Abstracts

Rationale: Profound astrogliosis occurs throughout the hippocampus in animal models and in human temporal lobe epilepsy (TLE). While the membrane properties of astrocytes have been evaluated in the hippocampus from normal rodents and in resected hippocampus from patients with TLE, to our knowledge, very few studies have examined, in brain slices, the electrophysiological properties of astrocytes in the kainic acid (KA)-induced status epilepticus (SE) model of TLE. The present study uses patch clamp and immunohistochemical techniques to characterize the membrane properties of glial fibrillary acidic protein (GFAP)-positive and GFAP-negative glial cells in the CA1 region of the hippocampus in animals subjected to KA-induced SE.Methods: Brain slices were prepared from male Sprague-Dawley rats 7-14 days after systemic KA treatment (Smith et al., 2007). Following decapitation, horizontal brain slices (300 µm) were cut on a vibroslicer and placed in oxygenated standard artificial cerebrospinal fluid (ACSF) at 37 C for one hour. Slices were then incubated at room temperature until transferred to the recording chamber. In some experiments, 10µM glycine was added and MgCl2 was omitted from the recording solution. Glial cells in the stratum radiatum of the hippocampus were visualized by IR-DIC microscopy and recorded in the whole cell configuration with biocytin-containing electrodes (5-9 MΩ).Results: Voltage clamp experiments revealed heterogeneity in the resting membrane potentials, input resistances, and voltage dependent membrane currents of hippocampal glial cells. In a subset of cells, post-hoc staining for biocytin and GFAP revealed two classes of glia, GFAP-positive and GFAP-negative, which were correlated with differences observed in membrane properties. Interestingly, cells with linear current-voltage profiles and low input resistance (GFAP-positive) were found to have recurrent spontaneous inward currents in the high glycine, low MgCl2 ACSF (n=5) at a frequency of 0.17 ± 0.07 Hz (mean ± st.dev.). In contrast, spontaneous recurrent inward currents were never observed in the other type of glial cell (with high input resistance, non-linear current-voltage relationships, and GFAP negative).Conclusions: Astrocytes with a linear current profile from the hippocampus of KA-treated rats were found to be GFAP-positive and dye-coupled with other glial cells. These cells were also found to exhibit recurrent, spontaneous inward currents under permissive conditions for neuronal bursting. In contrast, glial cells with a non-linear current-voltage relationship were often GFAP-negative, were not dye-coupled, and did not exhibit recurrent spontaneous inward currents. Ongoing experiments will identify the cellular mechanism underlying the spontaneous inward currents observed in the GFAP-positive cells. It is anticipated that an increased understanding of the role of astrocytes in TLE will provide innovative molecular targets for the treatment of this frequently therapy-resistant seizure disorder. (Sources of funding: KSW: R01 NS44210, DKT: EFA pre-doctoral fellowship)