Anxiogenic stimuli in the elevated plus-maze

Abstract

Untreated rats normally avoid the open arms of the “elevated plus-maze,” preferring instead the closed arms, whereas rats treated with antianxiety drugs (e.g., diazepam) show far less open-arm avoidance. Although it has often been assumed that rats avoid the open arms because of novelty, height, or open space, the anxiogenic role of these stimuli in the plus-maze has not been systematically examined. In Experiment 1, rats were repeatedly exposed to the elevated plus-maze with the expectation that their “fear” of the open arms would habituate over trials. Instead, open-arm avoidance actually increased on the second trial and showed no evidence of habituating after 18 trials. In Experiment 2, three 30-min sessions of confinement to the open arms (“flooding”) failed to decrease rats' open-arm avoidance. Instead, rats that had received flooding avoided the open arms significantly more than control rats during the first test. Experiment 3 showed that although diazepam-treated rats avoided the open arms less than vehicle-controls on the first test.this difference dissipated across test trials. Further, diazepam had no carryover effect on rats' subsequent avoidance of the open arms in a nondrugged state. In Experiment 4, plus-maze height was varied from 50 to 6 cm, but rats did not display more open-arm activity as maze height decreased. In Experiment 5, height cues were manipulated by placing a “floor” 8 cm beneath one open arm while leaving the floor of the other open arm at 50 cm. Rats did not avoid the “low” open arm less than the “high” open arm. In Experiment 6, rats explored an open arm with a raised Plexiglas edge more than an open arm with standard flat edges despite identical height cues under these arms. Thus, it appears that open space, rather than height or novelty, is the anxiogenic stimulus in the elevated plus-maze.