Kindling epileptogenesis in immature rats leads to persistent depressive behavior

Abstract

Depression is a frequent comorbidity in epilepsy patients. A variety of biological factors may underlie epilepsy-associated depression. We examined whether kindling-induced chronic increase in seizure susceptibility is accompanied by behavioral symptoms of depression. Three-week-old Wistar rats underwent rapid kindling: 84 initially subconvulsant electrical stimulations of ventral hippocampus delivered every 5 minutes, followed by depression-specific behavioral tests performed 2 and 4 weeks later. Kindled animals exhibited a sustained increase in immobility time in the forced swim test and the loss of taste preference toward calorie-free saccharin, as compared with controls. Initial loss of preference toward the intake of calorie-containing sucrose was followed by the increased consumption at 4 weeks. At both time points, animals exhibited enhanced seizure susceptibility on test stimulations of the hippocampus. We conclude that neuronal plastic changes associated with the kindling state are accompanied by the development of depressive behavior.

Figure 1. Experimental design

At postnatal day 21, rats were subjected to rapid kindling followed 24 hours later by test stimulations to measure afterdischarge properties and seizure response. Two weeks after kindling, afterdischarge properties and seizure response to test stimulation were examined in a subset of these animals (n=5). The remaining nine animals underwent behavioral tests two and four weeks after kindling; in these rats, response to test stimulation was examined four days after the completion of behavioral studies. In addition, behavior was examined in five sham and six naïve animals. The sequence of tests during each behavioral trial is indicated in the bottom inset.

Figure 2. Afterdischarge threshold (A), duration (B) and seizure responses (C) in the animals before and after rapid kindling

Data before and one day after kindling are pooled for 14 animals for presentation purposes. Afterdischarge and seizure parameters two weeks (n=5) and four weeks (n=9) after kindling are compared for respective animals. Animals were divided for 2 and 4 weeks tests in such a way, that there were no differences in the examined parameters between the two subgroups. X-axis refers to time points with the reference to kindling. *-these animals were not used for behavioral tests. Data are presented as Mean±SEM. †- p<0.05 vs. “Before kindling” (Friedman+Wilcoxon test). ‡- p<0.05 vs. One day after kindling (Wilcoxon test).

Figure 3. Forced swim test in the animals before and after rapid kindling

A. Mean±SEM values two and four weeks after kindling. *-p<0.05 vs. both naïve and sham (One-way ANOVA+Bonferroni test). There were no statistical differences (p>0.1) between two and four weeks for any of groups (paired t-test). Seizure score in individual animals in response to threshold stimulation one day (B) and four weeks (C) after kindling is plotted against immobility time in FST 2 weeks (open circles) and 4 weeks (black circles) after kindling. Coefficient of correlation (r) was calculated using Spearman test, is indicated for each of the data set; positive correlation was statistically significant in all cases (p<0.05).

Figure 4. Saccharin (A, B) and sucrose (C, D) consumption over twenty-four hour period two and four weeks after rapid kindling

A. No differences were observed at any age in total fluid (tap water +saccharin) intake among control and experimental groups. B. While naïve and sham animals exhibited strong preference towards saccharin versus tap water, kindled animals exhibited loss of taste preference towards saccharin both 2 and 4 weeks after kindling. C. No differences were observed in total fluid (tap water + sucrose) intake between naïve and sham animals. However, the total volume of consumed fluid was significantly larger in kindled animals. D. Both naïve and sham animals preferred sucrose over tap water. Kindled animals did not show sucrose preference 2 weeks after kindling. However, at four weeks, kindled animals consumed significantly more of sucrose solution as compared to both sham and naïve rats. Data in A and C are presented as Mean±SEM, and on B and D- as Mean. *-p<0.05 vs. both Naïve and sham (One way ANOVA + Bonferroni); †- p<0.05 of per cent of either saccharin or sucrose (bottom bars in the stacks) versus percent of water (top bars in the stacks, T test).