Mg2+-dependent modification of the N-methyl-D-aspartate receptor following graded hypoxia in the cerebral cortex of newborn piglets

The Effect of Maternal Antihypertensive Drugs on the Cerebral, Renal and Splanchnic Tissue Oxygen Extraction of Preterm Neonates

BACKGROUND

Drugs with antihypertensive action are frequently used in obstetrics for the treatment of preeclampsia (labetalol) and tocolysis (nifedipine) or for neuroprotection (MgSO4), and may affect the hemodynamics of preterm born neonates.

OBJECTIVE

The aim of this study was to assess whether maternal antihypertensive drugs affect multisite oxygenation levels of the neonate.

METHODS

Eighty preterm neonates of ≤32 weeks of gestational age were monitored using near-infrared spectroscopy. Mean cerebral, renal and splanchnic fractional tissue oxygen extractions (cFTOE, rFTOE and sFTOE) were calculated for the first 5 postnatal days. We determined the effect of various maternal antihypertensive drugs on cFTOE and rFTOE using multilevel analysis, and on sFTOE using Kruskal-Wallis and Mann-Whitney U tests.

RESULTS

Eleven infants were exposed to labetalol ± MgSO4, 7 to nifedipine ± MgSO4, 20 to MgSO4 only, and 42 to no maternal antihypertensive drugs. The infants exposed to labetalol ± MgSO4 had a lower cFTOE on days 1 (0.14, p = 0.031), 2 (0.13, p = 0.035) and 4 (0.18, p = 0.046) than nonexposed infants on the corresponding days (0.22, 0.20 and 0.24, respectively). On day 2, cFTOE was also lower in infants exposed to nifedipine ± MgSO4 (0.11, p = 0.028) and to MgSO4 only (0.15, p = 0.047). sFTOE was higher in infants exposed to labetalol ± MgSO4 on days 1 (µ = 0.71) and 2 (µ = 0.82) than in nonexposed infants (µ = 0.26, p = 0.04 and µ = 0.55, p = 0.007, respectively). Maternal antihypertensive drugs did not affect rFTOE.

CONCLUSIONS

Low neonatal cFTOE found with maternal antihypertensive drug exposure may relate to either increased cerebral perfusion or neurologic depression induced by the medication, or preferential brain perfusion associated with preeclampsia placental insufficiency. Concomitantly high sFTOE found with labetalol exposure supports the latter, while renal autoregulation may explain rFTOE stability.

Total plasma magnesium in healthy and critically ill foals

Abnormalities in total Mg (tMg) concentration in plasma and/or serum are common in critically ill humans, and the association with increased mortality has been documented in several clinical studies in adults and newborns with hypoxic-ischemic encephalopathy. Abnormalities in tMg were studied in hospitalized dogs, cats, and adult horses. Newborn foals were scarcely studied with regard to Mg concentration. The aims of the present study were: (1) to compare two analytical methods for the determination of tMg in plasma: the automated colorimetric method and the atomic absorption spectrometry; (2) to measure plasma tMg in healthy foals during the first 72 hours after birth and in sick foals during the first 72 hours of hospitalization; (3) to compare total plasma Mg concentration among healthy foals, foals affected by perinatal asphyxia syndrome (PAS), prematurity and/or dismaturity, and sepsis; (4) to evaluate tMg plasma concentration in surviving and non-surviving foals. One hundred seventeen foals were included in the study: 20 healthy and 97 sick foals. The automated method used in clinical practice probably overestimates plasma tMg. Due to its higher sensitivity and specificity, the atomic absorption spectrometry should be considered the method of choice from an analytical point of view, but requires an instrumentation not easily available in any laboratory and specific technical skills and competencies. Plasma tMg in healthy foals were included in the range 0.52 to 1.01 mmol/L and did not show any time-dependent change during the first 72 hours of life. In sick foals, tMg evaluated at T0 was statistically higher than tMg measured at subsequent times. Foals affected by PAS had a tMg at T0 significantly higher (P < 0.01) than healthy, septic, and premature and/or dysmature foals. The t test found significantly higher (P < 0.01) plasma tMg measured at T0 in non-surviving than in surviving foals. Plasma tMg could be a useful parameter for the diagnosis of PAS and the formulation of the prognosis in critically ill foals.

Delayed treatment with magnesium: reduction of brain infarction and improvement of electrophysiological recovery following transient focal cerebral ischemia in rats

Object. The authors examined whether delayed treatment with Mg++ would reduce brain infarction and improve electrophysiological and neurobehavioral recovery following cerebral ischemia—reperfusion.

Methods. Male Sprague—Dawley rats were subjected to right middle cerebral artery occlusion for 90 minutes followed by 72 hours of reperfusion. Magnesium sulfate (750 µmol/kg) or vehicle was given via intracarotid infusion at the beginning of reperfusion. Neurobehavioral outcome and somatosensory evoked potentials (SSEPs) were examined before and 72 hours after ischemia—reperfusion. Brain infarction was assessed after the rats had died.

Before ischemia—reperfusion, stable SSEP waveforms were recorded after individual fore- and hindpaw stimulations. At 72 hours of perfusion the SSEPs recorded from ischemic fore- and hindpaw cortical fields were depressed in vehicle-injected animals and the amplitudes decreased to 19 and 27% of baseline, respectively (p < 0.001). Relative to controls, the amplitudes of SSEPs recorded from both ischemic fore- and hindpaw cortical field in the Mg++-treated animals were significantly improved by 23% (p < 0.005) and 39% (p < 0.001) of baselines, respectively. In addition, Mg++ improved sensory and motor neurobehavioral outcomes by 34% (p < 0.01) and 24% (p < 0.05), respectively, and reduced cortical (p < 0.05) and striatal (p < 0.05) infarct sizes by 42 and 36%, respectively.

Conclusions. Administration of Mg++ at the commencement of reperfusion enhances electrophysiological and neurobehavioral recovery and reduces brain infarction after cerebral ischemia—reperfusion. Because Mg++ has already been used clinically, it may be worthwhile to investigate it further to see if it holds potential benefits for patients with ischemic stroke and for those who will undergo carotid endarterectomy.

Expression and Phosphorylation of N-Methyl-D-Aspartate Receptor Subunits during Graded Hypoxia in the Cerebral Cortex of Newborn Piglets

The present study tests the hypothesis that during graded hypoxia, N-methyl-D-aspartate (NMDA) receptor expression and phosphorylation are altered in the cerebral cortex of newborn piglets. Studies were performed in anesthetized, ventilated piglets, 6 normoxic and 9 exposed to different lengths of decreased fractions of inspired oxygen to achieve varying biochemical levels of phosphocreatine (PCr). P(2) membrane proteins were immunoprecipitated with antiphosphoserine, antiphosphotyrosine, or antiphosphothreonine antibodies and separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Proteins were transblotted and probed with NMDA receptor subunit 1 (NR1), NR2A or NR2B antibodies. As tissue PCr levels decreased from 3.5 to 0.5 micromol/g brain during hypoxia, NR1, NR2A and NR2B protein increased by 84, 56 and 38%, respectively. Phosphorylated serine, tyrosine and threonine residues also increased during hypoxia on the three subunits. However, the increase in subunit protein exceeded the increase in phosphorylated residues for all three subunits. Therefore, the ratio of phosphorylated/dephosphorylated serine, tyrosine and threonine residues decreased with worsening hypoxia. We speculate that an alteration in the ratio of phosphorylated/dephosphorylated residues of the NMDA receptor may regulate receptor activation during hypoxia.

Glutamate-induced differential mitochondrial response in young and adult rats

Excitatory amino acid glutamate is involved in neurotransmission in the nervous system but it becomes a potent neurotoxin under variety of conditions. However, the molecular mechanism of excitotoxicity is not known completely. We have studied the influence of glutamate on intracellular calcium and mitochondrial functions in cortical slices from young and adult rats. The slices from both the age groups exhibited comparable intracellular calcium changes upon glutamate stimulation. Glutamate treatment caused a decrease in adenosine 5'-diphosphate/adenosine 5'-triphosphate (ADP/ATP) and an increase in nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide reduced form (NAD/NADH) ratio in both the age groups but the magnitude and the nature of temporal change was different. Glutamate-induced decrease in ATP/ADP and increase in NAD/NADH ratio was significantly higher in slices from the adult as compared to the young rats. The slices from young rats elicited slightly higher mitochondrial depolarization than adult rats. However, the formation of reactive oxygen species (ROS) and lactate dehydrogenase (LDH) release were significantly higher in adult rats as compared to young rats. Glutamate-induced mitochondrial depolarization, ROS formation and LDH release were highly dependent on the presence of Ca(2+) in the extracellular medium. The treatment of slices with mitochondrial inhibitors rotenone and oligomycin inhibited ROS formation and LDH release substantially. Our results suggest that the glutamate-induced increase in intracellular calcium is not the only factor responsible for neuronal cell death but the mitochondrial functions could be crucial in excitotoxicity.

Delayed treatment with 5-nitro-6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione, a glycine site N-methyl-D-aspartate antagonist, protects against permanent middle cerebral artery occlusion in male rats

The glycine site, N-methyl-D-aspartate antagonist 5-nitro-6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione (ACEA1021) was previously tested only in models of transient stroke with pre-treatment paradigms. We therefore tested whether it would protect in two models of permanent stroke in two rat strains with delayed treatment. Intravenous ACEA1021 reduced cerebral infarction by 62% (15 min treatment delay) and 42% (2 h treatment delay), relative to vehicle-injected rats, when subjected to a modified Tamura and permanent intraluminal filament model of stroke, respectively. In comparison, intravenous nicotinamide (500 mg/kg), which was tested in separate animal cohorts, had no significant effect on infarction. These data show that ACEA1021 protects against permanent focal cerebral ischemia, even with a 2 h post-treatment delay. Characterization of the therapeutic window with longer outcome times including infarction and neurobehavioral endpoints is needed.

Magnesium-DNA interactions and the possible relation of magnesium to carcinogenesis. Irradiation and free radicals

Magnesium deficiency causes renal complications. The appearance of several diseases is related to its depletion in the human body. In radiotherapy, as well as in chemotherapy, especially in treatment of cancers with cis-platinum, hypomagnesaemia is observed. The site effects of chemotherapy that are due to hypomagnesaemia are decreased using Mg supplements. The role of magnesium in DNA stabilization is concentration dependent. At high concentrations there is an accumulation of Mg binding, which induces conformational changes leading to Z-DNA, while at low concentration there is deficiency and destabilization of DNA. The biological and clinical consequences of abnormal concentrations are DNA cleavage leading to diseases and cancer. Carcinogenesis and cell growth are also magnesium-ion concentration dependent. Several reports point out that the interaction of magnesium in the presence of other metal ions showed that there is synergism with Li and Mn, but there is magnesium antagonism in DNA binding with the essential metal ions in the order: Zn>Mg>Ca. In the case of toxic metals such as Cd, Ga and Ni there is also antagonism for DNA binding. It was found from radiolysis of deaerated aqueous solutions of the nucleoside 5'-guanosine monophosphate (5'-GMP) in the presence as well as in the absence of magnesium ions that, although the addition of hydroxyl radicals (*OH) has been increased by 2-fold, the opening of the imidazole ring of the guanine base was prevented. This effect was due to the binding of Mg2+ ions to N7 site of the molecule by stabilizing the five-member ring imitating cis-platinum. It was also observed using Fourier Transform Infrared spectroscopy, Raman spectroscopy and Fast Atom Bombardment mass spectrometry that *OH radicals subtract H atoms from the C1', C4' and C5' sites of the nucleotide. Irradiation of 5'-GMP in the presence of oxygen (2.5 x 10(-4) M) shows that magnesium is released from the complex. There is spectroscopic evidence that superoxide anions (O2-*) react with magnesium ions leading to magnesium release from the complex. From radiolysis data it was suggested that magnesium ions can act as radiosensitizers in the absence of oxygen, while in the presence of oxygen they act as protectors and stabilizers of DNA.

NMDA receptor and neonatal hypoxic brain injury

The NMDA-type glutamate receptor is a predominant mediator of excitotoxicity in the immature brain due to overexpression of the receptor in the developing brain. Within the development period however, the extent of NMDA receptor mediated processes including hypoxia-induced excitotoxicity may depend on the ontogeny of the NMDA receptor recognition and modulation sites, and subunits leading to altered function of the ion-channel comples. The function of the receptor may be modified by intracellular mechanisms such as phosphorylation/dephosphorylation, nitration, and generation of free radicals including nitric oxide. The susceptibility of the developing brain to hypoxia depends on several factors: the lipid composition of the brain cell membrane; the rate of membrane lipid peroxidation and the status of anti-oxidant defenses; the development and modulation of the NMDA receptor sites; the intracellular Ca(2+) influx mechanisms; expression of apoptotic and antiapoptotic genes such as Bax and Bcl-2; and the activation of initiator caspases and caspase-3, the "executioner" of cell death. The developmental status of these cellular mechanisms and their response to hypoxia determine the fate of the hypoxic cell in the developing brain in the fetus and the newborn.

Effect of graded hypoxia on the high-affinity CPP binding site of the NMDA receptor in the cerebral cortex of newborn piglets

Previous studies have shown that the N-methyl-D-aspartate (NMDA) receptor is modified during hypoxia in the cerebral cortex of newborn piglets. The present study tests the hypothesis that the NMDA receptor 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) high-affinity binding site is modified during hypoxia and that the degree of modification correlates with the progressive decrease in cerebral cellular energy metabolism and increase in lipid peroxidation induced by hypoxia. Studies were conducted in twelve anesthetized, ventilated newborn piglets, five normoxic and seven hypoxic which were exposed to decreased fraction of inspired oxygen (FiO2) to achieve varying phosphocreatine (PCr) levels. 3[H]-CPP binding was performed with CPP concentrations ranging from 0.5 to 1500 nM at 23 degrees C for 40 min in P2 membrane fractions. Brain tissue PCr levels were determined biochemically. Conjugated dienes (CDs) were measured as an index of lipid peroxidation. In the normoxic group, B(max) (receptor number) for the CPP binding site was 329+/-93 fmol/mg protein and Kd (dissociation constant) 137+/-44 nM, the mean PCr value was 2.5+/-0.4 micromol/g brain and the CD level was 0.0 nmol/g brain. As tissue hypoxia worsened, there was a gradual decline in tissue PCr as well as receptor B(max) and K(d) values, and there was an increase in conjugated dienes. Both the receptor B(max) (r=0.90) and Kd (r=0.72) decreased in a linear relationship as PCr decreased. As the levels of CDs increased both the receptor B(max) (r=0.88) and Kd (r=0.68) decreased in a linear fashion. The data show that there is not a critical hypoxic threshold for modification of the CPP binding site of the NMDA receptor, but that modification is coupled to a gradual decrease in brain cell energy metabolism and increase in lipid peroxidation. We speculate that hypoxia-induced modification of the NMDA receptor is mediated not only by changes in the receptor recognition site but also by an alteration of brain cell membrane structure secondary to conjugated diene formation.

Hypoxia-induced generation of nitric oxide free radicals in cerebral cortex of newborn guinea pigs

Previous studies have shown that brain tissue hypoxia results in increased N-methyl-D-aspartate (NMDA) receptor activation and receptor-mediated increase in intracellular calcium which may activate Ca++-dependent nitric oxide synthase (NOS). The present study tested the hypothesis that tissue hypoxia will induce generation of nitric oxide (NO) free radicals in cerebral cortex of newborn guinea pigs. Nitric oxide free radical generation was assayed by electron spin resonance (ESR) spectroscopy. Ten newborn guinea pigs were assigned to either normoxic (FiO2 = 21%, n = 5) or hypoxic (FiO2 = 7%, n = 5) groups. Prior to exposure, animals were injected subcutaneously with the spin trapping agents diethyldithiocarbamate (DETC, 400 mg/kg), FeSO4.7H2O (40 mg/kg) and sodium citrate (200mg/kg). Pretreated animals were exposed to either 21% or 7% oxygen for 60 min. Cortical tissue was obtained, homogenized and the spin adducts extracted. The difference of spectra between 2.047 and 2.027 gauss represents production of NO free radical. In hypoxic animals, there was a difference (16.75+/-1.70 mm/g dry brain tissue) between the spectra of NO spin adducts identifying a significant increase in NO free radical production. In the normoxic animals, however, there was no difference between the two spectra. We conclude that hypoxia results in Ca2+-dependent NOS mediated increase in NO free radical production in the cerebral cortex of newborn guinea pigs. Since NO free radicals produce peroxynitrite in presence of superoxide radicals that are abundant in the hypoxic tissue, we speculate that hypoxia-induced generation of NO free radical will lead to nitration of a number of cerebral proteins including the NMDA receptor, a potential mechanism of hypoxia-induced modification of the NMDA receptor resulting in neuronal injury.

Differential expression of glutamate receptor subtypes in human brainstem sites involved in perinatal hypoxia-ischemia

This study delineates the development of N-methyl-D-aspartate (NMDA) and non-NMDA receptor binding in the human brainstem, particularly as it relates to issues of the trophic effects of glutamate, the glutamate-mediated ventilatory response to hypoxia, and regional excitotoxic vulnerability to perinatal hypoxia-ischemia. We used tissue autoradiography to map the development of binding to NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA), and kainate receptors in brainstem sites involved in the glutamate ventilatory response to hypoxia, as well as recognized sites vulnerable to perinatal hypoxia-ischemia. NMDA receptor/channel binding was virtually undetectable in all regions of the human fetal brainstem at midgestation, an unexpected finding given the trophic role for NMDA receptors in early central nervous system maturation in experimental animals. In contrast, non-NMDA (AMPA and kainate) receptor binding was markedly elevated in multiple nuclei at midgestation. Although NMDA binding increased between midgestation and early infancy to moderately high adult levels, AMPA binding dramatically fell over the same time period to low adult levels. High levels of kainate binding did not change significantly between midgestation and infancy, except for an elevation in the infant compared with fetal inferior olive; after infancy, kainate binding decreased to negligible adult levels. Our data further suggest a differential development of components of the NMDA receptor/channel complex. This baseline information is critical in considering glutaminergic mechanisms in human brainstem development, physiology, and pathology.

Effect of hypoxia on DNA fragmentation in different brain regions of the newborn piglet

DNA fragmentation has been studied in different regions of the newborn piglet brain following different times of normobaric hypoxia (5% O(2), 95% N(2)). After 1 hr of hypoxia, fragmented DNA was observed in cerebellum, cortex, hippocampus, and striatum but not in hypothalamus. More fragmentation occurred in these areas of the brain when the animals were kept under hypoxia for times up to 8 hr 45 min. When the animals were submitted to hypoxia for two and a half hours, integrity of DNA was recovered respectively after 3 hr of exposure to the ambient atmosphere in hippocampus and striatum, but 4 hr of recovery were necessary for cerebellum and cortex. These results are discussed in terms of the consequences of neonatal hypoxia and apnea for newborn infants and economical impact for farm animals.