Abstract
The hippocampus is important for declarative memory in humans. Long-term potentiation (LTP) is a form of synaptic plasticity, which has been studied extensively in the rodent hippocampus. However, information about LTP in the primate hippocampus is limited, which is primarily due to difficulties in the precise implantation of electrodes within deep structures of the primate brain. We have been developed a technique to record evoked potentials in the monkey hippocampus, overcoming the difficulty by combination of MRI with stereotaxic procedure. Using this model, we investigated the induction and maintenance of hippocampal LTP in monkeys and compared its properties with those of rats. Stimulation and recording electrodes were implanted in the perforant pathway and dentate gyrus, respectively. The effects of high-frequency stimulation (HFS, 400 Hz, 20 pulses x 20) on evoked potentials were tested in an awake condition. As results, HFS reliably induced LTP in the hippocampus of both species. LTP was maintained stably for more than 4 weeks in monkeys, whereas it declined to the basal level within a week in rats. These data suggest that 1) the present experimental model of evoked potential recording is suitable for testing long-term synaptic plasticity, and 2) synaptic plasticity in the hippocampus is more stably maintained in monkeys than in rats, which could be involved in the specific difference of duration for which temporary graded retrograde amnesia is observed. [J Physiol Sci. 2007;57 Suppl:S169]
