Electrophysiological identification of medial and lateral perforant path inputs to the dentate gyrus

• Both medial and lateral path inputs to the dentate gyrus reliably show paired-pulse facilitation.
• The lateral perforant path exhibits a greater paired-pulse ratio than the medial perforant path.
• Paired-pulse ratios in the LPP and MPP were reduced by elevating Ca2+ or reducing bath temperature.
• Paired-pulse depression is observed in the MPP and LPP in association with population spikes.
• The MPP and LPP could be differentiated using either visualized microscopy or waveform analysis.

The medial perforant path (MPP) and lateral perforant path (LPP) inputs to the hippocampal dentate gyrus form two distinct laminar inputs onto the middle and distal aspects of granule cell dendrites. Previous evidence indicated that paired stimuli reliably produced paired-pulse depression (PPD) in the MPP and paired-pulse facilitation (PPF) in the LPP. Despite this, several years of practical experience in our laboratory questioned the utility of using paired-pulse administration to reliably differentiate the MPP and LPP in vitro. Using visualized field and whole-cell recordings in male Sprague–Dawley rats, we demonstrate that both pathways show net PPF of the excitatory postsynaptic potential (fEPSP) at 50-ms interpulse intervals. LPP afferents did reliably exhibit greater PPF than MPP afferents. Thus, the LPP reliably exhibits a greater paired-pulse ratio than the MPP. The magnitude of the paired-pulse ratio was reduced in both afferents by raising calcium levels or lowering the temperature of the recording chamber. PPD of MPP-evoked fEPSPs was only reliably detected at moderate to high stimulus intensities when population spike activity was evident. PPD was more evident in whole cell voltage clamp recordings but nonetheless was not completely diagnostic as PPD was occasionally observed with LPP stimulation as well. We found the MPP and LPP could be reliably identified using conventional microscopy with hippocampal slices, and that they could be distinguished through the analysis of evoked waveform kinetics. This work refines our knowledge of electrophysiological differences between MPP and LPP projections and will help to facilitate the selective activation of these pathways.