In vivo [31P]NMR studies on the influence of age on rat brain hypoxia

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.

The effect of age on susceptibility to brain damage in a model of global hemispheric hypoxia-ischemia

Stroke occurs in all age groups, ranging from the newborn to the elderly. The immature brain is generally believed to be more resistant to the damaging effects of cerebrovascular compromise compared to the more mature brain. However, recent experiments suggest that the correlation between brain damage and age is not linear. To determine the effects of age and development on hypoxic-ischemic brain damage, we developed a model whereby rats of increasing age received identical cerebrovascular insults, and assessed neuropathologic outcome. Male Wistar rats of 1, 3, 6, and 9 weeks and 6 months underwent unilateral common carotid artery ligation and exposure to 12% oxygen for 35 min. Animals were all spontaneously breathing under light halothane anesthesia (0.5%). Core temperatures were maintained at 37 degrees C. Blood pressures were monitored via indwelling carotid artery catheters on the side ipsilateral to the carotid artery ligation. Cerebral blood flow was assessed in separate groups utilizing Laser Doppler flowmetry. Physiologic monitoring revealed that under these experimental conditions, mean arterial blood pressure and cerebral blood flow decreased to the same extent in each of the age groups, verifying that all animals experienced an identical insult. Neuropathologic assessment at 7 days of recovery showed that brain damage was most severe in the 1 and 3 week old animals followed by those that were 6 months. The 6 and 9 week old groups had significantly less injury than the other 3 age groups. Hippocampal damage was most severe in the 3 week and 6 month old rats compared to all other age groups. Our findings contrast previously held beliefs regarding the enhanced tolerance of the immature brain to hypoxic-ischemic damage and demonstrates that, in a physiologically controlled in vivo model of hemispheric global ischemia, (1) the immature brain is, in fact, less resistant to hypoxic-ischemic brain damage than its adult counterpart, (2) the brain damaging effects of hypoxic-ischemia are age dependent, but do not increase linearly with advancing age and development, and (3) the intermediate age groups are more tolerant to hypoxic-ischemic brain injury than either very young or more mature ages.

Influence of post-hypoxia reoxygenation conditions on energy metabolism and superoxide production in cultured neurons from the rat forebrain

Brain reperfusion and/or reoxygenation may be of particular importance in the etiology of neuronal damage after hypoxic-ischemic insult in neonates, especially with reference to the generation of free radicals. To investigate this issue, the influence of either standard reoxygenation or transient hyperoxia was studied on the consequences of severe hypoxia in a model of cultured neurons isolated from the fetal rat brain. Culture dishes were exposed for 6 h to hypoxia (95% N2/5% CO2). They were then placed under normoxia (95% air/5% CO2) or hyperoxia (95% O2/5% CO2) for 3 h, and finally returned to normoxia. Control cultures were kept under normoxic conditions. Cell morphology, protein concentrations, lactate dehydrogenase leakage, energy metabolism, as reflected by specific transport and incorporation of 2-D-[3H]deoxyglucose, as well as superoxide radical formation were analyzed as a function of time. Po2 values in the cell incubating medium were decreased by 78% by hypoxia and increased by 221% by hyperoxia. No morphologic alteration could be noticed before 72 h posthypoxia, when cell degeneration became apparent, with a concomitant reduction in protein contents. Hypoxia-reoxygenation induced a transient cellular hypermetabolism, as shown by a 36% increase in 2-D-[3H]deoxyglucose uptake 24 h after hypoxia, and then a 23% decrease below control values at 72 h. It also led to a sharp increase in the formation of superoxide radicals (+108%). Transient hyperoxia during reoxygenation did not exacerbate these events, and thus would not enhance their deterimental effects on cell integrity.

In vivo 31P NMR studies of orientation effects upon rat brain metabolism during mild hypoxia

Energy metabolites in rat brain under the same level of hypoxia were monitored by 31P NMR in both horizontal and vertical magnets. The changes in PCr, Pi, and pHi in the vertical setting toward the end of hypoxia were significantly larger and the recovery in the horizontally held animals was more complete. The results demonstrated quantitatively that the stress of the alignment is superimposed on the stress of hypoxia in the vertical magnet.

Effect of aging on phosphate metabolites of rat brain as revealed by the in vivo and in vitro 31P NMR measurements

Changes of phosphate metabolism in brains of neonate, weaning and adult rats were compared using both in vivo and in vitro nuclear magnetic resonance spectra. Ratios of phosphocreatine/nucleoside triphosphate (PCr/NTP) were the same in neonatal brain in both in vivo and in vitro studies, but not in weaning and adult brains. This discrepancy may have resulted from extended cerebral hypoxia due to slowed freezing of the brain by the increased skull thickness and brain mass in the weaning and adult rats. Variations in in vitro extraction condition for this age-related study may lead to systematic errors in the adult rats. Nevertheless, the phosphomonoester/nucleoside triphosphate (PME/NTP) ratios in extracts of brain from neonatal rats were higher than those obtained in vivo. In addition, the glycerophosphorylethanolamine plus glycerophosphorylcholine/nucleoside triphosphate (GPE+GPC/NTP) ratios, which were not measurable in vivo, showed age-dependent increase in extracts of rat brain. Some of the phosphomonoester and phosphodiester molecules in rat brain may be undetectable in in vivo NMR analysis because of their interaction with cellular components. The total in vitro GPE and GPC concentration in brain from neonatal rat was estimated to be 0.34 mmole/g wet tissue.

Age-related modifications of potassium homeostasis and synaptic transmission during and after anoxia in rat hippocampal slices

Age-related changes in the capacity of the brain to survive short anoxic episodes were studied in stratum pyramidale (region CA1) of hippocampal slices from control (6-7 months) and aged (26-27 months) rats. Our primary interest was in how aging affected the ability of slices to maintain or to recover extracellular potassium ion (K+o) homeostasis and orthodromically-stimulated field potentials during and after anoxia. During anoxia, K+o homeostasis was lost faster in slices from aged rats. Following anoxia, K+o homeostasis recovered more slowly, and synaptic transmission recovered less completely, in aged slices. These studies provide what is believed to be the first demonstration that aging diminishes the capacity of brain tissue to maintain K+o during anoxia and to recover K+o homeostasis and synaptic transmission following anoxia, and support suggestions that the aged brain is more vulnerable to anoxia.