The current dogma states that the medial perforant path (MPP) and lateral perforant path (LPP) inputs to the hippocampal dentate gyrus can be differentiated electrophysiologically using the response to paired-pulse stimuli. Stimulation at 50 ms intervals produces paired-pulse depression (PPD) in the MPP, whereas these same stimuli produce paired-pulse facilitation (PPF) in the LPP (McNaughton 1980). Several years of practical experience in our laboratory has led us to question the utility of paired-pulse administration as a reliable means to differentiate the perforant path subdivisions in vitro. Using field recordings in male Sprague Dawley rats, we demonstrate both subdivisions of the perforant pathway show predominantly PPF at low stimulus intensities. Activation of the LPP registered significantly greater net PPF (24.97±4.08%) relative to the MPP (13.76±3.86%) at the 50 ms interpulse interval. These results were independent of the position in the dorsoventral axis from which the hippocampal slice was obtained but elevating the calcium concentration (2mM to 4mM) or decreasing the temperature (300C to 230C) reduced the paired-pulse ratio. Increasing the magnitude of the applied stimulus could result in PPD in both paths in a manner that was correlated with the emergence of population spikes (r> -0.90). Partial blockade of AMPA receptors reduced the ability of high stimulus intensities to induce PPD and restored PPF in most cases. A comparison of field excitatory postsynaptic potential (fEPSP) characteristics demonstrated MPP waveforms could be differentiated by their significantly shorter peak latency and half-width times, greater total decay time, and the presence of a more reliable bi-exponential decay phase function relative to LPP waveforms. This research helps to refine our view of functional differences between the MPP and LPP, revealing more subtle differences in paired-pulse plasticity and distinct fEPSP waveform parameters as reliable features to distinguish these pathways.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/3113 |
| Date | 16 November 2010 |
| Creators | Petersen, Ross |
| Contributors | Christie, Brian R. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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