Control of in vivo ictogenesis via endogenous synaptic pathways

Abstract

The random nature of seizures poses difficult challenges for epilepsy research. There is great need for a reliable method to control the pathway to seizure onset, which would allow investigation of the mechanisms of ictogenesis and optimization of treatments. Our hypothesis is that increased random afferent synaptic activity (i.e. synaptic noise) within the epileptic focus is one endogenous method of ictogenesis. Building upon previous theoretical and in vitro work showing that synaptic noise can induce seizures, we developed a novel in vivo model of ictogenesis. By increasing the excitability of afferent connections to the hippocampus, we control the risk of temporal lobe seizures during a specific time period. The afferent synaptic activity in the hippocampus was modulated by focal microinjections of potassium chloride into the nucleus reuniens, during which the risk of seizure occurrence increased substantially. The induced seizures were qualitatively and quantitatively indistinguishable from spontaneous ones. This model thus allows direct control of the temporal lobe seizure threshold via endogenous pathways, providing a novel tool in which to investigate the mechanisms and biomarkers of ictogenesis, test for seizure threshold, and rapidly tune antiseizure treatments.

Chemical biomarkers of epileptogenesis and ictogenesis in experimental epilepsy

“…These experiments utilized the method described in Luna-Munguia et al (2017). Briefly, 56-days-old rats were randomly assigned to a group that received an injection of pilocarpine hydrochloride (340 mg/kg ip; Sigma-Aldrich, St. Louis, MO) 20 min after a single intraperitoneal injection of methylatropine bromide (5 mg/kg; Sigma-Aldrich, St. Louis, MO).…”

“…Procedures for affixing the subdural and depth electrodes were performed as previously described (Luna-Munguia et al, 2017), with the sole exception being the introduction of the microdialysis probe. Briefly, Pilocarpine (n = 15) and Control (n = 15) rats were anesthetized with a ketamine/xylazine mix (70/10 mg/kg ip, respectively) and placed in a stereotactic frame (David Kopf Instrument, Tujunga, CA) 14 days after the pilocarpine injection.…”

“…The sockets were fitted into a 6-pin electrode pedestal (MS363; PlasticsOne, Roanoke, VA) and the entire apparatus was secured with dental cement. For local drug injection into the nucleus reuniens , we followed our previously described procedure (Luna-Munguia et al, 2017). Here, a 26-gauge guiding cannula (C315GA/SPC; PlasticsOne, Roanoke, VA) was stereotactically implanted (AP −1.8, ML −1, DV −7.2, with a 15° arm angle from vertical axis) to guide a 33-gauge injecting cannula (C315IA/SPC; PlasticsOne, Roanoke, VA).…”

“…A major limitation is that there were previously no methods to control seizure threshold experimentally in a spontaneous seizure model. Prior work has utilized the kindling model, which provides excellent control of seizure timing, but is not a model of spontaneous epilepsy (Bertram, 2007), limiting its ability to inform of the chemical changes of epileptogenesis and ictogenesis (Luna-Munguia et al, 2017).…”

“…Therefore, it is difficult to evaluate the changes preceding seizures, especially with labor-intensive microdialysis. We recently developed a novel model of ictogenesis, which allows control of seizure threshold through physiological pathways (Luna-Munguia et al, 2017). This model utilizes rats that have developed spontaneous seizures several weeks after pilocarpine-induced status epilepticus , and increases the risk of those seizures by increasing the random afferent synaptic activity to the hippocampus.…”