The ventral hippocampus and fear conditioning in rats - Different anterograde amnesias of fear after tetrodotoxin inactivation and infusion of the GABA(A) agonist muscimol

Studies on the involvement of the rat hippocampus in classical fear conditi oning have focused mainly on the dorsal hippocampus and conditioning to a c ontext. However, the ventral hippocampus has intimate connections with the amygdala and the nucleus accumbens, which are involved in classical fear co nditioning to explicit and contextual cues. Consistently, a few recent lesi on studies have indicated a role for the ventral hippocampus in classical f ear conditioning to explicit and contextual cues. The present study examine d whether neuronal activity within the ventral hippocampus is important for the formation of fear memory to explicit and contextual cues by classical fear conditioning. Tetrodotoxin (TTX; 10 ng/side), which completely blocks neuronal activity, or muscimol (1 mug/side), which increases GABA(A) recept or-mediated inhibition, were bilaterally infused into the ventral hippocamp us of Wistar rats before the conditioning session of a classical fear-condi tioning experiment. Conditioning to a tone and the context were assessed us ing freezing as a measure of conditioned fear. TTX blocked fear conditionin g to both tone and context. Muscimol only blocked fear conditioning to the context. The data of the present study indicate that activity of neurons in the ventral hippocampus is necessary for the formation of fear memory to b oth explicit and contextual cues and that neurons in the ventral hippocampu s that bear the GABA, receptor are important for the formation of fear cond itioning to a context. In addition, both bilateral muscimol (0.5 mug/side a nd 1 mug/side) and TTX (5 ng/side and 10 ng/side) infusion into the ventral hippocampus dose-dependently decreased locomotor activity in an open-field experiment.