Research reportThe effects of a mild stressor on spontaneous alternation in mice
Introduction
The deleterious effects of stressful experiences on cognitive functions have been investigated under many experimental conditions in both humans and animals [5], [18], [24], particularly in memory processes [8], [10], [12], [20], [22], [37]. The psychobiological consequences of stressful events can be reduced after administration of benzodiazepines [14] or changes in environmental conditions [41]. Some of the effects of stressful events may be mediated by the direct action of glucocorticoids on brain functions, particularly in the hippocampus, a region with a high density of glucocorticoid receptors [25] and crucially involved in spatial memory [17], [28], [31], [32], [33], [39]. Increases of plasma glucocorticoid concentrations either by endogenous [23], [34], [35], [40] or exogenous [3], [4], [7], [26], [30] means or the administration of glucocorticoid receptor agonists [9], [42] impaired different learning tests. Moreover, chronic corticosterone administration impaired spontaneous alternation [2]. Nevertheless, chronic corticosterone administration did not cause spatial learning deficits so severe as a prolonged psychosocial stressful event, indicating that corticosterone is not the sole mediator of the behavioral consequences of stressful events [18]. On the other hand, short-term glucocorticoid administration sometimes facilitates learning, for example during active or passive avoidance learning [11] and the same effect can be seen after inescapable stress as well, for example during acquisition of the classically conditioned eyeblink response [38] and spatial learning [21].
In the present study the effects of a mildly stressful event (exposure to a novel bright lit open field), as indicated by increases of plasma concentrations of corticosterone, were evaluated on two-trial spontaneous alternation. After a forced trial to one of two arms in a T-maze, mice have a strong tendency to choose the opposite arm. This behavior requires spatial working memory and is sensitive to bilateral lesions of widespread areas of the brain including the hippocampus [6], [13], [17]. The stressful event occurred at different periods but always before the choice trial. It could either be escapable (temporary entries into an adjacent dark chamber) or inescapable (continuous exposure). By such means we wished to investigate whether a stressful event impairs storage or retrieval processes and whether a differential effect can be demonstrated as a function of cognitive mediation of this event.
Section snippets
Animals
The animals used in this study were 120 male mice of the C57BL/6 strain, 3–5 months old, weighing between 27 and 32 g, obtained from our own colony. They were reared five per cage in standard conditions (12 h light/12 h dark: 08:00–20:00/20:00–08:00; 20–22°C; food and water available ad libitum). The animals were regularly handled before the experiment in order to minimize handling-related stress and then split into three groups: escape stress, inescapable stress, and unstressed controls.
Open field
The
Plasma corticosterone concentrations
A significant group effect was seen for plasma corticosterone concentrations immediately after exposure to the open field during 12–14 min (F(2,27)=21.709, P<0.0001) or after this 12–14 min exposure was followed by a 10 min delay period spent in the home cage (F(2,27)=16.709, P<0.0001). Tukey HSD test indicated that both the escapable (P<0.001) or the inescapable (P<0.001) stressful event increased corticosterone levels either with or without the delay interval (Fig. 2).
Spontaneous alternation
Irrespective of the
Discussion
The alternation rate of unstressed controls was significantly above chance irrespective of the retention interval, lasting up to 24 min. Mice can alternate even at retention intervals of at least 1 h [6]. The presence of alternation presupposes an intact short-term memory for the previously visited arm [17], [32], [33] and the motivation to explore the novel arm instead of the previously explored arm.
The purpose of the present study was to evaluate the effects of a mildly stressful event
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