Abstracts

RATIONALE: Prior studies from our laboratory have revealed that status epilepticus on postnatal day 20 (P20) result in abnormal place cell function and spatial learning in the adult rat (P75-90). As a first step in understanding the time course of the changes in place cell physiology that occurred following status epilepticus, we have systematically studied the dynamic changes of hippocampal pyramidal cell activity that occurs during development in male Sprague-Dawley (SD) rats not subjected to status epilepticus.
METHODS: Four tetrodes consisting of 16 insulated nichrome wires were implanted into the CA1 region of the right hippocampus of P23 rats (n = 6). The tetrodes were implanted about 300 microns above the CA1 region and then advanced 80 microns/day starting with the first recording session on P25. Once a place cell was isolated, the tetrode was no longer advanced until that cell was no longer well isolated. During a recording session, the rat was free to move within a cylinder (diameter = 76 cm) surrounded by a 2.5 m long curtain for up to 16 minutes per session. Similar to our previous studies, rats were subjected to four recording sessions/24 hour period divided into two pairs separated 4-6 hours with 2-3 minutes separating each session in a pair. During a recording session, the rats would chase 20-30 food pellets that were dropped at random times at random position in the recording cylinder.
RESULTS: Twenty-one hippocampal pyramidal cells in the CA1 region with complex spikes and peak-to-peak amplitude of 150 [mu]V were recorded. No place cell was isolated prior to P32. Place cell fields were relatively stable during the period of cell isolation. Place cells isolated prior to P55 tended to have a higher average firing frequency and a larger average firing area, and the information content and coherence were lower than the place cells in adult control SD rats. Place cell isolated on P55 and after were similar to those isolated in adult control rats of age P75 and greater.
CONCLUSIONS: This study demonstrates (1) the place cells can be recorded from the hippocampal pyramidal cells in the freely moving developing rat, (2) the developing place cells offers less information content and coherence than the mature place cell, and (3) an adult place cell profile is obtained just prior to the end of the 2nd postnatal month. Our failure to isolate place cells prior to P32 may result from the recording technique. These findings suggest that place cell function in the developing rat will be a useful measure in assessing seizure-induced injury.
[Supported by: This work was supported by the Emily P. Rogers Research Fund, a grant to GLH from the NINDS (NS27984), and a Mental Retardation Research Center Grant from NIH (HD18655-19). XZL was supported by a fellowship from the American Epilepsy Foundation. HPG received support from the NIH (32NS07473).]