Abstracts

Rationale: Gliosis is a pathological hallmark of many epileptic foci. Persistent change of morphology and loss of astrocytic domain organization has been found in the epileptic brain (Oberheim, et al., 2008). However, there is little known about the functional changes of chronic reactive astrocytes. In our previous studies, we observed dramatic changes in morphology, immunohistochemical staining and electrophysiological properties of acute reactive astrocytes following pilocarpine-induced seizures. We propose that reactive astrocyte changes persist chronically, and may be important contributors to epileptogenesis. Methods: Whole-cell patch-clamp recording was performed on astrocytes in rat hippocampus and piriform cortex one month to three month after pilocarpine-induced seizures. Intercellular gap junction coupling was examined by filling cells with 0.2% Lucifer yellow and/or 0.5% biocytin. Brain slices were fixed for further immunohistochemical staining. Results: One to three months after pilocarpine insult, many chronic reactive astrocytes (RAs) were observed in the vicinity of brain lesions in hippocampal CA1-CA3 region and piriform cortex. Most chronic RAs were found to have fewer fine processes and enlarged branches, with a high level of GFAP expression (n=10). Some fibrous-like astrocytes with long, radial and non-branched extensions could also be seen in the area proximal to the brain lesion. In most chronic RAs, intercellular gap junction coupling was dramatically reduced. Resting membrane potential of chronic RAs was lower than control astrocytes. They showed passive current-voltage (IV) pattern as observed in control astrocytes. However, they had impaired glutamate transport function. Glutamate transporter current was reduced from 574.28 68pA (control, n=9) to 163.33 34.8pA (n=6) (p<0.05). We also observed some chronic RAs with a moderately high level of GFAP expression (n=5), but with cellular appearance similar to that of control protoplasmic astrocytes, with highly ramified, spongiform processes. These chronic RAs were coupled to surrounding cells. Their passive membrane properties were similar to control astrocytes, while glutamate transporter current was only slightly reduced in these RAs (370 128.08 pA, n=4). Conclusions: Our results demonstrate that morphological and physiological changes of acute severe RAs are persistent in many chronic RAs one to three months after pilocarpine insult. Some chronic RAs develop a fibrous-like cellular appearance, similar to that seen in scar astrocytes in human sclerotic cortical and hippocampal tissues. Our findings are consistent with acute severe RA cells progressing to become glial scar cells, while acute mild RA cells may recover morphologically and functionally over time. Further study will determine if such dynamic changes of chronic RAs can be inhibited by agents such as olomoucine or rapamycin.