Diazepam effects on the exploratory behaviour of rats in an elevated runway: evidence for biphasic effects of benzodiazepines.

The purpose of this study was to test the validity of the fear-reduction model of benzodiazepine (BZ) action on the exploration of novelty. According to this hypothesis an animal given a tranquilizer should selectively increase the amount of investigative behaviour in the more novel portion of an elevated maze. To permit comparison of the same behaviours at both ends of the maze, an elevated runway was built with a wall running lengthwise along the midline of one end. In the first experiment, male Sprague-Dawley rats treated with diazepam (2.0 mg/kg, i.p., -30 min) compared to saline-treated animals, increased the time spent exploring the open end of the runway but not the wall end of the runway, thus supporting the fear-reduction model. However, saline-treated animals, made less fearful by repeated prior exposure to the runway, did not show a similar increase in open-end exploration. Instead, they habituated to the novelty of the runway, as grooming and sitting still replaced investigation. In Experiment 2, exploration was rewarded by adding to the open end of the runway a patch of litter soiled by a female rat. This produced a behavioural pattern in naive saline-treated rats very similar to that seen in naive diazepam-treated rats in the first experiment. In Experiment 3, diazepam potentiated the habituation of rats previously familiarized with the runway. The initial increase and subsequent decrease in exploration caused by diazepam were encompassed by the biphasic model of BZ action more adequately than either the fear-reduction or reward-enhancement models.