Ventral Hippocampal Muscimol Disrupts Context-specific Fear Memory Retrieval After Extinction in Rats

Research aimed at understanding Pavlovian fear memory extinction has yielded considerable insight into the conditions under which fear memories may become inhibited. After extinction, Pavlovian fear memory retrieval is context-specific. Fear memories are not expressed in the extinction context, but they are expressed in every other context. Research indicates that the dorsal hippocampus mediates the context-specific expression of fear memory, but the role of the ventral hippocampus in mediating this process is unknown. Insofar as the ventral hippocampus is involved in the acquisition and expression of both context and tone fear, we asked whether GABA systems in the ventral hippocampus mediate context-specific fear memory retrieval after extinction. Experiment 1 showed that ventral hippocampal inactivation with muscimol disrupted context-specific fear memory retrieval. Experiment 2 showed that rats infused with muscimol can discriminate a context in which they were shocked from a neutral context. Nonetheless, they do appear to have a mild impairment in this task. Experiment 3 showed that ventral hippocampal muscimol did not disrupt locomotor activity, but did result in a slight increase in freezing and grooming, an effect that cannot explain the context-specific retrieval deficit demonstrated in experiment 1. These data are consistent with a role for the ventral hippocampus in mediating context-specific fear memory retrieval.