Striking differences in synaptic facilitation along the dorsoventral axis of the hippocampus

Abstract

Hippocampus displays functional heterogeneity along its long axis which has been interpreted in terms of segregation of inputs. Recent evidence has shown that there are also important differences in the organization of the local neuronal circuitry between the dorsal (DH) and the ventral hippocampus (VH). Synaptic plasticity is a crucial factor for the function of the hippocampal circuit. In this study I compared the synaptic facilitation of the CA1 excitatory postsynaptic potential (EPSP) between dorsal and ventral rat hippocampal slices using field recordings and paired-pulse stimulation delivered at varying inter-pulse intervals (IPIs). The facilitation of the EPSP-slope displayed an exponential decline with increasing stimulation strength or IPI. Furthermore, the facilitation of threshold EPSP-slope was significantly higher in DH than in VH at all IPIs. Most remarkably, the facilitation of the area of EPSP displayed a prominent peak at around 200ms in DH but not VH. This optimal facilitation declined abruptly at a position located two thirds of the way along the dorsoventral axis. N-methyl-d-aspartic acid receptors (NMDARs) contributed to the facilitation of EPSP-area in an IPI-selective manner in DH but not VH. Furthermore, NMDARs participated to the single-pulse-evoked EPSP-area more in VH than in DH. Blockade of GABAB receptors (GABABRs) eliminated the prominent facilitation at around 200ms and abolished the large dorsoventral difference in the facilitation of EPSP-area. Blockade of GABAA receptors (GABAARs) increased the maximum area of EPSP more in VH than in DH and reversed facilitation into GABABR-dependent depression that was more robust in DH than in VH. I conclude that interactions between the synaptic actions of GABABR, GABAAR, and NMDAR contribute to diversifying short-term synaptic plasticity along the dorsoventral axis of the hippocampus. It is hypothesized that this diversification has important implications for the information processing performed by the local circuitries of the two hippocampal segments.