Animals
In the present study, forty male Wistar rats weighing 250–300 g were used. Animals were placed in the animal laboratory in a controlled environment, a cycle of 12 h of darkness-light and temperature 22 ± 2°C, with free access to food and water. All experiments were approved by the Ethics Committee of Tehran University of Medical Sciences (Project number: 98-01-139-41863, Approval ID: IR.TUMS.NI.REC.1398.002).
Establishment of sepsis by the cecal ligation and puncture (CLP) model
First, during isoflurane inhalation, the abdominal cavity of animals was exposed by a 2-cm midline incision. Then, separation of the cecum was carefully performed without any injury to the blood vessels. Next, cecum ligation was stoutly made at the base of it with a 4-silk suture. After that, an 18-gauge needle was used to puncture underside of the ileocecal valve twice. To extrude a small amount of feces into the peritoneal cavity, the perforations were gently pressed. Finally, the incision was closed in two layers by a 4 − 0 silk suture and for fluid resuscitation; saline (3 mL/100 g body wt) was injected subcutaneously. Animals were placed into their cages and 0.86 mg/kg ketorolac was administrated via intra-muscular injection to relieve pain (16).
Study design
Animals were randomly assigned to four groups of ten in each: 1) Sham; in this group, the lower part of the animals abdomen was incised in order to expose the cecum without induction of sepsis. 2) Sham + Tannic acid; 6, 12 and 18 h after sham operation, animals were given 20 mg/kg tannic acid intraperitoneally (17). 3) Sepsis; animals underwent the CLP surgery. 4) Sepsis + Tannic acid; in this group, 6, 12 and 18 h after the sepsis induction, animals received tannic acid (20 mg/kg) via intraperitoneal injection (17).
Sample collection and preparation
Twenty four hours after the CLP surgery in rats, systolic blood pressure was measured by PowerLab Tail cuff system. Then, sepsis score was evaluated 12 and 24 h after the sepsis induction. Next, animal's anxiety-related behaviors were assessed using the elevated plus-maze and light-dark transition tests. Finally, animals were anesthetized via intraperitoneal injection of ketamine hydrochloride (100 mg/kg) and xylazine (10 mg/kg) and the brain tissue samples were collected for assessment of inflammatory markers (tumor necrosis factor-alpha [TNF-α] and interleukin-6 [IL-6]), oxidative stress parameters (malondialdehyde [MDA] and superoxide dismutase [SOD] activity) and protein levels (GABAA receptors and interleukin-1 beta [IL-1β]).
Measurement of systolic blood pressure
Systolic blood pressure was measured non-invasively by PowerLab Tail cuff system at the end of the study prior to tissue sampling. For this purpose, a rat restrainer was used to place a conscious rat. The animal's tail was cleaned with moisten gauze, then, the tail cuff was located on. Next, the non-invasive blood pressure sensor was placed below the tail-cuff. After deflating the tail cuff, the blood pressure was monitored and recorded for three times consecutively.
Sepsis score
In all animals, 12 and 24 h after the sepsis induction, sepsis scores were assessed by one of the investigators who was not aware of the experimental groups. Each variable evaluated in this assessment (piloerection, consciousness level, amount of activity, response to auditory stimulus or touch, eyes, respiration rate and respiration quality) was given a score ranging from 0 to 4 (18). There was a positive correlation between the severity of sepsis and the sepsis score amount.
Behavioral tests
Elevated plus-maze test
In the present study, anxiety-related behaviors were evaluated using the elevated plus-maze (19). The instrument consisted of four arms which were 35 cm long and 5 cm wide. The edges of two open arms were 0.5 cm high and the dark walls of two closed arms were 15 cm high. The maze height from the floor was 50 cm. Each rat was located solely at the center of the maze in front of the open arm. During 5 min, spending time in the open arms, the open arm entries percentage and total entries were monitored and recorded. After each session, cleaning of arms was performed using 70% ethanol.
The spending time in the open arms (time spent in the open arms/total time spent in all arms × 100) and the open arm entries percentage (the open arm entries number/total entries into all arms × 100) were calculated for all animals. Reduction of anxiety-related behaviors was shown by open arm activity (entry and time). For locomotor activity assessment, the total entries (sum of the number of entries into the open and closed arms) were calculated.
Light-dark transition test
The light-dark transition test was used to assess anxiety-related behaviors (20). The box contained two Plexiglas compartments, one of them with white walls illuminated by a 60-watt bulb, the other one with the black walls which had no illumination. The size of the compartments was the same (30 × 40 × 40 cm). The time spent in the light side was recorded for 5 min.
Assessment of the brain tissue oxidative stress markers (MDA level and SOD activity)
MDA level was evaluated according to the Esterbauer and Cheeseman technique (21). This measurement was based on the reaction of MDA with thiobarbituric acid which produced a pink substance with the maximum absorption at 532 nm.
To assess SOD activity, an ELISA kit (Navand Salamat Co., Iran) was provided. Briefly, prior to the assay, all materials, reagents and the brain tissue samples were placed in the room temperature. Then, 100 mg brain tissue was homogenized with 500 µl lysing buffer. Then, this solution was centrifuged at 12,000 rpm for 4 min at 4°C and then, collected supernatants were used to evaluate SOD enzyme activity. First, 50 µl the supernatants were added to the wells of the micro-titer plate. Then, 50 µl deionized water was added to the control wells. Next, 200 µl R1 reagent and then 50 µl R2 reagent were added to all wells respectively. After that, the ELISA kit was incubated at room temperature away from light for 5 min. The samples' light absorption were read by an ELISA reader at 405 nm. Finally, the level of enzyme activity was calculated using the following formula:
SOD activity (U/mg protein) = OD Test/OD Control × 200
Assessment of the brain tissue inflammatory markers (TNF-α and IL-6)
The measurement of TNF-α and IL-6 levels in the brain tissues was performed using specific rat ELISA kits (Zellbio Co, Germany). All samples were evaluated in duplicate and according to the instructions provided by the manufacturer. The absorbance of samples was measured by a micro plate reader (Biotek, USA) and then obtained data were compared to the standard curve and the concentrations were calculated.
Assessment of the hippocampal GABAA receptors and IL-1β protein levels
To isolate lysate, the hippocampus was homogenized in lysis buffer (50 mM Tris-HCl pH 7.5, 137 mM NaCl, 0.5% Triton X-100, EDTA-free 1 × complete protease inhibitor mixture, Roche, Massachusetts, USA) and kept on ice for 15 min prior to centrifugation at the maximum speed in an Eppendorf centrifuge at 4°C for 10 min. The total protein concentration was evaluated by the Bradford’s method. The standard plot was provided by using bovine serum albumin. Then, the 12% SDS-PAGE gels (Bio-Rad Laboratories, USA) were used to electrophorese whole proteins and conveyed to polyvinylidene fluoride (PVDF) (Millipore, USA) membranes and probed with GABAA and IL-1β antibodies (ABNOVA, Taiwan) and then placed in secondary antibody (Cell Signaling Technology, USA). Bands were detected by chemiluminescence applying the electro-chemiluminescence reagent kit. (Amersham Bioscience, USA). It was provided to recognize immunoreactive polypeptides and consequent autoradiography. The PVDF membranes were stripped and reused by applying an anti-β-actin antibody (Sigma-Aldrich, USA) to normalize protein loading and transfer. The density of bands on the radiography film was quantified by Image J software.
Statistical analysis
All data were presented as mean ± SEM. Sepsis score was analyzed by repeated measures ANOVA and the other variables were analyzed by one way ANOVA followed by Tukey’s post hoc test. p < 0.05 was considered statistically significant.