Abstracts

Rationale: Seizures and brain injury result in water and Cl- accumulation in neurons. Neocortical neuronal membranes have a low water permeability as they lack aquaporins necessary to move free water. Instead, neurons use cotransport of ions, including Cl- to move water. Thus, increasing the extracellular osmolarity during seizures should result in an outward movement of water and salt, reducing [Cl-]i and improving GABAA receptor-mediated inhibition. Methods: We tested the effects of hyperosmotic therapy with a clinically relevant dose of mannitol (20 mM) on epileptiform activity, spontaneous multiunit activity (sIPSCs), [Cl-]i, and neuronal volume in layer IV/V of the developing neocortex. We used electrophysiological and multiphoton imaging techniques in acute brain slices (post-natal day 7-12), and organotypic neocortical slice cultures from C57BL/6 and Clomeleon mice.  Results: Mannitol application resulted in: 1) decreased epileptiform activity, 2) decreased neuronal volume and [Cl-]i through CCCs, 3) decreased spontaneous multi-unit activity frequency but not amplitude, and 4) restored the anticonvulsant efficacy of the GABAA receptor modulator diazepam.  Conclusions: We conclude that an increase in extracellular osmolarity by mannitol mediates the efflux of [Cl-]i and water through CCCs, which results in a decrease in epileptiform activity and enhances benzodiazepine actions in the developing neocortex in vitro. Novel treatments aimed to decrease neuronal volume may concomitantly decrease [Cl-]i and improve seizure control.  Funding: NIH/NINDS 1K08NS091248, NIH/NINDS 5R01NS40109-14