D2 receptor imaging in neonates using I-123 iodobenzamide brain SPECT

PURPOSE

Hypoxic-ischemic injury induces early changes in cerebral energy that later lead to the presence and extension of brain damage and subsequently to severe neurodevelopmental impairments such as the dyskinetic form of cerebral palsy, which is associated with damage to the striatum. The purpose of the current study was to evaluate the viability of D2 receptors in the perinatal period using I-123 iodobenzamide brain SPECT and to correlate this with early neurologic status.

METHODS

After obtaining informed parental consent, 12 full-term neonates with hypoxic-ischemic events were included. I-123 iodobenzamide brain SPECT was performed 1 week after birth, corresponding to a gestational age of 39.2+/-1.7 weeks. Images were acquired using a brain-dedicated gamma camera 1 hour after intravenous injection of 30 MBq (0.8 mCi) I-123 iodobenzamide. Magnetic resonance images (T2 weighted sequence: repetition time/echo time: 2,000/30 to 150) of the brains of the same neonates were acquired on the same day.

RESULTS

The right and left striatum:cerebellum activity ratios were between 1.28 and 2.25, with the greatest concentration of I-123 iodobenzamide occurring in the striatum area. A tendency of the striatum:cerebellum ratio to decrease was observed as the severity of the perinatal hypoxic-ischemic event increased despite striatal hypersignal on magnetic resonance imaging in only two neonates.

CONCLUSIONS

This study, which confirms that I-123 iodobenzamide could be used in the neonatal period, shows the biochemical maturation of D2 receptors as early as 1 week after birth and also suggests the deleterious effect of perinatal hypoxic-ischemic events on D2 receptors.

Imaging anoxic depolarization during ischemia-like conditions in the mouse hemi-brain slice

Focal ischemia evokes a sudden loss of membrane potential in neurons and glia of the ischemic core termed the anoxic depolarization (AD). In metabolically compromised regions with partial blood flow, peri-infarct depolarizations (PIDs) further drain energy reserves, promoting acute and delayed neuronal damage. Visualizing and quantifying the AD and PIDs and their acute deleterious effects are difficult in the intact animal. In the present study, we imaged intrinsic optical signals to measure changes in light transmittance in the mouse coronal hemi-brain slice during AD generation. The AD was induced by oxygen/glucose deprivation (OGD) or by ouabain exposure. Potential neuroprotective strategies using glutamate receptor antagonists or reduced temperature were tested. Eight minutes of OGD (n = 18 slices) or 4 min of 100 microM ouabain (n = 14) induced a focal increase of increased light transmittance (LT) in neocortical layers II/III that expanded concentrically to form a wave front coursing through neocortex and independently through striatum. The front was coincident with a negative voltage shift in extracellular potential. Wherever the LT front (denoting cell swelling) propagated, a decrease in LT (denoting dendritic beading) followed in its wake. In addition the evoked field potential was permanently lost, indicating neuronal damage. Glutamate receptor antagonists did not block the onset and propagation of AD or the extent of irreversible damage post-AD. Lowering temperature to 25-30 degrees C protected the tissue from OGD damage by inhibiting AD onset. This study shows that anoxic depolarization evoked by global ischemia-like conditions is a spreading process that is focally initiated at multiple sites in cortical and subcortical gray. The combined energy demands of O(2)/glucose deprivation and the AD greatly exacerbate neuronal damage. Glutamate receptor antagonists neither block the AD in the ischemic core nor, we propose, block recurrent PID arising close to the core.

Calcitonin potentiates oxygen-glucose deprivation-induced neuronal death

Calcitonin is a hormone that decreases plasma calcium through inhibition of osteolysis. It is used in the treatment of osteoporosis and other bone disorders. Salmon calcitonin is typically utilized in individuals for whom use of estrogen is contraindicated, for example, women at high risk for breast cancer. In addition to actions on bone, calcitonin may have effects on the central nervous system. Receptors for calcitonin are present on central neurons, and salmon calcitonin has been shown to alter neuronal activity. Since salmon calcitonin is used clinically, and it can have actions on neurons, the present studies were designed to determine whether salmon calcitonin could alter death of cortical neurons. The effects of salmon calcitonin on neuronal death induced by exposure of murine cortical cultures to serum deprivation, staurosporine, oxygen-glucose deprivation, kainate, and NMDA were tested. Salmon calcitonin had no effect on apoptotic cell death in cortical cultures. However, acute treatment with salmon calcitonin (1-1000 nM) caused significant potentiation of neuronal death induced by oxygen-glucose deprivation. Similarly, salmon calcitonin potentiated cell death induced by exposure to kainate. In contrast, it did not potentiate cell death induced by exposure to NMDA. In fact, addition of a high concentration (1000 nM) of salmon calcitonin attenuated NMDA toxicity. These results indicate that calcitonin is not a survival factor for cortical neurons; however, it can alter excitotoxic cell death. The most interesting, and disturbing, effect is the ability of low concentrations of salmon calcitonin to potentiate oxygen-glucose deprivation-induced cell death.

Impairment of mitochondrial respiration after cerebral hypoxia-ischemia in immature rats: relationship to activation of caspase-3 and neuronal injury

Mitochondrial damage may play a key role in the development of necrotic and apoptotic hypoxic-ischemic (HI) brain damage. It has previously been shown that mitochondrial respiration is depressed in the cerebral cortex after HI in neonatal animals. The aim of the present study was to further characterize the time course of the mitochondrial impairment during reperfusion and the correlation between the respiratory control ratio and brain injury and activation of caspase-3. Rat pups were subjected to unilateral carotid artery ligation and exposed to hypoxia (7.7% oxygen). Mitochondrial respiration was measured 0-72 h after HI in a mitochondrial fraction isolated from cerebral cortex. Microtubule associated protein-2 (MAP2) and caspase-3 were analyzed with immunoblotting in cerebral cortex homogenates. In addition, the time course of caspase-3 activation was measured as DEVD cleavage. The mitochondrial respiratory control ratio in cerebral cortex decreased immediately after HI followed by a partial recovery at 3-8 h. Thereafter, a secondary drop occurred with a minimum reached at 24 h of reperfusion. The secondary loss of respiratory function was accompanied by depletion of MAP2, cleavage of caspase-3 and an increased caspase-3 -like activity at 3-24 h after the insult. In conclusion, the primary phase of mitochondrial dysfunction was paralleled by a moderate decrease of MAP2 and a limited activation of caspase-3. The secondary mitochondrial impairment was associated with neuronal injury and pronounced activation of caspase-3.

Alterations in glutathione and amino acid concentrations after hypoxia-ischemia in the immature rat brain

Hypoxic-ischemic brain injury involves an increased formation of reactive oxygen species. Key factors in the cellular protection against such agents are the GSH-associated reactions. In the present study we examined alterations in total glutathione and GSSG concentrations in mitochondria-enriched fractions and tissue homogenates from the cerebral cortex of 7-day-old rats at 0, 1, 3, 8, 14, 24 and 72 h after hypoxia-ischemia. The concentration of total glutathione was transiently decreased immediately after hypoxia-ischemia in the mitochondrial fraction, but not in the tissue, recovered, and then decreased both in mitochondrial fraction and homogenate after 14 h, reaching a minimum at 24 h after hypoxia-ischemia. The level of GSSG was approximately 4% of total glutathione and increased selectively in the mitochondrial fraction immediately after hypoxia-ischemia. The decrease in glutathione may be important in the development of cell death via impaired free radical inactivation and/or redox related changes. The effects of hypoxia-ischemia on the concentrations of selected amino acids varied. The levels of phosphoethanolamine, an amine previously reported to be released in ischemia, mirrored the changes in glutathione. GABA concentrations initially increased (0-3 h) followed by a decrease at 72 h. Glutamine levels increased, whereas glutamate and aspartate were unchanged up to 24 h after the insult. The results on total glutathione and GSSG are discussed in relation to changes in mitochondrial respiration and microtubule associated protein-2 (MAP2) which are reported on in accompanying paper [64].

NBQX attenuates excitotoxic injury in developing white matter

The excitatory neurotransmitter glutamate is released from axons and glia under hypoxic/ischemic conditions. In vitro, oligodendrocytes (OLs) express non-NMDA glutamate receptors (GluRs) and are susceptible to GluR-mediated excitotoxicity. We evaluated the role of GluR-mediated OL excitotoxicity in hypoxic/ischemic white matter injury in the developing brain. Hypoxic/ischemic white matter injury is thought to mediate periventricular leukomalacia, an age-dependent white matter lesion seen in preterm infants and a common antecedent to cerebral palsy. Hypoxia/ischemia in rat pups at postnatal day 7 (P7) produced selective white matter lesions and OL death. Furthermore, OLs in pericallosal white matter express non-NMDA GluRs at P7. Unilateral carotid ligation in combination with hypoxia (6% O(2) for 1 hr) resulted in selective, subcortical white matter injury with a marked ipsilateral decrease in immature and myelin basic protein-expressing OLs that was also significantly attenuated by 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX). Intracerebral AMPA demonstrated greater susceptibility to OL injury at P7 than in younger or older pups, and this was attenuated by systemic pretreatment with the AMPA antagonist NBQX. These results indicate a parallel, maturation-dependent susceptibility of immature OLs to AMPA and hypoxia/ischemia. The protective efficacy of NBQX suggests a role for glutamate receptor-mediated excitotoxic OL injury in immature white matter in vivo.

Ischemia-induced phosphorylation of initiation factor 2 in differentiated PC12 cells: role for initiation factor 2 phosphatase

An in vitro model of ischemia was obtained by subjecting PC12 cells differentiated with nerve growth factor to a combination of glucose deprivation plus anoxia. Immediately after the ischemic period, the protein synthesis rate was significantly inhibited (80%) and western blots of cell extracts revealed a significant accumulation of phosphorylated eukaryotic initiation factor 2, alpha subunit, eIF2(alphaP) (42%). Upon recovery, eIF2(alphaP) levels returned to control values after 30 min, whereas protein synthesis was still partially inhibited (33%) and reached almost control values within 2 h. The activities of the mammalian eIF2alpha kinases, double-stranded RNA-activated protein kinase, mammalian GCN2 homologue, and endoplasmic reticulum-resident kinase, were determined. None of the eIF2alpha kinases studied showed increased activity in ischemic cells as compared with controls. Exposure of cells to cell-permeable inhibitors of protein phosphatases 1 and 2A, calyculin A or tautomycin, induced dose- and time-dependent accumulation of eIF2(alphaP), mimicking an ischemic effect. Protein phosphatase activity, as measured with [(32)P]phosphorylase a as a substrate, diminished during ischemia and returned to control levels upon 30-min recovery. In addition, the rate of eIF2(alphaP) dephosphorylation was significantly lower in ischemic cells, paralleling both the greatest translational inhibition and the highest eIF2(alphaP) levels. The endogenous phosphatase activity from control and ischemic extracts showed different sensitivity to inhibitor 2 and fostriecin in in vitro assays, inhibitor-2 effect in ischemic cells being lower than in control cells. Together these results indicate that an eIF2alpha phosphatase, probably protein phosphatase 1, is implicated in the ischemia-induced eIF2(alphaP) accumulation in PC12 cells.

NOC/oFQ PKC-dependent superoxide generation contributes to hypoxic-ischemic impairment of NMDA cerebrovasodilation

This study determined whether nociceptin/orphanin FQ (NOC/oFQ) generates superoxide anion (O(2)(-)) in a protein kinase C (PKC)-dependent manner and whether such production contributes to hypoxic-ischemic (H-I) impairment of N-methyl-D-aspartate (NMDA)-induced pial artery dilation in newborn pigs equipped with closed cranial windows. Superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was an index of O(2)(-) generation. Under non-H-I conditions, topical NOC/oFQ (10(-10) M, concentration present in cerebrospinal fluid after I or H-I) increased SOD-inhibitable NBT reduction from 1 +/- 1 to 20 +/- 3 pmol/mm(2). PKC inhibitors staurosporine and chelerythrine (10(-7) M) blunted NBT reduction (1 +/- 1 to 7 +/- 2 pmol/mm(2) for chelerythrine), whereas the NOC/oFQ receptor antagonist [F/G]NOC/oFQ (1-13)-NH(2) (10(-6) M) blocked NBT reduction. [F/G]NOC/oFQ(1-13)-NH(2) and staurosporine also blunted the NBT reduction observed after I or H-I. NMDA (10(-8), 10(-6) M)-induced pial artery dilation was reversed to vasoconstriction after H-I. The NOC/oFQ antagonist staurosporine and free radical scavengers partially prevented this impaired dilation (sham: 9 +/- 1 and 16 +/- 1; H-I: -5 and -10 +/- 1; H-I staurosporine pretreated: 3 +/- 1 and 6 +/- 1%). These data show that NOC/oFQ increased O(2)(-) production in a PKC-dependent manner and contributed to this production after insult and that NOC/oFQ contributed to impaired NMDA-induced pial artery dilation after H-I, suggesting, therefore, that PKC-dependent O(2)(-) generation by NOC/oFQ links NOC/oFQ release to impaired NMDA dilation after H-I.

Chemical preconditioning with 3-nitropropionic acid in gerbil hippocampal slices: therapeutic window and the participation of adenosine receptor

Ischemic tolerance induced by a subtoxic dose of neurotoxin, 3-nitropropionic acid (3-NPA), was recently reported as "chemical preconditioning." We electrophysiologically investigated the therapeutic window and the effect via action at the adenosine receptor using a gerbil hippocampal slice model of the tolerance phenomenon. 3-NPA at the dose of 4 mg/kg was administered intraperitoneally at 3, 24, and 72 h prior to slice preparation. Prolonged delay to hypoxic depolarization (HD) and improvement of the field excitatory postsynaptic potential recovery following a fixed period of hypoxia (8 min) were observed in the groups pretreated at 3 and 24 h compared with the control (P < 0.05). Correlation between the delay to HD and the recovery was highly significant (r = 0.37, P < 0.001). These effects were completely reversed by administration of theophylline (20 mg/kg), an adenosine receptor blocker. These findings indicate that chemical preconditioning with 3-NPA induces early onset (3 h) and long-lasting (24 h) tolerance of hypoxic damage to excitatory synaptic mechanisms in the hippocampus by delayed calcium entry, and the activation of adenosine receptor contributes to this neuroprotective effect.

Developmental downregulation of ATP-sensitive potassium conductance in astrocytes in situ

In many neural and non-neural cells, ATP-sensitive potassium (K(ATP)) channels couple the membrane potential to energy metabolism. We investigated the activation of K(ATP) currents in astrocytes of different brain regions (hippocampus, cerebellum, dorsal vagal nucleus) by recording whole-cell currents with the patch-clamp technique in acute rat brain slices. Pharmacological tools, hypoglycemia and specific compounds in the pipette solution (cAMP, UDP), were used to modulate putative K(ATP) currents. The highest rate of K(ATP) specific currents was observed with a pipette solution containing cAMP and external stimulation with diazoxide (0.3 mM). The diazoxide-activated current had a reversal potential negative to -80 mV and was inhibited by tolbutamide (0.2 mM). We found that not all cells activated a K(ATP) current, and that the portion of cells with functional K(ATP) channel expression was developmentally downregulated. Whereas diazoxide activated K(ATP) currents in 57% of the astrocytes in rats aged 8-11 days (n = 21), the rate decreased to 38% at 12-15 days (n = 29) and to 8% at 16-19 days (n = 12). No significant difference was observed for the three brain regions. In recordings without cAMP in the internal solution, only 21% (12-15 days; n = 19) or none (16-19 days; n = 7), respectively, showed a potassium current upon diazoxide application. This metabolically regulated potassium conductance may be of importance, particularly in immature astrocytes with a complex current pattern, which have a relatively high input resistance: K(ATP) currents activated by energy depletion may hyperpolarize the cells, or stabilize a negative resting potential during depolarizing stimuli mediated, e.g., by glutamate receptors and/or uptake carriers.

Enhancement of whole cell calcium currents following transient MCAO

Cerebral infarctions have been shown to cause widespread changes of neuronal excitability in non-infarcted tissue. Calcium currents are major determinants of neuronal behavior, and pathological modulation of Ca(2+)-channels is known to lead to altered excitability states in a variety of paradigms. In the present study we addressed the question to what extent whole cell calcium currents are altered after middle cerebral artery occlusion (MCAO) in both the ipsi- and contralateral sensory cortex. Transient middle cerebral artery occlusion was induced for 1 h in rats using the intraluminal thread model. After 7 or 28 days survival, whole cell patch clamp studies were carried out on freshly isolated neurons of the ipsi- and contralateral sensory cortex, and high voltage activated (HVA) calcium currents were examined. In lesioned animals, we found a significant increase of calcium current amplitude and maximal conductance in the sensory cortex contralateral to the infarcts. This was paralleled by a prominent positive shift of the potential of half-maximal activation (V(h,a)) in these cells. Changes were long-lasting and at least stable for the following 28 days. These alterations were present in animals with lesions of moderate size, but not in those with massive infarction, and only in the cortex contralateral to the lesion. Following cortical infarctions, changes of calcium current properties are selectively observed in neurons contralateral to the lesion. At the behavioral level, compensatory mechanisms involving the unaffected hemisphere may induce this alteration of calcium current properties.

Role of cAMP and K(+) channel-dependent mechanisms in piglet hypoxic/ischemic impaired nociceptin/orphanin FQ-induced cerebrovasodilation

This study was designed to determine the role of altered cAMP and K(+) channel-dependent mechanisms in impaired pial artery dilation to the newly described opioid, nociceptin/orphanin FQ (NOC/oFQ) following hypoxia/ischemia in newborn pigs equipped with a closed cranial window. Recent studies have observed that NOC/oFQ elicits pial dilation via release of cAMP, which, in turn, activates the calcium sensitive (K(ca)) and the ATP-dependent K(+) (K(ATP)) channel. Global cerebral ischemia (20 min) was induced via elevation of intracranial pressure, while hypoxia (10 min) decreased pO(2) to 35+/-3 mm Hg with unchanged pCO(2). Topical NOC/oFQ (10(-8), 10(-6) M) induced vasodilation was attenuated by ischemia/reperfusion (I+R) and reversed to vasoconstriction by hypoxia/ischemia/reperfusion (H+I+R) at 1 h of reperfusion (control, 9+/-1 and 16+/-1%; I+R, 3+/-1 and 6+/-1%; H+I+R, -7+/-1 and -12+/-1%). Such altered dilation returned to control values within 4 h in I+R animals and within 12 h in H+I+R animals. NOC/oFQ dilation was associated with elevated CSF cAMP in control animals but such biochemical changes were attenuated in I+R animals and reversed to decreases in cAMP concentration in H+I+R animals (control, 1037+/-58 and 1919+/-209 fmol/ml; I+R, 1068+/-33 and 1289+/-30 fmol/ml; H+I+R, 976+/-36 and 772+/-27 fmol/ml for absence and presence of NOC/oFQ 10(-6) M, respectively). Topical 8-Bromo cAMP (10(-8), 10(-6) M) pial dilation was unchanged by I+R but blunted by H+I+R (control, 10+/-1 and 20+/-1%; I+R, 11+/-1 and 20+/-2%; H+I+R, 0+/-1 and 0+/-2%). Pituitary adenylate cyclase activating polypeptide and cromakalim, adenylate cyclase and K(ATP) channel activators, respectively, elicited dilation that was blunted by both I+R and H+I+R while NS1619, a K(ca) channel activator, elicited dilation that was unchanged by I+R but blunted by H+I+R. These data indicate that impaired NOC/oFQ dilation following I+R results form altered adenylate cyclase and K(ATP) channel-dependent mechanisms. These data further indicate that impaired NOC/oFQ dilation following H+I+R results not only from altered adenylate cyclase and K(ATP) channel but also from altered cAMP and K(ca) channel-dependent mechanisms.

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.

Effects of brain oxygenation on metabolic, hemodynamic, ionic and electrical responses to spreading depression in the rat

The effect of cortical spreading depression (CSD) on oxygen demand (extracellular K(+)), oxygen supply (cerebral blood flow - CBF) and oxygen balance (mitochondrial NADH) was studied by a special multiprobe assembly (MPA), during hypoxia and partial ischemia. The MPA was constructed and applied to monitor the CSD wave from its front line until complete recovery, continuously and simultaneously. CSD under hypoxia or partial ischemia led to an initial increase in NADH levels and a further decrease in CBF during the first phase of the CSD wave, indicating a decrease of tissue capability to compensate for an increase in oxygen demand. Furthermore, the special design of the MPA enabled identifying the close interrelation between oxygen demand, supply and balance during CSD propagation. In conclusion, brain oxygenation was shown to have a clear effect on tissue responses to CSD.

Production of macrophage inflammatory protein-2 following hypoxia/reoxygenation in glial cells

Polymorphonuclear neutrophils (PMNs) are known to mediate brain inflammation following hypoxia/reoxygenation (H/R), but the precise mechanisms leading to PMN recruitment are undefined. The alpha-chemokine macrophage inflammatory protein-2 (MIP-2) has specificity for the recruitment of PMNs. In this study, we found that 8 or 12 h of hypoxia followed by 24-h reoxygenation (H8/R24 or H12/R24) induced MIP-2 secretion in cultures of enriched microglia or mixed glia, respectively. Microglia, however, could not survive longer duration (>12 h) of hypoxia. Astrocytes did not produce any significant amount of MIP-2 even though astrocytes maintained 98-99% viability following H12/R24. We also found that microglia survived the H/R treatment better (following H24) in the presence of astrocytes (mixed glial culture) than in microglia-enriched culture. Reoxygenation for prolonged periods (3 and 5 days) following H24 resulted in progressively larger increases in MIP-2 production (20- and 60-fold, respectively) in mixed glial cultures. Immunocytochemical staining revealed that the cells expressing MIP-2 in response to H/R were microglia rather than astrocytes in mixed glial cultures. Examination of MIP-2 mRNA expression showed that H/R upregulated MIP-2 gene expression. Taken together, our data suggest that microglial cells are an important source of MIP-2 production and suggest a potential injury mechanism involving brain-derived production of MIP-2 in H/R.

N-methyl-D-aspartate but not glutamate induces the release of hydroxyl radicals in the neonatal rat: modulation by group I metabotropic glutamate receptors

Although they likely involve activation of N-methyl-D-aspartate (NMDA) receptors, the mechanisms giving rise to perinatal hypoxic-ischemic-induced damages remained unclear. The purpose of the present study was to investigate in vivo the mechanisms regulating the glutamate-induced release of toxic hydroxyl radicals (.OH) in neonatal rat. Anesthetized 7-day-old Wistar rat pups bearing a microdialysis cannula implanted in the striatum were perfused with a solution containing salicylate as an.OH trap. Hydroxyl radicals formation was evaluated, after a 3 hr postoperative delay, by measuring the 2,3-DHBA levels by HPLC/EC before, during and over 3 hr after the administration of glutamatergic agonists or antagonists. Administration of NMDA and of ibotenate dramatically increased the efflux of.OH, 17-fold and sixfold, respectively. Glutamate, used at the same concentration did not produce any significant increase in the.OH release and may even decrease this efflux when given at larger concentrations. The NMDA-induced.OH response was partially but progressively reduced by glutamate coinjection and completely blunted by DHPG [(RS)-3, 5-dihydroxyphenylglycine], a group I metabotropic glutamate receptor agonist. Conversely, AIDA [(RS)-1-aminoindan-1,5-dicarboxylic acid], an antagonist of the same receptors, unmasked an.OH response to glutamate. These results are evidence that the glutamate-induced activation of a group I metabotropic glutamate receptor normally protected the neonatal brain from any glutamate activation of NMDA receptor, which otherwise would produce the release of toxic hydroxyl radicals. Targeting group I metabotropic glutamate receptors and/or.OH might contribute to protecting the neonatal brain against perinatal hypoxic-ischemic induced lesions.

31P-MRS-based determination of brain intracellular and interstitial pH: its application to in vivo H+ compartmentation and cellular regulation during hypoxic/ischemic conditions

In the last decade, significant progress has been made in the characterization of pH regulation in nervous tissue in vitro. However, little work has been directed at understanding how pH regulatory mechanisms function in vivo. We are interested in how ischemic acidosis can effect pH regulation and modulate the extent of post-ischemic brain damage. We used 31P-MRS to determine normal in vivo pH(i) and pH(e) simultaneously in both the isolated canine brain and the intact rat brain. We observed that the 31P(i) peak in the 31P-MRS spectrum is heterogeneous and can be deconvoluted into a number of discrete constituent peaks. In a series of experiments, we identified these peaks as arising from either extracellular or intracellular sources. In particular, we identified the peak representing the neurons and astrocytes and showed that they maintain different basal pH (6.95 and 7.05, respectively) and behave differently during hypoxic/ischemic episodes.

Sublethal hypoxia up-regulates corticotropin releasing factor receptor type 1 in fetal hippocampal neurons

We analysed the influence of oxygen-glucose deprivation (OGD) on the expression of corticotropin releasing factor (CRF) receptors (CRF-R1 and CRF-R2) in fetal hippocampal neurons in vitro. A 2 h exposure of neurons to OGD resulted in death of 18+/-2.8% cells at 24 h following exposure, which was considered sublethal hypoxia. Expression of both receptors was quantitated by competitive reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting. Levels of mRNA for CRF-R1 were increased 3.2-fold compared to control neurons, while CRF-R2 mRNA levels remained unchanged. The increase of CRF-R1 mRNA levels was observed at 6 h and peaked at 24 h. CRF-R1 protein levels were also increased by 2.4-fold and 1.7-fold at 24 h and 48 h, respectively. These data suggest that the effects of CRF on neuronal survival are mediated in part through the induction and expression of CRF-R1 following a hypoxic/ischemic insult.

Two distinct components of initial glutamate release synchronized with anoxic depolarization in rat global brain ischemia

Numerous reports have suggested that anoxic depolarization is a critical event in the pathogenesis of cerebral ischemia. Extracellular glutamate concentration ([Glu]e) is closely related to the pathogenesis of ischemia. Therefore, these pathogenic mechanisms merit study, especially the relationship between [Glu]e elevation and the ionic basis of early changes in membrane potential after ischemic insult in vivo. It is often presumed from electrophysiological studies that a causal relationship exists between impaired glutamate uptake and/or progressive glutamate increase and anoxic depolarization, but few in vivo reports have found any sign of a progressive increase of [Glu]e elevation preceding anoxic depolarization. Recently, we reported the application of an oxygen-independent real-time technique for monitoring glutamate levels in the extracellular space during in vivo ischemia, and demonstrated that the massive glutamate release during ischemia is biphasic. In the present study, using this real-time monitoring system, we carried out a more detailed analysis of the initial events in the first phase of glutamate release during ischemia-induced anoxic depolarization. The shape of the rising slope that forms the peak of the first phase suggested two components. The second component was approximately 10 times steeper than the first, with two different components of the rise on the way to the peak of the biphasic [Glu]e elevation. This is the first report to demonstrate these components of the initial glutamate peak, and suggests a progressive second component of the [Glu]e increase preceding Ca2+-dependent release from synaptic vesicles with anoxic depolarization, in vivo.

NMDA blockade attenuates caspase-3 activation and DNA fragmentation after neonatal hypoxia-ischemia

The aim was to study the effects of an NMDA receptor antagonist on caspase-3 activation and DNA fragmentation after hypoxia-ischemia (HI) in 7-day-old rats. Animals were treated with vehicle or MK-801 (0.5 mg/kg) directly after HI and sacrificed 8, 24 or 72h later. MK-801 reduced injury (by 53%), cells positive for active caspase-3 (by 39%) and DNA fragmentation (by 79%) in the cerebral cortex. Furthermore, MK-801 significantly decreased caspase-3 activity, and Western blots revealed a tendency towards decreased proteolytic cleavage of the caspase-3 proform. The data imply that NMDA receptors are involved in the activation of apoptotic processes in the immature brain after HI.

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

Role of hypoxia-inducible factor-1 in hypoxia-induced ischemic tolerance in neonatal rat brain

Hypoxia-inducible factor-1 (HIF-1) is a heterodimer composed of HIF-1alpha and HIF-1beta protein subunits. This transcription factor is essential for the activation of hypoxia-inducible genes like erythropoietin, some glucose transporters, the glycolytic enzymes, and vascular endothelial growth factor. Because HIF-1 activation may promote cell survival in hypoxic tissues, we studied the effect of hypoxic preconditioning on HIF-1 expression in neonatal rat brain. Hypoxic preconditioning (8% O2 for 3 hours), a treatment known to protect the newborn rat brain against hypoxic-ischemic injury, markedly increased HIF-1alpha and HIF-1beta expression. To support the role of HIF-1 in protective preconditioning, we also studied the effect of two other known HIF-1 inducers, cobalt chloride (CoCl2) and desferrioxamine (DFX), on HIF-1 expression and neuroprotection in newborn brain. HIF-1alpha and HIF-1beta protein levels were markedly increased after intraperitoneal injection of CoCl2 (60 mg/kg) and moderately increased after intraperitoneal injection of DFX (200 mg/kg) 1 to 3 hours after the injections. Preconditioning with CoCl2 or DFX 24 hours before hypoxia-ischemia afforded 75 and 56% brain protection, respectively, compared with that in vehicle-injected littermate controls. Thus, HIF-1 activation could contribute to protective brain preconditioning, which could be used in high-risk deliveries and other clinical situations.

BDNF protects the neonatal brain from hypoxic-ischemic injury in vivo via the ERK pathway

Neurotrophins activate several different intracellular signaling pathways that in some way exert neuroprotective effects. In vitro studies of sympathetic and cerebellar granule neurons have demonstrated that the survival effects of neurotrophins can be mediated via activation of the phosphatidylinositol 3-kinase (PI3-kinase) pathway. Neurotrophin-mediated protection of other neuronal types in vitro can be mediated via the extracellular signal-related protein kinase (ERK) pathway. Whether either of these pathways contributes to the neuroprotective effects of neurotrophins in the brain in vivo has not been determined. Brain-derived neurotrophic factor (BDNF) is markedly neuroprotective against neonatal hypoxic-ischemic (H-I) brain injury in vivo. We assessed the role of the ERK and PI3-kinase pathways in neonatal H-I brain injury in the presence and absence of BDNF. Intracerebroventricular administration of BDNF to postnatal day 7 rats resulted in phosphorylation of ERK1/2 and the PI3-kinase substrate AKT within minutes. Effects were greater on ERK activation and occurred in neurons. Pharmacological inhibition of ERK, but not the PI3-kinase pathway, inhibited the ability of BDNF to block H-I-induced caspase-3 activation and tissue loss. These findings suggest that neuronal ERK activation in the neonatal brain mediates neuroprotection against H-I brain injury, a model of cerebral palsy.

Caspase-cleaved actin (fractin) immunolabelling of Hirano bodies

Hirano bodies are eosinophilic rod-like inclusions that are found predominantly in neuronal processes in the hippocampal CA1 sector with increasing age and are particularly numerous in Alzheimer's disease. They contain a variety of cytoskeletal epitopes, especially actin and actin-binding proteins. Actin cleavage by cysteinyl aspartate-specific proteases (caspases) is a feature of apoptosis. Cleavage at aspartate 244 generates N-terminal 32 kDa and C-terminal 15 kDa actin fragments. This has led to the development of a rabbit polyclonal antibody specific for caspase-cleaved actin, directed to the last five C-terminal amino acids of the 32 kDa fragment of actin ('fractin'). Fractin immunohistochemistry was performed on hippocampal sections from Alzheimer's disease and control cases containing numerous Hirano bodies, in addition to immunolabelling with CM1 antiserum which recognizes activated caspase-3. The Hirano bodies showed strong diffuse fractin immunoreactivity. They did not label with CM1 antiserum, perhaps reflecting too low a level of activated caspase-3 for immunodetection, or involvement of a different member of the caspase family. The finding of fractin immunoreactivity of Hirano bodies suggests that caspase-like cleavage of actin may play a role in their formation and further supports caspase-like activity in neuronal processes, distinct from that associated with acute perikaryal apoptosis.