Synaptic plasticity and memory storage in the hippocampal formation of monkeys

Long-term potentiation (LTP) and long-term depression (LTD) are both forms of synaptic plasticity which are recognized as a neural substrate for memory. LTP and LTD have been studied extensively in the hippocampus using rodent models. However, there has been no information about synaptic plasticity in the primate hippocampus. Therefore, we addressed this issue in the present study. In the first fiscal year, we exploited a model for recording evoked potentials in the hippocampus of monkeys, precisely implanting stimulation and recording electrodes in the perforant pathway and dentate gyrus, respectively, based on the coordinates determined by MRI images. In the second fiscal year, using this animal model, we tested the effects of high-frequency or low-frequency repetitive stimulation (HFS or LFS) on evoked potentials, and found that only the HFS with the highest frequency (400 Hz) induced LTP, while all LFS conditions failed to induce LTD. In the third fiscal year, we compared the induction and maintenance of LTP between monkeys and rats, and found that LTP was stably maintained more than 1 month without decay in monkeys, while it decayed rapidly and returned to the baseline level within a week in rats. Furthermore, we induced LTP in senile and young monkeys, and tested the relations between LTP induction or maintenance and memory task performance. The senile monkey tended to show worse task performance than the younger monkey. However, no clear correlations between LTP induction or maintenance and task performance were observed. These data indicate that 1) LTP can be a ubiquitous model for long-term synaptic plasticity, and 2) the difference of LTP maintenance between the 2 species is involved in the difference of hippocampal-dependent duration necessary for memory consolidation between the species (i.e., longer in primates and shorter in rodents).