Rapid Volume Pulsations of the Brain's Extracellular Space Underlying Seizures

Abstract

It has been established that during epileptic seizures the brain’s extracellular space (ECS) undergoes a shrinkage of about 35% for their duration. This slow ECS shrinkage (SES) may be a target to inhibit them. However, by utilizing a technique called probe transients quantification (PTQ), we have discovered transient shrinkage events of the ECS that are correlated with local field potentials during seizure activity. The discovery of these rapid volume pulsations (RVPs) of the ECS led to the two main aims of this thesis: first, to extensively characterize RVPs during seizure activity, and second, to identify the channels and transporters that govern these RVPs. The PTQ technique allowed for the detection of RVPs across multiple mouse seizure models in vitro (hyaluronan synthase 3 knock-out, Picrotoxin, Bicuculline, 4-Aminopyridine) and a pharmacological mouse seizure model in vivo (Bicuculline Methiodide). In the in vivo model, these RVPs reach an average of 14.8% shrinkage of the ECS. By using real-time iontophoresis (RTI) in conjunction with PTQ, we determined that 4-Aminopyridine (4-AP) induced epileptiform activity caused a SES that resulted in 36% shrinkage, and caused RVPs that on average caused 4.4% shrinkage of the ECS. To investigate the channels and transporters required for the generation of RVPs and to determine whether blocking them would stop epileptiform activity, we utilized the PTQ technique in the 4-AP in vitro model and applied pharmacological blockers of cellular transporters. Pharmacological blockade of the electrogenic sodium-bicarbonate cotransporter 1 (NBCe1) by using 4,4'-Diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS, 300 μM) led to the elimination of RVPs and epileptiform activity. RTI during 4-AP and DIDS application revealed that DIDS blocked the SES induced by 4-AP epileptiform activity. Based on these results, RVPs are a ubiquitous component of ECS dynamism during seizures, and the mechanism of their generation serves as a possible target for epilepsy therapy.