Expression pattern of Arc in the hippocampus of a rat model of epilepsy and depression comorbidity

Two key factors associated with the comorbidity of epilepsy and depression (EAD), activity regulated cytoskeletal protein (Arc) and homer protein homolog 1 (Homer1), were previously identified by our group through bioinformatics methods (Yu et al., 2022). The expression of Arc and Homer1 were verified through animal experiments.

Six-week-old male specific pathogen-free grade Sprague Dawley rats (weighing 200 ± 20 g) received intraperitoneal injection of lithium chloride (LiCl)-pilocarpine for status epilepticus (SE) induction. SE was terminated after 30 min by intraperitoneal injection of diazepam, and spontaneous SE in rats was monitored by video for 2 weeks. The control group (Con group) was injected with an equal dose of sterile normal saline. Subsequently, EAD rats (EAD group) were selected from rat models of LiCl-pilocarpine-induced chronic epilepsy according to the immobility time of the forced swimming test on day 14 after LiCl-pilocarpine induced epilepsy. The remaining rats were included in the epilepsy group (EP group). Depression-like behaviors were evaluated using sucrose preference, open-field, and forced swimming tests. Body weight, sucrose preference percentage, the total distance of the open-field test, the average speed, the number of upright times, and the immobility time of the forced swimming test were assessed 14 and 28 days after LiCl-pilocarpine induced epilepsy. Rats in the EAD and EP groups were monitored by video for 2 weeks, and the frequency, grade, and duration of chronic spontaneous epileptic seizures were recorded. Epileptic seizures were compared between the EAD and EP groups. The expression of activity-regulated cytoskeletal protein (Arc) and Homer protein homolog 1 (Homer1) in the hippocampus of each group was detected by real-time quantitative PCR and western blot analysis. The fluorescence intensity of Arc in the hippocampus of each group was detected by immunofluorescence (IF) assay.

Compared with the Con and EP groups, rats in the EAD group exhibited a decreased body weight on day 28, a significant decrease in sucrose preference percentage on days 14 and 28, significantly extended immobility time, and significantly reduced total travel, average speed, the number of upright times. No significant differences in the number, grade, and duration of seizures were observed between the EAD and EP groups. Meanwhile, the expression level of Arc in the hippocampus was significantly decreased in the EAD group compared with the Con and EP groups; however, the expression level of Homer1 showed no significant change. IF results showed that Arc was mainly expressed in the cytoplasm, and the fluorescence intensity of Arc in hippocampal CA1, DG, and CA3 was lower in the EAD group than in the Con and EP groups.

The expression of Arc in the hippocampal tissue of EAD rats is significantly decreased, suggesting that Arc is associated with EAD.