Dorsal, ventral, and complete excitotoxic lesions of the hippocampus in rats failed to impair appetitive trace conditioning

Three experiments examined appetitive trace and delay conditioning of the licking response (LR). In Experiment 1, normal rats were trained in trace conditioning using different trace intervals (2, 4, or 8 s) and in delay conditioning (i.e., with a 0-s trace) in order to determine an appropriate trace interval for the following lesion experiments. Only the rats trained with a 2-s trace interval ultimately reached the same level of learning as rats trained in delay conditioning. In Experiments 2A and 2B, the performance of rats with dorsal, ventral, and complete excitotoxic hippocampal lesions was compared to that of sham-operated rats in LR conditioning with a 2-s trace. In Experiment 2B, the performance of rats in trace LR conditioning was also compared to that of rats tested in the delay paradigm. In both experiments, acquisition did not differ in lesioned and sham-operated rats and, in Experiment 2B, it was faster in the delay than in the trace paradigm. These results contrast with those showing that aversive trace conditioning is impaired after hippocampal damage. Experiment 3 examined whether the differential effects of hippocampal lesions on aversive and appetitive trace conditioning could be related to a parametric difference, that is, the relative durations of the conditional stimulus and of the trace interval. Again, hippocampal damage failed to produce a learning impairment. It is suggested that the procedure of aversive, but not of appetitive, trace conditioning is context- specific and that an intact hippocampus is required only in these situations. (C) 2007 Elsevier B.V. All rights reserved.