Repetitive one-per-day seizures induced in otherwise normal rats by the volatile convulsant
flurothyl decrease the accuracy of locating a hidden goal without changing the mean
location of goal selection. We now show that an 8-d series of such seizures degrades
the spatial signal carried by the firing of hippocampal pyramidal cells and specifically
reduces the information conveyed by the place cell subset of pyramidal cells. This
degradation and a concomitant slowing of the hippocampal theta rhythm occur over time
courses parallel to the development of the behavioral deficit and plausibly account
for the impairment. The details of how pyramidal cell discharge weakens are, however,
unexpected. Rather than a reduction in the precision of location-specific firing distributed
evenly over all place cells, the number of place cells decreases with seizure number,
although the remaining place cells remain quite intact. Thus, with serial seizures
there is a cell-specific conversion of robust place cells to sporadically firing (<0.1
spike/s) "low-rate" cells as opposed to gradual loss of place cell resolution. This
transformation occurs in the absence of significant changes in the discharge rate
of hippocampal interneurons, suggesting that the decline in the number of place cells
is not a simple matter of increased inhibitory tone. The cumulative transformation
of place cells to low-rate cells by repetitive seizures may reflect a homeostatic,
negative-feedback process.
