Simultaneous induction of long-term potentiation in the hippocampus and the amygdala by entorhinal cortex activation: mechanistic and temporal profiles.

The medial temporal lobe, including the entorhinal cortex, the amygdala and the hippocampus, has an important role in learning and memory, and its circuits exhibit synaptic plasticity (long-term potentiation [LTP]). The entorhinal cortex is positioned to exert a potent influence on the amygdala and the hippocampus given its extensive monosynaptic projections to both areas. We therefore studied the effects of activation of the entorhinal cortex with simultaneous recording of LTP in the hippocampus and amygdala in the anesthetized rat. theta Burst stimulation of the lateral entorhinal cortex induced LTP simultaneously in the basal amygdaloid nucleus and in the dentate gyrus. However, the mechanisms involved in the induction of LTP in the two areas differed. The N-methyl-D-aspartate receptor antagonist 3-[(+/-)-2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid delivered 1 h before LTP induction (10 mg/kg, i.p.), blocked LTP in the dentate gyrus but not in the amygdala. In addition we found that the basal amygdala as well as the dentate gyrus sustained late-phase LTP (10 h) which may participate in memory encoding and/or modulation processes. Overall, the results suggest a coordinating role for the entorhinal cortex by simultaneously modulating activity and plasticity in these structures, albeit through different mechanisms. Interactive encoding of this sort is believed to endow memories with a different, more integrative, quality than when either pathway is activated alone.