Effect of anoxia on striatal monoamine metabolism in immature rat brain compared with that of hypoxia: an in vivo microdialysis study

Tetrahydrobiopterin in antenatal brain hypoxia-ischemia-induced motor impairments and cerebral palsy

Antenatal brain hypoxia-ischemia, which occurs in cerebral palsy, is considered a significant cause of motor impairments in children. The mechanisms by which antenatal hypoxia-ischemia causes brain injury and motor deficits still need to be elucidated. Tetrahydrobiopterin is an important enzyme cofactor that is necessary to produce neurotransmitters and to maintain the redox status of the brain. A genetic deficiency of this cofactor from mutations of biosynthetic or recycling enzymes is a well-recognized factor in the development of childhood neurological disorders characterized by motor impairments, developmental delay, and encephalopathy. Experimental hypoxia-ischemia causes a decline in the availability of tetrahydrobiopterin in the immature brain. This decline coincides with the loss of brain function, suggesting this occurrence contributes to neuronal dysfunction and motor impairments. One possible mechanism linking tetrahydrobiopterin deficiency, hypoxia-ischemia, and neuronal injury is oxidative injury. Evidence of the central role of the developmental biology of tetrahydrobiopterin in response to hypoxic ischemic brain injury, especially the development of motor deficits, is discussed.

Effect of perinatal asphyxia and carbamazepine treatment on cortical dopamine and DOPAC levels

Background

One of the most important manifestations of perinatal asphyxia is the occurrence of seizures, which are treated with antiepileptic drugs, such as carbamazepine. These early seizures, combined with pharmacological treatments, may influence the development of dopaminergic neurotransmission in the frontal cortex. This study aimed to determine the extracellular levels of dopamine and its main metabolite DOPAC in 30-day-old rats that had been asphyxiated for 45 min in a low (8%) oxygen chamber at a perinatal age and treated with daily doses of carbamazepine. Quantifications were performed using microdialysis coupled to a high-performance liquid chromatography (HPLC) system in basal conditions and following the use of the chemical stimulus.

Results

Significant decreases in basal and stimulated extracellular dopamine and DOPAC content were observed in the frontal cortex of the asphyxiated group, and these decreases were partially recovered in the animals administered daily doses of carbamazepine. Greater basal dopamine concentrations were also observed as an independent effect of carbamazepine.

Conclusions

Perinatal asphyxia plus carbamazepine affects extracellular levels of dopamine and DOPAC in the frontal cortex and stimulated the release of dopamine, which provides evidence for the altered availability of dopamine in cortical brain areas during brain development.

Pre-existing hypoxia is associated with greater EEG suppression and early onset of evolving seizure activity during brief repeated asphyxia in near-term fetal sheep

Spontaneous antenatal hypoxia is associated with high risk of adverse outcomes, however, there is little information on neural adaptation to labor-like insults. Chronically instrumented near-term sheep fetuses (125 ± 3 days, mean ± SEM) with baseline PaO2 < 17 mmHg (hypoxic group: n = 8) or > 17 mmHg (normoxic group: n = 8) received 1-minute umbilical cord occlusions repeated every 5 minutes for a total of 4 hours, or until mean arterial blood pressure (MAP) fell below 20 mmHg for two successive occlusions. 5/8 fetuses with pre-existing hypoxia were unable to complete the full series of occlusions (vs. 0/8 normoxic fetuses). Pre-existing hypoxia was associated with progressive metabolic acidosis (nadir: pH 7.08 ± 0.04 vs. 7.33 ± 0.02, p<0.01), hypotension during occlusions (nadir: 24.7 ± 1.8 vs. 51.4 ± 3.2 mmHg, p<0.01), lower carotid blood flow during occlusions (23.6 ± 6.1 vs. 63.0 ± 4.8 mL/min, p<0.01), greater suppression of EEG activity during, between, and after occlusions (p<0.01) and slower resolution of cortical impedance, an index of cytotoxic edema. No normoxic fetuses, but 4/8 hypoxic fetuses developed seizures 148 ± 45 minutes after the start of occlusions, with a seizure burden of 26 ± 6 sec during the inter-occlusion period, and 15.1 ± 3.4 min/h in the first 6 hours of recovery. In conclusion, in fetuses with pre-existing hypoxia, repeated brief asphyxia at a rate consistent with early labor is associated with hypotension, cephalic hypoperfusion, greater EEG suppression, inter-occlusion seizures, and more sustained cytotoxic edema, consistent with early onset of neural injury.

Influence of acute progressive hypoxia on cardiovascular variability in conscious spontaneously hypertensive rats

The purpose of this study is to examine the influence of acute progressive hypoxia on cardiovascular variability and striatal dopamine (DA) levels in conscious, spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). After preparation for measurement, the inspired oxygen concentration of rats was decreased to 10% within 5 min (descent stage), maintained at 10% for 10 min (fixed stage), and then elevated back to 20% over 5 min (recovery stage). The systolic blood pressure (SBP) and heart rate (HR) variability at each stage was calculated to evaluate the autonomic nervous system response using the wavelet method. Striatal DA during each stage was measured using in vivo microdialysis. We found that SHR showed a more profound hemodynamic response to progressive hypoxia as compared to WKY. Cardiac parasympathetic activity in SHR was significantly inhibited by acute progressive hypoxia during all stages, as shown by the decrease in the high frequency band of HR variability (HR-HF), along with transient increase in sympathetic activity during the early hypoxic phase. This decrease in the HR-HF continued even when SBP was elevated. Striatal DA levels showed the transient similar elevation in both groups. These findings suggest that acute progressive hypoxic stress in SHR inhibits cardiac parasympathetic activity through reduction of baroreceptor reflex sensitivity, with potentially severe deleterious effects on circulation, in particular on HR and circulatory control. Furthermore, it is thought that the influence of acute progressive hypoxia on striatal DA levels is similar in SHR and WKY.

Regulation of cytochrome oxidase redox state during umbilical cord occlusion in preterm fetal sheep

The preterm fetus is capable of surviving prolonged periods of severe hypoxia without neural injury for much longer than at term. To evaluate the hypothesis that regulated suppression of brain metabolism contributes to this remarkable tolerance, we assessed changes in the redox state of cytochrome oxidase (CytOx) relative to cerebral heat production, and cytotoxic edema measured using cerebral impedance, during 25 min of complete umbilical cord occlusion or sham occlusion in fetal sheep at 0.7 gestation. Occlusion was followed by rapid, profound reduction in relative cerebral oxygenation and EEG intensity and an immediate increase in oxidized CytOx, indicating a reduction in electron flow down the mitochondrial electron transfer chain. Confirming rapid suppression of cerebral metabolism there was a loss of the temperature difference between parietal cortex and body at a time when carotid blood flow was maintained at control values. As occlusion continued, severe hypotension/hypoperfusion developed, with a further increase in CytOx levels to a plateau between 8 and 13 min and a progressive rise in cerebral impedance. In conclusion, these data strongly suggest active regulation of cerebral metabolism during the initial response to severe hypoxia, which may help to protect the immature brain from injury.

Endogenous alpha2-adrenergic receptor-mediated neuroprotection after severe hypoxia in preterm fetal sheep

Central alpha-adrenergic receptor activity is important for fetal adaptation to hypoxia before birth. It is unclear whether it is also important during recovery. We therefore tested the hypothesis that an infusion of the specific alpha(2)-adrenergic receptor antagonist idazoxan (1 mg/kg/h i.v.) from 15 min to 4 h after profound hypoxia induced by 25 min umbilical cord occlusion in fetal sheep at 70% of gestation (equivalent to the 28-32 weeks in humans) would increase neural injury. After 3 days' recovery, idazoxan infusion was associated with a significant increase in neuronal loss in the hippocampus (P<0.05), expression of cleaved caspase-3 (P<0.05), and numbers of activated microglia (P<0.05). There was no significant effect on other neuronal regions or on loss of O4-positive premyelinating oligodendrocytes in the subcortical white matter. Idazoxan was associated with an increase in evolving epileptiform electroencephalographic (EEG) transient activity after occlusion (difference at peak 2.5+/-1.0 vs. 11.7+/-4.7 counts/min, P<0.05) and significantly reduced average spectral edge frequency, but not EEG intensity, from 54 until 72 h after occlusion (P<0.05). Hippocampal neuronal loss was correlated with total numbers of epileptiform transients during idazoxan infusion (P<0.01; r(2)=0.7). In conclusion, endogenous inhibitory alpha(2)-adrenergic receptor activation after severe hypoxia appears to significantly limit evolving hippocampal damage in the immature brain.

Dopamine transporters are involved in the onset of hypoxia-induced dopamine efflux in striatum as revealed by in vivo microdialysis

Although many studies have revealed alterations in neurotransmission during ischaemia, few works have been devoted to the neurochemical effects of mild hypoxia, a situation encountered during life in altitude or in several pathologies. In that context, the present work was undertaken to determine the in vivo mechanisms underlying the striatal dopamine efflux induced by mild hypoxaemic hypoxia. For that purpose, the extracellular concentrations of dopamine and its metabolite 3,4-dihydroxyphenyl acetic acid were simultaneously measured using brain microdialysis during acute hypoxic exposure (10% O(2), 1h) in awake rats. Hypoxia induced a +80% increase in dopamine. Application of the dopamine transporters inhibitor, nomifensine (10 microM), just before the hypoxia prevented the rise in dopamine during the early part of hypoxia; in contrast the application of nomifensine after the beginning of hypoxia, failed to alter the increase in dopamine. Application of the voltage-dependent Na(+) channel blocker tetrodotoxin abolished the increase in dopamine, whether administered just before or after the beginning of hypoxia. These data show that the neurochemical mechanisms of the dopamine efflux may change over the course of the hypoxic exposure, dopamine transporters being involved only at the beginning of hypoxia.

Effect of moderate hypercapnic hypoxia on cerebral dopaminergic activity and brain O2 uptake in intrauterine growth-restricted newborn piglets

There is evidence that intrauterine growth restriction (IUGR) is associated with altered dopaminergic function in the immature brain. Compelling evidence exists that in the newborn brain, specific structures are especially vulnerable to O2 deprivation. The dopaminergic system is shown to be sensitive to O2 deprivation in the immature brain. However, the respective enzyme activities have not been measured in the living neonatal brain after IUGR under hypercapnic hypoxia (H/H). Therefore, 18F-labeled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography was used to estimate the aromatic amino acid decarboxylase activity of the brain of seven normal weight (body weight 2078 +/- 434 g) and seven IUGR newborn piglets (body weight 893 +/- 109 g). Two positron emission tomography scans were performed in each piglet. All animals underwent a period of normoxia and moderate H/H. Simultaneously, cerebral blood flow was measured with colored microspheres and cerebral metabolic rate of O2 was determined. In newborn normal-weight piglets, the rate constant for FDOPA decarboxylation was markedly increased in mesostriatal regions during H/H, whereas brain oxidative metabolism remained unaltered. In contrast, moderate H/H induced in IUGR piglets a marked reduction of clearance rates for FDOPA metabolites (p <0.05), which was accompanied by a tendency of lowering the rate constant for FDOPA conversion. Furthermore, IUGR piglets maintained cerebral O2 uptake in the early period of H/H, but during the late period of H/H, a significantly reduced cerebral metabolic rate of O2 occurred (p <0.05). Thus, IUGR is accompanied by a missing activation of dopaminergic activity and attenuated brain oxidative metabolism during moderate H/H. This may indicate endogenous brain protection against O2 deprivation.

Alterations in metabolism and gap junction expression may determine the role of astrocytes as ?good samaritans? or executioners

Our knowledge of astroglia and their physiological and pathophysiological role(s) in the central nervous system (CNS) has grown during the past decade, revealing a complex picture. It is becoming increasingly clear that glia play a significant role in the homeostasis and function of the CNS and that neurons should no longer be considered the only cell type that responds, both rapidly and slowly, to electrochemical activity. We discuss recent advances in the field with an emphasis on the impact of hypoxia and ischemia on astrocytic metabolism and the functional relationship between glucose metabolism and gap junctions in astrocytes. We also address the controversy over whether astrocytic gap junctions mediate protection or killing of neurons during or after hypoxic or ischemic insults.

Perinatal nicotine attenuates the hypoxia-induced up-regulation of tyrosine hydroxylase and galanin mRNA in locus ceruleus of the newborn mouse

The effect of perinatal nicotine exposure on the hypoxic response in the newborn mouse was examined, with special reference to the catecholaminergic system. We studied transcripts for the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) and the neuropeptide galanin (GAL) in locus ceruleus (LC) and adrenal medulla at different times after birth and postnatal hypoxia. We thereafter investigated how perinatal nicotine affected these mRNA levels, as well as the ability of the newborn to survive severe hypoxia. TH mRNA levels increased postnatally in both LC and adrenals, reaching peak values at 24 h postnatally and thereafter stabilizing at lower levels. GAL mRNA also increased in the LC but did not decrease after 24 h. Acute hypoxia (5% O(2) for 60 min) elicited increases in TH and GAL mRNA levels in the LC after 24 h. However, TH mRNA levels in the adrenals did not change. Perinatal nicotine exposure increased mortality after hypoxia (from 0% to 16.9%). Moreover, hypoxia-induced increases in TH and GAL mRNA levels in the LC were not observed in nicotine-treated pups. Nicotine also decreased basal TH mRNA levels in the adrenals. The present results suggest (1) that the postnatal increases in adrenal TH mRNA levels are not directly due to hypoxia at birth, and (2) that the increased mortality seen after hypoxia in nicotine pups concurs with a perturbed LC function in these animals. A deficient catecholamine synthesis in the adrenals may also contribute to the detrimental effect of prenatal exposure to nicotine on the response to hypoxia.

Effect of hypoxia/hypercapnia on metabolism of 6-[(18)F]fluoro-L-DOPA in newborn piglets

There is evidence that the dopaminergic system is sensitive to altered p(O(2)) in the immature brain. However, the respective enzyme activities have not been measured in the living neonatal brain together with brain oxidative metabolism. Therefore 18F-labelled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography (PET) was used to estimate the activity of the aromatic amino acid decarboxylase (AADC) in the brain of fifteen newborn piglets (2-5 days old). Two PET scans were performed in each piglet. Eleven animals underwent a period of normoxia and moderate hypoxia/hypercapnia (H/H). The remaining four animals were used as an untreated control group. Simultaneously, the brain tissue p(O(2)) was recorded, the regional cerebral blood flow (CBF) was measured with colored microspheres and the cerebral metabolic rate of oxygen (CMRO(2)) was determined. In addition, in four untreated and six H/H treated piglets the relative amounts of fluorodopamine and the respective metabolites were determined in brain tissue samples using HPLC analysis. H/H conditions were induced by lowering the inspired fraction of oxygen from 0.35 to 0.10 and adding CO(2) to the inspired gas resulting in an arterial p(CO(2)) between 74 and 79 mmHg. H/H elicited a more than 3-fold increase of the CBF (P<0.05) so that the CMRO(2) remained unchanged throughout the H/H period. Despite this, the brain tissue p(O(2)) was reduced from 19+/-4 to 6+/-3 mmHg (P<0.05). The permeability-surface area product of FDOPA (PS(FDOPA)) was unchanged. However, the transfer rate of FDOPA (k(3)(FDOPA)) of the nigrostriatal dopaminergic system and the relative amounts of fluorodopamine and the respective metabolites were significantly increased (P<0.05). It is suggested that H/H induces an increase of AADC activity. However, an H/H-induced CBF increase maintains bulk O(2) delivery and preserves CMRO(2).

Striatal perfusion of indomethacin attenuates dopamine increase in immature rat brain exposed to anoxia: an in vivo microdialysis study

Using in vivo microdialysis and HPLC, we examined the effects of indomethacin on extracellular dopamine (DA) in the striatum of immature rats submitted to anoxia. Rat pups in two indomethacin groups received intrastriatal perfusion of either 1 mM or 5 mM indomethacin throughout the experiment. The DA level reached 1185+/-400% of the basal level during anoxia; in contrast, the peak levels of DA were only 307+/-63%, 153+/-35% in indomethacin groups (p<0.05). We consider that this suppression would be one of the mechanisms of the protective effect of indomethacin on hypoxic ischemic encephalopathy.

Anoxic and hypoxic immature rat model for measurement of monoamine using in vivo microdialysis

The immature brain is considered relatively resistant to anoxia and ischemia. Although hypoxia without ischemia has not been considered to produce brain damage in immature rats as well as in adult rats (S. Levine, Anoxic-ischemic encephalopathy in rats, Am. J. Pathol., 36 (1960) 1-17 [8]; D.E. Levy, J.B. Brieley, D.G. Silverman, F. Plum, Brief hypoxia-ischemia initially damages cerebral neurons, Arch. Neurol., 32 (1975) 450-456 [9]; J.E. Rice, R.C. Vannucci, J.B., Brieriey, The influence of immaturity on hypoxic-ischemic brain damage in rat, Ann. Neurol., 9 (1981) 131-141 [14]), hypoxia in postnatal period is possible to cause a functional brain damage (T. Hender, P. Lundborg, Regional changes in monoamine synthesis in the developing rat brain during hypoxia, Acta. Physiol. Scand., 106 (1979) 139-143 [3]; W. Ihle, J. Gross, R. Moller, Effect on chronic postnatal hypoxia on dopamine uptake by synaptosomes from striatum of adult rats, Biomed. Biochem. Acta., 44 (1985) 433-437 [7]; A. Lun, J. Gross, M. Beyer, H.D. Fischer, C. Wustmann, J. Schmidt, K. Hecht, The vulnerable period of perinatal hypoxia with regard to dopamine release and behavior in adult rats, Biomed. Biochem. Acta., 45 (1986) 619-627 [10]). Using microdialysis, we studied the anoxic or hypoxic effect on catecholamine metabolism in immature rat brain by measuring extracellular concentrations of norepinephrine (NE), dopamine (DA), and its metabolites and also 5-hydroxyindole-3-acetic acid (5-HIAA), the serotonin metabolite. DA is a well established excitatory neurotransmitter (R.C. Vannucci, Experimental biology of cerebral hypoxia-ischemia: relation to perinatal brain damage, Pediatr. Res., 27 (1990) 317-326 [16]), and in the previous report using hypoxic 7-day-old rat pups increase of DA was not detected without additional stimulations (K. Gordon, D. Johnston, M.V. Robinson, T.E. Statman, J.B. Becker, F. Silverstein, Transient hypoxia alters striatal catecholamine metabolism in immature brain: An in vivo microdialysis study, J. Neurochem., 54 (1990) 605-611 [2]). Whereas recently in newborn piglets, hypoxic hypoxia produced increase of extracellular DA (C.-C. Huang, N.S. Lajevardi, O. Tammela, A. Pastuszko, Relationship of extracellular dopamine in striatum of newborn piglets to cortical oxygen pressure, Neurochem. Res., 19 (1994) 649-655 [6]; Olano, M., Song, D., Murphy, S., Wilson, D. F. and Pastuszko, A., Relationships of dopamine, cortical oxygen pressure, and hydroxyl radicals in brain of newborn piglets during hypoxia and posthypoxic recovery, J. Neurochem., 65 (1995) 1205-1212 [13]). We consider that hypoxic ischemic brain damage of human newborns that we can treat is a damage, which does not show overt neuropathological changes. We therefore tried to show that transient anoxia and hypoxia caused biochemical alteration if the exposure did not produce marked morphological changes. This rodent model is adequate to study perinatal asphyxia and alteration of monoamine level could be useful for evaluation of brain damage, even if it is not detected histologically.

Effect of N-methyl-D-aspartate and potassium on striatal monoamine metabolism in immature rat: an in vivo microdialysis study

Effects of N-methyl-D-aspartate (NMDA) and potassium on 5-day-old rat's brain were examined. We measured extracellular striatal monoamines such as dopamine (DA), 3,4 dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindole-3-acetic acid (5-HIAA) using intracerebral microdialysis. After 3 h stabilization, pups received varying concentrations of NMDA (1-3 mM) and potassium (200-800 mM) by intrastriatal perfusion for 32 minutes. Increasing the concentration of NMDA and potassium induced a dose related DA increase (p < 0.001), whereas DOPAC, HVA, and 5-HIAA decreased significantly. Five days later the same animals were sacrificed and the weight reduction of their cerebral hemispheres was measured. The weight of the drug perfused side was significantly reduced compared with that of the contralateral one. We examined next the relationship between the level of maximum DA and the relative hemisphere weight reduction. The DA peak was highly correlated with the hemisphere weight reduction (r = 0.70, n = 52, p < 0.001 in the NMDA group, r = 0.83, n = 30, p < 0.001 in the potassium group, respectively). These data show that each treatment alter striatal monoamine metabolism in immature rat brain and that the extracellular DA peak is a potential early indicator to estimate brain injury.

Short-term fasting alters neonatal rat striatal dopamine levels and serotonin metabolism: an in vivo microdialysis study

Although frequent feeding is necessary for neonatal brains, rat pups were usually separated from their dams throughout a microdialysis experiment. First, in 5-day-old rats, we examined the effect of probe insertion on initial fluctuation of extracellular striatal monoamines using in vivo microdialysis and subsequent HPLC. Second, fasting effect on monoamine metabolism was examined with or without fasting; the latter was regarded as controls. Extracellular striatal DA in the fasting group decreased promptly to 60% of the basal level in the first 2 h, and reached 50% by the end of the experiment. Dopamine in the fasting group decreased more markedly than in the control group (P < 0.01 by ANOVA) which also decreased to about 80% of the basal level. Extracellular 5-hydroxyindole-3-acetic acid (5-HIAA) continuously increased (P < 0.01), and the serum concentration of tryptophan also increased in the fasting group (P < 0.001). We showed that extracellular striatal monoamine levels fluctuated especially in the first 2 h and fasting altered monoamine metabolism. Therefore, it should take at least 2 h after surgery to stabilize the animals and obtain adequate basal levels. In addition, we should consider that these alterations occur when we use fasting animals as controls in microdialysis studies.

Magnesium attenuates a striatal dopamine increase induced by anoxia in the neonatal rat brain: an in vivo microdialysis study

We evaluated the effects of magnesium on extracellular dopamine (DA) and its metabolites in the striatum of 5-d-old rats submitted to 16 min of anoxia using microdialysis and HPLC. Rat pups were divided into three groups and received either 1) intrastriatal perfusion (IS) of MgSO4, 2) intraperitoneal injection (IP) of MgSO4, and 3) NaCl and Ringer's solution, respectively in place of MgSO4. After stabilization, Mg2+, saline, and Ringer's solution were administered; then, 114 animals were exposed to 100% nitrogen for 16 min. Anoxia induced a DA surge, an acutely marked increase of DA, in both the control and the IP group. In contrast, the DA surge was significantly suppressed in the IS group (p < 0.01, analysis of variance). During anoxia, the plasma Mg2+ in the IP group, but not in the IS group, maintained a significantly higher level compared with the basal level. On the other hand, Mg2+ in the perfusates in the IS group, but not in the IP group, maintained a significantly high level during anoxia. Alterations induced by anoxia in other metabolites, 3,4-dihydroxyphenylacetic acid, homovanillic acid, norepinephrine, and 5-hydroxyindole-3-acetic acid, did not significantly differ among the three groups. We propose that elevated levels of Mg2+ in the striatum had inhibitory effects on the DA surge during anoxia.