Influence of Propofol on Neuronal Damage and Apoptotic Factors after Incomplete Cerebral Ischemia and Reperfusion in Rats: A Long-term Observation

The experimental protocol used in this study was approved by the Ethics Committee for Animal Experimentation of the state of Bavaria in Munich, Germany. Fasted male Sprague-Dawley rats weighing 400 ± 60 g were anesthetized in a bell jar saturated with isoflurane. Rats were then tracheally intubated and mechanically ventilated (arterial carbon dioxide tension [Paco²], 38–42 mmHg) with 2.0 vol% isoflurane in nitrous oxide and oxygen (inspiratory oxygen fraction [Fio²], 0.33). Catheters were inserted into the right femoral artery and vein for blood withdrawal, blood sampling, and blood pressure measurement. Two catheters were inserted into the right jugular vein for drug administration. A loose ligature was placed around the right common carotid artery for later clamping. Temperature sensors were inserted into the rectum and the right temporal muscle. Pericranial temperature was maintained constant at 37.5°C using a servocontrolled overhead heating lamp and a heating pad. On completion of the surgical preparation, all surgical incisions were infiltrated with 0.5% bupivacaine. Respiratory parameters, pericranial temperature, arterial blood gases, arterial pH, and plasma glucose concentration were monitored and maintained constant during the experiment. An electroencephalogram was recorded continuously using subdermal platinum needle electrodes placed over both hemispheres at the parietotemporal versus  frontal cortex recording sites (AC/AD Strain Gage Amplifier, model P122; Grass Instruments Division, West Warwick, RI).

At the end of the preparation, the background anesthetic was discontinued, and the animals were assigned by random lists to one of the following treatment groups: Animals in group 1 (n = 32, control group) were anesthetized with fentanyl (intravenous bolus: 10 μg/kg, intravenous infusion: 25 μg · kg⁻¹· h⁻¹) and nitrous oxide in oxygen (Fio²= 0.33). Animals in group 2 (n = 32) received 0.8–1.2 mg · kg⁻¹· min⁻¹intravenous propofol sufficient to induce burst suppression on the electroencephalogram and oxygen in air (Fio²= 0.33). After an equilibration period of 45 min, cerebral ischemia was induced by clip-occlusion of the right common carotid artery in combination with hemorrhagic hypotension to a mean arterial blood pressure of 40 mmHg for 45 min. At the end of the ischemia, the clip was removed, and the withdrawn blood, which was kept at body temperature, was reinfused over 15 min. Vecuronium was given as a continuous infusion (0.1 mg · kg⁻¹· min⁻¹) to maintain neuromuscular blockade, to prevent the animals from breathing (Paco²control), and to avert a rigor of the animals that is caused by fentanyl. The physiologic variables were recorded at four time points: before hemorrhagic hypotension (baseline), at 45 min of cerebral ischemia (ischemia), 15 min after cerebral ischemia on reinfusion of the withdrawn blood (reperfusion), and 45 min after the end of reperfusion (recovery). After completion of cerebral ischemia, catheters were removed, and the wounds were closed. The animals were extubated and assigned to one of four time-control groups to survive 1, 3, 7, or 28 days according to a predefined random list. At the end of the observation period, brains were removed in deep anesthesia (halothane) and placed in tissue-freezing medium (Jung, Nussloch, Germany). Then the brains were frozen in methylbutan and later on dry ice before being stored at −70°C. The brains were cut into 7-μm slices and mounted on slides. Brains of eight untreated animals were prepared in an identical fashion and were used to measure the natural expression of the investigated proteins (group 3, nonischemic rats).

From each animal, two 7-μm coronal tissue sections were stained with hematoxylin and eosin. The absolute number of eosinophilic neurons was counted by a blinded investigator in the hippocampal areas CA1–3 of the ischemic hemisphere and compared to the total number of hippocampal neurons for each section. The hippocampus of the nonischemic hemisphere was also analyzed for eosinophilic neurons, but the total number of neurons was not counted. If neuronal damage was too severe (e.g. , formation of a scar after 28 days), hippocampal damage was assumed to be 100%.

For immunofluorescence staining, the tissue sections were fixed in ethanol, followed by a washing period using phosphate-buffered saline (PBS) with 0.1% Tween® 20 (Fulka Chemie, Buchs, Switzerland). The brain sections were then incubated in blocking buffer (10% fetal calf serum in PBS with Tween® 20), followed by incubation with the first antibody (rabbit polyclonal antibodies; Santa Cruz Biotechnology, Santa Cruz, CA), Bax antibody (I-19 and P-19), p53 antibody (FL-393), Bcl-2 antibody (C21), and Mdm-2 antibody (C18). After incubation, the brain sections were washed and incubated with the second antibody (Alexa Fluor® 488 goat anti-rabbit IgG antibody; Molecular Probes, Leiden, Netherlands), followed by another washing period. The tissue sections were covered with mounting medium (Vectashield® H-1000; Vectorlabs, Burlingame, CA) and cover slips and were stored at 4°C. With every staining, a negative control was performed by omitting the first antibody to detect nonspecific fluorescence. As a positive control, tissue sections of rat liver were used because Bax, p53, Bcl-2, and Mdm-2 are constantly expressed in this tissue. Within the next 24 h, the immunofluorescence intensity of the proteins (two sections per protein) in the hippocampal regions CA1–3 (four images per hemisphere) was recorded with a confocal laser scanning microscope (LSM 410; Carl Zeiss, Jena, Germany) by an investigator blinded to treatment condition. The constant intensity of the laser light was controlled with a power meter. Images were evaluated with the KS400-software (Carl Zeiss Vision, Jena, Germany). With the help of the software, hippocampal neurons were marked. The mean intensity of immunofluorescence (gray levels) that is proportional to the concentration of the fluorescence marker and therefore proportional to the mean protein concentration was measured within these marked neurons. Then the background intensity was subtracted.

Frozen brain sections were fixed in 4% paraformaldehyde. After being washed in PBS, the tissue sections were quenched using 3% H²O²in methanol at room temperature. Sections were washed again, and a blocking agent (70 μl; DAKO, Hamburg, Germany) was applied. Afterward, tissue sections were incubated with the first antibody diluted in blocking agent (purified rabbit antiactive caspase-3 monoclonal antibody, Clone C92-605; BD Pharmingen, San Jose, CA). After another washing period in PBS, sections were incubated for 30 min with the second antibody (Universal-LSAB TM Kit; DAKO). After being washed in PBS, the brain sections were incubated with streptavidin-conjugated horseradish peroxidase (Universal-LSAB TM Kit) and washed again in PBS. The sections were stained with one drop of chromogen substrate (DAKO) and were then rinsed gently in distilled water. Sections were washed and then incubated with purified mouse anti-neuronal nuclei monoclonal antibody NeuN; Chemicon International, Temecula, CA). The sections were washed before the second antibody (biotinylated horse anti-mouse antibody; Vector Laboratories, Burlingame, CA) was applied. Then the sections were incubated with streptavidin-conjugated alkaline phosphatase (Vector Laboratories) and washed again. The Vector red alkaline phosphatase substrate kit (Vector Laboratories) was used to stain neurons red. Finally, brain sections were counterstained with Mayer's hematoxylin, dehydrated in ascending alcohol concentrations, and mounted with Roti Histokitt (Roth, Karlsruhe, Germany). Using a light microscope, cells that were double positive for activated caspase-3 and NeuN were counted in the hippocampus. Their number was compared to the total amount of hippocampal cells.

Continuous variables are presented as mean ± SD. Bivariate comparison of continuous variables were performed via t  test or if the normality assumption was not met by the Mann–Whitney U test. Inferential statistics between factors were assessed by analysis of variance models in the context of general linear model and post hoc t  test using the Bonferroni adjustment. The Greenhouse-Geisser correction was applied if sphericity assumption was not met. In detail, the following models were performed:

Physiologic variables are analyzed via  a full factorial repeated-measures model using four repetitions (baseline, ischemia, reperfusion, and recovery) as the within-subject factor and day (four levels: 1, 3, 7, and 28 days) and intervention group (two levels: control and propofol) as between-subject factors.

The concentration of apoptosis-related proteins (Bax, p53, Bcl-2, and Mdm-2) was recorded on ischemic and nonischemic hemispheres. A full factorial repeated-measures model was applied to the within-subject factor hemisphere and between-subject factors group (control, propofol, untreated) and day (1, 3, 7, and 28).

For the analyses of eosinophilic cells and activated caspase-3/NeuN–positive cells, a univariate analysis of variance model was conducted. Group (control, propofol, untreated) and day (1, 3, 7, and 28) are treated as between-subject factors.

Statistical analyses were performed using SPSS 11.5 (SPSS Inc., Chicago, IL). All tests were performed two sided on a 5% level of significance.

The physiologic parameters mean arterial blood pressure, arterial blood gas tensions, hemoglobin concentration, and plasma glucose concentration are listed in table 1. There were no differences for control animals or for propofol-anesthetized animals between 1, 3, 7, and 28 days; therefore, these parameters were presented together for each treatment. Baseline mean arterial blood pressure was higher in control animals compared with propofol-anesthetized animals.

The number of eosinophilic neurons in relation to the total number of neurons in the ischemic hippocampus is shown in figure 1. Although no eosinophilic neurons were detected in animals anesthetized with propofol, 16–54% of the neurons in the hippocampus of control animals were eosinophilic over time. There was no injury in the nonischemic hemisphere of control animals or propofol-anesthetized rats.

Figure 2shows the concentration of the apoptosis-regulating proteins Bax, p53, Bcl-2, and Mdm-2 in the ischemic hemisphere. The concentration of Bax was significantly higher in control animals compared with propofol-anesthetized or nonischemic animals on days 1, 3, and 7 (fig. 2A). An influence of the p53 protein could not be detected for different treatments over time (fig. 2B). The Bcl-2 concentration was lower in control animals compared with animals treated with propofol (during the first 3 days) and compared with nonischemic animals (at days 1, 3, and 28; fig. 2C). The concentration of Mdm-2 was lower in the control group compared with propofol-anesthetized animals after the first day and was higher after 28 days (fig. 2D).

Figure 3shows the amount of hippocampal neurons (marker: NeuN) with a positive staining for activated capase-3. In both groups, 0.5–3.0% of all neurons were stained positive for activated caspase-3, with no difference between groups over time. In the nonischemic animals, no activated caspase-3–positive neurons were evident.

In a model of incomplete cerebral ischemia and reperfusion, propofol reduces the number of eosinophilic neurons and favorably modulates apoptosis-regulating proteins. This neuroprotective effect is sustained and differs from transient neuroprotection seen with isoflurane.3Cerebral ischemia also induces active caspase-3, an additional marker of apoptotic cell death. However, propofol did not inhibit the expression of activated caspase-3. This suggests that over a long-term observation period, propofol can completely inhibit neuronal damage after cerebral ischemia and also interferes with caspase-3–independent apoptotic cell death.

Postischemic neurons develop cytoplasmic eosinophilia in hematoxylin and eosin preparations. This is due to increased production of basic proteins before the death of the cell.4There is a general notion that eosinophilia represents irreversible damage. In a light and transmission electron microscopic study in neurons exposed to hypoglycemic injury, eosinophilic neurons displayed ultrastructural indicators of fatal cell injury, such as cytorrhexis and karyorrhexis.5Neurons exhibiting these electron-microscopic constellations underwent subsequent cytolysis. This justifies the use of eosinophilia as a morphologic endpoint of irreversible ischemic neuronal damage. In the current study, neuronal eosinophilia progressively increased with a peak at 3 days after ischemia and reperfusion. This temporal profile of neuronal damage is consistent with studies in rats after transient forebrain ischemia.6–8Anesthesia with propofol inhibited the ischemia-induced neuronal damage for the entire observation period of 28 days. This proves for the first time that neuroprotection previously observed with propofol1,2is not only a short-term rather than a sustained effect. The total suppression of neuronal damage by propofol might be related to the fact that, compared with other studies, a moderate form of cerebral ischemia was chosen to guarantee adequate survival of all animals for 1 month and to enable apoptotic cell death.

Apoptosis  refers to programmed cell death. This endogenous program is essential in the modeling of organs during fetal life, enabling self-renewing processes, and eliminating infected or injured cells in adults. Superfluous cells die by apoptosis, and their death is initiated by a genetic program. When these dying cells have disintegrated into apoptotic bodies, they are phagocytosed by macrophages without inflammatory response. Apoptosis can be activated by an extrinsic (death receptor–mediated) or intrinsic (mitochondrial) pathway.9The intrinsic pathway is initiated by binding of the proapoptotic protein Bax to the permeabilization-related protein adenine nucleotide translocator, which is located in the mitochondrial membrane.10This increases the permeability of mitochondrial membranes and permits the release of cytochrome C from the mitochondria into the cytosol.10,11Rats subjected to permanent middle cerebral artery occlusion or transient incomplete cerebral ischemia developed increased concentrations of the death effector Bax.12,13The initial up-regulation of the proapoptotic protein Bax can be counteracted by the protein Bcl-2, which binds to the Bax protein.14Accordingly, transduction of Bcl-2 by a herpes simplex virus amplicon into the hippocampus of gerbils 24 h before transient global cerebral ischemia reduces hippocampal neuronal damage.15The concentration of Bax is up-regulated by the p53 protein, which normally controls cell cycle and DNA repair.16,17This p53 protein can be inactivated by the Mdm-2 protein, thereby inhibiting the intrinsic apoptotic pathway.18In the current study, cerebral ischemia increased the concentration of Bax and suppressed Bcl-2 formation compared with nonischemic animals. Propofol inhibited the ischemia-induced increase of the apoptosis-promoting protein Bax. In addition, propofol stimulated the synthesis of the apoptosis-inhibiting protein Bcl-2. This confirms previous observations where propofol prevented the up-regulation of Bax 4 h from cerebral ischemia.19The protein Bcl-2 reacts slower than Bax, because Bcl-2 was not decreased at 4 h19rather than 24 h after injury. In contrast to other studies, in the current preparation, the proapoptotic protein p53 was not involved in apoptotic mechanisms after cerebral ischemia.20In generation of hypothesis, this might be related to different mechanisms of neuronal trauma (various ischemia models, radiation, excitotoxic substances)21or regional differences (hippocampus, striatum, cortex). Furthermore, it is possible that the temporal window of p53 detection was not adequate to the dynamic or the half-life of the p53 protein. Surprisingly, the antiapoptotic protein Mdm-2 seems to act independent of p53, because Mdm-2 was increased on day 1 in propofol-anesthetized animals. In conclusion, propofol might avoid apoptotic cell death by influencing the equilibration of proapoptotic and antiapoptotic proteins in favor of apoptosis-inhibiting proteins.

Cytochrome C activates the cytosolic caspases and thereby induces the apoptotic cell death. Caspases are constitutively expressed in the cytoplasm as procaspases. After activation of caspase-9 by cytochrome C, procaspase-3 is cleaved. Caspase-3 belongs to the group of downstream terminators that destroy essential structural and vital proteins of the cell, leading to apoptotic bodies.9Because propofol seems to have antiapoptotic properties by mechanisms related to the formation of Bax and Bcl-2, a reduced amount of neurons that are positive for activated caspase-3 was expected in propofol-anesthetized animals. However, 0.5–3.0% of all neurons were positive for activated caspase-3 regardless of the presence of propofol. This might be explained by the existence of Bax– and Bcl-2–independent pathways for activation of caspase-3. For example, within the intrinsic apoptotic pathway, the enzyme caspase-2 can directly induce the release of cytochrome C from the mitochondria into the cytoplasm.22The extrinsic apoptotic pathways are mediated by death receptors (e.g. , Fas-receptor) independent of mitochondrial activation.23Both mechanisms (caspase-2 and Fas-receptors) result in the activation of caspase-3 independent of Bax and Bcl-2. However, these results do not disprove the key role of Bax and Bcl-2 in the execution of apoptosis. Both proteins regulate other key proteins, such as the apoptosis-inducing factor, another promoter of apoptosis independent of caspase-3 activation.11,24This emphasizes the problem that no reliable marker of apoptotic cell death exists, because of the different pathways leading to apoptotic cell death.

In this model of incomplete cerebral ischemia with reperfusion, changes in the concentration of apoptosis-regulating proteins are similar in the hippocampus of both hemispheres. This may be due to a decrease in cerebral blood flow in the nonischemic hemisphere of 40–60%25caused by the hemorrhagic hypotension during ischemia. Although the extent of cerebral blood flow reduction is above the threshold to induce neuronal necrosis in the nonischemic hemisphere,26it apparently induces apoptotic signals in a vulnerable brain region such as the hippocampus.

It has been shown that after 3 h of isoflurane anesthesia, the tolerance toward permanent middle cerebral artery occlusion in rats was increased even when ischemia was induced 24 h after isoflurane application.27An effect of preconditioning with isoflurane cannot be ruled out in the current study because isoflurane was given for 1 h during the preparation period. However, isoflurane anesthesia was used in both treatment groups. Even if there was an overall effect of preconditioning, it should not have influenced the comparability of both groups.

The current study shows for the first time that propofol inhibits neuronal damage after incomplete cerebral ischemia with reperfusion for at least 28 days after injury. Propofol also seems to possess antiapoptotic qualities by influencing apoptosis-regulating proteins after cerebral ischemia. Interestingly, the well-known caspase-3–dependent apoptotic pathway was not affected by propofol. Therefore, further caspase-3–independent apoptotic pathways should be investigated to better understand the antiapoptotic properties of propofol.

The authors thank Doris Droese (Technician, Klinik für Anaesthesiologie, Technische Universität München, Munich, Germany) and Anne Frye (Technician, Klinik für Anaesthesiologie, Technische Universität München, Munich, Germany) for their technical assistance and Ernst Mannweiler, Ph.D. (Mathematician, Institut für Pathologie des GSF-Forschungszentrums, Neuherberg, Germany), for his expertise.