Abstracts

Rationale: Temporal lobe epilepsy accounts for a large segment of focal seizures and is typically characterized by loss of consciousness. Despite its impact on patient quality of life, the exact neural pathways by which this loss of consciousness occurs have yet to be clearly defined. Intracranial EEG data shows that focal limbic seizures depress cortical activation and cholinergic arousal, as demonstrated by the appearance of slow oscillations in cortex during the ictal state. In addition, electrostimulation of the lateral septum, a putative inhibitory region with connections to the hippocampus, has been shown to reduce cholinergic arousal and induce cortical slow-waves, suggesting that the lateral septum may be implicated in the cortical deactivation pathway. Considering that cholinergic arousal to cortex is mainly provided by input from the nucleus basalis (NB), we posit that hippocampal seizures inhibit cholinergic neurotransmission via subcortical pathways. Methods: First, we explored the function of NB cholinergic neurons to the cortex by developing an optogenetic rat model to restore cholinergic arousal in NB neurons during electrically-induced hippocampal seizures. Female Long Evans ChAT-Cre positive rats were injected with Cre-dependent viral vectors carrying channelrhodopsin bilaterally into nucleus basalis and an optic fiber was used to photostimulate ChAT neurons during seizures while local field potentials (LFP) were recorded in the orbitofrontal cortex. Then, we conducted retrograde and anterograde tracing studies to map any neuroanatomical connections between the nucleus basalis and subcortical regions. Finally, we recorded multiunit activity (MUA) and LFP from subcortical regions of interest to characterize neuronal firing during electrically induced partial seizures. Results: Our results showed that the slow waves found in cortex during partial seizures converted to fast waves following optogenetic stimulation of NB cholinergic neurons (n=3 animals), suggesting that the nucleus basalis is important for cortical arousal. Moreover, our tracing studies showed evidence of direct anatomical connections between the lateral septum and nucleus basalis, with further connections to and from midline thalamic nuclei, specifically the paratenial (PT) region of the thalamus (n=6 animals). Our in vivo PT multiunit activity recordings showed that during seizures, neuronal firing was suppressed in PT (n=10 seizures from 5 animals). Conclusions: These findings suggest that focal limbic seizures may depress cortical activation by disrupting subcortical networks that have excitatory outputs to cortex. One potential mechanism may be inhibitory inputs from the lateral septum reduce NB cholinergic arousal directly. In parallel, a second mechanism may be decreased excitatory output from PT to the nucleus basalis, leading to decreased overall cholinergic arousal. Continued investigation into these networks should provide novel therapeutic targets aimed at improving cortical arousal during and after seizures. Funding: NIH R01 NS066974 and R01 NS096088