The roles of the dorsal and ventral hippocampus in fear and memory are unclear. This study examined the effects of temporary inactivation of the dorsal or ventral hippocampus on unconditioned and conditioned fear, using the shock-probe test. In Experiment 1, rats received either dorsal or ventral hippocampal infusions of lidocaine or saline, before exposure to an electrified shock-probe (acquisition I). In Experiment 2, rats received lidocaine or saline infusions after exposure to the shock-probe (acquisition II). In both experiments, a retention test in the same apparatus was given 24 h later, at which time the hippocampus was no longer inactivated, and the probe was disconnected from the shock-source. Because ventral hippocampal inactivation impaired fear behaviour during acquisition, and dorsal hippocampal inactivation impaired fear behaviour (probe avoidance) during retention, we concluded that 1) the ventral hippocampus plays a role in the expression of untrained fear reactions whereas 2) the dorsal hippocampus plays a role in encoding memory of the fearful experience.
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