Epileptogenesis induces long-term alterations in intracellular calcium release and sequestration mechanisms in the hippocampal neuronal culture model of epilepsy

Abstract

Calcium and calcium-dependent processes have been hypothesized to be involved in the induction of epilepsy. It has been shown that epileptic neurons have altered calcium homeostatic mechanisms following epileptogenesis in the hippocampal neuronal cultur e (HNC) and pilocarpine models of epilepsy. To investigate the mechanisms causing these alterations in [Ca2+]ihomeostatic processes following epileptogenesis, we utilized the HNC model ofin vitro 'epilepsy' which produces spontaneous recurrent epileptiform discharges (SREDs). Using [Ca2+]iimaging, studies were initiated to evaluate the mechanisms mediating these changes in [Ca2+]ihomeostasis. 'Epileptic' neurons required much longer to restore a glutamate induced [Ca2+]iload to baseline levels than control neurons. Inhibition of Ca2+entry through voltage and receptor gated Ca2+channels and stretch activated Ca2+channels had no effect on the prolonged glutamate induced increase in [Ca2+]iin epileptic neurons. Employing thapsigargin, an inhibitor of the sarco/endoplasmic reticulum calcium ATPase (SERCA), it was shown that thapsigargin inhibited sequestration of [Ca2+]iby SERCA was significantly decreased in 'epileptic' neurons. Using Ca2+induced Ca2+release (CICR) cell permeable inhibitors for the ryanodine receptor (dantrolene) and the IP3 receptor (2-amino-ethoxydiphenylborate, 2APB) mediated CICR, we demonstrated that CICR was significantly augmented in the 'epileptic' neurons, and determined that the IP3 receptor mediated CICR was the major release mechanism altered in epileptogenesis. These data indicate that both inhibition of SERCA and augmentation of CICR activity contribute to the alterations accounting for the impaired calcium homeostati c processes observed in 'epileptic' neurons. The results suggest that persistent changes in [Ca2+]ilevels following epileptogenesis may contribute to the long-term plasticity changes manifested in epilepsy and that understanding the basic mechanisms mediating these changes may provide an insight into the development of novel therapeutic approaches to treat epilepsy and prevent or reverse epileptogenesis.