Effect of novel experiences on retention of inhibitory avoidance behavior in mice: the influence of previous exposure to the same or another experience.

Mice were trained and tested in a step-through inhibitory avoidance task with a 24-h interval between training and testing. At one of several intervals prior to the test session (9 h, 6 h, 3 h, or 6 min), they were given one of the following novel experiences: 4 min in a small Plexiglas box containing an empty water bottle, or 4 min hanging from the wire mesh ceiling of a large Plexiglas box. When given 3 h or 6 min before testing, both novel experiences enhanced retention test performance. The effect was antagonized by naltrexone and mimicked by an administration of beta-endorphin 6 min prior to testing. Thus, the findings are consistent with previous evidence suggesting that the effects of novel experiences on retention test performance are due to an activation of the brain beta-endorphin system. When one of the novel experiences given 3 h or 6 min prior to testing was preceded by the same experience given 6 h earlier retention test performance was not enhanced. Thus, the enhancing effect is obtained only if the experience is novel. Further, an experience given prior to retention testing did not affect performance if either the same or a different experience was given 3 h earlier. This finding is consistent with previous evidence indicating that following a novel experience, the brain beta-endorphin remains unresponsive for several hours. These results provide additional evidence that novel experiences prior to retention testing affect retention performance and provide additional support for the view that the effect may involve the release of beta-endorphin.(ABSTRACT TRUNCATED AT 250 WORDS)