Spontaneous hippocampal interictal spikes following local kindling: time-course of change and relation to behavioral seizures.

Spontaneous interictal spikes (SISs) were recorded in the hippocampus in freely behaving rats following hippocampal stimulations that resulted in afterdischarges (ADs). Hippocampal SISs were detected after an average of 5 (range 2-10) daily ADs. The rate of SISs typically increased minutes after a tetanus, and then decayed with time constants of approximately 70 min and 1.5 days. Seizure onset in the kindling paradigm was not related to a consistent change in SIS rate. Following the interruption of daily kindling, SIS rate invariably decreased to near zero by 4-8 days while seizure susceptibility, as tested by the ability to evoke generalized convulsions, remained unchanged. Despite having a low or zero SIS rate the hippocampus seemed to retain an excitability after kindling interruption, as demonstrated by the observation that an average of 1.7 rekindling stimulations resulted in a high SIS rate. In conclusion, changes in hippocampal SISs were closely time-locked to an AD, and not to evoked behavioral seizures. Hippocampal SISs probably reflect an excitability change that is more local than that necessary for evoking behavioral convulsions. The persistence of SISs in terms of hours and days suggests the involvement of long-term potentiation.