Morris water maze

All rats (n = 12 in each group) performed MWM test with the protocols adopted from our previous work (Kan et al., 2016). The MWM is a circular tank placed in a quiet and dimly lit room with visual clues around. The inner surface of the tank was painted in black and filled with water (24 ± 1°C) in about 20 cm in depth. A hidden platform was placed in one fixed quadrant hidden in about 1.5 cm below the water surface. On top of the circular tank is a video camera tracking the behavior performance of the rats. Around the circular tank is a series of large support holders with thick curtains to avoid sharp lights if necessary. Briefly, the MWM test contains two sections of spatial acquisition trials (day 1–5 and day 31–35) and probe trials (day 7 and day 37) to assess learning and spatial working memory in both short term (<7 days) and long term (>30 days). For the spatial acquisition trial, the rat was gently placed in one of the quadrants facing the wall of MWM circular pool and then was allowed to swim freely for 60 s to find the hidden platform. If the rat succeeds, it was allowed to stay on the platform for 5 s. Otherwise, it would be manually placed on the platform and allowed to stay for 20 s. Four spatial acquisition trials were performed in each day to let the rat entering the MWM from four different quadrants and the training session lasted for 5 consecutive days. On day 7, a probe trial was performed with the hidden platform removed. Rats were gently placed in the opposite quadrant to the original location of the platform and allowed to swim freely for 30 s. On day 6, the rats were allowed to have a rest. The latency to find the platform in spatial acquisition trials and the time spent and crossovers in platform quadrant in probe trials were recorded and analyzed using an animal behavior tracking system (EthoVision, Noldus Information Technologies, Netherlands). From days 31 to 35, another section of spatial acquisition trials was repeated followed with another probe trial on day 37. After finishing the behavior examination, the rats were sacrificed by over inhalation of isoflurane (ISO) with injection of potassium via the femoral vein. The timeline for MWM protocol is summarized in Figures 1B, ​,2A2A.

Lipopolysaccharide (LPS) exposure caused short- and long-term impaired learning and spatial working memory. (A) Morris Water Maze (MWM) test protocol. (B) Rats with LPS exposure had longer escape latency compared with those in the control group from day 2 to day 4. (C,D) Rats with LPS exposure spent shorter time in the target quadrant and performed fewer platform site crossovers compared with those in the control group. (E) Rats with LPS exposure had longer escape latency compared with those in the control group from day 34 to day 35. (F,G) Rats with LPS exposure spent shorter time in target quadrant and performed fewer platform site crossovers compared with those in the control group. Escape latency was analyzed by two-way ANOVA for repeated measurement followed by Bonferroni post hoc analysis. Time spent in the target quadrant and number of crossovers were analyzed using two-sample t-test. Data were presented as mean ± standard error of mean (SEM). ^*p < 0.05, ^**p < 0.01, ^***p < 0.001 vs. control group.