Research reportBehavioral profile of rats submitted to session 1-session 2 in the elevated plus-maze during diurnal/nocturnal phases and under different illumination conditions
Introduction
Circadian rhythm synchronizes [37] and influences a large number of physiological and behavioral processes, such as sleep [29], feeding [12], [36], drinking [53] and body temperature [55], as well as exploratory [53] and social behavior. Within the central nervous system, altered physiological parameters can also be found in the release of neurotransmitters [2], [11], [34], [51], activation of neurotransmitter receptors and associated secondary messenger systems [1], [10], [26], [28], [32], [36], [57] and in the expression of immediate-early genes [17], [20]. In the rat, the nocturnal phase of the circadian rhythm corresponds with the active, inquisitive and responsive period [30].
Recently, this circadian rhythmicity has been evaluated in pre-clinical models of anxiety [16], [27], [44]. Kelliher et al. [30] pointed out that, probably due to reasons of practicality, these behavioral investigations, whilst employing nocturnal species, are almost exclusively performed during the diurnal phase (lights on) of the circadian cycle. To overcome this situation, some laboratories have kept the subjects under a reversed light cycle, but this procedure may not guarantee that all the normal physiological functions will change along with the circadian cycle. In fact, very little of the rat's behavior profile has been studied to identify the minimum time required to elicit a complete behavioral and physiological adaptation to the natural situation. In the elevated plus-maze (EPM), a commonly used model [23], [43], [46] for assessing anxiolytic effects of compounds acting on γ-amino-butyric acid (GABA) [24], [48] and serotonin (5-HT) [19], [50] systems, the circadian phase of testing failed to consistently alter behavior in rats kept under a reversed light cycle [27]. However, it has been suggested that the rodent circadian rhythm could be a possible source of variability in the EPM. Indeed, discrepancies between laboratories in elucidating an anxiolytic effect of 5-HT1A agonists/antagonists in the EPM have been explained by circadian variations in drug activity [21], [44], [45]. For example, Rodgers et al. [44] found that LY297996, a 5-HT1A antagonist, was anxiolytic in the mid-dark phase but not in the mid-light phase. Such behavioral differences agree with the idea of a circadian rhythm in 5-HT1A receptor-mediated effects [12], [34].
Organismic variables, such as species, genetic strain [40], gender and age [26], as well as procedural variables, such as housing [33], prior handling [3], prior stress [52] and previous maze experience [6] have been shown to affect baseline behavior in the EPM. Although some results have shown that illumination conditions failed to alter the rat's behavior in the EPM [5], [14], [39], it may also be considered an aversive stimulus in the EPM [35], since rats tested under low illumination showed more open arm exploration and were generally more active than those tested under high illumination conditions [9], [18]. Test illumination conditions may also explain inconsistencies for 5-HT receptor-active compounds [8], since the same dose of the 5-HT1A agonist, 8-OH-DPAT, has produced anxiolysis under high illumination and anxiogenesis under low illumination conditions [22].
Taking into account these concerns, the purpose of this study was to evaluate the behavioral profile of rats, kept under a normal light cycle, submitted to the EPM in both session 1 (S1) and session 2 (S2), during the diurnal or nocturnal phase and under different illumination conditions.
Section snippets
Subjects
The subjects were 68 male Wistar rats weighing 250–300 g, aged 13–15 weeks at the time of testing, housed in groups of five to six per cage (50×30×15 cm), kept in a room (vivarium), under a normal light cycle (12:12 h light:dark phase; lights on at 06:00 h), in a temperature controlled environment (23±1 °C) and with free access to food and water. The subjects were reared in the above conditions from weaning and 48 h before the experiment, they were moved to an adjacent room under the same light
Full session changes in behavior profile
Table 1 illustrates the two-factor (group×session) repeated measures ANOVA results from full session analysis showing significant main effects and interactions. Briefly, the analysis revealed significant main effects (but no interactions) of the group and session factors for all parameters evaluated in the EPM.
Data illustrated in Fig. 1, Fig. 2 represent the effects of the circadian phases and illumination conditions in rats submitted to the EPM in both S1 and S2. Comparisons using the
Discussion
It has been demonstrated that circadian rhythm influences cognitive, physiological and behavioral patterns [4], [15], [37], [42], [56] as well as drug effects [38], [41], [44], [54] in both humans and animals. Moreover, behavioral responses in rodents may be altered by illumination conditions [9], [18], [22], [27], [35], which adds to the problem of inter-laboratory variability related to this animal model of anxiety.
In the present study, regardless of the circadian phase and the illumination
Acknowledgements
The Brazilian Government-CNPq supported this research; Leandro J. Bertoglio was in receipt of a doctoral fellowship and Antonio P. Carobrez, a research fellowship. The authors thank Gareth Cuttle for English corrections to the manuscript.
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