Identification of QTLs involved in the development of amygdala kindling in the rat

Abstract

Amygdala kindling is useful for modeling human epilepsy development. It has been known that genetic factors are involved in the development of amygdala kindling. The purpose of this study was to identify the loci that are responsible for the development of amygdala kindling. To achieve this, rat strains from a LEXF/FXLE recombinant inbred (RI) strain panel were used. The phenotypes of amygdala kindling-related parameters for seven RI strains and parental LE/Stm and F344/Stm strains were determined. They included the afterdischarge threshold (ADT), the afterdischarge duration (ADD), and the kindling rate, an incidence of development of kindling. Quantitative trait loci (QTL) analysis was performed to identify linkage relationships between these phenotypes and 1,033 SNP markers. Although no significant differences in pre-kindling ADT and ADD were observed, a significant difference in the kindling rate was found for the LEXF/FXLE RI strain. Two QTLs for the amygdala kindling rate (Agkr1 and Agkr2) were identified on rat chromosome 2. These findings clearly prove the existence of genetic influences that are involved in kindling development and suggest that substantial genetic components contribute to the progression of partial seizures into generalized seizures.

Fig.1.

Experimental schedule and representative EEG trace of afterdischarge. (A) Schedule for inducing kindling. At 10 weeks of age, each animal had an electrode implanted into its amygdala. At 12 weeks of age, the animals were subjected to pre-kindling ADT determination as indicated by the open arrowhead. The next day, daily kindling stimuli were applied to the animals as noted by the arrowhead to determine the ADD. (B) A representative EEG recoding of the AD. This AD lasted at least 13 seconds. Just after stimulation, the amplitude of the EEG was too great to be recorded.

Fig. 2.

Pre-kindling ADT of the LEXF/FXLE RI and parental strains. (A) Pre-kindling ADTs of the seven RI and two parental strains. There were no significant differences in the ADTs among the strains. (B) The ADDs of the seven RI and two parental strains, which were obtained when the pre-kindling ADTs were determined. There were no significant differences in the ADDs among the strains. The data are shown as the mean ± SEM.

Fig. 3.

Kindling development of the LEXF/FXLE RI and parental strains. (A) Average of the seizure severity stage that was observed during daily kindling stimulation for the RI and parental strains. The seizure sensitivity stage was classified as follows by behavior according to Racine’s criteria: stage 1, mouth and facial twitches; stage 2, clonic head movements; stage 3, forelimb clonus; stage 4, clonic rearing; stage 5, loss of postural control (falling) [14].

Fig. 4.

Kindling rates of the LEXF/FXLE RI and parental strains. The kindling rate was defined by the number of daily stimuli that were applied until the first stage 5 seizure. Rat strains that received a smaller number of stimuli were more susceptible to kindling development, while rat strains that received a larger number of stimuli were more resistant to kindling development. Strain F344/Stm showed a significant difference in the number of stimuli that it received compared with strains LEXF11, LEXF6B, LEXF2B, FXLE13, LE/Stm, and LEXF10C (^aP<0.05; ^aaP<0.01). Strain FXLE16 showed a significant difference in the number of stimuli that it received compared with strains LEXF2B, FXLE13, LE/Stm, and LEXF10C (^bP<0.05; ^bbP<0.01). Strain LEXF7B showed a significant difference in the number of stimuli it received compared with strains LE/Stm and LEXF10C (^cP<0.05; ^ccP<0.01). Strains LEXF11 and LEXF6B showed significant differences in the number of stimuli that they received compared with strain LEXF10C (^dP<0.01). The data are shown as means ± SEM.

Fig. 5.

Significant QTLs for the kindling rate. LOD score profiles of the kindling rate for rat chromosomes that were obtained for the seven RI and parental strains. The level of significance (LOD score was 3.0) is indicated by the horizontal solid line. Two QTLs, loci Agkr1 and Agkr2, were detected on Chr 2, as indicated by the arrows.