Muscimol inactivation of the dorsal hippocampus impairs contextual retrieval of fear memory

Some models of hippocampal function have suggested a role of the hippocampu s in contextual memory retrieval. We have examined this hypothesis by asses sing the impact of reversible inactivation of the dorsal hippocampus (DH) o n the context-specific expression of latent inhibition, a decrement in cond itional responding produced by preexposure to a to-be-conditional stimulus. In Experiment 1, rats received tone preexposure either in the context that would later be used for extinction testing (context A) or in a different c ontext (context C); a third group of rats did not receive tone preexposure. All rats then received fear conditioning, which consisted of tone-footshoc k pairings, in a third distinct context (context B). The following day cond itional fear to the tone was assessed in one of the preexposure contexts (c ontext A) by measuring freezing during a tone extinction test. Rats preexpo sed and tested in the same context exhibited less freezing to the tone than either rats preexposed and tested in different contexts or nonpreexposed r ats. These results indicate that the expression of latent inhibition is con text specific. In Experiment 2, DH inactivation eliminated the context-spec ific expression of latent inhibition. Compared with saline-infused rats, ra ts infused with muscimol into the DH exhibited low levels of tone freezing independent of whether they had received tone preexposure in the test conte xt or in a different context. Experiment 3 revealed normal contextual discr imination in rats after DH inactivation. These results suggest the DH is re quired for contextual memory retrieval in a latent inhibition paradigm.