Lesions of the ventral hippocampus, but not the dorsal hippocampus, impair conditioned fear expression and inhibitory avoidance on the elevated T-maze

The hippocampus has been suggested to be involved in spatial (or configural) memory and also in the inhibition of certain response or goal alternatives. An increasing number of anatomical, physiological, and behavioral studies indicate that the hippocampus is functionally heterogeneous along the dorsal-ventral axis. Identification of distinct behavioral roles for the dorsal (DH) and ventral (VH) hippocampus may resolve differences between the various theoretical accounts of hippocampal function. The present study examined the effects of electrolytic lesions restricted to the DH or VH on fear-conditioned freezing, passive avoidance on the elevated T-maze (ETM) test of anxiety, and general activity in male Sprague-Dawley (Charles-River derived) albino rats. We found that rats with lesions of the VH, but not DH showed reduced freezing to both context and tone conditioned stimuli (CS). Rats with VH lesions also showed a reduced latency to emerge from the enclosed arm on trials 2 and 3 of the ETM (indicating reduced anxious behavior), while having no effect on the latency to escape from the open arms on trial 4. There were no differences in activity between the groups. These results indicate that the VH and DH are differentially involved in passive avoidance on the ETM and conditioned freezing to context and tone CS. We suggest that the VH may be specifically involved in modulating goal-oriented, defensive behavior expression through hypothalamic and amygdaloid connections.