Bengt Nellgård

Cardiopulmonary Bypass Induces Neurologic and Neurocognitive Dysfunction in the Rat

Background Neurocognitive dysfunction is a common complication of cardiac surgery using cardiopulmonary bypass (CPB). Elucidating injury mechanisms and developing neuroprotective strategies have been hampered by the lack of a suitable long-term recovery model of CPB. The purpose of this study was to investigate neurologic and neurocognitive outcome after CPB in a recovery model of CPB in the rat. Methods Fasted rats (n = 10) were subjected to 60 min of normothermic (37.5°C) nonpulsatile CPB using a roller pump and a membrane oxygenator. Sham-operated controls (n = 10) were not subjected to CPB. Neurologic outcome was assessed on days 1, 3, and 12 after CPB using standardized functional testing. Neurocognitive outcome, defined as the time (or latency) to finding a submerged platform in a Morris water maze (an indicator of visual–spatial learning and memory), was evaluated daily from post-CPB days 3–12. Histologic injury in the hippocampus was also evaluated. Results Neurologic outcome was worse in the CPB versus the sham-operated controls at all three measurement intervals (P < 0.001). The CPB group also had longer water maze latencies compared with the sham-operated controls (P = 0.004), indicating significant neurocognitive dysfunction after CPB. No difference in histologic injury between groups was observed. Conclusions CPB caused both neurologic and neurocognitive impairment in a rodent recovery model. This model could potentially facilitate the investigation of CPB-related injury mechanisms and possible neuroprotective interventions.

Cerebral protection by AMPA- and NMDA-receptor antagonists administered after severe insulin-induced hypoglycemia

SummaryExcitatory amino acids are implicated in the development of neuronal cell damage following periods of reversible cerebral ischemia or insulin-induced hypoglycemic coma. To explore the importance of glutamate receptor activation in the posthypoglycemic phase, we exposed rats to 20 min of insulin-induced severe hypoglycemia. The rats were treated immediately after the hypoglycemic insult with four regimes of glutamate receptor antagonists: (1) the AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propriate)-receptor antagonist NBQX [2.3-dihydroxy-6-nitro-7-sulfamoyl-benzo (F) quinoxaline] given as a bolus dose of 30 mg · kg-1 i.p., followed by an i.v. infusion of 225 μg · kg-1 · min-1 for 6 h; (2) the non-competitive NMDA-receptor antagonist, dizocilpine (MK-801) 1 mg · kg-1 given i.v.; (3) a combined NBQX treatment, (a bolus dose of 10 mg · kg-1 i.p., followed by an i.v. infusion of 225 μg · kg-1 · min-1 for 6 h), with dizocilpine 0.33 mg · kg-1 given twice i.p. at 0 and 15 min after recovery and (4) the competitive NMDA-receptor blocker CGP 40116 [D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid] 10 mg · kg-1 given i.p.. In the striatum, all glutamate receptor blockers significantly decreased neuronal damage by approximately 30%. An approximately 50% decrease in neuronal damage was demonstrated in neocortex and hippocampus following the combined treatment with NBQX and dizocilpine, while protection was variable following the treatment with a single glutamate-receptor antagonist. We conclude that neuronal damage continues to develop in the striatum and in cortical brain regions in the posthypoglycemic period and that both NMDA- and AMPA-receptors contribute to this process, possibly by a change in the cellular response to both AMPA- and NMDA-receptor stimulation.

Lack of protection by the N-methyl-D-aspartate receptor blocker dizocilpine (MK-801) after transient severe cerebral ischemia in the rat.

Glutamate is an important factor in the mechanisms of neuronal damage following cerebral ischemia. Blockade of one type of glutamate receptor, the N-methyl-D-aspartate (NMDA) receptor, decreases brain infarct size in experimental models of permanent focal ischemia, but protection in models of transient reversible ischemia is ambiguous. We investigated the effect of the noncompetitive NMDA receptor antagonist dizocipiline (MK-801) on neuronal damage in the CA1 region of the rat hippocampus, using two models of reversible cerebral ischemia: 10 or 15 min of bilateral common carotid occlusion combined with hypotension, or 6-8.5 min of cardiac arrest. Histopathologic evaluation of neuronal damage was performed 7 days after the ischemic insults. Thirteen groups of rats (a total of 129 animals) were treated with saline or dizocilpine in single or multiple doses ranging from 0.1 to 5 mg.kg-1, given intravenously or intraperitoneally prior to and/or after the ischemic insult. In none of the dizocilpine-treated groups could neuronal protection be demonstrated in the CA1 region of the septal as well as dorsotemporal hippocampus, compared to a corresponding saline-treated group. We conclude that systemically administered noncompetitive NMDA receptor antagonists do not provide a marked protection against neuronal damage after a transient period of severe forebrain ischemia.

Sympathetic Ganglionic Blockade Masks Beneficial Effect of Isoflurane on Histologic Outcome from Near-complete Forebrain Ischemia in the Rat

BACKGROUND Isoflurane-anesthetized rats have better outcome from global cerebral ischemia than rats anesthetized with fentanyl and nitrous oxide. The authors wanted to determine whether circulating catecholamine concentrations depend on the anesthetic agent and whether sympathetic ganglionic blockade affects anesthetic-mediated differences in outcome from near-complete forebrain ischemia. METHODS For two different experiments, normothermic Sprague-Dawley rats that had fasted were assigned to one of four groups and subjected to 10 min of 30 mm Hg mean arterial pressure and bilateral carotid occlusion. Rats were anesthetized with 1.4% isoflurane or fentanyl (25 microg x kg(-1) x h(-1)) and 70% nitrous oxide, with or without preischemic trimethaphan (2.5 mg given intravenously). In experiment 1, arterial plasma catecholamine concentrations were measured before, at 2 and 8 min during, and after ischemia (n = 5-8). In experiment 2, animals (n = 15) underwent histologic analysis 5 days after ischemia. RESULTS In experiment 1, intraischemic increases in plasma norepinephrine and epinephrine levels were 28 and 12 times greater in the fentanyl-nitrous oxide group than in the isoflurane group (P<0.01). Trimethaphan blocked all changes in plasma catecholamine concentrations (P<0.02). In experiment 2, isoflurane reduced the mean +/- SD percentage of dead hippocampal CA1 neurons compared with fentanyl-nitrous oxide (43+/-22% vs. 87+/-10%; P<0.001). Trimethaphan abolished the beneficial effects of isoflurane (91+/-6%; P<0.001). Similar observations were made in the cortex. CONCLUSIONS Isoflurane attenuated the peripheral sympathetic response to ischemia and improved histologic outcome compared with fentanyl and nitrous oxide. This outcome benefit was reversed by sympathetic ganglionic blockade. The beneficial effects of isoflurane may result from a neuroprotective influence of an intermediate sympathetic response that is abolished by trimethaphan.

Anesthetic effects on cerebral metabolic rate predict histologic outcome from near-complete forebrain ischemia in the rat.

Background Although reduction of cerebral metabolic rate is thought to contribute to anesthetic neuroprotection, histologic evidence to support this concept has not been provided. In this study, histologic outcome was evaluated in rats subjected to different durations of severe forebrain ischemia while anesthetized with volatile anesthetics that have substantially different effects on cerebral metabolic rate. Methods Normothermic rats that underwent fasting were anesthetized with 0.75 minimum alveolar concentration (MAC) isoflurane–60% nitrous oxide (N2O) or 0.75 MAC halothane–60% N2O. Ischemia was induced with use of a combination of bilateral carotid occlusion and controlled hypotension. Rats in the isoflurane group were subjected to 6.5 min or 8.0 min ischemia, whereas the halothane group received 6.5 min ischemia. Histologic damage was assessed 4 days later. Results With 6.5 min ischemia, mean ± SD, hippocampal CA1 percent of dead (% dead) neurons was reduced with isoflurane–N2O (45 ± 18) versus halothane–N2O (60 ± 23, P = 0.023). Eight minutes of ischemia increased % dead neurons in the isoflurane–N2O group (60 ± 17, P = 0.017). There was no difference between the isoflurane 8.0-min and halothane 6.5-min groups (P = 0.935). A similar pattern was observed in hippocampal CA4 and the neocortex. Striatal damage was not affected by anesthetic or ischemic duration. Conclusions At 6.5 min ischemia, isoflurane provided improved outcome versus halothane. Previous research has shown that 0.75 MAC isoflurane–N2O increases the time to onset of ischemic depolarization by 1.5 min and reduces cerebral metabolic rate by 42%versus 0.75 MAC halothane–N2O. In the current study, when the duration of ischemia was increased by 1.5 min in the isoflurane–N2O group, histologic outcome became similar to that in halothane–N2O-anesthetized rats. These results provide evidence that cerebral metabolic rate reduction has an advantageous effect on outcome from severe brain ischemia, but also suggest that such benefit is likely to be small.

Periischemic Cerebral Blood Flow (CBF) Does Not Explain Beneficial Effects of Isoflurane on Outcome from Near-complete Forebrain Ischemia in Rats

BackgroundIsoflurane improves outcome from near-complete forebrain ischemia in rats compared with fentanyl–nitrous oxide (N2O). Sympathetic ganglionic blockade with trimethaphan abolishes this beneficial effect. To evaluate whether anesthesia-related differences in cerebral blood flow (CBF) may explain these findings, this study compared regional CBF before, during, and after near-complete forebrain ischemia in rats anesthetized with either isoflurane (with and without trimethaphan) or fentanyl–nitrous oxide. MethodsFasted, normothermic isoflurane anesthetized Sprague-Dawley rats were prepared for near-complete forebrain ischemia (10 min of bilateral carotid occlusion and mean arterial pressure = 30 mmHg). After surgery, rats were anesthetized with either 1.4% isoflurane (with or without 2.5 mg of trimethaphan intravenously at onset of ischemia) or fentanyl–nitrous oxide (25&mgr;g · kg−1 · h−1 · 70% N2O−1). Regional CBF was determined (14C-iodoantipyrine autoradiography) before ischemia, 8 min after onset of ischemia, and 30 min after onset of reperfusion. ResultsRegional CBF did not differ significantly among groups at any measurement interval. Ischemia caused a marked flow reduction to 5% or less of baseline (P < 0.001) in selectively vulnerable regions, such as the cortex, caudoputamen and hippocampus, whereas flow in the brain stem and cerebellum was preserved. Reperfusion at 30 min was associated with partial restoration of flow to 35–50% of baseline values in ischemic structures. ConclusionsThe results indicate that improved histologic–behavioral outcome provided by isoflurane anesthesia cannot be explained by differential vasodilative effects of the anesthetic states before, during, or after severe forebrain ischemia. This study also shows severe postischemic delayed hypoperfusion that was not affected by choice of anesthetic or the presence of trimethaphan. Mechanisms other than effects on periischemic CBF must be responsible for beneficial effects of isoflurane in this model.

Effects of isoflurane, ketamine, and fentanyl/N2O on concentrations of brain and plasma catecholamines during near-complete cerebral ischemia in the rat.

UNLABELLED We postulated that adrenergic responses to global cerebral ischemia are anesthetic-dependent and similar in both brain and arterial blood. Rats were anesthetized with isoflurane (1.4%), ketamine (1 mg x kg(-1) x min(-1)), or fentanyl (25 microg x kg(-1) x h(-1))/70% N2O. The carotid arteries were occluded for either 20 min with mean arterial pressure (MAP) 50 mm Hg (incomplete ischemia) or 10 min with MAP 30 mm Hg (near-complete ischemia). Norepinephrine was measured in hippocampal microdialysate. Norepinephrine and epinephrine were measured in arterial plasma. In both hippocampus and plasma, basal norepinephrine was similar among anesthetics. During incomplete ischemia, hippocampal norepinephrine was twofold greater with fentanyl/N2O than with isoflurane (P = 0.037), but plasma norepinephrine and epinephrine were similar and unchanged among all three anesthetics. During near-complete ischemia, hippocampal norepinephrine was threefold greater with ketamine than fentanyl/N2O (P = 0.005), whereas plasma norepinephrine and epinephrine were markedly greater with fentanyl/N2O than with ketamine (P < 0.0005) or isoflurane (P = 0.05). There was no correlation between norepinephrine concentrations in hippocampus and plasma for either incomplete or near-complete ischemia. This study demonstrates that adrenergic responses to global ischemia are anesthetic-dependent, particularly during more severe insults. The absence of a correlation between plasma and brain catecholamine concentrations indicates that adrenergic responses to ischemia are independent in brain and blood. IMPLICATIONS It has been proposed that anesthetics modulate cerebral ischemic outcome by influencing peripheral adrenergic responses to ischemia. This experiment demonstrates that anesthetics differentially modulate adrenergic responses to ischemia but that effects in plasma and brain are independent. This suggests that events detected in the peripheral circulation do not implicate direct mechanisms of action of catecholamines at the neuronal/glial level.

The effects of anesthetics on stress responses to forebrain ischemia and reperfusion in the rat

Rats exposed to forebrain ischemia have reduced injury when anesthetized with isoflurane versus fentanyl + N2O. The protection caused by isoflurane is reversed by trimethaphan. We hypothesized that these anesthetic-dependent effects on ischemic outcome can be associated with altered stress responses to ischemia. Rats were randomized to four treatments: isoflurane; fentanyl + N2O; isoflurane + trimethaphan; or isoflurane + metyrapone. Severe forebrain ischemia was then induced for 10 min. Plasma and brain corticosterone, tumor necrosis factor (TNF)-&agr;, and interleukin (IL)-6 were assayed. Plasma corticosterone concentrations were similar in the isoflurane and isoflurane + trimethaphan groups, but greater than in the fentanyl + N2O and isoflurane + metyrapone groups. Brain corticosterone was similar among all groups except isoflurane + metyrapone, in which values were markedly reduced. The addition of metyrapone to isoflurane also reduced plasma TNF-&agr;; however, values among other groups were similar. There were no differences among groups for brain TNF-&agr;. Plasma IL-6 concentrations were below the limit of detection. Brain IL-6 concentrations were increased by ischemia; however, there was no difference among groups. In conclusion, there were no differences between the isoflurane and isoflurane + trimethaphan groups for any of the measured stress markers. Further, there was little difference between the isoflurane and fentanyl + N2O groups, except for plasma corticosterone concentration. Accordingly, isoflurane neuroprotection and its reversal by trimethaphan appear to be independent of effects on the stress responses measured in this study. Implications Differential anesthetic effects on ischemic outcome are independent of effects on adrenergic/noradrenergic responses to ischemia. The absence of a consistent differential effect of anesthetics on either corticosterone or cytokine responses to ischemia serves to further refute the hypothesis that isoflurane neuroprotection can be attributed to dampening of adverse stress responses to ischemic insults.

Effect of intracerebral norepinephrine depletion on outcome from severe forebrain ischemia in the rat

Manipulations of plasma catecholamine concentrations influence outcome from ischemic brain insults. It has been suggested that these effects are mediated by influences on brain catecholamine concentrations. This study examined whether major changes in brain norepinephrine concentrations can alter outcome from severe forebrain ischemia. Sprague-Dawley rats were administered 50 mg/kg i. p. N-(chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) or were left untreated (control). One week later, these rats were subjected to either 7 or 8 min of normothermic forebrain ischemia (bilateral carotid occlusion and MABP=30 mmHg) and allowed to recover for 4 days. Histologic damage was then evaluated. In other control and DSP-4-treated animals, hippocampal microdialysate norepinephrine concentrations were measured before, during and after 8 min of forebrain ischemia. Norepinephrine concentrations were also determined in brain homogenates from non-ischemic DSP-treated and control rats. A 95% depletion of norepinephrine was observed in brain homogenates from non-ischemic DSP-4-treated rats compared with control. During ischemia, microdialysate norepinephrine concentrations increased in control but not in DSP-4-treated rats (P=0.002). For plasma, intra-ischemic epinephrine concentrations increased 8-10-fold and returned to baseline values post-ischemia with no differences between groups. Plasma norepinephrine values remained unchanged in both groups. Histologic damage resulting from either 7 or 8 min of ischemia in hippocampal structures, caudoputamen, and neocortex was similar between DSP-4-treated and control groups. This study could not identify any effect of major changes in brain norepinephrine concentrations on ischemic brain damage. These data indicate that peripheral catecholamine effects on near-complete forebrain ischemic outcome are unlikely to be mediated by effects on central catecholamine concentrations.

Pre-ischemic depletion of brain norepinephrine decreases infarct size in normothermic rats exposed to transient focal cerebral ischemia

This study examined the importance of brain norepinephrine concentration on outcome from a focal ischemic insult. Fasted temperature-controlled male Wistar rats pretreated with DSP-4, (N-(chloroethyl)-N-ethyl-2-bromobenzylamine), to selectively deplete brain norepinephrine, were subjected to reversible filament occlusion of the middle cerebral artery for 75 min in the awake state. After 3 days recovery, total infarct volume in DSP-4 treated rats (185 +/- 107 mm3) was reduced vs. untreated control animals (242 +/- 71 mm3, P = 0.04). Subcortical infarct volume was also smaller in the DSP-4 group (93 +/- 44 vs. 121 +/- 28 mm3, P = 0.02). Cortical infarct volume was not statistically different between groups. Neurologic function correlated with infarct-size. These findings suggest that brain norepinephrine affects stroke development either by direct neuronal toxicity and/or through influences on the penumbral circulation. Dampening of the central stress response induced by the onset of stroke may thus be advantageous.