Persistence of kindling: effect of partial kindling, retention interval, kindling site, and stimulation parameters

The kindling effect is generally thought to be highly persistent and possibly permanent, but little direct evidence is available to support this idea. Retention of amygdala kindling was examined after a 12-wk interval in groups of rats that had been electrically kindled to different seizure stages (stages 1, 3, or 5), or kindled by high intensity or low frequency (3 pulses per second) stimulation, or fully kindled and allowed a rest of 1-24 wk. The retention of hippocampal kindling after a 12-wk interval was also examined. Rekindling after a 12-wk rest in the groups initially kindled to different seizure stages indicated that although there was evidence of erosion of the kindling effect in all groups, there were savings in all groups. There was also evidence of greater erosion in the afterdischarge response than in the convulsive response to the first stimulation after the interval. Although there was evidence of erosion of kindling during the 1-24-wk intervals, there was evidence of savings in all groups, none of which required more than a mean of 2.2 afterdischarges to rekindle to stage 5. Seizures kindled in the hippocampus were retained as well as those kindled in the amygdala, and seizures kindled using low frequency stimulation were retained as well as those kindled using conventional 60 pulses per second stimulation. We conclude that the effects of kindling the amygdala and hippocampus are highly persistent, and that the effects of kindling with low frequency stimulation are as persistent as kindling with conventional stimulation.