Synaptic Modulation via Basolateral Amygdala on the Rat Hippocampus-Medial Prefrontal Cortex Pathway in Fear Extinction

The present study elucidated the functional role of modulatory effects of basolateral amygdala (BLA) on synaptic transmission in the rat hippocampus medial prefrontal cortex (mPFC) pathway, compared with the hippocampal dentate gyms (DG). Exposure to conditioned fear stress (CFS) or prior BLA activation enhanced tetanus-induced long-term potentiation (LTP) in DG. A similar synaptic response was found by low frequency stimulation (LFS) prior to tetanus. In mPFC, they did not affect LTP, but prior BLA activation, as well as pretreatment with the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (0.1 mg/kg, i.p.), suppressed LFS-primed LTP. This BLA-mediated synaptic pattern was mimicked by synaptic changes observed in the fear extinction process; prior BLA activation suppressed the synaptic potentiation responsible for extinction retrieval and attenuated decreases in fear-related freezing behavior. These data suggest that LFS-primed LTP in mPFC is related to the neural basis of extinction. Extinction-related synaptic potentiation did not occur in a juvenile stress model that exhibited extinction deficit. In addition, LFS-primed LTP was suppressed in this model, which was reversed by the NMDA-receptor agonist D-cycloserine (15 mg/kg, i.p.). These findings suggest that modulatory effects of BLA on synaptic function in the hippocampus mPFC pathway play a significant role in fear extinction in rats.