Behavioral impairments in rats with chronic epilepsy suggest comorbidity between epilepsy and attention deficit/hyperactivity disorder

Abstract

Attention deficit/hyperactivity disorder (ADHD) is encountered among patients with epilepsy at a significantly higher rate than in the general population. Mechanisms of epilepsy-ADHD comorbidity remain largely unknown. We investigated whether a model of chronic epilepsy in rats produces signs of ADHD, and thus, whether it can be used for studying mechanisms of this comorbidity. Epilepsy was induced in male Wistar rats via pilocarpine status epilepticus. Half of the animals exhibited chronic ADHD-like abnormalities, particularly increased impulsivity and diminished attention in the lateralized reaction-time task. These impairments correlated with the suppressed noradrenergic transmission in locus coeruleus outputs. The other half of animals exhibited depressive behavior in the forced swimming test congruently with the diminished serotonergic transmission in raphe nucleus outputs. Attention deficit/hyperactivity disorder and depressive behavior appeared mutually exclusive. Therefore, the pilocarpine model of epilepsy affords a system for reproducing and studying mechanisms of comorbidity between epilepsy and both ADHD and/or depression.

Keywords: 5-HT; ADHD; Attention deficit/hyperactivity disorder; Depression; EPMT; Epilepsy; FCV; FST; LC; LRTT; Lateralized reaction-time task; NE; Norepinephrine; PFC; RN; SE; Serotonin; TLE; attention deficit/hyperactivity disorder; elevated plus maze test; fast cyclic voltammetry; forced swimming test; lateralized reaction-time task; locus coeruleus; norepinephrine; prefrontal cortex; raphe nucleus; serotonin; status epilepticus; temporal lobe epilepsy.

Figure 1. Forced swimming test

Post-SE animals showed increase in cumulative duration of immobility time and increased non-cued struggling behavior. A. Group data are presented as Mean±SEM. *- p<0.5 vs. Naïve for each respective parameter (i.e. swimming, immobility and struggle); †- p<0.05 “Struggling” vs. “Immobile” groups. All vs. Naïve- t-test. “Immobile” vs. “Struggling” vs. Naïve- One Way ANOVA + Bonferroni multiple comparison test (active swimming F(2,22)=126.2, p<0.0001; immobility F(2,23)=78.13, p<0.0001; struggling F(2,22)=63.84, p<0.05). B. Plot of individual data showing the dichotomy between the immobility and struggle among post-SE animals.

Figure 2. Lateralized reaction time task

Post-SE animals showed increased impulsivity and diminished attention. Data are presented as Mean±SEM. A. Impulsivity *-p<0.05 vs. Naïve; †-p<0.05 “Non-impulsive” vs. “Impulsive” groups. All vs. Naïve-t-test. “Non-impulsive” vs. “Impulsive” vs. Naïve- One Way ANOVA + Bonferroni multiple comparison test (F(2,16)=53.58, p<0.0001). B. Attention. There was a consistent trend in the reduction of correct choices between Post-SE and naïve animals; however statistical significance was reached only at 0.2 s. *-p<0.05 vs. Naïve (t-test).

Figure 3. Individual plots of impulsivity vs. attention in the Lateralized reaction time task

A-C show percent of correct choices in response to stimuli of various durations. Only animals with the exacerbated impulsivity showed diminished attention. At 2.0 s and 0.5 s stimulus duration, “impulsive” post-SE animals showed statistically significant decrease in percent of correct choices compared with both naïve and “non-impulsive” rats (2.0s F(2,16)=7.962, p<0.05; 0.5 s F(2,16)=7.512, p<0.05).

Figure 4. Individual plots of impulsivity in the lateralized reaction time task vs. behaviors in forced swimming test

A. Impulsivity (LRTT) vs. immobility (FST). All post-SE animals showed increased immobility time (p<0.05 for “non-impulsive” vs. naïve and “impulsive vs. naïve); however the increased in the immobility was more pronounced in “non-impulsive” rats than in animals of “impulsive” group (p<0.05). One-Way ANOVA+Bonferroni multiple comparisons test (F(2,16)=28.31, p<0.0001). B. Impulsivity (LRTT) vs. non-cued struggle (FST).In “non-impulsive” rats, cumulative duration of non-cued struggle was in normal range (p>0.05 vs. naïve), while “impulsive” rats showed significant increase in the non-cued struggle vs. both naïve and “non-impulsive” animals (p<0.05; One way ANOVA + Bonferroni test for multiple comparisons, F(2,16)=12.27, p<0.005).

Fig. 5. Noradrenergic and serotonergic transmission with the reference to impairments in the lateralized reaction time task

A. Group data are presented as Mean±SEM. There was an overall decrease in both NE and 5-HT release in all post-SE animals as compared with controls (t-test). “Non-impulsive” animals showed suppressed 5-HT release and preserved NE release; “impulsive” animals showed preserved 5-HT release and suppressed NE release (One-Way ANOVA + Bonferroni multiple comparison test; NE F(2,16)=26.93, p<0.05; 5-HT F(2,16)=14.76, p<0.05). *- p<0.05 vs. Naïve; †- p<0.05”impulsive” vs. “non-impulsive” group. B and C: Individual plots. Individual impulsivity data (LRTT) are plotted against NE release (FCV, B) and against 5-HT release (FCV, C). “Non-impulsive” animals showed consistent suppression of 5-HT release, while “impulsive” rats showed consistently diminished NE release.

Fig. 6. Noradrenergic and serotonergic transmission with the reference to impairments in the forced swimming test

A. Group data are shown as Mean±SEM. NE release was significantly suppressed in all post-SE animals However, rats of the “struggling” group showed more pronounced suppression than “immobile” animals. There was an overall suppression of 5-HT release in post-SE rats; however, on the sub-group level, only “immobile” animals showed diminished 5-HT responses. *-p<0.05 vs. naïve; †- p<0.05, “struggling” vs. “immobile” group. (All vs. Naïve- t-test; “immobile” vs. “struggling” vs. naïve- One-way ANOVA+Bonferroni multiple comparison test; NE F(2,22)=12.62, p<0.01; 5-HT F(2,22)=98.01, p<0.0001). B. Individual plots whereby noradrenergic responses are plotted against serotonergic responses. Note the suppression of serotonergic responses in “immobile”, and noradrenergic- in “struggling” animals.