Differences in kindling development in seven outbred and inbred rat strains

The kindling phenomenon, i.e., the progressive development of focal and secondarily generalized seizures upon repeated electrical stimulation of a limbic brain region, occurs in various species, but with marked differences in kindling rate between species and also within the same species. In rats, differences in kindling rates have been reported within the same strain and between different strains, and both genetic and environmental influences are thought to be involved in this variability. In most studies on kindling in rats, outbred strains such as Sprague-Dawley have been used. In the present study, we compared rates of amygdala kindling development in two outbred (Sprague-Dawley, Wistar) and five inbred (Lewis, Fischer 344, ACI, Wistar-Kyoto, Brown Norway) rat strains, including several strains which have not been kindled before. We were particularly interested which parts of the stepwise progression of kindling differ among these strains. Furthermore, the sensitivity of the basolateral amygdala to electrical stimulation was determined before and after kindling. Once daily electrical stimulation of the basolateral amygdala resulted in marked interstrain differences in kindling rates, with Sprague-Dawley and Brown-Norway rats exhibiting the lowest number of stimulations to reach fully kindled (stage 5) seizures, and Lewis rats showing the highest number of the 7 strains. In contrast to the significant differences in number of stimulations to reach the fully kindled state, total (cumulative) afterdischarge duration (ADD) to reach stage 5 did not significantly differ among strains, substantiating that cumulative AD is the principal factor in the acquisition of kindled seizures. Marked differences in ADD of a stage 5 seizure were obtained between strains, with strains kindling rapidly exhibiting longer ADD than strains kindling slowly. Postkindling afterdischarge threshold (ADT) varied significantly among strains, but only 3 of the 7 strains showed a decrease of ADT compared to prekindling values. When the stepwise progression of kindling was evaluated, pronounced interstrain differences were determined in the time spent in the initial phase of kindling, i.e., stage 1 seizures, both in terms of stimulations and cumulative ADD, indicating that variations in kindling rates were predominantly due to the time needed to progress from stage 1 to subsequent stages of the kindling process. The data seem to indicate that inbred rat strains offer an interesting resource for dissecting the underlying genetic basis for phenotypic differences in epileptogenesis as induced by kindling, although the high variability of kindling rates seen within some inbred strains weakens this possibility.