Hypoosmolar conditions reduce extracellular volume fraction and enhance epileptiform activity in the CA3 region of the immature rat hippocampus
Article first published online: 21 APR 2006
Copyright © 2006 Wiley-Liss, Inc.
Journal of Neuroscience Research
Volume 84, Issue 1, pages 119–129, July 2006
How to Cite
Kilb, W., Dierkes, P. W., Syková, E., Vargová, L. and Luhmann, H. J. (2006), Hypoosmolar conditions reduce extracellular volume fraction and enhance epileptiform activity in the CA3 region of the immature rat hippocampus. J. Neurosci. Res., 84: 119–129. doi: 10.1002/jnr.20871
- Issue published online: 13 JUN 2006
- Article first published online: 21 APR 2006
- Manuscript Accepted: 8 MAR 2006
- Manuscript Revised: 7 MAR 2006
- Manuscript Received: 6 OCT 2005
- epileptic seizure;
- field potential recordings;
- extracellular space;
The osmolarity of the extracellular space (ECS) compartment is an important factor determining the excitability of neuronal tissue. In the adult hippocampus an important role of osmolarity and ECS diffusion parameters on the susceptibility to epileptic events is well established, but the influence of hypo- and hyperosmolar conditions on the immature hippocampus remains elusive. To investigate the influence of osmolarity on epileptiform activity, extracellular field potentials were recorded in the CA3 region of hippocampal slices of immature (postnatal days 4–7) Wistar rats. The ECS diffusion parameters were determined by the real-time tetramethylammonium (TMA+) iontophoretic method with ion-selective microelectrodes in immature hippocampal slices and showed a lack of diffusion anisotropy; a tortuosity of about 1.39; and a volume fraction, α, of 0.41 ± 0.01 (n = 10 slices). A reduction in osmolarity of –90 mOsm induced a decrease in α to 0.17 ± 0.02 (n = 4 slices). The frequency of epileptiform activity elicited in 10–50 μM 4-AP-containing low-Mg2+ solution was increased under –90 mOsm and –40 mOsm hypoosmolar conditions by 39.9% ± 8.1% (n = 16) and 24.1% ± 4.8% (n = 10), respectively, whereas hyperosmolar solutions decreased the frequency. A –90-mOsm reduction in the osmolarity of low-Mg2+ solution induced epileptiform activity in nine of 19 slices. In summary, these results demonstrate that hypoosmolar conditions increased excitability and susceptibility to epileptiform activity in immature hippocampal slices, suggesting a functional role of the larger α in suppression of seizures. © 2006 Wiley-Liss, Inc.