Abstracts

This is a Late-Breaking abstract.

Rationale: Post traumatic epilepsy (PTE) is a debilitating sequela of traumatic brain injury (TBI). Currently there is no effective method of prevention and once established, PTE is resistant to standard anti-seizure medications. PTE is understood to occur through multiple complex changes at the synaptic, cellular, and circuit level that increase excitability and lower seizure threshold. Astrocytes help regulate excitability by changing the volume of the extracellular space (ECS). One mechanism that regulates neuronal excitability is the electrogenic sodium-bicarbonate cotransporter 1 (NBCe1) which can influx sodium and bicarbonate into the cell causing the cell to swell and the ECS to shrink. Consequently, local concentrations of neuroactive substances in ECS increase and ephaptic interactions are enhanced, causing hyperexcitability of neuronal circuits. Studying chemo-convulsant models of epilepsy, our laboratory recently discovered that ECS volume undergoes Rapid volume pulsations (RVP) that transiently shrink ECS volume by about 15% during each epileptiform discharge in vivo. Furthermore, administration of the NBCe1 inhibitor DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid), both reversed ECS shrinkage and stopped epileptiform activity, restoring neuronal networks to their normal electrophysiological state. We propose that: (1) RVPs coincide with epileptiform activity in the controlled cortical injury (CCI) model of TBI, and (2) Blocking NBCe1 with DIDS prevents astrocytic swelling that drives RVPs, halting epileptiform activity after CCI._x000D_  
Methods: Severe TBI was induced by CCI in eight P24 age Sprague-Dawley rats. After 3-4 weeks recovery, animals were euthanized for ECS recordings of the neocortex. Probe transient quantification (PTQ) was used to detect transient tetramethylammonium (TMA) concentrations in the ECS of layer 5 neocortex. If spontaneous activity was not detected, a sub-convulsant concentration of 4-AP (5-10 µM) was added to perfusing aCSF. This concentration was determined by challenging the contralateral hemisphere with 4-AP to provoke activity (Figure 1). Once RVPs were detected, DIDS (300 µm) was then added to the perfusing aCSF to measure effects on RVPs and hyperexcitability. Total RVP volume change was calculated by converting voltage to TMA concentration using the Nicolsky slope and ISM interference.

Results: RVPs coincide with epileptiform activity in rat neocortex after CCI injury both spontaneously and with sub-convulsant 4-AP challenge in 8 rats. In both instances ECS volume shrank an average of 7% during RVP events (17 slices, 8 animals). Application of DIDS (300 µm) ceased spontaneous and 4-AP induced RVPs and epileptiform activity (3 slices, 3 animals) (Figure 2).

Conclusions: This study demonstrated that RVPs coincide with epileptiform activity in an in vivo model of TBI, and validates RVPs as a clinically relevant target for interventions to prevent or manage PTE. Moreover, spontaneous and 4-AP induced RVPs are sensitive to the NBCe1 blocker DIDS. By eliminating NBCe1’s ability to drive RVPs, neuronal circuits may recover to a subthreshold state.

Funding: Seed Award, Office of the Senior Vice President of Research SUNY Downstate Health Sciences University