Sex and estrous cycle stage shape left-right asymmetry in chronic hippocampal seizures in mice.

Lateralization of hippocampal function is indicated by varied outcomes of patients with neurological disorders that selectively affect one hemisphere of this structure, such as temporal lobe epilepsy (TLE). The intrahippocampal kainic acid (IHKA) injection model of TLE allows for targeted damage to the left or right hippocampus, enabling systematic comparison of effects of left-right asymmetry on seizure and non-seizure outcomes. Although varying non-seizure phenotypic outcomes based on injection side in dorsal hippocampus were recently evaluated in this model, differences in chronic seizure patterns in left- (IHKA-L) vs. right-injected (IHKA-R) IHKA animals have yet to be evaluated. Here, we assessed hippocampal seizure incidence in male and female IHKA-L and IHKA-R mice. Females displayed increased electrographic seizure activity compared to males at both 2 and 4 months post-injection. In addition, IHKA-L females showed higher seizure frequency than IHKA-R on diestrus and estrus at 2 months post-injection, but seizure duration and percent time in seizures were only higher in IHKA-L females on diestrus. These cycle stage-associated changes, however, did not persist to 4 months post-injection. Furthermore, this lateralized difference in seizure burden was not observed in males. These results indicate for the first time that the side of IHKA injection can shape chronic electrographic seizure burden. Overall, these results demonstrate a female-specific left-right asymmetry in hippocampal function can interact with estrous cycle stage to shape chronic seizures in mice with epilepsy, with implications for neural activity and behavior in both normal and disease states. Significance Statement Seizures in temporal lobe epilepsy often originate in the hippocampus, and patient outcomes can depend on whether the seizures initiate in the left or right hippocampus. Although rodent brain function appears less lateralized than in humans, emerging evidence indicates stronger lateralization of hippocampal function in mice than previously thought. Here, we systematically compared chronic epilepsy profiles in mice based on whether left or right hippocampus is the main site of seizure generation. Males did not show a left-right asymmetry in epilepsy severity, but females showed effects of seizure initiation side that varied with estrous cycle stage. These results thus suggest a female-specific lateralization of hippocampal function can interact with the estrous cycle to shape chronic seizures in mice with epilepsy.