Effects of fimbria lesions on trace two-way active avoidance acquisition and retention in rats.

The fimbria-fornix (FF) is the main subcortical input to the hippocampus. It has been shown that FF lesions facilitate performance on a standard-delay two-way active avoidance task (AA2), thought to involve implicit memory. The hippocampal region is required for explicit or relational memory. It has been proposed that the hippocampus and related structures might associate events that are separated in space or time and detect elements shared in common by such discontiguous episodes. Therefore, FF lesions would be expected to impair performance on a trace paradigm, which introduces an interval between the CS (conditioned stimulus) and the US (unconditioned stimulus) and is generally considered a model of explicit memory. We predicted that FF lesions would impair memory in a trace AA2 procedure, while the same lesions would facilitate memory in a standard delay version of the task. To test this hypothesis, two experiments were carried out in 102 male Wistar rats. The first experiment characterized the trace paradigm using this kind of conditioning and demonstrated that control rats were able to acquire and retrieve (24 h and 11 days postacquisition) the association between the CS (tone) and the US (electric foot shock) when a trace interval (5, 10, or 20 s) was interposed between both stimuli. In the second experiment, we investigated the effects of FF electrolytic lesions on the same task using delay and trace (10-s trace interval) paradigms. Surprisingly, FF lesions facilitated the acquisition and the 24-h retention of the AA2 not only on the standard delay paradigm, but also with the trace paradigm. We suggest that facilitative effects could be a result of impairment in contextual learning.