LINCs Are Vulnerable to Epileptic Insult and Fail to Provide Seizure Control via On-Demand Activation

Temporal lobe epilepsy (TLE) is notoriously pharmacoresistant, and identifying novel therapeutic targets for controlling seizures is crucial. Long-range inhibitory neuronal nitric oxide synthase-expressing cells (LINCs), a population of hippocampal neurons, were recently identified as a unique source of widespread inhibition in CA1, able to elicit both GABA(A)-mediated and GABA(B)-mediated postsynaptic inhibition. We therefore hypothesized that LINCs could be an effective target for seizure control. LINCs were optogenetically activated for on-demand seizure intervention in the intrahippocampal kainate (KA) mouse model of chronic TLE. Unexpectedly, LINC activation at 1 month post-KA did not substantially reduce seizure duration in either male or female mice. We tested two different sets of stimulation parameters, both previously found to be effective with on-demand optogenetic approaches, but neither was successful. Quantification of LINCs following intervention revealed a substantial reduction of LINC numbers compared with saline-injected controls. We also observed a decreased number of LINCs when the site of initial insult (i.e., KA injection) was moved to the amygdala [basolateral amygdala (BLA)-KA], and correspondingly, no effect of light delivery on BLA-KA seizures. This indicates that LINCs may be a vulnerable population in TLE, regardless of the site of initial insult. To determine whether long-term circuitry changes could influence outcomes, we continued testing once a month for up to 6months post-KA. However, at no time point did LINC activation provide meaningful seizure suppression. Altogether, our results suggest that LINCs are not a promising target for seizure inhibition in TLE. Significance Statement Novel treatments are needed for temporal lobe epilepsy, and altering inhibitory signaling may provide seizure control. Recently, a previously uncharacterized hippocampal cell population, long-range inhibitory neuronal nitric oxide synthase-expressing cells (LINCs), was found to provide strong, widespread inhibition in healthy tissue. Despite being a novel source of powerful inhibition, and therefore a promising candidate for seizure control, on-demand activation of LINCs in a mouse model of temporal lobe epilepsy did not substantially suppress seizure activity. LINC numbers were decreased in epileptic tissue, indicating that LINCs are a vulnerable cell population. The heterogeneity of inhibitory signaling, and how it changes in epilepsy, are important factors to consider when developing new temporal lobe epilepsy therapies.