Analysis of behavioral asymmetries in the elevated plus-maze and in the T-maze
Introduction
Asymmetries at the functional, as well as the cerebral level have been demonstrated not only in man, but also in laboratory animals, and here especially in rats (Gazzaniga et al., 1998, Glick, 1985, Walker, 1980). With respect to rat behavior, several test have been applied to gauge functional asymmetries, including turning in rotometers or open-fields, handedness, choice behavior in a T-maze, and others (e.g., Collins, 1985, Glick and Shapiro, 1985, Pisa and Szechtman, 1986, Schwarting et al., 1991, Tang and Reeb, 2004, Tang et al., 2003, Zimmerberg et al., 1974). Most of this work has been performed with measures of turning behavior showing that asymmetries can be demonstrated at the individual, as well as the population level. Furthermore, several factors have been identified which can determine the degree and laterality of behavioral asymmetry, including the strain and source of animals, their gender, developmental stage and hormonal status (e.g., Dark et al., 1984, Glick and Shapiro, 1985, LaHoste et al., 1988, Robinson et al., 1980). Besides, the conditions of testing have been found to be critical, that is, the time of testing (dark- versus light-phase), and the level of arousal, pre-experience, and stress (e.g., Denenberg and Yutzey, 1985, Myslobodsky and Braun, 1980, Tang and Reeb, 2004, Tang et al., 2003). Finally, it has repeatedly been shown that biases in turning can be induced or enhanced by several types of psychostimulatory drugs, like amphetamines, cocaine or phencyclidine. Most often, but not consistently, the direction of asymmetry was to the right (Ali et al., 1994, Christie and Crow, 1971, Glick and Cox, 1978, Glick et al., 1983).
Here, we investigated behavior in two types of tests, namely the elevated plus-maze and the T-maze. In both tests, the occurrence of spontaneous behavior towards the left or right is very likely due to the specific requirements of these tests, since the animals have to turn around when changing their direction within an arm, or when alternating between arms. Therefore, the tests might serve as useful tools for the detection of asymmetry. To our knowledge, the elevated plus-maze, which otherwise serves as a standard test in research on anxiety (e.g., Rodgers et al., 1997), has not yet been applied to analyze asymmetries in turning or choice behavior. Thus, and in a first step (study I), we took a relatively large sample of rats, exposed them to the plus-maze and tested for possible asymmetries in spontaneous behavior. In a second study, we investigated the effects of MDMA in the plus-maze. This drug has psychostimulatory actions (Callaway et al., 1990, McNamara et al., 1995, Marston et al., 1999) and might therefore serve as a tool to induce or enhance existing functional asymmetries. For this analysis, we took the data from an experiment published before (Ho et al., 2004), where we demonstrated the stimulatory and dose-dependent anxiogenic-like or anxiolytic-like effects of this drug. Here, and in contrast to our previous analysis, we focused our measures on those of possible behavioral asymmetry in the plus-maze.
Finally, we present data from a study (study III), where we asked for asymmetries in a T-maze. This analysis was initiated by several previous reports which showed behavioral lateralization in such kind of task (Rodriguez et al., 1992, Zimmerberg et al., 1974). Interestingly, the majority of these studies points at an asymmetry in favor of the right side (Andrade et al., 2001, LaHoste et al., 1988, Rodriguez and Alonso, 1993). For the present analysis, we took a sample of rats, which had underwent several test for other purposes before (open-field test, plus-maze, inhibitory avoidance learning; Schwarting and Borta, 2002). Therefore, this final study should be considered as of exploratory nature, in which we asked whether asymmetries might be detectable, and whether previously reported biases to the right might also be observed.
Section snippets
Animals
Eighty male Wistar rats (Harlan Winkelmann, Borchen, Germany) were housed in groups of five rats in acrylic cages (35 cm × 56 cm × 19 cm) in an animal room (22–24 °C) under a 12 h light:12 h dark cycle (lights on at 07:00 h) with food and water provided ad libitum. The mean body weight was 258.7 ± 0.7 g (mean ± S.E.M.) at the beginning of the experiment. Each animal was handled for several days (5 min each) prior to the experiment.
General procedure
All animals underwent a standard testing procedure in the lab, that is,
Data analysis
Statistical testing was performed using either paired or unpaired 2-tailed t-tests. The results of these tests, that is, the P-values were taken as empirical measures of effect. Therefore, exact P-values will be presented which were not interpreted in terms of statistical significance. P-values below 0.001 will not be presented as exact values, but as <0.001.
Turns within arms
As a first step, turning frequency was analyzed irrespective of direction (Table 1). Here, it was found that turns were more frequent in the closed than in the open arms (test 1: t[79] = 2.897, P = 0.005; test 2: t[79] = 6.899, P < 0.001). Furthermore, turns within open arms declined from test 1 to test 2 (t[79] = 5.332, P < 0.001), whereas turns within closed arms did not change substantially (t[79] = 1.399, P = 0.166).
As a next step, these turns were analyzed with respect to their location
Discussion
The aim of the present study was to test for possible behavioral asymmetries of male Wistar rats in the elevated-plus maze and in the T-maze.
Acknowledgements
This work was supported by a research grant from the Deutsche Forschungsgemeinschaft (Schw 559/5-3). The authors would like to thank Jeanette Löhn, Manuela Klein, Rahel Müller, Maximiliane Florentine Thöne and Valerian Carlos Cunderlik for their assistance in data collection.
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