they possibly result in severe developmental outcomes. The results are discussed in terms of using the newborn piglet as a model for the study of different forms of stress on the heat-shock protein expression during postnatal development.

Persistent c-fos induction by nicotine in developing rat brain regions: interaction with hypoxia

Prenatal nicotine exposure evokes postnatal CNS cell loss. We administered nicotine to pregnant rats throughout gestation and neonatal brains were examined for expression of c-fos, a nuclear transcription factor involved in differentiation and cell death. The nicotine group showed persistent c-fos overexpression in the forebrain long after termination of exposure; in the brainstem, overexpression was apparent both after birth and at the end of the second postnatal week. In contrast to these effects, postnatal administration on d 1-4 caused persistent c-fos only at systemically toxic doses and treatment at subsequent ages did not cause induction at all. We also determined whether prenatal nicotine exposure would sensitize the brain to a subsequent postnatal episode of hypoxia comparable to that experienced during parturition. Hypoxia evoked acute stimulation of c-fos with a regional selectivity and ontogenetic profile differing from those of prenatal nicotine and this acute response was reduced by prenatal nicotine treatment. Persistent c-fos elevation is a harbinger of cell death, a relationship that provides an underlying mechanism for eventual cell deficits that appear after fetal nicotine exposure. Nicotine's interference with the acute c-fos stimulation caused by a subsequent episode of hypoxia may indicate a further compromise of cellular repair mechanisms.

Hypoxia/reoxygenation induces apoptosis through biphasic induction of protein synthesis in cultured rat brain neurons

To investigate biochemical events accounting for the outcome of central neurons following hypoxia/reoxygenation, cultured neurons from fetal rat forebrain were exposed to hypoxia (95% N2/5% CO2) for 6 h, and then reoxygenated for up to 96 h. Time-dependent changes in macromolecular biosynthesis were analysed by incorporation of [3H]uridine and [3H]leucine and were coupled to cell viability and lactate dehydrogenase leakage. Morphological features of necrosis and apoptosis were scored following nuclear incorporation of the fluorescent dye 4,6-diamidino-2-phenylindole. Hypoxia led to a 36% reduction of cell viability at the end of the reoxygenation period, while 23% of the neurons exhibited apoptosis. A biphasic increase in the rates of protein synthesis was measured 1 h after the onset of hypoxia (77% above controls) and by 48-h postreoxygenation (72%). The presence of cycloheximide during hypoxia inhibited both peaks of synthesis and prevented the development of apoptosis. Protein electrophoresis outlined specific alterations in constitutive proteins, and immunohistochemistry revealed an overexpression of the pro-apoptotic gene products Bax and ICE. Therefore, hypoxia followed by reoxygenation would trigger sequential changes in synthesis of specific proteins, leading to delayed and mainly apoptotic neuronal death.

Cryptic brain cell injury caused by fetal nicotine exposure is associated with persistent elevations of c-fos protooncogene expression

Neurobehavioral teratogenesis caused by prenatal nicotine exposure is associated with deficiencies in brain cell numbers that reflect, in part, effects on cell replication but that also involve delayed cell loss. In the current study, pregnant rats were given nicotine by implanted minipump infusion either from gestational days 4-12 or 4-21 and fetal and neonatal brain regions were examined for expression of the mRNA encoding c-fos, a nuclear transcription factor that becomes chronically elevated when cell injury or apoptosis are occurring. Fetuses exposed to nicotine on gestational days 4-12 did not show elevations of c-fos mRNA on gestational day 18 whereas animals undergoing exposure through day 21 did. In the latter group, elevated c-fos expression was still present on postnatal day 2 despite the cessation of nicotine exposure on gestational day 21. In contrast to the elevation of c-fos seen with prenatal nicotine, postnatal nicotine injections given to 2-day-old rats did not cause acute stimulation of c-fos expression. The ability of injected nicotine to evoke acute rises in c-fos emerged by postnatal day 8 and initially displayed regional specificity paralleling the concentration of nicotinic cholinergic receptors. With increasing maturity, regional selectivity of the c-fos response to acute nicotine was lost, consistent with indirect actions that could be mediated through nicotine-induced hypoxia/ischemia. These results indicate that prenatal nicotine exposure causes chronic elevations of c-fos expression in fetal and neonatal brain that are distinguishable from the later onset of the ability of acute nicotine to cause short-term stimulation of c-fos. The critical period and dose threshold for these effects correspond to those of subsequent cell damage and cell loss identified in previous studies with fetal nicotine exposure. Given that chronic elevations of c-fos are known to be associated with cell injury and to evoke apoptosis in otherwise healthy cells, these results suggest that prenatal nicotine exposure evokes delayed neurotoxicity by altering the program of neural cell differentiation, and that elevated c-fos expression provides an early marker of the eventual deficits.

Prognostic value of neonatal electroencephalography in premature newborns less than 33 weeks of gestational age

In a prospective study of 417 premature neonates born before 33 weeks' gestational age, neonatal tracings were reviewed to evaluate the use of EEG in prognosis of neurological injuries. The population was divided into two groups: Group 1, infants who died before the age of 1, and Group 2, survivors in which two categories of motor development were considered. Category A, were abnormal, and Category B, were always normal. Positive rolandic sharp waves (PRSW), which reflect white matter injury, occurred equally in both groups, indicating a similar incidence of white matter damage in Groups 1 and 2. In Group 2, there was a significant correlation of PRSW with developmental motor sequelae (Category A). A frequency of PRSW above 2/min (suggesting more severe periventricular white matter injury) and seizures were significantly more prevalent in Group 1 than in Group 2 and in Category A of Group 2 than in Category B. Background abnormalities occurred equally in both subgroups of extremely premature infants (< or = 28 weeks' gestation) they were significantly more numerous in the subgroup of very premature infants (between 28 and 33 weeks' gestation) who died, than in the subgroup of very premature infants who survived. This study shows the potential utility of using neonatal EEG in association with transfontanellar ultrasonography in anticipating the neurological development of very (> 28 weeks' gestation) and extremely (< or = 28 weeks' gestation) premature newborns.

Chlorpyrifos interferes with cell development in rat brain regions

Chlorpyrifos, one of the most widely used pesticides, exhibits greater toxicity during development than in adulthood. We administered chlorpyrifos to neonatal rats in doses spanning the threshold for systemic toxicity and examined developing brain regions (brainstem, forebrain, cerebellum) for signs of interference with cell development using markers for cell packing density and cell number (DNA concentration and content) and cell size (protein/DNA ratio). Neonatal rats given 5 mg/kg of chlorpyrifos on postnatal days 1-4 showed significant mortality and the survivors exhibited severe cell loss in the brainstem; brainstem growth was maintained by enlargement of the remaining cells. This effect was not seen at 1 mg/kg, a dose that did not compromise survival or growth, nor was there any adverse effect at either dose in the forebrain, despite the fact that both brainstem and forebrain possess comparable cholinergic projections. When chlorpyrifos was administered later, on days 11-14, the major target for cell loss shifted from the brainstem to the forebrain and in this case, effects were seen at doses that did not compromise survival or growth. The loss of forebrain cell number occurred between 15 and 20 days of age rather than during the chlorpyrifos treatment. The cerebellum differed from the other regions in that it showed short-term elevations of DNA after chlorpyrifos exposure in either early or late postnatal periods; nevertheless, values then regressed to subnormal in parallel with the loss of cells in other regions. Thus, chlorpyrifos likely causes delayed cell death. Although regions rich in cholinergic projections, such as brainstem and forebrain, may be more affected than noncholinergic regions (cerebellum), the maturational timetable of each region (brainstem earliest, forebrain intermediate, cerebellum last) appears to be more important in setting the window of vulnerability. These results indicate that, even when growth or survival are unaffected, chlorpyrifos produces cellular deficits in the developing brain that could contribute to behavioral abnormalities.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Does concurrent or prior nicotine exposure interact with neonatal hypoxia to produce cardiac cell damage?

Cigarette smoking during pregnancy exposes the fetus to both nicotine and hypoxia/ischemia; postnatal exposure to second-hand smoke also involves substances that cause hypoxia (CO, HCN). Although developing cardiac cells are more resistant to hypoxia-induced damage than are mature cells, we examined whether nicotine affects this resistance, either when exposure is concurrent with hypoxia, or when animals are exposed to nicotine prenatally and receive subsequent hypoxic exposure. One, 8-, or 15-day-old rats exposed to 7% O2 for 2 hr all showed inhibition of cardiac DNA synthesis. By contrast, administration of nicotine at either low (0.3 mg/kg) or high (3 mg/kg) doses failed to alter DNA synthesis. To examine effects on cells that were not undergoing mitosis, we examined ornithine decarboxylase (ODC), an enzymatic marker for cell damage. One day old rats showed inhibition of ODC by hypoxia, a response that represents preservation of cell integrity; by 8 days of age, ODC was increased by hypoxia, evidence of cell damage. The high dose of nicotine evoked an increase in ODC at all ages and the low dose exacerbated the effects of hypoxia at 8 days of age. Prenatal nicotine exposure caused a transient inhibition of cardiac DNA synthesis but did not produce evidence of cell damage (ODC, protein synthesis markers) by itself, nor did it alter the effect of a subsequent postnatal exposure to hypoxia. These results suggest that cardiac cell damage could emerge as a consequence of concurrent, repeated exposures to nicotine and hypoxia. Such effects could contribute to the elevated incidence of perinatal morbidity/mortality and Sudden Infant Death Syndrome associated with smoking.

Ornithine decarboxylase activity in fetal and newborn rat brain: responses to hypoxic and carbon monoxide hypoxia

In response to acute maternal hypoxia, ornithine decarboxylase (ODC) activity increased significantly in fetal rat brain, peaking at 4 h. This was associated with increased ODC mRNA and elevated polyamine concentrations. To correlate this response with development, we measured ODC activity in the rat from gestational day E 17 to postnatal day P 10. We also examined to what extent hypoxia induces increased ODC activity in adult rat brains and whether the response to chronic hypoxia differed from that to acute hypoxia. To test the hypothesis that this increased activity is due to hypoxic hypoxia per se, we subjected pregnant dams to inspired carbon monoxide concentrations ranging from 150 to 1000 ppm and assayed ODC activity in the fetal brain 4 h later. In the fetus, ODC activity was elevated on E 17 in the cerebrum and cerebellum. It declined gradually to about one-tenth E 17 levels by E 21 and remained low thereafter except for a postnatal elevation in the cerebellum on P 3. In response to 10.5% O2, in the 3-day-old rat, ODC activity peaked between 2 and 3 h of hypoxia, increasing 3-fold in the hippocampus and 2-fold in cerebellum. Similar increases were seen in the hypoxic adult rat brain. In inspired oxygen dose-response studies, exposure of P 3 rat pups to 13.25% O2 for 2.5 h produced a 1.5-fold increase in ODC activity; 10.5% O2 produced a 2-3-fold increase while in response to 9% O2, ODC activity remained at baseline levels. With maternal CO-hypoxia, ODC activity increased in the fetal brain at 4 h, as seen with hypoxic-hypoxia. For example, in hippocampus, ODC activity doubled at 500 ppm and tripled at 600 ppm. We conclude: (1) apparently, the ability to respond thus is not lost as the animal ages and may represent an important cellular response to acute hypoxia; (2) the increase in hypoxic-induced ODC activity is relative to the already elevated activity seen from E 17 to E 20; a vast reserve for the induction of fetal ODC activity probably exists and may indicate the importance of this enzyme during this time frame for differentiation and growth promotion; and (3) the CO-hypoxia studies suggest that some aspects of the cellular responses to CO- and hypoxic-hypoxia are similar.

Early postnatal hypoxia induces long-term changes in the dopaminergic system in rats

A rat model of a mild, chronic, early postnatal hypoxia, characterized by long-term consequences in the behavioural outcome, was used to study long-term consequences in the dopaminergic system. Exposure of newborn rats to an early postnatal hypoxia (hypobaric hypoxia, 11 kPa pO2 in the inspiratory air, 2nd-10th day of life, 10 hours daily) brings about the following lasting neurochemical changes: an increased stimulated dopamine release rate from striatum slices by about 30%, an increased low affinity, high capacity dopamine uptake into striatum synaptosomes by about 100%. The critical period to produce an increased release rate of dopamine was estimated as day 2-6 postnatally. There are no long-term changes in the concentration of dopamine and its metabolites and in the tyrosine hydroxylase activity in consequences of this early postnatal hypoxia. Treatment of newborn animals with L-DOPA (10-50 micrograms/g body weight) previous to hypoxia normalizes the DA release rate.

Acute hypoxia increases ornithine decarboxylase activity and polyamine concentrations in fetal rat brain

The cellular responses to hypoxia are poorly understood. To test the hypothesis that ornithine decarboxylase (ODC; L-ornithine carboxy-lyase; EC 4.1.1.17) activity and polyamine concentrations change in response to acute hypoxia, we performed the following studies. Pregnant Sprague-Dawley rats inspired various O2 concentrations (9-21%) for various time periods (0.5-48 h) from days 15 to 21 of gestation. In fetal brains we measured the activity of ODC, ODC mRNA, and polyamines. In response to 4-h acute mild hypoxia, ODC activity in fetal rat brain (cerebrum, cerebellum, and hippocampus) increased to 330-450% from control values (P < 0.001), after which it declined to control levels in 6-8 h. The 4-h ODC response varied inversely with inspired O2 concentration and was not mimicked by beta 2 agonist or blocked by beta 2-antagonist administration. The ODC response was associated with an increase in fetal brain putrescine concentration to 190% above control at 4-6 h (P < 0.01) and an increase in the polyamines spermidine and spermine to about 115% above control at 6-8 h. We also observed that ODC mRNA increased significantly after 2-4 h of hypoxia. ODC activity and polyamine concentrations appear to be useful enzymatic markers for fetal brain hypoxia. The magnitude and time course of the acute hypoxic ODC increase were similar to responses to extracellular signals that result in differentiation or cell growth. Thus, the well-defined and regulated ODC activity response may represent a protective mechanism in brain to hypoxia.

Impact of fetal nicotine exposure on development of rat brain regions: critical sensitive periods or effects of withdrawal?

Fetal nicotine exposure evokes alterations in central nervous system structural, neurochemical, and behavioral development. In the current study, the relative importance of critical developmental exposure periods and withdrawal were examined by infusing nicotine to pregnant rats using osmotic minipumps beginning on the fourth day of gestation. Infusions were confined to either the first 8 days (withdrawal on gestational day 13), to nearly all of gestation (withdrawal on gestational day 21), or throughout gestation and continued into the first 2 postnatal weeks. Maternal weight gain was retarded by nicotine, with a hierarchy corresponding to the duration of nicotine exposure. Similarly, fetal and neonatal body weights were unaffected in the group receiving the shortest duration of nicotine exposure, and were less affected by the intermediate infusion regimen than by the longest regimen; brain region weights were reduced significantly only with the longest regimen. Using ODC activity, a sensitive marker for altered brain cell development, we found little change in animals exposed to nicotine in early gestation and undergoing withdrawal on day 13. However, in the groups receiving nicotine through the end of gestation or through gestation and into the postnatal period, ODC activity was significantly elevated. These results indicate that withdrawal from nicotine contributes little, if any, effect either to the growth deficits or to abnormalities of brain cell development. Instead, the most important factor appears to be exposure within the developmental period corresponding to the proliferation of nicotinic receptors and the timing of receptor control of cell replication and differentiation.

Influence of hypoxia on excitation and GABAergic inhibition in mature and developing rat neocortex

To analyze the functional consequences of hypoxia on the efficacy of intracortical inhibitory mechanisms mediated by gamma-aminobutyric acid (GABA), extra- and intracellular recordings were obtained from rat primary somatosensory cortex in vitro. Hypoxia, induced by transient N2 aeration, caused a decrease in stimulus-evoked inhibitory postsynaptic potentials (IPSPs), followed by a pronounced anoxic depolarization. Upon reoxygenation, the fast (f-) and long-latency (l-) IPSP showed a positive shift in the reversal potential by 24.4 and 14.9 mV, respectively. The peak conductance of the f- and l-IPSP was reversibly reduced in the postanoxic period by 72% and 94%, respectively. Extracellular field potential recordings and application of a paired-pulse inhibition protocol confirmed the enhanced sensitivity of inhibitory synaptic transmission for transient oxygen deprivation. Intracellular recordings from morphologically or electrophysiologically identified interneurons did not reveal any enhanced susceptibility for hypoxia as compared to pyramidal cells, suggesting that inhibitory neurons are not selectively impaired in their functional properties. Intracellularly recorded spontaneous IPSPs were transiently augmented in the postanoxic period, indicating that presynaptic GABA release was not suppressed. Developmental studies in adult (older than postnatal day 28), juvenile (P14-18), and young (P5-8) neocortical slices revealed a prominent functional resistance of immature tissue for hypoxia. In comparison with adult cortex, the hypoxia-induced reduction in excitatory and inhibitory synaptic transmission was significantly smaller in immature cortex. Our data indicate a hypoxia-induced distinct reduction of postsynaptic GABAergic mechanisms, leading to the manifestation of intracortical hyperexcitability as a possible functional consequence.

Fetal dexamethasone exposure sensitizes neonatal rat brain to hypoxia: effects on protein and DNA synthesis

Fetal exposure to glucocorticoids is known to produce long-term alterations in cell development within the central nervous system. The current study examines whether some of the adverse effects of prenatal dexamethasone treatment on brain development represent sensitization to hypoxia-induced damage. Pregnant rats were given 0.2 or 0.8 mg/kg of dexamethasone on gestational days 17, 18 and 19 and their offspring were challenged by exposure to 7% O2 on postnatal days 1 and 8. In control rats at 1 day of age, hypoxia evoked an acute decrease in protein synthesis, assessed by [3H]leucine incorporation, in both the midbrain + brainstem and forebrain. The decrease was also seen in animals receiving the low dose of dexamethasone, but was of smaller magnitude in the midbrain + brainstem than in the control cohort. At the higher dose of dexamethasone, hypoxia failed to evoke a decrease in protein synthesis; instead, protein synthesis was significantly increased. By 8 days of age, the animals receiving the lower dose of dexamethasone also displayed the anomalous increment in [3H]leucine incorporation during hypoxic challenge, whereas the effect in the high dose group was less notable. Similarly, parallel examination of incorporation of [3H]thymidine into DNA on postnatal day 1 indicated that control animals would reduce their macromolecule synthetic rate in a hypoxic environment, but that animals exposed to the high dose of dexamethasone would not; unlike the case with protein synthesis, however, the dexamethasone group never showed an increase in DNA synthesis during hypoxia. By 8 days of age, the interaction between the high dose of dexamethasone and hypoxia was no longer apparent for DNA synthesis.2

Acute stimulation of ornithine decarboxylase in neonatal rat brain regions by nicotine: a central receptor-mediated process?

Nicotine exposure during development alters central nervous system structure and function. In the current study, we examined the acute effects of nicotine (3 mg/kg) on developing rat brain by monitoring ornithine decarboxylase (ODC), a marker for perturbed cell development; ODC controls polyamine biosynthesis and thus regulates cell differentiation. Three brain regions were selected that differ both in their timetables for maturation and in nicotinic receptor concentrations: midbrain + brainstem (earliest development, highest receptor concentration), forebrain (intermediate profiles) and cerebellum (latest development and lowest receptor concentration). Nicotine caused stimulation of ODC within 1 h after drug administration, an effect that displayed both age- and region-dependence corresponding to the development of central nicotinic receptors: effects appeared earliest and were largest in magnitude in midbrain + brainstem and forebrain, and appeared last and with smaller magnitude in the cerebellum. Central receptor involvement was confirmed at 8 days postpartum by demonstrating desensitization of the response after repeated nicotine administration, and by evoking equivalent effects with direct introduction of a small dose of nicotine into the central nervous system. Later in development, acute stimulation of ODC by nicotine became less selective, reflecting secondary actions mediated through systemic hypoxia caused by the drug; this conclusion was confirmed by the absence of desensitization after repeated nicotine administration, and by the failure of centrally administered nicotine to evoke a full stimulatory response. Nicotine-induced ischemia did not contribute to stimulation of ODC seen at the 1 h time point: pretreatment with chlorisondamine, a ganglionic nicotinic antagonist, failed to alter the central stimulatory response.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in brain angiotensin II receptors in the rat

AII binding and distribution were measured in rat brain during development by autoradiographic techniques using radioiodinated [Sar1,Ile8]AII. At all ages, from 2 days to 7 weeks, binding was present in the circumventricular organs, and areas related to pituitary hormone secretion and modulation of sympathetic activity. At early stages of development, AII binding was transiently expressed in a number of motor- and sensory-related areas. These findings support a role for AII in the control of water intake and autonomic activity at all stages of development, and suggest that the peptide may be involved in the maturation of neuronal function during development.

Glucose homeostasis and hypothalamic-pituitary-adrenocortical axis during development in rats

Glucoprivation represents a model stress in which activation of different stress responses at different ages can be monitored both in vivo and in vitro. Physiological data indicate rat brain contains a liver/pancreas-type glucose sensor, yet no biochemical or immunocytochemical evidence exists for such a sensor. Young rats appear to lack normal hypothalamic glucose-sensing ability and do not show typical secretory patterns of corticotropin-releasing factor, adrenocorticotropic hormone, or corticosterone after experimentally induced glucoprivation. However, they hypersecrete catecholamines and glucagon (compared with adults) and thrive on fuel sources other than glucose that are abundant after birth. High steroid levels during the first 24 h after birth may be critical for inducing gluconeogenic enzymes and promoting differentiation of tissues like pancreas. Neonatal rats also have unique control systems to combat the damaging effects of other stresses like hypoxia; these systems may disappear in adults. Thus the definition of stress may change during development, and the compensatory mechanisms employed to combat stress change from neonatal to adult life and are intricately related to the metabolic needs of the animal.

Dual control of DNA synthesis by alpha- and beta-adrenergic mechanisms in normoxic and hypoxic neonatal rat brain

To examine how catecholamines influence cell replication in the developing brain, we examined regional [3H]thymidine incorporation into DNA after acute challenge with an alpha-adrenergic blocking agent (phenoxybenzamine) or a beta-blocker (propranolol). Phenoxybenzamine inhibited DNA synthesis in 1-day-old rat pups but the effect was less pronounced at 8 days; regional differences corresponded to transient expression of alpha-receptors and their subsequent maturational decline. Propranolol given at 1 day of age exerted a regionally selective, promotional effect on DNA synthesis; in contrast, at 8 days, propranolol inhibited DNA synthesis in all brain regions. Propranolol, but not phenoxybenzamine, also exacerbated the reduction in DNA synthesis caused by neonatal hypoxia, and again the effect was limited to the 1-day-old group. These results indicate that catecholamines exert a dual action on DNA synthesis; the effects are dependent upon maturational profiles of specific receptor populations which are either transiently expressed or which couple to cell replication only during a critical period.

Effects of acute hypoxia on neonatal rat brain: regionally selective, long-term alterations in catecholamine levels and turnover

Neonatal rats were exposed to 2 hr of hypoxia (7% O2) on the day after birth and examined for effects on development of noradrenergic and dopaminergic systems. Measurements were made of transmitter levels and turnover, the latter a biochemical index of neuronal activity. Hypoxia had a regionally selective effect, characterized by a long-lasting increase in turnover of norepinephrine and dopamine in midbrain and brainstem, with little or no effect in cerebral cortex or cerebellum. The effects of hypoxia were exacerbated when peripheral alpha-adrenergic receptors were blocked with phenoxybenzamine during the hypoxic exposure; in this case, the same abnormalities were then seen in the cerebral cortex as well. Thus, the release of peripheral catecholamines during the hypoxic insult, and their actions at alpha-adrenergic receptors, may play a role in protecting the neonatal nervous system from hypoxia-induced alterations.

Do catecholamines contribute to the effects of neonatal hypoxia on development of brain and heart? Influence of concurrent alpha-adrenergic blockade on ornithine decarboxylase activity

Hypoxia in the neonate releases catecholamines from the adrenal medulla, a response which is necessary to survive. This study examines whether a similar dependence exists for the ability of brain and heart tissue to recover from hypoxia-induced damage, as assessed by measurements of ornithine decarboxylase (ODC) activity. Hypoxia at either 1 day or 8 days of age produced a subsequent elevation of brain ODC which persisted for 1 week, a pattern known to be associated with recovery from tissue damage and delayed cellular maturation. Pretreatment of the rats with phenoxybenzamine, an alpha-receptor blocking agent, resulted in attenuation of the long-term ODC response, but did not interfere with effects on the enzyme during the hypoxia itself. In the heart, hypoxia at 8 days of age displayed similar effects, with long-term ODC elevations which were attenuated by phenoxybenzamine. Hypoxia at 1 day of age also produced long-term heart ODC stimulation, but in this case the effect was exacerbated by phenoxybenzamine, an effect consistent with the greater dependence of cardiac tissue on alpha-receptor-mediated responses to hypoxia at that age. These results suggest that alpha-receptor stimulation by catecholamines released during neonatal hypoxia play a role in the metabolic adjustment of brain and heart tissue to damage and may aid in subsequent recovery.

Role of ornithine decarboxylase and the polyamines in nervous system development: a review

Development of nervous tissue is controlled, in part, by the ornithine decarboxylase (ODC)/polyamine system. Each brain region possesses a unique ontogenetic pattern for ODC, with highest levels of the enzyme associated with periods of most rapid growth. For this reason, perturbation of the ODC profile has proven useful in examinations of teratologic mechanisms and detection of adverse environmental effects during development. More recently, the replication of neuronal cells in developing brain has been shown to require the maintenance of polyamine levels and consequently, depletion of polyamines by alpha-difluoromethylornithine (DFMO, an ODC inhibitor) arrests brain cell maturation. DFMO also interferes with neuronal migration, axonogenesis and synaptogenesis, leading to disruption of the cytoarchitectural organization of brain structures: these results imply a similarly important role for polyamines in post-replicative events. Indeed, [3H]DFMO-autoradiographic localization of ODC in developing cerebellar lamina indicates high levels of activity associated with neuropil, areas of axonal outgrowth, and post-mitotic granule cells. Axonal outgrowth during regeneration after nerve damage in the mature nervous system may display some of the same characteristics as in developing neurons, suggesting that the two processes share common polyamine-dependent mechanisms.