Gender Difference Of Learning Ability In Three Strains Of Mice Tested In Morris Water Maze

Learning and memory is the highly advanced function of the cerebralcortex. Morris first designed and used the water maze method to evaluate thespatial learning and memory ability of rodent. Nowadays, Morris water mazetest has gradually become a most frequently used method in the research onlearning and memory. Rats and mice are useful laboratory animals in Morriswater maze test. Most studies indicated that the ability of reference memory inmale rats is superior to that in female rats. However, several investigations onhuman object-location-memory reveal a superior performance by womenrelative to men. So far, a few studies reported gender difference in mouseperformance in Morris water maze test, and it is also not clear if genderdifference of spatial learning and memory exists in mice. The C57BL/6miceshow better performance at spatial learning in Morris water task than BALB/cmice. Albino Kunming strain mice, originally introduced from Swiss mice atthe Hoffkine Institute of India, are the most widely used laboratory animals inChina and frequently used in experiments in behavioral research. In thepresent study, we observed the performance in Morris water maze task usingC57BL/6, BALB/c and Kunming mice to analyze the behavioral genderdifferences in spatial reference memory test, spatial probe test and spatialworking memory test.Objective: Using C57BL/6, BALB/c and Kunming mice to analyze thebehavioral gender differences in spatial reference memory test, spatial probetest and spatial working memory test in Morris water maze.Methods: A total of36male mice and36female mice of three strains(C57BL/6, BALB/c and Kunming) were used. The experimenters were notallowed to walk about in the laboratory and keep the position of the laboratoryitems fixed. One day before experiments, each mouse was allowed to swim freely in the tank without platform for1min. The experimental designconsisted of three phases, spatial reference memory test, spatial probe test andspatial working memory test.Firstly, in the place navigation test the platform was in the NW quadrant1cm below the water surface. animals were trained with four tests to find theplatform on each day during6days of training. During the daily training, eachmouse facing the wall of the tank was gently released into water from SEquadrant for the first and the fourth tests of training, and released into waterfrom SW and NE quadrants randomly for the second and third tests. Micewere allowed to swim freely until they climbed onto the platform within60s.Data acquisition and processing by the computer system started as soon as themouse was released into water, and it stopped automatically when the mousefound the hidden platform. The time required to find the platform was definedas escape latency. Once the mouse found the platform, it was allowed toremain there for10s. Mice failed to find the platform within60s had to beplaced on the platform for10s by experimenter, and their performance score(escape latency) was marked as ‘‘60s’’. After each swim test, mice were driedwith a towel to prevent cold stress, and took60s of rest before the next test.After above6days of training, the animals received a spatial probe test.Mice were released into water from the starting point of quadrant SW andallowed to swim for60s in the pool without the platform. In the spatial probetest, percent quadrant time (PQT) refers to amount of time mice searched avirtual quadrant (i.e.,25%of total pool surface area) centered on the locationof the platform during training. Percent path length (PPL) refers to amount ofpath length mice searched a virtual quadrant centered on the location of theplatform during training. Crossings (Cs) refer to number of times mice crosseda virtual quadrant centered on the location of the platform during training.Values of percent quadrant time (PQT), Percent path length (PPL), Averagespeed (AS) and Crossings (Cs) were recorded by the computer systemautomatically.Lastly, we carried out a spatial working memory test in the mice after spatial probe test. There were4training days, and the platform position waschanged on a different day. On each training day the mouse received four tests,and the platform remained in the same place. Changes of the platform positionwere in a clockwise order each day. The mice were released into the pool fromopposite quadrant for the first and fourth test, and other2quadrants excepttarget quadrant were chosen for the second and third test at random. Escapelatency and swimming speed were recorded.Result:1Spatial Reference Memory Test1.1Changes in Average Escape Latencies in Three Strains of Mice during6days of TrainingThe average escape latencies in C57BL/6mice became significantlyshorter from Day3or Day4of training (P<0.01), and that in male Kunmingmice became significantly shorter from Day5of training (P<0.01), whencompared with BALB/c mice of the same sex. There was no significantdifference in the average escape latency between C57BL/6mice and Kunmingmice of the same sex (P>0.05). In comparison with Day1of training, theaverage escape latencies of C57BL/6mice of both sexes and female Kunmingmice became significantly shorter from Day3or Day4of training (P<0.05and P<0.01), however that of female BALB/c mice and male Kunming micebecame significantly shorter on Day6of training only (P<0.05).1.2Changes in Effective Search Strategy in Three Strains of Mice during6Days of TrainingIn comparison with Day1, there was no significant change in percentage ofeffective search strategy in the same BALB/c mice of both sexes during thefollowing daily training (P>0.05), however female C57BL/6mice and femaleKunming mice had significantly higher percentage of effective search strategyfrom Day2of training (P<0.05and P<0.01). On the other hand, maleC57BL/6mice had significantly higher percentage of effective search strategyfrom Day5of training (P<0.05), and the percentage of effective searchstrategy was significantly higher for female C57BL/6mice than female Kunming mice on Day4of training (P<0.01). Male Kunming mice hadincreasing trend of effective search strategy but this was not statisticallysignificant（P>0.05）.1.3Spatial Probe TestWhen compared with the parameters of BALB/c mice of the same sex,PQT、AS and PPL values in C57BL/6and Kunming mice of both sexes weresignificantly higher, and Cs values were significantly increased in maleKunming mice and C57BL/6mice of both sexes (P<0.05and P<0.01). PQTand PPL values in the female mice of C57BL/6and Kunming strains wereincreased slightly in comparison with male mice of the same strain, but thistrend was not statistically significant (P>0.05); however, PQT value wassignificantly increased in female BALB/c mice than male (P<0.05). Therewas no significant difference in all the parameters between C57BL/6andKunming mice of the same gender.2Spatial Working Memory TestIn comparison with Day1of training in the three strains of mice, theaverage escape latencies during the following daily training within the samemouse strain did not change significantly (P>0.05), and there was nosignificant difference in the average escape latency between male and femalemice within the same strain of BALB/c, Kunming or C57BL/6(P>0.05).However, the average escape latencies of female C57BL/6mice on Day2andmale C57BL/6mice on Day1and Day4of training were significantly shorterthan those of BALB/c mice of the same gender (P<0.05and P<0.01).In comparison with Day1of working memory test, the averageswimming speeds of female BALB/c mice significantly decreased from Day2to Day4(P<0.01), whereas those of female C57BL/6mice significantlyincreased from Day2to Day4(P<0.05and P<0.01). The averageswimming speeds of Kunming and C57BL/6mice of both sexes during4daysof experiments were significantly faster than those of BALB/c mice of thesame gender (P<0.05and P<0.01).Conclusion: Female C57BL/6and KM mice present a significantly better spatial reference memory performance than males, while BALB/c mice are notsuitable for use in the Morris water maze. Conventional experimental designsused currently in the Morris water maze are not easy to assess the workingmemory or short-term memory ability at least in the three strains of mice.