NEUROTOXIC LESIONS OF THE DORSAL HIPPOCAMPUS AND PAVLOVIAN FEAR CONDITIONING IN RATS

Electrolytic lesions of the dorsal hippocampus (DH) produce deficits i n both the acquisition and expression of conditional fear to contextua l stimuli in rats. To assess whether damage to DH neurons is responsib le for these deficits, we performed three experiments to examine the e ffects of neurotoxic N-methyl-D-aspartate (NMDA) lesions of the DH on the acquisition and expression of fear conditioning. Fear conditioning consisted of the delivery of signaled or unsignaled footshocks in a n ovel conditioning chamber and freezing served as the measure of condit ional fear. In Experiment 1, posttraining DH lesions produced severe r etrograde deficits in context fear when made either 1 or 28, but not 1 00, days following training. Pretraining DH lesions made 1 week before training did not affect contextual fear conditioning. Tone fear was i mpaired by DH lesions at all training-to-lesion intervals. In Experime nt 2, posttraining (1 day), but not pretraining (1 week), DH lesions p roduced substantial deficits in context fear using an unsignaled shock procedure. In Experiment 3, pretraining electrolytic DH lesions produ ced modest deficits in context fear using the same signaled and unsign aled shock procedures used in Experiments I and 2, respectively. Elect rolytic, but not neurotoxic, lesions also increased pre-shock locomoto r activity. Collectively, this pattern of results reveals that neurons in the DH are not required for the acquisition of context fear, but h ave a critical and time-limited role in the expression of context fear . The normal acquisition and expression of context fear in rats with n eurotoxic DH lesions made before training may be mediated by condition ing to unimodal cues in the context, a process that may rely less on t he hippocampal memory system. (C) 1997 Elsevier Science B.V.