FAST and SLOW amygdala kindling rat strains: comparison of amygdala, hippocampal, piriform and perirhinal cortex kindling

In our companion paper, we selectively bred offspring of a Long Evans Hoode d and Wistar rat cross for either fast or slow rates of amygdala kindling ( Racine et al., 1999. Development of kindling-prone and kindling resistant r ats: Selective breeding and electrophysiological studies, Epilepsy Res. 35, 183-195). Within 10 generations, there was no overlap in the distribution of kindling rates between these newly developed FAST and SLOW kindling stra ins. In the present report, we compared the local excitability, kindling ra tes, and convulsion profiles of kindling sites in either the amygdala, dors al hippocampus, piriform cortex or perirhinal cortex in the two strains. Lo cal excitability, measured as the local afterdischarge (AD) threshold and i ts duration, showed varied effects between structures and strains. Before k indling, the AD threshold was lower in the FAST than the SLOW rats in the h ippocampus, piriform and perirhinal cortices, but not the amygdala (the sel ection structure). Also, the duration of the AD threshold duration was sign ificantly longer in the FAST than in the SLOW rats in all structures, excep t the CA, hippocampus. Most of these differences were maintained after kind ling. Kindling itself was significantly faster in the FAST compared with th e SLOW rats in all structures; however, the different structural kindling r ates showed proportional differences between strains that were about five t imes different in the amygdala compared with only about two times different in the hippocampus. This suggested a selection bias for the amygdala and i ts networks. As in other rat strains, the fastest kindling rates were seen in the perirhinal cortex followed by the piriform cortex, amygdala and hipp ocampus in both FAST and SLOW rats. Other important differences between str ains and structures occurred in the stage-5 convulsion profiles, including latency to forelimb clonus, clonus duration and duration of associated loca l afterdischarges. The differences in kindling profiles between strains and structures were discussed with respect to possible underlying mechanisms, significance for epileptogenesis, and impact on other normal behaviours. (C ) 1999 Elsevier Science B.V. All rights reserved.