Animals. All procedures were approved by the Institutional Animal Use and Care Committee of School of Life Sciences, University of Science & Technology of China. Adults C57BL/6 male mice aged 6 weeks were used for all studies. Mice were obtained from Vital River Laboratory Animal Technology Co., Ltd. (Beijing, China). Simvastatin (S.C.) at 30 mg/kg was used. All mice were housed at 18–23℃ with 40–60% humidity under a 12-h dark/light cycle (lights off at 7 p.m) and free access to food and water.
Novel Object Recognition. The open-field apparatus consisted of an acrylic chamber (40 cm × 40 cm × 30 cm). Two different objects were prepared in duplicate: towers of rectangular Lego bricks (built from blue, green and yellow bricks) and circular Lego bricks (built from yellow and red bricks). The objects were placed 10 cm away from the walls and attached to the floor. Mice were tested in the dark (active phase between 7:00 p.m. and 7:00 a.m.). During the familiarization session, mice were allowed to freely explore two identical objects (rectangular Lego) placed into the arena at fixed locations for 3 min. The ANY-maze video-tracking system (Stoelting, Wood Dale, USA), which is based on nose-point detection, was used to record the time spent exploring objects. Active exploration was defined as mice sniffing or touching the object when the gap between the nose and the object was less than 2 cm. Climbing over the object or gnawing the object was not considered as exploratory activity. At the end of the test, each mouse was returned to its home cage, and the chamber and objects were cleaned using 75% ethanol, then air-dried for 3 min. After an intersession interval (ISI) of 24 h, one of the familiar objects was replaced by a novel object (circular Lego). The location of the novel object (left or right) was randomized among the mice and the groups tested. Object preference was calculated by using the following formula: preference % = (time to explore the individual object/total exploration time for both objects) × 100%). Data were excluded if the total of exploration time was less than 10 s.
Morris Water Maze (MWM). The spatial memory ability of mice was measured using MWM test. Mice of each group were trained in a large tank 120 cm in diameter and 40 cm in depth, which was divided into four quadrants. And a hidden 10-cm-diameter platform, 1 cm below the surface of warm water was placed in the center of one quadrant. The pool was covered with a black curtain with four visual cues on the wall of pool. Water was kept at 20 ° C and opacified with titanium dioxide. The trials were conducted 4 times daily at the same time point for 5 successive days. Mice were placed into four quadrants in order (spaced 20 min apart) to swim freely for a maximum of 60 s. If a mouse did not find the platform within a 60-second period, it was gently guided to the platform and allowed to stay on it for 15 s. The latency to find platform was recorded.
Rotarod Test. The rotarod training system (XR1514, Xinruan, Shanghai, China) was used for Rotarod test. Before the first training sessions, the mice were habituated to stay on a stationary rod for 2 min. A total of six trials for the rotarod test were carried out using an accelerating protocol from 4 to 60 rpm in 300 s with 20-min inter-trial intervals. After falling, the mice were immediately placed back to their home cages and the time to fall was automatically recorded by the rotarod software. Once the trial reached to 300 s, the mice were manually removed from the rod immediately. The apparatus and testing area were cleaned with 75% ethanol (w/v) after each trial.
Open Field Test. Open field test system (XR-XZ301, Xinruan, Shanghai, China) was used. The mice were individually transferred from home cage to the open field room (width, 45 cm; length, 45 cm; height, 45 cm) for locomotion test. The whole experiment lasted for 15 minutes. Locomotor activity was recorded by camera and the distance each mouse travelled was measured by ANY-MAZE software (Global Biotech Inc.).
Elevated Plus Maze. Anxiety and fear were measured using an elevated plus maze apparatus. The apparatus consisted of a cross-shaped maze (with 25 cm × 5 cm arms) elevated by a 60-cm support. Two opposite arms were surrounded by a 20-cm wall, while the other two were open (only with a 1-cm contention step). Mice were individually placed in the central area of the apparatus, facing one of the closed arms, and their mobility within the maze was assessed over 5 min. The exploration profile within the different areas of the maze (open arms, closed arms and center) was analyzed, and anxiety behavior was assessed by examination of the open arm exploration. Animals that fell from the apparatus had to be censored from the analyses. Arm preference was automatically analyzed using the ANYmaze video tracking software.
Hippocampal Slice Preparations and Electrophysiological Recordings. Coronal hippocampal slices (350 µm thick) from adult male mice were prepared with Leica Vibratome in ice-cold cutting solution containing (in mM) 30 NaCl, 26 NaHCO3, 10 Glucose, 194 sucrose, 4.5 KCl, 1.2 NaH2PO4, 1 MgCl2 and continuously bubbled with carbogen (95% O2-5% CO2). The slices were then recovered at room temperature for 1 hour. Slices were transferred into the recording chamber continuously perfused at 12 ml/min with artificial cerebrospinal fluid (ACSF) at 37 ℃. The constituent of ACSF are the followings: (in mM): 124 NaCl, 4.5 KCl, 1 MgCl2, 2 CaCl2, 1.2 NaH2PO4, and 26 NaHCO3, continuously bubbled in carbogen. Long-term potentiation (LTP) was induced by HFS (high frequency stimulation, 100 HZ, 1 s) in the CA3 area. Field excitatory postsynaptic potentials (fEPSPs) were recorded using a glass electrode (filled with NaCl, 3–6 MΩ) placed into the stratum radiatum of the CA1 area. Signals were amplified (gain 100) and filtered (3 kHz), then digitized (10–100 kHz; National Instruments). After a 20-min baseline period preceded attempted LTP induction, Recordings were continued for at least 50 min following LTP induction. LTP was quantified by comparing the mean fEPSP amplitude over the post period with the mean fEPSP amplitude during the baseline period and calculating the percentage change from baseline. Data were collected and analyzed on or off-line by using pClamp 10.4 software (Molecular Devices, Sunnyvale, CA) software.
paDESI-MSI. The mice were sacrificed after behavioral tests. The brain was immediately removed from the skull and flash frozen in liquid nitrogen for 15 s. the frozen mouse brain was transferred to the cryostat chamber of a Vibratome (VT 1200S, Leica, Germany) at − 20 °C. The brain tissue was cut into 16-µm-thick coronal sections and collected onto clean microscope slides. DESI/PI MSI System consisted of a DESI sprayer, a 2D scanning stage, and a postphotoionization interface. A solvent was infused at a flow rate of 3 µL/min through a DESI sprayer (50 µm i.d. and 150 µm o.d. inner fused silica capillary and a 250 µm i.d. and 350 µm o.d. outer fused silica capillary) and directed onto the surface of a tissue slice with a 53° angle of incidence with the assistance of the nebulizing N2 gas (120 psi). The flow of the solvent was driven by a syringe pump, and the metal needle tip was connected to a high-voltage power supply (3500 V for the positive ion mode and − 4000 V for the negative ion mode). The desorbed compounds were sucked in the heated transfer tube (i.d. 0.5 mm, o.d. 1/16 in.) with a 10° angle of collection, and the un-ionized neutral molecules were ionized in an ionization tube (i.d. 4 mm, o.d. 10 mm) by a coaxially oriented krypton DC discharge vacuum ultraviolet (VUV) lamp, which was positioned to shine toward the exit of the transfer tube. Then the ionized species was transferred into a capillary of mass spectrometer. In order to improve the transfer efficiency, an air-flow assisted transport arrangement was added in this interface, and a pneumatic diaphragm pump (60 L/min, model GM-1.0A, Jinteng Experimental Equipment Co., Ltd., Tianjin, China) was connected to the side port of the ionization tube. In experiments, the transfer tube and ionization tube were kept at 300 °C. Note that the krypton lamp was turned off in the DESI mode and turned on in the DESI/PI mode. All imaging data were collected on an Agilent 6224 Accurate-Mass TOF mass spectrometer (Agilent, USA). The flow rate and temperature of drying gas of the mass spectrometer were set at 5 L/min and 325 °C, respectively. A programmable motorized X-Y scanning stage (GCD- 203050M, Daheng, Beijing, China) was used for tissue imaging, and the scanning process was allowed to be synchronized with the Agilent mass spectrometer data acquisition by the customized stage control software. The sample surface was line scanned in the X direction with a stepper motor at a velocity of 370 µm/s while acquiring mass spectra every 0.5 s. The distance between adjacent scan lines in the Y direction was 200 µm. The acquired multiple scan lines were combined in one data file for ion distribution images by using the freely available standalone version of the MSiReader software. Two brain slices respectively from vehicle and simvastatin group on a microscope slice were scanned at the same time. The cholesterol intensity in simvastatin group were normalized with the slice from vehicle group which on the same microscope slice. There were 18 slices were calculated from 6 mice in each group. The identifications for most of these peaks were facilitated by accurate m/z values, comparison of isotope distribution patterns, and tandem mass spectrometry.
Statistics. In our study, no statistical methods were used to predetermine sample sizes, all experiments and data analysis were conducted in a blinded way. All statistical analyses for in vitro recording and behavioral experiments were performed using Prism7 software (GraphPad). Data were statistically compared by unpaired t test, as indicated in the specific figure legends. Average values are expressed as the mean ± SEM. P < 0.05 was considered significant.