Abstracts

Rationale: Reactive astrocytosis is a prominent pathological abnormality observed following chemoconvulsive induced status epilepticus (SE) in rodent models of limbic epilepsy. Reactive astrocytes are also seen in resected hippocampi in human mesial temporal lobe epilepsy. We hypothesize that reactive astrocytes have decreased uptake of glutamate and potassium in response to neuronal activity. These acute alterations may contribute to neuronal death and epileptogenesis following pilocarpine SE.Methods: Electrophysiological (whole cell patch-clamp) and immunohistochemical methods were used to study reactive astrocytes in rat hippocampus (stratum radiatum of CA1) from days 1-30 follwoing pilocarpine SE. Results: Reactive astrocytes were readily identified by morphological and electrophysiological criteria beginning 2 days after pilocarpine SE. These cells demonstrated prominent hypertrophy, with high levels of expression of GFAP, vimentin, and nestin. Electrophysiological recordings were carried out 4-7 days following pilocarpine SE. Recorded reactive astrocytes revealed high variability in measured parameters. Many had diminished inward currents in response to the application of 1mM glutamate or 30 mM potassium chloride, respectively. This diminution directly correlated with depolarization of the resting membrane potential (-79.1 ± 6.7 vs. -42.4 ± 4.8). The observed reduction of glutamate and potassium uptake was paralleled by declines in immunoreactivity for glutamate transporters (EAAT1 and EAAT2) and Kir 4.1. All recorded reactive astrocytes lost coupling with neighboring astrocytes, based on diffusion of Lucifer Yellow from the recording electrode. Immunohistochemically staining demonstrated a correspondingly significant reduction in the expression of connexin 30, but not connexin 43. Conclusions: Acutely reactive astrocytes in the pilocarpine model of epilepsy have impaired glutamate and potassium uptake functions. Simultaneous loss of intercellular coupling with neighboring astrocytes may deteriorate functional support of neurons by astrocytes and aggravate seizure related neuronal damage. Changes in astrocytes that acutely follow pilocarpine SE may be pro-epileptogenic.