[Effects of aversive stress during brain development on hippocampal synaptic and behavioral responses to emotional stress at postadolescence].

The aim of the present study was to examine whether early postnatal stress during brain development alters the synaptic response to emotional stress, focusing on the behavioral expressions to contextual fear conditioning (CFC) at postadolescnece. Pups were exposed to an aversive stimulus, foot shock (FS), at the postnatal period of the second week (2w-FS) and the third week (3w-FS). At the postadolescent period (10- to 12-week old), the evoked potential in the hippocampal CA1 field was measured with simultaneous determination of fear-related freezing behavior. CFC-induced freezing behavior decreased remarkably in 2w-FS, but not 3w-FS, compared to non-FS controls. The evoked potential in the CA1 field was reduced during exposure to CFC in controls. Meanwhile this synaptic inhibition was attenuated in 2w-FS. Furthermore, the suppression of synaptic responses, including long-term potentiation (LTP), mediated via 5-HT(1A) receptors was not observed in 2w-FS. These results suggest that the second postnatal week appears to be the critical period for developing "proper" synaptic response underlying regulation of emotional stress, in which the 5-HTIA receptor-mediated regulation may be involved.