Elsevier

Neuroscience

Volume 117, Issue 2, 21 March 2003, Pages 485-492
Neuroscience

Original contribution
Membrane properties of identified lateral and medial perforant pathway projection neurons

https://doi.org/10.1016/S0306-4522(02)00659-0Get rights and content

Abstract

The physiological characteristics of neurons that project to the hippocampus and dentate gyrus via the medial perforant pathway (projection neurons) are well known, but the characteristics of neurons that project to these areas via the lateral perforant pathway (projection neurons) are less well known. We have used retrograde tracing and whole-cell recording in brain slices to compare the membrane and firing properties of medial perforant pathway and lateral perforant pathway projection neurons in layer II of the medial and lateral entorhinal cortex. The properties of medial perforant pathway projection neurons were identical to those reported previously for spiny stellate neurons in the medial entorhinal cortex. In contrast, lateral perforant pathway projection neurons were characterized by a higher input resistance, a lack of time-dependent inward (anomalous) rectification, and a lack of prominent depolarizing spike afterpotentials. Voltage-clamp recordings suggest that the absence of anomalous rectification in lateral perforant pathway projection neurons is due to smaller hyperpolarization activated cation currents in these cells, and the lack of depolarizing afterpotential may be due to smaller low-threshold calcium currents. Persistent sodium current was also smaller in lateral perforant pathway projection neurons, but the difference in persistent sodium current between medial perforant pathway and lateral perforant projection neurons was much less pronounced than the difference in low voltage activated currents. These results underscore the functional differences between the medial entorhinal cortex and lateral entorhinal cortex, and may help to explain the differing abilities of these cortical areas to participate in certain types of network activity.

Section snippets

Retrograde labeling and cell identification

All procedures were carried out in strict adherence to international guidelines on the ethical use of animals and all efforts were made to minimize the number of animals used and their suffering. A combination of fluorescence labeling and whole-cell recording in brain slices was employed. Briefly, Sprague–Dawley rats (12 days old) were placed in a stereotaxic frame under barbiturate anesthesia. Approximately 0.5–1.0 μl of a 5% solution of 3,3′-dioctadecyloxacarbocyanine perchlorate (DiO;

Results

Injection of retrograde tracers (DiO or fluorescent polystyrene microspheres) into the dorsal hippocampus reliably labeled neurons in the superficial layers of the medial and lateral entorhinal cortices Dolorfo and Amaral 1998, Steward and Scoville 1976. MPP projection neurons were identified by fluorescence microscopy in layer II at the caudal pole of slices prepared in the horizontal plane. LPP projection neurons were identified in layer II, ventral to the rhinal sulcus, in rostral slices

Discussion

We have compared the membrane properties of layer II MPP and LPP projection neurons identified by retrograde tracing. The major differences between these neurons are: 1) LPP projection neurons have a higher input resistance, 2) MPP projection neurons have much more pronounced anomalous rectification, presumably due to their larger Ih currents, and 3) MPP projection neurons have large DAPs, fire action potential doublets, and can generate slow subthreshold depolarizing potentials, which

Acknowledgements

We thank John Dempster, Strathclyde University, for supplying data acquisition software (WinWCP). This work was supported by a grant from the NIH (NS36455) and a Department of Veterans Affairs Merit Review.

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