Brain biochemical and behavioral changes in 4 weeks old rats after neonatal oxygen deprivation

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.

Effect of hypoxia/ischemia on bicuculline-induced seizures in immature rats: behavioral and electrocortical phenomena

The relation between hypoxia/ischemia and subsequent alterations in seizure susceptibility in developing brain remains unclear. We assessed the behavioral and electrocorticographic (ECoG) effects of hypoxic/ischemic brain damage on bicuculline (BIC)-induced seizures in 7-day postnatal rats, and determined maturational changes in seizure susceptibility, behavior and ECoG activity. Rat pups were subjected to unilateral common carotid artery ligation, followed by exposure to 8% O2 at 37 degrees C for 2 h, an insult that produces brain damage in the cerebral hemisphere ipsilateral to carotid artery occlusion. The experimental group consisted of rat pups previously subjected to hypoxia/ischemia; control littermates received neither arterial ligation nor systemic hypoxia. Experimental animals received 4, 5, or 6 mg/kg BIC subcutaneously (s.c.) at 2 and 24 h, and at 3, 7, and 21 days of recovery from hypoxia/ischemia. Two animals at each interval of recovery, 1 each from the experimental and control groups, were used for ECoG monitoring. After BIC injection, animal behavior was observed for 2 h. Behaviors and seizures were classified in five categories based on severity, duration, and character: 1, mild irritability; 2, few clonic seizures and agitation; 3, few tonic-clonic seizures with swimming movements; 4, frequent tonic-clonic seizures with apneic episodes; 5, continuous tonic-clonic seizures and death. Rat pups previously subjected to hypoxia/ischemia had lesser seizure susceptibility than controls at 2-h recovery (p < 0.05) and greater susceptibility than controls at 24 h (p < 0.05). Tonic seizures were prominent at 2 and 24 h in both the experimental and control groups, whereas lesion-sided circling was prominent only in the hypoxic/ischemic rat pups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of early postnatal anoxia on adult learning and emotion in rats

Cognitive functioning, behavioural attention and anxiety were studied in adult male Wistar rats after early postnatal anoxia. Spatial memory performance in the hole board learning task was impaired in anoxic rats when compared with control animals. Attention assessed by the behavioural immobility response to a sudden reduction in background noise was tested in an open field. In anoxic rats this response was reduced compared to controls, which was further reflected in a higher ambulation score in anoxic animals. The emotional state in adult rats after postnatal anoxia was not affected as was demonstrated in the elevated plus-maze and in the shock probe/defensive burying task. The results indicate that adult cognitive deficits after neonatal anoxia are not related to changes in emotional behaviour. Disruption of behavioural attention or the capacity of concentration to task performance may, however, contribute to the observed cognitive impairment.

The effects of prenatal exposure to hypoxia on the behavior of rats during their life span

The aim of this study was to investigate the influence of moderate prenatal damage on adaptability during the juvenile, adult, and senile phases. Pregnant rats were exposed to a 12% normobaric hypoxia from day 1 to 17 postconception. Pregnancy was normal in both the treated animals and the controls. Erythrocytes, hemoglobin, and hematocrit did not increase in the treated pregnant animals. During the first 3 weeks, the F1 generation showed developmental deviations in physiological characteristics. Throughout subsequent ontogeny, motor performance, cognitive ability, and adaptability to physical stress were determined with a test battery of varying demands. Some of the differences (e.g., locomotor activity, learning ability) between juvenile untreated and treated rats disappeared during the adult phase. Motor and coordinative abilities, however, remained partially impaired in the old rats, especially under high demands. This study, and previous findings with alcohol (37), indicate that prenatal exposure to a noxa may result in a highly differentiated brain injury pattern. Depending on the different functions, damage may intensify age-dependent adaptive disorders or provoke impairment without influencing the course of development.

Altered catecholaminergic behavioral and hormonal responses in rats following early postnatal hypoxia

We have previously reported alterations in a battery of behavioral functions in the rat following both intermittent and chronic prenatal hypoxia. In this species, the critical brain growth spurt for the catecholaminergic neurotransmitter system takes place in the late gestational and early postnatal period. In addition, postnatal stress can modify adult hypothalamic-pituitary-adrenal responsiveness. Following a given stress, administration of dopaminergic/adrenergic agonists/antagonists may elucidate subtle changes that are not apparent in routine behavioral and endocrine tests. To test the hypothesis that early postnatal hypoxia affects development of the catecholaminergic system and, thus, alters functional outcome, we performed the following study. We exposed 25 litters of Sprague-Dawley rats, each consisting of 10 male pups, to hypoxia (10.5% inspired O2) for 6 h/day (0900 to 1500 h) from postnatal day (P) 2 to 10. We also had 25 control (C) litters. We then performed a series of behavioral tests in immature and mature animals. Body weights were significantly decreased in hypoxic (H) animals from P10 to P100. At P21 we tested locomotor activity in an open-field paradigm with drug challenge (apomorphine, a dopamine receptor agonist, 0.025 and 0.1 mg/kg; or haloperidol, a dopamine receptor antagonist, 0.2 and 0.4 mg/kg). Grooming activity was significantly decreased in H animals at both apomorphine concentrations, compared to controls. Moreover, rearing activity was significantly increased in H animals under basal conditions and when challenged with 0.1 mg/kg apomorphine. Apomorphine (1.0 mg/kg)-induced stereotypy at P39 was significantly increased in H animals compared to controls. Open-field activity at 80 days revealed no significant differences in drug responsiveness between H and C animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of monoamine systems after neonatal anoxia in rats

Neurochemical and morphological effects of neonatal anoxia on monoamine systems were studied after 100% N2 exposure for 25 min at 30 h postnatally (postnatal day 2-P2). At 20 min after anoxia, reductions of tissue levels of cerebellar noradrenaline (NA) and striatal dopamine (DA) and metabolites were seen, while 5-hydroxyindoleacetic acid (5-HIAA) was increased in cortex and cerebellum. At P7, NA increased in cerebellum, while serotonin (5-HT) and 5-HIAA decreased in cortex and cerebellum. At P21, increased hippocampal NA and striatal homovanillic acid (HVA) were found, while striatal 5-HT decreased and 5-HIAA increased in striatum and hippocampus. At P60, striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-HIAA levels were found to be enhanced. No effects were seen on 5-HT, tyrosine hydroxylase, or DARPP-32 immunostaining in cortex, hippocampus, and striatum. Thus, the neonatal anoxia induced both acute and persistent neurochemical abnormalities in monoamine systems that were not accompanied by morphological changes detectable with the methods used. The monoamine alterations found could be critically connected to the behavioral disturbances observed in rats after neonatal anoxia. The findings may also be of relevance to dysfunctions seen in humans after perinatal oxygen deficiency, e.g., the attention deficit hyperactivity disorder syndrome.

Effects of postnatal ganglioside administration and hypoxia-exposure on the dopamine release from striatal slices, the behaviour and the ganglioside pattern of 2-3 months old rats

Neonatal rats were injected with a mixture of bovine brain gangliosides (30 mg/kg body weight, s.c.) immediately before the exposure to hypoxia (pO2 = 10 kPa, 10 hrs daily) from the 2nd to the 11th day of life. At the age of 2-3 months the potential protective or restitutive effect of gangliosides on the radiolabelled dopamine release from striatal slices and on the conditioned avoidance learning was studied. No change in the content and the pattern of gangliosides of the rat striata was found after exposure to hypoxia and ganglioside administration, respectively. Both hypoxia exposure and ganglioside treatment of controls increased the dopamine release whereas hypoxic animals treated with gangliosides showed a diminished release. Hypoxia-induced impaired conditioned avoidance learning was improved by ganglioside treatment. The changed release of dopamine and the altered behavioural performance after ganglioside treatment alone indicate the necessity of a very cautious application of gangliosides to the developing brain.

Lasting effects of postnatal hypoxia and saline injection on the striatal dopamine transport and their modification by gangliosides

Hypobaric hypoxia (10 h daily, pO2 10 kPa) and saline administration (2.5 microliters/g body wt) from the 2nd till the 11th day of life both induced a long-lasting increase of the low-affinity dopamine (DA) uptake capacity in S1-fractions of the rat striatum. Additionally, the potassium-stimulated DA release was enhanced in adult control rats postnatally injected with saline. The administration of a mixture of bovine brain gangliosides (30 micrograms/g body wt) was found to prevent these effects. However, the kinetic constants of the DA uptake of hypoxic rats treated with gangliosides were reduced in comparison to untreated controls. Thus, the effects of gangliosides appear to differ between hypoxic and control conditions. The modification of the dopaminergic activity during brain development is discussed as a possible mechanism of the preventive effects of gangliosides against long-term cerebral dysfunctions following hypoxia or stress.

Perinatal hypoxia and subsequent development of seizures

Perinatal asphyxia has been implicated in the pathogenesis of persistent convulsive disorders later in life. Whether epilepsy is the result of oxygen deficiency alone or is due to the combined effect of hypoxia and ischemia is not known. In this report we studied the role of perinatal hypoxia alone on the development of epilepsy. One day and ten days old rat pups were exposed to prolonged hypoxia (6% O2). The subsequent susceptibility to focally elicited (kindled) or generalized (flurothyl) seizures was determined in the fourth week of age. Rats exposed to hypoxia were not more susceptible to the development of either type of seizures when compared to controls. Since the equivalent degree of hypoxia used for the 1 day old rat has previously been shown to result in lasting neurochemical and behavioral alterations and the degree of hypoxia used for the 10 day rat was lethal in over 35% of the animals, it is suggested that oxygen deficiency in and of itself may not be sufficient to lead to the development of epilepsy.

Alteration in the postnatal ontogeny of cerebellar norepinephrine content following chronic prenatal carbon monoxide

The postnatal ontogeny of norepinephrine content in the cortex and cerebellum was determined in rats exposed prenatally to a chronic low level of carbon monoxide (150 parts per million). In the cerebellum, norepinephrine concentration and total norepinephrine content among carbon monoxide-exposed rats were consistently elevated over that of control rats from the second through the sixth postnatal weeks. In the cortex, norepinephrine concentration and total norepinephrine content among carbon monoxide-exposed rats did not differ from that of control rats over the same period. These results identify the cerebellum as a region whose postnatal development is altered by prenatal exposure to low levels of carbon monoxide-induced hypoxia.

Persisting effects on adult brain monoamines of neonatal distress and carbon monoxide exposure

Perinatal factors causing distress or trauma to the neonate are often linked to behavioral abnormalities in development and later life; however, little is known of the long-term effects of these experiences on neurotransmitter systems. This study used carbon monoxide exposure to produce a slow onset and recovery hypoxic episode of two hours in 5-day-old rat pups. This required placement of the pups in a hypothermic environment (air exposure) isolated from the dam so the effects of this distressing experience were also studied. When the brains of these animals were assayed at adulthood, air exposure was found to have increased dopamine and reduced 5-hydroxytryptamine levels in the frontal cortex. Norepinephrine levels were reduced in both midbrain and pons-medulla but the 5-hydroxytryptamine level was increased in midbrain. Hypoxia, induced with carbon monoxide, prevented changes in 5-hydroxytryptamine levels in cortex and brain stem and modified effects on norepinephrine. However, hypoxia resulted in significant decreases in striatal dopamine levels accompanied in females by increased norepinephrine and 5-hydroxytryptamine levels. The data reported here indicate that neonatal distress can have considerable impact on developing neurotransmitter systems. Furthermore, brain region and sex influence the outcome. Carbon monoxide-induced hypoxia, under the same conditions, produced a different pattern of change with particular predeliction for the striatum. Again there were regional and sex differences in the neurotransmitter effects. It is concluded that this hypoxic model may be valuable in the study of developmental disorders.

Serotonin metabolism in neonatal rat brain during asphyxia and recovery

Neonatal rats were exposed to 20 or 30 min of total or partial oxygen deprivation. During asphyxia and subsequent recovery the endogenous levels of tryptophan and 5-hydroxytryptamine (5-HT, serotonin) were measured. The activity of tryptophan hydroxylase, the first and rate limiting enzyme in the 5-HT synthesis pathway, was studied in vivo by measuring the accumulation of 5-hydroxytryptophan (5-HTP) after inhibition of aromatic L-amino acid decarboxylase with NSD 1015. During asphyxia there was a decrease in tryptophan hydroxylase activity in the whole brain and various regions studied. The levels of tryptophan, 5-HTP and 5-HT all increased after 30 min of recovery from asphyxia. In the whole brain, 5-HTP and 5-HT levels were normal 2 h after anoxia while tryptophan levels normalized more slowly to reach control values after 6 h. In the regional brain study, the 5-HTP levels returned quickly to control levels after asphyxia in the striatum and midbrain but not in the brainstem and hemispheres regions. The whole brain 5-HTP and 5-HT levels did not differ from controls 24 to 48 h after the asphyxia. Although the neonatal nervous system exhibits a great resistance to asphyxia, the metabolism of the neurotransmitter 5-HT is affected already during a short period of asphyxia and subsequent recovery. As 5-HT is ascribed important neurotransmitter functions, this might be relevant to the neurological sequelae of human asphyxia neonatorum.