Differences in synaptic transmission between medial and lateral components of the perforant path.

The differences between the potentials recorded in the hilus of the dentate gyrus following test shocks applied separately to the medial perforant path (MPP) and the lateral perforant path (LPP) have been ascribed to the greater length of dendrite over which the LPP potentials are electrotonically conducted to the somata of the granule cells. We tested this hypothesis by recording MPP and LPP evoked potentials in the hilus and in the molecular layer of both in vivo and in vitro preparations. Analysis of field potential and current source density depth profiles in vivo indicated that different waveshapes occurred not only in the hilus but at the sites of synaptic contact in the molecular layer as well. In the in vitro study, paired stimulating and recording electrodes were stepped through the molecular layer and revealed a relatively sudden waveshape change around 225 micron from the cell layer, where the transitional zone between MPP and LPP terminal fields was expected to be located. Quantitative analysis of the differences between the potentials recorded in the molecular layer and the hilus revealed that electrotonic decay accounts for approximately 20% of the difference seen in the hilus between the MPP and LPP potentials. Our data therefore suggest that the differences between MPP and LPP hilar potentials are due mostly to differences between the two pathways in their properties of synaptic transmission and are due relatively little to the different sites of synaptic contact on the dendritic tree.